INTRAMOLECULAR H-MIGRATIONS IN CERTAIN TYPES OF PEROXY RADICALS IN
THE URBAN ATMOSPHERE
S. WANG1,3, R. WU1, T. BERNDT2, M. EHN3 and L. WANG4
1
School of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou
510640, China.
2
3
4
Leibniz Institute for Tropospheric Research, TROPOS, 04318 Leipzig, Germany
Department of Physics, University of Helsinki, P.O. Box 64, Helsinki 00014, Finland
Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China
University of Technology, Guangzhou 510006, China.
Keywords: Peroxy radicals; Intramolecular H-migration; Atmospheric Oxidation Mechanism; Mass
Spectrometry.
INTRODUCTION
Peroxy radicals (RO2) are formed in the atmospheric oxidation of VOCs. In the atmosphere, the RO2
radicals are often assumed to react with NO, HO2 and other peroxy radicals, and to lesser extent with NO3.
RO2 radicals are also formed in the low-temperature combustion of VOCs, where they undergo
intramolecular H-migration as ROO QOOH Products. Because of the high endothermicity and high
barriers, the H-migration are often excluded in the atmospheric oxidation mechanism of VOCs. In the past
few years, both experimental and theoretical studies showed that the H-migration in the peroxy radicals
might be important in the oxidation of biogenic hydrocarbons. (Ehn et al., 2014; Jokinen et al., 2014;
Rissanen et al., 2014; Peeters et al., 2014; Berndt et al., 2016) However, little research has focused on this
behavior in anthropogenic organic compounds. (Crounse et al., 2013; Jørgensen et al., 2015) Here we
report our recent studies on the role of intramolecular H-migrations in the atmospheric oxidation of certain
types of anthropogenic compounds, including ethers (Wang et al., 2016), substituted benzenes, and
carbonyl compounds. These kind of unimolecular reactions might be fast enough to compete with
bimolecular reactions in the urban atmosphere, and may result in formation of highly oxidized
multifunctional compounds (HOMs) and contribute to SOA formation. Moreover, the importance of Hmigrations would significantly increase with elevated temperatures, and some of the subsequent reaction
channels might regenerate OH radical.
METHODS
All the molecular geometries are optimized using density functional theory at the M06-2X/6311++G(2df,2p) level, and the electronic energies are calculated by using the complete basis set model
chemistry with both unrestricted (UCSB-QB3) and restricted (ROCBS-QB3) wave function for the openshell species. In kinetics calculations, we have treated the internal rotations as uncoupled hindered rotors,
and have obtained their potential energy profiles by fixing the corresponding dihedral angles with relaxing
all other coordinates in optimization. Asymmetric Eckart model is used for the tunneling correction
factors. The reaction rates/rate constants at high-pressure limit are estimated by transition state theory
(TST):
n 1

6
k    
  G   RT 10 
kBT
 exp   r     

h
 RT   P N A 
( E1)
For the pressure-dependence of the reaction kinetics, unimolecular rate theory coupled with the energygrained master equation (RRKM-ME) is employed to estimate the effect of collision by using the
MESMER code. (Glowacki et al., 2012) Single exponential-down model is employed to approximate the
collisional energy transfer. The collisional parameters are estimated by the method of Gilbert and Smith.
We also carried out experiments to test intramolecular H-migrations and formation of highly oxidized
products in the oxidation of alkylbenzenes. The experiments were performed in a free-jet flow system at a
temperature of 295  2 K, a pressure of 1 bar air and a reaction time of 7.9 s. The detection of highly
oxidized RO2 radicals and closed-shell products was carried out by means of CI-APi-TOF (chemical
ionization - atmospheric pressure interface-time-of-flight) mass spectrometry (Airmodus, Tofwerk,
resolving power >3000 Th/Th) using acetate as the reagent ion.
CONCLUSIONS
We have predicted theoretically the occurrence of intramolecular H-migrations in certain types of peroxy
radicals formed in the atmospheric oxidation of ethers, carbonyls and alkylbenzenes. The estimated rate
coefficients vary from the order of 102 s1 to 101 s1 with various substitute groups in different peroxy
radicals (Table 1), which might be fast enough to compete with the bimolecular reactions of these peroxy
radicals with NO and HO2 or RO2 radicals in the atmosphere. The predicted formation of highly oxidized
multifunctional products in the oxidation of alkylbenzenes, through intramolecular H-migrations, is
qualitatively supported by our experimental studies. This is shown for isopropylbenzene in Figure 1. To
summarize, intramolecular H-migration can result in the formation of HOMs, which could contribute to
the formation of SOA in urban areas. Moreover, recycling of OH radicals is proposed in certain cases,
which may sustain the oxidizing power to the atmosphere. Therefore, we suggest that it is necessary to
include the intramolecular H-migration of the RO2 radical in the atmospheric oxidation mechanism of
certain anthropogenic VOCs for air quality modelling.
Table 1. Reaction energies and barrier heights (
and
, in kJ/mol) for the intramolecular H-migrations in the
oxidation of ethers and carbonyls at UCBS-QB3 level; for alkylbenzenes at ROCBS-QB3 level; kEff,298K in s–1.
T/EB/IB-R4-BPRs represent bicyclic peroxy radicals in toluene, ethylbenzene and isopropylbenzene, respectively,
with OH additions to para-position
Figure 1. Mass spectra recorded from the reaction of OH radicals with isopropylbenzene, IB. The red spectrum
represents the background measured in absence of isopropylbenzene. Products are detected as adduct with acetate.
The spectrum depicted in part A was measured in absence of NO and that in part B with a NO concentration of 5 
1010 molecules cm3. Reactant concentrations (unit: molecules cm3): O3 = 6.6  1011, TME = 1.0  1011 and
isopropylbenzene = 1.64  1013.
ACKNOWLEDGEMENTS
This work was supported by the National Natural Science Foundation of China (No. 21177041&
21477038) and Public Welfare Project of Ministry of Environmental Protection of China (No.
201409019). SW thanks for the support from China Scholarship Council and University of Helsinki.
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