Dynamical Structures of Glycol and Ethanedithiol Examined by

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Dynamical Structures of Glycol and Ethanedithiol Examined by Infrared Spectroscopy, Ab Initio Computation, and Molecular Dynamics Simulations Xiaoyan Ma,† Kaicong Cai,‡ and Jianping Wang* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ABSTRACT: Infrared (IR) experiment, ab initio computations, and molecular dynamics (MD) simulations were used to examine the dynamical structures of ethylene glycol (EG) and 1,2-ethanedithiol (EDT) in carbon tetrachloride and deuterated chloroform. Using the O-H and S-H stretching modes as structural probes, EG and EDT were found to exhibit different conformational preferences, even though they share similar molecular formula. Results suggest that the gauche conformation of EG presents and is stabilized by the intramolecular hydrogen bond (IHB), while both the trans and gauche EDT are possible in the two solvents. Exchangeable IHB donor and acceptor pairs were predicted in the case of EG. Anharmonic vibrational frequencies, anharmonicities, and couplings of the O-H and S-H stretching modes were predicted and found to be structurally dependent. Linear IR and twodimensional IR spectra containing these structural signatures were simulated and discussed. These results demonstrate that a combination of the methods used here is very useful in revealing structural dynamics of small molecules in condensed phases.

1. INTRODUCTION Small molecules in solution phase often have quite flexible structures. Their fast structural dynamics usually take place on the time scale of femtosecond to picosecond. It has been quite a challenge as how to characterize such ultrafast structural dynamics. Nuclear magnetic resonance (NMR) is a successful solution-phase structural method;1 however, it is only sensitive to structural dynamics occurring on the nanosecond to microsecond or slower time scales.2-4 Vibrational spectroscopy,5-7 on the other hand, has been well-known as a powerful structural tool: the frequency and line shape of a given vibrational mode are quite sensitive to the structure and local environment of chemical groups involved in the mode. Very recently, along with the development and application of two-dimensional IR (2D IR) technique,8-11 new vibrational parameters, such as overtone anharmonicity and vibrational coupling, were proposed to be useful probes for molecular structures and dynamics.12 Linear vicinal diol presents a very interesting case of small molecules having floppy structures that can be studied by the vibrational spectroscopy.13-16 The O-H stretching vibrational frequency is known to be very sensitive to molecular structure, and there are two OH groups presented in close proximity in the smallest vicinal diol ethylene glycol (EG). The two OH groups may have different local chemical environments, and even form a weak intramolecular hydrogen bond (IHB) with one another. This would yield two O-H stretching bands in conventional linear IR (1D IR) spectroscopy. Similarly, dithiol is also believed to exhibit structurally r 2011 American Chemical Society

sensitive S-H stretching bands. Examining the vibrational properties of these two types of vibrational modes and how they reflect the structural dynamics forms the main theme of this work. There are extensive studies of the O-H and S-H stretching mode in the literature. The O-H stretching of pinacol having two vicinal OH groups has been studied using femtosecond mid-infrared pump-probe spectroscopy.17 The structures of 1,2-ethanedithiol (EDT) have been studied by vibrational spectroscopy,18 microwave spectroscopy,19 and ab initio calculations.20-24 It was proposed that 10 all-staggered rotameric forms are possible and four of these conformers are predominant.19,24 A single IR peak at 2570 cm-1 for the two S-H stretching vibrations in EDT has been reported.18 In addition, electron diffraction measurements showed the coexistence of anti and gauche conformers around the C-C bond of EDT in the vapor phase with relative abundances of 62 and 38%, respectively,25 with the anti form being about 0.8 kcal/mol more stable than the gauche form.25,26 Further, the S-H stretching modes in hydrogen sulfide (H2S) in rare-gas matrices and other conditions have been reported.27-30 Molecular dynamics (MD) simulation is an effective tool to reveal the structural dynamics of the small molecules in explicit solvent. MD simulations of EG in water31 and in carbon tetrachloride32 as well as in pure liquid33 have been performed Received: August 16, 2010 Revised: December 14, 2010 Published: January 5, 2011 1175

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The Journal of Physical Chemistry B

Figure 1. Low-energy conformations of EG and EDT predicted at the level of B3LYP/6-31þG* and computed transition dipole of the O-H and S-H stretching modes in monohydric ethanol and ethanediol: (A) EG, φ = 60°, ψ = -60°; (B) EDT, φ = 70°, ψ = -70°; (C) EDT, φ = 180°, ψ = -70°; (D) monohydric ethanol; (E) monohydric ethanediol.

under optimized potentials for liquid simulations (OPLS) force field (FF). Three-dimensional dynamical structures and hydrogenbonding (HB) dynamics for EG in solution phase were examined using the MD simulations. There are also MD simulations for poly(ethylene oxide) and its oligomers; the force fields used are based on ab initio electronic structure calculations of model molecules.34-37 Using these FFs, interactions between protein and oligo (EG) self-assembled monolayers have been investigated.38 However, although in the early 1960s a set of force constants of EDT have been determined in the modified Urey-Bradley force field,39 there are still no complete force field parameters available in OPLS or in CHARMM.40 Thus, molecular mechanics force fields for EDT are developed in this work. In this paper, we first examine the 1D IR signatures of nearby O-H or S-H stretching modes including their frequencies and line broadening of EG or EDT in nonpolar solvent CCl4 and in polar solvent CDCl3. Possible dynamical structures are deduced from the 1D IR results. The potential energy surface (PES) scan along backbone dihedral angles φ and ψ (defined in Figure 1) is carried out to search for stable conformations. Intramolecular charge distributions are obtained for several selected conformations. Harmonic and anharmonic frequencies, anharmonicities, and vibrational couplings, for the O-H stretching vibrational modes in EG and the S-H stretching vibrational modes in EDT, are computed to explore their structural sensitivities. 2D IR spectra are simulated for several representative conformations and their structural signatures are discussed. MD simulations in explicit solvent for EG and EDT are carried out to gain detailed insights into their structural dynamics.

