Ab Initio EOM-CCSD Investigation of One-Bond C-C, N-C, and N-N Spin-Spin Coupling Constants in Fluoroazines
Ab initio EOM-CCSD calculations were carried out to examine one-bond 1J (C−C), 1J(N−C), and 1J(N−N) spin−spin coupling constants in benzene, pyridine, the diazines, and selected triazines, tetrazines, and pentazine and their fluoro-substituted derivatives. Relative to benzene, 1J(C−C) decreases in the azines as N atoms are introduced into the ring, but this decrease does not exceed 5 Hz. In the fluoro-substituted derivatives, 1J(C−C) may increase only slightly if the coupled carbon atoms form C−H bonds, or increase dramatically if either or both of the coupled atoms participate in C−F bonds. The value of 1J(C−C) also depends on the nature of the bonding of the coupled atoms in the ring. The largest increase is found when both carbons participate in C−F bonds, and both are ortho to N atoms. Relative to pyridine,1J(N−C) increases as N atoms are introduced into the ring, with the magnitude of the increase depending on the bonding of the coupled atoms. It is negligible if neither atom is bonded to another N, increases if one of the coupled atoms is bonded to another N atom, and increases further if both are bonded to other N atoms. Fluoro-substitution has an opposing effect on1J(N−C), making this coupling constant less positive or negative when the coupled C participates in a C−F bond. The decrease in 1J(N−C) relative to the parent molecule is enhanced if either of the coupled atoms is bonded to another N atom or to another C−F group. A further enhancement occurs if both coupled atoms are so bonded, with the largest increases associated with the bonding scheme in which the coupled C is bonded to another N and the coupled N to another C−F. Fluoro-substitution has a small effect on 1J(N−C) if the coupled C forms a C−H bond, and on 1J(N−N). Thus, the effects of fluoro-substitution on one-bond couplings tend to be localized.