Mass spectra of ethenol and its deutero analogues

Ethenol, 1‐ d ‐ethenol, O ‐ d ‐ethenol and Z ‐2‐ d ‐ethenol were prepared by pyrolysis of corresponding 5‐norbornenols at 800 ° C/2 × 10 −6 Torr. The most important fragments in the electron impact mass spectrum of ethenol are [C 2 H 3 O] + and CHO + and CH 3 ˙. The hydrogen atom eliminated from the molecular ion comes mainly from the hydroxyl group (68%) and to a lesser extent from C(1) (25%) and C(2) (7%). The loss of the hydroxyl hydrogen is preceded by rate‐determining migration of the hydrogen atom from C(1) onto C(2) to yield CH 3 COH + ˙ions that decompose to CH 3 CO + and H˙. The loss of deuterium from O ‐ d ‐ethenol shows a very small primary isotope effect (k H /k D =1.07), whereas a significant effect is observed for the loss of hydrogen from 1‐ d ‐ethenol (k H /k D =1.28). The appearance energy of [C 2 H 2 DO] + from 1‐ d ‐ethenol, AE =11.32 eV, gives a critical energy for the hydrogen loss, E =203 kJ mol −1 , which is 90 kJ mol −1 above the thermochemical threshold for CH 3 CO + +H˙. The appearance energy of CDO + from 1‐ d ‐ethenol was measured as 12.96±0.07 eV, which sets the barrier to isomerization to CH 3 CDO + ˙ at 1121 kJ mol −1 . The ionization energy of ethenol was found to be 9.22±0.03 eV.