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Decomposition mechanism of a-alkoxyalkylhydroperoxides in the liquid phase: temperaturedependent kinetics and theoretical calculations†
Mingxi Hu,a Kunpeng Chen, b Junting Qiu,a Ying-Hsuan Lin,
Kenichi Tonokura a and Shinichi Enami *cbOrganic hydroperoxides (ROOHs) play key roles in the atmosphere as a reactive intermediate species. Dueto the low volatility and high hydrophilicity, ROOHs are expected to reside in atmospheric condensed
phases such as aerosols, fogs, and cloud droplets. The decomposition mechanisms of ROOHs in theliquid phase are, however, still poorly understood. Here we report a temperature-dependent kinetics andtheoretical calculation study of the aqueous-phase decompositions of C12 or C13 a-alkoxyalkylhydroperoxides (a-AHs) derived from ozonolysis of a-terpineol in the presence of 1-propanol, 2propanol, and ethanol. We found that the temporal proﬁles of a-AH signals, detected as chlorideadducts by negative ion electrospray mass spectrometry, showed single-exponential decay, and thederived ﬁrst-order rate coeﬃcient k for a-AH decomposition increased as temperature increased, e.g.,k(288 K) ¼ (5.3  0.2)  104 s1, k(298 K) ¼ (1.2  0.3)  103 s1, k(308 K) ¼ (2.1  1.4)  103 s1 forC13 a-AHs derived from the reaction of a-terpineol Criegee intermediates with 1-propanol in thesolution at pH 4.5. Arrhenius plot analysis yielded an activation energy (Ea) of 12.3  0.6 kcal mol1. Ea of
18.7  0.3 and 13.8  0.9 kcal mol1 were also obtained for the decomposition of a-AHs (at pH 4.5)
derived from the reaction of a-terpineol Criegee intermediates with 2-propanol and with ethanol,
respectively. Based on the theoretical kinetic and thermodynamic calculations, we propose that
Received 27th September 2021
Accepted 17th January 2022

a proton-catalyzed mechanism plays a central role in the decomposition of these a-AHs in acidic
aqueous organic media, while water molecules may also participate in the decomposition pathways and

DOI: 10.1039/d1ea00076d

aﬀect the kinetics. The decomposition of a-AHs could act as a source of H2O2 and multifunctionalized

rsc.li/esatmospheres

species in atmospheric condensed phases.

Environmental signicanceThe decomposition of organic hydroperoxides (ROOHs) contributes to the formation of H2O2 and multifunctionalized species in atmospheric condensed
phases, which play central roles in modulating many atmospheric processes. Our results show that the stability of a-alkoxyalkyl-hydroperoxides (a-AHs) derived
from ozonolysis of a-terpineol in the presence of short-chain alcohols in aqueous organic media is markedly increased as the temperature is decreased, implyingthat the lifetimes of a-AHs in aerosols are aﬀected by local temperatures under diﬀerent environmental conditions such as altitude, day/night, season, andweather. The derived Ea values for the decomposition of the a-AHs could be incorporated in atmospheric modeling.

Introduction
Among a variety of reactive oxygen species, organic hydroperoxides (ROOHs) possessing single or multiple –OOH moieties(e.g., highly oxygenated organic molecules, HOM) area

Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha,
Kashiwa 277-8563, Japanb

Department of Environmental Sciences, University of California, Riverside, California
92521, USAc
National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba 305-8506,
Japan. E-mail: enami.shinichi@nies.go.jp; Tel: +81-29-850-2770† Electronic supplementary information (ESI) available: Additional experimental
data. See DOI: 10.1039/d1ea00076d© 2022 The Author(s). Published by the Royal Society of Chemistrya ubiquitous intermediate species formed by the oxidation ofvolatile organic compounds (VOCs).1–5 ROOHs are largely
formed via two major atmospheric processes, that is, oxidationsof VOCs initiated by OH-radical (RO2 chemistry) and ozone(Criegee chemistry). The former process involves the reaction
RO2 + HO2 / ROOH + O2 (ref. 6) and intramolecular autoxidation of RO2 leading to the formation of HOM,7 that containmultiple –OOH groups. The latter process involves reactive
carbonyl oxide zwitterions/biradicals, known as Criegee intermediates (CIs),8 generated by ozonolysis of unsaturated VOCs
possessing C]C bond(s) such as biogenic terpenes. CIs canrapidly react with OH-containing species such as water, alcohols and carboxylic acids to form ROOHs in the gas-phase or

