Kinetic Energy Release in the Loss of Water from n-Propanol Cation

Abstract

Unimolecular dissociations of ionized aliphatic alcohols have been investigated extensively. Interestingly, competition between loss of hydrogen atom and water is quite different for lower aliphatic alcohol cations with different chain lengths. Loss of H dominates in the dissociations of methanol and ethanol cations, while loss of H2O does in those of n-propanol and n-butanol cations. Theoretical studies agree that a H atom of the methyl group migrates to the O atom of the hydroxyl group prior to loss of H2O from these alcohol ions. In a recent computational study, the energy barriers for the H migration in the H2O loss were compared. The barrier decreases as the aliphatic chain increases. Very recently, we proposed a potential energy surface (PES) for the loss of H2O and H from n-propanol ion by density functional theory (DFT) molecular orbital calculations. Initially CH3CH2CH2OH +• isomerizes to CH2CH2CH2OH2 + via a five centered transition state (1,4-H shift). This is the rat-determining step in the H2O loss. CH2CH2CH2OH2 + undergoes further isomerizations to ion-dipole complexes, c-C3H6 •••H2O prior to loss of H2O. Based on the obtained PES, the experimental results of the metastable dissociation such as the branching ratio and the isotopic effect on the H loss could be well interpreted by the statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The kinetic energy release (KER), however, has not been investigated in detail, which provides valuable information on the exit channel in dissociation. In this work, the KER distribution (KERD) in the H2O loss from metastable npropanol cation has been investigated using mass-analyzed ion kinetic energy (MIKE) spectrometry. The nature of the dissociation is discussed by comparing the experimental result with statistical phase space theory (PST) calculations.