Hydrogen bonding in the benzene-ammonia dimer

Amines have long been characterized as amphoteric (acting as both donor and acceptor) in terms of their hydrogen-bond interactions in the condensed phase. With the possible exception of (NH₃)₂, however, no gas-phase complexes exhibiting hydrogen-bond donation by ammonia, the 'simplest amine', have been observed^1,2. Here we present high-resolution optical and microwave spectra of the benzene-ammonia dimer in the gas phase, which show that the ammonia molecule resides above the benzene plane and undergoes free or nearly free internal rotation. In the vibrationally averaged structure, the C₃ symmetry axis of NH₃ is tilted by about 58^° relative to the benzene C₆ axis, such that the ammonia protons interact with the benzene π-cloud. Our ab initio calcula-tions predict a 'monodentate' minimum-energy structure, with very low barriers to rotation of ammonia. The larger separation of the two molecular components, and the smaller dissociation energy, relative to the benzene-water dimer³ reflect the weak hydrogen-bond donor capability of ammonia, but the observed geometry greatly resembles the amino-aromatic interaction found naturally in proteins⁴.