Anharmonic Vibrational Frequencies Research Articles

Theoretical and NMR Studies of Deuterium Isotopic Perturbation of Hydrogen Bonding in Symmetrical Dihydroxy Compounds

Deuterium equilibrium isotope effects (EIEs) for a cage diol and 2,6-dihydroxyacylaromatics and complexes thereof containing intra- or intermolecular hydrogen bonds have been calculated using harmonic and anharmonic vibrational frequencies using Gaussian '03 and the HF, B3LYP, and MP2 levels of theory. The predicted isotope effects have been compared with experimental NMR data, and the origins of the isotope effects have been characterized in terms of zero-point vibrational energy differences and enthalpic and entropic contributions to the free energy difference between labeled species. Reliable free energy predictions based upon harmonic frequencies were found for systems whose isotope effects are governed by bond stretching effects and for systems whose isotope effects are determined by low-frequency vibrational modes. In contrast, thermochemical predictions based upon anharmonic frequencies were found to be far less consistent. Vibrational entropy is predicted to play an important role in modulating and, in some cases, governing isotopic site preferences in hydroxyl-derived intra- and intermolecular hydrogen bonds.

Anharmonic OH vibrations in brucite: Small pressure-induced redshift in the range 0-22 GPa

The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg(OH)2(brucite) has been calculated in the pressure range 0(DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations.The following findings emerged: (1) The calculated d with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg(OH) much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small d are pressed toward each other. (4) At high pressure, the OH with respect to the variation of the D quadrupole coupling constant is approximately –1 kHz/GPa. −22 GPa. Quantum-mechanical electronic structureν(OH)/dP slope is –4 cm–1/GPa, in agreement2]. (2) The calculated ν(OH) vs. R(O···O) correlation is linear and the slope isν/dP and dν/dR(O···O) slopes is the small electric field variation as the mineral layers− ions show some tendency to be tiltedc axis, and a larger tilt angle leads to a larger ν(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately –1 kHz/GPa.

Gas-Phase Infrared Spectra of Vinyl Selenol and Vinyl Tellurol

The infrared spectra (3500-500 cm(-1)) of gaseous vinyl selenol and vinyl tellurol have been recorded at 0.1 cm(-1) resolution. For the latter the spectra were obtained at room temperature, but for the former a temperature of -40 degrees C was required because of the chemical instability of vinyl selenol at room temperature. To compensate the very weak vapor pressure of vinyl tellurol at room temperature, a long optical path up to 136 m was necessary to record its spectrum. B3LYP density functional theory (DFT) calculations have been performed to assign the different absorption bands. Since an unambiguous assignment of the absorption bands requires a precise knowledge on the relative abundance of the syn and gauche rotamers of these compounds, their relative energies and their anharmonic vibrational frequencies were obtained using a very extended Def2-QZVP basis set. Two rotamers, the syn, which is planar, and a nonplanar gauche, were found to be local minima for both compounds. The gauche rotamer presents two degenerate conformers, which differ by the position of the SeH (TeH) hydrogen atom above or below the molecular plane. Our theoretical results are in good agreement with the main features of the experimental spectra. Fundamental bands and some combination bands of vinyl selenol and vinyl tellurol were assigned and compared with those of vinyl alcohol and vinyl thiol, whose spectra had been reported previously in the literature.

Theoretical investigation of the potential energy surface of the van der Waals complex CH4–N2

The interaction potential energy surface of the van der Waals CH(4)-N(2) complex has been calculated for a broad range of intermolecular separations and configurations in the approximation of rigid interacting molecules at the CCSD(T) and MP2 levels of theory using the correlation consistent aug-cc-pVTZ basis set. The BSSE correction was taken into account for all the calculations. The most stable configurations of the complex were found. Binding energies were calculated in the CBS limit with accounting for the molecular deformations. The harmonic and anharmonic fundamental vibrational frequencies and rotational constants for the ground and first excited vibrational states were calculated for the most stable configurations at the MP2 level of theory with BSSE correction. Fitting parameters were found for the most stable configuration for the Lennard-Jones and Esposti-Werner potentials.

