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The thermochemistry of gas-phase ion molecule interactions and the structures of various clusters between protonated glycine (GlyH+), glycine, and ammonia have been studied by high pressure mass spectrometry (HP-MS) and ab initio calculations. For the association reactions of GlyH+ with NH3, Gly(NH3)H+ with NH3, and (Gly)2H+ with NH3, the enthalpy changes experimentally determined are –23.2, –18.3, and –19.1 kcal mol–1 (1 cal = 4.184 J), respectively. For all clusters investigated, the measured binding enthalpies are in excellent agreement with those obtained from ab initio calculations at the B3LYP/6-311+G(d,p) level of theory. Different isomers of each of these clusters have been obtained and the corresponding binding energies have been computed. The potential energy surface for isomerization of the clusters of protonated glycine with ammonia has also been computed at the same level. For this cluster, the three most stable isomers all involve a proton transfer from protonated glycine to ammonia. According to the calculated potential energy surface, the barrier between GN4, the least stable isomer, and the most stable isomer (GN1) is 11.5 kcal mol–1 at 298 K. Thus, this isomerization will be facile given the exothermicity of the association reaction. Therefore, a statistical distribution of isomers will be present under thermal equilibrium conditions. Single point energy calculations at the MP2(full)/6-311++G(2d,2p)//B3LYP/6-311+G(d,p) level of theory reveal that the isomer GN2 in which glycine has a zwitterionic structure has the same energy as the most stable non-zwitterionic isomer GN1. NH4+ evidently may stabilize the zwitterionic structure of glycine. In contrast, N2H7+ and GlyH+ are not as effective in stabilizing the zwitterionic structure of glycine. This likely results from the more localized charge in NH4+ giving rise to stronger hydrogen bonds with the carboxylate moiety of zwitterionic glycine. This conjecture is supported by the computational results.Key words: high pressure mass spectrometry, glycine, gas-phase ion thermochemistry, ab initio calculations, cluster structure.

The formation of C6H7+ species in ion/molecule reactions in gaseous vinyl chloride was studied in a high pressure photoionization mass spectrometer and in a Fourier transform ion cyclotron resonance (FT-ICR) spectrometer. Collision-induced dissociation (CID) mass spectra of C4H5Cl+, C4H6Cl+, and C6H7+ species suggest a "butadiene-like" structure for the two former ions, and a non-benzenium structure for the last species. The C6H7+ ions are formed in a two-step mechanism involving C4H5+ as intermediate ions. These processes are in competition with condensation reactions leading to the formation of C6H7–9Cl+ species. Key words: ion–molecule reactions, gaseous vinyl chloride, collision-induced dissociation.