Understanding the interaction between hydrogen cyanide HCN and silicate surfaces is crucial for elucidating the prebiotic processes occurring on interstellar grain cores as well as in cometary and meteoritic matrices. In this study we characterized the adsorption features of HCN on crystalline forsterite Mg2SiO4 surfaces one of the most abundant cosmic silicates by combining experimental infrared spectra at low temperatures 100150 K with periodic DFT simulations. Results showed the coexistence of both molecular and dissociative HCN adsorption complexes as a function of the considered forsterite crystalline face. Molecular adsorptions dominate on the most stable surfaces while dissociative adsorptions occur predominantly on surfaces of lower stability catalyzed by the enhanced Lewis acidbase behavior of surfaceexposed Mg2O2 ion pairs. On the whole set of adsorption cases harmonic frequency calculations were carried out and compared with the experimental infrared bands. To disentangle each vibrational mode contributing to the experimental broad bands we run a best nonlinear fit between the predicted set of frequencies and the experimental bands. The outcome of this procedure allowed us to i deconvolute the experimental IR spectrum by assigning computed normal modes of vibrations to the main features of each band ii reveal which crystal faces are responsible of the largest contribution to the adsorbate vibrational bands giving information about the morphology of the samples. The present straigthforward procedure is quite general and of broad interest in the fine characterization of the infrared spectra of adsorbates on complex inorganic material surfaces.
['Condensed Matter - Materials Science', 'Astrophysics - Astrophysics of Galaxies']
Unraveling the Interface Chemistry between HCN and Cosmic Silicates by the Interplay of Infrared Spectroscopy and Quantum Chemical Modeling
{'Unraveling the interface chemistry between HCN and cosmic silicates by the interplay of infrared spectroscopy and quantum chemical modeling': 'Niccolò Bancone , † , ‡ Rosangela Santalucia , ‡ Stefano Pantaleone , ‡ Piero Ugliengo  , ‡ Lorenzo Mino  , ‡ Albert Rimola , ∗ , † and Marta Corno ∗ , ‡  \n† Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193, Catalonia, Spain  \n‡ Dipartimento di Chimica and Nanostructured Interfaces and Surfaces (NIS) Centre, Università degli Studi di Torino, via P. Giuria 7, 10125, Torino, Italy.  \n```\nE-mail: albert.rimola@uab.cat; marta.corno@unito.it Phone:  +34-935813723; +39-0116702439\n```', 'Abstract': 'Understanding  the  interaction  between  hydrogen  cyanide  (HCN)  and  silicate  surfaces  is  crucial  for  elucidating  the  prebiotic  processes  occurring  on  interstellar  grain cores,  as  well  as  in  cometary  and  meteoritic  matrices.  In  this  study,  we  characterized  the  adsorption  features  of  HCN  on  crystalline  forsterite  (Mg 2 SiO 4 )  surfaces,  one of  the  most  abundant  cosmic  silicates,  by  combining  experimental  infrared  spectra  at low temperatures (100-150 K) with periodic DFT simulations. Results showed the coexistence of both molecular and dissociative HCN adsorption complexes as a function of  the  considered  forsterite  crystalline  face.  Molecular  adsorptions  dominate  on  the  \nmost stable surfaces, while dissociative adsorptions occur predominantly on surfaces of lower stability, catalyzed by the enhanced Lewis acid-base behavior of surface-exposed Mg 2+ -O 2 -ion  pairs.  On  the  whole  set  of  adsorption  cases,  harmonic  frequency  calculations  were  carried  out  and  compared  with  the  experimental  infrared  bands.  To disentangle  each  vibrational  mode  contributing  to  the  experimental  broad  bands,  we run a best non-linear fit between the predicted set of frequencies and the experimental bands.  The outcome of this procedure allowed us to:  i) deconvolute the experimental IR  spectrum  by  assigning  computed  normal  modes  of  vibrations  to  the  main  features of  each  band;  ii)  reveal  which  crystal  faces  are  responsible  of  the  largest  contribution to  the  adsorbate  vibrational  bands,  giving  information  about  the  morphology  of  the samples.  