“Nibbering's C7H7N”: an ab initio study of the structure and electronic properties of benzaldimine and its protonated ion

Abstract

Imines are an interesting but relatively poorly-characterized class of organic compounds: for example, is the gas phase ion-derived C₇H₇N species observed by Nibbering et al. benzaldimine or troponimine? Proton affinities are important quantities for the understanding of organic compounds. We have investigated the structure, energetics and proton affinities of benzaldimine, troponimine, and their carbonyl and olefinic analogs (benzaldehyde, tropone, styrene, heptafulvene) using G2(MP2) computational theory. We find the benzenoid species (benzaldimine, benzaldehyde, styrene) to be more stable by 20–25 kcal mol⁻¹ than their nonbenzenoid isomers (troponimine, tropone, heptafulvene). From our calculations and analysis of literature data, we derive the enthalpy of formation and proton affinity of benzaldimine to be 39 ± 3 and 220 ± 3 kcal mol⁻¹ respectively and conclude Nibbering's C₇H₇N is, in fact, this species.