Glycine and Its Hydrated Complexes: A Matrix Isolation Infrared Study

Apr 20, 2010 - Clifton Espinoza, Jan Szczepanski, Martin Vala, and Nick C. Polfer*. Department of Chemistry and Center for Chemical Physics, UniVersit...
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J. Phys. Chem. A 2010, 114, 5919–5927

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Glycine and Its Hydrated Complexes: A Matrix Isolation Infrared Study Clifton Espinoza, Jan Szczepanski, Martin Vala, and Nick C. Polfer* Department of Chemistry and Center for Chemical Physics, UniVersity of Florida, GainesVille, Florida 32611-7200, USA ReceiVed: February 15, 2010; ReVised Manuscript ReceiVed: April 3, 2010

The hydration of glycine is investigated by comparing the structures of bare glycine to its hydrated complexes, glycine · H2O and glycine · (H2O)2. The Fourier transform infrared spectra of glycine and glycine · water complexes, embedded in Ar matrices at 12 K, have been recorded and the results were compared to density functional theory (DFT) calculations. An initial comparison of the experimental spectra was made to the harmonic infrared spectra of putative structures calculated at the MPW1PW91/6-311++G(d,p) level of theory. The results suggest that bare glycine adopts a Cs symmetry structure (G-1), where the hydrogens of the amino NH2 hydrogen-bond intramolecularly with the carboxylic acid CdO oxygen. Also observed as minor constituents are the next two lowest-energy structures, one in which the carboxylic acid (O-)H group hydrogenbonds to the amino NH2 group (G-2), and the other where intramolecular hydrogen bonding occurs between the NH2 and the carboxylic acid O(-H) groups (G-3). The abundances of these structures are estimated at 84%, 9% and 8%, respectively. The least favored structure, G-3, can be eliminated by annealing the matrix to 35 K. Addition of the first water molecule to G-1 takes place at the carboxylic acid group, with simultaneous hydrogen bonding of the water molecule to the carboxylic acid (C))O and (O-)H. The results are consistent with the predominance of this structure, although there is evidence for a small amount of a hydrated G-2 structure. Addition of the second water molecule is less definitive, as only a small number of intense infrared modes can be unambiguously assigned to glycine · (H2O)2. Anharmonic frequency calculations based on secondorder vibrational perturbation theory have also been carried out. It is shown that such calculations can generate improved estimates (i.e., ∼2%) of the experimental frequencies for glycine and glycine · H2O, provided that the potential energy surfaces are modeled with high-level ab initio approaches (MP2/aug-cc-pVDZ). 1. Introduction One of the most daunting challenges facing computational chemistry today is accurately predicting the secondary and tertiary structures of proteins, based on their primary structures. But the effort expended may also offer huge rewards, for example, in drug discovery and development.1,2 This task is particularly complex, given the large number of competing effects involving intra- and intermolecular interactions. Amino acids and small peptides can serve as useful benchmark systems for the comparison of theory to experiment, and thus aid in building confidence in the various computational approaches. Glycine, the simplest amino acid, is the subject of the present paper. Over the past 30 years, a plethora of theoretical studies have been published on bare glycine (see, for example, refs 3-6). These studies have shown that there are three distinct low-energy conformers (see Figure 1). Of these, the Cs symmetry conformer G-1, where the hydrogens of NH2 hydrogen bond with the acid CdO, lies at the global minimum. Experimental studies, using gas-phase microwave absorption spectroscopy,7,8 gas-phase electron diffraction,9 matrix isolation infrared absorption spectroscopy,10,11 and glycine-helium cluster experiments12 have confirmed this finding. Despite this shared conclusion, the ability of these different experimental approaches to distinguish between the different possible conformers (G-1, G-2, and G-3) is limited. Although microwave spectroscopy yields extremely precise structural information, the dipole moment of the species of interest needs to be sufficiently large, * To whom correspondence should be addressed. E-mail: polfer@ chem.ufl.edu. Fax: (352)-392-0872.

Figure 1. Geometries of stable isomers of glycine and the transition state (TS) structure connecting G-1 and G-3 conformers, all optimized at the MPW1PW91/6-311++G(d,p) level of calculations. The relative electronic energies (in kJ/mol) corrected for zero-point vibrational energies calculated at MPW1PW91/6-311++G(d,p) and CCSD(T)/ccpVTZ// MPW1PW91/6-311++G(d,p) (in italic type) are in parentheses.

as exemplified by the problems in initially detecting G-1.13,14 Since electron diffraction cannot detect the position of hydrogen atoms, the conformers G-2 and G-3 cannot be distinguished by this method.9 In matrix isolation infrared spectroscopy, the temperature of the matrix plays a role in the ratio of the various conformers formed.11 Finally, spectra from helium cluster infrared measurements were found to be reasonably broad and

