T. Darrah Thomas

Distinguished Professor of Chemistry

PROFESSOR EMERITUS

B.S., Haverford College, 1954

Ph.D., University of California, Berkeley, 1957

  T.Darrah.Thomas@oregonstate.edu

Dr. Thomas studies the energy spectra of electrons ejected from the inner shells of molecules, using facilities for high-resolution electron spectroscopy at the Advanced Light Source in Berkeley or the MAX-II synchrotron in Sweden. A major theme of this work has been a search for better understanding of chemical processes that involve the addition of charge to a molecule.[1] [2] [3] [4]  Other research has aimed at understanding the vibrational structure in photoelectron spectra,[5] determining lifetimes of molecules with inner-shell vacancies,[6] [7] [8] studying localization-delocalization effects that can occur in molecules with equivalent atoms (such as HCCH and CH3CH3),[9] [10] investigating molecular-field splitting, which, for ionization of levels with l>0, removes the degeneracy of levels such as 2p3/2 that are degenerate in the spherical case,[11] [12] and providing accurately measured carbon 1s ionization energies[13] and xenon Auger kinetic energies for calibration purposes.[14] 

Inner-shell ionization energies

Inner-shell ionization energies reflect the energy required to change the charge at a particular site in a molecule, as do such more common chemical properties as acidity, basicity, and rates of electrophilic reactions. Recent work has taken advantage of the high-resolution capability of synchrotron radiation to measure the carbon 1s ionization energies for hydrocarbons, where high-resolution makes it possible to resolve details that were previously inaccessible. A typical example can be seen in the carbon 1s photoelectron spectrum of propyne,[15] shown to the right. The contributions from the three inequivalent carbons as well as the unique vibrational structure associated with each carbon are clearly visible. Comparing the observed vibrational structure with that predicted theoretically makes it possible to assign the three peaks to the chemically inequivalent carbon atoms in propyne: the peak to the left arises from ionization of the HC≡ carbon and that to the right from ionization of the CH3 carbon.

 

The carbon 1s ionization energies reflect the electronegativity of the ligands attached to the carbon atom. This relationship is reflected in a correlation that has been observed between the ionization energies and electronegativity, as illustrated in the figure to the right.3 Here the carbon 1s ionization energies of halomethanes are seen to correlate linearly with the electronegativities of the halogens. Using such relationships as this, we have investigated the group electronegativities of CF3, SF5 and OSO2F.1,2

 

Correlations of carbon 1s ionization energies in 1,3-butadiene and 1,3-pentadiene with both proton affinities and reactitivities give insight into these processes as well as into the substituent effect of a methyl group.4

Vibrational excitation during core-ionization

Inner-shell ionization of molecules is generally accompanied by vibrational excitation, as can be seen in the spectrum for propyne. A more complex example, the carbon 1s photoelectron spectrum of 1,3-cyclohexadiene,5 which, like propyne, has three different types of carbon atom is shown to the left. In addition to the experimental data, shown as circles, the figure also shows the vibrational excitation spectra for each carbon (colored sticks) calculated ab initio using electronic structure theory, the same calculated spectra dispersed with the known line shape and experimental resolution (colored curves), and a least squares fit of these spectra to the data with only the energy position and the height of each group as adjustable parameters (solid line through the data). It is clear that this procedure provides an excellent description of the experimentally observed spectrum and that with this we can obtain accurate ionization energies for the inequivalent carbon atoms, even in a rather complex spectrum.

 



[1] Electronegativities from core-ionization energies: Electronegativities of SF5 and CF3,  J. E. True, T. D. Thomas, R. W. Winter, and G. L. Gard, Inorganic Chemistry, 42, 4437-4441 (2003). http://dx.doi.org/10.1021/ic0343298.

[2] Gas-phase structure, conformation, and sulfur 2p photoelectron spectroscopy of pentafluorosulfur fluorosulfonate, SF5OSO2F, C. Leibold, H. Oberhammer, T. D. Thomas, L. J. Sæthre, R. Winter, and G. L. Gard, Inorg. Chem. 43, 3942-3947 (2004). http://dx.doi.org/10.1021/ic035439h.

