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](+)). The formation of specific [M - H](+)cations in the API process is of significant interest for further investigation. Methods The ionization processes of 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate esters were investigated using electrospray ionization (ESI)-MS and atmospheric pressure chemical ionization-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 using ESI-MS. Interestingly, no [M + H](+)ion was obtained in the ESI-MS analysis, and acidification of the ESI solvent accelerated the formation of [M - H](+)rather than [M + H](+)ion. DFT calculations for the typical methyl 2-(diphenylsilyl)-1-phenyl-cyclopropanecarboxylate (1) indicated that the [1 + H](+)ion can thermodynamically and kinetically undergo facile H(2)elimination to generate [1 - H](+). Conclusions The favorable formation of [M - H](+)ions in these compounds is attributed to the unique diphenylhydrosilyl group in their structure. The [M + H](+)ion formed easily underwent H(2)elimination to produce the [1 - H](+)ion in the API source, and thus, acidification of the ESI solvent apparently accelerates the formation of the [1 - H](+)ion.