Infrared spectroscopic observation of the McLafferty rearrangement in ionized 2-pentanone

The McLafferty rearrangement is a well-known process in mass spectrometry. In ionization of organic molecules containing a carbonyl group, β cleavage occurs following transfer of a proton of aliphatic CH at the γ position to the carbonyl group. Although the McLafferty rearrangement has undergone numerous mass spectrometric investigations, no spectroscopic investigation of the enolized cation generated in the proton transfer has been carried out. 2-pentanone is the simplest ketone containing CH bonds at the γ position. In this study, infrared predissociation spectroscopy for both neutral and ionized 2-pentanone in the gas phase through the vacuum ultraviolet ionization detection is performed to investigate the ionization-induced isomerization and to observe the enolized product. An OH stretch band is observed in the infrared spectrum of ionized 2-pentanone, and this demonstrates its enolization accompanying the rearrangement of an alkyl proton. The enolization of ionized 2-pentanone is theoretically supported by the reaction path search based on the anharmonic downward distortion following method.