Svein Saebo

An efficient reformulation of the closed‐shell self‐consistent electron pair theory

The closed‐shell SCEP method is reformulated in terms of pairwise nonorthogonal configuration state functions with particle‐hole singlet coupling. The use of a single type of external pair function instead of the usual singlet and triplet coupled pairs results in the elimination of the internal coupling coefficients in the residual vector formulas, simplifying the latter considerably. The new CI coefficients are represented as unsymmetrical quadratic matrices which are naturally adapted to the description of long‐range correlation between spatially distant localized orbitals. The number of matrix multiplications required for the interorbital pair coupling terms is reduced by half. This should lead to a reduction in the total computational effort by about 30%, except for very small molecules where intraorbital correlation dominates the computation.

The weakly exothermic rearrangement of methoxy radical (CH3O⋅) to the hydroxymethyl radical (CH2OH⋅)

Although the CH3O⋅ and CH2OH⋅ radicals have long been considered critical intermediates in combustion and atmospheric processes, only very recently has the potential significance of the isomerization CH3O⋅→CH2OH⋅ been appreciated. This isomerization and related aspects of the CH3O⋅/CH2OH⋅ potential surface have been studied here using nonempirical molecular electronic structure theory with moderately large basis sets and with incorporation of electron correlation. The vibrational frequencies of CH3O⋅, CH2OH⋅ and seven other stationary points on the potential energy hypersurface have been predicted, both to compare with results from spectroscopy and to provide estimates of zero‐point vibrational corrections. In general, there is reasonable agreement with those vibrational frequencies of CH3O⋅ and CH2OH⋅ which are known from experiment. Our ab initio calculations predict that CH3O⋅ lies 5.0 kcal mol−1 higher in energy than CH2OH⋅ with a barrier to rearrangement to CH2OH⋅ of 36.0 kcal mol−1. Rearrangement of...

The nature of the C…C ring-opened form of the ethylene oxide radical cation

CAS SCF and Moller-Plesset perturbation calculations with polarized basis sets have been used to investigate the nature of the C…C ring-opened isomer of the ethylene oxide radical cation. The best calculations predict a planar C 2V structure for CH 2 OCH 2 + , lying 82 kJ mol −1 below the ring-closed ethylene oxide radical cation. However, the possibility of a slightly distorted structure, lying in a very shallow potential well, cannot be completely ruled out.

The structure of vinylamine

Abstract Theoretical structures for vinylamine have been obtained by ab initio molecular orbital calculations, using the minimal STO—3G, split-valence 3—21G, and split-valence plus d-polarization 6—31G* basis sets. The theoretical structures were corrected for systematic deficiencies in the basis sets and by use of experimental rotational constants to predict a complete r0 structure for vinylamine. Vinylamine is predicted to be non-planar with a pyramidal amino group.

The structure of 1,4-dithiin

Abstract Ab initio calculations with the STO-3G and 3-21G(*) basis sets have been used to determine optimized boat and planar structures for 1,4-dithiin. Improved relative energies have been obtained with the 6-31G* basis set. The predicted equilibrium structure for 1,4-dithiin is a boat, with an angle between the two SCCS planes of 136.6°. The calculated barrier to ring flapping is 8 kJ mol−1.

The structure of aminopropenenitrile (CH2C(NH2)CN)

Abstract Ab initio molecular orbital calculations with minimal STO—3G, split-valence 3-21G, and split-valence plus d -polarization 6-31G* basis sets have been used to obtain optimized structures for aminopropenenitrile. Systematic deficiencies in the geometry predictions with these basis sets have been taken into account with the aid of calculations on the smaller systems, vinylamine and propenenitrile. A complete r o structure for aminopropenenitrile is predicted which yields rotational constants in good agreement with experimental values. Aminopropenenitrile is found to be non-planar with a pyramidal amino group.