Direct observation of the gas phase reaction of the cyclohexyl radical with dioxygen using a distonic radical ion approach

Article abstract of DOI:10.1021/jp9073398

Alkylperoxyl radicals are intermediates in the oxidation of hydrocarbons. The reactive nature of these intermediates, however, has made them elusive to direct observation and isolation. We have employed ion trap mass spectrometry to synthesize and characterize 4-carboxylatocyclohexyl radical anions ( C₆H₁₀-CO₂^-) and observe their reactivity in the presence of dioxygen. The resulting reaction is facile (k = 1.8 × 10^-10 cm³ molecule^-1 s^-1 or 30% of calculated collision rate) and results in (i) the addition of O ₂ to form stabilized 4-carboxylatocyclohexylperoxyl radical anions ( OO-C₆H₁₀-CO₂^-), providing the first direct observation of a cyclohexylperoxyl radical, and (ii) elimination of HO₂ and HO radicals consistent with recent laser-induced fluorescence studies of the reaction of neutral cyclohexyl radicals with O₂. Electronic structure calculations at the B3LYP/6-31+G(d) level of theory reveal viable pathways for the observed reactions showing that formation of the peroxyl radical is exothermic by 37 kcal mol^-1 with subsequent transition states as low as -6.6 kcal mol ^-1 (formation of HO₂ ) and -9.1 kcal mol^-1 (formation of HO ) with respect to the entrance channel. The combined computational and experimental data suggest that the structures of the reaction products correspond to cyclohexenes and epoxides from HO₂ and HO loss, respectively, while alternative pathways leading to cyclohexanone or ring-opened isomers are not observed. Activation of the charged peroxyl radical OO-C₆H₁₀-CO₂^- by collision induced dissociation also results in the loss of HO₂ and HO radicals confirming that these products are directly connected to the peroxyl radical intermediate.

Full text of DOI:10.1021/jp9073398

1446
J. Phys. Chem. A 2010, 114, 1446–1456
Direct Observation of the Gas Phase Reaction of the Cyclohexyl Radical with Dioxygen
Using a Distonic Radical Ion Approach
Benjamin B. Kirk, David. G. Harman, and Stephen J. Blanksby*
School of Chemistry, UniVersity of Wollongong, NSW, Australia 2522
ReceiVed: July 31, 2009; ReVised Manuscript ReceiVed: NoVember 30, 2009
Alkylperoxyl radicals are intermediates in the oxidation of hydrocarbons. The reactive nature of these
intermediates, however, has made them elusive to direct observation and isolation. We have employed ion
-
trap mass spectrometry to synthesize and characterize 4-carboxylatocyclohexyl radical anions (•C6H10-CO2 )
and observe their reactivity in the presence of dioxygen. The resulting reaction is facile (k ) 1.8 × 10-10 cm3
molecule-1 s-1 or 30% of calculated collision rate) and results in (i) the addition of O2 to form stabilized
-
4-carboxylatocyclohexylperoxyl radical anions (•OO-C6H10-CO2 ), providing the first direct observation of
•
a cyclohexylperoxyl radical, and (ii) elimination of HO2 and HO• radicals consistent with recent laser-induced
fluorescence studies of the reaction of neutral cyclohexyl radicals with O2. Electronic structure calculations
at the B3LYP/6-31+G(d) level of theory reveal viable pathways for the observed reactions showing that
formation of the peroxyl radical is exothermic by 37 kcal mol-1 with subsequent transition states as low as
•
-6.6 kcal mol-1 (formation of HO2 ) and -9.1 kcal mol-1 (formation of HO•) with respect to the entrance
channel. The combined computational and experimental data suggest that the structures of the reaction products
•
correspond to cyclohexenes and epoxides from HO2 and HO• loss, respectively, while alternative pathways
leading to cyclohexanone or ring-opened isomers are not observed. Activation of the charged peroxyl radical
•
•OO-C6H10-CO2- by collision induced dissociation also results in the loss of HO2 and HO• radicals confirming
that these products are directly connected to the peroxyl radical intermediate.
Introduction
rearrangement of the peroxyl radical via hydrogen abstraction from
the hydrocarbon backbone (e.g., reaction 8) followed by subsequent
addition of oxygen are proposed as key chain-branching events in
the radical chain reaction6,7 leading to autoignition (the cause of
engine knock) in internal combustion engines.8 The importance of
the unimolecular chemistry of alkylperoxyl radicals has prompted
innumerable experimental and theoretical studies with a particular
focus on reactions of the simple, archetypal ethylperoxyl radical.