The  present  straigthforward  procedure  is  quite  general  and  of  broad  interest in the fine characterization of the infrared spectra of adsorbates on complex inorganic material surfaces.', 'Introduction': "Hydrogen cyanide (HCN) is recognized as an ubiquitous molecule in various astrophysical environments, ranging from diffuse and dense clouds, 1,2 protostellar hot cores, 3Ð5 cometary comas 6Ð8 and meteorites, 9 to planetary atmospheres, 10,11 including Pluto and Saturn's moon Titan. 12Ð14  \nSeveral studies have highlighted the importance of HCN in the synthesis of organic compounds relevant to the emergence of life, prompting experimental investigations into its reactivity under different conditions mimicking both interstellar and prebiotic environments, 15Ð19 which more recently have been extended to a theoretical side. 20Ð24 Within this context, the significance of HCN lays in its ability to polymerize, leading to the formation of complex organic molecules, among which the adenine nucleobase, a fundamental biomolecular building block for life. 25 The gas-phase polymerization involving only HCN has been reported to  be  hindered  by  high  kinetic  barriers,  hard  to  overcome  at  the  interstellar  conditions. 26  \nExplorations on HCN and HNC reactive pathways have demonstrated that the introduction of basic or acidic catalysts, and/or other molecules, leads to more feasible mechanisms. 24,27 In liquid-phase, pathways related to HCN polymerization seem to be more promising, 23 due to the presence of basic catalysts. 15,20 Among the few studies on the chemistry of HCN in the solid phase, water ices have been investigated as possible concentrator 28,29 of gaseous HCN and as catalysts for its dimerization. 30 On the other hand, the effectiveness of surface siloxyl radicals (SiO · ) as initiators for the polycyclization of HCN has been theoretically described, 31 thus introducing the potential role of silicate materials as heterogeneous catalysts for HCN prebiotic reactions.  \nSilicates  represent  the  most  abundant  minerals  in  the  Universe  as  they  are  the  major refractory  material  in  interstellar  grains,  cometary  nuclei,  meteoritic  chondrites  and  rocky planets;  as  an  emblematic  example,  they  represent  the  most  abundant  and  important  inorganic  material  in  Earth's  crust.  Silicates,  moreover,  possess  well-established  catalytic properties towards chemical reactions of interstellar 32,33 and prebiotic interest. 34 Therefore, investigating  the  activation  of  HCN  upon  silicate  adsorption  and  the  subsequent  catalytic reactivity (particularly in the absence of external energy inputs) is of interest in the astrochemistry, cosmochemistry and prebiotic chemistry fields.  \nIn a previous work by some of us, spectroscopic experimental measurements of HCN in interaction with synthetic Mg-rich silicate surfaces (both amorphous and crystalline) were performed,  with  the  aim  of  investigating  their  catalytic  properties  in  the  HCN  polymerization. 36 The chosen temperature range (i.e., 100-300 K) aligned well with the conditions found in common prebiotic environments, including interplanetary regions, meteorites, and comets.  The  study  employed  innovative  methods  for  the  safe  production  and  controlled dosing of pure HCN onto solid samples, followed by Fourier-transformed infrared (FT-IR) spectroscopy and high-resolution mass spectrometry (HR-MS) analysis to characterize the reaction products.  For what concerns the adsorption of gaseous HCN, results showed that above 150 K the HCN/silicates complexes undergo an intricate chemistry mainly due to the  \nFigure 1: Representations of the optimized geometries (focusing on the binding region). All distances  are  in  Å.  Adapted  with  permission  from  35.  Copyright  2023  Royal  Society  of Chemistry.  \n<!-- image -->  \npresence of surface-exposed Mg 2+ -O 2 -ion pairs, as they present an enhanced Lewis acidbase behavior 37 that enable the deprotonation of adsorbed HCN, which in turn triggers their polymerization.  However, the first steps of the HCN adsorption process were not investigated, and accordingly the study lacked a detailed and a thorough atomistic description of the molecule-surface interactions.  \nDue to that, a recent computational work 35 on the HCN adsorbed on Mg 2 SiO 4 (forsterite) surfaces was published, being dedicated to characterize the structures and energetics of all possible HCN adsorption complexes (see Figure 1), while vibrational frequencies were only marginally discussed. Indeed, the lack of an experimental counterpart made impossible the assignment of the contribution of each adsorption mode.  At the same time, the experimental spectra were rather intricate, with probably overlapping bands difficult to be assigned.  \nThe present work aims at overcoming the above-mentioned drawbacks, by combining in  \na more thorough way the experimental and theoretically predicted infrared spectra involving the  adsorbate  modes.  The  main  outcome  allows  a  fine  interpretation  of  the  experimental bands, providing a band deconvolution as least biased as possible, and also some hints about the crystal morphology of the forsterite samples and its affectation in the HCN adsorption.", 'Materials': 'Crystalline  forsterite  was  obtained  by  heating  the  pristine  amorphous  magnesium  silicate at  1073  K  for  24  h  in  air.  The  specific  surface  area  was  of ≈ 26  m 2 /g.  Before  HCN dosage, the samples were outgassed at 673 K for 1 hour under high vacuum (residual pressure 5 × 10 -4 mbar) to obtain a dehydrated/dehydroxylated surface. More information about the properties and the synthesis of the two materials are available in ref. 37.  The HCN used in the experiment was synthesized using a safe and efficient procedure developed by our group, which is detailed in the supporting information of ref.  36.', 'Experimental  methodology': 'For the transmission IR spectroscopic measurements, the powder sample was pressed in the form of self-supporting pellet and placed into a quartz cell equipped with IR-transparent KBr windows and a valve for connection to a vacuum line (residual pressure 5 × 10 -4 mbar). This setup allows in situ adsorption/desorption experiments to be conducted under controlled atmosphere. Additionally, the cell is designed for low-temperature experiments (approximately 100 K) by cooling the sample with a reservoir of liquid nitrogen.  In the experiment, HCN (5 mbar equilibrium pressure) was dosed into the cooled cell.  Under these conditions, the gas condensed on the walls of the cell. Subsequently, the system temperature was allowed to increase freely, causing HCN to be slowly released and reach the sample as its temperature approached ≈ 150 K. The initial spectra sequence, discussed in this study, can be essentially  \ndescribed as the dosing of increasing amounts of HCN at a constant temperature (150 K). The HCN adsorption was monitored in  situ by continuously acquiring IR spectra using a Bruker EQUINOX 55 FTIR spectrometer equipped with a DTGS detector.  Typically, 128 interferograms (at a resolution of 2 cm -1 ) were acquired for each spectrum to ensure a good signal-to-noise ratio.  Further details are reported in ref.  36.', 'Computational  methodology': 'All geometry optimizations and vibrational frequency computations were performed with the CRYSTAL17 periodic quantum mechanical code. 38 The DFT-PBE level of theory was used, complemented with the revised version of the Grimme\'s D2 empirical term (namely, the D*N correction, specifically customized for periodic systems containing Mg 2+ cations) 39Ð41 to account for dispersion interactions. The Ahlrichs-VTZ 42 basis set augmented with polarization functions for describing the atoms of HCN and the smaller basis set proposed by Bruno et al. 43 for forsterite atoms (8-511G* For Mg, 8-6311G* for Si, and 8-411G* for O) were used. The same methodology as that of ref.  35 was used, so that we strongly refer to that work for further computational details.  \nVibrational frequencies were computed numerically within the harmonic approximation and  at  the Γ point,  thus  including  vibrations  in  the  unit  cell,  only.  Each  Hessian  matrix element  was  calculated  through  the  second  derivatives  of  the  potential  energy  of  the  stationary points by displacing each atom from its equilibrium geometry along each cartesian coordinate by ± 0 . 