The hydrogen-bridged radical cation [H2O⋯H⋯OCOH·+: A combined experimental and theoretical study of its stability and dissociation chemistry

Proton mobility in water clusters

Proton mobility in water occurs quickly according to the so-called Grotthuss mechanism. This process and its elementary reaction steps can be studied in great detail by applying suitable mass spectrometric methods to ionic water clusters. Careful choice of suitable core ions in combination with analysis of cluster size trends in hydrogen/deuterium isotope exchange rates allows for detailed insights into fascinating dynamical systems. Analysis of the experiments has been promoted by extensive and systematic quantum chemical model calculations. Detailed low-energy mechanistic pathways for efficient water rearrangement and proton transfer steps, in particular cases along short preformed "wires" of hydrogen bonds, have been identified in consistency with experimental findings.

Hydrogen Bonding Interactions of Pyridine•+ with Water: Stepwise Solvation of Distonic Cations

The solvation energies of the pyridine*+ radical cation by 1-4 H2O molecules were determined by equilibrium measurements in a drift cell. The binding energies of the pyridine*+(H2O)n clusters are similar to the binding energies of protonated pyridine-water clusters, (C5H5NH+)(H2O)n, which involve NH+..OH2 bonds and different from those of the solvated benzene radical cation-water clusters, C6H6*+(H2O)n, which involve CHdelta+..OH2 bonds. These relations indicate that the observed pyridine*+ ions have the distonic *C5H4NH+ structures that can form NH+..OH2 bonds. The observed thermochemistry and ab initio calculations show that these bonds are not affected significantly by an unpaired electron at another site of the ion. Similar observations also identify the 2-fluoropyridine*+ distonic ion. The distonic structure is also consistent with the reactivity of pyridine*+ in H atom transfer, intra-cluster proton transfer and deprotonation reactions. The results present the first measured stepwise solvation energies of distonic ions, and demonstrate that cluster thermochemistry can identify distonic structures.

A prototype for catalyzed amide bond cleavage: production of the [NH(3), H(2)O](*)(+) dimer from ionized formamide and its carbene isomer

The reaction of ionized formamide H(2)NCHO(*)(+) with water leads to an exclusive loss of CO from the complex. This contrasts with the unimolecular reaction of low-energy ionized formamide, which loses exclusively one hydrogen atom. The unimolecular loss of CO is not observed because it involves several H-transfers corresponding to high-energy barriers. Experimental and theoretical studies of the role of solvation by water on the fragmentation of ionized formamide leads to three different results: (i) In contrast with different systems previously studied, in which solvation plays only a role on one or two steps of a reaction, a molecule of water is efficient in the catalysis of the decarbonylation process because water catalyzes all the steps of the reaction of ionized formamide, including the final dissociation of the amide bond. (ii) The catalyzed isomerization of carbonylic radical cations into their carbene counterparts is shown to be an important step in the process. To study this step, a precise probe, characterizing the carbene structure by ion-molecule reaction, is for the first time described. (iii) Finally, decarbonylation of ionized formamide yields the [NH(3), H(2)O](*)(+) ion, which has not been generated and experimentally studied previously. By this method, the [NH(3), H(2)O](*)(+) ion is generated in abundance and with a low internal energy content, allowing one either to prepare, by ligand exchange, a series of other solvated radical cations or to generate covalent structures such as distonic ions. First results on related systems indicate that the conclusions obtained for ionized formamide are widespread.

Computational chemistry: a useful (sometimes mandatory) tool in mass spectrometry studies

In this review, we present a brief summary of the theoretical methods most frequently used in gas-phase ion chemistry. In subsequent sections, the performance of these methods is analyzed, paying attention to the reliability of geometries, vibrational frequencies, energies, and entropies. The possible pathologies of the different methods, in the form of instabilities of the wave function or spin contamination problems, are discussed. Several examples are presented to illustrate the usefulness of ab initio or density functional theory (DFT) methods to predict the existence of elusive molecules and/or to characterize non-conventional structures, and to rationalize the charge redistributions normally associated with ion-molecule interactions and which result in bond-weakening or bond-reinforcement effects. Finally, the role of non-classical structures in ion-molecule interactions is also illustrated with different examples.

The decarbonylation of ionized β-hydroxypyruvic acid: the hydrogen-bridged radical cation [CH2=O...H...==C-OH].+studied by experiment and theory

The intriguing gas-phase ion chemistry of β-hydroxypyruvic acid (HPA), HOCH2C(==O)COOH, has been investigated using tandem mass spectrometry (metastable ion (MI) and (multiple) collision-induced dissociation (CID) experiments, neutralization-reionization mass spectrometry (NRMS),18O and D isotopic labelling on both the acid and its methyl ester) in conjunction with computational chemistry (ab initio MO and density functional theories). HPA does not enolize upon evaporation, but it retains its keto structure. When ionized, decarbonylation occurs and, depending on the internal-energy content, this dissociation reaction proceeds via two distinct routes. The source-generated, high-energy ions lose the keto C==O, not via a least-motion extrusion into ionized glycolic acid, HOCH2COOH.+, but via a rearrangement that yields the title H-bridged radical cation CH2==O...H...O==C-OH.+for which Δ Hf0= 99 ± 3 kcal/mol. The long-lived low-energy ions enolize prior to decarbonylation and lose the carboxyl C==O. Again, this is not a least-motion extrusion (which would produce the most stable isomer, HOC(H)==C(OH)2.+Δ Hf0= 73 kcal/mol) but a rearrangement yielding the ion-dipole complex HOC(H)C==C==O.+/H2O. The methyl ester of HPA behaves analogously, yielding CH2==O...H...O==C-OCH3.+and HOC(H)C==C==O.+/ CH3OH upon decarbonylation of the high- and low-energy ions, respectively. Decarboxylation into the ylidion CH2OH2.+characterizes the dissociation chemistry of both the title H-bridged ion and its glycolic acid isomer HOCH2COOH.+. A computational analysis of this reaction (which satisfies the experimental observations) leads to the proposal that the decarboxylation of the acid occurs via CH2-O(H)...H...==C==O.+as the key intermediate, whereas the title H-bridged ion follows a higher energy route that involves ion-dipole rotations leading to the ionized carbene HO(H2)CO-C-OH.+and the distonic ion H2O-C(H2)-O-C==O.+as key intermediates.Key words: tandem mass spectrometry, hydrogen-bridged radical cation, hydroxypyruvic acid, ab initio calculations, keto-enol tautomerization,18O labelling.