Intramolecular competition between n-pair and π-pair hydrogen bonding: Microwave spectrum and internal dynamics of the pyridine-acetylene hydrogen-bonded complex

a-type rotational spectra of the hydrogen-bonded complex formed from pyridine and acetylene are reported. Rotational and ¹⁴N hyperfine constants indicate that the complex is planar with an acetylenic hydrogen directed toward the nitrogen. However, unlike the complexes of pyridine with HCl and HBr, the acetylene moiety in HCCH - NC₅H₅ does not lie along the symmetry axis of the nitrogen lone pair, but rather, forms an average angle of 46° with the C₂ axis of the pyridine. The a-type spectra of HCCH - NC₅H₅ and DCCD - NC₅H₅ are doubled, suggesting the existence of a low lying pair of tunneling states. This doubling persists in the spectra of HCCD - NC₅H₅, DCCH - NC₅H₅, indicating that the underlying motion does not involve interchange of the two hydrogens of the acetylene. Single ¹³C substitution in either the ortho- or meta-position of the pyridine eliminates the doubling and gives rise to separate sets of spectra that are well predicted by a bent geometry with the ¹³C on either the same side ("inner") or the opposite side ("outer") as the acetylene. High level ab initio calculations are presented which indicate a binding energy of 1.2 kcal/mol and a potential energy barrier of 44 cm^-1 in the C_2v configuration. Taken together, these results reveal a complex with a bent hydrogen bond and large amplitude rocking of the acetylene moiety. It is likely that the bent equilibrium structure arises from a competition between a weak hydrogen bond to the nitrogen (an n-pair hydrogen bond) and a secondary interaction between the ortho-hydrogens of the pyridine and the π electron density of the acetylene.