The Liquid Side

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Research Summary


The current goal of this research group is to assemble a detailed microscopic understanding of chemical reaction dynamics in solution.  The most basic reactions in solution share certain reaction processes, which include dissociation, recombination, solvent caging, intramolecular morphology changes, and system-solvent interaction. These molecular events which underlie our basic understanding of reaction dynamics occur on an ultrafast (femtosecond-nanosecond) timescale.  This research group utilizes advanced experimental techniques along with modern computational methods to study these fundamental issues. The femtosecond infrared lasers developed in our group enable us to study a variety of chemical systems in the time and spectral domain.  These setups allow us to follow elementary chemical reaction events as they occur. There are also workstations capable of carrying out large-scale, high-level quantum chemical calculations and molecular dynamics (MD) simulations. We also have access to the supercomputer facilities at the Lawrence Berkeley National Laboratory. Theoretical methods including ab initio and density functional theory (DFT) as well as MD are utilized to assist interpretation and realization of experimental results.

Current chemical systems under study include alkane functionalization by transition metal intermediates, molecular rearangement at transition states in solution as well as chemical disproportionation and ring slip mechanisms in organometallic reaction dynamics. The properties of a solvent are also altered in our experiments by using high pressure and supercritial fluids. A new femtosecond laser system has been acquired in order to study novel chemical dynamics, which, in conjunction with our extensive computational resources, will allow us to address what physical principals are responsible for dynamical barrier crossing in condensed phase media. In this respect our projects are unified in that they explore different dynamical aspects of the same general principals that are responsible for most of the chemistry that is of current industrial and academic interest.

This material is based upon work supported by the National Science Foundation.
Experimental Setup

Experimental Setup Diagram
Schematic of the system setup using commercial laser output.

A new commercial laser system (Spectra-Physics) has recently been set up to run ultrafast UV-pump IR-probe experiments.  The system uses a Ti:Sapphire oscillator (800 nm, 1.0 W, 80 MHz) coupled to a Ti:Sapphire regenerative amplifier to give stable femtosecond pulses (110 fs, 800 nm, 1.0 W, 1.0 kHz).  UV pulses are generated using a series of two crystals to triple the laser output giving pump pulses centered at 266 nm.  A home-built OPA is used to generate the mid-IR probe pulses tunable from 3.0-6.0 microns.  Pump and probe beams are then focused onto the sample.  The timing between the pump and the probe beam can be varied using a computer controlled translational stage to obtain information about the system dynamics.  The IR probe beam is split into signal and reference lines in order to monitor shot-to-shot noise fluctuations and variations in sample conditions are monitored using an optical chopper in the pump line.  Changes in signal absorption are detected using a 32-element MCT array detector.  

        fig1                
Example of difference spectra obtained in pump probe experiments.

Electronic Structure Calculations


                                       fig2
Example of some optimized structures obtained using DFT.

Ab initio and density functional theory (DFT) are used to elucidate experimental data.  Geometry optimizations can be used to gain a perspective on the processes of interest and facilitate chemical intuition.  Frequency calculations are used to anticipate experimental results and to make peak assignements in conjunction with those results.  
md
One proposed mechanism for solvation studied using simulations.
Once experimental data has been collected on a particular system, the work of constructing a model to explain that data begins.  The model will provide a complete description of the reaction dynamics that were observed.  An ideal way of testing such a proposed framework is to construct a molecular dynamics simulation.  Such a simulation will probe the fundamental assertions of the model and conclusions about validity can be made.
Systems Being Studied

The liquid side of the Harris Group is interested in looking at organometallic reactions in the condensed phase.  These systems are ideally suited to UV-pump IR probe spectroscopy.  Reactions are initiated using ultraviolet light and the reaction dynamics are following using infrared light.  Carbonyl groups are excellent indicators of electron density at metal centers due to backbonding properties and are also strong IR absorbers.  Using token ligands such as the carbonyl group changes that occur in the reaction sequence can be tracked.  
backbonding

Recent Publications
(click on title to view paper)
  1. "Ligand Rearrangement Reactions of Cr(CO)6 in Alcohol Solutions: Experiment and Theory" J.E. Shanoski, E.A. Glascoe, and C.B. Harris, J. Phys. Chem. B. 110, p.996 (2006).
  1. "Nature and Role of Bridged Carbonyl Intermediates in the Ultrafast Photoinduced Rearrangement of Ru3(CO)12" E.A. Glascoe, M.F. Kling, J.E. Shanoski, and C.B. Harris, Organometallics 25, p. 775, (2006).
  1. "19-Electron Intermediates and Cage-Effects in the Photochemical Disproportionation of [CpW(CO)3]2 with Lewis Bases" J.F. Cahoon, M.F. Kling, S. Schmatz, and C.B. Harris, J. Am. Chem. Soc. 127, p. 12555 (2005).
  1. "Ultrafast Infrared Mechanistic Studies of the Interaction of 1-Hexyne with Group 6 Hexacarbonyl Complexes" J.E. Shanoski, C.K. Payne, M.F. Kling, E.A. Glascoe, and C.B. Harris, Organometallics, 24, p. 1852 (2005).

  2. "Mechanism of Ligand Exchange Studied Using Transition Path Sampling" P.T. Snee, J.E. Shanoski, and C.B. Harris, J. Am. Chem. Soc., 127, p.1286 (2005).

