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

Abstract

Mass spectrometry and ab initio calculations have been employed to investigate the unimolecular decompositions of proton-bound dimers consisting of acetonitrile with n- and i-propanol. Common to both systems is a competition between dissociation of the hydrogen bond in the proton-bound dimer and isomerization to (CH₃CNR)(H₂O)⁺ [R = CH₂CH₂CH₃ and CH(CH₃)₂]. The minimum-energy-reaction pathways for the isomerization in these two systems, as well as those for the dimers containing methanol and ethanol, are presented and compared. The dominant isomerization pathway for these ions is an internal S_N2 reaction that proceeds via a stable intermediate CH₃CN•••ROH₂⁺ ion [R = CH₃, CH₂CH₃, CH₂CH₂CH₃ and CH(CH₃)₂]. The mass spectra for the four butanol-containing dimers (n-, s-, i- and t-butanol) follow similar behavior.