Structures of the X-Y-NO molecules and homolytic dissociation energies of the Y-NO bonds (Y = C, N, O, S)

High-level theoretical methods (UB3LYP/6-311++g(2df,p), RMP2/6-311++g(d.p), CBS-4M, CBS-Q, and G3) were used to study the structures and bond dissociation energies (BIDE) of the X-Y-NO molecules. The data were used to evaluate the previous experimental and theoretical results. It was found that the syn conformation is favored by CH3-Y-NO, C2H5-Y-NO, and CH3O-Y-NO (Y = C, N, 0, S), whereas the anti conformation is favored by CH3CO-Y-NO and Ph-Y-NO (Y = C N, 0). For Ph-S-NO, the syn conformation is preferred because of the long S-N bond. When X is an alkyl substituent, the Y-NO BDEs increase in the order X-S-NO (similar to30 kcal/mol) < X-CH2-NO (similar to40 kcal/mol) < X-O-NO (similar to43 kcal/ mol) < X-NH-NO (48 kcal/mol). When X is an aromatic substituent, the Y-NO BDEs increase in the order X-O-NO (similar to21 kcal/mol) < X-S-NO (26 kcal/mol) < X-CH2-NO (similar to30 kcal/mol) < X-NH-NO (similar to35 kcal/mol). The solvent effects of acetonitrile on the free energy change of C-NO and N-NO homolysis are significant, which are about 3-5 kcal/mol. The solvent effects of acetonitrile on the free energy change of O-NO and S-NO homolysis are relatively small. which are about 1-2 kcal/mol. Finally, we found that the remote substituent effects on C-NO, N-NO, O-NO. and S-NO BDEs have rho(+) values of -0.4similar to-0.9, 1.7-1.8. 3.2-3.9, and 1.2-1.7 kcal/mol. These values are significantly different from those on the C-H (0.4-0.6 kcal/mol), N-H (3.4-4.6 kcal/mol). O-H (4.1-5.7 kcal/mol), and S-H (2.0-3.8 kcal/ mol) BDEs. Therefore, the ground effects are important for the net substituent effects on BDEs.