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liquid-phase and at the air–liquid interface.8–10 The reactions of
CIs with ammonia, amine, HCl, H2S also produce ROOHs.8
Among the reaction partners of CIs, water (H2O)n$1 is expected
to be the most important reactant because of its predominant
abundance in both the gas-phase and the condensed phases. At
relative humidity (RH) ¼ 50%, for example, the concentration of
gaseous water dimer (H2O)2 reaches 3.0  1014 molecule per
cm3 and it could be a dominant reactant for CH2OO (thesmallest CI), which converts to hydroxymethyl hydroperoxide
within 0.5 ms.11 The hydration of CIs produces a-hydroxyalkylhydroperoxides (a-HHs) possessing –OOH and –OH moieties.
Because of the low volatilities and high hydrophilicities,
these ROOHs are expected to be taken up into atmosphericcondensed phases. We have found that the a-HHs in aqueous
phase generated by ozonolysis of terpenes decompose intofunctionalized geminal diols and the decompositions occur
with lifetimes (s1/e) that depend on the water content, the pH,
and the temperature of the medium as well as the chemical
structure of the a-HH.1,12–15 Recently, we investigated the fates of
a-alkoxyalkyl-hydroperoxides (a-AHs), an important ROOH
derived from the reaction of CIs with alcohols, in the liquid
phase.16 a-AHs possess –OOH and –OR0 groups attached to thesame carbon-atom. Given that atmospheric mixing ratios ofshort-chain alcohols exceed a few parts per billion by volume(ppbv) at forest sites,12,15,16 CIs could be partly converted byalcohols into a-AHs under specic conditions (e.g., at low relative humidity). In addition to the direct source, alcohols areformed during the oxidation of VOCs, such as incorporation of
OH into C]C double bonds.17,18 a-AHs can be formed by ozonolysis of VOCs with alcohols not only in the gas-phase but also
at the air–water interface.19,20 Interestingly, b-caryophyllene/ahumulene CIs can react with saccharides, a major componentof biomass burning particles, to form a-AHs at the air–waterinterface.19 Previously, we successfully detected C11–C13 a-AHs,
as their chloride-adducts by on-line electrospray mass spectrometry, generated from ozonolysis of C10 monoterpenealcohol a-terpineol (a-Tp) in solutions with the presence ofC1–C3 alcohols. a-Tp is a representative monoterpene alcoholthat is emitted from plants and household products.21,22 Amongterpenes, we selected a-Tp because of its high solubility in neatwater (103 times more soluble than a-pinene) and its structural similarity to other monoterpenes such as a-pinene and Dlimonene.23 We found the decay rate of C13 a-AHs derived fromthe a-Tp CIs + 1-propanol reaction increased as pH decreased
from pH 5.9 to 3.8, implying an acid-catalyzed decomposition
mechanism.16 A primary decomposition product of a-AHs in anacidic aqueous solution was found to be a hemiacetal, thatpossesses an –OH and an –OR0 attached to the same carbonatom. However, the decomposition mechanisms of a-AHs in
the liquid phase are not fully understood yet.
Here, we extend our research by performing new experiments of a-AH decomposition in aqueous organic solutions at
diﬀerent temperatures, in addition to theoretical calculations
for elucidation of the decomposition mechanism for details. We
measured temporal proles of a-AHs in solutions at T ¼ 288–
308 K derived from ozonolysis of a-Tp in the presence of C3 1propanol, 2-propanol or C2 ethanol (Fig. 1). Direct monitoring
242 | Environ. Sci.: Atmos., 2022, 2, 241–251

Paper

Fig. 1 Chemical structures and molecular weights (MWs) of aterpineol, ethanol, 1-propanol, and 2-propanol.

of the temporal proles of a-AHs enabled us to derive rate
coeﬃcients (k) at T ¼ 288–308 K and activation energies (Ea) of
a-AHs decomposition in the liquid phase for the rst time, thatwill be useful for atmospheric modeling and interpretation of
results obtained from eld measurements.
Theoretical calculations of Gibbs free energy proles andreaction rate coeﬃcients were also performed to evaluate therole of proton (H+)-catalyzed and water-catalyzed decomposition mechanisms by taking a-AHs in 1-propanol:water and 2propanol:water solutions as the model compounds. Results ofthe theoretical calculations may further shed light on thegeneral decomposition mechanism for ROOHs in atmosphericcondensed phases.