2. METHODS 2.1. FTIR Spectra. FTIR spectra of EG and EDT were measured in CCl4 and CDCl3. Briefly, a Nicolet 6700 spectrometer (Thermo Electron, USA) was used. The samples were held between two CaF2 windows by a 950 μm thick Teflon spacer. The FTIR spectra were collected at ambient temperature (23 °C), at 1 cm-1 spectral resolution for 64 scans. EG (purity g95%, Beijing Chemical) and EDT (purity g98%, Fluka Chemical) were used without further purification. The concentration of EG was 1.8 mM to minimize the formation of intermolecular HB species. The concentration of EDT was 12.0 mM. The IR absorption bands of the O-H and S-H stretching modes were all fitted with Lorentzian functions. IR spectrum of

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5.0 mM EDT in CCl4 was found to be almost identical to that at 12.0 mM, suggesting no intermolecular aggregation formed at these concentrations. 2.2. Computations. 2.2.1. Ab Initio Calculations. Geometry optimization, PES scanning, and normal-mode frequency calculation of EG and EDT were carried out at the level of B3LYP/6-31þG*. Tight convergence was forced during geometric optimization. This combination of theory and basis sets is known to be a reasonable choice for predicting gas-phase harmonic vibrational frequencies of midsized biomolecules.41 The PES scan was carried out for a fixed dihedral angle (σ = 180°, Figure 1), with the aim of having a “free” OH or SH group. Angles φ and ψ were scanned from -180° to 180° at 10° intervals, so there are totally 1369 structures in each case. The PES was also carried out at the level of MP2; however, a very similar energy profile was found. To be consistent, only DFT results were given. Anharmonic frequency calculations were performed for representative conformations at the level of B3LYP/6-31+G*. Natural population analysis (NPA) and natural bond orbital (NBO) analysis were carried out at the same level of theory. 2.2.2. Vibrational Couplings. The vibrational coupling constants between the two O-H stretching modes in EG and two S-H stretching modes in EDT were calculated using two methods: the wave function demixing scheme42-44 and transition dipole coupling (TDC) scheme.7,45,46 The former yields the total coupling, including the through-bond and through-space contributions, and the latter yields the through-space electrostatic contribution. In order to compute the coupling constant using TDC model, the transition dipole moment of both the O-H and S-H stretching modes were determined using quantum chemistry computation at the level of B3LYP/6-31þG*. The calculation of ethanol yields the magnitude of local O-H mode in EG as 0.047 D, with 20.1° to the O-H bond direction (Figure 1). Similarly, the transition dipole of local S-H mode in EDT was obtained from ethanethiol, whose magnitude is 0.058 D, tilting 9.2° away from the S-H bond direction. The polarizable continuum (PCM)47 solvation model was utilized to compute the transition dipole moment of the O-H stretching mode in ethanol and the S-H stretching mode in ethanethiol. All quantum chemical computations were carried out using Gaussian 03.48 2.2.3. Anharmonicity and 1D IR, 2D IR Simulations. 1D IR and 2D IR spectra in the O-H and S-H stretching region were simulated for representative structures of EG and EDT. For the 1D IR simulation, ab initio computed anharmonic transition frequencies and intensities were used. The spectra were broadened by experimentally determined full width half-maximum (fwhm) without intensity normalization. The polarized 2D IR spectra were simulated by using the simulation protocol described recently,49,50 which is based on ab initio calculation and third-order nonlinear IR response functions. Anharmonic vibrational transition frequencies, anharmonicities of overtone and combination transitions, and transition dipole moments were utilized in the 2D IR spectral simulations. The anharmonic parameters were obtained by performing ab initio anharmonic frequency computation at the level of B3LYP/6-31þG* for representative structures of EG and EDT. 2.2.4. Molecular Dynamics Simulations. MD simulations were performed for EG and EDT in explicit solvent models of CCl4 and CDCl3. CHARMM all-atom force field40 for EG was used. Parameters of CDCl3 and CCl4 were transferred from OPLS-AA force field51 with proper unit conversions. The parameters of EDT were developed based on CHARMM general force field.52 Force-field development for EDT is described in section 3.6. 1176

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Table 1. Frequency (ν, in cm-1), Full Width at HalfMaximum (ν1/2, cm-1), Frequency Separation (Δν, cm-1), and Ratio of Integrated Peak Area (R = Slow/Shigh) of the Two O-H Absorption Bands of EG and Those of the Two S-H Absorption Bands of EDT Obtained from the FTIR Spectra Shown in Figure 2 O-H or S-H species EG EDT

Figure 2. FTIR spectra of EG in the O-H stretching region and those of EDT in the S-H stretching region. Lorentzian fittings are shown.

To perform the MD simulations, cubic solvent boxes were constructed first. For CDCl3, the side length was set to 28 Å, containing 144 CDCl3 molecules. For CCl4, the side length was set to 34 Å, containing 216 CCl4 molecules. One center-located EG or EDT molecule was solvated in the boxes. The solvent molecules with its heavy atom within 2.4 Å of any heavy atom of the solute were removed. A distance of 12 Å was set for the nonbonded cutoff, and the long-range electrostatic interaction was considered by the particle-mesh Ewald (PME) summation.53 Each MD system was first energy minimized by 10 000 cycles of conjugate gradient minimization followed by a 20 ps heating process to reach 298 K. The system was then equilibrated for 1 ns at a step of 1 fs. The MD production simulations were performed at 298 K for 1 ns with a step of 1 fs. The Langevin piston NoseHoover method,54,55 which is a combination of the Nose-Hoover constant pressure method with piston fluctuation control implemented using Langevin dynamics, was used under the isothermalisobaric ensemble (NPT). Totally, 500 000 snapshots were extracted evenly from the 1 ns MD trajectories for conformational and vibrational frequency analysis. Instantaneous normal-mode (INM) vibrational frequency analysis of the O-H and S-H stretching modes were carried out on the nuclear configurations of the solute molecule extracted from the MD trajectories using VIBRAN.56 The INM method has been used quite extensively, for example, to evaluate vibrational modes of peptides.57,58 The correlations of the frequency fluctuations between the two vibrational chromophores in each case were examined using the following equation: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi D E 2 2 ð1Þ Fij ¼ δνi δνj = δνi δνj Here Fij varies from þ1 (fully correlated), 0 (uncorrelated) to -1 (fully anticorrelated). δvi is the variance of frequency distribution of vi.

