GAS-PHASE TAUTOMERIC EQUILIBRIUM OF 2-PYRIDINONE AND 2-HYDROXYPYRIDINE BY MICROWAVE SPECTROSCOPY

The tautomerism of 2-hydroxypyridine/2-pyridinone has been investigated by microwave spectroscopy using both a conventional spectrometer and also a jet-cooled millimeter-wave spectrometer. We have observed spectra attributable to both the (Z)-hydroxy tautomer and the pyridinone tautomer and also their monodeutero isotopomers, the relative abundances in both spectrometers being about 3:1 in favor of the hydroxy form. From relative intensity and dipole moment measurements, we estimate the energy difference between the vibrational ground states of the two tautomers to be DELTAE0,0 = 270 (30) cm-1. The rotational constants for 2-pyridinone yield an inertial defect that demonstrates the planarity of this tautomer, in contrast to recent reports of the nonplanarity of pyridinone. We can detect no lines that could be assigned to such a conformer. There are no lines corresponding to the E isomer of 2-hydroxypyridine, indicating that its relative abundance in the jet is less than approximately 5% of the Z isomer. The rotational spectra of several vibrational satellites have also been assigned. Identification of species is based on comparison of rotational constants determined experimentally with the values predicted from molecular orbital calculations and independently confirmed by comparison of observed and theoretically predicted hyperfine multiplets. The observed rotational constants for both tautomers are within 2% of the values predicted by ab initio molecular orbital calculations at the basis set level of 6-31G*, a degree of agreement similar to that found in some previous studies of nitrogen heterocycles. Nitrogen quadrupole coupling constants have been determined for both species: (Z)-2-hydroxypyridine, chi(aa) = 0.4 (3) MHz, chi(bb) = -2.3 (3) MHz; 2-pyridinone, chi(cc) = -2.8 (2) MHz. These values provide an independent confirmation of the identification of the two species of the molecule.