Experimental section
Fig. S1† shows the schematic procedure for preparing a-AHs inalcohol : water solutions. 2 mM a-Tp and 0.4 mM NaCl weredissolved in 10 mL of neat alcohol in a glass vial (25 mL) ina Peltier-type circulating water bath (AS ONE corporation, CTB1). Aqueous O3 solutions were prepared by sparging 10 mL ofwater in a 25 mL vial (kept in the water bath in advance) for 7–
30 s with O3(g) generated by means of a commercial ozonizer(KSQ-050, Kotohira, Japan) fed with ultra-high-purity O2(g)(>99.999%). The output gases from the ozonizer were carried to
the vial by means of Teon tubing (3 mm i.d.) at a ow rate of 1
L min1 (regulated by a digital mass-ow controller, Horiba
STEC). The initial O3 concentrations in the solutions, [O3(aq)]0,
were 0.06  0.01 mM, determined with a UV–vis spectrometer(Agilent 8453) using the reported O3 molar extinction coeﬃcientat 258 nm (3258 nm ¼ 3840 M1 cm1 in water24).
Ozonolysis reactions were initiated by mixing the a-Tp +NaCl in an alcohol solution and the O3 in an aqueous solution(10 mL each) in a 25 mL glass vial in the water bath. The
temperature of the reactants and reaction mixtures weremaintained within 1.0 K. Then, hydrochloric acid (HCl) was
added to the mixtures for experiments at pH of 4.0 or 4.5. The
pH values of the a-Tp + NaCl + HCl solutions were measured
with a calibrated pH meter (LAQUA F-74, Horiba) in separate
experiments. The [a-Tp]0/[O3(aq)]0 ratio was kept constant at
z17, ensuring that O3 is consumed exclusively by a-Tp ([a-Tp]0© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Environmental Science: Atmospheres¼ 1 mM, k ¼ 9.9  106 M1 s1) within a lifetime s1/e of 0.1
ms.25 The reaction mixtures were immediately injected at 100
mL min1 via a glass syringe (5 mL, covered with aluminum foil)
and a syringe pump (Pump 11 Elite, Harvard apparatus) into anelectrospray mass spectrometer (ESMS, Agilent 6130 Quadrupole LC/MS Electrospray System housed at the National Institute for Environmental Studies, Tsukuba, Japan).
Adding sub-millimolar amounts of NaCl to the sample
solutions enables us to detect Cl adducts of specic ROOHs,
including a-HHs and a-AHs, and other chemical species by
ESMS. We demonstrated that species possessing at least threefunctional groups, including a peroxide (–OOH/–OOR), an
alcohol (–OH), or a ketone (–RC]O) are MS-detectable as theCl adducts.1,12,13,26 The temporal proles of the ion signals for(a-AHs + Cl) were recorded via ESMS with a digital stopwatch.
The mass spectrometer was operated under the following
conditions: drying gas (N2) ow rate, 12 L min1; drying gas
temperature, 340  C; inlet voltage, +3.5 kV relative to ground;fragmentor voltage, 60 V. All solutions were prepared in ultrapure water (resistivity $ 18.2 MU cm at 298 K) from a Millipore
Milli-Q water purication system. a-Tp (95%, Tokyo Chemical
Industry), ethanol (99.5%, Wako), 1-propanol (99.5%, Tokyo
Chemical Industry), 2-propanol (99.5%, Tokyo Chemical
Industry), HCl (37%, ACS reagent-grade, Sigma-Aldrich), andNaCl (99.999%, Sigma-Aldrich) were used as received.

00

k ¼ kHþ þ kH2 O ¼ k Hþ ½Hþ  þ k H2 O½H2 On(3)

where [H+] is the concentration of H+ and is set as 3.16  105 M(corresponding to pH ¼ 4.5) for all the calculations, [H2O] is themolar concentration of water molecules, which is 55.5 M at thestandard condition, n is the number of water molecules
00involved in the unit reactions. k Hþ and k H2 O are the reaction
rate coeﬃcients of a-AHs decomposition corresponding to H+and water-catalyzation, which can be estimated by the Eyring
equation32 (eqn (4)).
0k ¼

kB T DGs =ðRTÞ
kB T DSs =R DH s =ðRTÞ
e
e
¼
e
h
h

(4)

where DGs, DSs, and DHs are the activation changes of Gibbs
free energy, entropy, and enthalpy, respectively. kB is theBoltzmann constant; h is the Planck constant; T is the temperature; and R is the gas constant. For reactions with Gibbs freeenergy barriers but without transition states, DGs, DSs, andDHs were calculated by the diﬀerence of G, S, and H between
products and reactants. The activation energy (Ea) for reactions
was estimated using eqn (5) according to the correlation
between the Arrhenius equation and the transition state theory:Ea ¼ DHs + RT

(5)

Results and discussion
Theoretical calculations
Calculations for the Gibbs free energy barriers were performed
with the Gaussian 16 program (revision C. 01).27 The relaxedscan of the potential energy surface between reactants andproducts was carried out with a step size of 0.5 Bohr to identify
possible transition states by using the M06-2X functional28 andthe 6-31G(d,p) basis set (i.e., M06-2X/6-31G(d,p)).29 Geometrical
optimization and frequency analysis of reactants, transition
states and products were computed with the same method. The
Gibbs free energy (G) and enthalpy (H) were calculated by eqn(1) and (2).
G ¼ ESP + Gcorr(1)

H ¼ ESP + Hcorr(2)

Gcorr and Hcorr are thermal corrections of Gibbs free energy
and enthalpy, which were estimated by M06-2X/6-31G(d,p),while ESP is the single-point energy rened by using the M062X functional implemented with Grimme's third-generation
empirical dispersion correction (D3)30 and the 6-311G(2d,p)basis set29 (i.e., M06-2X-D3/6-311G(2d,p)). The SMD implicit
solvation model was employed to simulate the water environment in all of the calculations.31 The temperatures for calculating the Gibbs free energy, enthalpy, and entropy for all thereactants, transition states, and products were set at 288, 298,
308, and 318 K. Pseudo-rst-order kinetics was assumed for all
the H+-catalyzed and water-catalyzed decomposition processes,
and the total reaction rate coeﬃcients (k) of a-AHs decay were
evaluated by their linear combination (eqn (3)).