The available data have been critically reviewed by Rienstra-
Kiracofe et al.,9 and the most significant reaction pathways are
summarized in reactions 6-10.
In the gas phase, alkylperoxyl radicals are formed from the
combination of alkyl radicals and dioxygen during the oxidation
of hydrocarbons in the atmosphere and during combustion.1,2
Tropospheric processing of alkanes is initiated predominantly
by hydroxyl radicals via hydrogen abstraction reactions, resulting
in the formation of alkyl radicals and water (reaction 1). In the
presence of oxygen, the nascent alkyl radical undergoes rapid
addition to form an alkylperoxyl radical (reaction 2).3,4 This
reaction is highly exothermic (e.g., ∆rxnH298[CH3CH2• + O2 f
•
CH3CH2O2 ] ) 36 kcal mol-1)5 and thus, depending on the
•
•
capacity of the peroxyl radical to accommodate the excess
energy, a third body collision is usually required to stabilize
the product. In the troposphere, alkylperoxyl radicals oxidize
NO to NO2, which is photolabile in sunlight, leading to
production of ozone (reactions 4 and 5). Given that NO and
hydrocarbons are both byproducts of combustion, reactions 1-5
form a crucial part of the chemistry responsible for the formation
of photochemical smog in urban areas.3,4
CH3CH2 +O2 f C2H4 + HO2
(6)
(7)
•
•
•
CH3CH2 + O2 f CH3CH2O2 f C2H4 + HO2
•
•
•
CH3CH2 + O2 f CH3CH2O2 f CH2CH2O2H f
•
C2H4 + HO2
(8)
(9)
•
•
•
CH3CH2 + O2 f CH3CH2O2 f CH2CH2O2H f
c-C2H4O + HO•
•
RH + OH f R• + H2O
(1)
(2)
(3)
(4)
(5)
•
•
•
CH3CH2 + O2 f CH3CH2O2 f CH3 CHO2H f
CH3CHO + HO•
R• + O2 + M f ROO• + M
(10)
•
•
ROO• + NO f RO• + NO2
The weight of evidence suggests that direct abstraction of a
hydrogen atom from the ethyl radical (reaction 6) does not play
a significant role in hydrocarbon oxidation at low temperatures
(below ca. 800 K). Two mechanisms are proposed for the
experimentally observed formation of ethylene and the hydro-
peroxyl radical; the first involves direct elimination from the
ethylperoxyl radical (reaction 7), while the second involves
rearrangement facilitated by a 1,4-hydrogen shift to form the
2-hydroperoxylethyl radical followed by elimination (reaction
8). The rearranged 2-hydroperoxylethyl radical can give rise to
•NO2 + sunlight f NO + O(3P)
•
O2 + O(3P) + M f O3 + M
During combustion, the unimolecular chemistry of the interme-
diate alkylperoxyl radical is critical in controlling product distribu-
tion and ultimately the efficacy of the process. In particular,
† Part of the “W. Carl Lineberger Festschrift”.
* Corresponding author. E-mail: blanksby@uow.edu.au.
10.1021/jp9073398  2010 American Chemical Society
Published on Web 12/29/2009

Gas Phase Reaction of the Cyclohexyl Radical with O2
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1447
SCHEME 1
oxirane and hydroxyl radical (reaction 9) while an analogous
1,3-hydrogen atom transfer can result in acetaldehyde with
concomitant loss of hydroxyl radical (reaction 10). Reactions
7-10 are competitive processes and all occur through the
intermediacy of the ethylperoxyl radical. In recent investigations
of longer chain alkylperoxyl radicals (e.g., propyl-, butyl-, and
pentylperoxyl radicals) hydrogen atom transfer from more
remote sites on the carbon chain have been shown to play an
important role.6,10
Despite the apparent importance of alkylperoxyl radicals in
directing reaction outcomes in alkane oxidation, relatively few
experimental studies of these intermediates have been reported.