  3. "The role of odd-electron intermediates and in-cage electron transfer in ultrafast photochemical disproportionation reactions in Lewis bases" M.F. Kling, J.F. Cahoon, E.A. Glascoe, J.E. Shanoski, and C.B. Harris, J. Am. Chem. Soc., vol. 126, p. 11414 (2004).

  4. "Ultrafast UV Pump / IR Probe Studies of CH Activation in Linear, Cyclic and Aromatic Hydrocarbons" M. C. Asplund, P. T. Snee, J. S. Yeston, M. J. Wilkens, C. K. Payne, H. Yang, H. Frei, R. G. Bergman, and C. B. Harris, J. Am. Chem. Soc., vol. 124, p. 10605, (2002).

  5. "Intramolecular Rearrangements on the Ultrafast Timescale: Femtosecond Infrared Studies of Ring Slip in (C5Cl5)-Mn(CO)5 " C. K. Payne, P. T. Snee, H. Yang, K. T. Kotz, L. Schafer, T. D. Tilley and C. B. Harris , J. Am. Chem. Soc., vol. 123, p. 7425, (2001).<><>

  6. "Dynamics of Photosubstitution Reactions of Fe(CO)5, An Ultrafast Infrared Study of High Spin Reactivity. " P. T. Snee, C. K. Payne, S. D. Mebane, and C. B. Harris , J. Am. Chem. Soc., vol. 123, p. 6909, (2001).

  7. "Femtosecond Infrared Study of the Dynamics of Solvation and Solvent Caging" H. Yang, P. T. Snee, K. T. Kotz, C. K. Payne, and C. B. Harris , J. Am. Chem. Soc., vol. 123, p. 4204, (2001).

  8. "High Spin Reactivity Under Ambient Conditions: An Ultrafast UV-Pump IR-Probe Study " P. T. Snee, C. K. Payne, K. T. Kotz, H. Yang, and C. B. Harris , J. Am. Chem. Soc., vol. 123 p. 2255-2264, (2001).

  9. "Femtosecond infrared studies of ligand rearrangement reactions: silyl hydride products from Group 6 carbonyls " K. T. Kotz, H. Yang, P. T. Snee, C. K. Payne and C. B. Harris , J. Organometallic Chem., vol. 596, p.16303-16327, (2000).

  10. "Ultrafast Infrared Studies of the Reaction Mechanism of Silicon-Hydrogen Bond Activation by CpV(CO)4," P. T. Snee, H. Yang, K. T. Kotz, C. K. Payne, and C. B. Harris, JPC-A, vol. 103, p.10426-10432, (l999).

  11. "Femtosecond Infrared Studies of a Prototypical One-Electron Oxidative-Addition Reaction: Chlorine Atom Abstraction by the Re(CO)5 Radical ," H. Yang, P. Snee, K. Kotz, C. K. Payne, H. Frei, and C. B. Harris, J. Am. Chem. Soc., vol. 121, p.9227-9228, (l999).

  12. "Ultrafast Infrared Studies of Bond Activation in Organometallic Complexes ," H. Yang, K. Kotz, M. C. Asplund, M. J. Wilkens, and C. B. Harris, Acc. Chem. Res., vol. 32, p.151-160, (l999). 

  13. "The Reaction Mechanism of Silicon- Hydrogen Bond Activation Studied Using Femtosecond to Nanosecond IR Spectroscopy and Ab Initio Methods," H. Yang, M. Asplund, K. Kotz, M. M. Wilkens, H. Frei and C. B. Harris, J. Am. Chem. Soc, vol. 120, p.10154-10165, (l998).

  14. "The Mechanism of C-H Bond Activation Reaction in Room Temperature Alkane Solutions," S. E. Bromberg, H. Yang, M. Asplund T. Lian, B. McNamara, K. Kotz, J. S. Yetson, M. Wilkens, H. Frei, R. Bergman and C. B. Harris, Science, Vol. 278, p. 260, (l997). (cover)

  15. "Femtosecond Infrared Studies of Silane Silicon-Hydrogen Bond Activation," H. Yang, K. Kotz, M. Asplund and C. B. Harris, J. Am. Chem. Soc, vol. 119, 40, p.9564-9565, (l997)."Femtosecond IR Studies of Solvation by Probing the Solvent, " T. Lian, H. Yang, M. Asplund, S. Bromberg and C. B. Harris, Ultrafast Phenomena X. P. Barbara et al. ed. Springer Verlag Berlin, p. 300, (l996).

  16. "Femtosecond IR Studies of C-H Bond Activation by Organometallic Compounds," C. B. Harris, T. Lian, S. Bromberg, H. Yang, M. Asplund, R. Bergman, Ultrafast Phenomena X , P. Barbara et al. ed. Springer Verlag Berlin, p. 237, (l996).

  17. "Femtosecond Infrared Studies of the Dissociation of Metal Carbonyls in Solution," T. Lian. S. E. Bromberg, M. Asplund, H. Yang and C. B. Harris, J. Phys. Chem. 100, 29, 11994, (l996).

  18. "Femtosecond IR Studies of Alkane C-H Bond Activation by Organometallic Compounds: Direct Observation of Reactive Intermediates in Room Temperature Solultions," T. Lian, S. E. Bromberg, H. Yang, G. Proulx, R. G. Bergman and C. B. Harris, J. Am. Chem. Soc, 118, 15, p. 3769-3770, (l996).


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