© 2022 The Author(s). Published by the Royal Society of Chemistry

Decays of a-AHs in alcohol:water solutions at diﬀerent
temperatures
Time-dependent negative-ion mass spectra of solutions obtained by the reaction of O3(aq) + a-Tp/NaCl(aq) in 1-propanol : water (1 : 1 ¼ vol/vol) at pH 4.5 adjusted by addition of
HCl at T ¼ 288, 298 and 303 K are shown in Fig. 2.
Ozonolysis of a-Tp begins with incorporation of O3 into theC]C double bond of a-Tp, resulting in the formation ofa primary ozonide,26,33 which then isomerizes to CIs in thesolution. Then, the CIs in the alcohol : water (1 : 1 ¼ vol/vol)mixture isomerize,25,34–36 react with (H2O)n$1 to produce aHHs,12,13,36 or react with alcohol to produce a-AHs (Scheme 1).
The doublet peaks at m/z 255; 257 and m/z 297; 299 in Fig. 2
were assigned to the C10 a-HH–Cl and C13 a-AH–Cl derived
from a-Tp: 255; 257 ¼ 154 (a-Tp) + 48 (O3) + 18 (H2O) + 35; 37(Cl), and 297; 299 ¼ 154 (a-Tp) + 48 (O3) + 60 (1-propanol) + 35;
37 (Cl).12,16 From the mass-specic and reaction timedependent characterization of the products formed in 1propanol:H218O and 1-propanol:D2O solutions, we previously
identied the a-HH-Cl and a-AH-Cl and corresponding
decomposition products.12,16 We note that in the absence of HCl(pH 5.9), the m/z 297; 299 signals decreased to less than 20% of
their maximum value and persisted even aer several hours,16
implying the decomposition of a-AHs is H+-catalyzed. We
previously found that the decay of the signal at m/z 297; 299
became faster as pH decreased from 5.9 to 3.8 in the solution atroom temperature.16 The peaks at m/z 239; 241 were assigned to
the C10 geminal diols, a decomposition product from C10 aHHs. The doublet peaks at m/z 281; 283 in Fig. 2 were assigned

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Temporal dependence of negative-ion mass spectra of
mixtures obtained by ozonolysis ([O3]0 ¼ 0.06  0.01 mM) of solutionsof a-terpineol (1 mM) and NaCl (0.2 mM) in a 1-propanol : water (1 : 1¼ vol/vol) solution at T ¼ 288 K (A), 298 K (B) or 303 K (C) acidiﬁed by
0.05 mM HCl at pH 4.5. Ion signals at m/z 239; 241, m/z 255; 257, m/z281; 283, and m/z 297; 299 correspond to Cl adducts of C10 geminal
diols, C10 a-HHs, C13 hemiacetals, and C13 a-AHs, respectively. The
small ion signals at m/z 267; 269; 271; 273 correspond to Na4Cl5
clusters. See our previous study16 for assignments using D2O andH218O.
Fig. 2

to C13 hemiacetals, a decomposition product of C13 a-AHs;
hemiacetals ¼ 297; 299 (a-AH–Cl)  16 (O), where the –OOH of
the a-AHs is replaced by the –OH from water (Scheme 1).
Experiments for the reaction of O3(aq) + a-Tp/NaCl(aq) inaqueous mixtures with ethanol and 2-propanol conrmed the

244 | Environ. Sci.: Atmos., 2022, 2, 241–251

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formation of C12 and C13 a-AHs, respectively. In our previous
work, the a-AH signals appearing at m/z ¼ 154 (a-Tp) + 48 (O3) +alcohol's MW + 35/37 (Cl) and corresponding hemiacetal
products (16 Da) were also conrmed for methanol and 2propanol-d8 in alcohol : water (1 : 1 ¼ vol/vol) solutions.16
Therefore, it is evident that the a-Tp CIs react with the C#3
alcohols to form the a-AHs, that decompose into corresponding
hemiacetals in the liquid phase.
We previously showed, by using D2O and H218O solvents,that the hemiacetals can be formed via an H+-catalyzed
decomposition involving an H2O2 emission and an H2O addition; a-AH (+H+)  H2O2 + H2O (H+).16 The hemiacetals are
expected to further transform into MS-silent lactols (Scheme 1).
We will discuss the decomposition mechanism in the theoretical calculation section.
Fig. 3 shows time series of the proles of the a-AH signals
obtained for the reaction of O3(aq) + a-Tp/NaCl(aq) in 1-propanol : water (1 : 1 ¼ vol/vol) at pH 4.5 adjusted by 0.05 mM HCl
at T ¼ 288 K, 293 K, and 298 K. The time proles of a-AHs
showed a single-exponential decay that did not go to zero under
the conditions of the present study.
The decays of the m/z 297 signals were tted with a singleexponential function with a baseline S ¼ S0 + A exp(kt). The
decays of a-AHs derived from a-Tp CIs + 2-propanol and ethanol
were also tted with the single-exponential function witha baseline. As previously discussed,16 the baseline (S0) was
attributed to inert isomers of the m/z 297; 299, such as the
specic a-AHs R1C(–H)(–OOH)(–OR0 ) possessing a secondary–OOH (A in Scheme 1) and/or cyclic peroxyhemiacetals that
could not decompose at the current timescales. It has been reported that tertiary hydroperoxides can produce corresponding
tertiary alcohols under acidic conditions, due to the stabilization of the corresponding carbenium ion.37 In that case, theobserved 1st order decays and corresponding k values were
attributed to a-AHs possessing a tertiary –OOH (B in Scheme 1).
This interpretation is supported by the theoretical calculations
as shown below. The observation that the larger S0 values wereobtained at lower temperatures (Fig. 3) implies that the relative
yield of a-AHs possessing a secondary versus tertiary –OOH (A/B
in Scheme 1) increases as temperature decreases. The obtained
k values for diﬀerent temperatures are summarized in Table 1.
The k values are means  SDs for three independent
experiments.
Fig. 4 shows an Arrhenius plot (ln k vs. 1/T) for the decomposition of a-AHs derived from ozonolysis of a-Tp in 1-propanol:water solutions at pH 4.5. The linear regression yieldeda preexponential factor (A) of 1.2  106 s1 (ln A ¼ 14.0  1.0)
and an Ea value of 12.3  0.6 kcal mol1. The uncertainties wereobtained from the standard errors of the intercept and slope,
respectively. Temperature dependent k was expressed as k ¼ 1.2
 106 exp[(51.5  2.5)  103/RT] s1, where R is the gas
constant.
The temporal proles of a-AHs derived from a-Tp CIs + 1propanol in solution at pH 4.0 (adjusted by adding 0.1 mM HCl)
were measured to determine k. The obtained k and the Arrhenius plot at pH 4.0 are shown in Table S1 and Fig. S2.† The© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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The formation and decomposition of a-AHs (A; secondary –OOH, B; tertiary –OOH) and a-HHs derived from ozonolysis of aterpineol in 1-propanol : water (1 : 1¼vol/vol) solutions. The likely isomers are shown.16

