Quantum vibrational analysis of hydrated ions using an ab initio potential

We present full-dimensional potential energy surfaces (PESs) for hydrated chloride based on the sum of ab initio (H(2)O)Cl(-), (H(2)O)(2), and (H(2)O)(3) potentials. The PESs are shown to predict minima and corresponding harmonic frequencies accurately on the basis of comparisons with previous and new ab initio calculations for (H(2)O)(2)Cl(-), (H(2)O)(3)Cl(-), and (H(2)O)(4)Cl(-). An estimate of the effect of the 3-body water interaction is made using a simple 3-body water potential that was recently fit to tens of thousands of ab initio 3-body energies. Anharmonic, coupled vibrational calculations are presented for these clusters, using the "local monomer model" for the high frequency intramolecular modes. This model is tested against previous "exact" calculations for (H(2)O)Cl(-). Radial distribution functions at 0 K obtained from quantum zero-point wave functions are also presented for the (H(2)O)(2)Cl(-) and (H(2)O)(3)Cl(-) clusters.     

Quantum calculations of vibrational energies of H3O2- on an ab initio potential

We report a full-dimensional potential energy surface for H3O2-, based on fitting 66,965 ab initio electronic energies. A major feature of this potential is a barrier of roughly 200 cm-1 to internal rotation of the two hydroxyl groups about a line connecting the two oxygen atoms and the bridging hydrogen atom. The potential is used in calculations of vibrational energies, performed with the "Reaction Path" version of the code "MULTIMODE". The results are compared to recent infrared messenger experiments and are used to propose interpretations of the experimental results.     

Matrix and ab initio infrared spectra of germanium carbonyls

Germanium monocarbonyl has been prepared by high-temperature vaporization of Ge with excess carbon powder or CO co-condensation and trapped in various matrices at 12 K. Fourier transform infrared spectra, ab initio (MP2 and CISD) and density function (Becke3LYP) calculations suggested a linear species with the formula GeCO. A weak feature appearing only in annealed N₂ and CO matrices containing GeCO has tentatively been assigned to germanium dicarbonyl on grounds of isotopic substitution and theoretical calculations. SCF and CI methods predict a bent structure for this complex.     

A chemometric analysis of ab initio vibrational frequencies and infrared intensities of methyl fluoride

Factorial design and principal component analyses are applied to CH₃F infrared frequencies and intensities calculated from ab initio wave functions. In the factorial analysis, the quantitative effects of changing from a 6–31G to a 6–311G basis, of including polarization and diffuse orbitals, and of correcting for electron correlation using the second-order Møller-Plesset procedure are determined for all frequencies and intensities. The most significant main effect observed for the frequencies corresponds to the shift from Hartree-Fock to MP2 calculations, which tends to lower all frequency values by approximately 100 cm⁻¹. For the intensities, the main effects are larger for the CF stretching and the CH₃ asymmetric stretching modes. Interaction effects between two or more of the four factors are found to be of minor importance, except for the interaction between correlation and polarization. The principal component analysis indicates that wave functions with polarization and diffuse orbitals at the second-order Møller-Plesset level provide the best estimates for the harmonic frequencies, but not for the intensities. For the frequencies, the first principal component distinguishes between MP2 and Hartree-Fock calculations, while the second component separates the wave functions with polarization orbitals from those without these orbitals. For the intensities, the separation is similar but less well defined. This analysis also shows that wave function optimization to calculate accurate intensities is more difficult than an optimization for frequencies. © 1996 by John Wiley & Sons, Inc.     

Raman and infrared spectra, ab initio and DFT calculations, and vibrational assignments for 2,3-cyclopentenopyridine

The structural properties of 2,3-cyclopentenopyridine (pyrindan) have been investigated using several spectroscopic and computational techniques. The Raman and infrared spectra of the molecule have been recorded and a full vibrational assignment was proposed on the basis of experimental and theoretical results. The vapor-phase Raman spectrum was successfully obtained at 260 degrees C without sample decomposition. Density functional theory (DFT) and M?ller-Plesset (MP2) calculations predict that the presence of the nitrogen atom in the six-membered ring has almost no effect on the barrier to inversion (587 cm(-1)) and puckering frequency (139 cm(-1)) as compared to the values previously determined (488 cm(-1) and 143 cm(-1)) for the indan molecule.     

Full-dimensional vibrational calculations for H5O2+ using an ab initio potential energy surface

We report quantum diffusion Monte Carlo (DMC) and variational calculations in full dimensionality for selected vibrational states of H(5)O(2) (+) using a new ab initio potential energy surface [X. Huang, B. Braams, and J. M. Bowman, J. Chem. Phys. 122, 044308 (2005)]. The energy and properties of the zero-point state are focused on in the rigorous DMC calculations. OH-stretch fundamentals are also calculated using "fixed-node" DMC calculations and variationally using two versions of the code MULTIMODE. These results are compared with infrared multiphoton dissociation measurements of Yeh et al. [L. I. Yeh, M. Okumura, J. D. Myers, J. M. Price, and Y. T. Lee, J. Chem. Phys. 91, 7319 (1989)]. Some preliminary results for the energies of several modes of the shared hydrogen are also reported.     

