| A variety of systems in which resonance interactions have been invoked were examined via ab initio theory in order to ascertain the source of stabilization. In substituted carbonyl compounds such as amides and esters and also in multiple-substituted methanes such as tetrafluoromethane, Coulombic interactions proved more important than previously believed, and resonance stabilization somewhat less so. In particular, no evidence was found for fluorine acting as a {dollar}pi{dollar}-donor, and instead the stability of acetyl fluoride and tetrafluoromethane were found to derive from cooperative Coulombic stabilization. This electrostatic analysis also predicted stability in tetrasilylmethane and 1,3-disilapropene, but instability in acetyl silane and difluorodisilylmethane, and the predictions were verified by ab initio calculations.; Changes occurring in the molecular charge density distribution during rotation about the C-N and C-O bonds of amides, carboxylic acids, vinylamine, thioamides and related species were calculated via ab initio theory and visualized graphically. Charge transfer in the {dollar}pi{dollar}-systems occurred from nitrogen or oxygen to the terminal atom of the double bond in the ground state relative to the transition state for bond rotation. Charge transfer in the {dollar}sigma{dollar}-systems took place in the reverse direction. However, the quantity of charge transfer did not in general correlate with the magnitude of the rotational barriers.; The solvent effect on bond rotation in dimethyl amides and vinylogous amides was examined experimentally via NMR determination of the rate constants for methyl group exchange. The process was also studied computationally via self-consistent reaction field theory. The energetic barriers increased in proportion to the polarity of the solvent as reflected in the value of the Onsager dielectric function, and extrapolated to the gas-phase values determined in other laboratories. Chlorinated, aromatic and hydrogen-bond donating solvents caused an additional increase in the barrier, with the latter having the most pronounced effect. This deviation was attributed to specific solvent-solute interactions. The experimental barriers for the amides agreed well with the calculated values in the non-associating solvents. |