Scheme 1

Table 1 Rate coeﬃcients for decay of the C13 a-AHs derived from aTp ozonolysis in 1-propanol:water solutions at pH 4.5 at diﬀerent
temperaturesa

Temperature (K)kpH 4.5 (s1)s1/e (minutes)

288
293
298
303
308

(5.3  0.2)  104
(7.5  1.4)  104
(1.2  0.3)  103
(1.6  0.7)  103
(2.1  1.4)  103

31
22
14
10
8

Experimental conditions: [a-Tp]0 ¼ 1 mM, [NaCl]0 ¼ 0.2 mM, [O3]0 ¼
0.06 mM, pH 4.5. The k values are means  SDs for three independent
experiments. s1/e ¼ 1/k.
aDecay proﬁles of the Cl adducts of the C13 a-AHs (m/z 297)
generated by ozonolysis of a-Tp (1 mM a-terpineol, 0.2 mM NaCl) at[O3]0 ¼ 0.06 mM in a 1-propanol : water (1 : 1 ¼ vol/vol) solution at T ¼
288 K (circles), 293 K (triangles) or 298 K (squares) at pH 4.5 adjusted by
0.05 mM HCl. Lines indicate ﬁttings of signal intensities (S) to singleexponential functions with baselines. See the text for details.
Fig. 3derived Ea for pH 4.0 was 14.4  0.6 kcal mol1, which is slightly(by 2.1 kcal mol1) larger than that for pH 4.5.
We also obtained a series of the proles of the a-AH signals
for the reaction of a-Tp CIs with 2-propanol and ethanol at pH
4.5 at diﬀerent temperatures. All the decays of a-AHs showedsingle-exponential functions S ¼ S0 + A exp(kt), consistent
with the case of a-AHs derived from CIs + 1-propanol. Derived kvalues were summarized in Tables 2 and 3.
Fig. 5 and 6 show Arrhenius plots for the decomposition of aAHs derived from a-Tp CIs + 2-propanol and a-Tp CIs + ethanol© 2022 The Author(s). Published by the Royal Society of Chemistryat pH 4.5, which provide Ea of 18.7  0.3 and 13.8 
0.9 kcal mol1, respectively.
Theoretical calculations for the decay of a-AHs in the aqueous
phase
Here, we employed rst-principle calculations to estimate the
Gibbs free energy barriers of H+-catalyzed decomposition of aAHs (Fig. 7 and 8) and the subsequent formation of hemiacetals(Fig. S3 and S4†). The Cartesian coordinates for the structuralgeometry used in the calculations are summarized in Table S2.†
Since a-Tp potentially produces two kinds of CIs (Scheme 1) andeach CI leads to two chiral isomers of a-AHs in the reaction withalcohols, the detected formula of each a-AH may have four
possible structures (A, B, C, D in Fig. 7 and 8).
All the Gibbs free energy barriers in Fig. 7 and 8 exhibit low
values (<10 kcal mol1), supporting the high feasibility of H+catalyzed decomposition mechanisms. The Gibbs free energy

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Fig. 4 Arrhenius plot of the rate coeﬃcients (k) for the decay of the
C13 a-AHs generated by ozonolysis of aqueous a-terpineol in thepresence of 1-propanol at pH 4.5.

Arrhenius plot of the rate coeﬃcients (k) for the decay of the C13
a-AHs generated by ozonolysis of aqueous a-terpineol in the presence of 2-propanol at pH 4.5.
Fig. 5

Table 2 Rate coeﬃcients for decay of C13 a-AHs derived from a-Tp
ozonolysis in 2-propanol:water solutions at pH 4.5 at diﬀerent
temperaturesa

Temperature (K)kpH 4.5 (s1)s1/e (minutes)

293
298
303
308

(4.0  0.5)  104
(7.0  1.4)  104
(1.1  0.2)  103
(1.9  0.4)  103

42
24
15
9

a
Experimental conditions: [a-Tp]0 ¼ 1 mM, [NaCl]0 ¼ 0.2 mM, [O3]0 ¼
0.06 mM, pH 4.5. The k values are means  SDs for three independent
experiments. s1/e ¼ 1/k.

