Add time:08/03/2019         Source:sciencedirect.com

Intensities of [M + 1]+ ions in mass spectra of methyl esters of C8–C14 acids exceeded theoretical values, deviation was greatest for the shortest chain esters and for methyl decanoate was four times the calculated intensity. Similar effects were also observed in spectra of decanoates of C2–C6 alcohols. The abnormal results were due to the presence of MH+ ions and mode of formation of this ion was investigated using all the methyl gem dideuterated decanoates, methyl[10−2H3]- and [3,5,6-2H6]- decanoates and [2H3] methyl decanoate. A hydrogen radical migrates from one of the carbons in the chain to the ionized carbonyl oxygen and the resultant ion, on collision with a molecule, transfers the hydrogen, as a proton, giving MH+. The transferred hydrogen was derived from those same carbons that provide hydrogen in the first stages of fragmentation to the mass spectrum of methyl decanoate. Thus 37% came from C-4, 15% from C-5, 9% from C-6 and lesser percentages from the other carbons. The results do not exclude possible formation of MH+ by hydrogen transfer from ions m/z 74 and 87. Modes of formation of ions m/z 103, 117 and 131 were also studied using the deuterated decanoates. They were formed by a mechanism similar to that involved in production of ions m/z 87 and 143 but less specifically. Hydrogen transfer from C-3, C-4 or C-5 was followed by cleavage of the bond between the carbons α- and β to that carbon. Some initial migration from carbons 5,6 and 7 also ocurred.

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