Matrix-Isolation Infrared and Theoretical Studies of the Glycine Conformers

Transcription

1 J. Phys. Chem. A 1998, 102, Mtrix-Isoltion Infrred nd Theoreticl Studies of the Glycine Conformers S. G. Stepnin,, I. D. Rev, E. D. Rdchenko, M. T. S. Rosdo, M. L. T. S. Durte, R. Fusto, nd L. Admowicz*, Deprtment of Chemistry, UniVersity of Arizon, Tucson, Arizon 85721, Institute for Low Temperture Physics nd Engineermg, Ntionl Acdemy of Sciences of Ukrine, 47 Lenin AVenue, KhrkoV Ukrine, Deprtment of Chemistry, Fculty of Science, ljniversity of Lisbon, R. Ernesto de Vsconcelos, ed. C Lisbo, Portugl, nd Deprtment of Chemistry, UniVersity of Coimbr, P-3049 Coimbr, Portugl ReceiVed: October 21, 1997 The IR spectr of nonionized glycine nd its deuterted derivtives isolted in the low-temperture rgon mtrices hve been mesured, nd for the first time the infrred spectrl chrcteristics of the three most stble conformers hve been determined nd ssigned. Correlted level b initio nd density functionl theory (DFT) clcultions of IR frequencies nd intensities with extended bsis sets were performed nd their results were employed to seprte the bnds of the glycine conformers in the experimentl spectr nd to ssist the ssignment. The intrmoleculr interconversion, conformer III w conformer I, which is observed in the mtrices t tempertures higher thn 13 K, ws found to cuse significnt decrese of the bnd intensities of conformer III in the spectr. This phenomenon ws used to distinguish the vibrtionl bnds of this conformer from the bnds of the other conformers. The relibility of the Møller-Plesset second-order perturbtion theory (MP2) method nd the DFT method with the three-prmeter density functionl, Becke3LYP, in the prediction of the IR spectr of the nonionized glycine conformers ws exmined. We found tht the DFT/Becke3LYP method with ug-cc-pvdz bsis set yields vibrtionl frequencies of the glycine conformers very similr to the MP2 results. Both DFT nd MP2 results re in the excellent greement with the experimentl dt. 1. Introduction The simplest minocid, glycine, is one of the most importnt biologicl compounds. In the solid stte nd in solutions glycine is known to exist in the zwitterion form, 1 NH + 3 -CH 2 -COO -, but in the gs-phse glycine exists in nonionized form, 2 NH 2 - CH 2 -COOH. The internl rottion bout the C-C, C-N, nd C-O bonds results in severl glycine conformers. The determintion of the spectrl nd structurl chrcteristics of the conformers of glycine, s well s other nturl minocids, is of gret interest becuse of their reltion to the minocid units in peptides. Moreover, the ssignment of the gs-phse glycine spectrum is of importnce to rdiostronomy in identifiction of interstellr minocids. 3 The investigtion of the gseous glycine is difficult due to its therml instbility. Similr to other minocids, glycine decomposes before melting. Only few experimentl investigtions of the gseous glycine hve been crried out during the pst two decdes. The first microwve spectroscopic results were presented by Brown et l. 4 nd Suenrm et l. 5 These studies identified conformer II (Figure 1) s the only gs-phse form of glycine nd disgreed with the results of the erlier b initio clcultions which concluded tht conformer I should be the lowest energy form. 6 The microwve spectrum of this conformer ws observed few yers lter by Suenrm et l. 7 Accurte vlues of the rottionl constnts nd dipole moments of the two low-energy glycine conformers, I nd II, were * Corresponding uthor. University of Arizon. Ntionl Acdemy of Sciences. University of Lisbon. University of Coimbr. determined in the more recent microwve studies. 8,9 The difficulty in the observtion of conformer I in the microwve spectr ws cused by its significntly lower dipole moment thn the dipole moment of conformer II. Since the other lowenergy conformers of glycine lso hve low dipole moments, their identifiction in microwve spectr is difficult. Thus, the theoreticlly predicted conformer III is hrdly registered by the microwve spectroscopy. The moleculr structure of the gseous glycine were determined by Iijim et l. 2 in n electron-diffrction study. Agin conformer I ws proven to be the most stble form in the gs phse, but t temperture of 219 C, bout 24% of the compound ws detected to pper s mixture of minor conformers which were supposed to be conformers II nd III. 2 However, these conformers re different from ech other only in the position of the hydrogens nd they re indistinguishble by their electron scttering intensities. 2 Thus, the question of the conformtionl structure of glycine in the gs phse hs remined open. The conformtionl behvior of glycine hs been the subject of very extensive theoreticl studies All clcultions hve been consistent in predicting tht the conformer I is the most stble form. However the stbility order nd the structures of the other glycine conformers depended on the level of theory nd the bsis set used in the clcultions. The comprison of the reltive energies of the glycine conformers clculted t the HF nd MP2 levels shows tht the former method overestimtes the energy gp between conformer I nd the other conformers. 12,17 The DFT clcultions of the structure nd reltive stbilities of the glycine conformers 11,14 produced results which re very similr to the results of the MP2 method. The S (97) CCC: $ Americn Chemicl Society Published on Web 01/22/1998

