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Farneth, Dixon, et al. / Proton Affinities of Ethylidenimine(4) Hydrogen-bond formation in open water-2-R-pyridine dimers leads to charge transfer from the substituted pyridine molecule to the water molecule, to characteristic changes in the electron densities of the 0, H, and N1 atoms which form the O-H-N1 hydrogen bond, and to a redistribution of the zelectron density of the pyridine ring. Abstract:The proton affinity of ethylidenimine has been determined by ion cyclotron resonance to be 21 3.2 zk 2 kcal/mol relative to the value for NH3 of 203.6 kcal/mol. This value is combined with the known heat of formation of the protonated species to give a value of 2 f 4 kcal/rnol for the heat of formation (298 K) of the neutral imine. The proton affinity for C-protonation of vinylamine was calculated using a b initio molecular orbital theory with a DZ + D basis set to be 7.0 kcal/mol higher than that of the imine, giving 220.2 kcal/mol. The proton affinity for N-protonation of vinylanline was calculated to be 18.7 kcal/ mol lower than that for C-protonation. The calculated proton affinities for ammonia, methylamine, dimethylamine, and trimethylamine using the same basis set are also given.Abstract: Gaseous (2-phenylethy1)benzenium and (3-pheny1propyl)benzenium ions 1 and 2 are generated easily by mass spectrometric loss of CO2H from the positive molecular ions of the corresponding 1 -(a-phenylalkyl)-1,4-dihydrobenzoic acids 4 and 5. The major secondary fragmentation is loss of benzene from 1 and 2. It is shown by deuterium labeling that 1 and 2 ions undergo repeated ring-to-ring proton transfer reactions, equilibrating all of the 11 "aromatic" hydrogen atoms within s without involving those from the aliphatic chain. A competition between the ring-to-ring (quasi-intermolecular) proton transfer and proton shifts within the ring ("ring walks") is discussed.

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