Most mechanistic insight into these gas phase reactions is the
result of kinetic analysis of the formation of detectable reaction
products. Direct observation of the peroxyl radical intermediates
in situ has been undertaken by UV absorption but this is often
complicated by overlapping absorptions from the wide range
of peroxyl and peroxide species present in the reaction mixture.11
Alternatively, high resolution spectroscopy of a range of
alkylperoxyl radicals has been undertaken in the gas phase using
cavity ring down techniques,12-16 while matrix isolation has
provided detailed infrared signatures for these intermediates.17
These studies provide essential insight into the molecular
structure and energetics of alkylperoxyl radicals, but they are
not well suited to careful mechanistic investigation of their
reactions. Similarly, indirect methods to probe the thermochem-
istry of peroxyl radicals5,18 are essential but do not provide for
direct observation of peroxyl radical reactions. As such, the most
detailed mechanistic insight into the reactions of alkylperoxyl
radicals relies on theory with a raft of computational studies
undertaken on the processes 6-10,9 as well as higher alkyl-
peroxyl radicals.10,19 In this paper we outline a new approach
to the formation, isolation and the direct observation of reactions
of alkylperoxyl radicals in the gas phase using distonic radical
anions.
outlined in Scheme 1, addition of oxygen to the 3-carboxyla-
toadamantyl radical anion (A) yields the 3-carboxylatoadaman-
tylperoxyl radical anion (B), which upon dissociation gives the
oxirane C via loss of the hydroxyl radical (cf., reaction 9). In
this system, however, no hydroperoxyl radical loss is observed
(cf., reactions 7 and 8) presumably due to the excessive ring
strain in formation of the resulting adamantene D.
In the present study we have selected the 4-carboxylato-
cyclohexyl distonic radical anion as a case study for the
unimolecular reactivity neutral alkyl radicals with dioxygen. This
molecule is significantly less strained than the adamantyl system
but retains sufficient rigidity to spatially separate the radical
and negative charge. Cycloalkanes are a major component of
gasoline and diesel fuels32 and as such the peroxyl radicals
formed from these compounds are of significant interest in both
combustion and atmospheric oxidation processes. Detailed
kinetic modeling of the oxidation of cyclohexane has recently
been reported and forms an excellent reference point for this
study.32,33
Methods
Mass Spectrometry. Experiments were performed on a
modified ThermoFisher LTQ (San Jose, CA) linear quadrupole
ion trap mass spectrometer34 fitted with a conventional IonMax
electrospray ionization source and operating Xcalibur 2.0 SUR1
software. Ions were generated by infusion at 3-5 µL min-1 of
a methanolic solution (10-50 µM) into the electrospray ion
source. In some instances, aqueous ammonia was added to raise
the pH and facilitate the formation of dianions. Typical
instrumental settings were spray voltage -2.5 to +3.5 kV,
capillary temperature 200-250 °C, sheath gas flow between
10 and 30 (arbitrary units), and sweep and auxiliary gas flow
set at between 0 and 10 (arbitrary units). For collision experi-
ments, ions were mass-selected with a window of 1-4 Th, using
a Q-parameter of 0.250 and the fragmentation energy applied
was typically 10-45 (arbitrary units) with an excitation time
of 30 ms (unless otherwise noted). Modifications to the mass
spectrometer to allow the introduction of neutral gases into the
ion trap region of the instrument have been previously de-
scribed.30 Briefly, liquid and gas reagents are introduced into a
flow of Ultra High Purity (UHP) helium (3-5 psi) via a heated
septum inlet (25-250 °C). The reagent flow (e.g., 18O2, NO) is
controlled using a syringe pump, while the mixture is supplied
via a variable leak valve to provide a total ion gauge reading of
∼0.9 × 10-5 Torr representing an estimated trap pressure of
2.5 mTorr. The temperature of the vacuum manifold surrounding
the ion trap was measured at 307 ( 1 K, which is taken as
being the effective temperature for ion-molecule reactions
observed herein.35 Reaction times of 30-10000 ms were set
using the excitation time parameter within the control software
using a fragmentation energy of 0 (arbitrary units). All spectra
presented represent an average of at least 50 scans. Reaction
Distonic radical ions were originally defined as ions arising
from the one-electron oxidation or reduction of biradicals, ylides,
or zwitterions20,21 but are more commonly considered as radical
ions in which the charge and radical are separated.22 This
separation between charge and radical provides these ions with
unique reactivity and the pioneering work of Kentta¨maa and
others has demonstrated that for systems with a sufficiently inert
charge, distonic radical ions can provide effective probes of
neutral radical reactivity.23-25 In such instances, the charge acts
as a spectator to the radical reactivity but provides an effective
handle to allow isolation and detection of reactants and products
by mass spectrometry. Putative distonic peroxyl radicals have
been previously reported, with the addition of 32 Da to
N-ethylpyridinium radical cations observed by mass spec-
trometry.26,27 More recently, McLuckey has reported on the
addition of 32 Da to radical cations formed from the electron
transfer dissociation of proteins.28 While it is likely that these
ions do possess a peroxyl radical moiety, their structures do
not serve to provide clear spatial and electronic separation of
charge and radical sites and as such they do not present good
models for neutral reactivity. We have previously reported the
formation and characterization of an adamantylperoxyl radical
where the adamantyl cage provides for rigid separation between
the charge and unpaired electron (Scheme 1).29,30 In this instance,
however, the inherent ring strain of the adamantyl system
significantly perturbs the intrinsic reactivity of the nascent
carbon-centered radical31 and further, the restricted geometry
prohibits the observation of important unimolecular reactions
of the peroxyl radical following addition of oxygen.29 As

1448 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Kirk et al.