Structure and conformational dynamics of the dicyclopropyl ketone in the ground electronic state

Geometric parameters, harmonic and anharmonic vibrational frequencies, conformer energy differences and barriers to internal rotation were obtained for dicyclopropyl ketone (DCPK) in the ground electronic state through MP2, DFT, CCSD and CCSD(T) calculations. VFPA was used to improve the estimations of conformer energy differences and heights of barriers to internal rotation. The ab initio calculations demonstrated that there are three stable conformations of DCPK: the cis–cis, the cis–trans and the gauche–gauche. The energy of the gauche–gauche conformer is noticeably higher than the energy of the two other conformers, so this conformer was not found experimentally. To study the conformational dynamics of the DCPK molecule, one- and two-dimensional sections of the potential energy surface corresponding to the rotations of the cyclopropyl groups were calculated. These sections were used to calculate torsion transition energies and vibrational wave functions in anharmonic approach. It was found that there is a strong coupling of large-amplitude torsion motions in the area of the cis–cis and gauche–gauche conformers.

Spectroscopic investigation of the à A1″-X̃ A1′ electronic transition of HSiNCO

The first spectroscopic observation of the previously unknown species HSiNCO has been reported. HSiNCO was generated by the fragmentation of trimethylsilylisocyanate with a high-voltage discharge source. The 000 band of the à A1″-X̃ A1′ transition has been recorded with full rotational resolution using laser-induced fluorescence spectroscopy and ground and excited state rotational and centrifugal distortion constants determined. Ten additional vibrational bands belonging to HSiNCO have also been observed in the laser-induced fluorescence spectrum and have been assigned based on predicted anharmonic vibrational frequencies. Due to the large change in geometry upon excitation, a number of axis-rotation peaks have been observed in the 000 band and the axis-rotation angle (θT) has been estimated to be 0.6°±0.2°. Dispersed fluorescence spectroscopy has been carried out and ν8 (the N–C–O out-of-plane bending mode) and a number of overtones of ν4 (the Si–H wagging mode) have been observed in the ground electronic state.

A combined ab initio, Fourier transform microwave and Fourier transform infrared spectroscopic investigation of β-propiolactone: The ν8 and ν12 bands

The pure rotational spectrum of β-propiolactone (c-C 2H 4COO) has been recorded between 7 and 21 GHz using a pulsed jet Fourier transform microwave spectrometer. The resulting ground state spectroscopic constants guided the analysis of the rotationally-resolved infrared spectra of two bands that were collected using the far infrared beamline at the Canadian Light Source synchrotron. The observed modes correspond to motions best described as ring deformation ( ν 12) at 747.2 cm −1 and CO ring stretching ( ν 8) at 1095.4 cm −1. A global fit of 4430 a- and b-type transitions from the microwave spectrum and the two infrared bands provided an accurate set of ground state and excited state spectroscopic parameters. To complement the experimental results, the harmonic and anharmonic vibrational frequencies of all 21 infrared active modes of β-propiolactone have been calculated using the DFT B3LYP method (6-311+G(d,p), 6-311++G(2d,3p) basis sets).

Vibrational spectroscopic studies, conformations and ab initio calculations of 1,2‐bis(trifluorosilyl)ethane (SiF3CH2CH2SiF3)

AbstractInfrared spectra of 1,2‐bis(trifluorosilyl)ethane (SiF3CH2CH2SiF3) were obtained in the vapour and liquid phases, in argon matrices and in the solid phase. Raman spectra of the compound as a liquid were recorded at various temperatures between 293 and 270 K and spectra of an apparently crystalline solid were observed. The spectra revealed the existence of two conformers (anti and gauche) in the vapour, liquid and in the matrix. When the vapour was chock‐frozen on a cold finger at 78 K and annealed to 150 K, certain weak Raman bands vanished in the crystal. The vibrational spectra of the crystal demonstrated mutual exclusion between IR and Raman bands in accordance with C2h symmetry. Intensity variations between 293 and 270 K of pairs of various Raman bands gave ΔH(gauche—anti) = 5.6 ± 0.5 kJ mol−1 in the liquid, suggesting 85% anti and 15% gauche in equilibrium at room temperature. Annealing experiments indicate that the anti conformer also has a lower energy in the argon matrices, is the low‐energy conformer in the liquid and is also present in the crystal. The spectra of both conformers have been interpreted, and 34 anti and 17 gauche bands were tentatively identified. Ab initio and density functional theory (DFT) calculations were performed giving optimized geometries, infrared and Raman intensities and anharmonic vibrational frequencies for both conformers. The conformational energy difference derived in CBS‐QB3 and in G3 calculations was 5 kJ mol−1. Copyright © 2009 John Wiley & Sons, Ltd.

