Substituent Effects on Cl•••N, S•••N, and P•••N Noncovalent Bonds

Cl, S, and P atoms have previously been shown as capable of engaging in a noncovalent bond with the N atom on another molecule. The effects of substituents B on the former atoms on the strength of this bond are examined, and it is found that the binding energy climbs in the order B = CH3 < NH2 < CF3 < OH < Cl < NO2 < F. However, there is some variability in this pattern, particularly for the NO2 group. The A center dot center dot center dot N bonds (A = Cl, S, P) can be quite strong, amounting to as much as 10 kcal/mol. The binding energy arises from approximately equal contributions from its induction and electrostatic components, although the former becomes more dominant for the stronger bonds. The induction energy is due in large measure to the transfer of charge from the N lone pair to a B-A sigma* antibonding orbital of the electron-acceptor molecule containing Cl, S, or P. These A center dot center dot center dot N bonds typically represent the lowest-energy structure on each potential energy surface, stronger than H-bonds such as NH center dot center dot center dot F, CH center dot center dot center dot N, or SH center dot center dot center dot N.