rate coefficients were determined as previously described and
collision rates for the ion-molecule reactions were estimated
by average dipole orientation (ADO) theory for 307 K36 using
an available Fortran routine.37 Branching ratios were calculated
using the method of Grabowski and Zhang.38
45.1 mmol) added slowly. Further tetrahydrofuran (1 mL) was
added to aid stirring of the mixture, after which it was allowed
to warm to room temperature over 1 h. The mixture was then
extracted with dichloromethane (3 × 2 mL), the combined
extracts dried over magnesium sulfate, and the solvent removed
in vacuo to yield a colorless oil that was used without further
purification.
Synthesis of 1,4-Dideuteriocyclohexane-1,4-dicarboxylic
Acid. A solution of lithium hydroxide (148.7 mg, 3.5 mmol)
in water (2.5 mL) was added to a stirred solution of dimethyl
1,4-dideuteriocyclohexane-1,4-dicarboxylate (104.6 mg, 0.5
mmol) in tetrahydrofuran (2.5 mL). The mixture was allowed
to stir overnight, after which it was washed with dichlo-
romethane (3 × 2 mL) and acidified to pH 1 with dilute
hydrochloric acid. The mixture was then extracted with ethyl
acetate (3 × 5 mL), the combined extracts were dried over
magnesium sulfate, and solvent was removed in vacuo to yield
a white powder that was used without further purification.
Electrospray ionization mass spectrometry gives [M - H]- 173
(100%), 172 (84), 171 (22); D2:D1:D0 (13C-isotope corrected)
4.5:3.8:1.
Synthesis of Methyl Cyclohexanecarboxylate. To a stirred
solution of cyclohexanecarboxylic acid (2.102 g, 16.3 mmol)
in methanol (50 mL) was added sulfuric acid (10 drops,
concentrated). When addition was complete, the solution was
heated at reflux (2.5 h), after which it was allowed to cool to
room temperature and left overnight. The reaction mixture was
diluted with water (50 mL) and extracted with dichloromethane
(3 × 30 mL). The combined extracts were dried over magnesium
sulfate, and the solvent was removed in vacuo to yield a pale
yellow oil (2.196 g, 99% yield), which was used without further
purification. 1H NMR (500 MHz, CD3CD): δ 1.11-3.59 (10H,
m, ring protons), 3.59 (s, 3H, CH3). 13C NMR (125 MHz,
CDCl3): δ 25.40, 25.71, 28.98 (CH2), 43.07 (CH), 51.39 (CH3),
176.54 (CO).
Synthesis of Methyl 1-Deuteriocyclohexanecarboxylate.
n-Butyllithium (2.4 mL, 3.10 mmol) was added to a stirred
solution of diisopropylamine (434 µL, 3.1 mmol) in tetrahy-
drofuran (2 mL) under dry nitrogen at 0 °C. The mixture was
cooled to -70 °C, after which methyl cyclohexanecarboxyate
(200 mg, 1.4 mmol) was slowly added. The mixture was allowed
to warm to room temperature over 3 h, after which it was cooled
to -70 °C and D2O (1 mL, 45.1 mmol) added slowly. The
mixture was allowed to warm to room temperature, after which
it was extracted with dichloromethane (4 × 3 mL), the combined
extracts were dried over magnesium sulfate, and the solvent
was removed in vacuo to yield a yellow oil, which was used
without further purification. Electron ionization mass spectrom-
etry gives [M•+] 143 (100%) and 142 (65%); D1:D0 (13C-isotope
corrected) 1.45:1.