[3] Carbon 1s photoelectron spectroscopy of halomethanes. Effects of electronegativity, hardness, charge distribution, and relaxation, T. D. Thomas, L. J. Sæthre, K. J. Børve, J. D. Bozek, M. Hutttula, and E. Kukk, J. Phys. Chem. A 108, 4893-4990 (2004). http://dx.doi.org/10.1021/jp049510w.

[4] Reactivity and core-ionization energies in conjugated dienes. Carbon 1s photoelectron spectroscopy of 1,3-pentadiene, T. D. Thomas, L. J. Sæthre, K. J. Børve, M. Gundersen, and E. Kukk, submitted to J. Am. Chem. Soc., 2004.

[5] Carbon 1s photoelectron spectroscopy of six-membered cyclic hydrocarbons, V. M. Oltedeal, K. J. Børve, L. J. Sæthre, T. D. Thomas, J. D. Bozek, and E. Kukk, Phys. Chem. Chem. Phys. 6, 4254-4259 (2004). http://dx.doi.org/10.1039/b405109b.

 

[6] Carbon 1s photoelectron spectroscopy of CF4 and CO: Search for chemical effects on the carbon 1s hole-state lifetime,  T. X. Carroll, K. J. Børve, L. J. Sæthre, J. D. Bozek, E. Kukk, J. A. Hahne, and T. D. Thomas,  J. Chem. Phys. 116, 10221 (2002).

 

[7] Anomalous natural linewidth in the 2p photoelectron spectrum of SiF4,  T. D. Thomas, C. Miron, K. Wiesner, P. Morin, T. X. Carroll, and L. J. Sæthre,  Phys. Rev. Lett., 89, 223001 (2002).

 

[8] Line shape and lifetime in argon 2p electron spectroscopy, T. X. Carroll, J. D. Bozek, E. Kukk, V. Myrseth, L. J. Sæthre, and T. D. Thomas, J. Electr. Spectrosc. Relat. Phenom., 120, 67 (2001).

 

[9] Vibronic structure in the carbon 1s photoelectron spectra of HCCH and DCCD,  K. J. Børve, L. J. Sæthre, T. D. Thomas, T. X. Carroll, N. Berrah, J. D. Bozek, and E. Kukk, Phys. Rev. A 63, 012506 (2001).

[10] Vibrational structure and vibronic coupling in the carbon 1s photoelectron spectra of ethane and deuteroethane,  T. Karlsen, L. J. Sæthre, K. J. Børve, N. Berrah, E. Kukk, J. D. Bozek, T. X. Carroll, and T. D. Thomas,  J. Phys. Chem. 105, 7700 (2001).

[11] Molecular-field splitting and vibrational structure in the phosphorus 2p photoelectron spectrum of PF3,  K. J. Børve, L. J. Sæthre, J. D. Bozek, J. True, T. D. Thomas,  J. Chem. Phys., 111, 4472 (1999).

 

[12] Molecular-field splitting of the 2p3/2 peak in x-ray photo­electron spectroscopy of second-row atoms: A theoretical study of phosphine and phosphorus trifluoride,  K. J. Børve and T. D. Thomas,  J. Chem. Phys. 111, 4478 (1999).

[13] Adiabatic and vertical carbon 1s ionization energies in representative small molecules, V. Myrseth, J. D. Bozek, E. Kukk, L. J. Sæthre, and T. D. Thomas, J. Electr. Spectrosc. Relat. Phenom., 122, 57 (2002).

[14] Xenon N4,5OO Auger spectrum – a useful calibration source, T. X. Carroll, J. D. Bozek, V. Myrseth, L. J. Sæthre, T. D. Thomas, and K. Wiesner, J. Electr. Spectrosc. Relat. Phenom., 125, 127 (2002).

 

[15] Chemical insights from high-resolution x-ray photoelectron spectroscopy and ab initio theory: Propyne, trifluoropropyne, and ethynylsulfur pentafluoride,  L. J. Sæthre, N. Berrah, J. D. Bozek, K. J. Børve, T. X. Carroll, E. Kukk, G. L. Gard, R. Winter, and T. D. Thomas,   J. Am. Chem. Soc. 123, 10729 (2001).