Larry P. Davis

The structure and stability of neutral pentacoordinated silicon compounds

Abstract Ab initio and semi-empirical wavefunctions are used to examine the stability of neutral pentacoordinated compounds with the formula H 3 NSiXY 3 , where X and Y are the axial and equatorial ligands, respectively, and both ligands can be H, F, Cl. It is found from both levels of theory that a relatively weakly bound ligand (e.g. Cl) in the axial position facilitates complexation of the Si with NH 3 due to a more balanced three-center bond. Very electronegative ligands (e.g. F) are also stabilizing, since they increase the positive charge on Si.

Thermochemical decomposition of explosives. I. TNT kinetic parameters determined from ESR investigations

Abstract Electron spin resonance techniques have been applied to the thermochemical decomposition kinetics of 2,4,6-trinitrotoluene in the pure liquid state. Direct observation of intermediate and product species in the reaction mixture allowed determination of activation energies which agreed well with previously-determined values utilizing other methods. The autocatalytic nature and complexity of the reaction were demonstrated.

MINDO/3, MNDO and AM1 calculations for nitro compounds

Abstract Although the MINDO/3, MNDO and AM1 molecular orbital programs accurately predict physical properties for a wide variety of classes of chemical compounds, their ability to estimate the physcial properties of nitro-compounds has not been rigorously tested. This paper compares MINDO/3, MNDO and AM1 calculations to each other and to available experimental data for 105 nitro-compounds – both aliphatic and aromatic. Properties evaluated include heats of formation, dipole moments, ionization potentials and molecular geometries. In general MINDO/3 predicts heats of formation, dipole moments and ionization potentials more accurately than MNDO and AM1. All three semi-empirical methods accurately predict molecular geometries.

Decomposition pathways leading to HONO for nitroalkenes

Abstract The reaction profiles for the dissociation of HONO from 1-nitro-2-methyl-1-propene have been examined for two distinct reaction pathways: a. The unimolecular dissociation of HONO from the intermediate CH2=C(CH3)CH=NOOH which in turn is formed by a unimolecular transfer of a hydrogen atom from the methyl group located cis to the nitro group in the parent molecule. b. The unimolecular dissociation of HONO from the intermediate formed by a bimolecular transfer of a hydrogen atom between two parent molecules. The overall activation enthalpies for the two processes are 62 and 50 kcal mol−1 respectively. The electronic nature of the transition states involved are discussed.

Theoretical studies of hypervalent silicic acid compounds

Abstract Hypervalent silicon compounds play a vital role in polymerization and sol-gel processes. Ab initio all electron calculations are performed to characterize the nature of the species involved in the reaction Si(OH) 4 +[ OH ] − → [ Si(OH) 5 ] − → [ Si(OH) 5 ] −‡ →[ Si(OH) 3 O ] − + H 2 O . Since the hydration of the reactants may aid in the polymerization process, we have determined both minima and transition state structures for the complexes between Si(OH) 4 and H 2 O. An effective drying agent in the sol-gel process involves the formation of a complex between Si(OH) 4 and HF and we have performed ab initio calculations on this system. Finally, the energetics and bonding between Si(OH) 4 and NH 3 is examined.

A Theoretical Study of Silanol Polymerization

We have applied state-of-the-art semi-empirical molecular orbital methods to a study of the anionic polymerization of silanols to form silica. In particular, we have considered nucleophilic attack on silanols and subsequent reactions of the products. Hydroxide addition proceeds without activation to form five-coordinate silicate anions. Five-coordinate structures can also be formed by oligomerization following the attack of hydroxide on neutral silanols to abstract a proton. These five-coordinate structures are predicted to play a key role as intermediates in the polymerization process. Water can be eliminated from these anions, but with a substantial activation barrier. The activation barrier appears to be lower for the larger, more complex systems. These predictions are consistent with a rapid pre-equilibrium to form dimer anions followed by the slower reaction to form higher oligomers.