Proton transfer in benzoic acid crystals: Another look using quantum operator theory

Abstract

Articles you may be interested inNovel quantum mechanical/molecular mechanical method combined with the theory of energy representation: Free energy calculation for the Beckmann rearrangement promoted by proton transfers in the supercritical water Isotope effects associated with tunneling and double proton transfer in the hydrogen bonds of benzoic acid Double proton transfer in the complex of acetic acid with methanol: Theory versus experimentWe present a calculation of the rate of synchronous double proton transfer in benzoic acid crystals. Experiments on these systems have been performed over a wide range of temperatures ͑roughly 10-400°K͒. Even though the energetic barrier for proton transfer is rather high, the observed activation energy is low, while kinetic isotope experiments seem to indicate classical transfer. The system exhibits significant quantum character even at high temperatures and we show that the observed low activation energies can be reproduced assuming that the reaction is ''assisted'' by a low-frequency intramolecular mode, as has been suggested in different contexts by Benderskii ͓V. A. Benderskii, S. Yu. Grebenshchikov, and G. V. Mil'nikov, Chem. Phys. 194, 1 ͑1995͔͒, Hynes ͓D. Borgis and J. Hynes, J. Chem. Phys. 94, 3619 ͑1991͔͒ and Silbey ͓A. Suarez and R. Silbey, J. Chem. Phys. 94, 4809 ͑1991͔͒. We use our previous work on the quantum Kramers problem to perform a fully quantum calculation that incorporates symmetric coupling to the intramolecular mode and coupling to the condensed environment to all orders. We calculate the activation energies for hydrogen and deuterium transfer and we show that our results are in quantitative agreement with the experiment.

Barrier passage and protein dynamics in enzymatically catalyzed reactions

“…Note that in this case, the oscillator that is symmetrically coupled, represented by the last term in Eqn (3), is in fact a physical oscillation of the environment. We were able to develop a theory [38] of reactions mathematically represented by the Hamiltonian in Eqn (3), and using this method and experimentally available parameters for the benzoic acid proton transfer potential, we were able to reproduce experimental kinetics as long as we included a symmetrically coupled vibration [39]. The results are shown in Table 1 below.…”

A 13C field-cycling NMR relaxometry investigation of proton tunnelling in the hydrogen bond: Dynamic isotope effects, the influence of heteronuclear interactions and coupled relaxation

“…They interchange by double proton transfer within the bridging hydrogen bonds of the dimer. The mechanism underlying the motion is dominated by phonon-assisted tunnelling and this is well established as the model system for translational molecular tunnelling [5,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. We have made 99% 13 C substitution of the carboxycarbon which now acts as a Ôspy nucleusÕ monitoring the concerted motion of the two hydrogen atoms in the hydrogen bonds that bridge the BA dimer.…”

Protein Dynamics and Enzymatic Chemical Barrier Passage

“…We realized that what was missing was a specific motion in the system that symmetrically moved the proton donor and acceptor closer to each other. Mathematically, we showed that a symmetric coupling to an environmental bath mode, rather than an antisymmetric coupling as in the Zwanzig picture characterized this motion 9 . The fact that this symmetric coupling gave essentially different physics was not a surprise – Marcus had discussed just such an effect in gas phase dynamics many years earlier 10 .…”