Computational study of substituent effects on the gas‐phase stabilities of N‐phenylguanidinium ions

Abstract

Relative gas‐phase stabilities of ring‐substituted N‐phenylguanidinium ions were determined at the B3LYP/6‐311+G(2d,p) level of theory, and the substituent effects were analyzed by the Yukawa–Tsuno equation. The total stabilization energies (TSEs) of the ring‐substituted cations were determined by the chloride‐transfer reaction. The substituent‐effect analysis showed a considerable amount of contamination from the chloride salts in the TSEs. The cation stabilization energies (CSEs) were then determined by an isodesmic reaction where the neutral species are substituted benzenes. The substituent‐effect analysis gave r+ = −0.04, which indicates a very small through‐resonance. The dihedral angle φ between the guanidinium and benzene ring moieties was found to be orthogonal in the fully optimized cation. The r+ value decreased with the decrease in φ, to give r+ = −0.15 in the planar cation. The degree of the through‐resonance decreased as the cation approached the planar structure. A detailed investigation with NBO analyses indicated through‐resonance mechanisms concerned by the π–σ* interaction between the benzene π‐electron system and the adjacent bonds as well as the direct interaction between the benzene π‐electron system and the distant p‐orbital at the β‐position. The former becomes dominant as the structure of the cation approaches an orthogonal conformation (φ = 90°), while the latter becomes dominant as the structure approaches the planar conformation (φ = 0°). The r+ value changed with the dihedral angle φ, reflecting the sum of these interactions. The mechanisms of stabilization and delocalization caused by the substituents attached to the guanidinium ion unit are discussed. Copyright © 2016 John Wiley & Sons, Ltd.