The 6-31 G* and 6-31 G•• basis sets previously introduced for first-row atoms have been extended through the second-row of the periodic table. Equilibrium geometries for one-heavy-atom hydrides calculated for the twobasis sets and using Hartree-Fock wave functions are in good agreement both with each other and with the experimental data. HF/6-31G• structures, obtained for two-heavy-atom hydrides and for a variety of hypervalent second-row molecules, are also in excellent accord with experimental equilibrium geometries. No large deviations between calculated and experimental single bond lengths have been noted, in contrast to previous work on analogous first-row compounds, where limiting Hartree-Fock distances were in error by up to a tenth of an angstrom. Equilibrium geometries calculated at the HF /6-31 G level are consistently in better agreement with the experimental data than are those previously obtained using the simple split-valance 3-21G basis set for both normal-and hypervalent compounds. Normal-mode vibrational frequencies derived from 6-31G• level calculations are consistently larger than the corresponding experimental values, typically by \0%-15%; they are of much more uniform quality than those obtained from the 3-21G basis set. Hydrogenation energies calculated for normal-and hypervalent compounds are in moderate accord with experimental data, although in some instances large errors appear. Calculated energies relating to the stabilities of single and multiple bonds are in much better accord with the experimental energy differences.

A new algorithm for fitting atomic charges to molecular electrostatic potentials is presented. This method is non-iterative and rapid compared to previous work. Results from a variety of gaussian basis sets, including STO-3G, 3-21G and 6-31G*, are presented. Charges for a representative collection of molecules, comprising both first and second row atoms and anions are tabulated. The effects of using experimental and optimized geometries are explored. Charges derived from these fits are found to adequately reproduce SCF dipole moments. A small split valence representation, 3-21G, appears to yield consistently good results in a reasonable amount of time.

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The present work examines the conditioning of the least-squares matrix for obtaining potential derived charges and presents a modification of the CHELP method for fitting atomic charges to electrostatic potentials. Results from singular value decompositions (SVDs) of the least-squares matrices show that, in general, the least-squares matrix for this fitting problem will be rank deficient. Thus, statistically valid charges cannot be assigned to all the atoms in a given molecule. We find also that, contrary to popular notions, increasing the point density of the fit has little or no influence on the rank of the problem. Improvement in the rank can best be achieved by selecting points closer to the molecular surface. Basis set has, as expected, no effect on the number of charges that can be assigned. Finally, a well-defined, computationally efficient algorithm (CHELP-SVD) is presented for determining the rank of the least-squares matrix in potential-derived charge fitting schemes, selecting the appropriate subset of atoms to which charges can be assigned based on that rank estimate, and then refitting the selected set of charges. 0 1996 by John Wiley & Sons, Inc.

The 19 F 19 F nuclear spin-spin coupling constants J FF for a set of eighteen compounds related structurally to 1,8-difluoronaphthalene were measured by 19 F NMR spectroscopy. The FF distances d FF in these compounds were determined by ab initio 3-21G* molecular orbital calculations. Consistent with the lone-pair overlap theory of the origins of through-space 19 F 19 F coupling, an exponential relationship is found between J FF and d FF (regression coefficient r 2 ) 0.991), and a linear relationship is found between J FF and the extent of the overlap interaction between the in-plane fluorine 2p lone-pair orbitals (regression coefficient r 2 ) 0.993). The magnitudes of these lone-pair interactions were estimated from molecular orbital energies obtained by ab initio 6-31G* calculations for a model consisting of a pair of HF molecules separated by various distances.

Hartree-Fock perturbation theory is used to derive an expression for the correction to the molecular electrostatic potential due to the polarization of the target charge distribution. HF/STO-3G, HF/3-21G, and HF/6-31G* wave functions are used; the dependence of the correction on basis set is examined. Variations in the polarization potential with type of reaction site (electrophilic or nucleophilic) are investigated. The utility of the corrected molecular electrostatic potentials for predicting sites of nucleophilic attack, an application for which uncorrected potentials have not been generally useful, is explored.

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