10.1021/jp1014115  2010 American Chemical Society Published on Web 04/20/2010

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were furthermore limited to the tuning range of the optical parametric oscillator (OPO) laser.12 Parallel to these experimental studies, concomitant theoretical studies of glycine hydration have been pursued over the years. In one of the first such investigations, Gordon and Jensen addressed the question of how many water molecules were necessary to stabilize the glycine zwitterionic structure.15 Although it is well-established that in the condensed phase glycine adopts the zwitterionic form, H3N+CH2CO2-,16,17 in the gas phase the neutral form, H2NCH2COOH, is preferred. An exact theoretical description of amino acid solvation is complicated by the many degrees of conformational freedom and “floppy” nature of these noncovalently bound systems. Numerous theoretical studies have addressed this challenge.18-22 A number of experimental studies have also addressed the question of the neutral-to-zwitterion transition and the number of surrounding waters necessary to stabilize the ionic conformer.23-27 Intriguing photoelectron results by Bowen and co-workers on discrete glycine · water cluster anions generated in a supersonic expansion showed a sudden onset of Gly · (H2O)5-complexes; this was interpreted as the onset of the zwitterionic form of glycine at five water molecules.23 However, later studies by Johnson and co-workers showed that smaller complexes, such as Gly · (H2O)1,2- could be generated,24,25 indicating that the onset of particular complexes is due to source kinetics, as opposed to dipole-induced electron capture rates. In a related study using infrared ion dip spectroscopy on neutral tryptophan · water and tryptophan · methanol complexes in a supersonic expansion, Oomens and co-workers showed that the zwitterionic form starts appearing at five water molecules,28 analogous to the initial suggestion by Bowen. Finally, in matrix-isolation FT-IR measurements, Maes and co-workers suggested that the zwitterionic form of glycine is stabilized with only three water molecules.26 Few experimental studies have addressed the detailed structures that are formed in the hydration of glycine. Very accurate structural information on gas-phase glycine · H2O was obtained by microwave absorption spectroscopy by Alonso et al.27 Because the FT-IR spectra recorded by Maes and co-workers were surprisingly broad,26 a unique separation of vibrational bands was not possible, although interesting frequency shifts upon hydration were noted. Maes and co-workers ascribed their inability to accurately predict the band positions from harmonic calculations to anharmonicities resulting from hydrogen bonding. It is general practice in the comparison of experimental vibrational spectra with harmonic quantum chemical calculations to use empirical scaling factors to account for inaccuracies in the calculational approach. The use of scaling factors can, however, be circumvented by including anharmonic effects with either second-order vibrational perturbation theory or vibrational self-consistent field (VSCF) methods.29,30 Extensions of the VSCF method, such as the correlation-corrected VSCF (ccVSCF) method,31 have refined the latter model. As Barone has described,32 second-order perturbative treatment can now be routinely carried out by using an automated code embedded in the Gaussian software.33 For these refinements to become accepted, it is essential to benchmark the theoretical results with experimental findings, which in the past have usually been in the form of infrared absorption measurements of molecules trapped in rare gas matrices.6,19,34 Hobza and co-workers carried out full-dimensional anharmonic calculations (using secondorder vibrational perturbation theory) on glycine with ab initio approaches (MP2/aug-cc-pVDZ), which they compared to the matrix isolation experiments from Stepanian et al.11 Chaban and Gerber performed anharmonic calculations on glycine ·

Espinoza et al. H2O,19,35 employing ab initio calculations (MP2/DZP) and their own cc-VSCF methodology.31,36 Both of these theoretical studies display differences in the predicted band positions for glycine. This underscores the importance of the choice of basis set in the potential surface calculations, as well as the algorithm used in the anharmonicity calculations. In this study, we use matrix isolation infrared absorption spectroscopy to probe the structures of bare glycine and its hydration products, glycine · H2O and glycine · (H2O)2. Analysis of the various structures observed is carried out by comparison with harmonic and anharmonic frequency calculations using density-functional theory (MPW1PW91/6-311++G(d,p)) and ab initio theory (MP2/aug-cc-pVDZ). Our results are compared with the ab initio anharmonic calculations by Hobza6 and by Chaban and Gerber.19 2. Experimental Procedures Since the experimental apparatus used has been described previously,37 only details important for this experiment are given here. Glycine powder (purity >99%, Sigma-Aldrich) was evaporated under vacuum from a small quartz oven (at 100-140 °C). Matrices were prepared by trapping the vaporized glycine on a cryogenically cooled (12 K) CsI window with the Ar isolant gas (99.999%, Matheson). The Ar gas contained 0.1-1.0% H2O by volume and was delivered from a vacuum port via a stainless steel (SS) flow-controller needle valve. Temperatures were recorded by a Au-Fe thermocouple attached to the copper window holder. Different concentrations of water in Ar and rates of flow were used to optimize the formation of glycine · H2O or glycine · (H2O)2 complexes. Preparatory experiments showed that the concentration of water in Ar matrices was generally lower than that measured manometrically (i.e., 0.1-1.0%). This is not unexpected because of the adhesion of water to the walls of the SS tubes, needle valve, and cryostat. It is anticipated that the formation of the hydrated complexes occurs primarily at the matrix surface during deposition, where the temperature is higher than 12 K. To increase the mobility of water in the matrix and to augment the formation of glycine · water complexes, in some experiments the matrix was thermally annealed (to 35 K and then cooled to 12 K). The diffusion coefficient of a mobile species such as water is high in Ar, due to its low melting temperature (Tm(Ar) ) 83.85 K).38 Infrared absorption spectra of the trapped molecular species were measured using a Nicolet Magna 540 Fourier transform infrared (FT-IR) spectrometer (equipped with a DTGS detector) in the range 500-4000 cm-1 at a resolution of 0.5 cm-1. 3. Computational Methods For geometry-optimized structures, the harmonic and anharmonic frequencies of vibrational fundamental modes for water and various conformers of glycine, glycine · H2O, and glycine · (H2O)2 complexes were initially run using density functional theory B3LYP and MPW1PW91 hybrid functionals with a 6-311++G(d,p) basis set, using the Gaussian 03 program.33 All computed geometries presented here are summarized in detail in Tables S1-S3 in the Supporting Information file, tabulating their Cartesian coordinates, as well as their zero-point corrected electronic energies. A comparison of the experimental IR absorption spectra with the predicted harmonic spectra for glycine showed that the MPW1PW91 functional yielded better agreement in terms of relative integral band intensities (Figure S4, Supporting Information).