2 1042 J. Phys. Chem. A, Vol. 102, No. 6, 1998 Stepnin et l. Figure 1. The lowest energy glycine conformers I, II, nd III. sophisticted investigtions of the potentil energy surfce of glycine 12,15,16,19 llowed identifiction of s mny s eight minimum-energy conformers nd the estimtion of the brriers of the conformer interconversions. Some of these brriers were predicted to be quiet smll. 15,19 The results of quntum-chemicl clcultions of the reltive stbilities of the glycine conformers hve shown tht, besides the erly observed conformers I nd II, t lest one more conformer, nmely conformer III (Figure 1), my exist in the gs phse. As it is seen in the moleculr structures of the lowenergy conformers in Figure 1, they differ in the intrmoleculr H-bonds: bifurcted NH 2 OdC H-bond in conformer I, N H-O H-bond in conformer II nd bifurcted NH 2 O-C H-bond in conformer III. IR spectroscopy could be very suitble tool to investigte the glycine conformtionl structure, since the IR spectr re very sensitive to the intrmoleculr H-bonding. However, the low therml stbility of glycine precludes IR study in the gs phse becuse it is not possible to obtin concentrtion of the glycine in the gs phse which is sufficient to register its IR spectr. In such situtions combined use of the IR spectroscopy nd mtrix isoltion technique cn be very helpful 21 since it llows seprtion of the mtrix deposition process, which my be very long, from the registrtion of the IR spectrum of the smple. Also, the inert-gs mtrix environment hinders the rottion of the molecules studied nd excludes the presence of rottionl structures in the IR bnds, simplifying the interprettion of the mtrix IR spectr in comprison with gseous ones. Our first ttempts to freeze complete set of the glycine lowenergy conformers in the Ar mtrices were unsuccessful. In these experiments the smples were deposited t K. From our previous experiments we found tht this temperture rnge is optiml to prepre smples with miniml scttering. We supposed tht the low energy brriers between the glycine conformers predicted theoreticlly 19 might led to esy interconversion of the conformers in the low-temperture mtrices. To elucidte this question further, we performed serch for optiml experimentl conditions which llow us to deposit complete set of the low-energy glycine conformers in the mtrices. 22 In the course of this study, IR spectr were mesured for smples deposited t tempertures rnging from 19 to 5.5 K. We found tht, for smples deposited t tempertures below 13 K, dditionl bnds pper in the IR spectr of mtrix-isolted glycine. These bnds my correspond to new glycine conformer which did not pper t higher tempertures becuse it immeditely interconverted to lower energy rotmers. Vrious mtrix gses (Ne, Ar, nd Kr) were used to prove tht the observed phenomenon is indeed due to the presence of n dditionl glycine conformer nd not to mtrix splittings. In this study 22 we lso explined why the mtrix IR spectr of glycine obtined by Grenie et l. 23 becuse the smples deposited t 20 K did not contin the conformtionl splittings which ppered t lower tempertures. In continution of these studies, in this work we undertook detiled investigtion of the conformtionl composition of the gseous glycine isolted in low-temperture mtrices. In this pper we present results of simultneous observtion of the three low-energy glycine conformers. The IR spectrl chrcteristics of these conformers (conformers I, II, nd III) re ssigned on the bsis of the comprison with the dt clculted using the DFT nd MP2 methods. The relibility of these methods in predicting vibrtionl frequencies nd intensities of nonionized minocids is discussed. 2. Experimentl Detils The therml instbility of glycine cretes certin problems with the smple preprtion. The Knudsen cell ws used in the present work to evporte glycine. It ws necessry to first determine the optiml evportion temperture, which is high enough to yield smples with sufficient concentrtion of the compound but still sufficiently low to prevent decomposition of the compound. A low-temperture qurtz microblnce ws used to mesure the intensity of the gseous flow of glycine. In our design of the deposition system, the miniml temperture t which the flow of glycine ws found sufficient for the smple preprtion ws 160 C. The qurtz microblnce ws used to mesure the gseous flow of the substnce studied nd of the mtrix gs. It llowed us to control the concentrtion of the glycine in the mtrix nd to void the ppernce of utossocites. The mesurements were crried out for glycine (glycine-d 0 ) nd for its deuterted derivtives: C,C-d 2 -glycine, N,N,O-d 3 - glycine, nd the fully deuterted glycine which will be denoted s glycine-d 2, glycine-d 3, nd glycine-d 5, respectively. The mtrix smples were prepred by simultneous deposition of the substnce nd the mtrix gs onto cooled CsI substrte. The mtrix gs ws 99.99% Ar. The substrte temperture ws K during mtrix preprtion, nd tht llowed us to prevent interconversion of the glycine conformers in the mtrices. Commercilly vilble glycine nd its isotopomers

3 Glycine Conformers J. Phys. Chem. A, Vol. 102, No. 6, Figure 2. The high-frequency regions of the mtrix IR spectr of the glycine isotopomers: glycine-d 0 (A), glycine-d 2 (B), glycine-d 3 (C), glycine-d 5 (D). The spectr were recorded for the smples deposited t 13 K. Mtrix rtio is 1:1000 for glycine-d 0 nd 1:750 for other isotopomers. were used in the experiments. They were evported from the Knudsen cell t 170 C for glycine, 161 C for glycine-d 2, 176 C for glycine-d 3, nd 169 C for glycine-d 5. The thermosttting ccurcy ws 0.2 C. Ar flow stbility ws chieved with stble gs pressure over the solid rgon t 77 K nd ws djusted by needle vlve. The fill-up helium cryostt used in our mtrix-isoltion IR experiments ws described elsewhere. 24 The IR spectr were registered with the updted SPECORD IR 75 spectrometer in the rnge cm -1 with the resolution of3cm -1 in the rnge cm -1 nd with the resolution of1cm -1 in the rnge cm -1. The spectrometer ws seled nd blown through with dry nitrogen during the experiment to exclude ny influence of the tmospheric H 2 O nd CO 2 vpors. Firstly, the IR spectr were recorded for glycine-d 0, glycine-d 2, glycine-d 3, nd glycine-d 5 immeditely fter smple deposition. These spectr re shown in Figures 2 nd 3. They do not contin ny bnds of the glycine decomposition products. Only trce mounts of CO 2 re found in the smples (the chrcteristic doublet bnd of the CO stretching vibrtions t 2344/2339 cm -1 ) which is most probbly due to CO 2 bsorbed on the surfce of the solid compounds used for the experiment. To seprte the IR bnds of conformer III from other conformers, dditionl experiments were crried out for ll isotopomers. In these experiments, the smples deposited t K were heted up to 20 K nd fter 30 min they were chilled nd their spectr were recorded. At 20 K rgon mtrix remins rigid enough so tht the trnsltionl diffusion of the guest molecules is negligible. Thus, neither dimer formtion of the molecules studied nor the sufficient repcking of the mtrix environment were possible. However, the temperture of 20 K ws high enough for conformer III to interconvert to the lower energy conformer I. Compring the spectr obtined before nd fter nneling, we were ble to identify the bnds due to conformer III. Figure 3. The fingerprint regions of the mtrix IR spectr of the glycine isotopomers: glycine-d 0 (A), glycine-d 2 (B), glycine-d 3 (C), glycined 5 (D). The spectr were recorded for the smples deposited t 13 K. Mtrix rtio is 1:1000 for glycine-d 0 nd 1:750 for other isotopomers. 3. Theoreticl Methods The min gol of the clcultions in the present study ws to simulte the IR spectrl chrcteristics of the most stble glycine conformers, which were used to identify the conformers in the experimentl mtrix IR spectr. To chieve this gol we first serched for energy minim on the potentil energy surfce of glycine corresponding to the lowest energy conformers nd then we clculted the IR frequencies nd intensities for the minim using the hrmonic pproximtion. Glycine, s the smllest minocid, is suitble model to exmine relibility of the theoreticl methods used in the clcultions of IR spectr. This kind of nlysis will be helpful in our future mtrix-isoltion IR studies on other minocids nd smll peptides. Among the Hrtree-Fock (HF), Møller-Plesset second-order perturbtion theory (MP2) nd DFT methods used to study the structure of the glycine conformers, the two ltter methods yield the results which re in best greement with the results of microwve nd electron diffrction studies. 2,4,5,7-9 The HF method filed to predict the correct reltive energies nd structure of the glycine conformers with the N H-O intrmoleculr H-bond. 12,17 Therefore, in the present study we hve used the MP2 25 nd the DFT methods for the hrmonic frequency clcultions of the glycine conformers. The DFT clcultions were crried out with Becke s grdient exchnge correction, 26 the Lee, Yng, Prr correltion functionl, 27 nd the Vosko, Wilk, Nusir correltion functionl. 28 This three-prmeter density functionl, designted s B3LYP, ws found to produce the most ccurte glycine structurl prmeters compred with the microwve results. 11,14 For geometry optimiztion nd hrmonic frequency clcultions, we used three Dunning s correltion-consistent double-ζ bsis sets. 29,31 The smllest bsis set used in this study ws

4 1044 J. Phys. Chem. A, Vol. 102, No. 6, 1998 Stepnin et l. TABLE 1: Optimized Geometry Prmeters (Angstroms nd Degrees) of the Glycine Conformers Obtined in the DFT/ B3LYP/ug-cc-pVDZ nd MP2/ug-cc-pVDZ Clcultions I II III exptl DFT MP2 DFT MP2 DFT MP2 C-N C-C CdO C-O C-H C-H N-H N-H O-H N-C-C C-CdO C-C-O C-C-H C-C-H H-C-H C-N-H C-N-H H-N-H C-O-H N-C-CdO N-C-C-O OdC-C-H OdC-C-H C-C-N-H > C-C-N-H C-C-O-H From ref 2. In ref 2 some prmeters were tken from HF/4-21G clcultions. 7b They re shown in itlics. the stndrd cc-pvdz bsis set. The two other sets were obtined from the first one by dding diffuse functions. The second bsis set, denoted here s cc-pvdz++, included only n s diffuse function on hydrogens nd s nd p diffuse functions on hevy toms (the exponents of the diffuse functions were the sme s in the stndrd ug-cc-pvdz bsis). The third bsis set ws the stndrd ug-ccpvdz bsis, which includes s nd p diffuse functions on hydrogens nd s, p, nd d diffuse functions on hevy toms. The previous theoreticl studies of the potentil energy surfce of glycine identified eight minimumenergy geometries. 12,15,16,19 In this study we crried out frequency clcultions for only the most stble conformers I, II, nd III (Figure 1), which re supposed to exist in the gs phse. All clcultions in this work were performed on IBM RS6000 worksttions using the Gussin94 32 quntum-mechnicl progrm. 4. Results nd Discussion 4.1. Structure nd Reltive Stbilities of the Glycine Conformers. The geometries of the three glycine conformerssi, II, nd IIIswere fully optimized t the DFT/B3LYP nd MP2 levels with the cc-pvdz, cc-pvdz++, nd ug-cc-pvdz bsis sets (i.e., six sets of structurl prmeters were clculted for ech conformer). In this section the structurl prmeters nd rottionl constnts of the glycine conformers clculted t the DFT/B3LYP nd MP2 levels of theory with the different bsis sets re compred with the vilble experimentl dt to determine which method yields the most ccurte results. For this comprison we use the experimentl geometry prmeters of the conformer I 2, s well s the rottionl constnts of the conformers I nd II of the glycine-d 0 nd glycine-d 2, nd the rottionl constnts of the conformer II of the glycine-d 3. 8,9 The experimentl nd clculted geometries of conformers I, II, nd III re presented in Tble 1. Among the experimentl prmeters, those shown in itlics in Tble 1 were not derived from the experimentl dt but were ssumed on the bsis of previous b initio clcultions t the HF/4-21G level. 