EDITORIAL pubs.acs.org/JPCL
Chemical Dynamics of Electrons, Liquids, and Molecules near Dissociation he field of chemical dynamics brings to mind elegant studies in crossed molecular beams in which the details of reactive collisions are studied under single-collision conditions by controlling the reactants and probing the products in state-specific detail. While such studies remain a vibrant area of current research, the field of chemical dynamics continues to evolve in new directions that extend into an ever widening array of environments and time scales. In this issue, four Perspectives illustrate this fact in dramatic fashion. The Perspective from Arthur Suits1 has the closest link with molecular beam studies. In this Perspective, Suits describes a new pathway to reaction products involving roaming trajctories that is increasingly being understood as a pervasive part of the dynamics of molecules very near their dissociation limits. At these energies, the dissociating fragment can undergo complicated trajectories in which energy exchange with the rest of the molecule can bring it far from the molecule before returning to attack another part of the molecule, leading to molecular products unanticipated in the absence of such roaming trajectories. The internal energy distributions of products formed by this pathway provide characteristic signatures that Suits describes in some detail, with state-of-the-art imaging methods playing an important role in resolving these signatures experimentally. The synergy with theory is also evident here as trajectory calculations that capture these processes require accurate multidimensional potential energy surfaces at high energies. Several recent Letters have also had roaming reactions as a theme, including both experimental and theoretical studies on roaming in stable molecules and free radicals.2-4 Molecular beams also play a role in the Perspective written by Dempsey, Brastad, and Nathanson,5 which describes groundbreaking work on the chemical dynamics of interfacial protontransfer reactions occurring when a strong acid like DCl collides with protic liquid surfaces such as H2O or glycerol. By monitoring both DCl and HCl products emanating from the surface as a function of time, these authors are able to develop an atomicscale picture of processes occurring at the interface based on direct determination of quantum yields for bulk uptake, interfacial proton exchange, and unreactive desorption. Building on this theme of new probes of interfacial processes, Winter, Seidel, and Th€urmer present studies of X-ray photoelectron spectroscopy of aqueous solutions using liquid microjets.6 The seeming incongruity of detecting the unperturbed kinetic energies of photoelectrons from volatile liquids seems now to be possible by clever experimental designs which probe liquid microjets close to their surfaces. Here, the chemical dynamics is the electronic relaxation dynamics that accompanies photoexcitation in the liquid and subsequent ejection of the electron into vacuum. Both charge and energy exchange between solute and solvent are at play in this dynamics, which is being uncovered for the first time. Finally, the Perspective by Coppens7 has as its subject timeresolved pump-probe X-ray diffraction of solids. Molecular
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spectroscopy has traditionally determined structural changes occurring upon electronic excitation via the optical transition itself or using various forms of transient spectroscopy with infrared or optical detection. Exciting developments in the generation of X-ray pulses with picosecond or femtosecond time resolution are opening up new schemes for directly determining electronic excited-state geometries even of very short-lived excited states using time-resolved X-ray diffraction. These last two Perspectives illustrate the recent explosion of activity involving new X-ray light sources, which includes several other recent papers in The Journal of Physical Chemistry Letters.8,9 It is exciting to think about future developments in the everexpanding field of chemical dynamics that will be first described in the pages of this journal. Timothy S. Zwier Senior Editor Purdue University, West Lafayette, Indiana, United States
’ REFERENCES (1) Herath, N.; Suits, A. G. Roaming Radical Reactions. J. Phys. Chem. Lett. 2011, 2, 642–647. (2) Harding, L. B.; Klippenstein, S. L. Roaming Radical Pathways for Decomposition of Alkanes. J. Phys. Chem. Lett. 2010, 1, 3016–3020. (3) Kamarchik, E.; Koziol, L.; Reisler, H.; Bowman, J. M.; Krylov, A. I. Roaming Pathway Leading to Unexpected Water þ Vinyl Products in C2H5OH Dissociation. J. Phys. Chem. Lett. 2010, 1, 3058–3065. (4) Chen, C.; Braams, B.; Lee, D. Y.; Bowman, J. M.; Houston, P. L.; Stranges, D. Evidence for Vinylidene Production in the Photodissociation of the Allyl Radical. J. Phys. Chem. Lett. 2010, 1, 1875–1880. (5) Dempsey, L. P.; Brastad, S. M.; Nathanson, G. M. Interfacial Acid Dissociation in Diverse Environments following Collisions of DCl with Salty Glycerol and Salty Water. J. Phys. Chem. Lett. 2011, 2, 622–627. (6) Seidel, R.; Thurmer, S.; Winter, B. Photoelectron Spectroscopy Meets Aqueous Solution: Studies from a Vacuum Liquid Microjet. J. Phys. Chem. Lett. 2011, 2, 633–641. (7) Coppens, P. Molecular Excited-State Structure by Time-Resolved Pump-Probe X-Ray Diffraction. What Is New and What Are the Prospects for Future Progress? J. Phys. Chem. Lett. 2011, 2, 616–621. (8) Azia, E. F. X-ray Spectroscopies Revealing the Structure and Dynamics of Metalloprotein Active Centers. J. Phys. Chem. Lett. 2011, 2, 320–326. (9) Kim, J.; Kim, K. H.; Kim, J. G.; Kim, T. W.; Kim, Y.; Ihee, H. Anisotropic Picosecond X-ray Solution Scattering from Photoselectively Aligned Protein Molecules. J. Phys. Chem. Lett. 2011, 2, 350–356.
Published: March 17, 2011 681
dx.doi.org/10.1021/jz200176w | J. Phys. Chem. Lett. 2011, 2, 681–681