Table 3 Rate coeﬃcients for decay of C12 a-AHs derived from a-Tp
ozonolysis in ethanol:water solutions at pH 4.5 at diﬀerent
temperaturesa

Temperature (K)kpH 4.5 (s1)s1/e (minutes)

288
293
298
303

(7.2  0.5)  104
(1.1  0.2)  103
(1.8  0.2)  103
(2.3  0.3)  103

23
15
9
7

a
Experimental conditions: [a-Tp]0 ¼ 1 mM, [NaCl]0 ¼ 0.2 mM, [O3]0 ¼
0.06 mM, pH 4.5. The k values are means  SDs for three independent
experiments. s1/e ¼ 1/k.

proles of the subsequent hemiacetal formation also show
small barriers (<4 kcal mol1) or even no barriers (Fig. S3 andS4†), supporting the rapid hydration of carbenium ions. Since
water molecules are ubiquitous in the aqueous environment,
the hemiacetal formation is expected to be much faster than thecarbenium ion formation. Hence, the H+-catalyzed decomposition of a-AH to carbenium ions should be the rate-determining
steps of our proposed H+-catalyzed mechanisms of a-AH decay,
and the reactions in Fig. 7 and 8 can be used to determine the Ea

246 | Environ. Sci.: Atmos., 2022, 2, 241–251

Fig. 6 Arrhenius plot of the rate coeﬃcients (k) for the decay of the
C12 a-AHs generated by ozonolysis of aqueous a-terpineol in thepresence of ethanol at pH 4.5.

and the pseudo-rst-order reaction rate coeﬃcients at pH 4.5(Table 4). However, the theoretical Ea values are about 4–
15 kcal mol1 lower than the apparent activation energy
measured by experiments, while the calculated reaction rate
coeﬃcients are 6–11 orders of magnitude larger than theexperimental results. This implies that other pathways of H+catalyzation with tight concerted transition states may exist andlead to slower a-AH decomposition observed in our experiments. As the a-AHs are surrounded by ubiquitous watermolecules in the solution, it is possible that water molecules
participate in the H+-catalyzed channels and decelerate the aAH decay by increasing the energy barriers. However, the exact
reaction rate with explicit participation of water molecules is
hard to estimate owing to the highly exible conformation ofliquid water, which may generate a variety of transition statesand hence require further studies.

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Fig. 7 Gibbs free energy proﬁles of H+-catalyzed decomposition of C13 a-AHs (a-terpineol CIs + 1-propanol).

Fig. 8 Gibbs free energy proﬁles of H+-catalyzed decomposition of C13 a-AHs (a-terpineol CIs + 2-propanol).

To better constrain the inuence of water participation in aAH decay, kinetics of water catalyzation is further explored.
Since the number of water molecules is much larger than H+, itis necessary to compare the water-catalyzed kinetics with thoseof the H+-catalyzed channels. Fig. 9 shows four possible pathways for the water-catalyzed reactions, where two water molecules (i.e., n ¼ 2 in eqn (3)) are assumed to be involved according
to our previous study.14 Here we only present the mechanisms at© 2022 The Author(s). Published by the Royal Society of Chemistry

the aldehyde group since the ketone group has a sterichindrance on the formation of the six-membered transitions
states. All the Gibbs free energy barriers exhibit high values(>45 kcal mol1), and all the activation changes of entropy are
negative values, that are in sharp contrast with those in the H+catalyzed reactions (Table 4). The theoretical Ea values are about
15–24 kcal mol1 higher than the apparent activation energy
measured by experiments, while the corresponding reaction

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Theoretical activation changes of entropy (DSs), activation energies (Ea) and pseudo-ﬁrst-order reaction rate coeﬃcients at pH 4.5 (kH+)
for the eight reactions in Fig. 7 and 8

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Table 4Ea (kcal mol1)kH+ (s1)

ReactionDSs(kcal mol1 K1)

288 K

298 K

308 K

318 K

288 K

298 K

308 K

318 K

1A
1B
1C
1D
2A
2B
2C
2D

0.013
0.009
0.004
0.005
0.011
0.009
0.005
0.006

8.9
7.1
3.7
5.4
8.5
5.8
3.4
5.3

8.9
7.2
3.7
5.4
8.6
5.8
3.5
5.3

9.0
7.2
3.8
5.5
8.6
5.8
3.5
5.4

9.0
7.2
3.8
5.5
8.7
5.8
3.5
5.4

1.4  106
1.8  105
2.6  107
5.0  106
1.1  106
7.9  104
3.8  107
7.6  106

1.6  106
2.3  105
2.9  107
5.7  106
1.3  106
1.1  105
4.1  107
8.6  106

1.9  106
2.9  105
3.1  107
6.5  106
1.5  106
1.5  105
4.4  107
9.7  106

2.1  106
3.6  105
3.3  107
7.3  106
1.8  106
1.9  105
4.6  107
1.1  107

Fig. 9 Gibbs free energy proﬁles of water-catalyzed decomposition of C13 a-AHs from a-terpineol CIs + 1-propanol (A and B) and a-terpineol
CIs + 2-propanol (C and D).

