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The tautomerism of 2-hydroxypyridinef2-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:l 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 = 270 (30) cm-I. 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 correspondingto the E isomer of 2-hydroxypyridine, indicating that its relative abundance in the jet is less than -5% of the 2 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 thevalues 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, xOO = 0.4 (3) MHz, Xbb = -2.3 (3) MHz; 2-pyridinone, xcc = -2.8 (2) MHz. These values provide an independent confirmation of the identification of the two species of the molecule.

It has been found possible to understand the observed relative abundances of the several different tautomers and conformers observed in the millimeter-wave jet spectroscopy of histamine by using ab initio energy calculations at the MP2/6-311++G(d,p) level and including an estimate of thermal free energies. Conformational relaxation, previously found to be important for several molecules of comparable complexity, has been found to have a significant role in determining the relative abundances of histamine species detectable in the cooled jet, the present example being indicative that this phenomenon is expected to influence generally the interpretation of multiconformational jet spectra. The need to include free-energy corrections is also noteworthy. The thermal equilibration of histamine tautomers in the gas phase within the millimeter-wave spectrometer prior to jet expansion has been confirmed experimentally by comparing abundances measured with high surface area and low surface area inlet systems. Thus, it is valid to use relative abundances to indicate relative free energies. The presented ab initio calculations yield spectroscopic constants (e.g., planar moments) that tighten the identification of the four species detected in jet spectroscopy. In particular, the identity of the least abundant species (3 G-Ic) is confirmed. Nitrogen quadrupole coupling constants computed at the MP2/6-311++G(d,p) level are in noticeably better agreement with experiment than previously reported calculations, it being noteworthy that they require no scaling for the 14N nuclear quadrupole moment.