Energetics of a model NH–π interaction: the gas phase benzene–NH3 complex

The dissociation energy of the benzene–ammonia complex formed in a supersonic expansion has been determined (D0=1.84±0.12 kcal mol–1) from the features of its photoionisation curve as obtained by mass-resolved two-color resonant two-photon ionisation. The complex structure, stabilised by a -type hydrogen bond between the benzene ring and the ammonia molecule located above, has also been obtained by a semi-empirical model. The neutral structure found is in good agreement with experiment and the best ab initio calculations in the literature. The ionic structures calculated enable us to interpret the slowly increasing photoionisation curve as a consequence of a large equilibrium geometry change between neutral and ion. The present study also shows that the benzene–ammonia complex is less bound than its homologue with water by ca. 0.6 kcal mol–1. However, the value found indicates that the NH– interaction can be taken into account when modelling the structure of biological systems.