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).