Distonic ions as reacting species

Abstract

The characteristic reactions of a family of distonic radical cations show that the gas-phase chemistry of these species differs considerably from that of the isomeric conventional molecular ions. Fifteen distonic radical cations, derived from 'CH2CHZNH3+ by introduction of alkyl substituents at C or N, have been demonstrated to be stable species whose reactions are characteristic of their structure. The predominant fragmentation and isomerization processes are the following: (i) simple cleavage of a C-N bond with formation of alkene ions or alkene molecules, (ii) 1,2-migration of protonated amino groups, and (iii) formation of ammonium ions by elimination of alkenyl radicals. Chain length permitting, these reactions may be preceded or acconpanied by specific intramolecular hydrogen abstraction reactions, which lead to isomeric distonic ions with characteristically different reactions. The hydrogen abstraction reactions are generally reversible; in the presence of long alkyl groups isomerization to amine molecular ions (et vice versa) can occur. Correspondingly, the reactions of many low-energy amine molecular ions require isomerization to distonic intermediates prior to fragmentation. A considerable number of recent papers have demonstrated the existence in the gas phase of stable distonic radical cations,'-I4 and distonic ions are with increasing frequency being suggested as key intermediates in unimolecular fragmentation The collision-induced reactions of these species have often been used to establish their structural integrity, but the question of how the particular properties of distonic ions influence their reactions has not been addressed in the literature. We have studied the isomerization and dissociation reactions of a family of distonic amine ions, in order to discover the characteristic reactions of these species and to examine the difference between these reactions and those of the corresponding conventional molecular ions. The term distonic ion was coined by Radom and co-workersZ3 to describe radical cations arising (formally) by ionization of zwitterions or diradicals. The difference between the properties of stable distonic ions and those of conventional molecular ions generated by direct ionization of neutral molecules can be con- siderable, and Radom's neologism has rapidly gained acceptance among gas-phase ion chemists. However, the properties of distonic ions often vary with the number of heavy atoms separating the charge and radical bearing sites. In the present paper we employ the adjectives a-distonic, P-distonic, y-distonic, etc., to denote respectively species with charge and radical at adjacent atoms (a-distonic ions or ylidions, e.g., l), with charge and radical separated by one heavy atom (P-distonic ions, e.g. 2), by two heavy atoms (y-distonic ions), etc. Recent experimental and ab initio st~dies~'-~~~~~~ ~~~~~ of the various CHSN'+ and C2H7N'+ isomers have shown that a- and