feature observed at 5603 cm−1 is also indicative of
an out-of-plane asymmetric CH stretch (n13); a simulation of the band contour is shown in fig. S3.
Most notably, the lowest-energy feature at
5603 cm−1 sets an upper limit of 16.0 kcal mol−1
for the effective barrier to unimolecular decay
to OH products. This experimental determination of the barrier height is nearly 2 kcal mol−1
lower than recent theoretical predictions for the
transition state separating the syn-CH3CHOO
Criegee intermediate from VHP and OH products of 17.9 kcal mol−1 (including zero-point
corrections) (12, 14); similar barrier heights are
predicted for several alkyl-substituted Criegee
intermediates with a-hydrogens (12, 17). This
lower estimate for surmounting and/or tunneling through the barrier reveals that unimolecular
decay to OH products is more facile than predicted by current theoretical models.
An intrinsic reaction coordinate (IRC) has been
computed to obtain insight on the potential
energy and structural changes required to move
from the minimum-energy configuration of syn-
CH3CHOO to the transition state leading to OH
products. The IRC is displayed in Fig. 4 as a
function of decreasing separation between the
transferring a-hydrogen and terminal oxygen
(ROH). Other important structural changes along
the IRC are a heavy-atom backbone motion to
close the HCCOO ring, denoted by the distance
between the methyl carbon and terminal oxygen
(RCO); the a-hydrogen to methyl carbon bond
length (RCH); and a HCCO torsion (t) associated
with internal rotation of the methyl group that
moves the a-hydrogen toward the heavy-atom
plane. The IRC also reveals that as the a-hydrogen
is brought closer to the terminal oxygen, first the
CH3 group rotates and then the CH bond elongates.
Further theoretical analysis (see supplementary
materials) identifies the vibrational excitations re-
quired to distort syn-CH3CHOO from the minimum-
energy configuration toward the transition state
for the hydrogen transfer reaction (table S3). Four
modes are most relevant: the out-of-plane CH stretch
vibrations (n3 and n13), the mode associated with
closing of the HCCOO ring (n12), and the HCCO
torsion (n18). Of these, many quanta of mode n12
are required to reach the transition state, indicating
that this mode plays an important role in the uni-
molecular decomposition mechanism. Excitation of
the in-plane CH stretches (n1 and n2), however, does
not lead directly toward the transition state. Never-
theless, analysis of quartic coupling terms in the
expansion of the potential (see table S4) shows ap-
preciable coupling among the four CH stretch vi-
brations (modes n1, n2, n3, and n13), consistent with
the mixed nature of many of the observed bands.
These CH stretches are further coupled to states
with excitation in the ring-closing (n12) or HCCO
torsion (n18) mode (see tables S5 to S7). These
higher-order couplings enable overtones and com-
bination bands involving all four CH stretch vi-
brations to access the transition-state region
required for reaction.
The theoretical analysis supports the experimental finding that energized Criegee intermediates,
prepared here through vibrational activation of CH-stretching vibrations, undergo direct 1,4-hydrogen
transfer to produce OH radicals. Both in-plane
and out-of-plane CH-stretching vibrations of syn-CH3CHOO, identified as overtone and combination
bands by using IR action spectroscopy with OH
radical detection, are shown to be coupled to the
reaction coordinate. The effective barrier to reaction
established by the lowest-energy feature observed
at 5603 cm−1 indicates that unimolecular decay of
methyl-substituted Criegee intermediates, and likely
other alkyl-substituted carbonyl oxides with an
a-hydrogen, should be more facile in producing
OH radicals than anticipated from current models
of alkene ozonolysis in the troposphere.
REFERENCES AND NOTES
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ACKNO WLEDGMEN TS
Additional details of the experiments and calculations are presented in
the supplementary materials. This research was supported, in part,
through NSF grants CHE-1112016 (M. I. L.), CHE-1362835 (M.I. L.), and
CHE-1213347 (A.B.M.). J.M.B. acknowledges support through the
Dreyfus Postdoctoral Program in Environmental Chemistry
(EP-12-025). Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for partial support of
this research (ACS PRF 53320-ND6). A.S.P. acknowledges support
from the U. S. Air Force Office of Scientific Research (USAFOSR)
PECASE award under AFOSR grant FA9950-13-1-0157.
Materials and Methods
Figs. S1 to S3
Tables S1 to S7
9 June 2014; accepted 20 August 2014
1598 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204
Fig. 3. Expanded
views of 5951
and 6081 cm−1
features in the
are superimposed with
(red), indicative of rapid (∼3 ps) IVR and/or reaction dynamics upon CH stretch overtone excitation,
and at the laser bandwidth (blue).
Fig. 4. IRC from the minimum-energy configuration of the Criegee intermediate to the transition
state leading to OH products. The IRC is displayed as a function of the terminal oxygen to a-hydrogen
distance (ROH). Representative geometric structures along the path (red points) are shown with the methyl
carbon to terminal oxygen distance (RCO), the a-hydrogen to methyl carbon bond length (RCH), and HCCO
torsional angle (t, with t = 0 corresponding to the a-hydrogen lying in the heavy-atom plane) indicated.