Ion rearrangement at the beginning of cluster formation: methyl substitution effects on the internal S(N)2 reaction in the proton-bound dimers of acetonitrile and alcohols

Mass spectrometry and ab initio calculations have been employed to investig ate the unimolecular decompositions of proton-bound dimers consisting of ac etonitrile with n- and i-propanol. Common to both systems is a competition between dissociation of the hydrogen bond in the proton-bound dimer and iso merization to (CH3CNR)(H2O)(+) [R = CH2CH2CH3 and CH(CH3)(2)]. The minimum- energy-reaction pathways for the isomerization in these two systems, as wel l as those for the dimers containing methanol and ethanol, are presented an d compared. The dominant isomerization pathway for these ions is an interna l Sr,2 reaction that proceeds via a stable intermediate CH3CN . . . ROH2+ i on [R = CH3, CH2CH3, CH2CH2CH3, and CH(CH3)(2)]. The mass spectra for the f our butanol-containing dimers (n-, s-, i- and t-butanol) follow similar beh avior.