Formation of the exceptional [M-H]+ cation in atmospheric pressure ionization-mass spectrometry analysis of 2-(diphenylsilyl) cyclopropanecarboxylate esters.

RATIONALE In general, ionization of analytes in atmospheric pressure ionization-mass spectrometry (API-MS) in positive ion mode results in the formation of protonated molecules ([M+H]+ ), and/or cationized molecules (e.g., [M+Na]+ ). Formation of specific [M-H]+ cations in the API process is of sufficient interest for further investigation to be performed. METHODS The ionization processes of 2-(diphenylsilyl)-1-phenyl cyclopropane- carboxylate esters were investigated by electrospray ionization mass spectrometry (ESI-MS) and atmospheric pressure chemical ionization mass spectrometry (APCI-MS) in positive ion mode. Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311+G(2d,p) level. RESULTS The anomalous [M-H]+ ion and the regular [M+Na]+ ion were both observed by ESI-MS. Interestingly, no [M+H]+ ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent promotes formation of [M-H]+ rather than [M+H]+ . DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl- cyclopropanecarboxylate (1) indicated that the [1+H]+ ion can thermodynamically and kinetically undergo facile H2 elimination to generate [1-H]+ . CONCLUSIONS The favorable formation of [M-H]+ in these compounds is ascribed to the unique diphenyhydrogenlsilyl group in their structure. The formed [M+H]+ ion easily undergoes H2 elimination to give [1-H]+ in the API source, and thus acidification of the ESI solvent apparently promotes formation of [1-H]+ .