7b The experimentl nd clculted rottionl constnts re summrized in Tble 2. The inclusion of the diffuse functions in the bsis set leds to significnt improvement of the clculted structure of the min glycine conformer with respect to the experimentl geometricl prmeters t both the DFT/B3LYP nd MP2 levels. The comprison of the DFT/B3LYP/ug-cc-pVDZ nd MP2/ ug-cc-pvdz geometries with the experimentl dt (Tble 1) shows tht the DFT method is ble to produce geometries with similr ccurcy s the MP2 method. The men differences between the clculted t the DFT/B3LYP level nd observed geometry prmeters re Å nd 1.6 for the bond lengths nd the bond ngles, respectively. The corresponding vlues for the MP2 method re Å nd 1.5. The comprison of the clculted nd the observed rottionl constnts (Tble 2) lso shows tht the two methods give very similr results. The best greement with experiment is obtined when the ug-ccpvdz bsis set is pplied in conjunction with the DFT/B3LYP method. In this cse, the rottionl constnts re in even better greement with the experiment thn the MP2 results (Tble 2). When nlyzing the clculted geometries of the three glycine conformers, we pid specil ttention to the conformtion of the hevy tom bckbone. The entry-level HF clcultions with smll bsis sets 19 predicted the plnr bckbone structure for ll three conformers. Incresing the bsis, by dditionl polriztion functions nd ccounting for the electron correltion effects, led to nonplnr structures of the conformers II nd III, 10,12,16,17 while conformer I still remins plnr in ll clcultions. As it is seen from Tble 1, t both the DFT/ B3LYP nd MP2 levels we obtin plnr structures for conformer I nd nonplnr structures for conformer II. In the ltter cse the torsion ngle, NCCdO, is nd t the B3LYP nd MP2 levels, respectively. On the other hnd,

5 Glycine Conformers J. Phys. Chem. A, Vol. 102, No. 6, TABLE 2: Rottionl Constnts (MHz) nd Dipole Moments (Debye) of the Glycine Conformers Observed nd Clculted with the ug-cc-pvdz Bsis Set I II III exptl DFT MP2 exptl DFT MP2 DFT MP2 glycine-d 0 A c B c C c µ 1.15b b glycine-d 2 A c B c C c glycine-d 3 A c B c C c The rottionl constnts of the glycine-d 0, glycine-d 2, nd glycine-d 3 re tken from ref 8. b From ref 9. determintion of whether conformer III hs plnr or nonplnr structure is not s cler. The potentil energy surfce of this conformer is extremely flt ner the minimum. The devition from the plnrity determined in the clcultions is less thn 1. The vlues of the torsion ngle NCCdO clculted t the DFT/B3LYP nd MP2 levels with the ug-cc-pvdz bsis set re nd , respectively. It is notble tht the energy difference between the nonplnr structures of the conformers II nd III nd their plnr counterpirs is rther smll. For exmple, the energy difference between the more stble non-plnr form of the conformer II nd its plnr counterprt ws predicted to be 0.21 kj mol -1 t the B3LYP/ DZP level, kj mol -1 t the MP2/6-31++g** level, kj mol -1 t the CISD/DZP level, 15 nd 0.42 kj mol -1 t the CCSD (T)/DZP level. 15 For conformer III, the clcultion crried out t the CISD/DZP 15 nd DFT/B3LYP/DZP levels 11 predicted the plnr conformtion to be the true minim, but t the sme time the clcultion t the MP2/ G** level, 16 s well s our clcultions t the MP2/ug-cc-pVDZ nd B3LYP/ug-cc-pVDZ levels, predict tht nonplnr conformtion is the true minimum. The flt potentil energy surfce ner the equilibrium point predicted for the conformer III requires tighter thresholds in the geometry optimiztion. Therefore, for conformer III, we crried out n dditionl structure clcultion using the keyword OPT)TIGHT, decresing the convergence criteri 30 times. In this clcultion we did not observe ny significnt chnges in the energy; it decresed by less thn 0.1 kj mol -1, nd in the geometry, except for the vlue of the dihedrl NCCdO ngle, which incresed by few degrees. The DFT/B3LYP/ug-cc-pVDZ frequency clcultions crried out for conformer III t the geometry obtined fter the optimiztion with the conventionl thresholds did not produce ny imginry frequencies. Nevertheless, the energy difference between the plnr nd nonplnr form of these conformers is lower thn the energy of the zero-point twisting vibrtion, nd from the experimentl point of view, the plnr structure is likely to be the most probble conformtion. The energies, zero-point energies (ZPEs), nd the reltive stbilities of the three most stble glycine conformers re summrized in Tble 3. In the first plce, the importnce of the ZPE contribution in the prediction of the reltive stbilities of the conformers, especilly in the cse of conformer II, should be noticed. As it is seen, the ZPE energies of conformer II, t both the DFT/B3LYP nd MP2 levels, re higher thn those of the conformers I nd III. This leds to destbiliztion of the conformer II by 1.20 kj mol -1 t the DFT/B3LYP/ug-cc-pVDZ level nd by 1.72 kj mol -1 t the MP2/ug-cc-pVDZ level. At the sme time, the ZPE only slightly chnges the reltive energy of the conformer III with respect to the min conformer. The DFT/B3LYP nd MP2 methods yield very similr vlues of the reltive energies of the glycine conformers. The reltive energies of conformers II nd III with respect to the min conformer re 3.60 nd 6.84 kj mol -1 t the DFT/B3LYP/ugcc-pVDZ level nd 3.96 nd 7.14 kj mol -1 t the MP2/ugcc-pVDZ level. On the bsis of nlysis of the dt presented in the Tbles 1-3, we my conclude tht the DFT/B3LYP method produces structures of the glycine conformers, which re in very good greement with the experimentl dt nd lmost identicl to the MP2 results. It llows us to hope tht this method is lso ble to produce the vibrtionl frequencies nd infrred intensities with high ccurcy IR Spectrl Chrcteristics of the Glycine Conformers. The electron diffrction 2 nd microwve 4,5,7-9 studies demonstrted the presence of glycine conformers I nd II in the gs phse. In our preliminry mtrix isoltion IR study, 22 we observed in the IR spectr some mnifesttions of n dditionl glycine conformer which, bsed on the results of the quntum-chemicl clcultions, is now supposed to be conformer III. If ll three conformers re present in the smples, the experimentl IR spectrum is superposition of their spectr. To determine the spectrl chrcteristics of the individul conformers, we hve to seprte their vibrtionl bnds. This seprtion is bsed on the following dt: (i) The results of the theoreticl simultions of the IR spectr of the three glycine conformers. (ii) The elimintion of the bnds of conformer III from the experimentl spectrum. As ws mentioned before, the low energy brrier of the conformer III w conformer I interconversion results in the disppernce of conformer III from the mtrix t temperture bove 13 K. This phenomenon ws used to distinguish the bnds of conformer III by compring the IR spectr recorded before nd fter mtrix nneling. (iii) The experimentl IR spectr mesured for the glycine deuterted derivtives. The nlysis of the isotopomer frequency shifts nd their comprison with clculted shifts cn provide dditionl support for the ssignments. The hrmonic frequency clcultions of the glycine conformers were crried out t the DFT/B3LYP level with the cc-pvdz, cc-pvdz++, nd ug-cc-pvdz bsis sets, s well s t the MP2 level with the cc-pvdz nd cc-pvdz++ bsis sets. The disk spce limittion precluded clcultions t the MP2 level with the ug-cc-pvdz bsis set. The verge divergences