Glycine and Its Hydrated Complexes

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TABLE 1: Summary of Computational Results and Comparison to Previous Studies Using Different Theoretical Approachesa level of theory

MPW1PW91/6-311++G(d,p) CCSD(T)/cc-pVTZ//MPW1PW91/ 6-311++G(d,p) B3LYP/6-31++G(d,p)b MP2/aug-cc-pVDZc

G-1 (kJ mol-1)

G-2 (kJ mol-1)

G-3 (kJ mol-1)

0.0 0.0

1.45 4.19

6.71 7.20

0.0 0.0

2.82 3.96

6.30 7.14

a Zero-point energy (ZPE)-corrected energies are shown as relative energies (in kJ mol-1). b Results from Maes and co-workers.26 c Results from Adamowicz and co-workers.11

From a comparison of the observed and predicted (MPW1PW91/6-311++G(d,p)) band positions for glycine, empirical scaling factors of 0.933 for the OH, CH, and NH stretching modes and 0.96 for other modes were employed. The same scaling factors were also used for glycine · H2O and glycine · (H2O)2. For the glycine conformers, the electronic energies were also calculated at the CCSD(T)/cc-pVTZ level based on the MPW1PW91/6-311++G(d,p) equilibrium geometry. The CCSD(T)/cc-pVTZ method, a coupled cluster approach (with singles and doubles) supplemented with a quasi-perturbative estimate of the effect of triple excitation, is expected to be more reliable for energy predictions than MPW1PW91/6-311++G(d,p), especially when utilizing Dunning’s correlation-consistent polarized valence triple-ζ (cc-pVTZ) basis set. The CCSD(T)/ccpVTZ// MPW1PW91/6-311++G(d,p) energies serve as an indicator on the reliability of the MPW1PW91/6-311++G(d,p) energy calculations for glycine · H2O and glycine · (H2O)2 complexes. The MPW1PW91/6-311++G(d,p) and CCSD(T)/ccpVTZ// MPW1PW91/6-311++G(d,p) energies were corrected for vibrational zero point energies without scaling, whereas in the CCSD(T)/cc-pVTZ// MPW1PW91/6-311++G(d,p) calculation the MPW1PW91/6-311++G(d,p) zero-point correction energies were used. In the anharmonic frequency calculations, a second-order vibrational perturbation theory approach was used, as implemented in the Gaussian03 software package and described by Barone.32 In Tables 2-3, those frequencies are compared to the anharmonic frequency calculations obtained using the correlation-corrected vibrational self-consistent field (CC-VSCF) method of Chaban and Gerber.19 4. Results and Discussion 4.1. Glycine. Computations on the glycine conformers, shown in Figure 1, were performed at different levels of theory, namely MPW1PW91/6-311++G(d,p) and CCSD(T)/cc-pVTZ// MPW1PW91/6-311++G(d,p). The results are summarized in Table 1 where they are also compared to previous theoretical results using B3LYP/6-31++G(d,p)26 and MP2/aug-cc-pVDZ approaches.11 In all calculations the relative energy ranking is G-1 < G-2 < G-3. However, it appears that the lower-level approaches, that is, MPW1PW91/6-311++G(d,p) and B3LYP/ 6-31++G(d,p), underestimate the energy gap between G-1 and G-2. Interestingly, only G-1 adopts a Cs symmetry, whereas nonplanar geometries are calculated for G-2 and G-3. This is related to the flat potential energy surfaces (PES) found for G-2 and G-3,39 as also reported previously.11 The observed absorption spectra for glycine in the mid-IR energy range are compared to the predicted spectra in Figure 2 (as well as in Figures S7 and S8). The best agreement for the

major bands is for the lowest energy conformer, G-1. In the CdO stretch region, two weak bands are observed and assigned to G-2 and G-3, as previously done by Adamowicz and coworkers.11 Assuming that the relative predicted integral band intensities are reliable for the CdO stretching modes, the relative abundances of the conformers are found to be G-1 (84%), G-2 (9%), and G-3 (8%). The vibrational modes for glycine are summarized in Table 2, where they are also compared to previous assignments. After landing in the 12 K matrix, the glycine molecules are expected to be fully thermalized (i.e., to have lost their excess energy to the matrix bath). The cooling rate is expected to be fast, hence it is not surprising that multiple conformers are produced. Nevertheless, it is possible to induce an interconversion of conformers by matrix annealing. The interconversion of G-3 to the lowest energy G-1 conformer during matrix annealing to 35 K was reported earlier.10,11 This is confirmed here in the experimental spectra in Figure 3, where the population of the G-3 isomer is depleted (Figure 3 D). To more fully understand the interconversion of G-3 to G-1, a calculation was performed on the rotation of the COOH group around the C-C bond (Figure S5, Supporting Information). The barrier for the rotation was found to be 5.0 kJ/mol (MPW1PW91/6-311++G(d,p)) and 4.5 kJ/mol (CCSD(T)/cc-pVTZ //MPW1PW91/6-311++G(d,p)). Note that in the rotamerization calculation, all degrees of freedom of the glycine molecule (other than the torsion angle) were optimized at each 5° step. Although the G-3 f G-1 isomerization naturally has a lower barrier than the reverse process, it is still much higher than the estimated thermal energy of glycine trapped at 35 K (∼0.29 kJ mol-1). This suggests that the interconversion is due to tunneling processes. Such tunneling processes have for instance been suggested for the OH rotation in cisoid 3-hydroxypropadienylidene trapped in solid Ar.40,41 4.2. Glycine · H2O. Observed matrix IR spectra were collected with increasing concentrations of water. Although many spectra were recorded, for the sake of clarity only three are shown in Figure 3, panels A-C. The spectrum obtained after matrix thermal annealing is displayed as well (Figure 3D). In the annealing experiment, water mobility is increased, thus boosting the concentration of glycine · water complexes. It can be seen that some bands increase in intensity at higher water content. On the basis of comparison to computed structures and frequencies for hydrated glycine · water complexes, it is proposed that many of these bands (denoted by larger dots in Figure 3D) be assigned to the glycine · H2O complex. Spectra at lower and higher frequencies are shown in Figure S7 and S8. Six stable conformers were found on the potential energy surface for glycine · H2O (see Figure 4). In Figure 5, the computed spectra for five of these complexes are compared to the experimental data. For the sake of clarity, a so-called experimental “synthetic” spectrum was created, which includes the frequencies and intensities of the bands marked by dots in Figure 3D, as well as Figure S7 and S8. Clearly, the lowestenergy conformer, G-1-W-1, yields the closest match to the experimental spectrum. None of the higher-energy conformations give convincing matches. The observed bands for glycine · H2O are summarized in Table 3, along with an assignment of the calculated bands for G-1-W-1. The G-1-W-1 complex is formed when a water molecule attaches to the carboxylic acid group, via double H-bonding (see Figure 4). The estimated electronic energy binding of water to glycine in this complex (corrected for zero-point vibrational energies) at MPW1PW91/6-311++G(d,p) level is 34.6 kJ/mol

