Comparison of substitution effects of F and methyl groups adjoined to C and B atoms in hydrogen bonds

Ab initio calculations have been performed for the complexes of pyridine with 1,2-dihydro-1,2-azaborine (1), 1,3-dihydro-1,3-azaborine (2), 1,4-dihydro-1,4-azaborine (3), and their F and methyl derivatives to compare the influence of both substituents adjoined to the C and B atoms on the strength of hydrogen bonding. The NH⋯N hydrogen bond is enhanced by the substituent F whether it is connected to C or B atom, and the enhancing effect is bigger for one bonded to the C atom. The role of methyl group depends on the position of the substitution. NBO analyses showed that H, F, and methyl substituents are electron-withdrawing in the formation of complexes, but the electron-withdrawing ability of the methyl group bonded to the B atom is smaller than that of the corresponding H atom. The cooperativity between different types of interactions in 1 and its derivatives has also been discussed in the change of interaction energy and binding distance.