6 1046 J. Phys. Chem. A, Vol. 102, No. 6, 1998 Stepnin et l. TABLE 3: Energies (u), Reltive Stbilities ( E)(kJ mol -1 ), Zero-Point Vibrtionl Energies (ZPE ) (u), Totl Energies Including the Zero-Point Vibrtionl Energies (u) nd Reltive Stbilities Including the Zero-Point Vibrtionl Energy (kj mol -1 ) of the Three Glycine Conformers. Vlues Tken from the Full Optimiztion nd Frequency Clcultions of the Conformers t the DFT/B3LYP nd MP2 Levels of Theory method glycine I glycine II glycine III DFT/cc-pVDZ energy E(DFT) ZPE totl energy E(DFT + ZPE) DFT/cc-pVDZ++ energy E(DFT) ZPE totl energy E(DFT + ZPE) DFT/ug-cc-pVDZ energy E(DFT) ZPE totl energy E(DFT + ZPE) MP2/cc-pVDZ energy E(MP2) ZPE totl energy E(MP2 + ZPE) MP2/cc-pVDZ++ energy E(MP2) ZPE totl energy E(MP2 + ZPE) MP2/ug-cc-pVDZ energy E(MP2) E(MP2 + ZPE b ) Zero-point vibrtionl energies were scled pplying the scling fctors SF1 nd SF2 listed in Tble 4. b ZPE from the MP2/cc-pVDZ++ clcultion ws used here. TABLE 4: Men Divergences (cm -l ) between the Clculted nd Observed Frequencies of the Four Isotopomers of the Conformer I glycine-d 0 glycine-d 2 glycine-d 3 glycine-d 5 method bsis set SF1 SF2 A B A B A B A B DFT cc-pvdz cc-pvdz ug-cc-pvdz MP2 cc-pvdz cc-pvdz The men divergences clculted for the nonscled nd scled frequencies re listed in the columns A nd B, respectively. For ech bsis set nd for the DFT/B3LYP nd MP2 levels of theory, the men divergences were clculted for ll frequencies (presented in the first row) nd for the frequencies below 2000 cm -1 (presented in the second row). between the clculted nd observed frequencies were clculted for the four isotopomers of the glycine I conformer to determine which theoreticl method yields the most ccurte isotopomer frequencies. The comprison is presented in Tble 4. The dt shown in column A of Tble 4 were obtined with nonscled clculted frequencies. For ll the levels of theory used in this study, we determined the optiml scling fctors which llowed us to obtin the miniml men divergencies between the clculted nd observed frequencies. Two scling fctors, SF1 nd SF2, were determined for the frequencies clculted with ech bsis set: SF1 for the X(C,N,O)-H stretching vibrtions nd SF2 for ll other vibrtions. The men divergences clculted for the scled theoreticl frequencies re presented in column B in Tble 4 long with the vlues of the scling fctors. Tble 4 includes two rows for ech bsis set. In the first row the men divergences were clculted for ll observed frequencies. In the second row the men divergences were clculted only for the frequencies below 2000 cm -1 (i.e., for ll frequencies, excluding X(C,N,O)-H stretching vibrtions). The following conclusions my be derived from the nlysis of the dt presented in Tble 4: (i) The best greement between the clculted nd observed frequencies is observed when the DFT/B3LYP method with the ug-cc-pvdz bsis set is employed. The men divergence is only bout 10 cm -1. This vlue ws obtined using scled frequencies with the scling fctors SF1 ) 0.96 nd SF2 ) 0.99 (Tble 4). (ii) The MP2 method lso yields frequencies which re in good greement with the experimentl dt fter scling. The men divergence clculted for the MP2/cc-pVDZ++ frequencies is bout 13 cm -1, lthough the scling fctors employed here re smller then the ones obtined for the DFT/B3LYP/ ug-cc-pvdz frequencies.

7 Glycine Conformers J. Phys. Chem. A, Vol. 102, No. 6, Figure 4. The region of the CdO stretching vibrtions of glycine-d 0 conformers. (1) IR spectrum recorded fter mtrix deposition t 13 K. (2) IR spectrum recorded fter mtrix nneling t 20 K for 30 min. Asterisks (*) mrk the bnds ssigned to conformer II. Down rrows (V) mrk the bnds ssigned to conformer III. (iii) Augmenttion of the bsis set by dditionl diffuse functions significntly improves the greement between the clculted nd observed frequencies t both the DFT/B3LYP nd MP2 levels of theory. As mentioned bove, the inclusion of the diffuse functions is prticulrly importnt to predict correctly the reltive energy nd the structure of the conformer II. This is most probbly due to the strong intrmoleculr N H-O H-bonding interction present in this conformer whose description requires diffuse orbitls in the bsis set. For the sme reson, the ugmenttion of the bsis set with diffuse functions cn be expected to produce better greement between the clculted nd observed IR frequencies. (iv) In lmost ll the cses considered incresing the number of the deuterium toms in the glycine molecule led to decrese of the men frequency divergences (Tble 4). This my be simply explined by the decrese of most frequency vlues upon deuterition, which results in decrese of the men divergence. The nlysis of the men divergences presented in Tble 4 shows tht the DFT/B3LYP/ug-cc-pVDZ method yields the most ccurte vibrtionl frequencies of ll the considered deuterted derivtives of the min glycine conformer I. These frequencies hve been used to identify the bnds of the minor conformers in the experimentl spectr. The observed nd clculted (t the DFT/B3LYP/ug-ccpVDZ level) IR frequencies nd intensities of