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The integral equation formulation of the polarizable continuum model is extended to the calculation of solvent effects on vibrational Raman optical activity (ROA) spectra. Results indicate that solvent effects on the ROA differential scattering intensities are substantial, and vary in sign and magnitude for different vibrational modes.

The nuclear magnetic resonance parameters in porphyrin and porphycene are calculated to investigate their changes during the process of proton exchange, using density-functional theory for both the spin-spin coupling constants and the shielding constants. Both the isotropic shielding of the exchanged protons and the shielding anisotropy exhibit a minimum close to the transition states.

Research is done on the scalar spin--spin coupling constants, both intra- and intermolecular, which were evaluated for the models of biological systems using the MCSCF method. In conclusion, the changes in the shieldings of the protons engaged in the hydrogen bond formation and proton acceptors are observed to decrease fast with increase in the hydrogen bond length.

Presented here are the gas-phase measurements of the 13C and 1H magnetic shielding and spin-spin coupling coupling of 1,2-C13-enriched acetylene in the gas phase.

We report the implementation of nuclear magnetic resonance (NMR) shielding tensors within the four-component relativistic Kohn-Sham density functional theory including non-collinear spin magnetization and employing London atomic orbitals to ensure gauge origin independent results, together with a new and efficient scheme for assuring correct balance between the large and small components of a molecular four-component spinor in the presence of an external magnetic field (simple magnetic balance). To test our formalism we have carried out calculations of NMR shielding tensors for the HX series (X = F, Cl, Br, I, At), the Xe atom, and the Xe dimer. The advantage of simple magnetic balance scheme combined with the use of London atomic orbitals is the fast convergence of results (when compared with restricted kinetic balance) and elimination of linear dependencies in the basis set (when compared to unrestricted kinetic balance). The effect of including spin magnetization in the description of NMR shielding tensor has been found important for hydrogen atoms in heavy HX molecules, causing an increase of isotropic values of 10%, but negligible for heavy atoms. [ABSTRACT FROM AUTHOR]

The linear and second-order nonlinear susceptibilities of the urea crystal have been evaluated by applying the supermolecule approach. Calculations performed at the time-dependent Hartree–Fock (TDHF) level using the Austin model (AM1) semiempirical Hamiltonian have first demonstrated the almost additive character of the essential polarizability and first hyperpolarizability components. In fact, the only exception concerns the χcc(1) component when stacking urea molecules along the c axis, i.e., the axis of the hydrogen bonds. This behavior has been confirmed by ab initio calculations on small clusters. The macroscopic quantities have then been determined by adopting the multiplicative scheme and by correcting the TDHF/AM1 values for missing electron correlation by means of density functional theory and coupled cluster method. The reliability of the multiplicative scheme was demonstrated for clusters as large as 3a×3b×3c. While the electron correlation correction factors are similar for a single molecule and different small clusters, the global performance of the scheme differs for the linear and nonlinear responses. For the second-order nonlinear susceptibility, our predictions are in good agreement with experiment, while for the linear susceptibility and the associated refractive index, our predictions underestimate the experimental values. The limitations of our approach may be attributed to its inability to account for more subtle cooperative effects, like those associated with a network of hydrogen bonds. Together with other works, the supermolecule calculations confirm that the sign of χabc(2) is negative, contrary to an estimate from band structure calculation. [ABSTRACT FROM AUTHOR]

Electronic circular dichroism (ECD) parameters of the disulphide chromophore have been calculated for dihydrogen disulphide, dimethyl disulphide, and cystine using density-functional theory, coupled-cluster theory, and multiconfigurational self-consistent field theory. The objective is twofold: first, to examine the performance of the Coulomb-attenuated CAM-B3LYP functional for the calculation of ECD spectra; second, to investigate the dependence of the ECD parameters on the conformation around the disulphide bridge. The CAM-B3LYP functional improves considerably on the B3LYP functional, giving results comparable to CCSD theory and to MCSCF theory in an extended active space. The conformational dependence of the ECD parameters does not change much upon substitution, which is promising for the application of ECD in structural investigations of proteins containing disulphide bridges. [ABSTRACT FROM AUTHOR]

Second harmonic generation hyperpolarizabilities and their geometric derivatives have been calculated for HF, HCl, CO, and LiF, using the coupled cluster hierarchies, CCS, CC2, CCSD, CC3, and large correlation-consistent basis sets. The full configuration interaction results have been used to test the accuracy of the coupled cluster results. The CCS and CC2 methods do not improve on the Hartree-Fock results while CCSD is robust and gives significant improvements compared to CCS and CC2. The effects of triples in CC3 are in some cases substantial. Higher order correlation effects are significant for LiF. Including core-valence correlation effects is required only if high accuracy is desired. The coupled cluster results have been used as benchmarks for the results obtained by means of density functional theory using various exchange-correlation functionals. For the hyperpolarizability B3LYP was found to perform best, i.e., to give the results closest to the CC3 ones, while for the geometric derivatives none of the considered functionals was able to give a consistent description for all the considered molecules. [ABSTRACT FROM AUTHOR]

