J. Am. Chem. Soc. 1997, 119, 5067-5068
5067
N
Photoinitiation of Gas-Phase S 2 Reactions through
the Evans-Polanyi Excited State Surface
Caroline E. H. Dessent and Mark A. Johnson*
Sterling Chemistry Laboratory
Yale UniVersity, P.O. Box 208107
New HaVen, Connecticut 06520-8107
ReceiVed February 24, 1997
In the classic Evans-Polanyi treatment1,2 of the SN2 reaction
-
-
N
Figure 1. Schematic potential energy surface of the gas phase S 2
X + CH Y f CH X + Y
(1)
3
3
-
-
reaction I + CH
3
NO
2
f NO
2
3
+ CH I along the reaction coordinate.
the Walden inversion transition state3 is recovered Via an
avoided crossing between diabatic curves that arise from the
hypothetical transfer of the methyl group without concomitant
electron transfer. These curves are sketched in Figure 1 for
The surface for the corresponding neutral reaction is shown at the top
of the diagram, with its associated dipole-bound anionic excited state
(dashed line).
-
4
We carry out these experiments using a tandem time-of-flight
the reaction I + CH3NO2. The matrix element splitting these
8
-
mass spectrometer, where the X ‚CH3NO2 complexes are
curves is related to the electron transfer rate (i.e., resonance
-
2
generated in a free jet expansion by association onto X ions
energy) at the transition state (∼0.5 eV). The avoided crossing
created by secondary electron attachment to CCl4, CH2Br2, or
results in a low-lying electronically excited charge transfer (CT)
-
-
-
-
-
3
CH I (for Cl , Br , and I , respectively). The nitromethane
state correlating to I‚[CH3NO2] and [CH3I] ‚NO2 over the
geometries of the (gas phase) ion-dipole minima. The excited
state has a deep minimum in the transition state region of the
SN2 reaction and should therefore provide a unique platform
for carrying out direct spectroscopic observations of the transi-
(40 Torr) was seeded along with the halide precursors in 4 atm
of Ar. The UV bands of the complexes were scanned using
the frequency-doubled or sum frequency mixed (with 1.165 eV)
signal beam from a â-barium borate (BBO) optical parametric
oscillator (Spectra Physics MOPO-710). Fragment ions were
separated using a reflectron, and action spectra for particular
photoproducts were obtained by selectively detecting a fragment
ion with a boxcar averager while the laser was scanned.
5
tion state below. We have reported several attempts to access
these excited states by photoexcitation of ion-dipole complexes
and have found that while they were not directly accessed from
the ground state, they could be reached indirectly through the
diffuse, dipole-bound excited state lying just below the electron
-
The charge transfer excited states, X‚[CH NO ] , are ex-
3
2
6
-
5
continuum. In the best studied case of I ‚CH3I, however,
pected to lie close to the vertical detachment energies (VDE)
indirect excitation of the CT state failed to drive the SN2
reaction, presumably due to the very repulsive nature of CH3I
which leads to prompt dissociation along the [ICH3I] anti-
symmetric stretch coordinate (i.e., to I + CH3 + I).
In this paper, we report the successful initiation of the
of these complexes since the vertical electron affinity of CH -
3
-
7
-
NO is approximately 0. The I ‚CH NO photoelectron spec-
trum displayed very little vibrational fine structure, consistent
with the other I ‚CH Y (Y ) CN, I, Br) complexes we have
studied, where the ion is calculated to sit in the “pocket”
behind the methyl group. The location of the band was first
established by monitoring the fast “photoneutrals” which
accompany photodestruction of the parent ion. A strong band
appeared just below the VDE (3.604 ( 0.01 eV for I ‚CH3NO2,
recall that the electron affinity of the iodine atom is 3.06 eV)
with the characteristic shape for excitation of a dipole-bound
2
3
2
-
-
-
3
5
,6
4
reactions
-
-
2
X + CH NO f NO + CH X (X ) I, Br, Cl) (2)
3
2
3
-
-
by photoexcitation of the stabilized X ‚CH3NO2 reaction
-
intermediates to the X‚[CH3NO2] charge transfer excited states.
