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Scholarly Interest Report
         
Philip R. Brooks
Professor
Professor of Chemistry
 
e-mail:brooks@rice.edu
 
  • Ph.D. (1964) University of California, Berkeley, Berkeley, CA
  • B.S. (1960) California Institute of Technology, Pasadena, CA
 
Primary Department
   Department of Chemistry
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Department Affiliations
 
  • Rice Quantum Institute
  •  
    Websites
     Phil Brooks
     
    Research Areas
     Physical Chemistry
     

    Research Statement 2009

     

    Electrons are the glue that binds molecules together, and the transfer of an electron is one of the first steps in making a chemical bond. Well-known example of these processes range from those occurring in automobile storage batteries to those occurring in the eye when light is detected. We wish to understand how the electron is transferred. Is the molecular negative ion stable or does it break up? Can we think of the electron being transferred to a localized site on the molecule? If so, does transfer to different sites result in different negative ions? We are able to gain some insight into these questions by studying electron transfer collisions in molecular beams. A beam of fast donor atoms collides with acceptor molecules which have been oriented in space and the electron transfer can be probed for collision at one end of a molecule, or at the other. The negative ions can be identified by mass spectrometry. A schematic diagram of the apparatus is shown in Fig 1. Molecules such as CH3Br have quantum states corresponding to "up" and "down" orientations and these can be separated in an inhomogeneous hexapole electric field. The molecules emerging from the focusing field are oriented along the direction of a uniform (weak) electric field and the molecular orientation can be changed by changing the direction of the weak field. The incoming atoms can then attack either the Br or CH3 end of the molecule. brooks Fig1 Fig. 1


    Our recent results on electron transfer to the haloforms, CF3H, CCl3H and CBr3H, serve to illustrate what we can learn. Fig 2 shows the steric asymmetry factor, G, measured for the formation of the respective halide ion (F-, Cl-, or Br--) for each molecule. G is defined at the difference in signal for attacking the different ends divided by the sum of the signals, G=(S--S+)/(S-+S+). If reaction occurs at only the negative end, G will be +1, (-1 for the positive end) and if the ends are equally reactive G=0. As shown in Fig 2, G is nearly zero for bromoform and chlorform because they are almost spherical. But for fluoroform, high energy collisions strongly favor formation of F- for attack at the F-end of the molecule. The steric specificity rises as the energy is lowered, and suddenly reverses, showing that F- is formed preferentially on attack at the H end. At low energies the electron enters the LUMO and if the energy is close enough to threshold the nascent F- will combine with the nascent K+ donor to form KF, which is undetectable in these experiments. F- production will favor electron transfer on the far side of the molecule, near the H atom where the production of KF is negligible. Only fluoroform shows this behavior because the LUMO in CF3H is the σ*CH orbital and the electron can be transferred to this orbital when the K atom attacks the H end of the molecule, far from the nascent F-. The LUMOs for CCl3H and CBr3H are the σ*CX orbitals, not the σ*CH orbital and there is no possibility for electron transfer when the ions are far apart. brooks Fig2a Fig. 2

     
    Selected Publications
     Refereed articles
     

    P. R. Brooks "Electron Transfer, Harpooning Reagent Orientation and Chemical Intuition." Mol. Phys. (2011) Submitted

     
     

    P. R. Brooks "How Orientation of Small Molecules Affects Chemical Reaction." J. Vac. Soc. Japan, 53 (2010) : 635.

     
     

    Peter W. Harland & Philip R. Brooks "Crossed-beam studies of electron transfer to oriented trichloronitromethane, CCl3NO2, molecules ." J. Chem. Phys., 132 (2010) : 044307.

     
     

    P. R. Brooks "Experimental Evidence for the Role of the π CO orbital in Electron Transfer to Gas Phase Acetic Acid, CH3CO2H. Effects of Molecular Orientation ." J. Chem. Phys., 130 (2009) : 14296.

     
     

    Philip R.Brooks "Electron Transfer Collisions with Oriented Trifluoroacetic Acid (CF3CO2H)." J. Phys. Chem. , 113 (2009) : 14296.