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Espinoza et al.

TABLE 2: Calculated Harmonic and Anharmonic Infrared Mode Frequencies and Intensities (In Parentheses) Compared to Experimental Observed IR Bands for Glycine Isolated in Ar Matrix at 12 K

No.

harmonic (ω/cm-1)a

harmonic scaled (ωsc/cm-1)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

3817.0 (0.20) 3632.7 (0.02) 3557.1 (0.00) 3107.9 (0.01) 3069.2 (0.05) 1853.3 (1.0) 1686.2 (0.07) 1455.6 (0.05) 1413.8 (0.08) 1389.7 (0.00) 1313.9 (0.04) 1189.1 (0.00) 1185.3 (0.28) 1153.1 (0.64) 921.4 (0.01) 919.9 (0.31) 833.4 (0.34) 655.2 (0.30) 646.1 (0.01) 510.2 (0.11)

3664.3 3487.3 3414.8 2983.5 2946.4 1779.1 1618.7 1397.3 1357.2 1334.1 1261.3 1141.5 1137.8 1106.9 884.5 883.1 800.0 628.9 620.2 489.7

anharm. (νanh/cm-1) anharm. anharm. MPW1PW91/ anharm. (νanh/cm-1) (νanh/cm-1) (νanh/cm-1) exp. (νexp/cm-1)g (Ar matrix) mode description 6-311++G(d,p)c MP2/aug-cc-pVDZ d ref 6e ref 19f 3612.2 3466.5 3448.7 2960.5 2964.2 1822 1712.2 1438 1377.8 1351.6 1286.0 1160.2 1150.4 1116.3 908.3 852.3 818.4 613 637.7 490.3

3541.1 3423.9 3368.1 2991 2959.5 1757.2 1629.0 1417.08 1374.57 1345.3 1279.05 1152.81 1127.93 1089.11 902.93 887.12 807.12 623.57 615.71 497.9

3538 3416 3342 2991 2959 1762 1599 1418 1360 1345 1257 1153 1131 1089 904 889 808 624 616 495

3598 3382 3343 2986 2959 1805 1669 1473 1410 1377 1290 1185 1167 1122 970 943 847 633 613 514

3564.5h 3448.7, 3410.0 3357.3 2939.4 1780.9 (1.0) 1632.4 1430.9 1391.7

1131.6 1102.6, 1100.7 883.3 801.9, 798.7 620.2, 618.2 501.9

OH str NH as str NH s str CH as str CH s str CdO str NH2 bend CH2 bend CC str CH2-NH2 twist COH,OCC bend CH2-NH2 twist CO str, COH bend CN str, CO str CH2-NH2 twist CC str CC str, NC str OH wag OdC-O bend OH wag

The calculated (MPW1PW91/6-311++G(d,p)) integral intensity for the most intense mode for glycine is found at 1853.3 cm-1 (329 km/mol). b The 0.933 scaling factor was applied for the OH, CH, and NH stretching modes and the 0.96 for other modes. c Calculated at MPW1PW91/6-311++G(d,p), frequencies unscaled. d Calculated at MP2/aug-cc-pVDZ, frequencies unscaled. e Calculated at MP2/ aug-cc-pVDZ using quartic force field approximation, frequencies unscaled (see text). f Calculated at MP2/VDZ using CC-VSCF approach, frequencies unscaled (see text). g Experimental spectrum of water with its band assignments is taken from ref 26 and the spectrum of glycine is from this work, Figure 2 and Figures S7 and S8. h Observed bands due to four-sites at 3559.4, 3564.5 (most intense), 3566.5, and 3569.3 cm-1. a

TABLE 3: Calculated Harmonic and Anharmonic Infrared Mode Frequencies and Relative Intensities (In Parentheses) Compared to Experimentally Observed IR Bands (in Ar matrix, 12 K) for Glycine · H2O Complex

No.

harmonic (ω/cm-1)a

harmonic scaled (ωsc/cm-1)b

anharm. (νanh/cm-1) MPW1PW91/6-311++G(d,p)c

anharm. (νanh/cm-1) MP2/aug-cc-pVDZ d

anharm. (νanh/cm-1) ref 19 e

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

3950.6 (0.14) 3700.3 (0.46) 3631.9 (0.01) 3556.4 (0.00) 3408.6 (1.0) 3108.4 (0.01) 3069.8 (0.03) 1804.6 (0.43) 1685.0 (0.03) 1618.6 (0.18) 1469.8 (0.04) 1454.0 (0.03) 1388.5 (0.00) 1384.0 (0.03) 1263.0 (0.35) 1188.2 (0.00) 1167.5 (0.04) 934.3 (0.16) 927.9 (0.10) 915.9 (0.05) 852.4 (0.20) 674.5 (0.03) 627.8 (0.28) 562.4 (0.00) 496.8 (0.03)