3. RESULTS AND DISCUSSION 3.1. FTIR Absorption Spectra. FTIR spectra of EG and EDT in CCl4 and CDCl3 respectively are given in Figure 2. Only the absorption bands in the O-H and S-H stretching region are shown. Peak fitting results are also shown, with the fitting parameters, including frequency (ν), fwhm (ν1/2), frequency separation (Δν), and ratio of integrated peak area (R, = Slow/Shigh) of the lowfrequency and high-frequency components, listed in Table 1. As can be seen, there are two main peaks in each case, having different peak

O-H or S-H

νhigh

ν1/2

νlow

ν1/2

Δν

R

CCl4

3642.8

24.4

3607.2

34.1

35.6

1.18

CDCl3

3629.0

29.0

3602.0

35.0

27.0

1.14

CCl4

2577.9

22.0

2566.6

19.2

11.3

1.00

CDCl3

2580.0

24.1

2565.2

33.0

14.8

1.26

solvent

widths and peak intensities. In CCl4, the two Lorentzian components are located at 3642.8 and 3607.2 cm-1 for EG, and at 2577.9 and 2566.6 cm-1 for EDT. In CDCl3, the two peaks are located at 3629.0 and 3602.0 cm-1 for EG and 2580.0 and 2565.2 cm-1 for EDT. The component near 2630 cm-1 in Figure 2B,D could be due to the overtones of CH2 wagging as discussed previously.59 Such a high-frequency weak band was also observed and unassigned in a Raman study.60 For the O-H stretching in EG, the high-frequency band arises from the free O-H stretching motion and the lowfrequency band from the H-bonded O-H stretching mode. For EG in CCl4, the value of ν1/2 of the low-frequency mode is larger than that of the high-frequency mode, indicating a larger structural fluctuation of the low-frequency O-H mode. The frequency separation between the two S-H stretching modes of EDT in CCl4 is about 11.3 cm-1, which is smaller than that of EG (35.6 cm-1). The two S-H stretching components of EDT in CCl4 are nearly degenerate, suggesting an insignificant H-bonding between the two S-H groups. Spectrum of 5.0 mM for EDT in CCl4 (data not given) shows a similar absorption band signature with proportionally lower peak heights, indicating that there is no intermolecular HB formed at these concentrations. In CDCl3, EDT has a broader ν1/2 for the low-frequency component (Table 1), suggesting the presence of a weak IHB that may cause an inhomogeneous structural distribution. This is clearly different from the case of CCl4. Further, it can also be seen that Δν for EG in CCl4 is larger than that in CDCl3 by 8.6 cm-1. The value of R is larger for EG in CCl4 than in CDCl3, suggesting somewhat stronger IHB in the former. As for EDT, on the contrary, the Δν is slightly larger in CDCl3, accompanied by a slightly larger R, suggesting a slightly stronger IHB in CDCl3. This shows the effect of solution polarity on the structural preferences of EG and EDT. A slight change of solvent polarity may alter the structural preference. Solvent-induced structures are examined by the MD simulations in section 3.7. Further, the IR results suggest that the S-H stretching mode has smaller IR absorption cross sections than the O-H mode; this is supported by the transition dipole moment computations using the PCM implicit solvent model as discussed in section 3.4. 3.2. PES, NPA, and NBO Analyses. Contour plots of the PES scans along dihedral angles φ and ψ for EG and EDT are given in Figure 3. Several structures corresponding to stationary points on the PES of EG and EDT are located and marked. Overall, the topologies of the two PES’s are somewhat similar; however, locations of local minima are different. In Figure 3A, there are two lower-energy points representing the gauche conformations; one is located at (φ = 60°, ψ = -60°), while the other is symmetrically located at (-60°, 60°). Four conformations at (0°, 0°), (60°, -60°), (180°, -80°), and (180°, 180°), denoted 1177



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Figure 3. Potential energy surface in the φ-ψ space at σ = 180°: (A) EG and (B) EDT. Symbol X denotes lower energy conformations with labelings. Relative energy (color bar, in kcal/mol) is shown, subtracted by 144 400 kcal/mol.

Table 2. NPA Charge Analysis for Selected Conformations of EG and EDTa charge on the σ side species EG1 EG2

a

charge on the ψ side

(φ, ψ)

qΟ or qS

qH

qΟ or qS

qH

ΔqΟ or ΔqS

energy diffb

(0, 0) (60, -60)

-0.79723 -0.79635

0.50959 0.50120

-0.78443 -0.77880

0.51574 0.50381

0.01280 0.01755

6.28 0.00 3.14

EG3

(180, -80)

-0.78132

0.49695

-0.77414

0.49165

0.00718

EG4

(180, 180)

-0.78156

0.49698

-0.78156

0.49698

0.00000

2.51

EDT1

(0, 0)

-0.07238

0.15396

-0.05641

0.14536

0.01597

10.04

EDT2

(70, -70)

-0.07652

0.15007

-0.05385

0.14963

0.02267

0.63

EDT3

(180, -70)

-0.07357

0.14764

-0.04361

0.13935

0.02996

0.00

EDT4

(180, 180)

-0.06275

0.14771

-0.06275

0.14771

0.00000

1.26

Charge in e, dihedral angle in degrees, energy in hartrees. b Energy difference is calculated with respect to EG2 and EDT3, respectively.

Table 3. NBO Analysis for the Selected Conformations of EG and EDT (φ and ψ, deg) species

a

(φ, ψ)

entity

energy of bonds and lone pairsa

important admixtures: entity and stabilizationb

EG1

(0, 0)

O1 n lone pair

-0.32863

C1C2* (1.55), C1H4* (6.57), C1H3* (6.53), O9H10* (6.55)

EG2

(60, -60)

O1 n lone pair

-0.31769

C1C2* (1.26), C1H4* (6.57), C1H3* (7.14), O9H10* (0.63)

EG3

(180, -80)

O1 n lone pair

-0.30956

C1C2* (1.50), C1H4* (7.14), C1H3* (7.32), O9H10* (0.00)

EG4

(180, 180)

O1 n lone pair

-0.30660

C1C2* (1.48), C1H4* (7.05), C1H3* (7.06), O9H10* (0.00)

EDT1

(0, 0)

S1 n lone pair

-0.26116

C1H4* (3.53), C1H3* (3.53), S9H10* (3.03)

EDT2

(70, -70)

S1 n lone pair

-0.25669

C1H4* (3.94), C1H3* (3.87), S9H10* (0.00)

EDT3

(180, -70)

S1 n lone pair

-0.25619

C1H4* (4.02), C1H3* (3.90), S9H10* (0.00)

EDT4

(180, 180)

S1 n lone pair

-0.26116

C1H4* (3.89), C1H3* (3.89), S9H10* (0.00)

The energies of NBO entities are reported in hartrees. b The atoms are listed in Figure 1. The stabilization energies (in parentheses) are in kcal/mol. Interactions of “germinal” entities (sharing a common atom) and those smaller than 0.5 kcal/mol are not reported.

