1,2-Eliminations from (CH3)2NH+CH2CH3 and (CH3)2NH2+: guided dissociations.

1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH(2)=CH(2) and CH(3)CH(3) from (CH(3))(2)NH(+)CH(2)CH(3) (1) and of CH(4) from (CH(3))(2)NH(2)(+) are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH(3))(2)NH(2)(+) + CH(2)=CH(2) from 1. CH(3)CH(3) elimination by H-transfer to C(2)H(5)(+) to form CH(3)NH(+)=CH(2) + CH(3)CH(3) also takes place. (CH(3))(2)NH(2)(+) eliminates methane by CN bond extension followed by beta-H-transfer to give CH(2)=NH(+) + CH(4). Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complex-mediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,1-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry.