halogen bonding ( xb ) 
 refers to the attractive noncovalent interaction arising between the 
 electron - deficient -hole of a polarized halogen atom , such 
 as bromine or iodine , and an electron - rich lewis base .
while xb has been well - developed in the context of solid - state crystal 
 engineering and in the design of functional materials , the utilization of xb in solution is relatively underexploited 
 with seminal applications in the areas of reactivity , catalysis , self - assembly , transport , and medicinal chemistry 
 only recently being reported .
anion supramolecular 
 chemistry has experienced rapid growth over the past few decades stimulated 
 by the crucial roles that anions play in many chemical , biological , 
 and environmental processes . while supramolecular interactions such as hydrogen bonding ( hb ) 
 have dominated the field of anion coordination , recognition , and transport , halogen bonding ( xb )
has only recently begun to emerge as a powerful 
 alternative interaction capable of coordinating anions in competitive 
 solvent media .
the strength of xb in solution has been demonstrated to be comparable 
 to hb , while the stringent linear directionality 
 of xb and ph - independence is advantageous for the design of selective 
 receptors for anions with particular geometries .
it is noteworthy 
 that the relatively few examples of acyclic and macrocyclic xb anion 
 receptors reported to date all display significantly contrasting anion 
 recognition behavior compared to hb analogues .
furthermore , rotaxane 
 interlocked host structural frameworks containing convergent xb triazolium 
 donor groups have been demonstrated to selectively recognize and sense 
 halide anions in highly competitive aqueous media .
herein we report a bis - iodotriazole - pyridinium 
 motif , incorporated within both an acyclic and a novel catenane - based 
 receptor , capable of binding anions via two convergent r  ix halogen bonds .
the catenane host structural framework , 
 prepared via chloride anion templation , is demonstrated to bind halide 
 anions with impressive selectivity in competitive organic 
in addition , we report for the first time 
 cl and br k - edge xas analysis of the interactions observed in these 
 receptors with anions , revealing significant charge transfer from 
 the halides to the iodotriazole xb donors , and the first measure of 
 covalency in these systems .
these data allow for the first quantitative 
 evaluation of the degree of covalency in halogen - bonded species .
pre - edge 
 features in the xas spectra are modeled and supplemented with dft 
 calculations and tddft simulations of the xas data which reproduce 
 the experimentally observed pre - edge features .
the synthesis of the acyclic bis - triazole - pyridinium 
 receptors is shown in scheme 1 .
3,5-diethynylpyridine 1 was prepared by deprotecting 3,5-bis(trimethylsilylethynyl ) 
 pyridine with koh in methanol , while azides 2a  c were prepared via a modification of 
 a literature procedure .
the bidentate bis - triazole - pyridinium acyclic 
 precursor , 3a , was prepared via a cuaac  click  

reaction between 1 and 2.2 equiv of hexylazide , prepared in situ from the corresponding bromide .
the bidentate bis - iodotriazole - pyridinium acyclic receptor 
 precursors , 3b and 3c , were prepared using 
 a modification of a cuaac  click 
ci which were characterized by high - resolution 
 esi mass spectrometry , h , c nmr , and f as well as p nmr spectroscopy where appropriate . 

acyclic receptors 4a and , for solubility reasons , 4b were anion exchanged to their corresponding noncoordinating 
 hexafluorophosphate salts in preparation for anion titration experiments , 
 while 4cx was used in x - ray crystallographic structure 
 analysis ( x = cl ) and x - ray absorption spectroscopic 
 analysis ( x = cl , br , pf6 ) .
reagents 
 and conditions : ( i ) x = h : cuso45h2o , 
 ascorbic acid , na2co3 , nan3 , hexyl - bromide , 3a : 79% ; x = i : nai , cu(clo4)26h2o , tbta , dbu , thf , 3b : 76% , 3c : 79% ; ( ii ) ch3i , dcm , 4ai : 81% , 4ci : 80% ; ( iii ) nh4pf6(aq ) , 4apf6 : 89% , 4bpf6 : 53% .
the anion recognition properties 
 of acyclic receptors 4apf6 and 4bpf6 were investigated using h nmr anion titration experiments by adding aliquots of anions as 
 their tetrabutylammonium salts to solutions of the receptor .
initial 
 titrations with tbacl in acetone - d6 determined 1:1 stoichiometric association constants of > 10 m for both receptors .
consequently , titrations 
 were repeated in the more competitive cd3cn affording the 
 data in table 1 for hb receptor 4apf6 ; however , association constants for xb receptor 4bpf6 were still > 10 m. as a result , titrations for 4bpf6 were performed in the even more competitive solvent dmso - d6 indicating the superior anion - binding capability 
 of the xb receptor .
addition of anions typically caused downfield 
 perturbations of 4apf6 triazole proton d , directly involved in anion coordination , and the 4bpf6 pyridinium proton c incumbent on anion binding ( figure 1 ) .
wineqnmr2 analysis of the titration data determined the 
 1:1 stoichiometric association constants shown in table 1 .
anion binding titrations with 4bpf6 following proton c in dmso - d6 : experimental titration data ( circles ) with fitted binding 
 isotherms indicated as lines ( 298 k ) .
calculated using wineqnmr2 monitoring the chemical shift data of 4apf6 proton d and 4bpf6 proton c ; estimated standard errors given 
 in parentheses ( 298 k ) .
the anion binding selectivity of prototriazole receptor 4apf6 follows anion basicity trends , as would be expected 
 for an anion receptor in an aprotic solvent , with the larger halides 
 complexed more weakly than chloride .
similarly , for the xb acyclic 
 receptor 4bpf6 , selectivity follows 
 anion basicity trends for the halides , with oac being the most strongly associated anion ; the association constant , ka , is 2.5 times larger in magnitude than chloride . 
 interestingly ,
no binding was observed for h2po4 and no3 , while 
 so4 was bound at > 10 m , presumably due to the greater negative charge . 
 to the best of our knowledge ,
this is only the second example of an 
 xb oxoanion - selective anion receptor .
crystals suitable for x - ray diffraction structural analysis were 
 obtained from the chloride salt of the octyl - appended acyclic receptor 4ccl .
 iodotriazole 
 heterocyclic motif , despite the large size of the iodine substituents 
 ( figure 2 ) .
the chelating xbs vary 
 in length from 3.121(2 ) to 3.195(2 )  corresponding to 8486% 
 of the sum of the van der waals halide radii , which is indicative 
 of strong xb interactions to the anion .
perspective 
 ( left ) and space - filling representation ( right ) views of the crystal 
 structure of 4ccl .
gray = carbon , blue = nitrogen , purple = iodine , green = chlorine . as a result of the enhanced 
 anion recognition properties of the acyclic xb receptor 4bpf6 , in comparison to the hb analogue , 4apf6 ,
the bis - iodotriazole - pyridinium motif was integrated 
 into a catenane host structural framework . to this end
, macrocycle 
 precursor 6 was prepared initially , by reaction of 1 with 2.2 equiv of vinyl - appended azide 5 , via the same modified cuaac methodology ( scheme 2 ) .
methylation was achieved using ch3i , and thereafter , the vinyl - appended bis - iodotriazole - pyridinium - containing 
 macrocycle precursor was anion exchanged to the corresponding chloride 
 salt by repeated washing with nh4cl(aq ) to afford 7cl .
reagents and conditions : ( i ) nai , 
 cu(clo4)26h2o , tbta , dbu , thf , 
 56% ; ( ii ) ch3i , dcm , 99% ; ( iii ) nh4cl(aq ) , 66% .
synthesis of catenane 9cl was accomplished using chloride templation via a ring - closing 
 metathesis strategy .
an initial interpenetrated assembly was prepared 
 by mixing equimolar amounts of macrocycle 8(71 ) with 7cl in dry ch2cl2 .
addition of grubbs ii ring - closing metathesis 
 catalyst afforded the catenane in 41% yield following purification 
 by preparative thin layer chromatography .
catenane 9cl was characterized by h , c and 2d 
 cosy nmr experiments and high - resolution mass spectrometry .
notable 
 shifts in the pyridinium and isophthalamide protons c and 3 , respectively , were observed in addition to the splitting 
 and upfield perturbation of the hydroquinone protons 5 and 6 consistent 
 with inter - ring donor  acceptor interactions between the electron - rich 
 hydroquinone groups and the electron - deficient pyridinium group , which 
 confirms the interlocked nature of the catenane ( figure 3 ) .
