Descripción: The resolution of molecular electric dipole polarizabilities into atomic contributions was studied in the context of methyl and ethyl derivatives, CH3-X and CH3-CH2-X. The reliability of the partitioning schemes allowing for adhoc quantum chemical tools was also tested. Results depicted a good degree of the transferability from molecule to molecule as shown by the evaluation of gross atomic isotropic contributions for carbon, hydrogen, and heteroatoms.

Descripción: Two additive schemes for resolving average molecular electric dipole polarizabilities into atomic contributions, based on the acceleration gauge for the electric dipole, are outlined. Extended calculations have been carried out for a few terms of the alkane series to test the reliability of the partition method. Gross atomic isotropic contributions evaluated for carbon, αAvC≈5.7a.u., and hydrogen, αAvH≈2.7a.u., are actually transferable from molecule to molecule, and can be used to predict fairly accurate average polarizabilities of higher homologous molecules in the alkane series. © 1998 American Institute of Physics.

Descripción: A procedure, based on a continuous transformation of the origin of the (quantum mechanical) current density that sets the diamagnetic contribution to zero (CTOCD-DZ) all over the molecular domain, is applied to determine shielding polarizabilities to first order in a perturbing electric field. In any calculations relying on the algebraic approximation, irrespective of size and quality of the (gaugeless) basis set employed, all the components of the CTOCD-DZ magnetic shielding polarizability are origin independent, and the constraints for charge and current conservation are exactly satisfied. The effects of a static uniform electric field on the nuclear magnetic resonance (NMR) shielding of H2O2, F2, H2C2, H2CO, NH3, HCN, and HNC molecules have been investigated within the CTOCD-DZ method, and compared with the conventional results evaluated via the same basis sets, and with theoretical results taken from the literature. © 2000 American Institute of Physics.

Descripción: Energetics, structural features, polarity, and melting transitions in water clusters containing up to eight molecules were studied using ab initio methods and empirical force field models. Our quantum approach was based on density functional theory performed at the generalized gradient approximation level. For the specific case of (H2O)6, we selected five conformers of similar energy with different geometries and dipolar moments. For these cases, the cyclic arrangement was found to be the only nonpolar aggregate. For (H2O)8, the most stable structures corresponded to nonpolar, cubic-like, D2d and S4 conformers. Higher energy aggregates exhibit a large spectrum in their polarities. The static polarizability was found to be proportional to the size of the aggregates and presents a weak dependence with the number of hydrogen bonds. In order to examine the influence of thermal fluctuations on the aggregates, we have performed a series of classical molecular dynamics experiments from low temperature up to the melting transition using two different effective pseudopotentials: the TIP4P and MCY models. Minimum energy structures for both classical potentials were found to reproduce reasonably well the results obtained using ab initio methods. Isomerization and phase transitions were monitored by following changes in dipole moments, number of hydrogen bonds and Lindemann's parameter. For (H2O)6 and (H2O)8, the melting transitions were found at Tm≈50 and 160 K, respectively; for both aggregates, we observed premelting transitions between well differentiated conformers as well. © 1999 American Institute of Physics.

Descripción: In the present work, we study the effect of translational-rotational hydrodynamic coupling on the stationary electric linear dichroism of DNA fragments. The theoretical resolution of the problem has, so far, been dealt with analytic methods valid only in the limit of low electric fields. In this work, we apply numerical methods that allow us to study the problem and also consider electric fields of arbitrary strength. We use the bent rod molecules model to describe DNA fragments with physical properties characterized by their electric charge, electric polarizability tensor, rotational diffusion tensor, and translation-rotation coupling diffusion tensor. The necessary orientational distribution function to calculate electric dichroism is obtained by solving the Fokker-Planck equation through the finite difference method. We analyze the different contributions due to electric polarizability and translational- rotational coupling to the electric dichroism. © 2011 American Institute of Physics.