Computational Chemistry. Previous studies of the reaction
coordinates for alkylperoxyl radicals have found good agreement
between B3LYP methods and more sophisticated approaches
such as CBS-QB310 and CCSD(T).9 Importantly, the compu-
tationally economical B3LYP method faithfully reproduced the
transition state and reaction energies of the high level approaches
and thus can be expected to provide reliable predictions as to
the lowest energy reaction pathways. Indeed, the hybrid density
functional B3LYP approach has been recommended for studying
the unimolecular potential energy surfaces of large alkylperoxyl
radicals.10 On the basis of these recommendations, all calcula-
tions were undertaken using the hybrid density functional theory
B3LYP method39,40 and the 6-31+G(d) basis set within the
GAUSSIAN03 suite of programs.41 All stationary points on the
potential energy surface were characterized as either minima
(no imaginary frequencies) or transition states (one imaginary
frequency) by calculation of the frequencies using analytical
gradient procedures. All reported energies include zero-point
energy corrections. Minima were confirmed to join a transition
state by intrinsic reaction coordinate calculation.42,43 Cartesian
coordinates, raw energies, and frequencies for all minima and
transition states are provided as Supporting Information.
Materials. 1,1-Cyclohexanedicarboxylic acid (95%) was
obtained from Matrix Scientific (Columbia, SC). trans-1,2-
Cyclohexanedicarboxylic acid (95%), 1,3-cyclohexanedicar-
boxylic acid (98%), and 1,4-cyclohexanedicarboxylic acid (99%)
were purchased from Aldrich (Milwaukee, WI) with the latter
two diacids obtained as mixtures of both cis and trans stereoi-
somers. Isotopically labeled oxygen 18O2 (95%) was obtained
from Cambridge Isotope Laboratories (Andover, MA). Methanol
(HPLC grade) and ammonia solution (28%, AR grade) were
obtained from Ajax (Sydney, Australia). All commercial
compounds were used without further purification. 4-Oxocy-
clohexane carboxylic acid and 3,4-epoxycycolohexane carboxy-
lic acid were prepared by literature procedures.44,45 Nitric oxide
was freshly prepared in a disposable syringe as previously
reported.30,46
Synthesis of Dimethyl Cyclohexane-1,4-dicarboxylate. To
a stirred solution of 1,4-cyclohexanedicarboxylic acid (2.002
g, 11.6 mmol, mixture of cis and trans diastereomers) in
methanol (50 mL) was added sulfuric acid (10 drops, concen-
trated). When addition was complete, the mixture was heated
at reflux overnight. The solvent was removed in vacuo and the
residue diluted with water (50 mL) and extracted with dichlo-
romethane (3 × 30 mL). The combined extracts were dried over
magnesium sulfate, and the solvent was removed in vacuo to
yield a colorless oil (2.604 g, 85%), which was purified by
distillation under reduced pressure before further use. 1H NMR
(500 MHz, CDCl3): δ 1.43-2.48 (10H, m, ring protons), 3.68
(6H, s, CH3). 13C NMR (125 MHz, CDCl3): δ 25.84, 27.86
(CH2), 40.48, 42.23 (CH), 51.39, 51.42 (CH3), 127.26 (CO).
Results and Discussion
Gas-Phase Synthesis and Characterization of the 4-Car-
boxylatocyclohexyl Radical Anion (E). Electrospray ionization
(ESI) of a methanolic solution of 1,4-cyclohexanedicarboxylic
acid yields abundant ions at m/z 171 and 85 corresponding to
the [M - H]- and [M - 2H]2- molecular ions, respectively.
Isolation of the dianion within the ion trap mass spectrometer
and application of collision induced dissociation (CID) results
in formation of an [M - 2H - CO2]•- ion at m/z 126 consistent
with oxidative decarboxylation processes, as previously reported
for dicarboxylate dianions (Scheme 2a).29,47
Synthesis of Dimethyl 1,4-Dideuteriocyclohexane-1,4-di-
carboxylate. n-Butyllithium (1.700 mL, 2.2 mmol) was added
to a stirred solution of diisopropylamine (308 µL, 2.20 mmol)
in tetrahydrofuran (2 mL) under dry nitrogen at 0 °C. The
mixture was cooled to -70 °C, after which dimethyl cyclohex-
ane-1,4-dicarboxylate (100 mg, 0.50 mmol) was slowly added.
The mixture was allowed to warm to room temperature over
3 h after, after which it was cooled to -70 °C and D2O (1 mL,
It has previously been demonstrated that oxidative decar-
boxylation of dicarboxylate dianions produces distonic radical

Gas Phase Reaction of the Cyclohexyl Radical with O2
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1449
SCHEME 2
tration in the helium buffer gas (Scheme 4). The resulting proton
transfer is estimated to be exothermic49 by ca. 10 kcal mol-1
and produced abundant [M - H]- molecular anions at m/z 141.
