ALBERT

Notes: Protonation/deprotonation reactions are represented by H++B(arrow-right-and-left)HB+. The ionization potential of H (13.6 eV) is higher than that of B for organic and most inorganic molecules (it is 10.166 eV for NH3), and the separated pair H+B+ will be lower in energy than the closed-shell pair H++B. The reaction path involves, therefore, an avoided crossing, and its theoretical study requires multideterminant methods. The reaction with B=NH3 (or R1R2R3N) is of interest in several fields, and its study is described here. The multireference coupled-cluster method (MR-CCM) and multireference double-excitation configuration interaction (MRD-CI) were used. At each (H3N---H)+ separation, from 1 to 11 bohr, the ground state MRD-CI energy was optimized with respect to the angle θ between the NH bond in the NH3 group and the C3 axis; MR-CCM and MRD-CI calculations were performed for the two lowest 1A1 states and the lowest 3A1. Two different reference determinants had to be used for the MR-CCM calculations at different regions, but this created no difficulties and the transition was smooth. Close agreement (a few mhartree) was obtained between MRD-CI and MR-CCM results. The avoided crossing, near R(H3N---H)+=4 bohr, is manifested by the rapid change in the CI and coupled-cluster method (CCM) mixing coefficients and by the transition of the NH3 group from pyramidal at small R to planar at large R. The lowest 1A1 state dissociates adiabatically to NH+3(2A1)+H(2S), whereas the single determinant self-consistent field (SCF) function dissociates to NH3(1A1)+H+.

Notes: Abstract Polarization functions are added in two steps to a split-valence extended gaussian basis set: d-type gaussians on the first row atoms C. N, O and F and p-type gaussians on hydrogen. The same d-exponent of 0.8 is found to be satisfactory for these four atoms and the hydrogen p-exponent of 1.1 is adequate in their hydrides. The energy lowering due to d functions is found to depend on the local symmetry around the heavy atom. For the particular basis used, the energy lowerings due to d functions for various environments around the heavy atom are tabulated. These bases are then applied to a set of molecules containing up to two heavy atoms to obtain their LCAO-MO-SCF energies. The mean absolute deviation between theory and experiment (where available) for heats of hydrogenation of closed shell species with two non-hydrogen atoms is 4 kcal/mole for the basis set with full polarization. Estimates of hydrogenation energy errors at the Hartree-Fock limit, based on available calculations, are given.

Notes: Abstract A variation-perturbation procedure for the evaluation of dispersion interaction, originally proposed by Jeziorski and van Hemert, has been reformulated to include basis set extension effects on an equal footing with the SCF interaction energy, corrected for basis set superposition error (BSSE). This approach has been tested for He2, (H2)2, (H2O)2, and (C2H4)2 complexes.

Notes: Ab initio MODPOT/VRDDO/MERGE calculations were carried out on carcinogenic 3-methylcholanthrene (3-MCA) and its metabolites. The results for 3-MCA were compared to our earlier similar calculations for carcinogenic benzo(a)pyrene (BP). Both compounds 3-MCA and BP are carcinogenic and are metabolically activated by similar mechanisms but in different positions. Both the calculated wave functions for 3-MCA and BP and the electrostatic molecular potential contour maps generated from these wave functions correctly reflect the similarity of mechanisms of metabolic activation and the differences in position. Our calculated results both for BP and for 3-MCA reflect accurately their experimentally observed behavior. Thus this combination of theoretical techniques can be used with confidence to describe the behavior of the polycyclic aromatic hydrocarbons (PAH's) and their metabolites. The ab initio MODPOT/VRDDO method incorporates two very desirable options into our fast ab initio Gaussian programs: MODPOT-ab initio effective core model potentials - and a charge-conserving integral prescreening approximation which we named VRDDO (variable retention of diatomic differential overlap). For orbital energies and population analysis the MODPOT/VRDDO results agree to essentially three decimal places with completely ab initio calculations using the same valence atomic basis set. For this series of very closely related congeners our recent MERGE technique which allows reuse of integrals from a common skeletal fragment was used. The ab initio MODPOT/VRDDO/MERGE calculations were carried out for 3-MCA, 3-MCA oxides, 3-MCA dihydrodiols, and 3-MCA dihydrodiolepoxides. The metabolites investigated were 3-MCA 9,10-oxide; 3-MCA 7,8-oxide; 3-MCA 9,10-dihydrodiol [trans(axial, axial); trans(equatorial, equatorial); cis(axial, equatorial); cis(equatorial, axial)]; and 3-MCA 9,10-dihydrodiol-7,8-epoxide [for both conformations A and B of the dihydrodiol and for all stereoisomers of the dihydrodiolepoxides relative to below and above the plane: ααα, and ααβ αβα αββ βαα βαβ ββα and βββ (most stable)]. Calculations were also carried out for opening of the C7—O—C8 epoxide ring both towards C7 and C8 for the most stable isomer Aβββ (above the ring). Opening the epoxide ring between C7 and O leads to a more stable intermediate than opening the epoxide ring between C8 and O. Again, however, as with opening the epoxide ring in BP 7,8-dihydrodiol-9,10-epoxide there is no buildup of positive charge on C7 in the 3-MCA metabolites as postulated by some cancer researchers, but rather the C7 becomes slightly more negative. Nor is there a buildup of negative charge on the O atom, but rather it becomes slightly more positive. As the epoxide ring is opened further than 90° for the O—C7—C8 or O—C8—C7 angles, there appears to be a possible mixing of configurations that is being investigated further.

Notes: Ab-initio MODPOT/VRDDO/MERGE SCF calculations were carried out on 2,6-dimethyl-N-nitroso morpholine and on four isomers of α-hydroxy substituted derivatives. The NNO bond angle is found to be 1135°, in agreement with that observed in dimethyl nitrosamine. The two methyl groups are found to be most stable in the equatorial position. The axial position is found to be the most stable orientation for the α-hydroxy group in both the syn and the anti isomers. This is in agreement with the observed axial orientation of straight chain alkyl substituents at the α position in N-nitroso piperidine. An investigation was made on the effects of internal rotation in the parent compound. A 90° rotation about the N—N bond raises the energy ∼.0526 a.u. or ∼33 kcal/mol. In the 90° orientation, population analysis shows a decrease of ∼0.1 e in the gross atomic population (GAP) of the nitroso oxygen and in the total overlap population for the N—N bond. There is a corresponding increase of ∼0.05 e in the GAP on the amino nitrogen. The GAP on other heavy atoms and overlap in other bonds show little effect from internal rotation of the nitroso group. Electrostatic molecular potential energy contour maps indicate differences as a function of conformation.