further evidence for the interlocking of the two macrocyclic 
 components was obtained by 2d roesy nmr which revealed through - space 
 interactions including those between hydroquinone and pyridinium protons 
 of the two macrocycles ( see supporting information ( si ) ) . truncated h nmr spectra of 7cl ( top ) , 9cl
( middle ) , and 8 ( bottom ) ( 300 mhz , 
 293 k , solvent : cdcl3 ) .
the chloride anion template was removed by repeated washing 
 with nh4pf6(aq ) to afford 9pf6 for anion recognition studies and was similarly fully characterized 
 ( scheme 3 ) .
reagents and conditions : 
 ( i ) grubbs ii ( 10 wt % ) , dcm , 41% ; ( ii ) nh4pf6(aq ) , 86% .
association constants for the formation 
 of 1:1 stoichiometric host / guest complexes of catenane 9pf6 with various anions were determined using h nmr titration experiments , monitoring shifts in the internal 
 isophthalamide proton 3 upon addition of aliquots of anions added 
 as their tetrabutylammonium salts ( figure 4 ) .
a preliminary titration of 9pf6 with tbacl in 1:1 cdcl3/cd3od resulted in an association constant ka > 10 m , much greater 
 than the value obtained for the previously reported analogous prototriazole - containing 
 hb catenane ( table 2 ) .
consequently , all subsequent titrations were conducted in the aqueous 
 solvent mixture 10:45:45 d2o / cdcl3/cd3od , which is more competitive .
the association constants shown in 
 table 2 were determined by wineqnmr2 analysis of the titration data .
d2o / cdcl3/cd3od : experimental 
 titration data ( circles ) with calculated fit indicated as the line 
 ( 298 k ) . calculated using wineqnmr2 monitoring the internal pyridinium proton ; estimated standard errors 
 given in parentheses ( 293 k , solvent : 10:45:45 d2o / cdcl / cd3od ) .
1:1 cdcl3/cd3od , 298 k. nb : no 
 binding :  < 0.07 ppm . very strong binding of the halides
indeed , 
 association constants determined for br and i are both > 10 m which 
 highlights the fact that binding is dramatically enhanced in the xb 
 catenane when compared to the hb catenane analogue . furthermore , in contrast , the xb host demonstrates a preference 
 for the larger halides despite the voluminous iodine xb donors .
indeed , 
 a reversal in selectivity for the halides is observed for 9pf6 in comparison to the acyclic receptor 4bpf6 due the interlocked host s unique 
 three - dimensional binding cavity . despite the oac selectivity observed for the acyclic xb receptor 4bpf6 , no evidence for oxoanion binding was discerned 
 for oac or h2po4 for the xb catenane 9pf6 .
this may 
 be attributed to the oxoanions large size and unfavorable 
 geometry to bind and be encapsulated by the unique interlocked catenane 
 host cavity ( vide infra ) .
crystals suitable for x - ray diffraction structural analysis were 
 obtained for catenane 9 with chloride , bromide , 
 iodide , dihydrogen phosphate , and sulfate counteranions .
all structures 
 crystallize in p1 with 1:1 complexes obtained 
 for chloride ( figure 5 ) , bromide , and iodide , 
 which are isomorphous , and 2:1 complexes were obtained for dihydrogen 
 phosphate and sulfate , which in turn are isomorphous . while both hydrogen 
 and halogen bonds are observed between the host 9 and the anionic guest , the catenane curiously adopts an  open  

conformation in the solid state rather than completely encapsulating 
 the anion ; presumably this is due to the large iodo - substituted triazole 
 and the most energetically favorable conformation for crystallization . 
 while a varying degree of disorder for the polyether ends of the macrocycles 

is observed in each complex , it does not affect the anion - binding 
 cavity of the catenane and is omitted for clarity here .
perspective 
 ( left ) and space - filling representation ( right ) views of the crystal 
 structure of 9cl .
triazole  icl halogen bonds ( 3.110(2)3.300(3 )  ) are 
 indicated as red dashed lines .
gray = carbon , blue = nitrogen , red = oxygen , purple 
 = iodine , green = chlorine , white = hydrogen . for further information 
 regarding the isomorphous 9br and 9i structures , see si .
examining the halide anion structures , strong halogen 
 bonds are observed indicated by bond lengths significantly shorter 
 than the sum of the van der waals radii and are summarized in table 3 . calculated errors 
 given in parentheses .
