137
J. Org. Chem. 1991,56, 137-142 and in solution that is due to the small change in molecule size (which is much more important for charged species because of the Born ionic solvation term in total energy) can be noticed. However, this is not observed experimentally" (cf. Table 11),which indicates either that there is practically no change in the volume of the acetate ion due to the C-H bond cleavage or the gas-phase experiment is in error. It is worth while to note in this connection that the AM1 prediction for the solution is in a very good agreement with experimental BDE, whereas the data for the isolated species are more inconsistent (difference between calculated and experimental BDE is 6.6 kcal/mol). the additional stabilization of As mentioned the captodative radicals in solution is supported by the increase of their dipole moments. However, the data presented in Table I11 on the calculated dipole moments of molecules and radicals do not stress the specifically enhanced dipole moment change of cyanohydroxymethyl radical in comparison with other species.18 Remarkably enough, the largest change (by 84% as calculated using AM1 parametrization) is obtained for the closed-shell cyanomethanol molecule. The large increase of AM1, (18) Remarkably enough, the largest change (by 84% as calculated wing AM1 parametrization was obtained for the closed-shell cyanomethanol molecule. The large increase of the AM1, PM3, and MNDO calculated dipole moments of the radical resulting from the C-H bond dissociation of thylene and MNDO calculated dipole moment of ethylkene glycol itself are mainly due to conformational change in these species (from trans conformation of OH groups in gas phase to gauche conformation in solution), but not to the electron redistribution in this radical.
PM3, and MNDO calculated dipole moments of the radical resulting from the C-H bond dissociation of ethylene glycol and MNDO calculated dipole moment of ethylene glycol itself are mainly due to the conformationalchange in these species (from trans conformation of OH groups in gas phase to gauche conformation in solution), but not to the electron redistribution in this radical. Altogether the calculated dipole moments of the closed-shell molecules are higher than the corresponding radicals (cf. Table 111). In part this is caused by significant geometry changes at the carbon atom from a nearly tetrahedral configuration in the molecule to a planar configuration in the radical. In conclusion, our results show that the AM1 SCRF ROHF method describes satisfactorily the homolytic dissociation energies (however, not the dissociation paths., cf. ref 19) of carbon-hydrogen bonds in polar solvents. More direct experimental data on bond cleavage energies are needed to validate the prediction of the merostabilization of captodative radicals in these media. Registry No. H3COH, 67-56-1; HO'CH2, 2597-43-5; H3CCHZOH, 64-17-5; H,CC'CHOH, 2348-46-1; (H&)ZCHOH, 67-63-0; (H&)2C'OH, 5131-95-3; (HZC)3COH, 75-65-0; (CHJZ(C'Hz)COH, 5723-74-0;CH3CN, 75-05-8; 'CHZCN, 2932-82-3;H&CO