conformers I, II, nd III re presented in Tbles 5-8 for glycine-d 0, glycine-d 2, glycine-d 3 nd glycine-d 5, respectively. In these tbles the results of the potentil energy distribution (PED) nlysis re lso given. If severl of the clculted frequencies belonging to the different conformers re ssigned to single observed bnd, the PEDs presented in the tbles correspond to the min glycine conformer I Seprtion of the Vibrtionl Bnds of the Glycine Conformers. CdO Stretching Vibrtions. The seprtion of the conformer bnds in the mtrix IR spectr strts with the nlysis of the most intensive spectrl fetures. The region of the CdO stretching vibrtions of glycine-d 0 is presented in Figure 4. In this region of the spectrum, one very intensive bnd, s well the less intensive bnds, re observed. The intensive bnds t 1779, 1776, 1768, nd 1769 cm -1 in the Figure 5. IR spectrl regions of the glycine isotopomers: glycine-d 2 (A nd B), glycine-d 3 (C), nd glycine-d 5 (D). (1) IR spectrum recorded fter mtrix deposition t 13 K. (2) IR spectrum recorded fter mtrix nneling t 20 K for 30 min. Asterisks (*) mrk the bnds ssigned to conformer II. Down rrows (V) mrk the bnds ssigned to conformer III. spectr of the d 0, d 2, d 3, nd d 5 isotopomers, respectively, re ssigned to the CdO stretching vibrtions of the min conformer I. The ssignment of the less intensive bnds is bsed on the results of the frequency clcultions (Tbles 5-8). For ll isotopomers, the high-frequency experimentl bnd in the region of the CdO stretching vibrtion is ssigned to the CdO vibrtion of conformer II, nd the low-frequency bnd is ssigned to the CdO vibrtion of conformer III. This ssignment is confirmed by comprison of the spectr recorded before nd fter mtrix nneling. The intensity of the bnds of conformer III in the spectr of ll glycine isotopomers must decrese significntly fter smple heting due to the conformer III w conformer I interconversion. These bnds re mrked in Tbles 5-8. Some relevnt regions of the spectrum of glycine-d 0 re presented in Figures 4-6. As seen in Figure 4 nd in Tbles 5-8, the intensity of the bnds ssigned to the CdO stretching vibrtions decreses significntly fter mtrix heting, which supports their proposed ssignment. In the cse of the most intensive experimentl bnds, it ws very importnt to prove tht the origin of these bnds is not due to site splitting. The region of the CdO stretching vibrtions ws studied in our preliminry work, where we used different mtrix gses, nd we demonstrted tht the observed phenomenon is not cused by site splitting. 22 It is lso importnt to mention tht the site splitting is observed in the glycine spectr nd some bnds or, more often, bnd shoulders re definitely due to the mtrix splitting. For exmple, the lowfrequency shoulder t 1773 cm -1 on the bnd of the CdO stretching vibrtion of the conformer I (Tble 5, Figure 4) is probbly due to site splitting. The comprison of the clculted nd observed intensities of the CdO stretching vibrtions of the three glycine conformers

8 1048 J. Phys. Chem. A, Vol. 102, No. 6, 1998 Stepnin et l. TABLE 5: Observed nd Clculted (t the DFT/B3LYP/ug-cc-pVDZ level) IR Frequencies (cm -1 ) nd Intensities of the Glycine-d 0 clculted observed glycine I glycine II glycine III ν A b I c obs ν I d clcd ν I d clcd ν I d clcd PED e OH str [100] NH 8 str [57], NH 9 str [43] NH 8 str [50], NH 9 str [50] NH 9 str [57], NH 8 str [42] NH 8 str [50], NH 9 str [50] OH str [98] CH 6 str [50], CH 7 str [50] CH6 str [50], CH7 str [50] CdO str [84] CdO str [85] 1773* * CdO str [86] HNH bend [56], CNH bend [43] HNH bend [60], CNH bend [39] HCH bend [93] COH bend [70], C-O str [10] CCH bend [52], C-O str [12], COH bend [11], CC str t11] CCH bend [47], CCH bend [44] 1339* COH bend [33], C-O str [14] CCH bend [69] CCH bend [70] COH bend [45], CCH bend [28] C-O str [46], CC str [20], GOH bend [18] CCH bend 162], CNH bend 132] 1147* C-O str [46], COH bend [42] CN str [37], C-O str [18] CN str [41], C-O str [36], COH bend [15] CN str [80] CCH bend [47], CNH 8 bend [17] CNH bend 183] CNH bend [22], CC str [18] CC str [13], CCOH tor [13], C-O str [13] CNH bend [54], GO str [17] 852* CCOH tor [79] CC str [41], C-O str [10] CNH bend [31], CC str [23] * CC str [30], C-O str [24] 771* * CCOH tor [68], NCCdO tor [19] CCOH tor [62], NCC-O tor [14] CCdO bend [60], NCC bend [16] NCCdO tor [57], NCC-O tor [22] CCOH tor [38], CCH bend [28] CC-O bend [66], CC str [16] NCC bend [49], O-CdO bend [47] CCNH tor 184], NCC-O tor [17] NCC-O tor [53], NCCdO tor [18] Ar mtrix deposited t 13 K. Mtrix rtio 1:1000. Asterisks (*) mrk the bnds which decresed fter the mtrix nneling t 20 K for 30 min. b A, experimentl reltive pek intensities. c I obs, experimentl reltive integrl intensities mesured for the single bnds or for the groups of the merged bnds. d I clcd, clculted intensities in km mol -1. e PED contributions [%]. Only contributions g10% re listed. Abbrevitions: str, stretching; bend, bending; tor, torsion. lso llows us to evlute pproximtely the conformtionl composition of the gseous glycine t the deposition temperture of 170 C. Under this condition, bout 70% of the compound exists s the min conformer I. The contributions of conformers