Spectroscopic Investigation of the Electronic Ã1A′′−X̃1A′ Transition of HSiNC

The first spectroscopic investigation of the A1A''-X1A' transition of HSiNC has been reported. The 0(0)(0) band of the A1A''-X1A' transition has been rotational resolved using laser-induced fluorescence spectroscopy, and ground- and excited-state rotational and centrifugal distortion constants were evaluated. Ten additional vibrational bands belonging to HSiNC have also been observed in the laser-induced fluorescence spectrum and have been assigned based on predicted anharmonic vibrational frequencies. Because of the large change in geometry upon excitation, a number of axis-rotation peaks have been observed in the 0(0)(0) band, and the axis-rotation angle (thetaT) has been estimated to be 1.0 +/- 0.2 degrees. Dispersed fluorescence spectroscopy has also been carried out, and a number of overtones of the nu3 fundamental (Si-H wagging mode) have been observed in the ground state, and its anharmonic parameter (x(e)) was evaluated.

A laboratory and theoretical study of protonated carbon disulfide, HSCS+

The rotational spectrum of protonated carbon disulfide, HSCS(+), has been detected in the centimeter-wave band in a molecular beam by Fourier transform microwave spectroscopy. Rotational and centrifugal distortion constants have been determined from ten transitions in the K(a)=0 ladder of the normal isotopic species, HS(13)CS(+), and DSCS(+). The present assignment agrees well with high-level coupled cluster calculations of the HSCS(+) structure, which, like earlier work, predict this isomer to be the ground state on the HCS(2) (+) potential energy surface; HCSS(+), an isomer with C(2v) symmetry, is predicted to lie more than 20 kcal/mol higher in energy. Other properties of HSCS(+) including its dipole moment, anharmonic vibrational frequencies, and infrared intensities have also been computed at the coupled cluster level of theory with large basis sets. Because carbon disulfide possesses a fairly large proton affinity, and because this nonpolar molecule may plausibly exist in astronomical sources, HSCS(+) is a good candidate for detection with radio telescopes in the submillimeter band where the stronger b-type transitions of this protonated cation are predicted to lie.

Carbon−Deuterium Vibrational Probes of Amino Acid Protonation State

The protonation state of titratable amino acid residues has profound effects on protein stability and function. Therefore, correctly determining the acid dissociation constant, pK(a), of charged residues under physiological conditions is an important challenge. The general utility of site-specific carbon-deuterium (C-D) vibrational probes as reporters of the protonation state of arginine, aspartic acid, glutamic acid, and lysine amino acid side chains was examined using density functional theory (DFT) calculations. Substantial shifts were observed in the anharmonic vibrational frequencies of a C-D(2) probe placed immediately adjacent to the titratable group. Lysine exhibited the largest C-D(2) frequency shifts upon protonation, 44.9 cm(-1) (symmetric stretch) and 69.5 cm(-1) (asymmetric stretch). Furthermore, the predicted harmonic intensities of the C-D(2) probe vibrations were extraordinarily sensitive to the protonation state of the nearby acidic or basic group. Accounting for this dramatic change in intensity is essential to the interpretation of an infrared (IR) absorption spectrum that contains the signature of both the neutral and charged states.

Conformers of Gaseous Cysteine.

Structures, accurate relative energies, equilibrium and vibrationally averaged rotational constants, quartic and sextic centrifugal distortion constants, dipole moments, (14)N nuclear quadrupole coupling constants, anharmonic vibrational frequencies, and double-harmonic infrared intensities have been determined from ab initio electronic structure computations for conformers of the neutral form of the natural amino acid l-cysteine (Cys). A systematic scan located 71 unique conformers of Cys using the MP2(FC)/cc-pVTZ method. The large number of structurally diverse low-energy conformers of Cys necessitates the highest possible levels of electronic structure theory to determine their relative energies with some certainty. For this reason, we determined the relative energies of the lowest-energy eleven conformers, accurate within a standard error (1σ) of about 0.3 kJ mol(-1), through first-principles composite focal-point analyses (FPA), which employed extrapolations using basis sets as large as aug-cc-pV(5+d)Z and correlation treatments as extensive as CCSD(T). Three and eleven conformers of l-cysteine fall within a relative energy of 6 and 10 kJ mol(-1), respectively. The vibrationally averaged rotational constants computed in this study agree well with Fourier-transform microwave spectroscopy results. The effects determining the relative energies of the low-energy conformers of cysteine are analyzed in detail on the basis of hydrogen bond additivity schemes and natural bond orbital analysis.

