Configuration energies (CE) of the d-block elements (Groups 3−11) are electronegativities evaluated from the formula CE = (pεs + qεd)/(p + q). εs and εd are the multiplet-averaged one-electron energies of the s- and d-orbitals of atoms which are in the lowest energy of the configurations s n d m and s n -1d m +1, and whose highest known oxidation state is (p + q). The orbital energies are obtained from spectroscopic data. Configuration energies generally increase across a row, with the highest values occurring at nickel, silver, and gold; all are lower than the CE of silicon, the least electronegative nonmetal (except for gold which has a CE equal to that of silicon). Down the groups configuration energies invariably decrease from the first row to the second row; for Groups 7−12, the third-row element has a CE higher than that of the second-row element, due to increasing relativistic stabilization of the 6s orbitals.

A selective classified bibliography of symbolic computation in some areas of chemistry is provided together with some examples of computer algebra algorithms and techniques to facilitate future joint work of chemists and computer scientists.

It is shown that the Hohenberg–Kohn–Levy density functional theory of molecular structure is not restricted by the Born–Oppenheimer approximation. The existence of the corresponding ground-state density functionals for the case of the exact nonadiabatic, nonrelativistic, field-free Hamiltonian of a molecular system, in terms of the one-particle electronic and nuclear densities, is proven and the associated Euler equations are discussed. Extensions to the case of the system in an external electric field and to the bound excited states, are briefly examined. As an example the non-Born–Oppenheimer Hartree–Fock theory of Thomas is discussed from the density functional viewpoint. Possible applications of the theory in the analysis of molecular structure and chemical reactivity are identified.

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The Challenger mechanism for the methylation of arsenic is a repeating sequence of a two-electron reduction of pentavalent arsenic As(V) species to trivalent arsenic As(III) species followed by a methylation-oxidation reaction forming the successive methyl As(V) species. This unusual oxidation-reduction sequence prompted an examination of the thermodynamics of these reactions. Quantum chemical methods are employed to estimate the thermodynamic parameters for the methyl arsenic species. The sequence is thermodynamically favored at neutral pH for redox potentials with pe < 0 and methyl cation activities pCH3+ < -3 to -7 depending on the precise situation analyzed. The observed distribution of methyl arsenic species in human urine, which is remarkably constant across many studied populations, can be reproduced using an equilibrium model if the formation of TMA species is prevented. The estimated thermodynamic parameters are sufficiently accurate to evaluate questions of thermodynamic plausibility but not the precise details of speciation.

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ABSTRACT:The reaction profile of the Bergman cyclization of enediynes is written as a cubic polynomial expansion about the critical distance. Using the "simple sewing" approximation, a relationship is derived that expresses the energy of activation as a function of the critical distance, with the effect of other geometric and electronic factors accounted for in the expansion coefficients. A training set of 10 representative enediynes were selected and input parameters were calculated by density functional theory at the B3LYP/6-311G* level of theory. All calculations were corrected for zero-point energy. All singlet biradical calculations were also checked for wave function stability. Calibration curves were then constructed that correlate the activation energy of cyclization of the molecules in the test set with the critical distance parameter.