ELECTRON-IMPACT IONIZATION OF ETHYLENE METHANOL HETEROCLUSTERS - STABLE CONFIGURATIONS AND MECHANISMS IN INTRACLUSTER ION-MOLECULE REACTIONS

Abstract

Reactions that proceed within mixed ethylene-methanol cluster ions were studied using an electron impact time-of-flight mass spectrometer. The ion abundance ratio, [(C2H4)n(CH3OH)mH+]/[(C2H4)n(CH3OH)m+], shows a propensity to increase as the ethylene/methanol mixing ratio increases, indicating that the proton is preferentially bound to a methanol molecule in the heterocluster ions. The results from isotope-labelling experiments indicate that the effective formation of a protonated heterocluster is responsible for ethylene molecules in the clusters. The observed (C2H4)n(CH3OH)m+ and (C2H4)n(CH3OH)m-1CH3O+ ions are interpreted as a consequence of the ion-neutral complex and intracluster ion-molecule reaction, respectively. Experimental evidence for the stable configurations of heterocluster species is found from the distinct abundance distributions of these ions and also from the observation of fragment peaks in the mass spectra. Investigations on the relative cluster ion distribution under various conditions suggest that (C2H4)2(CH3OH)mH+ ions with n+m less-than-or-equal-to 3 have particularly stable structures. The result is understood on the basis of ion-molecule condensation reactions, leading to the formation of fragment ions, CH2=OCH3 and (CH3OH)H3O+, and the effective stabilization by a polar molecule. The reaction energies of proposed mechanisms are presented for (C2H4)n(CH3OH)mH+(n+m less-than-or-equal-to 3) using semi-empirical molecular orbital calculations