Unimolecular and bimolecular reactions of the β-distonic ion CH₃CH₂OH⁺CH₂CH₂ ^•: An experimental and theoretical study

In the gas phase, the unimolecular reaction of the metastable β-distonic ion 1, CH₃CH₂OH⁺CH₂CH₂ ^•, yields a CH₃CHOH⁺ fragment ion. Experiments using isotopomers of 1 and ab initio calculations show that two pathways lead to its dissociation: (i) a 1,4-H migration leading to the α-distonic ion 2, CH₃CH^•OH⁺CH₂CH₃, with subsequent elimination of C₂H₅^•; (ii) a H transfer within a [C₂H₅OH, C₂H₄]^•+ intermediate complex 3 followed by loss of C₂H₅•. Two isomerization processes can occur prior to dissociation: a rapid permutation of the CH₂ groups in the radical chain and a reversible 1,5-H migration. The calculated transition state for 1,5-H migration and those arising on the fragmentation pathways are close in energy and correspond to relatively high energy barriers (93-100 kJ/mol), in agreement with the isotope effects observed. Finally, the bimolecular reactions of 1 with several neutral molecules are shown to be accompanied by the same 1,4- and 1,5-H migrations. The structures of the different isomeric ions thus formed were characterized by their specific ion-molecule reactions. The energy required for the various processes observed arises from the stabilization in the initial encounter complexes between 1 and each neutral molecule.