The analogous experiment with methyl 1-deuteriocyclohexan-
ecarboxylate was found to yield m/z 141 via exclusive abstrac-
tion of deuterium from the R-carbon thus confirming the
ester-enolate structure of this anion. CID of the precursor anion
gave a small amount of m/z 126 arising from the neutral loss
of a methyl radical (Scheme 4). Literature precedent for
homolytic dissociation of ester-enolate anions50,51 suggests that
the structure of the product ion corresponds to H: the most stable
of the four carboxylatocyclohexyl radical anions (Scheme 3).
Ion-molecule reactions were undertaken to probe the struc-
tural differences between the three isomeric cyclohexyl radicals
E, F, and H. Mass-selected m/z 126 ions formed via the three
independent syntheses (Schemes 2a,b and 4) were allowed to
react with both dioxygen and nitric oxide under comparable
conditions in the ion trap and examples of the resulting mass
spectra after reaction times of 2000 ms are presented in Figure
1. Addition of 32 Da to a radical ion upon reaction with oxygen
has previously been cited as evidence for distonic character26,27,29,30
and indeed the putative distonic ions E and F both undergo
addition of dioxygen to form adduct ions at m/z 158 under these
conditions (Figure 1a,b, respectively). The ions observed at m/z
141 and 125 are assigned as products arising from the addition
anions in a nondestructive and regiospecific manner,29,47 and
thus the 1,4-carboxylatocyclohexyl radical anion (E) is expected
to be the major product of this reaction. It is important to
recognize, however, that there are another three possible cyclic
isomers (F, G, and H) that might arise from rearrangement of
E through rapid hydrogen transfer reactions in addition to a
ring-opened radical that could be formed via ꢀ-scission (Scheme
3). Of the cyclic isomers, two are distonic (F and G) and would
thus be expected to exhibit similar behavior to E, while H is
resonance stabilized and might thus be expected to behave as a
conventional radical anion. In an effort to assess the energetic
accessibility of these structures, electronic structure calculations
were conducted at the B3LYP/6-31+G(d) level of theory to
identify minima on the potential energy surface as well as any
transition states for rearrangement. The results are summarized
in Scheme 3 and suggest that the distonic radical anions E, F,
and G are essentially isoenergetic, while the conventional radical
anion H is the global minimum: ca. 8 kcal mol-1 lower in
energy. Ring-opening of the radical E yields a primary radical
some 17 kcal mol-1 higher in energy. Computed activation
energies for isomerization of E range from 26.8 to 44.2 kcal
mol-1 and represent significant barriers to rearrangement.
Indeed, activation energies of this magnitude are expected to
be more than sufficient to prevent rearrangement of E under
the experimental conditions given that (i) there is little reverse
activation energy in the oxidative decarboxylation process to
provide excess internal energy to the nascent radical anion, (ii)
any excess energy is partitioned between the radical anion,
carbon dioxide, and the departing electron, (iii) CID in an ion-
trap instrument does not produce energetic secondary collisions,
and (iv) ions within the trap are rapidly thermalized by collisions
with the helium buffer gas.
Although calculations suggest that rearrangement of the
4-carboxylatocyclohexyl radical anion (E) is unlikely, it is
prudent to investigate this possibility to provide confidence in
the structural assignment of the ion at m/z 126. The [M - 2H]2-
molecular ion at m/z 85 was successfully generated from the
1,3-cyclohexanedicarboxylic acid, and CID of this ion yields a
radical anion at m/z 126 with the putative structure of F (Scheme
2b). Attempts were made to generate the isomers G and H using
the same oxidative decarboxylation pathway but, not surpris-
ingly, efforts to generate the requisite dianions by electrospray
ionization of the 1,1-cyclohexanedicarboxylic acid and 1,2-
cyclohexanedicarboxylic acid were unsuccessful: presumably
due to excessive Coulombic repulsion between the closely
spaced carboxylate moieties.48 As an alternative synthesis for
H, CD3O- anions, generated by negative ion electrospray
ionization of CD3OD, were allowed to react with neutral methyl
cyclohexanecarboxylate added to the ion trap at a low concen-
of dioxygen with concomitant loss of HO• and HO2 radicals,
•
respectively. A detailed discussion of the mechanistic origin of
these ions is presented later but importantly the relative
abundance of these ions in the spectra of the two isomers are
significantly different, indicating that the two isomers are
structurally distinct. In addition, the CID spectra of the two m/z
158 ions (Figure S1, Supporting Information) show very distinct
fragment ion abundances further differentiating the precursor
radicals E and F. In contrast to the distonic ions, the 1-car-
boxylatocyclohexyl radical anion (H) shows no appreciable
addition of O2 at m/z 158 but instead exhibits a major product
ion at m/z 60 (Figure 1c). As far as we are aware, this
transformation has not previously been reported, and it seems
likely to result from the formation of the carbonate radical anion
•-
CO3 via the mechanism outlined in Scheme 5a. Support for
this proposal is provided by the observation of the product ion
at m/z 62 when the reaction is carried out in the presence of
18O2 (Figure S2, Supporting Information). The absence or very
low abundance of the m/z 60 ion in Figure 1a,b suggests that
most of the m/z 126 ions in these experiments are indeed the
putative distonic ions E and F and rearrangement to the global
minimum during formation or subsequent reaction only occurs
to a very minor degree if at all.
