The linear polarizability, a, the first hyperpolarizability, ß, and the second hyperpolarizability, y, of three different classes of basic heterocyclic organic structures have been calculated by the ab inio time-dependent coupled Hartree-Fock-Roothaan method using a 4-31G basis set augmented by diffuse p and d functions.For the a and y coefficients, the calculated orders thiophene > pyrrole > furan for the five-membered rings; pyridine > pyrazole > s-triazine for the aromatic bases; and benzothiazole > oxazole for the fused ring systems show a qualitative relationship with the corresponding order in Ae(HOMO-LUMO) of the respective group of molecules. The ß values do not appear to show a discernible trend. The dispersion of (-; ) and those of y(-;0,0, ), (-\ , ,-), y(-2< ;0, , ), and y(-3 ; , , ) in the case of the five-membered rings are also reported.

No abstract

No abstract

The dipole moment, µ, linear polarizability, a, first hyperpolarizability, ß, and second hyperpolarizability, 7, of intramolecular charge transfer (ICT) molecules derived from the alkyl substitution of 4-amino-j8-nitrostyrene (ANST) have been calculated by ab initio self-consistent-field method using a double-zeta valence Gaussian basis set. The calculated results for , ß, and 7 show strong dependence on µ and the energy difference At between the lowest unoccupied and highest occupied molecular orbitals. For the (dimethylamino)-/3-nitrostyrene and (diethylamino)-/3-nitrostyrene, the calculated results for ß(-2 \ , ) are in qualitative agreement with the experimental values. The changes in the values of 7 for various third-order effects among the various alkylsubstituted ANST are parallel to the corresponding changes in the values of ß. The results indicate that the mechanism which leads to the enhancement of ß is also responsible for the enhancement of 7 values of ICT molecules and that these molecules may provide an alternative to conjugated polymers as a third-order nonlinear optical material.

The minimum-energy structures and bonding properties of the hydrogenated lithium clusters, Li4H2 and Li7H, have been investigated by means of an ab initio Monte Carlo simulated annealing method. The minimum-energy structures of Li,H2 and Li7H are found to resemble those of the triangular planar (D3h) isomer of Li6 and Td isomer of Lig clusters, respectively.