Conformers of gaseous threonine†

Following an extensive search on the potential energy surfaces (PES) of the natural amino acid L-threonine (Thr) and its allotropic form L-allo-threonine (aThr), 56 and 61 conformers of Thr and aThr, respectively, have been located with the help of density functional theory (DFT). Accurate structures, relative energies, rotational as well as quartic and sextic centrifugal distortion constants, dipole moments, 14N nuclear quadrupole coupling constants, anharmonic vibrational frequencies and double-harmonic infrared intensities have been determined from ab initio electronic structure calculations for the five most stable Thr and aThr conformers. The global minimum, Thr-I, has a cyclic triple H-bond motif with strong OH ··· N, C=O ··· HO, and a weaker NH ··· OH H-bond, where the latter two involves the side chain OH, and an energetically unfavourable trans-COOH arrangement. The best relative energies of the conformers, accurate within ±1 kJ mol−1, have been determined through the first-principles composite focal-point analysis (FPA) approach. There are four and three conformers of Thr and aThr, respectively, within a relative energy of 5 kJ mol−1. Similarly to other amino acids investigated, lower levels of electronic structure theory, especially the Hartree–Fock level, are unable to determine the correct relative energies of the conformers. The rotational, the quartic and sextic centrifugal distortion, and the 14N nuclear quadrupole coupling constants as well as the anharmonic vibrational fundamentals and double-harmonic infrared intensities, all determined using DFT, should aid identification and characterization of the conformers of threonine and allo-threonine by rotational and vibrational spectroscopies, respectively. †This paper is dedicated to Professor Fritz Schaefer on the occasion of his 65th birthday.

Anharmonic vibrational frequencies and vibrationally-averaged structures of key species in hydrocarbon combustion: HCO+, HCO, HNO, HOO, HOO–, CH3 +, and CH3†

A general scheme to predict anharmonic vibrational frequencies and vibrationally-averaged structures and rotational constants of molecules is presented with applications to some key species in hydrocarbon combustion (some also of importance in atmospheric and/or interstellar chemistry): HCO+, HCO, HNO, HOO, HOO–,, and CH3. A combination of coupled-cluster singles and doubles (CCSD), CCSD with a second-order perturbation correction in the space of triples [CCSD(2) T ] and in the space of triples and quadruples [CCSD(2) TQ ], and a correlation-consistent basis set series has been employed to achieve the complete-correlation, complete-basis-set limits of the potential energy surfaces (PESs) of these species near equilibrium geometries. A new, compact representation of PESs that combines two existing representations, namely, a fourth-order Taylor expansion and numerical values on a rectilinear grid, has been proposed and shown to yield accurate frequencies, when combined with vibrational general-order configuration-interaction method. The predicted frequencies (and the observed in parentheses, when available) of the fundamentals are as follows: 823 (830), 2175 (2184), and 3083 (3089) cm−1 in HCO+; 1079 (1081), 1874 (1868), 2432 (2434) cm−1 in HCO; 1503 (1501), 1572 (1565), and 2683 (2684) cm−1 in HNO; 1121 (1098), 1399 (1392), and 3447 (3436) cm−1 in HOO; 739, 1088, and 3587 cm−1 in HOO–; 1383 (1359 ± 7), 1384 (1370 ± 7), 2940, and 3096 (3108) cm−1 in ; 565 (606), 1377, 3002 (3004), and 3139 (3161) cm−1 in CH3. The mean absolute deviation in the predicted frequencies is 11 cm−1. †Dedicated to Professor Henry F. Schaefer III on the occasion of his 65th birthday.

Study of N, N-dimethyl(carboethoxymethyl)-3-phthalimidopropylammonium chloride dihydrate by DFT calculations, NMR and FTIR spectroscopy

N, N-dimethyl(carboethoxymethyl)-3-phthalimidopropylammonium chloride dihydrate ( 1) and N, N-dimethyl(carboxymethyl)-3-phthalimidopropylammonium hydrochloride ( 3) have been obtained in reaction of N, N-dimethyl-3-phthalimidopropylamine with ethyl chloroacetate and chloroacetic acid, respectively. N, N-dimethyl(carboethoxymethyl)-3-phthalimido-propylammonium chloride dihydrate ( 1) has been characterized by FTIR and NMR spectroscopy. Moreover, for ( 1) and ( 3) B3LYP calculations have been carried out. The optimized bond lengths, bond angles and torsion angles calculated by B3LYP/6-31G(d,p) approach have been presented. Both FTIR and Raman spectra of ( 1) are consistent with the calculated structures in the gas phase. Correlations between the experimental 1H and 13C NMR chemical shifts ( δ exp) of investigated compound in D 2O, and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors ( σ calc), δ exp = a + b σ calc, are reported. The assignments of the anharmonic experimental solid state vibrational frequencies of ( 1) based on the calculated B3LYP/6-31G(d,p) harmonic frequencies have been made. Linear correlations between the experimental 1H and 13C chemical shifts and the computed screening constants confirm the optimized geometry.

