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The authors thank the Swiss Federal Office for Energy (PECHouse
Competence Center, contract SI/500090–02), Nano-Tera NTF
project TANDEM, the PECDEMO project, cofunded by Europe’s
Fuel Cell and Hydrogen Joint Undertaking under Grant Agreement
621252, the European Union for supporting the following projects:
NANOMATCELL, under grant agreement 308997, MESO, and
GLOBASOL, under the grant agreement 577490. In addition, we
are grateful to the Swiss National Science Foundation and Swiss
National Center of Competence in Research Molecular Ultrafast
Science and Technology for financial support. M.G. thanks the
European Research Council for financial support under the
Advanced Research Grant (ARG 247404) “Mesolight” and
acknowledges his affiliation as a visiting faculty member with
Nanyang Technological University Singapore; the King Abdulaziz
University Jeddah, Saudi Arabia; and the Advanced Institute for
Nanotechnology at SKKU, Suwon, Korea. M.K.N. acknowledges
his affiliation as a visiting faculty member to the King Abdulaziz
University Jeddah, Saudi Arabia. H.J.F is thankful for the support
from Singapore-Berkeley Research Initiative for Sustainable Energy
(SinBeRISE) program. J.-H.I and N.-G.P are grateful to the National
Research Foundation of Korea grants funded by the Ministry of
Science, ICT & Future Planning (MSIP) of Korea under contracts
NRF-2012M1A2A2671721 and NRF-2012M3A6A7054861 (Global
Frontier R&D Program on Center for Multiscale Energy System).
Materials and Methods
Figs. S1 to S6
Movies S1 and S2
3 July 2014; accepted 20 August 2014
reaction of CH3CHOO Criegee
intermediates to OH radical products
Fang Liu,1 Joseph M. Beames,1 Andrew S. Petit,1 Anne B. McCoy,2 Marsha I. Lester1*
Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals
in the troposphere, proceeds through energized Criegee intermediates that undergo
unimolecular decay to produce OH radicals. Here, we used infrared (IR) activation of cold
CH3CHOO Criegee intermediates to drive hydrogen transfer from the methyl group to
the terminal oxygen, followed by dissociation to OH radicals. State-selective excitation
of CH3CHOO in the CH stretch overtone region combined with sensitive OH detection
revealed the IR spectrum of CH3CHOO, effective barrier height for the critical hydrogen
transfer step, and rapid decay dynamics to OH products. Complementary theory
provides insights on the IR overtone spectrum, as well as vibrational excitations, structural
changes, and energy required to move from the minimum-energy configuration of
CH3CHOO to the transition state for the hydrogen transfer reaction.
Hydroxyl (OH) radicals, often termed the atmosphere’s detergent, initiate the oxi- dative breakdown of most trace species in the lower atmosphere (1). Photolytic sources dominate the production of OH
radicals in the daytime through solar photoly-
sis of ozone, which generates O(1D) atoms that
react with H2O to form OH radicals, and nitrous
acid, with sizable amounts of the latter being
produced under high-NOx conditions. The prin-
cipal nonphotolytic source of atmospheric OH
radicals is alkene ozonolysis, which is an impor-
tant OH radical initiator in low-light conditions,
urban environments, and heavily forested areas
(2, 3). Recent field campaigns indicate that
alkene ozonolysis accounts for ~30% of tropo-
spheric OH radicals in the daytime and essen-
tially all of the smaller, yet appreciable, OH
radical concentration at night (4, 5).
Alkene ozonolysis occurs by cycloaddition of
ozone across the C=C double bond and subse-
quent decomposition of the resultant primary
ozonide, releasing ~50 kcal mol−1 of excess energy,
to produce an energized carbonyl oxide spe-
cies, known as the Criegee intermediate, and
an aldehyde or ketone product (6). Further uni-
molecular decay of the Criegee intermediate
leads to formation of OH radicals (7, 8). The OH
yield from ozonolysis changes substantially with
alkene structure, increasing from ~10% for eth-
ene via the simplest Criegee intermediate CH2OO
to more than 60% for ozonolysis of trans-2-butene
(9, 10), which proceeds through the methyl-
substituted Criegee intermediate CH3CHOO, the
focus of the present study. (See table S1 for chem-
ical structures of relevant species.) Concurrent
detection of Criegee intermediates and OH products
by photoionization mass spectrometry also shows
a large increase in OH yield for alkyl-substituted
Criegee intermediates compared to CH2OO (11, 12).
The efficient production of OH radicals upon
ozonolysis of alkenes has been proposed to fol-
low a 1,4-hydrogen atom shift mechanism for alkyl-
substituted Criegee intermediates. The computed
reaction coordinate, depicted in Fig. 1 for the
more stable syn-conformer of CH3CHOO, involves
passage over a transition state with a five-membered,
ringlike structure and migration of a hydrogen
on the methyl group (an a-hydrogen) to the
terminal oxygen to generate vinyl hydroperoxide
(VHP, H2C=CHOOH). This leads directly to O-O
bond breakage, yielding OH radical and vinoxy
products. A different mechanism is predicted for
CH2OO (and anticonformers of Criegee intermedi-
ates), with a substantially higher barrier to reaction
that leads to dioxirane (13, 14) and, based on kinetic
studies, a much smaller yield of OH products under
laboratory and atmospheric conditions (9).
This study focuses on infrared (IR) activation
of cold Criegee intermediates to drive unimolecular
decay to OH products. Specifically, we used IR
excitation of syn-CH3CHOO in the CH stretch
overtone (2nCH) region near 6000 cm−1 to surmount
the barrier associated with 1,4-hydrogen transfer
1596 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204
1Department of Chemistry, University of Pennsylvania,
Philadelphia, PA 19104-6323, USA. 2Department of
Chemistry and Biochemistry, The Ohio State University,
Columbus, OH 43210-1173, USA.
*Corresponding author. E-mail: email@example.com