Dehydration of ionized propanol in the gas phase

Experimental and theoretical reexamination of the dehydration reaction of i onized propanol, 1, indicates that both ionized cyclopropane and ionized pr opene are produced as fragment ions. Tandem mass spectrometry experiments i ncluding charge stripping, neutralization-reionization, and ion-molecule re actions with ammonia and NO are best interpreted by the formation of a mixt ure of [C3H6](.+) fragment ions. The composition of this mixture is nearly insensitive to the internal energy of the precursor ions 1. Molecular orbit al calculations, conducted at the G2(MP2,SVP) level, confirm that the first step of the reaction is the 1,4-hydrogen migration [CH3CH2CH2OH](.+), 1 -- > [CH2CH2CH2OH2](.+), 2; excellent agreement is found with the experimental critical energy barrier for this reaction. The distonic ion 2 may lead to ionized cyclopropane via a slightly stabilized ion-neutral complex, 3; the energy determining step of the cyclopropane formation is the dissociation o f complex 3. The second dissociation process of 2, leading to ionized prope ne, is associated with a 1,2-hydrogen migration leading to the distonic ion [CH3CH(OH2)CH2](.+), 4; the isomerization 1 --> 4 is the energy determinin g step of this reaction. Starting from 2, formation of ionized cyclopropane needs 69 kJ/mol whereas the critical energy for ionized propene formation is only 54 kJ/mol, From a strictly energetic point of view, both reactions are allowed for ions 2 coming from 1, moreover propene ion formation should be favoured. Statistical rate constant calculations using an orbiting tran sition state model for the dissociation of the loose structure 3, demonstra te that the two reaction channels in competition at low, as well as at high , internal energy of the precursor ions 1, (C) 2000 Elsevier Science B.V.