Isotope effects associated with tunneling and double proton transfer in the hydrogen bonds of benzoic acid

Abstract

The isotope effects associated with double proton transfer in the hydrogen bonds of benzoic acid (BA) dimers have been measured using field-cycling (1)H NMR relaxometry and quasielastic neutron scattering. By studying mixed isotope (hydrogen and deuterium) samples, the dynamics of three isotopologues, BA-HH, BA-HD, and BA-DD, have been investigated. Low temperature measurements provide accurate measurements of the incoherent tunneling rate, k(0). This parameter scales accurately with the mass number, m, according to the formula k(0)=(E/m)e(-Fm) providing conclusive evidence that the proton transfer process is a strongly correlated motion of two hydrons. Furthermore, we conclude that the tunneling pathway is the same for the three isotopologue species. Measurements at higher temperatures illuminate the through barrier processes that are mediated via intermediate or excited vibrational states. In parallel with the investigation of proton transfer dynamics, the theoretical and experimental aspects of studying spin-lattice relaxation in single crystals of mixed isotope samples are investigated in depth. Heteronuclear dipolar interactions between (1)H and (2)H isotopes contribute significantly to the overall proton spin-lattice relaxation and it is shown that these must be modeled correctly to obtain accurate values for the proton transfer rates. Since the sample used in the NMR measurements was a single crystal, full account of the orientation dependence of the spin-lattice relaxation with respect to the applied B field was incorporated into the data analysis.

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

“…For a recent account of this work the reader is referred to the study of Horsewill et al [74] who published the correlation times of the HH, HD and DD reactions given by…”

“…x 1 and x 2 represent the molefractions. The resulting Arrhenius diagram where the forward rate constants k 12 of Horsewill et al [74] are plotted as a function of the inverse temperature is depicted in Fig. 6.32.…”

Multiple Proton Transfer: From Stepwise to Concerted

“…Recently these measurements were extended to crystals in which all or part of the benzoic acid molecules carry a mobile deuteron instead of a mobile proton. In this way it was possible to measure rate constants as a function of temperature not only for HH but also for HD and DD transfer [49][50][51]. The resulting Arrhenius plots, illustrated in Fig.…”

“…The resulting Arrhenius plots, illustrated in Fig. 29.8, 923 Figure 29.8 Comparison of spin-lattice correlation rates measured by NMR relaxometry [51] with those calculated by AIM/DOIT [7] for solid benzoic acid isotopomers with mobile HH (top), HD (center), and DD (bottom) pairs. Measurements are depicted by symbols; the broken and dot-dash lines represent theoretical results for two limiting cases, the solid curve being their geometric mean [7].…”

Where are Protons and Deuterons in KH p D1 – p CO3? A Neutron Diffraction Study

“…Since there is no segregation of H and D atoms into separate domains or clusters, it might seem straightforward to suppose that the crystal is a mixture of HH, HD, DD dimers, with probabilities [19,20]. A random distribution of such dimers should destroy the translational invariance of the crystal, as well as-needless to say-the space group symmetry, and this should be evidenced by neutron diffraction.…”