3685.9 3452.4 3388.6 3318.1 3180.2 2900.1 2864.1 1732.4 1617.6 1553.8 1411.0 1395.8 1332.9 1328.6 1212.4 1140.6 1120.8 896.9 890.8 879.3 818.3 647.5 602.7 539.9 476.9

3764.0 3499.1 3467.1 3425.5 3187.7 2959.2 2935.2 1772.0 1626.2 1564.1 1412.9 1421.8 1358.0 1333.1 1228.2 1168.1 1134.2 876.0 893.7 888.9 790.2 670.2 547.0 559.4 489.9

3692.5 3455.9 3423.4 3366.1 3192.2 2989.8 2955.6 1723.9 1591.8 1572.1 1405.3 1397.5 1340.6 1316.6 1189.2 1151.4 1121.9 906.5 897.2 815.6 813.9 648.5 540.4 534.9 479.7

3724 3510 3411 3350 3125 3006 2966 1770 1668 1608 1469 1459 1378 1376 1245 1188 1157 979 947 944 872 671 688 580 493

exp. (νexp/cm-1)

mode description

f

OH as str H2O OH s str H2O NH as str NH s str OH str Glc CH as str CH s str CdO str NH2 bend H2O bend COH bend CH2 bend CH2-NH2 twist C-C str C-O str CH2-NH2 twist CN str COH bend C-C str CH2-NH2 twist CC str, NC str OdC-O bend OH wag H2O CH2-NH2 twist C-C-O bend

f f f

3205.0 (0.7) 1743.6 (1.0) 1577.5 (0.10) 1430.7 (0.18) 1423.6 (0.12) 1362.9 (0.13) 1202.0 (0.58) 1120.3 (0.15) 895.0 (0.17) 813.0 (0.58) 620.3g (∼0.25)

a The calculated integral intensity for the most intense mode of 3408.6 cm-1 is 713 km/mol. b The 0.933 scaling factor was applied for the OH, CH and NH stretching modes (modes 1-7) and the 0.96 for other modes (8-25). c Calculated at MPW1PW91/6-311++G(d,p) using second-order perturbation theory, frequencies unscaled. d Calculated at MP2/aug-cc-pVDZ using second-order perturbation theory, frequencies unscaled. e Calculated at MP2/VDZ using CC-VSCF approach, frequencies unscaled (see text). f The predicted bands overlapped with strong bands of water clusters. g Position of OH wag H2O probably affected by matrix effects.

(34.4 kJ/mol at MP2/aug-cc-pVDZ). This rather strong hydrogen bonding is expected to affect the band positions in glycine · H2O

compared to bare glycine. This is supported by the considerable red-shifting of the carboxylic group O-H and CdO stretches

Glycine and Its Hydrated Complexes

Figure 2. Comparison of (A) experimental mid-IR absorption spectrum of glycine isolated in solid Ar matrix (12K) to predicted IR spectra of stable isomers of glycine displayed in Figure 1: (B) G-1, (C) G-2, and (D) G-3. The bands in the experimental spectrum annotated by open circles are assigned to the most stable isomer G-1, and the weak side bands at 1774.7 cm-1 and 1791.6 cm-1 are assigned to the higherenergy isomers G-2 and G-3. The starred bands are assigned to the bending mode of “free” isolated water monomeric rotomers.

Figure 3. (A-C) Experimental mid-IR absorption spectrum of glycine deposited in solid Ar at 12K with increasing content of water in Ar. (D) Spectrum recorded after matrix annealing at 35 K of (C). Bands annotated by large dots are assigned to the most stable isomer of glycine · H2O complex (G-1-W-1, see Figure 4), and small dots correspond to a second glycine · H2O complex conformer (G-2-W-1, see Figure 4). The bands with open squares denote glycine · (H2O)2 complexes (G-1-2-W-1, see text). The starred bands are due to vibrations in isolated water clusters (D ) dimer, T ) trimer, LP ) larger water cluster).

to 3205 (Figure S9) and 1743.6 cm-1, compared to 3564.5 and at 1780.9 cm-1 for “free” glycine (see Tables 2 and 3). Conversely, for modes located relatively far from the water complexation site (e.g., C-C and C-N stretching), the effect of hydration is much less pronounced (1362.9 vs 1391.7, and 1120.3 vs 1102.6 cm-1). The predominant formation of G-1-W-1 is consistent with the high abundance found for the bare glycine conformer G-1. The lower abundance of G-2 (estimated at 9%) suggests that G-2-water complexes might be formed, albeit with much lower abundance. In the predicted lowest-energy structure, G-2-

