Noncovalent Interactions and Internal Dynamics in Pyridine-Ammonia: A Combined Quantum-Chemical and Microwave Spectroscopy Study

The 1:1 complex of ammonia with pyridine is characterized by using state-of-the-art quantum-chemical computations combined with pulsed-jet Fourier-transform microwave spectroscopy. The computed potential energy landscape indicates the formation of a stable sigma-type complex, which is confirmed experimentally: analysis of the rotational spectrum shows the presence of only one 1:1 pyridine-ammonia adduct. Each rotational transition is split into several components owing to the internal rotation of NH3 around its C-3 axis and to the hyperfine structure of both N-14 quadrupolar nuclei, thus providing unequivocal proof that the two molecules form a sigma-type complex involving both a N-HN and a C-HN hydrogen bond. The dissociation energy (BSSE- and ZPE-corrected) is estimated to be 11.5kJmol(-1). This work represents the first application of an accurate yet efficient computational scheme, designed for the investigation of small biomolecules, to a molecular cluster.