Structures of the X−Y−NO Molecules and Homolytic Dissociation Energies of the Y−NO Bonds (Y = C, N, O, S)

Abstract

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 (BDE) 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, O, S), whereas the anti conformation is favored by CH3CO−Y−NO and Ph-Y−NO (Y = C, N, O). 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 (∼30 kcal/mol) < X−CH2−NO (∼40 kcal/mol) < X−O−NO (∼43 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 (∼21 kcal/mol) < X−S−NO (26 kcal/mol) < X−CH2−NO (∼30 kcal/mol) < X−NH−NO (∼35 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 ρ+ values of −0.4∼−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.