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5923 W-1 (Figure 4), the water binds only to the CdO group, since the OH of the carboxylic acid group is already bonded to the amino group. The calculated spectrum exhibits an intense COH bending mode at 1395 cm-1 (352 km/mol), as well as a CdO stretching mode at 1777 cm-1. Inspection of the experimental spectrum reveals two bands (annotated by small black dots in Figure 3) located at 1409.9 and 1762.6 cm-1 that have plausible frequencies and integral intensity ratio compared to the predicted bands. Similar results on solvated tryptophan isomers have been observed by Oomens and co-workers in infrared ion dip experiments,28 where the characteristically intense COH bending mode of the OH-N hydrogen-bonded structure served as a diagnostic band. Note that the interpretation of the high-energy spectrum (Figure S8) is complicated by heavy congestion due to modes associated with water clusters. Thus, mode assignments in this region are at best tentative. In the previous study by Ramaekers et al.26 on glycine · H2O, two OH stretching modes of water were in fact observed (3410 and 3690 cm-1). Nonetheless, a larger total number of bands could be assigned to glycine · H2O in this study. It is interesting to note that all band positions are shifted to higher energies by a few cm-1 compared to Ramaekers et al., and some display significant differences, such as for instance the OH bending mode of the carboxylic acid group (895 vs 930 cm-1). Moreover, the FT-IR bandwidths presented here are considerably narrower than in the Ramaekers et al. study (1-3 cm-1 vs >5 cm-1). We suspect that some of these differences may be due to insufficient isolation of glycine molecules in the Ar matrix, as indicated by glycine cluster formation in their study. 4.3. Mode Anharmonicities. The use of scaling factors is commonly accepted as a method to relate harmonic calculated infrared spectra to experimentally observed ones and thus account for any inaccuracies in the calculations. An alternative approach is to directly compute the anharmonic potential energy surfaces (PES) for each vibration and use these to predict the vibrational frequencies. The reliability of such predicted anharmonic vibrational modes is investigated here for glycine and glycine · H2O. The observed matrix isolation infrared bands are compared to the predicted anharmonic frequencies (at MPW1PW91/6-311++G(d,p) and MP2/aug-cc-pVDZ levels), using an automated second-order vibrational perturbation theory approach32 in the Gaussian 03 software.33 These values are also compared to previous theoretical studies on glycine by Hobza and co-workers,6 and on glycine and glycine · H2O by Chaban and Gerber.19 Whereas Hobza employed a second-order pertubative approach in conjunction with MP2/aug-cc-pVDZ, the latter study made use of a correlation-corrected second-order perturbation theory vibrational self-consistent field (CC-VSCF) approach in combination with a MP2/DZP level of theory. To simplify a comparison over different wavelength regions, the frequency deviations are expressed as a percentage in the differences between anharmonic and experimental mode frequencies, (νanharm - νexpt)/νexpt (based on the data extracted from Tables 2 and 3). Figure 6 displays this comparison to harmonic calculations for glycine G-1, showing only those modes for which an experimental value could be determined. All frequencies are summarized in Table 2, and a positive frequency deviation indicates a blue-shifted position relative to the experimental band. For the higher-frequency O-H, N-H, and C-H stretches, all anharmonic calculations give a reasonable estimate of the experimental frequency, most being within 2% of the observed frequencies. In the mid-IR, the deviation between experiment and theory is greater, particularly for the DFT calculations and the MP2/VDZ predicted frequencies by Chaban and Gerber.

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Espinoza et al.

Figure 4. Geometries of the stable isomers of glycine · H2O complexes optimized at the MPW1PW91/6-311++G(d,p) level. The relative electronic energies (in kJ/mol) corrected for zero point vibrational energies calculated at MPW1PW91/6-311++G(d,p) are in parentheses.

Figure 5. Comparison of (A) experimental synthetic mid-IR absorption spectrum of glycine isolated in solid Ar/H2O matrix at 12 K (bottom) to (B-F) predicted IR spectra (MPW1PW91/6-311++G(d,p)) of stable glycine · H2O complexes, displayed in Figure 4.

The MP2/VDZ calculations consistently overestimate the vibrational frequencies, whereas the DFT calculations show no consistent pattern. Conversely, anharmonic calculations by Hobza and co-workers using a correlation-consistent polarized double-ζ Dunning basis set (MP2/aug-cc-pVDZ) still agree reasonably well over this range. Only one band exhibits a deviation greater than 2%. This suggests that the higher level of theory in Hobza’s calculations is important in predicting the frequencies more accurately. We conducted anharmonic calculations of glycine G-1 at MP2/aug-cc-pVDZ, making use of the second-order perturbation approach described by Barone.32 A comparison of the anharmonic frequencies determined in both studies is summarized in Figure S11. Although not all frequencies are identical, the general agreement between experiment and theory is highly comparable. Recent studies have shown

Figure 6. Comparison of experimental frequencies to anharmonic frequency calculations for glycine G-1 performed at different levels of theory: MPW1PW91/6-311++G(d,p) (this study), MP2/VDZ (Chaban and Gerber)19 and MP2/aug-cc-pVDZ (Hobza and co-workers).6 Frequency deviations are calculated as (νanharm - νexpt)/νexpt and are indicated as a percentage.

that ab initio approaches for anharmonic calculations yield enhanced estimates of the experimental frequencies compared to DFT.42,43 Those conclusions seem to be warranted here as well, provided that the potential energy surface is modeled using a high basis set (e.g., MP2/aug-cc-pVDZ). Figure 7 shows the deviation of the computed anharmonic frequencies to experimental bands for glycine · H2O, based on the G-1-W-1 structure. All frequencies are summarized in Table 3. A smaller number of modes could be experimentally assigned in this case, especially in the high-frequency region. The same trends are confirmed as for bare glycine. MP2/VDZ generally overestimates the frequencies, although no clear trend can be

Glycine and Its Hydrated Complexes

Figure 7. Comparison of experimental frequencies to anharmonic frequency calculations for glycine · H2O G-1-W-1 performed at different levels of theory: MPW1PW91/6-311++G(d,p) (this study), MP2/VDZ (Chaban and Gerber)19 and MP2/aug-cc-pVDZ (this study). Frequency deviations are presented as (νanham - νexpt)/νexpt and are indicated as a percentage.

established for the DFT calculations. Once again, the correlationcorrected polarized Dunning basis set is shown to give predicted frequencies within ∼2% of the experimental values, with the exception of the C-C stretching mode at 1362 cm-1. Note that the very poor agreement between experiment and theory for the H2O OH wag mode at 620.3 cm-1 (see Table 3) leads us to believe that this band may be subject to matrix effects. Vibrations associated with the water molecule in glycine · H2O are much more likely to be affected by the presence of the argon matrix, as opposed to vibrations that are mainly localized on the glycine molecule. In summary, the reliability of anharmonic calculations for predicting vibrational frequencies appears to be improved for ab initio approaches over DFT, provided that a high basis set