We present an implementation of the polarizable continuum model for the calculation of solvent effects on electronic circular dichroism spectra. The computational model used is density functional theory in the length-gauge formulation, and gauge-origin independence is ensured through the use of London atomic orbitals. Results of calculations carried out for methyloxirane and bicyclic ketones, camphor, norcamphor, norbornenone, and fenchone are presented, and the theoretically obtained solvent effects are compared with experimental observations. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Second derivatives of nuclear shielding constants with respect to an electric field, i.e., shielding polarizabilities, have been calculated for the noble gas atoms from helium to xenon. The calculations have been carried out using the four-component relativistic Hartree-Fock method. In order to assess the importance of the individual relativistic corrections, the shielding polarizabilities have also been calculated at the nonrelativistic Hartree-Fock level, with spin–orbit and scalar (Darwin and mass-velocity) effects having been established by perturbative methods. Electron correlation effects have been estimated using the second-order polarization propagator approach. The relativistic effects on the tensor components of the shielding polarizabilities are found to be larger and changing less regularly with the atomic number than for the shielding constant itself. However, there is a partial cancellation of the contributions to the parallel and perpendicular components of the shielding polarizability and as a consequence the mean shielding polarizability is far less affected than the individual components. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structural, energetic, and spectroscopic properties of the dihydrogen-bonded complexes LiH...H[sub 2], LiH...CH[sub 4], LiH...C[sub 2]H[sub 6], and LiH...C[sub 2]H[sub 2] are investigated. In particular, the interaction energy is decomposed into physically meaningful contributions, and the calculated vibrational frequencies, the magnetic resonance shielding constants, and inter- and intramolecular spin–spin coupling constants are analyzed in terms of their correlation with the interaction energy. Unlike the other three complexes, which can be classified as weak van der Waals complexes, the LiH...C[sub 2]H[sub 2] complex resembles a conventional hydrogen-bonded system. The complexation-induced changes in the vibrational frequencies and in the magnetic resonance shielding constants correlate with the interaction energy, as does the reduced coupling [sup 2h]J[sub HX] between the proton of LiH and hydrogen or carbon nucleus of the proton donor, while [sup 1h]J[sub HH] do not correlate with the interaction energy. The calculations have been carried out using Møller–Plesset perturbation theory, coupled-cluster theory, and density-functional theory. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The magnetically induced axial birefringence of six closed-shell chiral molecules (methyloxirane, C[SUB3]H[SUB6]O, fluoro- and methylcyclopropanone, C[SUB3]H[SUB3]OF and C[SUB4]H[SUB6]O, carvone, C[SUB10]H[SUB14]O, limonene, C[SUB10]H[SUB16], and proline, C[SUB5]H[SUB9]NO[SUB2]) is determined at the Hartree-Fock wave-function level by evaluating the frequency dependent quadratic response functions entering the molecular property expression, according to Barron and Vrbancich [Mol. Phys. 51, 715 (1984)]. Both the magnetic dipole and the electric quadrupole contributions are taken into account and their relative importance is discussed. A proof of the origin independence of the magnetochiral birefringence is presented for the exact wave function and the dependence on the origin is investigated in finite basis set calculations at the Hartree-Fock level. For carvone, limonene, and proline the results are compared with recent experimental data obtained by two different experimental groups, which are in disagreement with respect to the magnitude of the magnetochiral effect. A parallel study of the natural optical rotation shows that in the three larger molecules the optical rotatory strengths are strongly affected by changes in conformations. Nonetheless the magnetochiral birefringence computed for various different conformers--although varying remarkably--is much smaller in absolute value than experimentally observed. The disagreement--of more than three orders of magnitude--between some experimental data and theory appears to be hard to reconcile and to attribute entirely to limitations in the computational approach. [ABSTRACT FROM AUTHOR]

The linear response coupled cluster singles and doubles (CCSD) method has been used to evaluate electric dipole polarizabilities, their geometric derivatives and Q-branch Raman scattering cross sections for five diatomic molecules: N[sub 2], CO, HF, HCl, and Cl[sub 2]. A systematic study of basis set convergence has been carried out. Geometric derivatives of the frequency dispersion coefficients have been computed. This allowed to evaluate Raman intensities for arbitrary laser frequencies outside the resonance region. The geometric derivatives of polarizabilities, and therefore Raman intensities, exhibit stronger basis set dependence and dispersion effects than the polarizabilities themselves. Whereas the calculated polarizabilities are very close to experiment, the agreement of their geometric derivatives and of the Raman scattering cross sections with experiment is less excellent. We attribute this mainly to the limitations of Placzek’s theory (for Raman intensities) and to the low precision of the reference experimental data. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The Hartree Fock and density-functional theory with the Becke three-parameter Lee-Yang-Parr (B3LYP) functional employing analytical response theory the natural optical activity of two chiral amino acids, alanine and proline are calculated. Investigations are done of the dependence of the optical activity on the molecular conformation in the gas phase.

Potential energy surface of the simplest carbohydrate, glycolaldehyde, is investigated at the second-order Moller-Plesset (MP2) level of theory, and the rotation barriers between them are calculated. The resulting expressions may help to determine the confirmation of carbohydrates and their derivatives from the spin-spin coupling measurements.

The results of the linear response SCF calculations of Raman and vibrational Raman optical activity (VROA) spectra for D-lactic acid (two conformers), D-lactate, and D-glyceraldehyde (six conformers) are presented. The VROA parameter of the stretching vibration of the carbonyl group is shown to be a promising parameter for structural investigations as it changes sign when a short internal hydrogen bond OH...OC is formed.