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

• Published in: Rapid communications in mass spectrometry 2020-10, Vol.34 (19), p.e8866-n/a
• Main Authors: Wang, Ruoqian, Zhao, Zhiyuan, Zhang, Huarong, Sheng, Chunqi, Wang, Yan, Jiang, Kezhi
• Format: Article
• Language: English
• Subjects: Acidification; Atmospheric pressure; Cations; Density functional theory; Esters; Ionization; Ions; Mass spectrometry; Mathematical analysis; Positive ions; Scientific imaging; Sodium; Solvents; Spectroscopy
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.8866

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 H2 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 H2 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.