J. Phys. Chem. A, Vol. 114, No. 18, 2010 5925 is used. These trends are confirmed both for glycine, as well as for the much floppier glycine · H2O complex. 4.4. Glycine · (H2O)2. For relatively large concentrations of water in the Ar matrix, water forms clusters (see Figure 3 and Figure S8D, Supporting Information) such as dimers (D), trimers (T), and larger polymers (LP). The water association process is enhanced when water becomes more mobile in the matrix due to an increase in diffusion during annealing. Three new bands (marked by squares) located at 1251.9, 1720.2 (Figure 3), and 3137 cm-1 (Figure S8) grow in intensity during annealing. To test the hypothesis that the carrier of these three bands are the glycine · (H2O)2 complex, the structures, electronic energies, and infrared spectra for various conformers were calculated. Six stable conformers for glycine · (H2O)2 are displayed in Figure 8. In the lowest-energy isomer, G-1-2W-1, the extra water molecule binds to the amino N site of the lowest-energy G-1-W-1 glycine · H2O complex (Figure 4). A plethora of other geometries are possible. Only some of these have been considered here. Calculated infrared absorption spectra for these six conformers are compared to the synthetic spectrum in Figure S12 and S13. The observed band positions at 1251.9, 1720.2, and 3137 cm-1 are consistent with the computed spectra for glycine · (H2O)2 complexes. Due to the limited spectral information, it is not possible to exclude the higher-energy conformers, and it is possible that a mixture of structures is in fact formed. However, given the strong presence of G-1W-1 for the singly hydrated glycine complex, it is likely that the majority of glycine · (H2O)2 complexes exhibit the simultaneous hydrogen bonding of one water molecule to two sites of the carboxylic acid group. 5. Conclusions In this work, glycine · water complexes have been generated in a solid Ar matrix (12 K), and their vibrational absorption spectra were recorded. With the help of supporting calculations of equilibrium geometries, energies, and vibrational spectra, the following conclusions can be drawn. (1) For glycine molecules isolated in an Ar matrix, the interconversion of the higher-energy conformer G-3 into the

Figure 8. Geometries of stable isomers of glycine-two-water complexes optimized at the MPW1PW91/6-311++G(d,p) level. The relative electronic energies (in kJ/mol) corrected for zero-point vibrational energies calculated at the same level are in parentheses.

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lowest energy conformer G-1 during matrix annealing (to 35 K) has been confirmed experimentally. The potential energy surface for rotamerization of the COOH group is calculated to be 4.20 kJ/mol (MPW1PW91/6-311++G(d,p)), which is consistent with a matrix annealing temperature of 35 K. (2) The harmonic vibrational absorption spectra calculated at the MPW1PW91/6-311++G(d,p) level for various conformers indicates that in the lowest-energy glycine · H2O complex, the water molecule binds to the carboxylic acid site of glycine by hydrogen bonding to the CdO and OH groups. Most of the bands (11 in all) in the experimental spectrum match the calculated frequencies and intensities of the G-1-W-1 conformer well. A second conformer of glycine · H2O is identified based on two additional bands observed at 1409.9 and 1762.6 cm-1 (COH bending and CdO stretching modes, respectively). In the corresponding equilibrium structure water binds to the CdO group of the glycine G-2 conformer. (3) The increase in band intensities at 1251.9, 1720.2, and 3137 cm-1 during matrix annealing suggests the presence of a glycine · (H2O)2 complex. A number of geometries have been considered theoretically, and the lowest of these involves binding of the second water to the amino nitrogen. Athough this structure is consistent with the observed band positions, other structures cannot be excluded due to the limited spectral information. (4) The vibrational mode anharmonicities for the lowestenergy conformers of glycine and glycine · H2O have been calculated at MPW1PW91/6-311++G(d,p) and MP2/aug-ccpVDZ levels using second-order vibrational perturbation theory. The predicted frequencies have been compared to the experimentally determined matrix isolated band. A comparison is also made to previous anharmonic ab initio calculations by Hobza and co-workers (MP2/aug-cc-pVDZ using a quartic field), as well as Chaban and Gerber (MP2/VDZ in combination with cc-VSCF). Typically, for higher-energy modes, all anharmonic calculations constitute an improvement over using a static scaling factor in harmonic frequency calculations. In the lowerenergy range (500-2000 cm-1), the anharmonic frequencies at the DFT and lower-level ab initio level (MP2/VDZ) display considerable discrepancies with experiment. Conversely, the higher-level ab initio calculations at MP2/aug-cc-pVDZ yield a considerably better match with experiment (i.e., within 2% of experiment). This shows that an accurate prediction of vibrational frequencies for relatively floppy systems is possible, provided that anharmonic calculations make use of high ab initio basis sets that include correlation-corrected polarization and the complete s,p space. Acknowledgment. The UF HPC Center is acknowledged for providing computational resources and support. N.P. thanks the University of Florida for generous start-up funds. Supporting Information Available: Tables S1-S3 give Cartesian coordinates of the structures calculated as different levels of theory, as well as their zero-point corrected electronic energies in Hartrees. Figure S4 shows a comparison of the midIR spectrum of glycine to harmonic frequency calculations of G-1 at MPW1PW91/6-311++G(d,p) and B3LYP/6-311++ G(d,p). Figure S5 shows the potential energy surface for the G3 f G1 isomerization. The experimental FTIR spectra for glycine in the high (Figure S6 + Figure S7) and mid-IR (Figure S8) are shown. The synthetic high-energy spectrum for glycine · H2O is compared to calculated spectra (Figure S9). The experimental FT-IR results for glycine · H2O from a previous study by Ramaekers et al. (ref 26) are contrasted to the results