Characterization of the HSiNHNSi system in its electronic ground state

The electronic ground states (X (1)Sigma(+)) of HSiN, HNSi, and the transition state connecting the two isomers were systematically studied using configuration interaction with single and double (CISD) excitations, coupled cluster with single and double (CCSD) excitations, CCSD with perturbative triple corrections [CCSD(T)], multireference complete active space self-consistent field (CASSCF), and internally contracted multireference configuration interaction (ICMRCI) methods. The correlation-consistent polarized valence (cc-pVXZ), augmented correlation-consistent polarized valence (aug-cc-pVXZ) (X=T,Q,5), correlation-consistent polarized core-valence (cc-pCVYZ), and augmented correlation-consistent polarized core-valence (aug-cc-pCVYZ) (Y=T,Q) basis sets were used. Via focal point analyses, we confirmed the HNSi isomer as the global minimum on the ground state HSiN_HNSi zero-point vibrational energy corrected surface and is predicted to lie 64.7 kcal mol(-1) (22 640 cm(-1), 2.81 eV) below the HSiN isomer. The barrier height for the forward isomerization reaction (HSiN-->HNSi) is predicted to be 9.7 kcal mol(-1), while the barrier height for the reverse process (HNSi-->HSiN) is determined to be 74.4 kcal mol(-1). The dipole moments of the HSiN and HNSi isomers are predicted to be 4.36 and 0.26 D, respectively. The theoretical vibrational isotopic shifts for the HSiN/DSiN and HNSi/DNSi isotopomers are in strong agreement with the available experimental values. The dissociation energy for HSiN [HSiN(X (1)Sigma(+))-->H((2)S)+SiN(X (2)Sigma(+))] is predicted to be D(0)=59.6 kcal mol(-1), whereas the dissociation energy for HNSi [HNSi(X (1)Sigma(+))-->H((2)S)+NSi(X (2)Sigma(+))] is predicted to be D(0)=125.0 kcal mol(-1) at the CCSD(T)/aug-cc-pCVQZ level of theory. Anharmonic vibrational frequencies computed using second order vibrational perturbation theory are in good agreement with available matrix isolation experimental data for both HSiN and HNSi isomers root mean squared derivation (RMSD=9 cm(-1)).

Electronic circular dichroism of monomethyl [ 16O, 17O, 18O]-phosphate and [ 16O, 17O, 18O]-thiophosphate revisited

Phosphoryl-transfer reactions have long been of interest due to their importance in maintaining numerous cellular functions. A phosphoryl-transfer reaction results in two possible stereochemical outcomes: either retention or inversion of configuration at the transferred phosphorus atom. When the product is phosphate, isotopically-labeled [ 16O, 17O, 18O]-phosphate derivatives can be used to distinguish these outcomes; one oxygen must be replaced by sulfur or esterified to achieve isotopic chirality. Conventionally, stereochemical analysis of isotopically chiral phosphate has been based on 31P NMR spectroscopy and involves complex chemical or enzymatic transformations. An attractive alternative would be direct determination of the enantiomeric excess using chiroptical spectroscopy. ( S)-Methyl-[ 16O, 17O, 18O]-phosphate (MePi ∗), 7 and enantiomeric [ 16O, 17O, 18O]-thiophosphate (TPi ∗), 10, were previously reported to exhibit weak electronic circular dichroism (ECD), although with 10 the result was considered to be uncertain. We have now re-examined the possibility that excesses of 7 and 10 enantiomers can be detected by ECD spectrometry, using both experimental and theoretical approaches. 7 and both the ( R) and ( S) enantiomers of 10 ( 10a, 10b) were synthesized by the ‘Oxford route’ and characterized by 1H, 31P and 17O NMR, and by MS analysis. Weak ECD could be found for 7, with suboptimal S/N. No significant ECD could be detected for the 10 enantiomers. Time-dependent DFT (TDDFT) calculations of the electronic excitation energies and rotational strengths of the same three enantiomers were carried out using the functional B3LYP and the basis set 6-311G ∗∗. The isotopically-perturbed geometries were predicted using the anharmonic vibrational frequency calculational code in GAUSSIAN 03. In the case of 10, calculations were also carried out for the hexahydrated complex to investigate the influence of the aqueous solvent. The predicted excitation wavelengths are greater than the observed wavelengths, a not unusual result of TDDFT calculations. The predicted anisotropy ratios are 2.9 × 10 −5 for 7, −5.3 × 10 −6 for 10a/b, and 1.7 × 10 −6 for 10a/b⋅(H 2O) 6. For 7 the predicted anisotropy ratio approximates that observed in this work, 4.5 × 10 −5 at 208 nm. For 10a/b, the upper limits of the experimental anisotropy ratios (<5 × 10 −6 at 225 nm, pH 9; <5 × 10 −6 at 236 nm, pH 12) are comparable to the predicted magnitude of the value for 10a/b. The lower predicted value for 10a/b · (H 2O) 6 suggests that the aqueous environment affects the ECD significantly. Altogether, the TDDFT calculations together with a stereochemical analysis based on NMR and the MS data support the conclusion that the experimental ECD results for MePi ∗ and TPi ∗ may be reliable in order of magnitude.

