Proton Transfer To Solvent Research Articles

IR Spectroscopy of Protonated Acetylacetone and Its Water Clusters: Enol-Keto Tautomers and Ion→Solvent Proton Transfer.

Protonated ions of acetylacetone, H+(Hacac), and their argon-tagged analogues are produced via a pulsed discharge and cooled in a supersonic expansion. These ions are mass analyzed, selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy using the method of rare-gas atom tagging. Computational studies at the DFT/B3LYP level are employed to elucidate the structures and spectra of these ions, which are expected to exist as either enol- or keto-based tautomers. The protonated acetylacetone ion is found to form a single enol-based isomer. Adding one or two water molecules to this ion, for example, H+(Hacac)(H2O)1,2, produces primarily enol-based structures, although a small concentration of keto structures also contribute to the spectra. The vibrational patterns resulting from hydrogen bonding in these systems are not well-described by theory. Addition of a third water molecule to form the H+(Hacac)(H2O)3 ion causes a significant change in the spectroscopy, attributed to proton transfer from the H+(Hacac) ion into the water solvent.

Substrate specificity and kinetic mechanism of purine nucleoside phosphorylase from Mycobacterium tuberculosis

Purine nucleoside phosphorylase from Mycobacterium tuberculosis (MtPNP) is numbered among targets for persistence of the causative agent of tuberculosis. Here, it is shown that MtPNP is more specific to natural 6-oxopurine nucleosides and synthetic compounds, and does not catalyze the phosphorolysis of adenosine. Initial velocity, product inhibition and equilibrium binding data suggest that MtPNP catalyzes 2′-deoxyguanosine (2dGuo) phosphorolysis by a steady-state ordered bi bi kinetic mechanism, in which inorganic phosphate (P i) binds first followed by 2dGuo, and ribose 1-phosphate dissociates first followed by guanine. pH-rate profiles indicated a general acid as being essential for both catalysis and 2dGuo binding, and that deprotonation of a group abolishes P i binding. Proton inventory and solvent deuterium isotope effects indicate that a single solvent proton transfer makes a modest contribution to the rate-limiting step. Pre-steady-state kinetic data indicate that product release appears to contribute to the rate-limiting step for MtPNP-catalyzed reaction.

Characterization of Solvent and Deuterium Isotope Effects on Nonadiabatic Proton Transfer in the Benzophenone/N,N-Dimethylaniline Contact Radical Ion Pair

The dynamics of proton transfer within a variety of substituted benzophenones/N,N-dimethylaniline contact radical ion pairs are examined in a wide range of solvent polarities. The correlation of the rate constants with the thermodynamic driving force reveals both a normal and inverted region for proton transfer in solvents with an ET30 value of less than 43.1; in solvents with ET30 greater than 43.8, only the normal region is observed. Also, the kinetic deuterium isotope effect is examined. The solvent and isotope dependence for the transfer process is examined within the context of the Lee−Hynes model for nonadiabatic proton transfer. The theoretical analysis of the experimental data suggests that the reaction path for proton transfer involves tunneling. Conventional transition state theory with the inclusion of tunneling in the region of the transition state cannot account for the observed kinetic behavior.

Photochemistry of “Super” Photoacids. 2. Excited-State Proton Transfer in Methanol/Water Mixtures

Excited-state proton transfer to solvent (PTTS) of 5-cyano-2-naphthol was investigated in methanol/water mixtures. We have found that the time-resolved fluorescence data fit the solution of the Debye−Smoluchowski equation for the reversible geminate recombination of ions over the whole range of methanol/water concentration ratios. The rate constants of the elementary protolytic photochemical processes and their isotope effects were determined by a simultaneous analysis of the time-resolved fluorescence of the photoacid and its conjugated anion. The competition between adiabatic protonation and quenching of the excited naphtholate anion by the geminate proton was observed to diminish sharply near pure methanol. The dissociation rate coefficient near pure methanol depends on a power of the water concentration, which appears to decrease from 2 (for “ordinary” photoacids) to below 1 for “super” photoacids.

Surface enhanced raman scattering and photodimerization of pyridyl-substituted ethylenes at a silver electrode surface

Pyridyl-substituted ethylenes have been observed to photodimerize while adsorbed at a polycrystalline silver electrode surface. Surface Raman spectra reveal that vinylpyridine and bis-pyridyl ethylene dimerize to products in which ethylenic bonds have been reduced. The potential dependence of the photochemical threshold wavelength implicates the role of adsorbateto-metal charge transfer excitation in the dimerization mechanism.Adsorbate-to-metal charge transfer leaves a cation radical which initiates dimerization with nearest neighbor adsorbate molecules. Photochemistry occurs only under conditions of heavy surface coverage. A prerequisite to the topochemical dimerization of ethylenic compounds in the solid state requires that the separation between neighboring ethylene groups lie between 3.7 and 4.1 Å. The silver lattice parameter of 4.08 Å and the absence of photochemistry under light surface coverages appear to correlate well to the solid state topochemical prerequisite. Observed isotopic shifts in the surface Raman spectra of the photoproducts prepared in H 2O/D 2O electrolytes indicates that solvent proton transfer occurs during dimerization. Adsorbate surface orientation was determined and the structures of photoproducts are identified.