The mass spectra of organic compounds. 8th Communication. 1‐Buten‐3‐yn‐2‐ol. A new kinetically unstable C4H4O isomer

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Abstract
The unstable neutral enol, 1‐buten‐3‐yn‐2‐ol, and its deuterium‐labelled analogues were generated in the gas phase by high‐vacuum flash pyrolysis of the corresponding 2‐exo‐ethynylnorborn‐5‐en‐2‐ols, or alternatively from 1‐ethynylcyclobutanol. The ionization energy of the enol was measured as 8.92 ± 0.03 eV. The cation radical (HCC(OH)CH2)+· was prepared by electron‐impact ionization of the neutral enol, or by dissociative ionization of 1‐hexyn‐3‐ol and 1‐ethynylcyclobutanol, and its heat of formation was determined as 944 kJ·mol−1. The heat of formation of the neutral enol was assessed as 83 ± 7 kJ·mol−1 in good agreement with MNDO calculations. Cation radicals HCCC(OH)CH2+· undergo four main unimolecular fragmentations due to losses of the OH H‐atom, a CH3 group incorporating the C(1) methylene and the OH H‐atom, CO, and H2O. The structure of the neutral enol and the mechanisms of the ionic decompositions are discussed.