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The Raman and hyper-Raman spectra of acetonitrile and its deuterated analog have been investigated by combining experimental analysis and theoretical interpretation. It has been observed that the Raman spectra can easily be reproduced at both the Hartree-Fock and Mo\ller-Plesset second-order levels of approximation and that for these fundamental transitions, inclusion of anharmonicity effects is not essential. On the other hand, the hyper-Raman spectra are more difficult to simulate and interpret. In particular, electron correlation has to be included in order to describe properly the intensity of the CN stretching mode. Then, a pseudo-C∞v symmetry was assumed to better fit the experimental observations. This accounts for the fact that the a1- and e-symmetry modes correspond to time-decoupled vibrations. The e-symmetry modes, associated with nuclear motions perpendicular to the molecular axis are indeed subject to relaxation processes and, except the CCN bending mode, not visible in the hyper-Raman spectra of acetonitrile or of its deuterated analog. This assumption is supported by the gradual decrease of the phenomenon when going from acetonitrile to trichloroacetonitrile, where the presence of the heavier chlorine atoms in the latter reduces the relaxation processes. [ABSTRACT FROM AUTHOR]

Using theoretical simulations based on Hartree–Fock and density-functional theory calculations, the simulated vibrational Raman optical activity spectra of helical conformers of heptasilane are shown to present signatures sensitive to the helicity. These signatures are associated with collective wagging, twisting, and rocking motions. These simulated spectra have been obtained by combining analytical and numerical differentiation procedures to evaluate the geometry derivatives of the optical tensors entering into the expressions of the vibrational Raman optical activity intensities. From an investigation of basis set and electron correlation effects, it is shown that, like for local vibrations, diffuse functions are compulsory for evaluating the vibrational Raman optical activity intensities of collective vibrational motions. [ABSTRACT FROM AUTHOR]

Hyper-Raman spectra of pure carbon tetrachloride in the liquid phase are recorded for different combinations of the polarizations of the incident and scattered lights and are compared to ab initio time-dependent Hartree-Fock simulations. Both the calculated intensities of the Raman and hyper-Raman spectra give indeed a quite satisfactory agreement with polarized experimental spectra. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The second-order nonlinear optical coefficient of polyphosphazene oligomers of increasing size has been determined by using ab initio methods taking into account electron correlation and frequency dispersion effects. The calculated first hyperpolarizability per unit cell converges rapidly with respect to chain length. It attains an amplitude of about one-third of the one of classical push-pull systems. This amplitude can be strongly increased by replacing the nitrogen of the backbone by silicon. The effects of the side groups (H, CH3, F, Cl, Br, and OH) on the first hyperpolarizability have been investigated as well. The different results have been rationalized in terms of alternations of bond lengths and atomic charges. [ABSTRACT FROM AUTHOR]

Two analytical procedures based on the time-dependent Hartree-Fock (TDHF) scheme are elaborated to evaluate the first derivatives of the dynamic first hyperpolarizability with respect to atomic Cartesian coordinates. In the first scheme, the mixed third-order TDHF equations are solved iteratively to obtain the third derivatives of the linear combination of atomic orbitals coefficients twice with respect to external dynamic electric fields and once with respect to atomic Cartesian coordinates. The second takes advantage of the 2n + 1 rule and interchange relations to express the first derivatives of the dynamic first hyperpolarizability with respect to atomic Cartesian coordinates in terms of lower-order derivatives. Both procedures have been implemented in the GAMESS quantum chemistry package. Applications focus on the characterization of the frequency dispersion of the first derivatives of the first hyperpolarizability with respect to vibrational normal coordinates of H[sub 2]O, NH[sub 3], and CH[sub 4] as well as on the determination of nonresonant hyper-Raman intensities within the double harmonic oscillator approximation. [ABSTRACT FROM AUTHOR]

Investigates the vibrational hyperpolarizability of methane-[sub 4-n]fluorine [sub n] molecules. Implication of electronic and vibrational effects for nonlinear optical responses; Application of the Sadlej atomic basis set; Analysis of the potential energy surface of carbon-hydrogen and carbon-fluorine bonds.

Hyper-Raman spectra of several small systems have been simulated at the time-dependent density functional theory level of approximation and compared with their time-dependent Hartree–Fock analogues to address the effects of electron correlation. A numerical finite distortion scheme has been used to evaluate the first order-derivative of the dynamic first hyperpolarizability with respect to vibrational normal mode coordinates. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]

Analytical procedures based on the time-dependent Hartree-Fock (TDHF) scheme are elaborated to evaluate the frequency-dependent electric dipole-electric quadrupole polarizability and its derivatives with respect to atomic Cartesian coordinates. On one hand, the mixed second-order TDHF equations are solved iteratively to obtain the second-order derivatives of the linear combination of atomic orbitals coefficients, once with respect to atomic Cartesian coordinates and once with respect to external dynamic electric fields or electric field gradients. On the other hand, taking advantage of the 2n + 1 rule, the first-order derivatives of A are expressed with respect to atomic Cartesian coordinates in terms of lower-order derivatives. These procedures have been implemented in the GAMESS quantum chemistry package and have been illustrated in the case of several small molecules as well as adamantane.