the 
 structure of ( 9)2so4 ( figure 6 ) clearly shows the so4 anion situated between two hosts rather 
 than being encapsulated in the cavity , as it is too large to penetrate 
 the interlocked binding pocket .
furthermore , the anion is on the inversion 
 center in the asymmetric unit allowing it to adopt one of two symmetry - related 
 orientations ( see supporting information ) .
halogen 
 bonds are formed rather than bifurcated halogen bonds from one o to 
 each of the iodine xb donors . the dihydrogen phosphate structure ( see supporting information ) , while of lower quality , 
 shows a very similar structure ; i.e. , the large oxoanion binds outside 
 of the cavity .
crystal structure of ( 9)2so4 showing a 2:1 catenane / anion 
 complex with the anion ( so4 ) positioned 
 between the two catenane hosts rather than penetrating the binding 
 cavity .
gray = carbon , blue = nitrogen , 
 red = oxygen , purple = iodine , white = hydrogen , orange = phosphorus , 
 yellow = sulfur .
to establish the nature of the interactions 
 between the halide ( cl / br ) donor 
 and the iodine acceptor , we have exploited cl and br k - edge x - ray 
 absorption spectroscopy ( xas ) .
it has been clearly established that 
 cl k - edge xas can be used to evaluate the degree of delocalization 
 in chloro - metal bonds . in such cases ,
a pre - edge feature is observed in 
 the spectrum that formally corresponds to excitation of a cl1s electron to empty valence d - orbitals 
 on the transition metal acceptor .
the intensity of such transitions 
 is directly proportional to the amount of cl3p character mixed into these empty metal acceptor orbitals , 
 providing a convenient method of obtaining a quantitative measure 
 of the degree of mixing between the chloro donor and the metal acceptor . 

this methodology is equally applicable in cases where nonmetal acceptors 
 are under investigation , although to our knowledge it has not been 
 applied to other types of donor  acceptor systems . within 
 the context of this work
, one would expect a halide ion ( cl and/or br ) to exhibit simple k - edge xanes spectra 
 with no pre - edge features .
electric - dipole allowed cl3p  cl1s ( or br4p br1s ) transitions would not be observed since these valence p - states are filled ( nsnp ) .
covalent delocalization of the filled cl3p orbital with an empty acceptor orbital ( e.g. , via 
 hydrogen or halogen bonding ) would result in the possibility of a 
 new allowed transition corresponding to charge transfer from the chloride 
 to its bonding partner ( axcl *  cl1s ) .
given that the intensity of such transitions is directly proportional 
 to the amount of cl3p in the final state 
 wave function , axcl * , the intensity of any observable 
 pre - edge feature provides us a direct measure of the degree of covalency 
 in the halogen bond .
chlorine k - edge xas data were obtained 
 for a series of donor  acceptor complexes at beamline 4 - 3 at 
 the stanford synchrotron radiation lightsource .
the near - edge spectra 
 for 4cl and 9cl ( see figure 7 ) each exhibit an intense pre - edge feature that 
 is not present in ionic chloride salts .
the presence of this intense 
 feature can only result from charge transfer between the chloro donor 
 and its partners .
fitting of the pre - edge and edge 
 features allows experimental quantification of the degree of charge 
 donation in each of these species ( table 4 ) .
these results clearly demonstrate charge transfer from the donor 
 via halogen bonding in 4cl , and a combination of 
 xb and hb interactions in 9cl . for comparison , 

data were also collected on the h - bonding analogue of 4cl ( 10cl ) as well as a simple x - bonded adduct 
 5-iodo-3-methyl-1,4-dioctyl-1h-1,2,3-triazol-3-ium 
 chloride ( 11cl ) ( figure 7 ; see si for the synthesis of 10cl and 11cl ) .
these data provide a measure 
 of the total donation from the chloride anion to all of its bonding 
 partners , and thus the number of interactions must be considered when 
 interpreting these results .
normalized and background subtracted : ( a ) cl 
 k- and ( b ) br k - edge xas data for 11cl / br , 4cl / br , 10cl / br , and 9cl / br .
calculated errors 
 ( parentheses ) are obtained from statistical distribution of fit results 
 for > 50 fits for each data set .
total donation is calculated in comparison with cl k - edge xas data 
 on a cucl4 reference , errors include 
 an estimate of the error in charge transfer of this reference compound .