as EG1, EG2, EG3, and EG4, respectively, are selected for the vibrational analysis in the following context. For EDT, there are six lower-energy points, including both gauche and trans structures. Taking into account the symmetry, the lower-energy structures are (180°, (70°), (70°, -70°), and (-70°, 70°). Similarly, conformations at (0°, 0°), (70°, -70°), (180°, -70°), and (180°, 180°) are taken and denoted as EDT1, EDT2, EDT3, and EDT4, respectively. One can see that EDT1 is one of the most unstable conformations. However, this is not the case for EG1. These results clearly demonstrate that EG and EDT have different conformational preferences in the gas phase: EG is mainly in gauche form (EG2), while EDT is either in gauche (EDT2) or in trans (EDT3) form. The NPA analysis was performed for the selected conformations of EG and EDT to examine their intramolecular charge distribution. The obtained NPA charges for O or S atoms on the σ side and on the ψ side are given in Table 2. The charge

distributions show that the O-H bond is generally more polarized than the S-H bond. Based on the electrostatic considerations, EG is more easily to form IHB than EDT. The formation of IHB results in a more negatively charged O (or S) atom on the σ side (HB acceptor), and a more positively charged H atom on the ψ side (HB donor). The NBO computations were carried out and the results are given in Table 3. Significant admixtures are listed with their entities and stabilization energies for comparison. The atomic labeling is shown in Figure 1. The hyperconjugative interaction61 between the HB acceptor oxygen lone-pair orbital n(O) and the HB donor O-H σ antibond orbital σ* (O-H) was examined. The results show that EG1 and EG2 have the most significant hyperconjugative interactions, suggesting the presence of IHB. However, EG1 is not a stable conformation because of its repulsion of hydrogen atoms. Similar analysis shows that only EDT1 exhibit weak hyperconjugative interaction between sulfur lone-pair orbital n(S) 1178



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OH Figure 4. Harmonic frequencies in the φ-ψ space at σ = 180°: (A) on the σ side O-H mode (ωOH σ ) of EG; (B) on the ψ side O-H mode (ωψ ) of EG; OH OH SH SH SH (C) ΔωOH (= ωσ - ωψ ); (D) on the σ side S-H mode (ωσ ) of EDT; (E) on the ψ side S-H mode (ωψ ) of EDT; (F) ΔωSH (= ωσ - ωSH ψ ).

and the S-H σ antibond orbital σ* (S-H), suggesting a very weak IHB. However, EDT1 is not a stable conformation based on the PES result. In particular, even though EDT2 was a stable structure as shown by PES analysis, the NBO data suggest no IHB formation in this structure. 3.3. Dihedral Angle Dependence of the O-H and S-H Vibrational Frequencies. Conformational sensitivities of the harmonic O-H and S-H normal-mode frequencies were examined for EG and EDT. The φ-ψ dependence of the frequencies of EG and EDT are shown in Figure 4. For EG, we monitor the frequencies of the two O-H stretching modes as a function of the dihedral angles OH φ and ψ, and assign the two modes to ωOH σ and ωψ . It is possible to do so since we find that the O-H stretching mode is highly localized. Similar results are obtained for the S-H stretching vibrational SH frequencies of EDT (ωSH σ and ωψ ). Their frequency separations, OH SH SH ω ) and Δω ΔωOH (= ωOH σ ψ SH (= ωσ - ωψ ), are also displayed in Figure 4. As seen from the results, the calculated ΔωOH ranges from -55 to 80 cm-1 and the ΔωSH from -42 to 13 cm-1, suggesting a structural sensitive frequency separation in both. In -1 Figure 4 for EG, ωOH σ (0°,0°) has a high frequency (3785 cm ) and -1 OH ωψ (0°,0°) has a low value (3710 cm ), so ΔωOH is quite large (75 cm-1) as shown in Figure 4C. However, in Figure 4F for EDT, ΔωSH(0°,0°) is neither the largest nor the smallest. In Figure 4B, there is a highest frequency point along with ψ = 120° or ψ = -120° when φ dihedral changes from -180° to 180°, and the largest frequency separation value (-50 cm-1) in Figure 4C appears in the same region. A similar situation occurs on the EDT ωSH ψ and ΔωSH in Figure 4, E and F. The similarity descends gradually when φ and ψ change from -120° to 120°, indicating the different structural sensitivities of the O-H and S-H frequencies. Furthermore, for the EG2 conformation the frequency separation between two O-H modes is 34.3 cm-1. For EDT3, the frequency separation between two S-H modes is 8.0 cm-1. These values are on the same order as the FTIR observations (Table 1). 3.4. Vibrational Couplings. The vibrational coupling between the two O-H stretching modes and that between two S-H modes at various conformations have been examined. The total coupling calculated using the wave function demixing method and the through-space electrostatic coupling using the TDC model are

Figure 5. Conformation-dependent vibrational coupling (in cm-1) between the O-H stretching modes in EG and between the S-H SH OH SH stretching modes in EDT: (A) βOH total, (B) βtotal, (C) βTDC, and (D) βTDC. See text for details.

shown in Figure 5. Each panel shows roughly a C2 symmetry with respect to the center of the φ-ψ map (φ = 0°, ψ = 0°). It is found that the total coupling varies from -1.0 to þ1.5 cm-1 for EG and from -5.0 to 0.0 cm-1 for EDT. The mean value of the couplings are -0.04 and -0.34 cm-1, respectively, for EG and EDT, suggesting a generally weak coupling in these two cases. However, relatively strong positive coupling mostly occurs in the neighborhood of EG1 conformation whereas relatively strong negative coupling occurs in the neighborhood of EDT1. The TDC model calculation results for EG and EDT were obtained using transition dipole magnitude and orientation of 0.047 D and 20.1° (to the O-H bond direction) for the O-H mode, and those of 0.058 D and 9.2° (to the H-S bond direction) for the S-H mode. The profiles of the coupling showed in Figure 5C,D are somewhat similar. The TDC value ranges from -0.2 to þ0.5 cm-1 for EG, and from -0.2 to þ0.4 cm-1 for EDT. 1179



Figure 6. Simulated 1D IR spectra for the selected conformations of EG and EDT.

