Photoelectron spectra and gas phase tautomerism of some pyrazolones

The HeI photoelectron (PE) spectra of six 4‐substitued 3‐methyl‐1‐phenylpyrazolin‐5‐ones and the unsubstituted parent compound were measured. The interpretation of the low‐energy region of these spectra is attempted on the basis of HAM/3 molecular orbital calculations, making allowance for different tautomeric forms of the compounds. The results indicate preference of the CH versus the NH and/ or OH tautomer of these compounds in the gas phase at the temperatures employed (100–150°C).