Previous as well as recent observations by ISO Spitzer AKARI SOFIA JWST etc. have revealed various characteristics of midinfrared emission bands between 320 micron. Subsequently several forms of organics including Polycylic Aromatic Hydrocarbons PAHsPAHslike molecules are proposed as carriers for these bands. Deuterated PAH PAD is one such substituted PAH which is proposed as a potential candidate carrier for weak emission bands at 4.4 and 4.65 micron detected towards few astronomical targets and are characteristics of aromatic and aliphatic CD stretching modes in a PAD molecule respectively. However the 4.4 micron band is not widely detected. In order to validate PADs as carriers for midinfrared emission bands an additional alternative way is crucial. If PAHs are deuterated or deuteronated they should also possess an inherent signature from the CD out of plane CDoop vibrations which are at the longer wavelength side. In this report features due to CDoop modes belonging to PAHs with single to multiple deuteration in the same ring are reported by performing quantumchemical calculations. We found that some of the CDoop vibrations appear at 1419 micron range which does not overlap with the region attributed to CHoop modes. Also the strength of CDoop modes is not proportional to DH in PAHs. In addition new CHoop modes also appear due to the creation of new CH bonds upon deuteration of duet trio and quartet CH sites. We discuss the efficiency and usefulness of these bands to constrain the form of PAHs emitting midinfrared emission bands.
['Astrophysics - Astrophysics of Galaxies', 'Astrophysics - Solar and Stellar Astrophysics']
Investigating CDoop vibrational modes in PAHs as a tool to study interstellar deuteriumcontaining PAHs
{'No Header': ', 1-12 (2024)', 'Investigating C -D oop vibrational modes in PAHs as a tool to study interstellar deuterium-containing PAHs': 'Mridusmita Buragohain 1 ★ ,Amit Pathak 2 , Takashi Onaka 3 , Itsuki Sakon 4  \n1 School of Physics, University of Hyderabad, Hyderabad 500 046, India  \n- 2 Department of Physics, Banaras Hindu University, Varanasi 221 005, India\n- 3 Department of Astronomy, Graduate School of Science, The University of Tokyo, Japan  \n4 Institute of Astronomy, Graduate School of Science, The University of Tokyo, Japan  \nAccepted XXX. Received YYY; in original form ZZZ', 'ABSTRACT': 'Previous as well as recent observations by ISO , Spitzer , AKARI , SOFIA , JWST etc. have revealed various characteristics of midinfrared emission bands between 3 -20 𝜇 m. Subsequently, several forms of organics including Polycylic Aromatic Hydrocarbons (PAHs)/PAHs-like molecules are proposed as carriers for these bands. Deuterated PAH (PAD) is one such substituted PAH, which is proposed as a potential candidate carrier for weak emission bands at 4.4 and 4.65 𝜇 m, detected towards few astronomical targets and are characteristics of aromatic and aliphatic C -Dstretching modes in a PAD molecule, respectively. However, the 4.4 𝜇 mband is not widely detected. In order to validate PADs as carriers for mid-infrared emission bands, an additional alternative way is crucial. If PAHs are deuterated or deuteronated, they should also possess an inherent signature from the C -Dout of plane (C -D oop ) vibrations, which are at the longer wavelength side. In this report, features due to C -D oop modes belonging to PAHs with single to multiple deuteration in the same ring are reported by performing quantum-chemical calculations. We found that some of the C -D oop vibrations appear at 14 -19 𝜇 mrange, which does not overlap with the region attributed to C -H oop modes. Also, the strength of C -D oop modes is not proportional to D/H in PAHs. In addition, new C -H oop modes also appear due to the creation of new C -H bonds upon deuteration of duet, trio, and quartet C -H sites. We discuss the efficiency and usefulness of these bands to constrain the form of PAHs emitting mid-infrared emission bands.  \nKey words: astrochemistry, ISM: molecules, (ISM:) dust, extinction, molecular data, software: simulations', '1 INTRODUCTION': "Free flying gas phase Polycyclic Aromatic Hydrocarbon (PAH) molecules in the Interstellar Medium (ISM) have been so far the most attractive candidate carriers of the frequently detected broad mid-infrared emission bands, popularly known as 'Aromatic Infrared Bands' (AIBs) (Tielens 2008). These bands are manifestations of vibrational modes in PAHs on absorption of background FUV photons (Léger & Puget 1984; Allamandola et al. 1985). While, unadulterated and unsubstituted PAHs show roughly similar features, substitution of heteroatoms or even the slightest modification in the PAHs' geometry (for example: five membered ring PAHs, dehydrogenated PAHs, superhyderogenated PAHs etc.) can give rise to a set of new features or changes the intensity and position of the primary features, some of which can be even observationally detected. Deuterated PAH (PAD) (Bauschlicher et al. 1997; Hudgins et al. 2004) is one such substituted PAH, which is proposed as potential candidate carrier for weak bands at 4.4 and 4.65 𝜇 m towards Orion Nebula and M17 (Peeters et al. 2004b; Onaka et al. 2014; Doney et al. 2016) and are characteristics of aromatic and aliphatic C -D stretching modes present in a PAD molecule, respectively (Buragohain et al. 2020). Onaka et al. (2022) also reported excess emission at 4.4 𝜇 mtowards galactic cen-  \na massing young stellar object (AFGL 2006). JWST NIRSpec has recenlty detected a band at ∼ 4.65 𝜇 m, possibly from aliphatic deuterated PAHs, but it does not show any clear aromatic deuterated PAHsignature at ∼ 4 . 4 𝜇 m(Boersma et al. 2023; Peeters et al. 2024).  \nThe possible presence of deuterium-containing PAHs has been discussed in relation to the 'problem of missing D', which suggests that some primordial missing D might be sequestered in PAHs (Draine 2006; Allamandola et al. 2021). Deuterium is one of the elements formed immediately after the Big Bang and with the chemical evolution of the Universe, nuclear fusion at the stellar interiors convert them into heavier elements. The present lower value of [D/H] total ( gas plus dust ) from the primordial value [D/H] prim should be accounted for solely by chemical evolution model of our Galaxy. However, a wide range of [D/H] gas has been observed along different lines of sight within our own Galaxy which cannot be explained by the chemical evolution model (Linsky et al. 2006). In addition, the measured [D/H] gas seems to be correlated with the interstellar depletion of heavy elements of Fe, Si, Ti, etc., suggesting D depletion onto dust grains. Friedman et al. (2023) recently suggested that the spatial variability of D/H is not solely due to dust depletion. However, on account of the latest observations of deuterated features, some fraction of the missing D may still be attributable to dust depletion or depletion onto PAHs. Deuteration of PAHs may efficiently occur in low-temperature environments due to the difference in the zero-  \npoint energy (Sandford et al. 2001). However, the supposedly main diagnostic spectroscopic tool to confirm deuterated PAHs, i.e. C -D stretching features at 4.4 -4.65 𝜇 mmay not be sufficiently excited in low-temperature environments as it may require high-energy photons to be seen in emission. This makes it difficult to probe the deuteration of PAHs from the study of 4.4 -4.65 𝜇 m emission bands, even though D may be sequestered more in PAHs at low-temperatures or only in large PAHs, both of which are difficult to be detected at 4 𝜇 m. Thus, the search for longer wavelength features arising from deuterium substitution may be important as suggested in Onaka et al. (2022).  \nAmong the majorly observed AIBs, 11.2 𝜇 m feature is one of the most distinctive and examined bands in the AIB spectrum (Roche et al. 1989; Roelfsema et al. 1996; Sloan et al. 1999; Hony et al. 2001; Van Diedenhoven et al. 2004; Pasquini et al. 2024) and has been assigned to the C -Hout of plane (C -H oop ) bending mode of a neutral PAH molecule that contains a solo C -H group (Cohen et al. 1985; Witteborn et al. 1989; Hudgins & Allamandola 1999). A solo C -H group or non-adjacent C -H unit is one with no neighboring adjacent C-H units in the same ring. In a trend of increasing adjacency of C-H groups, PAHs are defined to have a duet (doubly-adjacent), trio (triply-adjacent), quartet (quadruply-adjacent) and quintet C -H (quintuply-adjacent) group (Hudgins & Allamandola 1999), each group giving rise to unique features. The wavelength of C -H oop vibration of a solo C -Hgroup matches well with that of the observed emission feature at 11.2 𝜇 mindicating abundance of PAHs with solo C -H group. A neighbouring feature at 11.0 𝜇 m is proposed to rise from solo C -H oop modes in a PAH cation (Hudgins & Allamandola 1999). If PAHs are deuterated or deuteronated, they should also have an inherent signature from the C -D out of plane (C -D oop ) vibrations, which are at longer wavelength side. Either these could be overlapping with other modes or are weak to be observationally detected. Studying these characteristic bands arising from deuterium substitution can constrain PAHs containing deuterium, if any. This work reports a Density Functional Theory (DFT) study on PAH molecules with a solo, duet, trio and quartet C -D groups to study the C -D oop vibrational modes and to see how deuteration at different adjacency site affects the spectra. The classification of these groups are made in similar ways as for C -Hgroups. Note that oops modes are relatively easily excited even in low-temperature regions and/or arise from larger PAHs. The paper is arranged in the following manner; i) Theoretical Approach, ii) Results, where PAHs with each type (solo, duet, trio and quartet C -H/C -D groups) are discussed separately, iii) Discussion & iv) Conclusion.", '2 THEORETICAL APPROACH': "Theoretical quantum chemical calculations, such as DFT has appeared to be reliable and time efficient for studying vibrational properties of PAHs, which is required to find correlation with the observed AIBs(Hudginsetal. 2001, 2004; Pathak & Rastogi 2005, 2006, 2007; Candian et al. 2014; Buragohain et al. 2015). For our purpose, we are performing DFT combined with B3LYP/6-311G** basis to calculate the optimized structure of the PAHs which will further be used to obtain the harmonic frequencies and intensities of vibrational modes of the molecules at the same level of theory. In order to bring down the computed frequency which is usually overestimated compared to experimental frequency, mode-dependent scaling factors are used (Buragohain et al. 2015, 2016). The harmonic approximation with scaling factors of the oop modes provides sufficiently accurate wavelengths and intensities for the purpose of the present study (Mackie  \net al. 2022; Lemmens et al. 2023; Esposito et al. 2024). Relative intensities (Int rel ) are obtained with respect to the integrated intensities of the C -H stretching modes, which is apparently the maximum intensity for neutral PAHs. The computed wavelength and Int rel of the lines are used to plot a Gaussian profile. Line width of FWHM 10 cm -1 is adapted so that individual bands can be seen discretely. We have used QChem (quantum chemistry package) to perform our calculation (Shao et al. 2015). For our study, molecules are chosen in terms of adjacency of C -H units which are then substituted with deuterium to form PAH molecule with solo, duet, trio and quartet C -D units. Previous observations by Doney et al. (2016) do not support high deuteration of PAHs' structure in the ISM. Considering that, PAHs of small to moderate size with duet, trio and quartet C -Dis comparatively less likely to exist. However, the motivation of this paper is to see the spectral features arising from C -D oop modes of different adjacency type and therefore, we consider PAHs with solo, duet, trio and quartet C-D unit. A molecule can have more than one unique position of solo, duet, trio and quartet C -D units and there is no significant energy difference depending on the position of deuterium indicating that substitution of D occurs with equal probability in all positions and for that reason, we consider any position of deuteration for our study. Note that a variation in the spectra is expected as we consider different isomers. Also, in the same molecule which exhibits different types of adjacency site simultaneously, for example: pyrene (it has both duet and trio C -H/C -D sites), there is no preferential site of deuterium substitution for the same number of substituting D.", '3 RESULTS': 'In the following sections, we will particularly focus on the C -D oop vibrational modes of deuterated PAHs in terms of adjacency of C -D bonds and explore how deuteration of the C -H adjacency group affects the existing oop bands and the overall spectra.', 'PAHs with solo C -Hgroup:': 'Anthracene (C 14 H 10 ) and pentacene (C 22 H 14 ) are PAH molecules with both solo and quartet C -Hsites. Here, we will discuss the characteristic behaviour of deuterated anthracene and deuterated pentacene, when D substitutes a H at the solo C -Hsite and later discuss the same molecules, when D substitution occurs at the quartet C -H site at subsequent steps resulting into solo, duet, trio and quartet C -D unit. Figure 1 shows the spectra of anthracene and pentacene along with their deuterated counterparts. We have marked the wavelength for relevant significant features as well as the C -D oop modes. Also, D/H 1 in the PAH species is mentioned in the figure. In a nondeuterated anthracene (Fig 1a), oop motion of solo C -Hgroups gives a feature at 11.51 𝜇 m(Int rel 0.38). This position is comparatively red shifted in comparison to that of the observed emission feature at 11.2 𝜇 m. The oop vibrational mode of quartet C -H groups, which is larger in number, gives an intense peak at 13.93 𝜇 m (Int rel 0.52). Whenadeuterium atom replaces a H atom at a solo C -Hsite, instead of a single intense peak, two comparatively moderate peaks arise at 11.38 (Int rel 0.12) and 11.97 𝜇 m(Int rel 0.14) which are characteristics of solo C -H oop and the combinational mode of both solo and quartet C -H oop respectively (Fig 1b). The intense quartet C -H oop vibration is slightly blueshifted from 13.93 𝜇 mto 13.67 𝜇 mwith a decrease in  \nFigure 1. Theoretical spectra of (a) anthracene, (b) singly-deuterated anthracene, (c) pentacene, (d) singly-deuterated pentacene. The location of deuterium substitution is fixed at a solo C -H site.  \n<!-- image -->  \nintensity to Int rel ∼ 0.44 in singly-deuterated anthracene along with the appearance of a feature at 12.78 𝜇 m (Int rel 0.1) due to quartet C -H oop modes. Another unique feature is 15.12 𝜇 mline (Int rel 0.09) which is characteristic of solo C -D oop motion antisymmetric with quartet C -H oop motion. The remaining small feature at ∼ 16.6 𝜇 m feature (Fig 1a-b) appear due to C -C -C inplane bending mode and are not associated with either C -H oop or C -D oop vibrational modes.  \nA larger PAH molecule, for example, pentacene (C 22 H 14 ), shows an intense solo C -H oop vibrational mode at 11.18 𝜇 m (Int rel 0.39), a weak antisymmetric solo C -H oop mode at 12.27 𝜇 m (Int rel 0.08) and a quartet C -H oop mode at 13.84 𝜇 m (Int rel 0.35) as shown in Fig 1c. Substituting a H at solo C -Hsite with a D (Fig 1d) results into two closeby solo C -H oop modes appearing at 11.22 𝜇 m(Int rel 0.22) and 11.43 𝜇 m (Int rel 0.14) and a quartet C -H oop mode retaining its position as its non-deutearted counterpart at 13.81 𝜇 m (Int rel 0.30). The small feature at 12.27 𝜇 minnon-deuterated pentacene continues to be present with a much reduced intensity alongside an additional feature at 11.97 𝜇 m (Int rel 0.05) in singly-deuterated pentance (Figure 1d). Deuteration also produces two additional minor C -H oop modes at 13.04 𝜇 m (Int rel 0.02) and 13.44 𝜇 m (Int rel 0.01) in Figure 1d. All these three features are due to combinations of solo and quartet C -H oop modes. The solo C -D oop vibrational mode, which is apparently the direct manifestation of deuteration of PAHs, is vaguely present at 15.25 𝜇 m with Int rel 0.01 (Fig 1d) in singly-deuterated pentance. The two minor features at ∼ 16.05 𝜇 m and ∼ 17.72 𝜇 m are present in the spectra of both the forms of pentacene and are due to C -C -C inplane bending modes. Note that we have marked the wavelengths (in all spectra) which are relevant for the discussion of this paper.', 'PAHs with duet C -Hgroup:': 'Pyrene (C 16 H 10 ) has both duet and trio C -H sites, at which we can substitute H with D. In this section, we discuss vibrational features of pyrene with D substituting H at the duet C -H site. A single D substitution will first give rise to a solo C -H and a solo C -D bond  \nat the substitution site, while two Ds will give rise to a duet C -D unit. Since, there is no preferential site for deuteration substitution, the placement of second deuterium can occur at any C -H site (either duet or trio C -H unit or in a different benzene ring), but we constrain ourselves to place the deuterium atoms at the duet C -H site in the same ring to generate a duet C -D unit. In case of nondeuterated neutral pyrene, the strong peak near 12 𝜇 m (Int rel 0.62) and 14.18 𝜇 m (Int rel 0.27) with another moderately intense peak at 13.65 𝜇 m (Int rel 0.09) are manifestations of duet and trio C -H oop vibrational modes (Figure 2a). The remaining much weaker features are due to C -C -C inplane bending. On substituting H at the duet C -H site with a single and double deuterium atoms, there is a small change in wavelength positions by 0.1 to 0.2 𝜇 m and intensities of these bands along with a few new features emerging (Figure 2b-c). The feature at 13.65 𝜇 m blueshifts to 13.22 𝜇 m with a reduced intensity in case of both singly and doubly-deuterated pyrene and a new feature due to C -H oop vibration appears at 14.7 𝜇 m(Int rel 0.06) only in case of singly-deuterated pyrene. The C -C -C inplane bending feature at 12.29 𝜇 m (Int rel 0.02) in non-deuterated pyrene shifts to 12.58-12.72 𝜇 m (Int rel 0.01) in singly-deuterated pyrene and to 12.83 𝜇 m (Int rel 0.02) in doubly-deuterated pyrene. New features include a few modes at 11.08 𝜇 m(Int rel 0.05), 11.31 𝜇 m(Int rel 0.03) and 11.45 𝜇 m (Int rel 0.03) that emerge in singly-deuterated pyrene. Unlike non-deuterated pyrene that lacks a solo C -H unit and does not show a feature at ∼ 11.2 𝜇 m, substituting by a single D at one of the two available duet C -H sites enables the pyrene molecule to gain a solo C -Hunit and produce solo C -H oop modes at these wavelengths. There also exists a C -D oop vibrational mode at 15.56 𝜇 m (Int rel 0.04) for singly-deuterated and 15.78 𝜇 m (Int rel 0.07) for a doubly-deuterated pyrene.  \nCoronene (C 24 H 12 ) is a compact, symmetric molecule with all the peripheral C -H sites being duet or doubly-adjacent and the corresponding spectra is ordinary compared to other PAHs (Fig 2d). The distinct features of a non-deuterated coronene in the range of 10 -20 𝜇 m are duet C -H oop modes at 11.75 𝜇 m (Int rel 0.68) and C -C -C oop modes (partially contributed by duet C -H oop modes)  \nFigure 2. Theoretical spectra of (a) pyrene, (b) singly-deuterated pyrene, (c) doubly-deuterated pyrene, (d) coronene, (e) singly-deuterated coronene, (f) doublydeuterated coronene. The location of deuterium substitution is fixed at a duet C -H site.  \n<!