003 Å along each cartesian axis (i.e., central difference formula). To speed up the calculations, only the HCN atoms and those belonging to the first layer of the surface models were allowed to displace.  \nComputed  vibrational  frequencies  suffer  from  a  systematic  error  intrinsic  to  the  DFT method and to the harmonic approximation.  As the main purpose of this work is to compare experimental spectra with the computed ones, each HCN computed frequency was corrected by two scaling factors s , one for the C-H and another for the C ≡ N stretching vibrations, by  \ncomparing the gas-phase HCN simulated and experimental frequencies. This allowed us to rescale the whole gas phase HCN spectrum by considering that the systematic errors are normal mode  dependent,  allowing  a  more  sensible  and  direct  comparison  with  the  experimental spectra.  Therefore, the following scaling factors were obtained:  \nS !" = 𝜈 #$% 𝜈 &\'(% = 3311 3370 = 0.9825  \nS !) = 𝜈 #$% 𝜈 &\'(% = 2097 2127 = 0.9859  \nAdditionally,  since  in  some  adsorption  complexes  HCN  dissociation  takes  place,  silanol (SiOH) surface groups are generated. For the OH stretching vibration of these surface SiOH groups,  the  scaling  factor  was  obtained  as  the  ratio  between  the  experimental  frequency of surface isolated silanols and the corresponding computed ones for those surfaces models exhibiting isolated SiOH groups, that is:  \nS *" = 𝜈 #$% 𝜈 &\'(% = 3744 3753 = 0.9976', 'Simulation of  a  global  IR  spectrum': 'i In  our  previous  computational  work, 35 we  identified  16  adsorption  complexes  considering  6 different crystalline forsterite surfaces. Thus, in order to simulate a single global spectrum taking into account the contribution of each adsorption complex j , we first associated each computed frequency ν i with a Lorentzian function L j ( ν ) , defined as:  \nL + , (𝜈) = I + , Γ + , [𝜋(𝜈-𝜈 + ) -+(Γ + , 2 / ) -] :  \ni i where I j is  the  PBE-D*N  computed  infrared  intensity  of  the  stretching  mode  corresponding to the ν i frequency and Γ j is the full width at half maximum (FWHM) of the peak.  \nThe experimental IR signals undergo a broadening in the 2500-3700 cm -1 zone,  due  \nFigure  2:  Example  of  the  effect  of  the  Huggins-Pimentel  correction 44 on  the  band-width of  the  computed  O-H  stretching  bands.  Top:  simulated  bands  with  a  fixed  FWHM  =  15 cm -1 . For each signal, the corresponding frequency in cm -1 is also reported. Bottom: peaks computed with a FWHM given by the Huggins-Pimentel formula, as adopted in this work. For each signal, the corresponding FWHM in cm -1 is also reported.  \n<!-- image -->  \ni to  the  H-bond  interactions  occurring  between  the  exposed  silanols  and  either  the  surfaceexposed O 2 -anions or the CN -adsorbates.  In order to better reproduce this broadening of the peaks, we computed Γ j for the O-H groups through the formula proposed by Huggins and Pimentel for the correction of the bandwidth as a function of the OH frequency shift with respect to the free SiOH group 44 (see Figure 2):  \ni In order to better reproduce this broadening of the peaks, we computed Γ j for the O-H groups  through  the  empirical  formula,  parametrized  on  OH  and  NH  H-bonded  systems, proposed by Huggins and Pimentel for the correction of the bandwidth as a function of the OH frequency shift with respect to the free SiOH group 44 (see Figure 2):  \ni Γ j = 0 . 72 ∆ i + 2 . 5 cm -1  \nwhere ∆ i represents the bathochromic shift of the stretching frequency of the i -silanol with  \ni respect to a free silanol, so that ∆ i = 3744 cm -1 -ν i .  For the peaks corresponding to the C-H and C ≡ N stretchings, we instead adopted a fixed Γ j = 15 cm -1 . Accordingly, we computed the IR spectrum of each single j -adsorption complex as a sum of the Lorentzian functions associated with its computed signals:  \nS , (𝜈) = ;L + , + (𝜈)  \nand generate the total spectrum T ( ν ) through a linear combination of each S j ( ν ) :  \nT(𝜈) = ;c , S , , (𝜈)  \nwhere c j are the weight coefficients of each spectrum, normalized for the case with the best match between experiment and theory.  \nWhen HCN deprotonates upon adsorption, the proton is transferred to the surface forming a SiOH group. The remaining CN -anion interacts with multiple surface-exposed Mg 2+ cations, forming different CN --Mg 2+ interactions of variable strength. 35 Simulations show that the freshly formed SiOH can engage H-bond interactions either with the CN -or nearby surface-exposed  O 2 -anions,  thus  experiencing  different  degrees  of  perturbation. 35,36 For these reasons, in this work, we treated the C ≡ N and O-H stretching vibrations separately. That is, we defined two independent S j ( ν ) spectra for C ≡ N and O-H.  \nBy proceeding this way, we generated a global spectrum T ( ν ) by linearly combining 23 single S j ( ν ) spectra, which include: 8 spectra given by the sum of C-H and C ≡ N signals (molecular  adsorptions),  8  spectra  given  by  the  C ≡ N  signals  alone  (dissociative  adsorptions) and 7 spectra by the O-H signals (dissociative adsorptions); the remaining one represents the free silanol used to compute the OH stretching scale factor. To match the experimental spectrum, we performed a non-linear best fit between T ( ν ) and the experimental one in the 3700-1300 cm -1 range, thus excluding any contribution given by the vibrations of the surface.  The best fit is performed by changing the relative weights c j of each spectrum  \nS j .  Therefore, only the relative intensities are changed, while keeping the frequencies of the bands fixed at the scaled computed values.  From now on, we will refer to the fitted simulated spectrum as T fit ( ν ) .  Therefore, the value of each S j ( ν ) contribution in T fit ( ν ) weighs  the  relevance  of  each  type  of  HCN/forsterite  adsorption  complex  contributing  to  the description of the experimental spectrum. In addition, this approach allows us to gauge the contribution  of  each  forsterite  crystal  face  in  contributing  to  the  final  spectrum,  disentangling the experimental bands, as discussed in the next section.', 'Results  and  Discussion': "Figure 3: Experimental IR spectra of HCN gas adsorbed at 150 K on crystalline Mg-silicate (forsterite) previously outgassed at 673 K. The series shows the effect of gradually increasing the HCN equilibrium up to 5 mbar (from black to red spectrum).  The spectrum of the bare activated  material  has  been  subtracted  from  all  spectra.  Negative  peaks  represent  species that  are  consumed  after  the  interaction  between  HCN  and  the  solid,  while  positive  peaks represent species that are formed.  \n<!-- image -->  \nThe experimental IR spectra obtained at 150 K at increasing HCN equilibrium pressure (from black to red) on dehydrated forsterite are reported in Figure 3, while Figure 4 shows the  global  computed  IR  spectrum T fit ( ν ) superimposed  to  the  low-coverage  experimental  \nFigure 4:  Experimental (blue) and fitted computed (red) IR spectra.  The letters correspond to  the  adsorption  complexes  which  mainly  contribute  to  the  experimental  spectrum.  In the assignments, the corresponding stretching mode, namely HC, CN and OH (silanol), is reported in superscript.  Atom color legend: H white, C ochre, N blue, O red, Mg cyan, Si yellow.  \n<!-- image -->  \none (blue line of Figures 3 and 4), as it better represents the isolated adsorptions modelled in the simulations.  Only 14 out of the 23 starting S j ( ν ) spectra contribute to T fit ( ν ) , of which here we report the best 10 cases in Figure 4 and in Table 1.  In Table S1 all the adsorption modes are reported with the corresponding vibrational frequencies and weight coefficients to the fitted spectrum.  \nUpon dosing HCN, there is a noticeable progressive reduction in the intensity of the band at ca.  3744 (3753 computed) cm -1 which represents the isolated silanols, accompanied by the simultaneous growth of two broad absorption bands at ca.  3500 cm -1 and 3200 cm -1 .  \nThis transformation is marked by the appearance of a distinct isosbestic point at ca.  3690 cm -1 .  Previous  studies  concerning  forsterite  characterization  and  the  interaction  between HCN gas and SiO 2 36 allow for an interpretation of these features, suggesting the formation of SiOH · · · NCH adducts involving the silanol groups present in the non-completely crystallized portions of the forsterite sample. 