9 Glycine Conformers J. Phys. Chem. A, Vol. 102, No. 6, TABLE 6: Observed nd Clculted (t the DFT/B3LYP/ug-cc-pVDZ level) IR Frequencies (cm -1 ) nd Intensities of the Glycine-d 2 clculted observed glycine I glycine II glycine III ν A b I c obs ν I d clcd ν I d clcd ν I d clcd PED e OH str [100] NH 8 str [57], NH 9 str [43] NH 8 str [50], NH 9 str [50] NH 9 str [57], NH 8 str [42] NH 8 str [50], NH 9 str [50] OH str [98] CD 6 str [50], CD 7 str [50] CD 6 str [50], CD 7 str [50] CdO str [84] CdO str [86] 1770* CdO str [86] HNH bend [56], CCH bend [44] HNH bend [60], CNH bend [40] COH bend [71], C-O str [10] * COH bend [41], C-O str [20] COH bend [37], C-O str [21] CNH bend [86] CN str [31], COH bend [30] C-O str [29], CNH 9 bend [18], CC str [16], COH bend [11] 1151* CN str [46], COH bend [18] 1141* C-O str [35], COH bend [26] CN str [34], CCD 6 bend [21] C-O str [35], CN str [32], COH bend [17] DCD bend [81] CCD bend [54], CN str [10] CCD bend [54], CN str [17] CCD bend [71], NCCdO tor [13] CN str [43], CCD bend [37] CCD bend [67], NCC-O tor [19] CNH bend [60], CN str [19] CNH bend [38], CCOH tor [37] CCOH tor [48], NCCO tor [13] 810* CNH bend [63], CN str [15] NCCdO tor [69] CC str [47], CC-O bend [10] CCD bend [45], CNH bend [30] 769* CC str [34], C-O str [32] * CCOH tor [83], NCCdO tor [11] CCdO bend [60], NCC bend [17] CCOH tor [82] * CC-O bend [63], NCC bend [19] CCdO bend [53], CC str [25] CCD bend [55], NCC-O tor [22] CC-O bend [63], CC str [18] NCC bend [16] NCC bend [48], OdC-O bend [47] CCNH tor [88], NCC-O tor [12] NCC-O tor [52], CCD bend [26] Ar mtrix deposited t 13 K. Mtrix rtio 1:750. Asterisks (*) mrk the bnds which decresed fter the mtrix nneling t 20 K for 30 min. b A, experimentl reltive pek intensities. c I obs, experimentl reltive integrl intensities mesured for the single bnds or for the groups of the merged bnds. d I clcd, clculted intensities in km mol -1. e PED contributions [%]. Only contributions g10% re listed. Abbrevitions: str, stretching; bend, bending; tor, torsion. II nd III re pproximtely even nd equl to 15%. This is in greement with the results of the electron diffrction study of glycine, 2 where the min glycine conformer ws determined to contribute 76% to the mixture. O-H (O-D) Stretching Vibrtions. In the high-frequency region of the spectr, the most intensive bnds t 3560 (Tble 5) nd 3564 cm -1 (Tble 6) re ssigned to the OH stretching vibrtions of glycine-d 0 nd glycine-d 2, respectively. The bnds t 2631 (Tble 7) nd 2630 cm -1 (Tble 8) re ssigned to the OD stretching vibrtions of the glycine-d 3 nd glycine-d 5. The

10 1050 J. Phys. Chem. A, Vol. 102, No. 6, 1998 Stepnin et l. TABLE 7: Observed nd Clculted (t the DFT/B3LYP/ug-cc-pVDZ level) IR Frequencies (cm -1 ) nd Intensities of the Glycine-d 3 clculted observed glycine I glycine II glycine III ν A b I c obs ν I d clcd ν I d clcd ν I d clcd PED e CH 6 str [50], CH 7 str [50] CH6 str [50], CH7 str [50] OD str [100] ND 8 str [54], ND 9 str [46] ND 8 str [50], ND 9 str [50] ND 9 str [53], ND 8 str [44] ND 8 str [50], ND 9 str [50] OD str [97] CdO str [87] CdO str [88] * CdO str [89] HCH bend [97] CCH bend [67], CC str [10] 1344* CCH bend [79] CCH bend [81] C-O str [44], CC str [16], COD bend [16] 1273* C-O str [38], CC str [22] CCH bend [77] DND bend [61], CN str [15] C-O str [25], CCH bend [22] CN str [66], CND bend [16] CCH bend [30], CND bend [76] CN str [70] COD bend [47], C-O str [31] COD bend [66], C-O str [14] 977* COD bend [57], C-O str [24] CC str [37], C-O str [31] CC str [39], NCC bend [17], COD bend [16] CC str [35], C-O str [24] CND bend [60] CND bend [54], CC str [19], COD bend [10] 663* CND bend [60], CC str [15] CND bend [63], NCC bend [12] CCOD tor [96] CC-O bend [32], NCC bend [17] NCCdO tor [46], NCC-O tor [23] CCH bend [34], NCC-O tor [25], NCCdO tor [14], CCOD tor [11] CCdO bend [55], C-O str [11] NCCdO tor [49], NCC-O tor [19] 535* CC-O bend [49], COD bend [16], 527* CCdO bend [14] CCdO bend [60], CC str [22] 481* CCdO bend [53], CC str [15] 477* CC-O bend [68], CC str [12] 419* CCOD tor [82], NCCdO tor [11] CCOD tor [89] NCC bend [51], O-CdO bend [42] CCND for [77], NCC-O for [24] NCC-O for [46], NCCdO for [22] Ar mtrix deposited t 13 K. Mtrix rtio 1:750. Asterisks (*) mrk the bnds which decresed fter the mtrix nneling t 20 K for 30 min. b A, experimentl reltive pek intensities. c I obs, experimentl reltive integrl intensities mesured for the single bnds or for the groups of the merged bnds. d I clcd, clculted intensities in km mol -1. e PED contributions [%]. Only contributions g10% re listed. Abbrevitions: str, stretching; bend, bending; tor, torsion.

11 Glycine Conformers J. Phys. Chem. A, Vol. 102, No. 6, TABLE 8: Observed nd Clculted (t the DFT/B3LYP/ug-cc-pVDZ level) IR Frequencies (cm -1 ) nd Intensities of the Glycine-d 5 clculted observed glycine I glycine II glycine III ν A b I c obs ν I d clcd ν I d clcd ν I d clcd PED e OD str [100] ND 8 str [54], ND 9 str [46] ND 8 str [50], ND 9 str [50] ND 9 str [53], ND 8 str [44] ND 8 str [50], ND 9 str [50] OD str [97] CD 6 str [50], CD 7 str [50] CD 6 str [50], CD 7 str [50] CdO str [88] CdO str [89] 1763* CdO str [90] C-O str [25], CC str [22], CCdO bend [16], CN str [13] C-O str [45], CC str [22], CCdO bend [14], COD bend [13] * C-O str [29], CC str [25], CCdO bend[12], CN str [10] 1220* DND bend [54], CN str [15] DND bend [53], CO str [14] DND bend [77], CN str [18] CN str [33], CCD bend [28] 1112* CCD bend [28], CN str [22] DCD bend [60], CCD bend [ CND bend [56] CCD bend [48], DCD bend [27] CND bend t68] COD bend [50], C-O str [26] * COD bend [59], C-O str [21] CCD bend [36], CN str [28] CCD bend [69], NCCdO tor [18] CN str [50], CCD bend [23] CC str [31], C-O str [26] CC str [27], CCD bend [24] CC str [26], C-O str [22] NCCdO tor [45], CND bend [40] CND bend [50], CC str [23], COD bend [12] 663* CND bend [58], CC str [16] CND bend [58J CCOD tor [96] CC-O bend [39], NCC bend [21] 528* CCOD tor [41], NCCdO tor [35] CCOD tor [33], NCC-O tor [21] CCdO bend [55], CC str [17] NCCdO tor [51], CCD bend [21] CC-O bend [66], CC str [16] CCOD tor [68], CCD bend [20] NCC bend [50], O-CdO bend [42] CCND tor [81], NCC-O tor [19] NCC-O tor [47], NCCdO tor [17] Ar mtrix deposited t 13 K. Mtrix rtio 1:750. Asterisks (*) mrk the bnds which decresed fter the mtrix nneling t 20 K for 30 min. b A, experimentl reltive pek intensities. c I obs, experimentl reltive integrl intensities mesured for the single bnds or for the groups of the merged bnds. d I clcd, clculted intensities in km mol -1. e PED contributions [%]. Only contributions g10% re listed. Abbrevitions: str, stretching; bend, bending; tor, torsion.