     
     

    P. R. Brooks "Experimental Evidence for the Role of the pi*co orbital in Electron Transfer to Gas Phase Acetic Acid, CH3CO2H. Effects of Molecular Orientation."  (2008) Submitted

     
     

    Philip R. Brooks, Peter W. Harland, Sean A. Harris, Terry Kennair, Crystal Redden, and Jack F. Tate "Steric Effects in Electron Transfer from Potassium to Pi-bonded Oriented Molecules CH3CN,CH3NC and CCl3CN." J. Am. Chem. Soc., 129 (2007) : 15572-15580.

     
     

    P. R. Brooks, P. W. Harland, C. E. Redden "Electron Transfer from Sodium to Oriented Nitromethane, CH3NO2: Probing the Spatial Extent of Unoccupied Orbitals." J. Am. Chem. Soc., 128 (2006) : 4773-4778.

     
     

    P. R. Brooks, P. W. Harland, C. E. Redden "Steric Asymmetry in Electron Transfer from Potassium Atoms to Oriented Nitromethane (CH3NO2) Molecules." J. Phys. Chem. A, 110 (2006) : 4697-4701.

     
     

    B. Jia, S. Harris, L. L. Lewis, J. Zhan, and P. R. Brooks "Threshold Behavior in Electron-Transfer Collisions between Rubidium Atoms and C2F5Cl or C2F5I Molecules." J. Phys. Chem. A, 109 (2005) : 9213-9219.

     
     

    P. R. Brooks, P. W. Harland and C. E. Redden "Electron Transfer from Sodium to Oriented Nitromethane, CH3: Probing the Spatial extent of Unoccupied Orbitals." J. Am. Chem. Soc. (2005) Submitted

     
     

    Beike Jia, Jonathan Laib, R. F. M. Lobo, and Brooks, P. R. "Evidence for Orbital-Specific Electron Transfer to Oriented Haloform Molecules." J. Am. Chem. Soc., 124 (2002) : 13896-13902.

     
     

    Brooks, P. R. and Sean A. Harris "Frontside versus backside reactivity in electron transfer to oriented tert butyl bromide and methyl bromide." J. Chem. Phys., 117 (2002) : 4220-4232.

     
     Other
     

    Sean A. Harris and Philip R. Brooks "Electron transfer to oriented molecules: Surprising steric effect in t-butyl bromide." J. Chem. Phys., 114 (2001) : 10569-10572.

     
     

    Sean A. Harris, Peter W. Harland and Philip R. Brooks "Electron Transfer Collisions with Spatially Oriented CH3CN ." Phys. Chem. Chem. Phys., 2 (2000) : 787-791.

     
     

    Peter W. Harland, Wan-Ping Hu, Claire Vallance and Philip R. Brooks "Spatial deorientation of upper-Stark-state-selected supersonic beams of CH3F, CH3Cl, CH3Br, and CH3I." Phys. Rev. A, 60 (1999) : 3138-3143.

     
     

    Sean A. Harris, Peter W. Harland and Philip R. Brooks "Electron Transfer Collisions with Spatially Oriented CH3CN." Physical Chemistry - Chemical Physics , 2 (2000)

     
     

    Sean A. Harris, Susan D. Wiediger, and Philip R. Brooks "Electron Transfer to SF6 and Oriented CH3Br." Journal of Physical Chemistry , 103 (1999) : 10035-10041.

     
    Presentations
     Invited Talks
     

    "How Chemical Reaction Depends on Orientation." Department of Physics, University of Texas, Austin, Texas. (April 1, 2012)

     
     

    "How Molecular Orientation Affects (Some) Reactions." University of Illinois, Chicago, Illinois. (October 11, 2010)

     
     

    "How Molecular Orientation Affects (Some) Reactions." Brown University, Providence, Rhode Island. (November 5, 2010)

     
     

    "Sn2 reactions, electron transfer, and orientation ." Yale University, New Haven, Connecticut. (November 9, 2010)

     
     

    "Electron Affinity of the C2F5 Radical." Gordon Research Conference, Ventura Beach, California. (March 1, 2005)

     
     

    "Orientation Effects in Electron Transfer Collisions." University of Massachusetts, Amherst, Massachusetts. (October 11, 2004)