The S-H vibrational couplings from the two methods have some similar φ-ψ dependences, suggesting the through-space interaction plays a dominant role. This could be due to the fact that the S-H bond length is relatively long (1.31 Å), so the interchromophore distance is large. While in the case of EG, the φ-ψ dependence of the coupling shown in Figure 5C is significantly different from that shown in Figure 5A, suggesting the coupling is dependent not only on the through-space but also on the through-bond interaction. This could be explained by the fact that the O-H bond length (0.96 Å) is much shorter than that of the S-H bond. The results show that TDC model is insufficient to describe coupling between adjacent O-H or S-H species. This is because TDC is known to break down at small distance.62,63 Further, it is known that solvent influences transition dipole of a vibrational mode. For example, the PCM implicit solvent model predicts that in CCl4 the dipole parameters become 0.080 D and 16.9° for the O-H mode and 0.020 D and 7.1° for the S-H mode. The variation of dipole from the gas phase to solution phase would not significantly change the TDC picture shown in Figure 5, C and D; however, the predicted small dipole of the S-H mode is in agreement with the IR results shown in Figure 2: the IR intensity of EDT is predicted to be ∼1/16th of that of EG at similar concentration in CCl4. 3.5. 1D IR and 2D IR Spectral Simulation for Stable Conformations. The simulated 1D IR spectra of EG2, EG3, EG4 and EDT2, EDT3, and EDT4 conformations are shown in Figure 6. The simulated 1D IR spectrum of EG2 shows two separated components and the high-frequency component has larger peak height, resembling the profile of the IR experiment (Figure 2). This suggests that in solution the most probable structure of EG is probably EG2-like. As for the EDT, the simulated spectrum of EDT3 has the larger frequency separation between the two components, whereas those of EDT2 and EDT4 have a much smaller frequency separation. The former is similar to the observed IR spectrum of EDT in CDCl3, while the latter is similar to the IR experimental result in CCl4. The simulated 2D IR spectra of EG2, EG4, EDT2, and EDT4 conformations are shown in Figure 7, in which structuredependent 2D IR features are shown. The two pairs of the diagonal peaks in the 2D IR spectrum of EG2 are clearly separated due to large diagonal anharmonicity. In the case of EG4, only one apparent pair of diagonal peaks is shown, suggesting a degenerated case: two O-H stretching frequencies are very close (as seen in Table 4). Both EG4 and EDT4 have a large off-diagonal anharmonicity Δij, which might serve as a probe for the specific conformations. In Figure 7D, EDT4 shows a rectangular-shaped diagonal peak due to a small diagonal anharmonicity. As shown above, anharmonic vibrational parameters such as the anharmonicity play a key role in determining the overall shape of a 2D IR spectrum. The computed parameters for the

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O-H and S-H stretching modes in the selected EG and EDT conformations are listed in Table 4. The diagonal anharmonicity for the gauche EG is estimated to be 170 cm-1 and about 110 cm-1 for both the gauche and trans EDT. It is interesting to note that for the trans conformation, the diagonal anharmonicity of EG4 and EDT4 decreases and the off-diagonal anharmonicity increases comparing to other conformations. The resultant 2D IR spectra might be useful for distinguishing different structures. 3.6. Force Field Parameters for EDT. The molecular mechanics (MM) force field parameters of EDT were partially taken from ethanethiol, including bond stretching, angle bending, torsional interaction, and van der Waals interaction. Two missing parameters are the torsions for φ and ψ (or σ). Here ψ and σ share the same dihedral parameters considering the molecular symmetry. The torsional parameter has the following form: X K χ 1 þ cos nχ - χ0 ð2Þ Vdihedral ¼ dihedral

In this equation, Kχ is force constant, n is multiplicity, χ0 is phase. The parametrization process includes first a scan of the potential energy at the B3LYP/6-31G* level, with the dihedral angle varying from 0° to 360° at the interval of 10°. Energy calculation at the MP2/6-31G* level was carried out for the 37 optimized structures. The MP2 method was used since it is known to include electron correlation and give excellent energy prediction of molecular systems. The MM energy calculation was performed for the optimized conformers in the TINKER module.64 The force constant, multiplicity and phase were adjusted to fit the MM results to the quantum mechanic (QM) energy profiles. The results are given in Figure 8, where three energy barriers as a function of φ and ψ (or σ) are shown. The FF parameters obtained for each dihedral are listed in Table 5, and the atomic charges used for the two molecules and two solvents are listed in Table 6. 3.7. Conformational Dynamics Revealed by MD. The conformational dynamics of EG and EDT in CCl4 and CDCl3 were examined by using the MD simulations. The distributions of the backbone dihedral angle φ in two different solvents obtained by using 500 000 MD structures are shown in Figure 9. The results suggest that in a nonpolar solvent such as CCl4, the backbone dihedral φ of EG is peaked at about -75° (Figure 9A), presenting a gauche conformation that facilitates the formation of IHB. The preferred angle is only different from the PES results by ca. 15° (Figure 3). A previous work using united atom approximation for CCl4 solvent model yielded very similar gauche conformation preferences in terms of the (σ, φ, ψ) angles.65 Two values (-75° and 75°) are found for φ of EG in CDCl3 (Figure 9C), differing from that in CCl4. For EDT in both CCl4 (Figure 9B) and CDCl3 (Figure 9D), it is seen that φ populates at -70° as well; however, there is also a significant population of φ at 180°, suggesting the coexistence of the trans and gauche conformers, and more trans conformation of EDT exists in CCl4 than in CDCl3. The conformational preferences of EG and EDT in the two solvents are in agreement with those inferred by the FTIR results. The distributions of dihedral angle ψ (or σ) of EG and EDT in both CCl4 and CDCl3 were also examined and the results are shown in Figure 10. Clearly, in all cases, the angle is mainly peaked at ca. 45° and -60° (EG) or ca. (70° (EDT), and has some population at (180°, even though the relative population differs from case to case. In EG, the former facilitates the formation of IHB, since the H atom of one OH group points to the O atom of the other OH group. This roughly corresponds to one of the energy minima in the PES (φ = 60°, ψ = -60°, Figure 3A). For EDT, only the gauche conformation 1180



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Figure 7. Simulated 2D IR spectra for the selected conformations of EG and EDT.

Table 4. Calculated Anharmonic IR Frequency (νi, cm-1) and Intensity (Ii, km/mol), Diagonal Anharmonicity (Δi, cm-1), and Mixed Mode off-Diagonal Anharmonicity (Δij, cm-1) of the Two O-H Modes of EG and Two S-H Modes of EDT with Selected Conformations (φ and ψ, deg), and Dipole Moment in Each Case species

(φ, ψ)

νi/Ii

νj/Ij

Δi

Δj

Δij

D 3.7460

EG2

(60, -60)

3552.9/35.6

3596.4/36.9

170.9

170.1

-0.10

EG3

(180, -80)

3574.2/20.9

3586.3/28.4

173.9

172.6

0.15

EG4

(180, 180)

3599.9/62.1

3600.6/0.0

84.8

84.5

169.06

EDT2

(70, -70)

2536.8/6.6

2524.5/14.7

113.1

111.7

0.48

2.1932

(70, 70)

2541.3/16.3

2648.0/22.5

110.9

106.8

0.39

2.1932

(180, -70) (180, 180)

2560.8/17.6 2526.7/35.1

2533.9/15.9 2526.7/0.0

107.1 57.9

114.3 57.9

0.01 115.73

1.4828 0

EDT3 EDT4

2.3568 0

Table 5. Dihedral Angle Parameters for EDT for the CHARMM Force Field Developed in This Work φ

ψ and σ

Figure 8. Dihedral angle energy scan on (A) φ and (B) ψ or σ in EDT. Quantum mechanical and molecular mechanical methods were used. See text for details.

with ψ = ( 70° may form IHB. The results show that the most probable conformation is quite different for EG and EDT in the two solvents; however, generally speaking the gauche conformation and IHB are preferred for EG in both solvents, while for EDT the population of the trans conformation is more significant in CCl4, and that of the gauche is more significant in CDCl3.

