[M−H]+ ions in 3-alkyl substituted indoles detected by electrospray mass spectrometry

•Anomalous [M−H]+ ions for indole derivatives were detected in positive-ion mode.•Possible pathways forming [M−H]+ were proposed.•MS/MS spectra and radical trapping experiment support direct loss of hydride anion.•Positions losing the hydride anion were confirmed by D-labeled and MS/MS spectra.

A series of six C-3 alkyl-substituted indole derivatives were investigated using electrospray, ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). Anomalous [M−H]+ ions, were detected. Three possible pathways forming [M−H]+ were proposed. MS/MS spectra and radical, trapping experiment support direct loss of hydride anion. Notably, the positions losing the hydride, anion were different for these compounds, and were tentatively confirmed by D-labeled and MS/MS, spectra. Both steric hindrance and electronic effect might contribute the difference.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (114 K)Download as PowerPoint slide