     
     

    "Steric Effects in Electron Transfer to pi-bonded Molecules." International Symposium on Stereodynamics in Chemical Reactions, Osaka, Japan. (November 28-December 3, 2004)

     
     

    "Effects of Molecular Orientation on Electron Transfer." Chemistry Department, Baylor University, Waco, Texas. (November 21, 2003)

     
     

    "Electron Transfer in Collisions with Oriented Molecules." XX International Symposium on Molecular Beams, Lisbon, Portugal. (June, 2003)

     
     

    "Frontside and Backside Attack in Electron Transfer Collisions." International Conference on Stereodynamics of Chemical Reactions, Schoorl, The Netherlands. (December 4, 2002) With Sean A. Harris

     
     Other
     

    "Spacial Orientation in Electron Transfer Collisions ." XVIIIth Conference on the Dynamics of Molecular Collisions, Copper Mountain, CO. (July, 2001) With Sean A. Harris

     
     

    "Steric Effects in Electron Transfer Collisions ." Gordon Research Conference on Gaseous Ions: Structures, Energetics & Reactions, Ventura Beach, CA. (February, 2001)

     
     

    "Effect of Orientation on Electron Transfer Collisions on Crossed Beams." Gordon Research Conference on Atomic & Molecular Interactions , New London, NH. (July, 2000) With Jiping Zhan, Sean Harris

     
     

    "Electron Transfer to Oriented Molecules ." University of Sydney, Sydney, Australia. (October, 2000) With Sue Weidiger, Sean Harris

     
     

    "Electron Transfer to Oriented Molecules." Seminar, Gothenburg University & Chalmers University of Technology GothenburgSweden . (19 May 1999) With Sean Harris & Susan D. Wiediger

     
     

    "Electron Transfer to Oriented Molecules." Seminar, Oak Ridge National Laboratory, Oak Ridge, TN. (16 April 1999) With Sean Harris & Susan D. Wiediger

     
     

    "Ion Production by Electron Transfer Collisions of Oriented Molecules in Crossed Beams." Gordon Research Conference on Gaseous Ions, Structure, Energy & Reactions, Venture, CA. (February 28-March 4, 1999)

     
     

    "Formation and identification of molecular negative ions in electron-transfer collisions with oriented molecules." American Chemical Society, Baton Rouge, LA. (November 1998) With Sean A. Harris, Susan D. Wiediger, Jiping Zhan, Lawrence L. Lewis

     
     Posters
     

    "Electron Transfer to Acetic Acid." Gordon Research Conference on Atomic & Molecular Interactions, New London, New Hampshire. (July 6-11, 2008)

     
     

    Presenter.  "Steric Effects in Electron Transfer from Potassium to Pi-bonded Oriented Molecules CH3CN,CH3NC and CCl3CN." Dynamics of Molecular Collisions Conference, Santa Fe, NM. (July 2007)

     
     

    "Steric Asymmetry in Electron Transfer Reactions." Gordon Research Conference on Atomic & Molecular Interactions, New London, New Hampshire. (July 9-14, 2006) With C. Redden

     
    Supervised Theses & Dissertations
     Beike Jia, Ph.D. Collisional Ionization Studies of Rubidium Atoms and Small Halogenated Molecules near Reaction Thresholds. (2006) (Thesis Director)

     Wei Zhao, Ph.D. Engineering Atomic Wavepackets in Very-High-n Rydberg Atoms. (2007) (Thesis Committee Member)

     Hardin Dunham, Ph.D. Angular Depenence of Xenon Rydberg Atom Ionization of Conducting and Semiconducting Surfaces. (2008) (Thesis Committee Member)

     Jeffrey Mestayer, PhD Rydberg Atom Wavepacket Dynamics in One and Two-Dimensions. (2009) (Thesis Committee Member)

     Yi Liu, PhD Study of Dipole-Bound Negative Ions: Formational Dynamics and Collisional Properties. (2009) (Thesis Committee Member)

     Dennis Neufeld, Ph.D. Interaction of Xenon Rydberg Atoms with Conductive Surfaces: The Effects of Stray Fields. (2011) (Thesis Committee Member)