Molecules in Motion: Chemical Reaction and Allied Dynamics in Solution and Elsewhere

Abstract

After my acceptance of the kind invitation from Todd Martínez and Mark Johnson, Co-Editors of this journal, to write this article, I had to decide just how to actually do this, given the existence of a fairly personal and extended autobiographical account of recent vintage detailing my youth, education, and assorted experiences and activities at the University of Colorado, Boulder, and later also at Ecole Normale Supérieure in Paris (1). In the end, I settled on a differently styled recounting of the adventures with my students, postdocs, collaborators, and colleagues in trying to unravel, comprehend, describe, and occasionally even predict the manifestations and consequences of the myriad assortment of molecular dances that contribute to and govern the rates and mechanisms of chemical reactions in solution (and elsewhere TRANSLATIONAL, ROTATIONAL, AND SOLVATION DYNAMICS IN SOLUTIONThis is basically where it all began for me. Beyond their fundamental interest, these three types of dynamics (translational, rotational, and solvation) can be important for chemical reactions in assorted regimes, although admittedly this is perhaps not always so obvious: Translation is relevant for diffusion-controlled and -influenced reactions, water rotation is important for proton transfers (PTs), for example, and solvation dynamics can have a significant effect on any reaction involving the redistribution or rearrangement of charges. I give only brief commentaries on our work at the University of Colorado, Boulder, on these types of dynamics in the 1970s and 1980s and end with more recent aspects carried out at Ecole Normal Supérieure (ENS).In the 1970s, it was thought by not a few that macroscopic continuum hydrodynamics-as reflected in, e.g., Stokes' friction laws for translation and rotation, the Stokes-Einstein equation for translational diffusion, and the Debye-Stokes-Einstein relation for rotational or reorientational diffusion-could actually apply in detail, and even quantitatively, at a molecular level. I myself have used (and continue to use) continuum hydrodynamic and dielectric models to provide concepts, perspectives, and guides for experiment. Clearly, I like such models greatly, but I just could not accept going this far. Together with Mike Weinberg and Ray Kapral (2-4), we addressed the issue with what we called a microscopic boundary layer approach, combining a molecular collisional picture for the immediate solvent neighborhood of a rotating or translating solute molecule with a hydrodynamic description of the remaining, outer solvent. We showed that for both rotational and translational friction (and thus for diffusion), the molecular aspects were indeed dominant. Although hydrodynamic influences could also enter, they only became the dominant story when the solute was far larger than the solvent molecules. We also found that the supposed agreement of assorted simulations and experiments with the hydrodynamic view disappeared upon suitable scrutiny. Solvation DynamicsThis flavor of dynamics came into it...

Dinitrogen Coupling to a Terpyridine-Molybdenum Chromophore Is Switched on by Fermi Resonance

“…This coupling was shown to be responsible for energy buildup via intramolecular energy flow at the O-O bond, eventually leading to its cleavage. 6,12 Furthermore, in the reaction of CHD 3 and Cl en route to H-Cl bond formation, both the C-H stretch and the CD 3 umbrella modes mutually couple to the reaction coordinate, thereby lowering the overall activation barrier. 11 In general, such coupling phenomena induce distortions that reduce structural and thermodynamic barriers for vibrational energy flow, which could ultimately induce selective photoreactivity in Mo 2 N 2 .…”

“…Identifying and energizing reaction-specific vibrational motions is an emerging strategy to achieve exquisite control over chemical reactions. [1][2][3][4][5][6][7][8][9][10][11][12][13] This approach of targeting specific vibrations provides a handle to surpass thermodynamic barriers via photoexcitation with more precision and at lower energy cost. Selective vibrational excitations of target functionalities have been found to promote reactivity in systems such as the gas-phase reaction of CH 4 and Cl atom, C-H bond cleavage in CHD 3 on a nickel surface, bond cleavage in NO on a Cu surface, and O-O bond cleavage in H 2 O 2 .…”

“…Selective vibrational excitations of target functionalities have been found to promote reactivity in systems such as the gas-phase reaction of CH 4 and Cl atom, C-H bond cleavage in CHD 3 on a nickel surface, bond cleavage in NO on a Cu surface, and O-O bond cleavage in H 2 O 2 . [1][2][3][4][5][6][7] These pioneering experimental and theoretical studies of low-dimensional, gas-phase small molecules demonstrated the role of intramolecular vibrations in facilitating the activation of specific bonds both directly through energetic excitations and indirectly via intramolecular vibrational redistribution (IVR). We seek to explore how, in large and complex systems, mechanisms could be implemented to enable controlled and directed energy flow into the reaction-specific motions for selective bond activations.…”

Atomistic characterization of the active-site solvation dynamics of a model photocatalyst

“…Solvation, the local organization of the solvent molecules around a solute, has a central role in the description of condensed phase chemical properties. The influence of the solvent on the solute structure and reaction dynamics has been extensively studied123, and the general success of continuum model descriptions bring into question the importance of short ranged molecular effects456. A diverse range of theoretical and simulation methods can decompose solvation dynamics and energetics into distinct short and long-range interactions, but few experiments provide robust tests of these models78.…”

CO2 capture in amine solutions: modelling and simulations with non-empirical methods

“…In conclusion, we believe that computer simulations can become an essential tool for materials and process design, not only by elucidating the reaction mechanisms and the specific role of solvent, 65,66 but also in providing quantitative information. Especially, the next "grand challenge" is the evaluation of the kinetic rates directly from the simulated trajectories.…”