The investigation of structural isomers and proton transfer phenomena of the 2-hydroxypyridine system in the cation ground state (D0) has been performed by two-color (1+1′) resonantly enhanced multiphoton ionization (REMPI) zero kinetic energy photoelectron spectroscopy. The adiabatic ionization energies of keto/enol tautomers have been determined to be 68137 ± 5 and 72093 ± 5 cm-1 for 2-pyridinone and 2-hydroxypyridinol, respectively. Although the structural isomers have been reported in the neutral excited state (S1) for the keto tautomer, the observed ionization energies for these isomers were found to be the same within our experimental accuracy. This indicates that no structural isomers of the keto tautomer exist in the D0 state. The keto tautomer adopts a planar geometry in the D0 state similar to that in the neutral ground state. In addition, no spectroscopic evidence for fast proton transfer from the enol form to the keto form in the D0 state has been obtained from the analysis of the spectral line width.
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