Torsional potential of 4,4'-bipyridine: ab initio analysis of dispersion and vibrational effects

Ab initio calculations using restricted Hartree-Fock, second-order M?ller-Plesset perturbation theory (MP2), density-functional theory (DFT), and coupled-cluster methods have been done to obtain the torsional potential-energy profile of the aza-aromatic molecule 4,4'-bipyridine. The torsional potential is evaluated adiabatically by fixing the normalized sum of the dihedral angles through the C-C inter-ring bond at several values along the torsional path and relaxing the remaining degrees of freedom. Previous discrepancies between MP2 and DFT internal rotation barrier heights are removed, and seen to be mostly due to the underestimation of the dispersion energy in the coplanar conformer by MP2 when using relatively small basis sets. The calculations indicate that the barrier height between the twisted global minimum and the 0 degrees conformer is around 1.5-1.8 kcal mol-1 while that corresponding to the 90 degrees one is about 2.0-2.2 kcal mol-1. This same relative energy ordering of the coplanar and perpendicular conformers was experimentally derived from nuclear magnetic resonance (NMR) measurements of 1H dipolar couplings on 4,4'-bipyridine solutions in a nematic liquid crystal, although the barrier heights are much lower than those estimated from NMR experiments in the gas phase. The DFT infrared spectrum and zero-point vibrational energy corrections to the torsional energy profile have also been calculated, the latter having a small influence on the torsional potential-energy profiles.     

Raman and infrared spectra, conformational stability, ab initio calculations and vibrational assignments for 2-chloroethyl silane

The infrared (3500–30 cm⁻¹) spectra of gaseous and solid and the Raman (3500–10 cm⁻¹) spectra of liquid with quantitative depolarization ratios and solid 2-chloroethyl silane, ClCH₂CH₂SiH₃, have been recorded. Similar data have been recorded for the Si–d₃ isotopomer. These data indicate that two conformers, trans and gauche, are present in the fluid states but only one conformer, trans, is present in the solid. The mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−55 to −100°C) has been recorded. The enthalpy difference between the conformers has been determined to be 181±12 cm⁻¹ (2.17±0.14 kJ/mol) with the trans rotamer the more stable form. From the isolated Si–H frequencies from the Si–d₂ isotopomer the r_o Si–H distances of 1.484 and 1.483 Å for the trans and 1.481 for the gauche conformers have been obtained. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G** from which structural parameters and conformational stabilities have been determined. With all the basis sets the trans form is predicted to be the more stable conformer which is consistent with the experimental results. These results are compared to the corresponding quantities for the carbon analogue.     

FTIR, FT-Raman spectra and ab initio DFT vibrational analysis of 2,4-dichloro-6-nitrophenol

The FTIR and FT-Raman spectra of 2,4-dichloro-6-nitrophenol (2,4-DC6NP) has been recorded in the region 4000-400 cm(-1) and 3500-100 cm(-1), respectively. The optimized geometry, frequency and intensity of the vibrational bands of (2,4-DC6NP) were obtained by the ab initio and DFT levels of theory with complete relaxation in the potential energy surface using 6-31G(d,p) and 6-311+G(d,p) basis sets. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FTIR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar type spectrograms.     

FTIR, FT-Raman spectra and ab initio DFT vibrational analysis of 2-amino-5-chloropyridine

The FTIR and FT-Raman spectra of 2-amino-5-chloropyridine (ACP) has been recorded in the region 4000-400 and 3500-100 cm-1, respectively. The optimized geometry, frequency and intensity of the vibrational bands of ACP were obtained by the ab initio and density functional theory (DFT) levels of theory with complete relaxation in the potential energy surface using 6-31G(d,p) and 6-311+G(2df,2p) basis sets. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FTIR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar type spectrograms.     

FT-IR, FT-Raman spectra and ab initio HF, DFT vibrational analysis of p-chlorobenzoic acid

The infrared, the Fourier transform infrared and Fourier transform Raman spectra of p-chlorobenzoic acid (p-CBA) has been recorded in the region 4000-600 cm(-1), 4000-400 cm(-1) and 4000-100 cm(-1), respectively. The optimized geometry, frequency and intensity of the vibrational bands of p-CBA were obtained by the ab initio HF and DFT (B3LYP) methods with complete relaxation in the potential energy surface using 6-311+G(d,p) basis set. The harmonic-vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar type spectrograms.     

