A finite-field method for the calculation of polarizabilities and hyperpolarizabilities is developed based on both a n energy expansion and a dipole moment expansion. This procedure is implemented in the MOPAC semiempirical program. Values and components of the dipole moment ( p ) , polarizability ( a ) , first hyperpolarizability (p), and second hyperpolarizability ( 7 ) are calculated as a n extension of the usual MOPAC run. Applications to benzene and substituted benzenes are shown as test cases utilizing both MNDO and AM1 Hamiltonians.

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Proton affinities (PA) were calculated for 60 compounds and deprotonation enthalpies (DPE) for 80 compounds by using the AMI semiempirical molecular orbital model. With few exceptions, the errors in the calculated DPEs and PAs, as well as in the calculated heats of formation of deprotonated and protonated species, are comparable with those in the calculated heats of formation of neutral precursors. Intramolecular hydrogen bonding in bifunctional bases is also effectively reproduced.The main problems involve anions in which the charge is largely concentrated on one atom and anions formed by the deprotonation of oximes. Systematic errors are indicated in the extension of alkyl chains by addition of methene groups and in substitution of amine and hydroxyl groups for methyl groups bonded to secondary or tertiary carbons in neutral molecules.Proton transfer reactions play a basic role in chemistry, in particular in biochemistry. A knowledge of the proton affinities (PA) of bases and of the deprotonation enthalpies (DPE) of acids is therefore essential. While major progress has been made in recent years in the development of experimental techniques for measuring PAs, these have necessarily been limited to the gas phase, and results are available only for a very limited number of ions and molecules.1 If PAs could be calculated theoretically with sufficient accuracy by some quantum chemical procedure, this would be of major value, because calculations, if feasible, can be carried out much more quickly and at much less cost than experiments, and they are, of course, not limited by the physical properties of the species being studied.

The chain extension step in the enzymatic synthesis of fatty acids by fatty acid synthase, involving a formal Claisen condensation of thio esters, has been clarified by theoretical calculations for model systems, using the modified neglect of diatomic overlap and Austin Model 1 parametric self-consistent field molecular orbital procedures. The reaction involves a free carbanion, formed by decarboxylation of a malonate ion. Formation of the carbanion and condensation with the fatty acid thio ester are not concerted. The decarboxylation is strongly endothermic. It is brought about by electrostatic interaction (field effect) with an ammonium ion derived from an adjacent lysine residue, the ions being far enough apart to inhibit proton transfer between them. Proton transfer would lead to an enol that is predicted not to be able to undergo the Claisen condensation. The formation of the ammonium ion is considered in terms of the pKa of the relevant groups. The bearing of this work on a recent interpretation of the activity and selectivity of enzyme reactions is discussed, and some misunderstandings concerning this interpretation are clarified.

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