A study of the metastable dissociations of formate esters: A McLafferty rearrangement to a distonic radical cation

The metastable dissociations of formate ester radical cations were studied by using both threshold photoelectron photoion coincidence and tandem mass spectrometry. Kinetic energy releases for the unimolecular fragmentations were obtained from both mass analyzed ion kinetic energy (MIKE) and time-of-flight spectra. An important dissociation property of propyl formate radical cation is that the appearance energies of the three major fragment ions [C₃H₆^•+, HC(OH)₂⁺, and m/z 60 (^•CH₂OCHOH⁺ and C₃H₇OH^•+)] are the same as the ionization energy of the propyl formate molecule, resulting in the absence of a molecular ion in the breakdown graph. The three major fragment ions are all formed via rearrangement processes (stepwise McLafferty rearrangement) involving a common distonic intermediate. The rearrangements are exothermic and occur without an activation energy, a conclusion in agreement with that of Benoit, Harrison, and Lossing for the formation of HC(OH)₂⁺. The heat of formation of the distonic intermediate formed by an β hydrogen atom transfer was determined to be <579 kJ mol^-1. The rearrangement also pertains to butyl formate and may be a characteristic of the unimolecular dissociations of larger alkyl formate ester radical cations at low internal energies.