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

Reactions that proceed within mixed ethylene-methanol cluster ions wer e studied using an electron impact time-of-flight mass spectrometer. T he ion abundance ratio, [(C2H4)n(CH3OH)mH+]/[(C2H4)n(CH3OH)m+], shows a propensity to increase as the ethylene/methanol mixing ratio increas es, indicating that the proton is preferentially bound to a methanol m olecule in the heterocluster ions. The results from isotope-labelling experiments indicate that the effective formation of a protonated hete rocluster is responsible for ethylene molecules in the clusters. The o bserved (C2H4)n(CH3OH)m+ and (C2H4)n(CH3OH)m-1CH3O+ ions are interpret ed as a consequence of the ion-neutral complex and intracluster ion-mo lecule reaction, respectively. Experimental evidence for the stable co nfigurations of heterocluster species is found from the distinct abund ance distributions of these ions and also from the observation of frag ment 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 str uctures. The result is understood on the basis of ion-molecule condens ation reactions, leading to the formation of fragment ions, CH2=OCH3 a nd (CH3OH)H3O+, and the effective stabilization by a polar molecule. T he 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