37 In our simulations we do not have this specific interaction, as the formation of silanol groups is associated with the HCN deprotonation.  Nevertheless, we modelled different degrees of perturbation of the SiOH, thus correctly reproducing the experimental bands in the OH region.  \nMoreover, according to our simulations, the band at 3200-3300 cm -1 is ascribed to ν (CH) modes (in addition to highly red-shifted ν (O-H)) coupled with the band at 2095 cm -1 of  the ν (C ≡ N).  This  assignments  fit  well  with  the  computed  cases a and b (c i =  0.5408 and 0.2173) which belong to the second most stable surface (120) and represent a slightly perturbed molecular HCN, ( ν (C-H) = 3305-3276 cm -1 , ν (C ≡ N) = 2099 cm -1 ).  The c case on the (010) surface ( ν (C-H) = 3195 cm -1 , ν (C ≡ N) = 2064 cm -1 , c i = 0.1688) undergoes a larger bathochromic shift of both features with respect to a and b ,  because of the stronger interaction  of  the  N  atom  with  a  Mg 2+ cation  and  the  formation  of  a  H-bond  with  a  silicate group Mg 2+ · · · NCH · · · OSiO 3 . This case well explains the C ≡ N feature below 2095 cm -1 while, for the C-H, the presence of a shoulder below 3000 cm -1 suggests that many structures  similar  to c could  contribute  to  the  observation  i.e.  differently  perturbed  C-H by H-bond donation to the surfaces.  The above mentioned cases are those contributing to the most intense bands of the experimental spectrum and, unsurprisingly, they represent the most stable  surfaces  of  forsterite, i.e. those  having  the  largest  surface  areas  in  crystalline nanoparticles (see  Figure  5).  This  means  that  our  simulated  IR  features  of  the  adsorption complexes, joint which the calculation of surface energies of the bare material, can provide important hints on the morphology obtained during the crystallization of forsterite.  \nThe broad band at 3500 cm -1 can be ascribed to interacting silanols, originated from the deprotonation of HCN. This can happen on defective and/or reactive sites, such as surface-  \nexposed Mg 2+ -O 2 -ion pairs.  From the computational standpoint, this situation can be somehow simulated by high energy forsterite surfaces, i.e. (111) and (021), where indeed the HCN deprotonation easily occurs, generating cases g and h (c i = 0.7301 and 0.2083).  \nHowever, we cannot exclude that strong interactions between silanols and deprotonated CN -anions can produce very large OH bathochromic shift that fall into the CH stretching zone (from 3200 cm -1 downwards), as shown by the case f ( ν (C ≡ N) = 3056 cm -1 , c i = 1.0000).  \nAs regards the CN -anion , only case i has an appreciable weight in the fitted spectrum ( ν (C ≡ N) = 2151 cm -1 ,  c i =  0.2016),  while  all  the  others  contributions  (from j to n )  are around 1% or less.  In this case the CN -is  bridged between two Mg 2+ cations forming a η 2 (C,N) adsorption mode (Mg-C ≡ N-Mg) resulting in a hypsochromic shift of the CN bond, which explains the experimental band at 2137 cm -1 .  \nTable 1:  Summary of all the contributions to the simulated IR spectrum after the best fit to the experimental one, grouped depending on the adsorption type, and ordered according to decreasing weight in the fitted linear combination. For each item we report the corresponding type of adsorption (molecular (M) or dissociative (D)), the involved stretching modes, the corresponding surface, the normalized weight in the linear combination (c i ) and the computed stretching frequency (in cm -1 .)  \nFinally,  it  is  worth  mentioning  case e ( ν (C=N)  =  1526  cm -1 ),  which,  even  if  it  does not  present  a  good  fit  with  the  experimental  IR  (c i =  0.0074),  we  believe  represents  an interesting case as well, possibly explaining the band at 1600-1700 cm -1 .  This is the region of the double bond stretching of C=N, among other moieties, and was assigned to a bridged  \n<!-- image -->  \nFigure 5: Left: surface energy of the six slab models adopted in this work calculated at the PBE-D*N level.  Right:  Wulff's  construction  of  a  forsterite  nano-crystal  at  0  K.  Adapted with permission from  35.  Copyright 2023 Royal Society of Chemistry.  \n<!