Abstract
Examination of the reactions of the long-lived (>0.5-s) radical cations of CD3CH2COOCH3 and CH3CH2COOCD3 indicates that the long-lived, nondecomposing methyl propionate radical cation CH3CH2C(O)OCH +·3 isomerizes to its enol form CH3CH=C(OH)OCH +·3 (ΔH isomerization ≃ −32 kcal/mol) via two different pathways in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. A 1,4-shift of a β-hydrogen of the acid moiety to the carbonyl oxygen yields the distonic ion ·CH2CH2C+ (OH)OCH3 that then rearranges to CH3CH=C(OH)OCH +·3 probably by consecutive 1,5- and 1,4-hydrogen shifts. This process is in competition with a 1,4-hydrogen transfer from the alcohol moiety to form another distonic ion, CH3CH2C+(OH)OCH ·2 , that can undergo a 1,4-hydrogen shift to form CH3CH=C(OH)OCH +·3 . Ab initio molecular orbital calculations carried out at the UMP2/6-31G** + ZPVE level of theory show that the two distonic ions lie more than 16 kcal/mol lower in energy than CH3CH2C(O)OCH +·3 . Hence, the first step of both rearrangement processes has a great driving force. The 1,4-hydrogen shift that involves the acid moiety is 3 kcal/mol more exothermic (ΔH isomerization=−16 kcal/mol) and is associated with a 4-kcal/mol lower barrier (10 kcal/mol) than the shift that involves the alcohol moiety. Indeed, experimental findings suggest that the hydrogen shift from the acid moiety is likely to be the favored channel.
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Pakarinen, J.M.H., Vainiotalo, P., Stumpf, C.L. et al. Methyl propionate radical cation. J Am Soc Mass Spectrom 7, 482–489 (1996). https://doi.org/10.1016/1044-0305(95)00707-5
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DOI: https://doi.org/10.1016/1044-0305(95)00707-5