Most carbamates are pseudoirreversible inhibitors of cholinesterases. Phenothiazine carbamates exhibit this inhibition of acetylcholinesterase but produce reversible inhibition of butyrylcholinesterase, suggesting that they do not form a covalent bond with the catalytic serine. This atypical inhibition is attributable to pi-pi interaction of the phenothiazine moiety with F329 and Y332 in butyrylcholinesterase. These residues are in a helical segment, referred to here as the E-helix because it contains E325 of the catalytic triad. The involvement of the E-helix in phenothiazine carbamate reversible inhibition of butyrylcholinesterase is confirmed using mutants of this enzyme at A328, F329, or Y332 that show typical pseudoirreversible inhibition. Thus, in addition to various domains of the butyrylcholinesterase active site gorge, such as the peripheral anionic site and the pi-cationic site of the Omega-loop, the E-helix represents a domain that could be exploited for development of specific inhibitors to treat dementias.

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Ab initio calculations employing the configuration interaction method have been applied to the recombination of methyl radicals. The methyl radicals have been restricted to a planar equilateral triangular geometry, but the carbon-carbon distance and five angles defining the mutual orientation of the radicals have been varied. Attractions between the radicals appear to be dominated by bonding between the singly occupied orbitals on the two radicals, although ir bonding also appears to contribute in some orientations. At short distances, repulsive forces between atoms on adjacent radicals are important.All these interactions have been represented by simple expressions, which have been combined to represent the potential energy as a function of the six coordinates varied. Eight parameters in the potential energy expression have been adjusted by least squares, fitting the ab initio points to within 1.4 kj mol'1 11. Canonical variation transition-state theory calculations have been performed on the resulting potential energy surfaces. A Monte Carlo method was used to integrate the Boltzmann factor over the five angles. The critical distance or bottleneck was found to decrease from a carbon-carbon distance of 4.0 Á at 200 K to 2.7 Á at 2000 K. The rate constant is 5 X 1010 L mol'1 s'1 at 300 K and decreases with increasing temperature.Recent experimental results parallel the theoretical ones but are 15-30% slower.

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Extensive configuration-interaction calculations with double-zeta plus polarization and near triple-zeta plus polarization basis sets are used to analyze the Jahn–Teller (JT) effect in the methane radical cation. Energy component analysis shows that the Jahn–Teller effect leads to a decrease in the expectation value of the electron-nuclear attraction energy, an increase in the expectation value of the interelectronic repulsion energy, and an increase in the internuclear repulsion energy. These observations are consistent with a contraction of the electron cloud. The dominant factor in the −0.0550 hartree Jahn–Teller distortion (Td→C2v) in CH+4 is the −0.5262 hartree change in the electron–nuclear attraction energy. The differences in all energy components are large in relation to the JT distortion. Interelectronic repulsion plays a dominant role in determining the relative energies of the possible JT distorted structures, but electron correlation effects are relatively unimportant.

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Configuration interaction calculations have been carried out for low-lying singlet and triplet states of ('•3 +), BH(UII), (1 11 ,3 "), and FH(UII), with the aim of evaluating singlet-triplet energy component differences. These calculations represent the first systematic study of the quantum mechanical interpretation of Hund's rule in a series of molecules. While early molecular work on Hund's rule suggested that the electron-nuclear attraction may always be deeper in the triplet than in the singlet, exceptions are found in the present work. Also, in certain cases, where configuration mixing and molecular charge separation play a key role, the singlet electron-electron repulsion exceeds that of the triplet. It is also observed that changes in electron-electron repulsion tend to be in the same direction as changes in kinetic energy, whereas changes in electron-nuclear attraction are of the opposite sign, for a given geometry.