Nitric oxide is expected to undergo rapid radical-radical
combination reactions with carbon-centered radicals, and indeed,
we have previously shown that this reaction occurs rapidly for
the distonic 3-carboxylatoadamantyl radical anion (A).30 When
trapped in the presence of nitric oxide, the putative distonic
ions E and F undergo rapid addition of 30 Da to form [M +
•NO]- product ions at m/z 156 (Figure 1d,e, respectively). In
contrast, reaction of the conventional radical anion H with NO
yields no significant formation of the adduct ion at m/z 156
and instead abundant product ions at m/z 112 are observed
corresponding to a net neutral loss of 14 Da (Figure 1f). This
observation can be rationalized as the displacement of CO2 by
NO (Scheme 5b) and has previously been observed by Wenthold
and Squires for the reaction of the analogous acetate radical
•
-
•
-
anion with nitric oxide (i.e., CH2CO2 + NO f CH2NO +

1450 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Kirk et al.
SCHEME 3
SCHEME 4
•
that correspond in mass to neutral losses of HO2 (33 Da) and
HO• (17 Da), respectively, and are consistent with several of
the pathways in the reaction manifold shown in Scheme 6. The
assignments of these ions are aided by the equivalent mass
spectrum obtained in the presence of 18O2 that shows a 4 Da
shift in the mass of the cyclohexylperoxyl radical (m/z 162), a
2 Da shift (m/z 143) consistent with the loss of H18O• and no
shift in the m/z 125 ion as expected following loss of H18O2
•
CO2).52 This observation lends further support to the putative
structure of the radical anion H.
(Figure S2, Supporting Information). Importantly, these labeling
data confirm that reaction products resulting from addition of
oxygen and loss of either hydroxyl or hydroperoxyl radicals do
not involve participation of carboxylate oxygens, thus indicating
that the charged moiety is a spectator to the observed free radical
reactivity.
The combination of computational and experimental data
provides compelling evidence for assigning distinct structures
for all three independently synthesized, isomeric m/z 126 ions.
Significantly, in the context of this study, the ions of m/z 126
formed by oxidative decarboxylation of 1,4-cyclohexanedicar-
boxylate dianion (Scheme 2a) can be confidently assigned to
the distonic 4-carboxylatocyclohexyl radical (E) and any
rearrangement of this ion during formation or isolation occurs
only to a very minor extent.
Branching ratios for the three major competitive reaction
channels were estimated from the data in Figure 2a to be 32%,
11%, and 54% for the product ions m/z 158, 141, and 125,
respectively with the remaining 3% made up of minor products.
These data confirm that all three are primary reaction products
and no significant secondary products were identified (e.g.,
secondary additions of O2). Considering the major reaction
pathway in the first instance, the formation of the 3-cyclohex-
enecarboxylate anion (J) at m/z 125, could arise from either
direct H-atom abstraction by molecular oxygen from the
cyclohexyl radical or dissociation of the nascent cyclohexyl-
peroxyl radical (I) via concerted elimination (Scheme 6i) or
via the intermediacy of the 2-hydroperoxlcyclohexyl radical (M,
Scheme 6ii). The observation of a significant abundance of the
m/z 125 ion in the CID spectrum (Figure 2b) of isolated
4-carboxylatocyclohexylperoxyl radical (I) confirms that the
cyclohexene product ion (J) can be formed from dissociation
of the peroxyl radical. This observation is consistent with
Reaction of a 4-Carboxylatocyclohexyl Radical Anion (E)
with Dioxygen. The reaction of cyclohexyl radical with
dioxygen has previously been studied in the context of the
combustion of cycloalkanes. Previous experimental work has
identified several of the stable even electron reaction products,53,54
while recent laser-induced fluoresence measurements have
•
quantified HO• and HO2 production.19,33 On the basis of these
data and ab initio calculations, a manifold for the c-C6H11
•
+
O2 reaction has been proposed.19,32,55 The most significant
reaction channels were used here to formulate the reaction
manifold for the corresponding distonic ion E (Scheme 6).