-
Unlike the CH3Y anions created upon photoexcitation of the
6
excited state. Such a band is expected since the dipole moment
-
-
X ‚CH3Y (Y ) halogen) complexes, the CH3NO2 ion is bound
of CH3NO2 (3.46 D) is much larger than the critical value
-
7
[
D0(CH3-NO2 ) ) 0.56 eV] and its stability enables the charge
required to support a bound state (µcrit ≈2-2.5 D).
transfer excited state to remain intact, leading to products
consistent with reaction on the ground state surface. This work
constitutes the first spectroscopic observation of this classic
Evans-Polanyi excited state surface.
A mass spectrum of the ionic photofragments produced at
-
the peak of the I ‚CH3NO2 absorption is presented in Figure
-
2
a, showing not only the expected CH3NO2 anion but also
-
-
the I and NO2 anions. Figure 3 displays the action spectrum
for production of the NO2 photofragment, illustrating that the
-
(
1) Evans, M. G.; Polanyi, M. Trans. Faraday Soc. 1938, 34, 11. Ogg,
R. A.; Polanyi, M. Trans. Faraday Soc. 1935, 31, 604.
band is sharply peaked just below the VDE of the complex
(
2) Shaik, S. S.; Schlegel, H. B.; Wolfe, S. Theoretical Aspects of
(
arrow in Figure 3) in the region where a dipole-bound state is
Physical Organic Chemistry: The SN2 Mechanism; Wiley-Interscience:
-
New York, 1992.
(
expected to occur. This indicates that the NO2 fragmentation
channel is enhanced through the dipole-bound excited state of
I ‚CH3NO2 (the I action spectrum tracks the NO2 channel).
A small progression is observed above the VDE; similar features
were observed and analyzed in the I ‚CH3I system in the
context of vibrational excitation arising from nonadiabatic
coupling between the free electron and CH3I. In the present
case, the 79 ( 3 meV spacing of the features in Figure 3 roughly
corresponds to either the NO2 bending or wagging mode in
3) (a) Olmstead, W. N.; Brauman, J. I. J. Am. Chem. Soc. 1977, 99,
4
(
219. (b) Graul, S. T.; Bowers, M. T. J. Am. Chem. Soc. 1991, 113, 9696.
-
-
-
c) DePuy, C. H.; Gronert, S.; Mullin, A.; Bierbaum, V. M. J. Am. Chem.
Soc. 1990, 112, 865. (d) Viggiano, A. A.; Morris, R. A.; Pashkewitz, J. S.;
Paulson, J. F. J. Am. Chem. Soc. 1992, 114, 10477.
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-
(
4) Tanaka, K.; McKay, G. I.; Payzant, J. D.; Bohme, D. K. Can. J.
Chem. 1976, 54, 1643.
(
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Chem. Phys. 1992, 97, 5911. (b) Cyr, D. M.; Bailey, C. G.; Serxner, D.;
Scarton, M. G.; Johnson, M. A. J. Chem. Phys. 1994, 101, 10507.
(
6) Dessent, C. E. H.; Bailey, C. G.; Johnson, M. A. J. Chem. Phys.
1
995, 102, 6335. Dessent, C. E. H.; Bailey, C. G.; Johnson, M. A. J. Chem.
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(
(8) Johnson, M. A.; Lineberger, W. C. Techniques in Chemistry; Farrar,
J. M., Saunders, W. H., Eds.; Wiley: New York, 1988; p 591.
(9) Dessent, C. E. H.; Bailey, C. G.; Johnson, M. A. J. Chem. Phys.
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S0002-7863(97)00583-0 CCC: $14.00 © 1997 American Chemical Society