-- image -->  \nspecies of non-deprotonated HCN where the bond order of C ≡ N decreases and a new covalent bond is formed between the C atom of HCN and one of the O atoms of the surface.  As for the HCN deprotonation, this particular chemisorption can occur on reactive/defective sites, i.e. surface-exposed Mg 2+ -O 2 -ion pairs, and high energy surfaces.", 'Conclusions': 'This combined experimental and computational study provides a comprehensive understanding of the adsorption behavior of hydrogen cyanide (HCN) on crystalline forsterite (Mg 2 SiO 4 ) silicate  surfaces.  Through  infrared  (IR)  spectroscopic  measurements  and  periodic  density functional  theory  (DFT)  simulations,  the  intricate  HCN/forsterite  interactions  occurring during the HCN adsorption on the crystalline forsterite surfaces at low temperatures (100150 K) were elucidated at an atomistic detail.  \nThe main achievement of this synergy is twofold: on one hand, we remove many biases related to the deconvolution of a complex spectrum by making a weighted convolution of all  \nthe simulated IRs, fitted on the experimental spectrum.  On the other hand, the exploration of all forsterite surface models allowed us to gauge the role of each specific crystal face in contributing to the vibrational features of the experimental spectrum which, in turn, allows to elucidate the crystal morphology of the forsterite sample.  \nResults reveal the prevalence of both molecular and dissociative adsorption complexes, the former dominating on stable surfaces, while the latter occurring predominantly on less stable surfaces. HCN dissociation occurs due to the enhanced Lewis acid-base behaviour of surface-exposed Mg 2+ -O 2 -ion pairs, but also to reactive silicate groups belonging to high energy surfaces, as it has been demonstrated by DFT simulations, which therefore present a non-negligible contribution to the exposed facets of experimental forsterite samples.  \nThese findings are useful to deepen our understanding on the interaction of HCN with silicate surfaces and, by extension, to the catalytic properties of silicates towards HCN activation, thereby contributing to improve our know-how of prebiotic chemistry in astrophysical environments. Our combined experimental-theoretical approach can also be used to explore the adsorption properties of other molecules on different inorganic material surfaces, thereby further advancing our knowledge on surface chemical processes such as adsorption and heterogeneous catalysis relevant to fundamental (like astrochemistry and prebiotic chemistry) and more applied fields.', 'Acknowledgement': "This project has received funding within the European Union's Horizon 2020 research and innovation program from the European Research Council (ERC) for the project 'Quantum Chemistry on Interstellar Grains' (Quantumgrain), grant agreement No.  865657.  The Italian Space Agency for co-funding the Life in Space Project (ASI N. 2019-3-U.O), the Italian MUR (PRIN 2020, Astrochemistry beyond the second period elements, Prot. 2020AFB3FX) are also acknowledged for financial support.  This research is also funded by the Spanish  \nMICINN (projects PID2021-126427NB-I00 and CNS2023-144902). This research acknowledges support from the Project CH4.0 under the MIUR program 'Dipartimento di Eccellenza 2023-2027'.  The authors thankfully acknowledge RES resources provided by UMA for the use of Picasso (activity QHS-2023-3-0032) and by IAC for the use of LaPalma (activity QHS2023-1-0020). The supercomputational facilities provided by CSUC and CINECA (ISCRAB projects) are also acknowledged.  The EuroHPC Joint Undertaking through the Regular Access call project no.  2023R01-112, hosted by the Ministry of Education, Youth and Sports of the Czech Republic through the e-INFRA CZ (ID: 90254) is also acknowledged.", 'Supporting  Information  Available': 'Electronic Supplementary Information (ESI) available: graphical representation of the forsterite slab models adopted in this work, IR spectrum with the isolated contributions to the fitting procedure  and  table  of  all  the  structures  contributing  to  the  fitting.', 'References': "- (1) Liszt,  H.;  Lucas,  R.  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