Accurate calculation of vibrational frequencies using explicitly correlated coupled-cluster theory

The recently proposed explicitly correlated CCSD(T)-F12x (x = a,b) approximations [T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys. 127, 221106 (2007)] are applied to compute equilibrium structures and harmonic as well as anharmonic vibrational frequencies for H(2)O, HCN, CO(2), CH(2)O, H(2)O(2), C(2)H(2), CH(2)NH, C(2)H(2)O, and the trans-isomer of 1,2-C(2)H(2)F(2). Using aug-cc-pVTZ basis sets, the CCSD(T)-F12a equilibrium geometries and harmonic vibrational frequencies are in very close agreement with CCSD(T)/aug-cc-pV5Z values. The anharmonic frequencies are evaluated using vibrational self-consistent field and vibrational configuration interaction methods based on automatically generated potential energy surfaces. The mean absolute deviation of the CCSD(T)-F12a/aug-cc-pVTZ anharmonic frequencies from experimental values amounts to only 4.0 cm(-1).

Structurally Sensitive Anharmonic Cα−D Stretch Vibration in Deuterated Peptides

In the present study, vibrational properties of the Calpha-D stretching mode in deuterated alanine dipeptide are examined by ab initio normal-mode calculations. A backbone conformational scan reveals a quite similar structural sensitivity of the harmonic and anharmonic vibration frequencies for both the fundamental and overtone transitions. The distributions of the frequencies are found to be linearly anticorrelated with that of the Calpha-D bond length, with the latter being found to be also structurally sensitive. Theory predicts that, as determined by the quantum mechanical anharmonic force field, the highly anharmonic (with a mean diagonal anharmonicity of 48.0 cm(-1)) and yet highly localized Calpha-D stretching mode shall be potentially useful in probing peptide local structures and dynamics.

Multiple Anharmonic Vibrational Probes of Sugar Structure and Dynamics

Quantum mechanical computations and molecular dynamics simulations are carried out for the simplest sugar glycolaldehyde to gain insight into the underlying force fields that determine the vibrational spectroscopic parameters relevant to structure and dynamics. The harmonic and anharmonic vibrational frequencies of the 3N--6 modes and their diagonal and off-diagonal anharmonicities are evaluated using the hybrid B3LYP functional in comparison with other high-level theories. Very good performance of B3LYP/6-31+G** is found in predicting the anharmonic frequencies by statistical analysis and by comparison to gas-phase experiments. Full cubic and semidiagonal quartic anharmonic force constants, the origin of the anharmonicities, the isotope dependence of the anharmonicities, and the polarizable continuum solvent effect on the anharmonicities are examined, in particular, for the CO, C-H, and O-H stretching modes. Site-dependent dynamical interactions between glycolaldehyde and water molecules in the hydration shells are examined by molecular dynamics simulations employing a set of molecular mechanical force fields developed on the basis of quantum mechanical computations. The statistical distributions and correlations of the fundamental transition frequencies and transition dipoles are obtained through instantaneous normal-mode analysis. The simultaneous assessment of multiple parameters of multiple vibrational probes shall prove useful in understanding the characteristics of sugar structure and dynamics expressed in two-dimensional infrared correlation spectra.