Thermochemical and mass spectrometric studies of gas-phase ions.

Abstract

Combining the available data related to phenyl substituted neutrals (molecules and radicals) and ions (molecular ions and even electron cations) with newly obtained data from electron impact appearance energy measurements, the effect of phenyl substitution on the heat of formation of these species was reviewed. In neutrals, the change in the heat of formation following replacement of an H atom by a phenyl moiety was found to be always destabilizing and the magnitude of the destabilizing effect depended on the functionality at which substitution took place. In the case of molecular ions and even electron cations, the relationship between the heat of formation and the number of phenyl groups was not straightforward and depended on the chemical nature of the species in which substitution took place. In most cases substitution of a first phenyl group significantly lowered the heat of formation of the ions. A monotonic rise in the heat of formation of the ions was observed. This was consistent with the heat of formation associated with the phenyl substituted neutrals becoming the predominant factor in determining the heat of formation of the ions. A controversy surrounding the thermochemical effect of methyl substitution in ketene was investigated by obtaining the heat of formation of ketene and its methyl substituted analogues using the appearance energy measurement method. From these measurements, methyl substitution in ketene was shown to decrease the heat of formation of the methyl substituted species by ca. 10 kcal mol$\sp{-1},$ which is in keeping with the effect of methyl substitution in structurally related systems. The dissociation characteristics of phenyl acetate, propanoate and isobutyrate were investigated. Metastable phenyl acetate and propanoate dissociated to ionized phenol and the corresponding neutral ketene. Metastable phenyl isobutyrate dissociated mainly by loss of ionized dimethyl ketene and neutral phenol but also by loss of ionized phenol and neutral dimethyl ketene. The relative intensity of these two peaks was not consistent with predictions based on thermochemical grounds. We proposed that ionization of phenyl isobutyrate produced the enol form of the ester which further rearranged to an ion-neutral complex, $\rm\lbrack (CH\sb3)\sb2CCO\sp{+\cdot}\cdots C\sb6H\sb5OH\rbrack$ before dissociation. The participation of both intermediates, the enol and the ion-neutral complex form of the ionized ester, was used to rationalize the relative abundance of the fragment ions in the metastable ion mass spectrum of phenyl isobutyrate. The 1-adamantyl (1Ad$\sp+$) cation is particularly interesting because the charge is thought to be formally located at the bridgehead position. Very little data concerning this ion and the 2-adamantyl cation (2Ad$\sp+$) in the gas phase, are available in the literature. A combination of mass spectrometric techniques showed that 1Ad$\sp+$ and 2Ad$\sp+$ ions occupied distinct potential energy wells, separated by an energy barrier high enough to prevent their interconversion. (Abstract shortened by UMI.)