| Abstract
| - The structure and relative stability of methanol complexes with various cyclic ketones, lactones, lactams, andN-methyl lactams from three- to seven-membered rings have been investigated using the density functionaltheory method. The geometries, harmonic frequencies, and energies were calculated at the B3LYP/6-311+G(d,p) level. Three stable structures, cis-a, cis-b, and trans, with respect to the ring oxygen (nitrogen) atom,were found to be local minima of the potential energy surface. For lactones and N-methyl lactams, the moststable structure is trans; it is stabilized, as in cyclic ketones, through the conventional hydrogen bond (HB)interaction between the basic carbonyl oxygen and the acidic methanolic hydrogen and an unconventionalHB interaction between the methanolic oxygen and the CH hydrogen, in the α position of the carbonyl group.For unsubstituted lactams, the cis-a structure, stabilized through a HB interaction between the NH group andthe methanol oxygen in addition to the conventional HB interaction, is the most stable. The topologicalproperties of the electron density ratify the existence of conventional (N,O−H· · ·O) and unconventional(C−H· · ·O) hydrogen bonding. A good correlation was found between the HB distances and the electrondensity at the HB critical point. The unsubstituted lactams yield more stable complexes with methanol thanN-methyl lactams, lactones, and cyclic ketones. In the most stable complexes, both components behavesimultaneously as a HB donor and as a HB acceptor.
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