rate coeﬃcients are negligibly small compared with themeasured values (Table 5). These results suggest that participation of water molecules may attribute to higher energy
barriers in a-AH decomposition. In addition, since H+-catalyzation leads to much faster kinetics than water-catalyzation,
the total reaction rate constants estimated by eqn (3) represent those of H+-catalyzation (Table 4). To further assess the
H2O-catalyzed reactions under realistic experimental conditions, we may need to include more than three water molecules(n $ 3) or use a diﬀerent theoretical approach. We also note thatthe presence of alcohol (e.g., 1-propanol at 6.7 M) could
modulate the activation energies of the a-AHs decomposition inalcohol:water mixtures.
Our theoretical calculations revealed that the reactions of aAHs R1C(–H)(–OOH)(–OR0 ) possessing a secondary –OOH (e.g.,
A in Scheme 1) leading to the formation of corresponding
carbenium ions are energetically less favorable compared to a-

248 | Environ. Sci.: Atmos., 2022, 2, 241–251

AHs R1C(–R2)(–OOH)(–OR0 ) possessing a tertiary –OOH (e.g., B
in Scheme 1). The present calculations also conrmed that
H2O2 could be an important byproduct of the decompositionof a-AHs in the aqueous phase, consistent with the previous
theoretical result for a-Tp a-HHs.15 A previous iodometry
analysis of a mixture of 1 mM a-Tp + 0.2 mM NaCl + 0.06 mM
O3 + 0.1 mM HCl revealed that approximately 70% of O3 wasconverted into H2O2 and other peroxides in water.38 Anotherexperimental study on the aqueous-phase decomposition of aacyloxyalkyl-hydroperoxides derived from a-pinene CIs withcarboxylic acids found that H2O2 is a key decomposition
product.39
Comparison of the activation energies for the decompositionof a-AHs and related ROOHs in aqueous solutions
The experimentally determined Ea values for the decompositionof a-AHs and related ROOHs in liquid phases are summarized© 2022 The Author(s). Published by the Royal Society of ChemistryView Article Online

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Theoretical activation changes of entropy (DSs), activation energies (Ea) and pseudo-ﬁrst-order reaction rate coeﬃcients (kH2O) for thefour reactions in Fig. 9Open Access Article. Published on 17 January 2022. Downloaded on 9/24/2024 4:39:21 AM.
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Table 5Ea (kcal mol1)

kH2O (s1)ReactionDSs(kcal mol1 K1)

288 K

298 K

308 K

318 K

288 K

298 K

308 K

318 K

3A
3B
3C
3D

0.039
0.047
0.047
0.048

35.5
36.6
33.7
34.7

35.5
36.6
33.7
34.8

35.5
36.7
33.8
34.8

35.6
36.7
33.8
34.8

8.4  1020
1.9  1022
3.3  1020
2.4  1021

1.3  1018
3.6  1021
5.3  1019
4.2  1020

1.7  1017
5.6  1020
7.2  1018
6.2  1019

1.8  1016
7.3  1019
8.4  1017
7.7  1018

in Table 6. The decay rate coeﬃcients for a-AHs, a-HHs, anda-acyloxyalkyl-hydroperoxides at pH  4.5 listed in the Table 6
are in the orders of 104 to 103 s1.
The Ea values for the decomposition of the a-AHs derived
from a-Tp CIs + alcohols examined in the present study range in
12–15 kcal mol1, except for a-AHs from 2-propanol (18.7 
0.3 kcal mol1). It can be argued that a steric hindrance of
isopropyl group would limit the accessibility of (H2O)nH+ to the–OOH moiety, resulting in a larger Ea. However, our theoretical
calculations show that the n-propyl and isopropyl groups havelittle inuence on the activation energy, revealing that theaccessibility of (H2O)nH+ to the –OOH group is not hindered.
This nding suggests that other mechanisms may also playa role in a-AH decomposition.
It was reported that the decomposition of tert-alkylhydroperoxides R1R2R3C–OOH yields the corresponding tertalkyl-alcohols R1R2R3C–OH under acidic conditions, due to thehigh stabilization of corresponding carbenium ions.37,41 The
assumed mechanism involves a protonation of the O-atom next to
the C-atom of R1R2R3C–OOH, releasing H2O2 by leaving a positive
charge on the C-atom. In that case, the obtained Ea values were
attributed to specic a-AHs possessing a tertiary –OOH (e.g., B in
Scheme 1). The formed carbenium ion is rapidly hydrated to forman oxocarbenium ion, which releases an H+ to form the corresponding alcohol, as supported by the low Gibbs free energy
barriers (0–4 kcal mol1) in our calculations (Fig. S3 and S4†).
Atmospheric implications
ROOHs possessing single or multiple –OOH moieties are a classof ubiquitous intermediate species that are formed by the