Kχ (kcal/mol)

n

χ0 (deg)

0.070

1

0.0

0.130

2

0.0

1.070 0.010

3 1

0.0 0.0

0.570

2

0.0

0.360

3

0.0

Further, to evaluate the spatial relationship between two IR probes, the joint distributions of the dihedral angle for the four systems were examined. The joint distributions φ and ψ angles are shown in Figure 11, and those of the σ-ψ angles are shown in Figure 12. Taking these two figures together, the most probable populations of the structures can be obtained. Take EG in CCl4 as one example: the (φ, ψ) pair is found to mostly populate at (-75°, 45°), indicating a double gauche conformation, whereas the (σ, ψ) pair mostly populates at (-60°, 45°). This leads to a single dominant peak in the σ-φ-ψ space (-60°, -75°, 45°). Similarly, for EG in CDCl3, one of the dominate populations is found at (-60°, 60°, 60°); whereas for EDT, a typical trans conformation is located at (45°, 180°, -70°) in CCl4, and a typical gauche conformation is located at (-60°, -60°, 60°) in CDCl3. This indicates that the solvent strongly influences the conformational preferences of the two IR chromophores in EG and EDT. 1181



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Figure 9. Distribution of the backbone dihedral φ of EG and EDT in two different solvents.

Figure 10. Distribution of the backbone dihedral ψ of EG and EDT in two different solvents.

Table 6. Charges of EG, EDT, CDCl3, and CCl4

the frequency distributions of the two O-H stretching modes of EG in CCl4 and CDCl3 are quite similar. The distribution mostly populates at (νlow = 3682 cm-1, and νhigh = 3686 cm-1) in both CCl4 and CDCl3, with subtle difference in their νlow values. In panels B and D, the distribution of the two S-H stretching frequencies is found to populate mostly at (νlow = 2576 cm-1, νhigh = 2576 cm-1), with a small component at slightly higher νhigh value in each case. It is seen that the population of the one at high-frequency side increases slightly in CDCl3 in comparison with that in CCl4. The projections of the joint distributions onto the νlow and νhigh axes of Figure 14 would yield static frequency distributions of the low- and high-frequency components respectively (data not shown). Basically the projected distributions have the same maxima for the νlow and νhigh components shown in Figure 14. It is noted that the separations of the maxima of the νlow and νhigh components are smaller than those found from fitting the experimental IR results in both cases of EG and EDT. However, larger separation is seen in EG than in EDT, which is in general agreement with the IR results (Figure 2). It is known that the profiles of these static frequency distributions have no straightforward correspondence with the IR absorption peaks, as also pointed by one of the referees. Frequency time-correlation function approach can be utilized to further predict the absorption profile66 but not the central frequency of each component. To predict the absorption profiles of the two O-H (or two S-H) stretching modes simultaneously requires a reasonably constructed instantaneous Hamiltonian, which is beyond the scope of the current work. The frequency correlation coefficient in each case is examined. Analysis shows that even though the joint frequency distribution appears to be overall negatively correlated in each case, the majority of the distribution (dashed rectangle in Figure 14) shows a slightly positive correlation. The correlation coefficient (Fij) obtained using eq 1 and relative population of the joint distribution in each case, are listed in Table 7. As seen in the table, the total frequency correlation coefficient is determined by a small portion of the joint distribution. For example, for EG in CCl4, 96.0% of the joint distribution (within the dashed rectangle) shows Fij = 0.082, while 4.0% of the joint distribution (outside the dashed rectangle) shows Fij = -0.630, and this leads to a total Fij = -0.332. These results suggest that the majority of the O-H or S-H stretching frequencies were uncorrelated, which is also generally consistent with a weak or non-H-bonding picture.

EG

atom

charge (e)

H(CH2) O

0.0900 -0.6500

C

H(OH) EDT

C H(CH2) S

0.0500

0.4200 -0.1100 0.0900 -0.2300

H(SH)

0.1600

CCl4

C

0.2480

CDCl3

Cl C

-0.0620 0.5609

Cl

-0.1686

H

-0.0551

Finally, the joint distributions of two pairs of H-bond donoracceptor distances in EG or in EDT (rH 3 3 3 O or rH 3 3 3 S) are examined using 500 000 instantaneous structures (Figure 13). For EG in both CCl4 and CDCl3, the two plots (panels A and C) are quite similar. The joint distribution of the two rH...O in each case populates mostly at (rH2O1 = 2.6 Å, rH1O2 = 3.9 Å) and (rH2O1 = 3.9 Å, rH1O2 = 2.6 Å). This means when one rH 3 3 3 O is short, the other is long, suggesting that the two OH groups tend to form mainly one HB at a time, with exchangeable H-bond donor and acceptor pairs. Double H-bonded species with two short rH...O only takes a negligible population. Quite in contrast, for EDT in both CCl4 and CDCl3, the joint distribution of the two rH...S populates mostly at the top right corner of both panels B and D (rH...S = ∼4.6 Å for both), a region that is unfavorable for the IHB formation. Population in other regions is mostly insignificant, except the case of EDT in CDCl3 where some population appears in the region of rS1H2 = 3.0 Å and rS2H1 = 3.0-4.5 Å, as well as in that of rS2H1 = 3.0 Å and rS1H2 = 3.0 - 4.5 Å. This indicates some chance of forming IHB in the EDT/CDCl3 system. This corresponds to the gauche conformation. 3.8. Instantaneous Vibrational Frequencies. We examine the INM frequencies of the two O-H stretching modes in EG and the two S-H stretching modes in EDT in the two solvents, respectively. The joint distribution of two INM frequencies in each case is shown in Figure 14. In panels A and C, it is seen that

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Figure 11. Joint distribution of φ and ψ angles in EG (A, C) and those in EDT (B, D).

Figure 12. Joint distribution of σ and ψ dihedral angles in EG (A, C) and those in EDT (B, D).

3.9. Instantaneous Vibrational Frequencies from SoluteSolvent Clusters. To further examine the vibrational properties

of the O-H or S-H stretching modes in the two solvents, representative MD structures containing solute and solvent

clusters were extracted. Initially, solvent molecules with their C atom within 5.0 Å from any of the solute atoms are selected. Five samples were chosen from the highly populated MD conformations shown in Figures 11 and 12. Only the three dihedral angles 1183



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Figure 13. Joint distribution of two possible H-bonding distances in EG (A, C) and that in EDT (B, D).

Figure 14. Joint frequency distribution of the two O-H stretching frequencies in EG and that of the two S-H stretching frequencies in EDT.

