Conical Intersections and Activation Energies

1
Addition of Peroxyl Radicals to Alkenes and the
Reaction of Oxygen with Alkyl Radicals
Moray Stark
Department of Chemistry University of York, York,
YO10 5DD, UK
Frontier Orbital Description of Radical Addition
to Alkenes This dependence of activation energy
for the However, the radical in the 2A
first addition reaction on the energy of the
first excited state correlates with the
excited state of the radical is what would be
2A ground state of the Cs saddle
expected if the radical and alkene approached
point (an ROHF representation of this
in the same plane (Cs symmetry). is
shown here). The peroxyl radical 2A ground
state correlates with an energetically
unfavourable 2A excited state of the Cs saddle
point for the addition.
Addition of Peroxyl Radicals to Alkenes One of
the most studied examples of radical reactions
with alkenes is peroxyl radical addition,
investigated in detail by Waddington et al. eg.1
(acetyl and alkylperoxyl radicals) and Baldwin
and Walker et al. eg.2 (hydroperoxyl
radicals). k1 k2 Hydroperoxyl
Ethene k-1 Transition
State Adduct Hydroxyl Ethene
Oxide Epoxides are the dominant product of these
reactions, with the initial addition being the
rate determining step (k2 gtgt k-1).1,2
Activation Energy vs. Alkene Ionisation
Energy It is well known that one of the factors
controlling This identifies the addition
as an the rate of addition of peroxyl radicals
to alkenes electrophilic reaction the
greater is the ionisation energy of the alkene
the reaction the charge transfer to the
radical being faster the lower the alkene
ionisation energy.1,2 during the
reaction, the faster the rate.
Conical Intersections and Activation Energies If
the radical and alkene approach in the
The conical intersection guarantees same plane
(Cs symmetry), then the 2A that for a
particular relative orientation and 2A surfaces
cross at a conical and separation of
the radical and intersection.
alkene, the energy of the system must
be higher than the
reactants. The transition state for the addition
reaction has C1 symmetry, and is The
proximity of the transition state to reached
from the conical intersection the
conical intersection ensures that it by moving
along the symmetry breaking too has an
energy higher than the co-ordinate of the
branching space. reactants, with a
barrier height related to the
energy of the first excited state of
the radical.
Charge Transfer During Reaction The parabola
model of Pearson and Parr allows Shown
here is how the systems the degree of charge
transfer at the transition energy
varies with charge transfer state for the
addition to be estimated by using for
the addition of acetylperoxyl the ionisation
energies (I) and electron affinities
radicals to 2-methyl-2-butene. (A) of
the isolated reactants.3 The
energy released (?EC) by the char
ge transfer (?NC) can be viewed
as the driving force for the
reaction.
Reaction of Oxygen with Alkyl Radicals Alkyl
radicals react with oxygen to give alkylperoxyl
radicals Ab-initio studies have shown
that which at low pressure or high temperature
can decompose to alkylperoxyl radicals
decompose an alkene and hydroperoxyl radical.
directly to the alkene HO2 via a
2A transition state and not
via the However, there has been controversy over
the mechanism of hydroperoxylalkyl
radical as once this decomposition, as the
reverse reaction, HO2 alkene, gives
thought.8 the epoxide OH and not the
alkylperoxyl radical as might be expected
on the grounds of reversibility.eg.2,7
This mechanism is combined here
with the above description for
peroxyl radical addition to alkenes.
Activation Energy vs. Charge Transfer Energy The
energy released by charge transfer
to The relationship also suggests
that the radical as it approaches the alkene
(?EC) the activation energy for a
reaction correlates very well with the activation
energy involving no charge transfer
would for the reaction4 (as indeed do data for
nitrate be 80 - 90 kJ mol-1. radical
addition).5 Coinciden
tally, 80 - 90 kJ mol-1 is This suggests that the
energy released by also typical of the
energy required to charge transfer lowers the
barrier for the promote these radicals
to their first addition approximately in
proportion, electronically excited
states.6 becoming barrierless for ?EC ? 60 kJ
mol-1.
Potential Energy Diagram for Ethyl
Oxygen Discussion of this class of reaction has
centred on A potential energy diagram
for this the most studied example, ethyl oxygen
, with system is suggested here, based
on disagreement over not just the mechanism, but
the above mechanism, and with also of
barrier heights for key steps in the
reaction. barrier heights chosen to be
as compatible as possible with
experimental observations.
References (1) Ruiz Diaz, R.
Selby, K. Waddington, D. J. J. Chem. Soc.
Perkin Trans. 2 1977, 360. (5) Wayne, R. P. et
al. Atmos. Environ. 1991, 25A, 1. (2) Stothard,
N. D. Walker, R. W. J. Chem. Soc. Faraday Trans.
1990, 86, 2115. (6) Stark, M. S. J. Am. Chem.
Soc. 2000, 122, 4162. (3) Parr, R. G. Pearson,
R. G. J. Am. Chem. Soc. 1983, 105, 7512. (7)
Wagner, A. F. Slagle, I. R. Sarzynski, D.
Gutman, D. J. Phys. Chem. 1990, 94, 1853. (4)
Stark, M. S. J. Phys. Chem. 1997, 101, 8296.
(8) Quelch, G. E. Gallo, M. M. Schaefer,
H. F. J. Am. Chem. Soc. 1992, 114, 8239.