Single and Multiple Hydrogen/Deuterium Transfer Reactions in Liquids and Solids

Abstract

OverviewIn this chapter, Arrhenius curves of selected single and multiple hydrogen transfer reactions for which kinetic data are available over a large temperature range are reviewed. The curves are described by a combination of formal kinetics of reaction networks and the one-dimensional Bell-Limbach tunneling model for each reaction step. The main parameters of this model are the barrier heights and barrier widths of the isotopic reactions, the tunneling masses, the pre-exponential factor and a minimum energy for tunneling to occur. This approach allows one to compare efficiently very different reactions studied in different environments and to prepare the kinetic data for higher-dimensional quantum-mechanical treatments. The first type of reactions discussed is concerned with those where the hydrogen bond geometries of the reacting molecules are well established and where kinetic data of the isotopic reactions are available over a large temperature range. Here, it is possible to study the relation between kinetic isotope effects and chemical structure. Examples are the tautomerism of porphyrin, of the porphyrin anion and related compounds exhibiting intramolecular hydrogen bonds of medium strength, and the solid state tautomerism of pyrazoles and of benzoic acid in cyclic associates. One main result is the finding of pre-exponential factors of the order of kT/h @ 10 13 s -1 , as expected by transition state theory for vanishing activation entropies. The barriers of multiple H-transfers are found to be larger than those of single H-transfers.The second type of reactions discussed refers mostly to liquid state solutions and involves major heavy atom reorganization. Here, equilibria between reactive and nonor less reactive molecular configurations may play a role. Several cases are discussed where the less reactive forms dominate at low or at high temperature, leading to unusual Arrhenius curves. These cases include examples from small molecule solution chemistry like the base-catalyzed intramolecular H-transfer in diaryltriazene, 2-(2¢-hydroxyphenyl)-benzoxazole, 2-hydroxy-phenoxyl radicals as well as an enzymatic system, thermophilic alcohol dehydrogenase. In the latter case, temperature dependent kinetic isotope effects are interpreted in terms of a transition between two regimes with different temperature independent kinetic isotope effects. 135Hydrogen-Transfer Reactions. Edited by

Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid

“…11. Comparison of the Arrhenius diagrams of the double proton and deuteron transfers in benzoic acid dimers [1,15], 2,4-diphenyl-3-brompyrazole dimers [31] and crystalline PCA. the forward rate constants k LL AB , LL = HH, HD, DD according to the analysis presented in Ref.…”

“…the forward rate constants k LL AB , LL = HH, HD, DD according to the analysis presented in Ref. [1].…”

“…Kinetic H/D isotope effects are then derived from longitudinal 2 H relaxation times at two different deuterium fractions in the mobile proton sites. Finally, the data are discussed in comparison with previous results on pyrazole [1,15] and benzoic acid [31] dimers.…”

“…As proton transfer is usually coupled to charge transfer, it is not surprising that there has been a special experimental [1][2][3] and theoretical interest [4][5][6] in neutral hydrogen transfer reactions, where one H is transferred along a resonance-assisted intramolecular hydrogen bond or where two or more hydrogen atoms are transferred in coupled hydrogen bonds. Of particular interest are (i) solid state measurements by which diffusion of reactants is avoided, and (ii) measurements of kinetic hydrogen/deuterium isotope effects of degenerate hydrogen transfer reactions which provide information about tunneling of the light hydrons (proton, deuteron, triton) and the contribution of heavy atom motions.…”

“…Such data can be obtained preferentially by solid state NMR line shape analyses for processes on the millisecond [7,8] and by relaxation studies for processes on the micro-to nanosecond time scale. 1 H relaxation of static powders is based on the modulation of homonuclear dipolar couplings upon motions of H in hydrogen bonds [9,10]. H jumps from and to nitrogen may not only modulate 15 N chemical shifts giving rise to typical line shape changes but also 1 H- 15 N heteronuclear dipolar couplings which may be observed by the measurement of 15 N longitudinal relaxation times under magic angle spinning (MAS) conditions and cross polarization (CP) techniques [11].…”