it is apparent that , with a 
 chloride donor , x - bonding interactions are significantly more covalent 
 in character than comparable h - bonding interactions .
these data are 
 consistent with previous reports that charge transfer is an important 
 factor in xb bonds .
these xas data , however , 
 allow us to quantify the degree of covalency in the xb bond and thus 
 compare the degree of charge transfer in different systems , which 
 to our knowledge is unprecedented . in 11cl , where 
 only a single halogen bonding interaction is possible , we find that 
 the degree of donation is consistent with 6% charge transfer 
 to the iodinated triazole acceptor .
the magnitude of charge donation / covalent 
 character in the xb bond is notable in that it is a similar magnitude 
 to that which is commonly observed in transition metal complexes , 
 where covalent contributions are considered to be of great importance 
 in defining chemical properties .
for example , in simple divalent metal 
 chlorides ( [ mcl4 ] , where m = cu , ni , 
 co , fe ) , bond covalencies have been determined to range from 6% to 
 9% . in the bis - triazole 4cl , the total charge donation from the halide ion almost doubles 
 to 11% , which implies that each of the halogen bonds are independent 
 from each other and additive . by contrast
, replacement of the iodine 
 acceptors for protons in the bis - triazole ( 10cl ) 
 leads to an almost complete loss of intensity in the cl k - edge xas 
 pre - edge feature , reflecting very poor charge donation through h - bonding 
 in this system .
data for the catenane 9cl , 
 where both h- and x - bonds are present , indicate that charge donation 
 from the chloride anion decreases substantially ( as compared to 4cl ) .
this presumably reflects weakened halogen bonding 
 due to competition with amide h - bond interactions .
qualitatively 
 similar data were obtained for the bromide donor complexes 11br , 4br , 10br , and 9br ( see figure 7 ) .
unfortunately , 
 lifetime broadening at the higher energy br k - edge , as well as smaller 
 energy separation between features , leads to poorer resolution of 
 the pre - edge features of interest .
the data indicate the presence 
 of a low - energy shoulder , indicating a pre - edge feature to that observed 
 in the cl k - edge data .
we note that the pre - edge shoulder is larger 
 for the x - bonded systems as compared to the h - bonded system ( 10br ) .
however , quantitative comparisons between the 
 different x - bonded systems are inconclusive due to large errors in 
 the associated fits ( see section s6 ) . to further explore the details of the covalent 
 nature of x - bonding in these systems , we performed a series of density 
 functional calculations , and tddft simulations of the xas data .
i in 4cl 
 are calculated to be 3.18(3 )  , which compare favorably with 
 that observed crystallographically ( 3.120(2)3.195(2 )  ) . 

tddft simulations of the cl k- and br k - edge xas data reproduce the 
 low energy pre - edge feature assigned to the axx *  
 x1s transition in each case .
the relevant 
 axx * orbitals for 11cl and 4cl are shown in figure 8 . in 
 each case , these antibonding acceptor orbitals reflect the covalent 
 nature of the x  i bond(s ) .
two relevant 
 acceptor orbitals are present in 4cl since two 
 x  i interactions are possible ; the splitting 
 of these two orbitals in 4cl is small ( 0.3 
 ev ) , and thus the resulting transitions can not be resolved experimentally .
all calculated values are for fully optimized structures at the b3lyp / def2-tzvp+zora 
 level of theory .
visual representation 
 of the important axx * kohn  sham acceptor orbitals 
 for 11 , 4 , and 9 .
tddft - based simulations of the 
 cl k- and br k - edge xas data are in good qualitative agreement with 
 the experimental data ( figure 9 ) .
as anticipated , 
 the intensities of the axx *  x1s pre - edge transitions correlate with the degree of 
 charge transfer from the halide to the acceptor .
the trends observed 
 in the tddft - calculated intensities of the transitions are in reasonable 
 agreement with those observed in the experimental data .
however , the 
 calculated pre - edge intensities ( and calculated xb charge donation ) 
 are in good agreement with those obtained from the cl k - edge xas data . 
 trends observed in pre - edge transition energies are also reproduced 
 in the dft results . a small shift to higher energy ( 0.4 ev ) 
 on going from a single xb acceptor ( 11cl ) to two 
 acceptors ( in 4cl ) is consistent with a similar 
 shift observed in the experimental data ( see table 4 ) .