12 1052 J. Phys. Chem. A, Vol. 102, No. 6, 1998 Stepnin et l. Figure 6. IR spectrl regions of the glycine-d 0. OH bending/c-o stretching vibrtion region (A), CCOH torsion/nh 2 bending vibrtion region (B), NH 2 scissors vibrtion region (C), NH 2 bending vibrtion region (D). (1) IR spectrum recorded fter mtrix deposition t 13 K. (2) IR spectrum recorded fter mtrix nneling t 20 K for 30 min. Asterisks (*) mrk the bnds ssigned to conformer II. Down rrows (V) mrk the bnds ssigned to conformer III. clcultions show tht the OH(D) stretching vibrtions of conformers I nd III re very close nd cnnot be seprted in the experimentl spectr. On the other hnd, the OH(D) stretching vibrtions of conformer II is significntly downshifted due to formtion of the intrmoleculr N H-O H-bond. The frequency shift is 360 cm -1 for the OH stretch (Tble 5) nd 240 cm -1 for the OD stretch (Tble 6). The clcultions predict considerble increse of the intensities of the OH(D) stretching vibrtions of the conformer II due to H-bonding nd this higher intensity llows us to identify the bnds of these vibrtions in the spectr of ll glycine isotopomers. For the OH stretching vibrtions, s well s for the CH nd NH stretches, the observed differences between the experimentl unhrmonic nd clculted hrmonic frequencies re lrger thn for other vibrtions. This difference is due to the significnt unhrmonicity of the lrge mplitude X(C,N,O)-H stretching vibrtions. The use of the uniform scling fctor of 0.96 for the DFT/B3LYP/ug-cc-pVDZ frequencies of X(C,N,O) stretches improves the greement between the clculted nd observed frequencies in ll cses except the OH stretching vibrtions of conformer II. The difference is more thn 100 cm -1 fter scling (Tbles 5 nd 6). We believe this is cused by significnt unhrmonicity of this vibrtion due to the strongest intrmoleculr N H-O H-bond in conformer II mong ll the conformers studied. This interction chnges the potentil energy surfce ner the equilibrium position of the hydroxy hydrogen. Anlysis of the spectr of the deuterted glycines supports this conclusion. The difference between the observed nd clculted frequencies of the OD stretching vibrtion, with much smller mplitude, is the sme s those for other vibrtions in this region. The reltively stronger unhrmonicity of the OH stretching vibrtions of the conformer II lso results in decrese of the isotope rtio for this vibrtion (ν OHstr /ν ODstr )- from the vlue of for conformers I nd III to O-H(O-D) Bending nd C-O Stretching Vibrtions. The PED nlysis of the vibrtions in the region cm -1 llowed us to identify in the spectr the bnds which cn be ssigned to mixed OH bending/c-o stretching vibrtions. Similr coupling hs lso been found in the spectr of the crboxylic cids. 33,34 The frequencies nd PEDs of these vibrtions re sufficiently different for the three glycine conformers to llow their ssignment. The intensive experimentl bnd t 1390 cm -1 in the spectrum of glycine-d 0 (Figure 5) is ssigned to the OH bending vibrtion of conformer II. This bnd is shifted towrd higher frequencies, s compred to the corresponding bnds of conformers I nd III. The OH bending vibrtion of conformer II is the most intensive vibrtion. This bnd my be esily recognized in the IR spectr nd my serve s one of the min spectrl mnifesttions of the conformer-ii-like conformtions of minocids. The other bnd t 1200 cm -1 is lso ssigned to the vibrtion of conformer II. This vibrtion hs very complex mode with contributions from the C-O stretching, the CC stretching nd the OH bending modes. Two bnds in this region with contributions from the OH bending nd the C-O stretching modes re ssigned to the vibrtions of conformer III. The intensities of these bnds decrese in the spectr fter mtrix nneling nd this fct supports the ssignment of these bnds. In the spectr of glycine-d 3 nd glycine-d 5 (Tbles 7 nd 8), the vibrtions with the contribution from the OD bending mode re observed in the rnge cm -1. This low-frequency shift of the OD bending vibrtion llows us to identify the vibrtions with lrge contributions from the C-O stretching mode in the spectr of the deuterted glycine. The ssignment nd PEDs of these bnds re presented in Tbles 7 nd 8. N-H (N-D) Stretching nd Bending Vibrtions. The NH stretching vibrtions, both symmetricl nd symmetricl, hve very low intensity nd in the experimentl spectr only the wek bnd of the NH symmetricl stretching vibrtion t 3410 cm -1 ws detected (Tble 5). At the sme time, the bnds ttributed to the NH bending vibrtions re more intensive. The NH scissors vibrtions re observed in the region cm -1 (glycine-d 0 nd glycine-d 2 ) or in the region cm -1 (glycine-d 3 nd glycine-d 5 ). The bnd of the NH scissors vibrtion of conformer II t 1622 cm -1 nd the corresponding bnd of conformer I t 1630 cm -1 re identified in the spectrum of glycine-d 0 (Tble 5, Figure 6). The smll difference (8 cm -1 ) between the frequencies of the NH bending vibrtions of conformers I nd II, s well s the reltively smll difference between the NH stretching vibrtions of these conformers (less thn 40 cm -1, Tble 5) demonstrte the wekness of the N-H O H-bonding interction in conformers I nd III. The corresponding vlues obtined for the OH bending nd stretching vibrtions re 100 nd 360 cm -1. This is the spectroscopic evidence of the significnt difference between the N-H O nd N H-O intrmoleculr interctions in the glycine conformers. It indictes tht the NH 2 group is good proton cceptor but less effective proton donor. The dt presented in Tbles 5-8 nd in Figures 7 nd 8 llows us to identify some other vibrtions of the minor glycine conformers in the region below 1000 cm -1. The OH torsionl vibrtions should be mentioned here in the first hnd. Three bnds in the spectr t 619, 852 nd 644 cm -1 re ttributed to the OH torsionl vibrtions of conformers I, II, nd III,