| Abstract
| - The enthalpy of formation of methylhydroxycarbene, CH3COH, has been determined from measurements ofthe threshold energy for collision-induced dissociation of protonated 2,3-butanedione in a flowing afterglow-triple quadrupole mass spectrometer and found to be 16 ± 4 kcal/mol, 57 ± 4 kcal/mol higher than that ofacetaldehyde. From the measured enthalpy of formation, the difference between the first and second C−HBDEs in ethanol is found to be 17 kcal/mol, which implies a singlet−triplet splitting of 28 kcal/mol in thecarbene. The activation energies for loss of ketene and carbon monoxide from protonated butanedione arefound to be 60 ± 4 and 50 ± 4 kcal/mol, respectively. On the basis of experimental and computationalresults, the loss of carbon monoxide is proposed to proceed through a tight transition state. Although calculationsalso suggest a tight transition state for loss of ketene, the experimental data indicate that it occurs via a loosetransition state, possibly forming by proton transfer along the direct dissociation pathway.
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