Table 6oxidation of VOCs. The OH oxidation of VOCs produces RO2
radicals, which then undergo bimolecular reactions or intramolecular H-atom abstractions and O2 addition to form HOM,
whereas the ozonolysis of VOCs with C]C double bond(s)produces CIs, which in most cases end up forming ROOHs withmultifunctionalities. The important point is that both OH andO3 oxidation of VOCs can result in the production of ROOHs,
although the quantitative yields of ROOHs via such processes
are not known yet. Because of the low volatilities and highhydrophilicities, ROOHs are readily taken up into atmosphericcondensed phases such as aerosols, fog/cloud droplets, and wet
lms of plants and soils. The kinetic data of ROOHs in atmospheric condensed phases is a key parameter in atmosphericmodeling. In this article, we determined the temperaturedependent decomposition rate coeﬃcients and the activationenergies of a-AHs [R1R2C(–OOH)(–OR0 )], a class of important
ROOHs generated from the reaction of CIs with alcohols, in theliquid phase. We found longer lifetimes for the a-AHs in solutions at lower temperatures. For example, the lifetime of C13 aAHs derived from a-Tp CIs + 1-propanol is changed from 8 min
at 308 K to 31 min at T ¼ 288 K. This result suggests that the
lifetimes of a-AHs in aerosols are aﬀected not only by the pH ofreaction media but also by local temperatures under diﬀerent
environmental conditions such as altitude, day/night, season,
and weather. Another implication is that, as in the case of aHHs and other ROOHs, the storage of aerosol samples at lower
temperatures would be a promising way to extend the lifetime of
a-AHs for analysis in eld measurements and chamber experiments.42 The derived Ea values for the decomposition of the aAHs of the a-Tp CIs + alcohols would be useful for atmospheric

Activation energies for decay of the a-AHs and related ROOHs in liquid phases

ROOH

Ea (kcal mol1)

C13 a-AHs (a-terpineol CIs + 1-propanol) at pH 4.5
At pH 4.0
C13 a-AHs (a-terpineol CIs + 2-propanol) at pH 4.5
C12 a-AHs (a-terpineol CIs + ethanol) at pH 4.5
C16 a-acyloxyalkyl hydroperoxides (a-pinene CIs + adipic acid) at pH 4.4a
C20 a-acyloxyalkyl hydroperoxides (a-pinene CIs + pinonic acid) at pH 4.4a
C10 a-terpineol a-HHs at pH 4.5b
C10 terpinen-4-ol a-HHs at pH 6.2bC1 hydroxymethyl hydroperoxide at pH 7.1c

12.3  0.6
14.4  0.6
18.7  0.3
13.8  0.9
15.0  1.0
14.5  1.6
19.2  0.5
17.1  0.2
22.9

aRef. 39. b Ref. 15. c Ref. 40.

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modeling as a representative value of ROOH possessing –OOH
and –OR0 .
Both our experiments and theoretical calculations revealed
that the H+-catalyzed conversion of ROOH into ROH, which isaccompanied by the formation of H2O2, may be a generalmechanism that produces H2O2 and multifunctionalized
species in atmospheric condensed phases3,43,44 and contributes
to the oxidative potential and toxicity of secondary organic
aerosols.45 The rate of the proposed H+-catalyzed decompositionof a-AHs in ambient aerosol particles could be faster than therate via photolysis by solar radiation or decomposition catalyzed by transition metals. We recently found that the reactionof a-Tp a-HHs with Fe2+ is outcompeted by H+-catalyzed
decomposition of a-HHs at ambient concentrations of Fe2+ andH+ found in atmospheric condensed phases.38 Further work totest the eﬀects of structures and functionalities on the decomposition of ROOHs in the aqueous phase is underway. Our
theoretical calculations imply the existence of diﬀerent reaction
pathways, such as water-catalyzed reactions, that could becorrelated with the fates of a-AHs in ambient particles undervariable relative humidity.

Conclusion
We report an experimental study of the liquid-phase decompositions of C12 or C13 a-alkoxyalkyl-hydroperoxides (a-AHs)
derived from ozonolysis of a-terpineol in the presence of 1propanol, 2-propanol, and ethanol as a function of temperature.
The temporal proles of a-AH signals, detected as chlorideadducts by negative ion electrospray mass spectrometry,showed single-exponential decay, and the derived rst-order
rate coeﬃcient k for a-AH decomposition increased as temperature increased, e.g., k(288 K) ¼ (5.3  0.2)  104, k(298 K) ¼
(1.2  0.3)  103, k(308 K) ¼ (2.1  1.4)  103 s1 for C13 aAHs derived from the reaction of a-terpineol Criegee intermediates with 1-propanol in solution at pH 4.5. Arrhenius plot
analysis yielded activation energy (Ea) of 12.3  0.6, 18.7  0.3
and 13.8  0.9 kcal mol1 for the decomposition of a-AHs
derived from the reaction of a-Tp CIs with 1-propanol, 2-propanol and ethanol, respectively. Theoretical kinetic and thermodynamic calculations revealed that H+-catalyzed mechanism
plays an important role in the decomposition of these a-AHs inacidic water, while water molecules may also participate in the
H+-catalyzed mechanism and inuence the kinetics. The rate of
the proposed H+-catalyzed decomposition of a-AHs in ambient
aerosol particles could be faster than that of other degradation
processes. The proposed decomposition of a-AHs could act asa source of H2O2 and multifunctionalized species in atmospheric condensed phases.

Author contributions
S. E. designed the research; M. H. and S. E. performed theexperiments; K. C. performed the theoretical calculations; S. E.
wrote the rst dra of the manuscript; and all of the authors
analyzed the data and contributed to revising the manuscript.

250 | Environ. Sci.: Atmos., 2022, 2, 241–251

Paper

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
S. E. is grateful for a JSPS KAKENHI (no. 19H01154). Computations were performed using the computer clusters and data
storage resources of the UCR High-Performance Computing
Center (HPCC), which were funded by grants from NSF (MRI1429826) and NIH (1S10OD016290-01A1).

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