(σ, φ, ψ) were kept unchanged during partial geometry optimization at the level of B3LYP/6-31þG*, followed by frequency computations. The structures of the selected clusters are shown in Figure 15, and the computed vibrational properties are given in Table 8. Figure 15 shows the first solvation layer in each case;

specific solvent-solute interaction can be seen. For example, two Cl-H distance in EG/CCl4 are 2.91 and 2.92 Å, suggesting weak H-bond interactions. In the CDCl3 system, even closer D-O distances can be seen (ca. 2.08 Å), indicating D-bond interactions. In the EDT system, relatively longer solvent-solute 1184



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Table 7. Correlation Coefficients of the Distributions of the O-H Stretching Frequencies in EG and Those of the S-H Stretching Frequencies in EDT EG Fij a

total

majorb a

CCl4 -0.332 0.082 (96.0%)

EDT CDCl3

-0.369 0.057 (95.8%)

CCl4 -0.430 0.149 (95.7%)

Table 8. Calculated IR Harmonic Frequency (νi, cm-1), Intensity (Ii km/mol), Frequency Separation (Δν, cm-1), and Ratio of Integrated Peak Area (R = Slow/Shigh) of the Two O-H Modes for Relative Stable EG and Two S-H Modes for EDT Conformations from MD Calculation Results (σ, φ, and ψ deg) species

CDCl3

EG

-0.484 0.127 (93.5%)

EDT

minorc -0.630 (4.0%) -0.619 (4.2%) -0.514 (4.3%) -0.480 (6.5%)

Total: all the joint distributions shown in Figure 13. b Major: joint distributions in dashed rectangles. c Minor: joint distributions outside the dashed rectangle.

Figure 15. Optimized solute-solvent clusters of EG and EDT extracted from the MD simulations in two different solvents. These structures were chosen for frequency and transition dipole computations for the O-H and S-H stretching modes. The distance between each solvent C atom and the nearest solute atom is shown. Dihedral angle in degrees and distance in angstroms.

distances are predicted, as shown in Figure 15, panels C-E. From Table 8, it is seen that the computed frequencies are relatively lower than those of gas phase shown in Figure 4 (panels A-D), indicating the influence of solvent clusters. For EG, the computed frequency separation (Δν) in the two solvents are comparable to experimental results shown in Table 1. However, the computed intensity ratios are somewhat different from experimental results, particularly for the EG/CDCl3 system. For EDT, the computed Δν given in first and third rows of Table 8 are also comparable to experimental results shown in Table 1. However, the trans conformation seems to have a much smaller Δν, which is not consistent with experimental result. This suggests that the trans conformation is unlikely a dominant component, which is not consistent with MD results shown in Figure 9. In comparison to the predictions of the frequency separations from the INM results shown in Figure 14, which were

solvent

(σ, φ, ψ)

νi/Ii

νj/Ij

Δν

R

CCl4 CDCl3 CCl4 CCl4 CDCl3

(-63, -68, 47) (-59, 68, 68) (-62, -61, 67) (50, 176, -65) (58, -62, -59)

3714.0/84.2 3714.1/154.1 2663.0/14.3 2677.0/11.7 2667.8/1.5

3736.4/54.2 3738.7/31.6 2678.5/2.3 2678.4/4.2 2677.2/0.9

22.4 24.6 15.5 1.4 9.4

1.55 4.88 6.22 2.79 1.67

discussed in the previous section, one notices that the DFT results are more reasonable, even though the latter do not completely reproduce the experimental IR results. However, the intensity ratios of EG/CDCl3 and EDT/CCl4 systems are found to be quite different from the experimental values. Since the transition intensities are closely related to the transition dipoles, the transition dipole magnitudes of the O-H and S-H stretching modes in the solute-solvent clusters were also obtained. The transition dipole (in debyes) for mode i can be easily evaluated using a simple formula:67 |Δμi| = (0.3989 Ii/vi)1/2. It is found that |Δμi| of the two O-H stretching modes are enhanced in both the EG/CCl4 and EG/CDCl3 systems (in the range of 0.058-0.129 D). The decrease of the |Δμi| for the two S-H stretching modes is also predicted, in the range between 0.012 and 0.045 D for the EDT/CCl4 and EDT/CDCl3 systems. Solvent influences on |Δμi| is clearly demonstrated in these solute-solvent cluster calculations, and the trend of the |Δμi|change from gas phase to solution phase is generally in agreement with the PCM results discussed earlier. These results suggest that predictions based on MD simulations and the DFT computations of the solute-solvent clusters do not completely reproduce the experimental IR results. This clearly indicates the limitations of the two methods. The INM frequencies, obtained from naked instantaneous solute structures, even though containing solvent influences,57 are in the classic limit and do not yield the O-H and S-H stretching transition frequencies of the diols and dithiols satisfactorily. On the other hand, since the solvent-solute interactions, being largely electrostatic in most cases, are important influential factors determining the transition frequencies, DFT computations of small cluster size do not yield reasonable results either. Further, the relative orientations of the two O-H groups or the two S-H groups in the selected solvent clusters may not be cases in the bulk solutions, perhaps due to the specific solvent-solute interactions. Larger change in the predicted intensity ratio of the two S-H stretching modes in the case of first EDT/CCl4 cluster showing in Table 7, for example, usually suggests a stronger intermode coupling. This is also the reason that DFT computations fails to predict the relative transition dipole strength. Nevertheless, these results have shed some lights on the dynamical structures.

4. CONCLUSION In this work, two small linear vicinal diol and dithiol molecules, namely ethylene glycol and 1,2-ethanedithiol, were chosen as model systems. Using the O-H or S-H stretching vibration as a structural probe, linear IR measurement in combination with quantum chemical computations and MD simulations was used to investigate their ultrafast structural dynamics in the solution phase. Our results show that EG and EDT have different 1185