Quantum wavepacket ab initio molecular dynamics: an approach for computing dynamically averaged vibrational spectra including critical nuclear quantum effects

We have introduced a computational methodology to study vibrational spectroscopy in clusters inclusive of critical nuclear quantum effects. This approach is based on the recently developed quantum wavepacket ab initio molecular dynamics method that combines quantum wavepacket dynamics with ab initio molecular dynamics. The computational efficiency of the dynamical procedure is drastically improved (by several orders of magnitude) through the utilization of wavelet-based techniques combined with the previously introduced time-dependent deterministic sampling procedure measure to achieve stable, picosecond length, quantum-classical dynamics of electrons and nuclei in clusters. The dynamical information is employed to construct a novel cumulative flux/velocity correlation function, where the wavepacket flux from the quantized particle is combined with classical nuclear velocities to obtain the vibrational density of states. The approach is demonstrated by computing the vibrational density of states of [Cl-H-Cl]-, inclusive of critical quantum nuclear effects, and our results are in good agreement with experiment. A general hierarchical procedure is also provided, based on electronic structure harmonic frequencies, classical ab initio molecular dynamics, computation of nuclear quantum-mechanical eigenstates, and employing quantum wavepacket ab initio dynamics to understand vibrational spectroscopy in hydrogen-bonded clusters that display large degrees of anharmonicities.     

Infrared spectra,vibrational assignment and ab initio calculations for 2,2,2-trifluoroethanimidamide

The infrared spectra of gaseous and solid 2,2,2-trifluoroethanimidamide, CF₃(NH₂)C=NH, have been recorded from 4000 to 80 cm^–1. A vibrational assignment for the normal modes is proposed based on group frequencies and normal coordinate calculations utilizing C₁ symmetry. The structures for both the cis [hydrogen atom of the =NH group is cis to the NH₂ group] and trans geometric isomers have been determined from ab initio Hartree-Fock gradient calculations employing the GAUSSIAN-82 program with the 3–21G basis set. The most stable conformer at this level of calculation is found to be a C₁, structure with a partially rotated CF₃ group and the hydrogen atom of the imine group trans to the NH₂ group. The calculated structural parameters have only very small differences between the conformers. Barriers to internal rotation for the NH₂ and CF₃ groups and vibrational frequencies have been calculated for the C₁ form. The results of this investigation are compared with similar data on some corresponding molecules.Taken in part from the thesis of T. G. Sheehan which was submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree, May 1990.     

The vibrational structure of the oxygen K-shell spectra in acenaphthenequinones: an ab initio study

The vibrational structure of the K-shell O1s → π? of acenaphthenequinone C(12)H(6)O(2) and its halogenated compound C(12)H(2)Br(2)Cl(2)O(2) has been simulated using an entirely ab initio approach. For both molecules, analysis of the calculated Franck-Condon factors confirm without ambiguity that, contrary to initial claims, the C-H stretching modes are not modified in the core states and are not excited. For C(12)H(6)O(2), the vibrational fine structure appears to be mainly due to three modes, involving C=O? asymmetric stretch and in-plane ring deformation modes, due to the symmetry breaking of the core state. For C(12)H(2)Br(2)Cl(2)O(2), the vibrational excitation arises essentially from the C=O? asymmetric stretch, with numerous secondary peaks arising from hot and combination bands. For both molecules, these bands are probably responsible for the asymmetry deduced in the experimental fits using a unique Morse potential and initially assigned to anharmonic effects.     

Comparison of experimental and ab initio HF and DFT vibrational spectra of benzimidazole

In this work, we will report a combined experimental and theoretical study on molecular and vibrational structure of benzimidazole. The laser Raman and Fourier transform infrared spectra of benzimidazole were recorded in the solid phase. The equilibrium geometry, harmonic vibrational frequencies, infrared intensities, Raman scattering activities, depolarization ratios and reduced masses were calculated by HF and density functional B3LYP method with the 6-311G(d,p) basis set. The scaled theoretical wavenumbers showed very good agreement with the experimental values. The thermodynamic functions of the title compound were also performed at HF/6-31G(d,p)/6-311G(d,p) and B3LYP/6-31G(d,p)/6-311G(d,p) levels of theory. A detailed interpretations of the infrared and Raman spectra of benzimidazole is reported. The theoretical spectrograms for FT-IR spectra of the title molecule have been constructed.     

FT-IR, FT-Raman spectra and ab initio HF, DFT vibrational analysis of 2,3-difluoro phenol

The FT-IR and FT-Raman spectra of 2,3-difluoro phenol (2,3-DFP) has been recorded in the region 4000-400 and 4000-100 cm(-1), respectively. The optimized geometry, frequency and intensity of the vibrational bands of 2,3-DFP were obtained by the ab initio HF and density functional theory (DFT) levels of theory with complete relaxation in the potential energy surface using 6-311+G(d,p) basis set. The harmonic vibrational frequencies were calculated and the scaled values have been compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. The experimental spectra also coincide satisfactorily with those of theoretically constructed bar type spectrograms.     