Espinoza et al. in this study in Figure S10. Anharmonic frequency calculations for G-1, performed at MP2/aug-cc-pVDZ, but using different anharmonic approaches are contrasted in Figure S11. The synthetic spectra for glycine · (H2O)2 are compared to calculated spectra in Figures S12 and S13. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Jorgensen, W. L. Science 2004, 303, 1813–1818. (2) Nanda, V.; Koder, R. L. Nature Chem. 2010, 2, 15–24. (3) Vishveshwara, S.; Pople, J. A. J. Am. Chem. Soc. 1977, 99, 2422. (4) Csaszar, A. G. J. Am. Chem. Soc. 1992, 114, 9568–9575. (5) Nguyen, D. T.; Scheiner, A. C.; Andzelm, J. W.; Sirois, S.; Salahub, D. R.; Hagler, A. T. J. Comput. Chem. 1997, 18, 1609–1631. (6) Bludsky, O.; Chocholousova, J.; Vacek, J.; Huisken, F.; Hobza, P. J. Chem. Phys. 2000, 113, 4629–4635. (7) Suenram, R. D.; Lovas, F. J. J. Am. Chem. Soc. 1980, 102, 7180. (8) Brown, R. D.; Crofts, J. G.; Godfrey, P. D.; McNaughton, D.; Pierlot, A. P. J. Mol. Spectrosc. 1988, 190, 185. (9) Ijima, K.; Tanaka, K.; Onuma, S. J. Mol. Spectrosc. 1991, 246, 257. (10) Reva, I. D.; Plokhotnichenko, A. M.; Stepanian, S. G.; Ivanov, A. Y.; Radchenko, E. D.; Sheina, G. G.; Blagoi, Y. P. Chem. Phys. Lett. 1995, 232, 141–148. (11) Stepanian, S. G.; Reva, I. D.; Radchenko, E. D.; Rosado, M. T. S.; Duarte, M. L. T. S.; Fausto, R.; Adamowicz, L. J. Phys. Chem. A 1998, 102, 1041–1054. (12) Huisken, F.; Werhahn, O.; Ivanov, A. Y.; Krasnokutski, S. A. J. Chem. Phys. 1999, 111, 2978–2984. (13) Suenram, R. D.; Lovas, F. J. J. Mol. Spectrosc. 1978, 72, 372. (14) Brown, R. D.; Godfrey, P. D.; Storey, J. W. V.; Bassez, M.-P. J. Chem. Soc. Chem. Commun. 1978, 547. (15) Jensen, J. H.; Gordon, M. S. J. Am. Chem. Soc. 1995, 117, 8159– 8170. (16) Albrecht, G.; Corey, R. B. J. Am. Chem. Soc. 1939, 61, 1087. (17) Almlof, J.; Kuick, A.; Thomas, J. O. J. Chem. Phys. 1973, 59, 3901. (18) Kassap, E.; Langlet, J.; Evleth, E.; Akacem, Y. J. Mol. Struct. (Theochem) 2000, 531, 267–282. (19) Chaban, G. M.; Gerber, R. B. J. Chem. Phys. 2001, 115, 1340– 1348. (20) Wang, W.; Pu, X.; Zheng, W.; Wong, N.-B.; Tian, A. J. Mol. Struct. (Theochem) 2003, 626, 127–132. (21) Chaudhari, A.; Sahu, P. K.; Lee, S.-L. J. Chem. Phys. 2004, 120, 170–174. (22) Bachrach, S. M. J. Phys. Chem. A 2008, 112, 3722–3730. (23) Xu, S.; Nilles, M.; Bowen, K. H. J. Chem. Phys. 2003, 119, 10696– 10701. (24) Diken, E. G.; Hammer, N. I.; Johnson, M. A. J. Chem. Phys. 2004, 120, 9899–9902. (25) Diken, E. G.; Headrick, J. M.; Johnson, M. A. J. Chem. Phys. 2005, 122, 224317. (26) Ramaekers, R.; Pajak, J.; Lambie, B.; Maes, G. J. Chem. Phys. 2004, 120, 4182. (27) Alonso, J. L.; Cocinero, E.; Lesarri, A.; Sanz, M. E.; Lopez, J. C. Angew. Chem., Int. Ed. Engl. 2006, 45, 3471–3474. (28) Blom, M. N.; Compagnon, I.; Polfer, N. C.; Von Helden, G.; Meijer, G.; Suhai, S.; Paizs, B.; Oomens, J. J. Phys. Chem. A 2007, 111, 7309– 7316. (29) Bowman, J. Acc. Chem. Res. 1986, 19, 202–208. (30) Ratner, M. A.; Gerber, R. B. J. Phys. Chem. 1986, 90. (31) Jung, J.-O.; Gerber, R. B. J. Chem. Phys. 1996, 105, 10332. (32) Barone, V. J. Chem. Phys. 2005, 122, 014108. (33) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.

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J. Phys. Chem. A, Vol. 114, No. 18, 2010 5927 (39) Note that the equilibrium geometry optimization for this isomer at MPW1PW91/6-311++G(d,p) requires using the Opt)Tight keyword, to prevent finishing the calculation with an imaginary frequency. (40) Liu, R.; Zhou, X.; Pulay, P. J. Phys. Chem. 1992, 96, 5748. (41) Szczepanski, J.; Ekern, S.; Vala, M. J. Phys. Chem. 1995, 99, 8002. (42) Chaban, G. M.; Gerber, R. B. Theor. Chem. Acc. 2008, 120, 273– 279. (43) Kabelac, M.; Hobza, P.; Spirko, V. Phys. Chem. Chem. Phys. 2009, 11, 3921–3926.

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