Gas-phase tautomeric equilibrium of 2-pyridinone and 2-hydroxypyridine by microwave spectroscopy

Abstract

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.

Base pair analogs in the gas phase

“…However, these constants are primarily sensitive to the separation of the two monomer units and to the dihedral angle between the two base planes; they are only secondarily sensitive to the detailed geometry of the two rings. The rotational constants of the two monomer units have been determined by both microwave (19,20) and high-resolution UV methods (21), and these are well reproduced by ab initio calculations. Therefore, we base our preliminary determination of the ground-state geometry of 2AP⅐2PY on the [B3LYP͞6 -31Gϩ(d,p)] theoretical structures of 2AP and 2PY and on the further assumption that these are unchanged on complex formation.…”

Gas‐Phase Watson–Crick and Hoogsteen Isomers of the Nucleobase Mimic 9‐Methyladenine⋅2‐Pyridone

“…[20] The microwave spectrum and the rotationally and vibrationally resolved UV spectra of 2-pyridone have been measured in molecular beams. [21][22][23] We have previously employed 2PY as a fluorescent UV chromophore in 2-pyridone·2-aminopyridine, where 2-aminopyridine was used to mimic the Watson-Crick binding site of adenine. [24,25] We investigate the intermolecular vibrations and binding energies of the supersonically cooled 9-methyladenine·2-pyridone (9MAd·2PY) complex using three different laser spectroscopic techniques and ab initio calculations.…”

Quantum Chemical Calculation of Tautomeric Equilibria

“…Gas-phase H T values range from 0.3 to 0.8 kcal mol −1 in favor of the hydroxy tautomer [108,[134][135][136][137][138]. Piacenza and Grimme have systematically studied the performance of various density functionals as well as wave-function-based methods for treating the tautomerism of DNA bases, including 2-hydroxypyridine as model compound [139].…”