Converged quantum dynamics calculations of vibrational energies of CH4 and CH3D using an ab initio potential

Exact variational calculations of vibrational energies of CH4 and CH3D are carried out using a two-layer Lanczos algorithm based on the ab initio potential energy surface of D. W. Schwenke and H. Partridge, Spectrochim. Acta, Part A 57, 887 (2001). The convergence of the calculated vibrational energies is discussed in detail. In addition, we report all well converged vibrational energy levels up to 6600 cm(-1) for CH4, and those up to 5000 cm(-1) for CH3D, respectively. These results clearly outperform previous theoretical calculations. And a comparison with experimental results available is also made.     

Far infrared spectra, conformational equilibria, vibrational assignments, ab initio calculations and structural parameters for 2-bromoethanol

The far infrared spectrum from 370 to 50 cm⁻¹ of gaseous 2-bromoethanol, BrCH₂CH₂OH, was recorded at a resolution of 0.10 cm⁻¹. The fundamental O–H torsion of the more stable gauche (Gg′) conformer, where the capital G refers to internal rotation around the C–C bond and the lower case g to the internal rotation around the C–O bond, was observed as a series of Q-branch transitions beginning at 340 cm⁻¹. The corresponding O–H torsional modes were observed for two of the other high energy conformers, Tg (285 cm⁻¹) and Tt (234 cm⁻¹). The heavy atom asymmetric torsion (rotation around C–C bond) for the Gg′ conformer has been observed at 140 cm⁻¹. Variable temperature (−63 to −100°C) studies of the infrared spectra (4000–400 cm⁻¹) of the sample dissolved in liquid xenon have been recorded. From these data the enthalpy differences have been determined to be 411±40 cm⁻¹ (4.92±0.48 kJ/mol) for the Gg′/Tt and 315±40 cm⁻¹ (3.76±0.48 kJ/mol) for the Gg′/Tg, with the Gg′ conformer the most stable form. Additionally, the infrared spectrum of the gas, and Raman spectrum of the liquid phase are reported. The structural parameters, conformational stabilities, barriers to internal rotation and fundamental frequencies have been obtained from ab initio calculations utilizing different basis sets at the restricted Hartree–Fock or with full electron correlation by the perturbation method to second order. The theoretical results are compared to the experimental results when appropriate. Combining the ab initio calculations with the microwave rotational constants, r₀ adjusted parameters have been obtained for the three 2-haloethanols (F, Cl and Br) for the Gg′ conformers.     

A three-dimensional ab initio potential energy surface and predicted infrared spectra for the He-N2O complex

We report a three-dimensional ab initio potential energy surface for He-N(2)O using a supermolecular method at the coupled-cluster singles and doubles with noniterative inclusion of connected triple level. Besides the intermolecular stretching and bending modes, we included the Q(3) normal mode for the nu(3) antisymmetric stretching vibration of N(2)O molecule in order to simulate the observed infrared spectra in the nu(3) region of N(2)O, especially to explain the frequency shift of the band origin in the infrared spectra. The harmonic oscillator approximation is used for the potential curve of the Q(3) mode of the isolate N(2)O molecule. The intermolecular potential energy surfaces are calculated for five potential-optimized discrete variable representation grid points of the Q(3) mode. The three-dimensional discrete variable representation method was employed to calculate the rovibrational states without separating the inter- and intramolecular nuclear motions. The calculated transition frequencies and line intensities of the rotational transitions in the nu(3) region of N(2)O for the van der Waals ground vibrational state are in good agreement with the observed infrared spectra. The calculated band shifts are found to be 0.1704 and 0.1551 cm(-1) for (4)He-N(2)O and (3)He-N(2)O, respectively, which agree well with the observed values of 0.2532 and 0.2170 cm(-1).     

The theoretical prediction of infrared spectra of trans- and cis-hydroxycarbene calculated using full dimensional ab initio potential energy and dipole moment surfaces

Accurate infrared spectra of the two hydroxycarbene isomers are computed by diagonalizing the Watson Hamiltonian including up to four mode couplings using full dimensional potential energy and dipole moment surfaces calculated at the CCSD(T)/cc-pVTZ (frozen core) and CCSD6-311G(**) (all electrons correlated) levels, respectively. Anharmonic corrections are found to be very important for these elusive higher-energy isomers of formaldehyde. Both the energy levels and intensities of stretching fundamentals and all overtone transitions are strongly affected by anharmonic couplings between the modes. The results for trans-HCOHHCOD are in excellent agreement with the recently reported IR spectra, which validates our predictions for the cis-isomers.