B.S. (1960) California Institute of Technology
Ph.D. (1964) University of California, Berkeley
Physical Chemistry
Email: brooks@rice.edu
Phone: (713) 348-3266
Office: Space Science & Tech. Bldg., 209
|
|
Philip Brooks
Professor of Chemistry
Research Statement
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. 
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.
 Fig. 2
Selected Publications
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.
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.
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." Physical Chemistry - Chemical Physics, 2 (2000).
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.
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
"Electron Transfer to Acetic Acid." Gordon Research Conference on Atomic & Molecular Interactions, New London, New Hampshire. (July 6-11, 2008)
"Steric Asymmetry in Electron Transfer Reactions." Gordon Research Conference on Atomic & Molecular Interactions, New London, New Hampshire. (July 9-14, 2006) With C. Redden.
"Electron Affinity of the C2F5 Radical." Gordon Research Conference, Ventura Beach, California. (March 1, 2005)
"Steric Effects in Electron Transfer to pi-bonded Molecules." International Symposium on Stereodynamics in Chemical Reactions, Osaka, Japan. (November 28-December 3, 2004)
"Orientation Effects in Electron Transfer Collisions." University of Massachusetts, Amherst, Massachusetts. (October 11, 2004)
|
