Jimmy W. Viers

Comments on steric effects in buttressed methyl pyridine species

Abstract Using MINDO/3 calculations, we have investigated the importance of gear effects and pointing H-H interactions in several polymethylpyridine systems. Gear clashing of methyl groups on adjacent ring positions was found to produce about 0.6 kcal mole -1 stabilization, while interaction between two pointing hydrogens destabilizes the molecule by about 1.1 kcal mo1e -1 . Calculated barriers for rotation of the isopropyl group in several 1-isopropyl-poly methyl pyridiniun cations were in good agreement with experimental results published recently by Roussel. The calculations indicate that the decrease in the rotational barrier upon additional buttressing of methyl groups can be attributed to destabilization of the ground rotational conformer for the more sterically hindered 1-isopropyl polymethyl pyridinium species.

A Full Variational Calculation Based on a Tensor ProductDecomposition

A new direct full variational approach exploits a tensor (Kronecker) product decomposition of the Hamiltonian. Explicit assembly and storage of the Hamiltonian matrix is avoided by using the Kronecker product structure to form matrix-vector products directly from the molecular integrals. Computation-intensive integral transformations and formula tapes are unnecessary. The wavefunction is expanded in terms of spin-free primitive kets rather than Staler determinants of configuration state functions, and the expansion is equivalent to a full configuration interaction expansion. The approach suggests compact storage schemes and algorithms which are naturally suited to parallel and pipelined machines.