Tautomerism and Microsolvation in 2-Hydroxypyridine/2-Pyridone

The Fourier transform microwave spectra of the hydrated forms of the tautomeric pair 2-pyridinone/2-hydroxypyridine (2PO/2HP) have been investigated in a supersonic expansion. Three hydrated species, 2PO-H2O, 2HP-H2O, and 2PO-(H2O)(2), have been observed in the rotational spectrum. Each molecular complex was confidently identified by the features of the N-14 quadrupole hyperfine structure of the rotational transitions. The presence of water affects the tautomeric equilibrium -N=C(OH)- <-> -NH-C(=O)-, which is shifted to the enol form for the bare molecules 2PO/2HP but to the keto tautomer for the hydrated forms.