-- image -->  \nat 18.15 𝜇 m (Int rel 0.17). The small features near ∼ 13.1 𝜇 m (Int rel 0.05) are due to C -C -C inplane bending modes. On deuteration, duet C -H oop mode at 11.75 𝜇 mis slightly shifted to 11.87 𝜇 m in both singly and doubly-deuterated coronene with Int rel ∼ 0.54 and ∼ 0.6, respectively (Figure 2e-f). Deuteration also produces additional C -H oop features at 12.3 𝜇 m (Int rel ∼ 0.04) in singly and doubly-deuterated coronene. The 18.15 𝜇 m in non-deuterated coronene shifts to 18.33 𝜇 m in singly-deuterated coronene with additional features of smaller intensities in the longward side. The same feature is present at 18.52 𝜇 mindoubly-deuterated coronene. A combination of C -C -C oop and C -D oop mode is present at 19.15 𝜇 min doubly-deuterated coronene (Fig 2f). Besides, deuterium substitution produces additional vibrational modes at ∼ 11.19 𝜇 m(Int rel ∼ 0.08) in singly-deuterated coronene for a similar reason as in singlydeuterated pyrene due to solo C -H oop (Figure 2e). There is no explicit feature solely due to C -D oop motion in either a singly or a doubly-deuterated coronene.', 'PAHs with trio C -Hgroup:': 'As mentioned in previous section, pyrene has trio C -H sites, too, apart from duet C -H sites and this section will discuss C -H oop vi-  \nbrational features, when single or multiple Ds substitute H atoms at the trio C -H sites of pyrene. The subsequent D substitutions at trio C -H sites in the same benzene ring will produce solo, duet and trio C -Dunits at subsequent steps. The calculated spectra are shown in Fig 3(a-d). The features of non-deuterated pyrene have already been discussed in previous section. A clear difference can be seen between the spectral features of a deuterated pyrene depending upon whether the site for D substitution is a duet or a trio C -H site (Figure 2 and 3), indicating that the position of D affects the spectra. Unlike non-deuterated pyrene that shows two distinct features between 13 -15 𝜇 m, deuteration at trio C -H sites would arise several features, which are majorly C -H oop vibrational modes (Figure 3bd). The comparatively intense feature at ∼ 12 𝜇 m in non-deuterated pyrene almost appears at a similar wavelength upon deuteration with a slight reduction in intensity (Figure 3b-d), giving off neighbouring features due to either more C -H oop modes or C -C -C inplane modes. The feature at 13.65 𝜇 m in non-deuterated pyrene shifts to ∼ 13.4213.43 𝜇 m in singly, doubly and triply-deuterated pyrene. The presence of features at 11.3 𝜇 m(Int rel ∼ 0.06) and 11.4 𝜇 m(Int rel ∼ 0.06) (Figure 3c) is clearly visible arising from C -H oop modesofsoloC -H unit, which is created when two deuterium atoms replace two H atoms at the trio C -H site. In addition, C -H oop combined with C -D oop  \nFigure 3. Theoretical spectra of (a) pyrene, (b) singly-deuterated pyrene, (c) doubly-deuterated pyrene, (d) triply-deuterated pyrene, (e) perylene, (f) singlydeuterated perylene, (g) doubly-deuterated perylene, (h) triply-deuterated perylene. The location of deuterium substitution is fixed at a trio C -H site.  \n<!-- image -->  \nmodes contribute to produce features at 14.76 𝜇 m(Int rel ∼ 0.09) in a singly-deuterated pyrene, at 13.28 𝜇 m (Int rel ∼ 0.07) and 14.31 𝜇 m (Int rel ∼ 0.16) in a doubly-deuterated pyrene (Fig 3c) and at 12.93 𝜇 m (Int rel 0.09) and 14.34 𝜇 m(Int rel ∼ 0.15) in a triply-deuterated pyrene (Fig 3d). A feature due to a solo C -D oop mode is present at 15.67 𝜇 m (Int rel ∼ 0.11, Fig 3b) in singly-deuterated pyrene (a single deuterium will correspond to solo C -D oop mode). The analogous duet and trio C -D oop modes appear at a longer wavelength position of 16.4 𝜇 m (Int rel ∼ 0.13, figure 3c) for a doubly-deuterated pyrene and 16.89 𝜇 m (Int rel ∼ 0.14, figure 3d) for a triply-deuterated pyrene, respectively.  \nAnother example of PAH molecule with trio C -Hsite is perylene (C 20 H 12 ) and Figure 3(e-h) shows the calculated spectra for nondeuterated perylene (C 20 H 12 ) and deuterated perylenes with number of D=1, 2 & 3, where D substitutes H at the trio C -H site. The most important features in a non-deuterated perylene in the 11 -15 𝜇 m are trio C -H oop modes that occur at 12.49 𝜇 m (Int rel ∼ 0.54) and 13.2 𝜇 m(Int rel ∼ 0.41) as shown in Fig 3e. There exists a less intense C -C -C inplane bending mode at 12.74 𝜇 m(Int rel ∼ 0.04). Remaining small features at 17.37 𝜇 m and 18.44 𝜇 m are due to C -C -C inplane bending and C -C -C oop bending, respectively. Similar to previous scenarios, on account of adding single or multiple Ds at the trio C -H site, these features slightly change the wavelength positions  \nalong with the appearance of new small features, which are mostly C -H oop modes or other C -H/C -C -C bond vibrations. An effect of deuterium substitution can be seen at 14.37 𝜇 m (Int rel ∼ 0.07) and 17.12 𝜇 m (Int rel ∼ 0.02) in a singly-deuterated perylene (Fig 3f), where C -D oop mode, coupling with other modes is present. Similar modesappearat15.15 𝜇 m(Int rel ∼ 0.06) and 17.27 𝜇 m(Int rel ∼ 0.03) for doubly-deuterated perylene and at 13.99 𝜇 m (Int rel ∼ 0.02), 15.29 𝜇 m (Int rel ∼ 0.04) and 19.14 𝜇 m (Int rel ∼ 0.08) for triplydeuterated perylene.', 'PAHs with quartet C -Hgroup:': 'In this section, we present the spectra of deuterated anthracene and deuterated pentacene when deuterium substitution occurs at quartet C -H site. The characteristics of the non-deuterated anthracene and non-deuterated pentacene are described before. Deuterium substitution at the quartet C -H site does not significantly change the position of the solo C -H oop vibrational modes in deuterated anthracenes (Fig 4b-e). Exception is a triply-deuterated anthracene (Fig 4d), in which an additional solo C -H site is created besides the existing solo C -Hsites because of deuterium substitution, which results into several solo C -H oop modes. These modes occur at 11.06, 11.37, and  \nFigure4. Theoretical spectra of (a) anthracene, (b) singly-deuterated anthracene, (c) doubly-deuterated anthracene, (d) triply-deuterated anthracene, (e) quadruplydeuterated anthracene, (f) pentacene , (g) singly-deuterated pentacene, (h) doubly-deuterated pentacene, (i) triply-deuterated pentacene & (j) quadruply-deuterated pentacene. The location of deuterium substitution is fixed at a quartet C -H site.  \n<!-- image -->  \n11.62 𝜇 m. The other intense peak at 13.93 𝜇 mdueto quartet C -H oop modes in non-deuterated anthracene appears at the range of 13.713.8 𝜇 m in deuterated anthracenes with a decrease in intensity. An additional weak feature due to C -H oop modes appears at 12.63 𝜇 m (Int rel ∼ 0.03) in singly-deuterated anthracene (Figure 4b) The effect of deuterium substitution is seen near 15.37 𝜇 m(Int rel 0.11) (Fig 4b) which is a C -D oop merged with C -H oop in the same ring. When we deuterate two of the four quartet C -H sites in the same ring, the same feature (antisymmetric and symmetric) appears at 14.24 𝜇 m (Int rel 0.05) and 16.02 𝜇 m (Int rel 0.15) respectively (Fig 4c). On further increasing the deuteration upto 𝐷 = 3 and 4 in the same ring, only the quartet C -D oop mode exists eliminating the quartet C -H oop from the combination and is redshifted to 16.96 𝜇 m ( Int rel 0.18, Fig 4d) and 17.03 𝜇 m ( Int rel 0.18, Fig 4e) respectively. In addition, for quadruply-deuterated anthracene, there is an excess combinational mode of quartet C -D oop and quartet C -H oop at 13.19 𝜇 m ( Int rel 0.05, Fig 4e). The 16.6 𝜇 m feature due to C -C -C inplane bending in non-deuterated anthracene persists to occur at a similar wavelength after deuteration, although, with increasing deuteration, its position overlaps with that of C -D oop modes and cannot be distinguished.  \nFig 4(f-j) presents the computational vibrational spectra of pen-  \ntacene along with its deuterated counterpart with number of D=1, 2, 3 and 4 when D substitution occurs at the quartet C -Hsites, resulting into solo, duet, trio and quartet C -D units at subsequent steps. The solo C -H oop modes at ∼ 11.18 𝜇 m remain invariant upon deuteration. Exception is a triply-deuterated pentacene (Figure 4i), in which triple deuteration in the same ring at the edge creates an additional solo C -Hunit, the oop vibration of which produces additional feature at 11.04 𝜇 m (Int rel 0.23), next to the 11.22 𝜇 m with similar Int rel . The quartet C -H oop mode appears at 13.78 𝜇 m upon deuteraion, close to the position as in a non-deuterated pentacene, with a reduction in relative intensity. A few new features appear mostly due to C -H oop modes and these are weak in intensity. A C -D oop mode arises which tends to be redshifted with the increasing number of deuterium (D=1,2,3,4) and appears at 15.46 𝜇 m (Int rel 0.04), 16.05 𝜇 m (Int rel 0.07), 17 𝜇 m (Int rel 0.09) and 17.05 𝜇 m (Int rel 0.09), respectively, as shown in Fig 4(g-j).', 'Large sized compact PAHs with deuterium:': 'This paper also reports C -D oop features from large sized compact PAHs, which are expected to survive the extreme condition of the interstellar environment more readily. As PAHs grow into a compact  \nFigure 5. Theoretical spectra of (a) circumcoronene, (b) singly-deuterated circumcoronene, (c) circumovalene, (d) singly-deuterated circumovalene. The location of deuterium substitution is fixed at a solo C -H site.  \n<!-- image -->  \nand large form, for example: circumcoronene and circumovalene, they would possess more solo and duet C -H sites and lack trio and quartet C -Hsites. Figure 5 represents the spectra of circumcoronene and cicrcumovalene along with their deuterated counterparts when D substitutes H at the solo C -H site. In a non-deuterated circumcoronene (Figure 5a), the intense peak at 11.3 𝜇 m (Int rel ∼ 0.4) arises majorly due to a solo C -H oop vibrational mode. The remaining small peaks in the range are at 12.05 𝜇 m (Int rel ∼ 0.02) due to a C -C -C inplane bending, 12.92 𝜇 m (Int rel ∼ 0.08) due to an antisymmetric oop motion of solo and duet C -H groups, 13.94 𝜇 m (Int rel ∼ 0.06) majorly due to a duet C -H oop motion, 15.56 𝜇 m (Int rel ∼ 0.02) due to a C -C -C inplane bending and 17.72 𝜇 m (Int rel ∼ 0.08) due to a symmetric oop motion of solo and duet C -H groups along with C -C -C oop . On deuteration of a solo C -H group, the intensity of the solo C -H oop mode at 11.3 𝜇 m is slightly reduced to Int rel ∼ 0.32, without changing the position and part of the intensity is redistributed to give an additional small feature at 12.29 𝜇 m (Int rel ∼ 0.03) and 12.58 𝜇 m (Int rel ∼ 0.01) etc., majorly due to a duet C -H oop and some C -D oop modes (Figure 5b), apart from the features that are common to its non-deuterated counterpart. A solo C -D oop mode partially combined with other modes is present at 18.39 𝜇 m(Int rel ∼ 0.02). For larger PAHs, say, circumovalene (Figure 5c-d), similar behaviour is seen, when we move from non-deuterated to deuterated PAH. The energy of solo C -H oop at a ∼ 11.3 𝜇 m is slightly reduced, which gets distributed to neighboring modes upon deuteration. A solo C -D oop mode combined with other modes is observed at 18.59 𝜇 m (Int rel ∼ 0.01) in singly-deuterated circumovalene. Remaining new features are mostly not contributed by the substituting C -D bond.  \nBoth circumcorone and circumovalene carry duet C -Hgroups and their spectra are shown for these species in Figure 6, when D substitutes H at the duet site at subsequent steps, producing a solo C -Dunit  \nfirst and then a duet C -Dunit. Similar to all previous scenarios, additional less intense features arise besides the already existing features in case of both circumcoronene and circumovalene (Figure 6a-f). Since Ds are at the duet site, it does not change the intensity and position of solo C -H oop mode at 11.3 𝜇 m. An additional solo C -Hunit is created because of single deuterium substitution at the duet C -H site of circumcoronene and circumovalene, which produces an additional solo C -H oop modeat 11.04 𝜇 m(Int rel ∼ 0.05) and at 11.02 𝜇 m (Int rel ∼ 0.05), respectively in singly-deuterated circumcoronene and singly-deuterated circumovalene. D substitution should somewhat affect the duet C -H oop modes. However, duet C -H oop modes are not the strongest in case of circumcoronene as well as circumovalene and any change in them is insignifcant apart from giving off additional features. A C -D oop mode combined with other modes is present at 18.05 𝜇 m (Int rel ∼ 0.02) and at 18.26 𝜇 m (Int rel ∼ 0.04) for singly and doubly-deuterated circumcoronene, respectively. The C -D oop modes in deuterated circumovalenes, however, are overpowered by other modes, making it less apparent.', '4 DISCUSSION:': "While the 4 . 4 and 4 . 65 𝜇 m features are considered as the standard tool to probe the deuterium-containing PAH molecules in the ISM, these are not observationally detected in abundance with significant intensity. This seeks for a supportive diagnostic tool for deuteriumcontaining PAHs at longer wavelength side and C -D oop could serve the purpose if any 'unique' C -D oop feature can be found out, which does not interfere with known C -H or C -C -C modes, even if it is weak. In this report, we have considered substitution at the solo, duet, trio and quartet C -Hsites of select PAHs by single or multiple Ds to understand the spectral features arising from these species.  \nFigure 6. Theoretical spectra of (a) circumcoronene, (b) singly-deuterated circumcoronene, (c) doubly-deuterated circumcoronene, (d) circumovalene, (e) singly-deuterated circumovalene, (f) doubly-deuterated circumovalene. The location of deuterium substitution is fixed at a duet C -H site.  \n<!-- image -->  \nTable 1. Band position and relative intensity of the existing solo C -H oop modes upon deuteration  \nIn addition to producing a C -D oop mode, replacing H by D at the periphery of PAH modifies the adjaceny group of principal C -H bond. For example: a duet C -H site is converted into a solo C -H site when a single D substitutes an H in the same ring producing a solo C -H and a solo C -D bond.  \nSince the existing C -H bonds undergo a modification in terms of number, symmetry and adjaceny classification upon deuteration, a change in the spectrum corresponding to the wavelength position  \nand intensity of the existing C -H oop mode is expected. Table 1-4 summarise the wavelength position and intensity of originally appearing dominant C -H oop modes in that particular molecule before and after deuteration. While the wavelength positions remain nearly the same, the intensities appear to be affected on account of deuterium substitutions. Deuterium substitution in the C -H site lowers the number of C -Hbonds in that molecule, resulting into a reduction in the overall intensity of C -H oop . Large-sized PAHs (for example:  \nTable 2. Band position and relative intensity of the existing duet C -H oop modes upon deuterationTable 3. Band position and relative intensity of the existing trio C -H oop modes upon deuteration  \nTable 4. Band position and relative intensity of the existing quartet C -H oop modes upon deuteration  \ncircumcoronene and circumovalene) are, however, insusceptible to this substitution in terms of intensity and wavelength position. This is possibly due to large number of C -Hbonds in larger PAHs, which are not affected by mere deuteration.  \nApart from any changes in the wavelength position and intensity of existing principal C -H oop vibrational modes, a few minor features may appear usually between 10 -15 𝜇 mafter deuterium substitution. These are new modes of different combinations of C -Hbonds. These are of weak to moderate intensities and are caused by the change in the PAHs' structure. While in smaller to mid-sized PAH molecules, these features may appear with visible intensity, spectra of large sized PAH molecules (C ≥ 50) are not greatly affected by mere deuteration and these modes, although present are feeble in intensity. D substitution may cause a duet C -H group to naturally convert into a solo C -H and a solo C -D bond. As a result, an additional solo C -H oop feature near 11.2 𝜇 m starts to appear because of the newly occurring solo C -H bond along with the already existing duet C -H oop modes. An example is a singly-deuterated pyrene or a singly-deuterated coronene in Figure 2b and 2f, in which additional features near ∼ 11.2 𝜇 mappear due to the solo C -H oop near the oop modes of remaining duet C -H bonds at 11.8 -12 𝜇 m. This feature was not present before deuterium substitution. However, this feature is less intense compared to what is obtained from a PAH originally comprising of many solo C -Hbonds. Other C -Hgroups can also be modified into some other forms, say a trio C -H site into a duet or a solo C -H site and a quartet C -H site into a trio, a duet or a solo C -site with increasing D substitution. Accordingly, new features start to appear resulting from these newly occurring groups.  \nFeatures due to C -D oop : This report presents a feature due to C -D oop bending mode in deuterated PAH molecules in addition to the changes in the spectra upon deuteration, which is discussed  \nabove. However, this mode can partially be combined with other C -H oop or C -C -C oop modes and cannot be counted as the sole effect of deuterium substitution. The C -H oop modes occur in a fairly distinctive gap of 11.1 -13.6 𝜇 m with increasing adjacency of C -H bonds. We can classify the C -D groups too as solo, duet, trio and quartet C -D group depending upon how many neighbouring C -D bonds are attached in the same ring. This will allow us to study the wavelength regions of solo, duet, trio and quartet C -D groups. Table 5 lists the positions and intensities of C -D oop modesfor a range of molecules that are classified in terms of increasing adjacency of C -D unit. Modes with Int rel ≥ 0.04 are only listed and those below this are considered as too feeble to be seen. The last column in the Table 5 indicates the location or adjacency type of C -Hunits at which the substitution of D or Ds takes place to produce solo, duet, trio and quartet C -D units. For example: the location could be a quartet C -H site, but single deuterium substitution at this site will convert it into one solo C -D unit and another trio C -H unit. Assuming the difference in C -H and C -D reduced mass (approximate ratio is 1.34), wavelengths corresponding to C -D oop vibrational modes can be estimated. This approach suggests that the solo C -D oop should be at around 15 𝜇 m, duet C -D oop at around 17 𝜇 m, trio C -D oop at around 18 𝜇 m and quartet C -D oop slightly further. The results in table 5 are near these ranges with small deviations. These deviation could be due to the interaction with other existing C -H or C -C -C vibrational modes and a simple estimation will not work to accurately calculate the positions of C -D oop vibrational modes.  \nWhilethesoloC -D oop modeappearsintherangeof14.4 -15.7 𝜇 m for our sample molecules, which does not overlap with the region attributed to solo C -H oop modes, with increasing adjacency of C -D units, the C -D oop modesgradually shift to longer wavelength side towards ∼ 17 𝜇 m. For coronene and perylene, this even goes to ∼ 19 𝜇 m  \ndue to duet C -D oop and trio C -D oop , respectively with moderate intensities. For very large sized compact PAHs, like circumcoronene and circumovalene with occupancy of solo and duet C -D units, the C -D oop feature of respective adjacency site is overpowered by other modes and is present with feeble intensities. For example: there exist solo C -D oop vibrational modes with contribution from other modes at 14.49 & 14.65 & 18.39 𝜇 m etc. in singly-deuterated circumcoronene, although these appear with negligible intensities. This occurs to other large sized PAHs too, in which presence of large number of C -Hmodes overpower the C -Dmodes. This makes it difficult to probe deuterated features in large-sized PAHs with minimal deuteration. Unlike C -H oop modes, which has a distinctive wavelength range depending upon the adjacency type of C -H units, the wavelength range of C -D oop modes belonging to different adjacency group may not be as distinctive as C -H oop modes, since C -D oop modes endure partial contribution from other modes. Nonetheless, the wavelengths of C -D oop modes do not overlap with the wavelength range attributed to C -H oop modes. With increasing adjacency of C -D units from solo to quartet signifying increasing number of deuterium from D=1 to 4 in the same benzene ring, the intensity of these modes increase. While deuterated features at 4.4 and 4.65 𝜇 mare detected towards limited sources (Doney et al. 2016) indicating that the incorporation of deuterium in PAHs could be rare with high dependency on the local conditions of the source, however, Dmay be sequestered only in large-sized PAHs, which is not yet observationally tested. For such PAHs, intensities of 4.4 and 4.65 𝜇 m are likely to be minimum. Hence, judging the deuteration process in PAHs only through the detections of 4.4 and 4.65 𝜇 mfeatures might not be appropriate. With growing size, compact symmetric PAHs tend to have more solo and duet C -H units compared to other adjacency type of C -H units, limiting the deuteration site in the same ring, which in turn would produce less intense deuterium associated vibrational modes. It is worth to mention that we consider cases in which D atoms are attached to the aromatic C atom, although, astronomical observations also suggest that D could be attached to an aliphatic C atom in a PAH molecules (Peeters et al. 2004b; Onaka et al. 2014; Buragohain et al. 2015), and in such a scenario, C -D oop modes might occur at other wavelengths. In this paper, we are not exploring the characteristic wavelengths from such species.  \nUsing the 3.3 and 4.4 𝜇 m bands, which are direct manifestations of aromatic C -H and aromatic C -D stretching in a PAH molecule, Yang et al. (2020, 2021) derived a D/H ratio in interstellar PAHs by computing the C -H and C -D stretching band strength (per bond) ratio and comparing that with observed intensities of 3.3 and 4.4 𝜇 m bands. The estimated degree of deuteration in PAHs (fraction of peripheral D atoms attached to aromatic C ring in PAH) was measured to be 2.4%. In a similar manner, band strength (per bond) of C -H oop modes and the analogus C -D oop modes may also be useful to estimate degree of deuteration in PAHs. From the sample molecules reported in this paper, we measure band strength of the C -D oop ( per bond ) band strength of the C -H oop ( per bond ) , which is found to vary among the sample species unlike the intrinsic band strength ratio of 3.3 and 4.4 𝜇 m. The measured ratio is in the range of 0.15 to 1.21. While, the band strength of C -H oop modes (per bond) is in the range of 12 -17 km mol -1 , the band strength of C -D oop modes (per bond) spans a wide range of 2 -17 km mol -1 . Measuring the intrinsic band strength of C -D oop modes on a per bond basis may not be as accurate as measuring the intrinsic band strength of C -D stretching modes, the reason being C -D oop modes are often found to be coupling with other modes and are difficult to isolate. Even if C -D oop modes are detected, these would suggest a  \nlower limit of D/H. This is in contrast to the C -H/C -D stretching modes, which are not coupled with other modes.  \nAstronomical implications: In addition to the well established major PAH mid-infrared bands, with its powerful sensitivity, Spitzer have also revealed secondary weaker bands spanning in the range of 15 -20 𝜇 m towards several sources including reflection nebulae, planetary nebulae, H /i.pc/i.pc regions, Herbig Ae/Be star etc. (Werner et al. 2004; Sellgren et al. 2007; Boersma et al. 2010; Peeters et al. 2012). The features are mainly at 15.8, 16.4, 17.0, 17.4, 17.8, 18.9 𝜇 m exhibiting spatial distributions. Studies following the detection suggested several probable independent species as carriers to explain these features, since these features are not related to each other. The 15 -20 𝜇 m wavelength in general corresponds to in-plane and out-of-plane ring bending modes of the carbon skeleton (C -C -C). Boersma et al. (2010) found no correlation between the detected features in the 15 -20 𝜇 m region and the mid-IR PAH features except for the 16.4 𝜇 m feature, which showed correlation with the 6.2 and 7.6/7.8 𝜇 mbands. PAHs consisting 'pendent' rings could reproduce the 16.4 𝜇 m feature, however, the absence of another feature at 13.5 𝜇 m feature expected from the same species poses a question upon this assignment. Peeters et al. (2012) found correlation of the bands at 15.8 𝜇 m and 15 -18 𝜇 m plateau with the 11.2 𝜇 m PAH band and 16.4 𝜇 m band with the 12.7 𝜇 m PAH band. Large sized neutral PAHs are proposed as carriers for the 15.8 𝜇 m band and the 15 -18 𝜇 m plateau, ionized PAHs for the 16.4 𝜇 m band, both neutral and cationic PAHs for the 17.8 𝜇 m band, C 60 for the 18.9 𝜇 m and the 17.4 𝜇 mfeature (Peeters et al. 2012, and references therein). Several other candidate carriers, for example: Diamonds, fullerenes, nanoparticles of specific minerology are too proposed as carriers for some of these bands (Sellgren et al. 2007; Werner et al. 2004). Recent JWST observations of the Orion Bar suggests the presence of a very weak feature at 14.21 𝜇 m(Chown et al. 2024). It is still challenging to detect faint features at wavelengths longer than ∼ 13 𝜇 meven with the JWST.  \nOur study presents several wavelengths between 14 -19 𝜇 m due to the C -D oop of weak to moderate intensities belonging to different C -Dadjacency category. This suggests that besides the already proposed carriers for the 15 -20 𝜇 m previously, deuterium containing PAHs could also be an elusive candidate carrier for these features. While the C -D oop wavelengths from our sample show a slight variation in wavelength to be precisely compared with the peak positions of the observed features in the 15 -20 𝜇 mregion, a good wavelength match for some molecules (for example: pyrene, perylene) make them viable to be included in the list of candidate carriers. It is worth to mention that isomerism also affects the position and intensity by some extent, although in our study, we restrict ourselves to limited PAHs and structures as a preliminary study. As an initial study on deuterated PAHs, we also restrict ourselves to small sized PAHs mostly and to include size effect, we consider only two large PAHs, circumcoronene (C 54 H 18 ) and circumovalene (C 66 H 20 ) in our sample. C -D oop modes in these two molecules give rise to a feature near ∼ 18 𝜇 m with much weak relative intensity. There exists a plateau component below the emission of the band in the 10 -15 𝜇 m (Hony et al. 2001; Peeters et al. 2004a), whose origin remains unclear. Minor features due to the combination of C -Hvibrational modes newly created by the change in PAHs' geometry may explain some of these plateau component features.  \nA practical approach to confirm any presence of deuteriumcontaining PAHs towards any astronomical environment is that the characteristic features due to C -D stretching and C -D oop bending should be observed together. However, detetcion of C -D stretching features at 4.4 -4.65 𝜇 m (Buragohain et al. 2015) is limited by few  \nTable 5. Band position and relative intensity of the C -D oop belonging to different adjacency group  \nobservations (Onaka et al. 2014; Doney et al. 2016; Boersma et al. 2023; Peeters et al. 2024). On the other hand, C -D oop bending features appear in a range rather than at a discrete position across molecules, making it complex to select the particular wavelengths that would help detection precisely. An additional weak to moderate ∼ 11.1 𝜇 mfeature is present resulting from oop mode of a converted solo C -H unit when deuteration occurs in PAHs with a duet, trio and quartet C-H sites at subsequent steps to produce a solo C -H unit. Its position may slightly differ from the observed position of 11.2 𝜇 m. However, PAH molecules with predominant trio and quartet C -H sites would acquire a solo C -H unit, when more than one deuterium would substitute the C -H sites. Owing to the rare detection of 4.4 -4.65 𝜇 m, single deuterium substitution is more likely to occur compared to multiple deuterium substitution. In order to investigate the possibility of deuteration of PAHs and the extent upto which a molecule can be deuterated, simultaneous evidence of these features may be useful. Previous reports suggest the presence of feature at 11.0 -11.1 𝜇 mdue to a characteristic band from PAH cations. A detail investigation is pivotal to understand if deuterated PAHs could contribute to this. If we assume minimal deuteration (no. of D=1) resulting into a solo C -D, there are additional new features (discussed above) that are nearly equal intense as the C-D oop feature, however, the position is not necessarily discrete across molecules and depends on shape and size. The features are marked in the figures 1-4. For circumcoronene and circumovalene, no such significant feature is observed except the feature near 11.0 𝜇 min case of single deuteration at the duet C -H site. Investigation on the simultaneous presence or absence of these features together with C -D oop features are critical to approve or disapprove the presence of deutered PAHs in the observed target. Note that compared to the C -Dstretching mode at 4.4 𝜇 m, the C -D oop mode can be excited in lower-temperature regions and in larger PAHs, which may provide a more favourable conditions for deuteration.", 'CONCLUSION:': 'Deuterium containing PAH molecules show a characteristic C -D oop vibrational feature depending on its adjacent group, size and % of deuteration. These features are moderate in strength. The range for C -H oop is 11.2 -13.6 𝜇 m and C -D oop is higher in wavelength than these. While it is difficult to assign a fixed feature in terms of position arising due to C -D oop and so is for C -H oop bending, but it is a range of wavelengths at which features due to C -D oop or C -H oop rise. The range for C -D oop is 14 -19 𝜇 m with the shorter wavelengths being dominated by solo C -D oop . This range does not overlap with the range dedicated to the analogous C -H oop vibrations. The intensity of inherent prevailing C -H oop modes of a PAH molecule are affected by the presence of deuterium at its peripheral sites upto some extent. Wehavecompared the spectra of non-deuterated and deuterated PAH molecules to identify such indirect effect. While the spectra of nondeuterated PAH molecule is comparatively simpler, on substituting one of the peripheral H atoms at any adjacent C -H site by D results into a comparatively complex spectra with several new more or less important characteristic modes.  \nBelow are a few observations from the study:  \n- (i) The positions of existing solo, duet, trio or quartet C -H oop does not change significantly upon deuteration and turn to appear in close neighbouring wavelengths with a few additional weak features. The shift is within the range of C -H oop vibrations of the respective adjacency group.\n- (ii) The intensity of solo, duet, trio or quartet C -H oop in nondeuterated PAHs reduces upon deuteration to give additional features in the neighbouring wavelengths. While the change in intensity is notable in smaller PAHs, it is comparatively less in large sized PAH.\n- (iii) Anadditional feature at ∼ 11.0 -11.2 𝜇 mmayappear in some deuterated-PAHs which originally do not exhibit a solo C -H unit. This feature is due to the creation of a solo C -Hunit upon deuteration of C -H site, resulting into solo C -H oop mode.\n- (iv) A very weak to moderate C -D oop feature is present in all  \ndeuterium-conaining PAHs, which is comparatively stronger in intensity in smaller PAHs as compared to large sized PAHs. With increasing deuteration, the feature is enhanced and red shifted in some molecules. Some of these wavelengths lie in the same region as a few detected features in the 15 -20 𝜇 m region by Spitzer, JWST etc (Peeters et al. 2012; Chown et al. 2024). Further studies are needed to make precise matching.  \nThis study adds to our existing knowledge on Aromatic Infrared Band carries. The region 15 -20 𝜇 m, which is usually attributed to C -C -C bending modes may also possess features from C -D oop vibrational modes from deuterated PAHs. Also, the well established bands, for example: 11.2 𝜇 m may also have a contribution from C -H oop in a deuterated PAHs, resulting from a modified solo C -H unit, if such molecules are present. The observational evidence of features at 14 -19 𝜇 mand also at 4.4 -4.65 𝜇 m, although few, captivates us to include deuterated PAHs in the list of candidate carriers. It is important to mention that deuteration of interstellar PAHs is considered as a rare process and therefore, features from deuterium are also expected to be less in strength. This makes high resolution high spatially resolved spectra a requisite to detect deuterated features, both C -D oop and C -D stretching in observations. JWST NIRSpec and MIRI spectra may potentially be used to search for these features, if present any.The favorable low temperature environment for deuteration process to occur efficiently will require the community to look for deuterated features in less excited regions, such as Horsehead nebula (Abergel et al. 2024). The Orion Bar may not be a good target to look for C -Dfeatures, since it may be too warm. In addition, data from a dataset including large sized PAHs is necessary for further invesitigation and firm conclusion.', 'ACKNOWLEDGEMENTS': 'MB(Mridusmita Buragohain) thanks the DST INSPIRE Faculty fellowship for awarding her research fellowship and grant. AP (Amit Pathak) acknowledges financial support from the IoE grant of Banaras Hindu University (R/Dev/D/IoE/Incentive/2021-22/32439), financial support through the Core Research Grant of SERB, New Delhi (CRG/2021/000907), financial support from the DST-JSPS grant 2024. MB and AP thank the Inter-University Centre for Astronomy and Astrophysics, Pune for visiting associateship. TO (Takashi Onaka) acknowledges the support by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP24K07087.', 'DATA AVAILABILITY': 'The data underlying this article are available from the authors upon request.', 'REFERENCES': 'Abergel A., et al., 2024, arXiv preprint arXiv:2404.15816  \n- Allamandola L. J., Tielens A. G. G. M., Barker J. R., 1985, ApJ, 290, L25 Allamandola L. J., Boersma C., Lee T. J., Bregman J. D., Temi P., 2021, ApJ, 917, L35 Bauschlicher C. W., Langhoff S. R., Sandford S. A., Hudgins D. M., 1997, J. Phys. Chem. A, 101, 2414 Boersma C., Bauschlicher C. 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