this shift correlates with decreased electron density on 
 the chloride , which leads to stabilization of the cl1s donor orbital .
tddft simulated cl k - edge ( left ) and br 
 k - edge ( right ) xas spectra for selected xb adducts .
a constant linear 
 shift was applied to the calculated energies 
 to better match experimental k - edge energies .
the bromide adducts show similar trends as those observed 
 in the chlorides ; the tddft simulations are also in reasonable agreement 
 with the br k - edge experimental data .
the calculated degree of charge 
 donation is very similar between the two halides , suggesting that 
 the degree of covalent mixing is similar in both cases .
investigations 
 are currently underway to further evaluate the specific factors that 
 affect the covalency of these xb interactions and the resultant binding 
 affinities in the catenanes .
in 
 conclusion , the xb bis - iodotriazole - pyridinium motif is shown to be 
 a potent anion - coordinating motif in competitive aqueous media , especially 
 when incorporated into a catenane host structural framework .
the 
 acyclic xb receptor displays a marked enhancement in anion recognition 
 over the hb analogue and the first instance of acetate selectivity . 

moreover , the xb catenane notably exhibits further augmentation 
 of anion recognition ability over its hb analogue , selectively binding 
 halides very strongly in competitive aqueous media with no binding 
 of oxoanions which is the reverse acetate selectivity trend of the 
 acyclic xb receptor .
this suggests the xb catenane s unique 
 interlocked host binding cavity is of complementary size and shape 
 for halides , whereas the oxoanions are too large and are of the wrong 
 geometry for encapsulation .
single crystal x - ray structural 
 analysis provided solid - state evidence for the association of anions 
 with the catenane host framework where halide anions are able to 
 penetrate the cavity of the catenane . by contrast oxoanions ( h2po4 and so4 ) are found outside the three - dimensional binding pocket in support 
 of the solution - state anion recognition binding observations .
cl and br k - edge xas revealed the presence of intense pre - edge features 
 characteristic of charge transfer between the halide donor and the 
 xb acceptor .
quantitative fitting of these pre - edge features provided 
 a direct measure of the degree of covalency in the halogen bonding 
 interaction , which is comparable to that observed in transition metal 
 complexes .
furthermore , we confirm that perpendicular xb interactions 
 are independent and additive , but that the degree of xb covalency 
 can be mitigated through the presence of hb donors .
these conclusions 
 are well supported and substantiated by tddft simulations of the xas 
 data , which indicate that the degree of covalency is essentially the 
 same for both cl and br donors .
most importantly , these results offer the first evidence of this 
 kind for covalency in halogen bonds of 
 anion receptors with halide anions .
furthermore , the presence of both 
 xb and hb donors in the catenane host framework provides interesting 
 insight into the interplay between these two competing interactions 
 in a single anion receptor system .
 the synthesis and anion binding properties 
 of novel halogen - bonding ( xb ) bis - iodotriazole - pyridinium - containing 
 acyclic and [ 2]catenane anion host systems are described . 
 the xb acyclic 
 receptor displays selectivity for acetate over halides with enhanced 
 anion recognition properties compared to the analogous hydrogen - bonding 
 ( hb ) acyclic receptor . 
 a reversal in halide selectivity is observed 
 in the xb [ 2]catenane , in comparison to the acyclic xb receptor , due 
 to the interlocked host s unique three - dimensional binding 
 cavity , and no binding is observed for oxoanions . 
 notable halide anion 
 association constant values determined for the [ 2]catenane in competitive 
 organic  aqueous solvent mixtures demonstrate considerable enhancement 
 of anion recognition as compared to the hb catenane analogue . 
 x - ray 
 crystallographic analysis of a series of halide catenane complexes 
 reveal strong xb interactions in the solid state . 
 these interactions 
 were studied using cl and br k - edge x - ray absorption spectroscopy 
 ( xas ) indicating intense pre - edge features characteristic of charge 
 transfer from the halide to its bonding partner ( axx *  x1s ) , and providing a direct measure 
 of the degree of covalency in the halogen bond(s ) . 
 the data reveal 
 that the degree of covalency is similar to that which is observed 
 in transition metal coordinate covalent bonds . 
 these results are supported 
 by dft results , which correlate well with the experimental data . 
Introduction
Results and Discussion
Conclusion