The Journal of Physical Chemistry B structural preferences in CCl4 and CDCl3. Different methods used in this work yield a generally consistent picture. Small molecules in nonpolar solvent at low concentration are believed to have structures similar to those in the gas phase. Under such circumstances, ab initio computation is useful in elucidating their lower-energy structures. This is clearly demonstrated in this study. Potential energy surface scan reveals several low-energy conformations. Among them, gauche EG and trans and gauche EDT are believed to be the preferred conformations in nonpolar solvent CCl4. Such preferred conformations are predicted by MD simulations. The frequency separations of the IR probes obtained from ab initio computations for these structures are in general agreement with the FTIR experimental observations. Further, a slight change of solvent polarity from nonpolar (CCl4, with a dielectric constant ε = 2.2) to polar (CDCl3, ε = 4.8) affects the structural preferences of the diol and dithiol. Polar solvent molecules interact with solute molecules more intensely than the nonpolar solvent molecules do, which may disrupt the stabilized structures in the latter. This is also clearly shown in their observed linear IR signatures particularly for EG. Different dihedral angle preferences for EG and EDT are predicted in CCl4 and in CDCl3 by the MD simulations. Dihedral angle joint distributions show that CDCl3 tends to reduce the occurrence probability of the trans EDT structure. In this case, the new force field parameters for EDT based on CHARMM general force field seem to yield a reasonable picture. Furthermore, instantaneous MD structural analysis shows that EG is easier to form IHB in both polar and nonpolar solvent, while generally speaking the IHB interaction between S-H groups of EDT in both solvents is unlikely to occur, even though it appears that there is some chance of forming the IHB in CDCl3 based on the H-bonding distance distributions. In addition, FTIR spectra show that the frequency separation of the O-H stretching modes for EG is generally larger than that of the S-H stretching modes for EDT, which is also correctly predicted by the MD simulations, as well as by the DFT computations on the selected solvent-solute clusters. The results suggest that neighboring OH or SH groups have very sensitive IR signatures of the dynamical molecular structure. They can be used as IR chromophores and can serve as local structural probes. The O-H and S-H stretching modes have structure-dependent overtone diagonal anharmonicities, as predicted by the ab initio and model calculations. The intermode vibrational couplings are also structurally sensitive, even though the couplings are generally small in comparison to those of strong IR transitions such as the CdO stretchings. In summary, the results in the present work form the theoretical bases for further studies of the ultrafast structural dynamics of the diol and dithiol molecules using linear and nonlinear IR experimental techniques.

’ AUTHOR INFORMATION Corresponding Author

*Tel: (þ86)-010-62656806. Fax: (þ86)-010-62563167. E-mail: [email protected]. Present Addresses †

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R.China. ‡ College of Chemistry and Materials Fujian Normal University, Fuzhou 350007, P. R. China.

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’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20773136 and 30870591), by the National Basic Research Program of China (2007CB815205), and by the Chinese Academy of Sciences (the Hundred Talent Fund). X.M. and K.C. thank Chen Han and Juan Zhao for technical help. ’ REFERENCES (1) Gallwey, F. B.; Hawkes, J. E.; Haycock, P.; Lewis, D. J. Chem. Soc., Perkin Trans. 1990, 2, 1979. (2) Palmer, A. G. Chem. Rev. 2004, 104, 3623. (3) Meier, B. H.; Ernst, R. R. J. Am. Chem. Soc. 1979, 101, 6441. (4) Jeener, J.; Meier, B. H.; Bachmann, P.; Ernst, R. R. J. Chem. Phys. 1979, 71, 4546. (5) Torii, H.; Tasumi, M. J. Chem. Phys. 1992, 96, 3379. (6) Torii, H.; Tasumi, M. Infrared Spectroscopy of Biomolecules; Wiley: New York, 1996. (7) Krimm, S.; Bandekar, J. Adv. Protein Chem. 1986, 38, 181. (8) Hamm, P.; Lim, M.; Hochstrasser, R. M. J. Phys. Chem. B 1998, 102, 6123. (9) Asplund, M. C.; Lim, M.; Hochstrasser, R. M. Chem. Phys. Lett. 2000, 323, 269. (10) Asplund, M. C.; Zanni, M. T.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 8219. (11) Rubtsov, I. V.; Wang, J.; Hochstrasser, R. M. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 5601. (12) Wang, J. J. Phys. Chem. B 2008, 112, 4790. (13) Singelenberg, F. A. J.; Van Der Maas, J. H. J. Mol. Struct. 1991, 243, 111. (14) Kuhn, L. P. J. Am. Chem. Soc. 1952, 74, 2492. (15) Sokolova, M.; Barlow, S. J.; Bondarenko, G. V.; Gorbaty, Y. E.; Poliakoff, M. J. Phys. Chem. A 2006, 110, 3882. (16) Buckley, P.; Giguere, P. A. Can. J. Chem. 1967, 45, 397. (17) Lock, A. J.; Gilijamse, J. J.; Woutersen, S.; Bakker, H. J. J. Chem. Phys. 2004, 120, 2351. (18) Hayashi, M.; Shiro, Y.; Oshima, T.; Murata, H. Bull. Chem. Soc. Jpn. 1965, 38, 1734. (19) Nandi, R. N.; Su, C.-F.; Harmony, M. D. J. Chem. Phys. 1984, 81, 1051. (20) Ohsaku, M.; Murata, H. J. Mol. Struct.: THEOCHEM 1981, 85, 125. (21) Buemi, G. Phosphorus, Sulfur, Silicon Relat. Elem. 1993, 84, 239. (22) Russo, N.; Sicilia, E.; Toscanoe, M. J. Mol. Struct.: THEOCHEM 1992, 257, 485. (23) Michael, W. S.; Kim, K. B.; Jerry, A. B.; Steven, T. E.; Mark, S. G.; Jan, H. J.; Shiro, K.; Nikita, M.; Kiet, A. N.; Shujun, S.; Theresa, L. W.; Michel, D.; John, A. M., Jr. J. Comput. Chem. 1993, 14, 1347. (24) Marstokk, K.-M.; Møllendal, H. Acta Chem. Scand. 1997, 51, 653. (25) Hargittai, I.; Schultz, G. J. Chem. Phys. 1986, 84, 5220. (26) Hargittai, I.; Schultz, G. J. Chem. Soc. 1972, 6, 323. (27) Khriachtchev, L.; Pettersson, M.; Isoniemi, E.; Rasanen, M. J. Chem. Phys. 1998, 108, 5747. (28) Isoniemi, E.; Pettersson, M.; Khriachtchev, L.; Lundell, J.; Rasanen, M. J. Phys. Chem. A 1999, 103, 679. (29) Vaittinen, O.; Biennier, L.; Campargue, A.; Flaud, J.-M.; Halonen, L. J. Mol. Spectrosc. 1997, 184, 288. (30) Naumenko, O.; Campargue, A. J. Mol. Spectrosc. 2001, 209, 242. (31) Widmalm, G.; Pastor, R. W. J. Chem. Soc., Faraday Trans. 1992, 88, 1747. (32) Hayashi, H.; Tanaka, H.; Nakanishi, K. Fluid Phase Equilib. 1995, 104, 421. (33) Saiz, L.; Padro, J. A.; Guardia, E. J. Chem. Phys. 2001, 114, 3187. (34) Tasaki, K. J. Am. Chem. Soc. 1996, 118, 8459. (35) Smith, G. D.; Jaffe, R. L.; Yoon, D. Y. J. Phys. Chem. 1993, 97, 12752. 1186


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