The effect of including polarization functions on the geometrical parameters calculated for pyridine.

Geometrical parameters for pyridine have been calculated using the 6-31G, 6-31G* (5D), 6-31G** (6D), and 6-31G(2 × 6D) basis sets. Comparisons are made with a microwave substitution structure and with results of other ab initio calculations reported in the literature. Particular attention is paid to the influence of polarization functions on the magnitude of the ring angle, 〈C6 N1 C2 , which is analogous to the ipso angle in monosubstituted benzene derivatives.

Interactions of Metal Ions with Water: Ab Initio Molecular Orbital Studies of Structure, Bonding Enthalpies, Vibrational Frequencies and Charge Distributions. 1. Monohydrates.

The formation and properties of a wide range of metal ion monohydrates, M(n)()(+)-OH(2), where n = 1 and 2, have been studied by ab initio molecular orbital calculations at the MP2(FULL)/6-311++G//MP2(FULL)/6-311++G and CCSD(T)(FULL)/6-311++G//MP2(FULL)/6-311++G computational levels. The ions M are from groups 1A, 2A, 3A, and 4A in the second, third, and fourth periods of the periodic table and the first transition series. Structural parameters, vibrational frequencies, bonding enthalpies, orbital occupancies and energies, and atomic charge distributions are reported. Trends in these properties are correlated with the progressive occupancy of the s, p, and d orbitals. Except for K(+)-OH(2) and Ca(2+)-OH(2), the O-H bond lengths and HOH angles are greater in the hydrates than in unbound water. The M-O bond lengths decrease proceeding from group 1A --> 4A but become larger in proceeding from the second --> fourth period. The bonding enthalpies, are found to be inversely linearly dependent on the M-O bond length M(n)()(+) according to equations of the form = A + B(1/M-O) for n = 1 and n = 2. Within each monohydrate the distribution of atomic charge reveals a small but definite transfer of charge from water to the metal ion. Compared to unbound water there is, in a metal-ion-bound water complex, an increase in the electronic (negative) charge on the oxygen atom, accompanied by a (significantly) larger decrease in the electronic charge on the hydrogen atoms. The bonding of the water molecule, although electrostatic in origin, is thus more complex than a simple interaction between a point charge on the metal ion, and the water dipole.

Binding of the two-valence-electron metal ions Sc(+), Ti(2+), and V(3+) to the second-row hydrides BeH(2), BH(3), NH(3), OH(2), and FH: a computational study.

We report results from a computational study of the binding in complexes formed from one of the transition-metal ions Sc(+), Ti(2+), or V(3+), each of which has two valence electrons outside an argon core, and one of the second-row hydrides FH, OH(2), NH(3), BH(3), or BeH(2). The complexes that involve the electron-rich ligands FH, OH(2), and NH(3) have strong ion-dipole components to their binding. There are large stabilization energies for sigma-interactions that transfer charge from occupied lone-pair natural bond orbitals on the F, O, or N atom of the (idealized) Lewis structure into empty non-Lewis orbitals on the metal ions; these interactions effectively increase electron density in the bonding region between the metal ion and liganded atom, and the metal ions in these complexes act in the capacity of Lewis acids. The complexes formed from the electron-poor hydrides BH(3) and BeH(2) consistently incorporate bridging hydrogen atoms to support binding, and there are large stabilization energies for interactions that transfer charge from the Be-H or B-H bonds into the region between the metal ion and liganded atom. The metal ions in Sc(+)-BeH(2), Ti(2+)-BeH(2), Ti(2+)-BH(3), and V(3+)-BH(3) act in the capacity of Lewis acids, whereas the scandium ion in Sc(+)-BH(3) acts as a Lewis base.

Soft computing in the design of nontoxic chemicals.

This research focuses on the use of soft computing to aid in the development of novel, state-of-the-art, nontoxic dyes which are of commercial importance to the U.S. textile industry. Where appropriate, modern molecular orbital (MO) and density functional (DF) techniques are employed to establish the necessary databases of molecular properties to be used in conjunction with the neural network approach. In this research, we focused on the following: (1) using molecular modeling to establish databases of various molecular properties of azo dyes required as input for our neural network approach; (2) designing and implementing a neural network architecture suitable to process these databases; and (3) investigating combinations of molecular descriptors needed to predict various properties of the azo dyes.