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Characterization of protonated formamide clusters by vibrational predissociation spectroscopy confirms theoretical predictions that O-protonation occurs in preference to N-protonation in formamide. The confirmation is made from a close comparison of the infrared spectra of H + [HC(O)NH 2 ] 3 and NH 4 + [HC(O)NH 2 ] 3 produced by a supersonic expansion with the spectra produced by ab initio calculations. For NH 4 + [HC(O)NH 2 ] 3 , prominent and well-resolved vibrational features are observed at 3436 and 3554 cm -1 . They derive, respectively, from the symmetric and asymmetric NH 2 stretching motions of the three formamide molecules linked separately to the NH 4 + ion core via three N-H + ‚‚‚O hydrogen bonds. Similarly distinct absorption features are also found for H + [HC(O)NH 2 ] 3 ; moreover, they differ in frequency from the corresponding vibrational modes of NH 4 + [HC(O)NH 2 ] 3 by less than 10 cm -1 . The result is consistent with a picture of proton attachment to the oxygen atom, rather than the nitrogen atom in H + [HC(O)NH 2 ] 3 . We provide in this work both spectroscopic and computational evidence for the O-protonation of formamide and its clusters in the gas phase.

Rearrangement of hydrogen bonds in the protonated methanol-water cluster ion H + (CH 3 OH) 4 H 2 O is analyzed. The analysis, based on ab initio calculations performed at the B3LYP/aug-cc-pVTZ//6-31+G* and MP4/ 6-311+G*//B3LYP/6-31+G* levels of computation, provides information about potential minima, transition states, and pathways for the hydrogen bond rearrangement processes. Results of the analysis are compared systematically to the experimental measurements for H + (CH 3 OH) 4 H 2 O, where two distinct charge-centered (H 3 O + and CH 3 OH 2 + ) isomers have been identified in a supersonic expansion by fragment-dependent vibrational predissociation spectroscopy (Chaudhuri et al. J. Chem. Phys. 2000, 112, 7279). Revealed by the calculations, the lowest energy pathway for the transition from an open noncyclic hydronium-centered isomer [H 3 O + (CH 3 OH) 4 ] to a linear methyloxoium-centered isomer [CH 3 OH 2 + (CH 3 OH) 3 H 2 O] involves three stable intermediates and four transition states. The transition can go through either all four-membered ring isomers or a mixture of four-membered and five-membered ring intermediates. The latter is an energetically more favorable process because of less strain involved in the five-membered ring formation. A barrier height of <2.5 kcal/mol (after zero-point energy corrections) is predicted, suggesting that rapid interconversions among different isomers can occur at room temperature for this particular cluster cation.

Competitive solvation of the excess proton in protonated mixed methanol−water clusters [H+(CH3OH) m (H2O) n , m + n = 4] has been characterized by vibrational predissociation spectroscopy in combination with density functional theory calculations. The solvation topology of the clusters can be classified as (1) the closed shell, in which a hydronium ion H3O+ is fully solvated by three neutral molecules forming a complete solvation shell, and (2) the open chain, where the excess proton is tugged between two mixed subunits in a linear chain. The existence of these two types of isomer is verified from a close examination of the characteristic free-OH and hydrogen-bonded-OH stretching modes in the spectra. It is found that sequential replacement of the water molecule in H+(H2O)4 by methanol redistributes the population between the closed-shell and the open-chain isomers. While the excess proton is preferentially taken by methanol (instead of water) in the chain configuration, it can be either localized as CH3OH2 + or delocalized as CH3OH···H+···CH3OH at m ≥ 2, depending sensitively on the number of the methanol molecules and the symmetry of the cluster isomers. In contrast to that of NH4 +(NH3) m (H2O) n , m + n = 4, previously studied, this work provides a clear picture of competitive solvation of a charge between the constituent solvent molecules within a cluster.

The ability of several species of birds to synthesize L-ascorbic acid is correlated with their phylogeny. In the more primitive species, synthesis of L-ascorbic acid occurs in the kidney. Among the highly evolved passeriform species, kidney and liver can synthesize L-ascorbic acid in some, whereas in others synthesis occurs in the liver. In still others, the capacity for the synthesis of L-ascorbic acid is apparently lost. The pattern of evolution of the ascorbic acid pathway among birds is thus similar to that among mammals.

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We report VUV-photoionization based photofragmentation-translational spectroscopy data, providing a comprehensive study of the collision free photochemistry of methyl azide (CH3N3) at 193 nm. We report the first observation of the production of methyl and the N3 radical and derive the translational energy release distribution of this reaction. The most probable translation energy is only 8%, and the maximum translational energy is only 60% of the available energy, taking CH3 + linear N3 as the zero of energy. However, the maximum translational energy release is quantitatively consistent with production of the higher energy isomer cyclic N3. Threshold photoionization of the N3 fragment using tunable synchrotron radiation shows results consistent with theoretical predictions of the cyclic N3 ionization potential. The secondary dissociation of N3 --> N(2D) + N2 is also observed and its translational energy release is derived. This distribution peaks at approximately 6 and extends to 11 kcal/mol as would be expected from the size of the exit channel barrier for spin-allowed dissociation of cyclic N3 (7 kcal/mol) and, furthermore, inconsistent with the barrier height of the spin-allowed dissociation of linear N3 (3 kcal/mol). A large fraction (approximately 45%) of the N3 does not dissociate on the microsecond time scale of the experiment suggesting methyl azide may be the most attractive photochemical precursor of cyclic N3 yet found.