In the present study, the mass spectra arising from the
ion-molecule reaction of the isolated 4-carboxylatocyclohexyl
radical (E) with dioxygen at reaction times ranging from 30 to
5000 ms are shown in Figure 2a. On the basis of these data
and previous measurements30 of background dioxygen concen-
trations in the ion trap of 3.0 × 109 molecules cm-3, a second-
order rate constant for the loss of the precursor ion of 1.8 ×
10-10 cm3 molecule-1 s-1 can be derived, representing an overall
reaction efficiency of 30%. Figure 2a reveals an abundant
product ion at m/z 158 that is observed to increase in abundance
at longer reaction times. The CID spectrum of mass-selected
m/z 158 (Figure 2b) reveals very little loss of dioxygen at m/z
126 and is thus consistent with a covalently bound alkylperoxyl
radical structure rather than any nonspecific ion-dipole adduct
that would be expected to dissociate to reactants upon collisional
activation. Two other abundant product ions are also observed
in the reaction of 4-carboxylatocyclohexyl radical E with
dioxygen (Figure 2a). These ions appear at m/z 125 and 141
•
previous theoretical studies that suggest that HO2 production
occurs via the alkylperoxyl radical intermediate and that direct
H-atom abstraction (cf. reaction 6) is not a significant process
at low temperatures.9
The reaction product ion at m/z 141 is found to be the minor
product with a branching ratio of 11%. The CID spectrum of
m/z 158 (Figure 2b) confirms that this ion can also be formed
directly from the isolated cyclohexylperoxyl radical precursor
(I) via loss of the HO• radical. Previous work has suggested at
least five mechanistic possibilities to account for this observa-
tion, and these are outlined in Scheme 6. To probe these
possibilities, deuterium labeling was undertaken to establish the
site(s) of hydrogen abstraction from the cyclohexane backbone
in generation of the HO• radical. Using 1,4-dideuterio-1,4-
cyclohexanedicarboxylic acid as a precursor, the 4-carboxylato-
1,4-dideuteriocyclohexyl radical anion was generated at m/z 128.

Gas Phase Reaction of the Cyclohexyl Radical with O2
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1451
SCHEME 5
SCHEME 6
Reaction of this isotopologue with dioxygen produced major
products at m/z 160, 143, and 127 corresponding to the labeled
cyclohexylperoxyl radical, with subsequent loss of HO• and
HO2 , respectively (Figure 2c). Loss of DO• at m/z 142 in this
•
spectrum was observed to be minor compared to HO• loss at
m/z 143; this suggests deuterium abstraction from the 1- and
4-positions in the dissociation of the peroxyl radical is a minor
pathway. The same trend is also observed in the CID spectrum
of the mass-selected peroxyl radical anions at m/z 160 (Figure
2d). These data are consistent with unimolecular dissociation
of the 4-carboxylatocyclohexylperoxyl radical anion (I) via loss
of HO• that does not result in significant formation of either
the cyclohexanone R or the 1,4-epoxycyclohexane Q (Scheme
6ix and Scheme 6viii).
Figure 3 compares the CID spectrum of the m/z 141 ion
formed from reaction of 4-carboxylatocyclohexyl radical anion
(Figure 3a) and dioxygen with the CID spectra of authentic
4-oxocyclohexanecarboxylate (R, Figure 3c) and 3,4-epoxycy-
clohexanecarboxylate (L, Figure 3b) anions formed by negative
ion electrospray ionization of the corresponding carboxylic acids.
The fragment ions observed at m/z 99 and 71 in the CID
spectrum of the 4-oxocyclohexanecarboxylate anion (R, Figure
3c) are absent in Figure 3a indicating that the ketone is not a
product of the reaction of the charge labeled cyclohexyl radical
with O2. This observation is consistent with the deuterium
labeling data (vide supra). The fragment ions observed in the
CID spectrum of the 3,4-epoxycyclohexanecarboxylate anion
(L, Figure 3b) show a greater similarity to the m/z 141 formed
from reaction (Figure 3a); however, there are significant
differences, particularly in the observation of a fragment ion at
Figure 1. Mass spectra obtained following isolation of isomeric
carboxylatocyclohexyl radical anions at m/z 126 in the presence of (a,
b, c) O2 and (d, e, f) NO. Reactions of (a, d) 4-carboxylatocyclohexyl
E, (b, e) 3-carboxylatocyclohexyl F, and (c, f) 1-carboxylatocyclohexyl
H are shown. It should be noted that adventitious oxygen is always
present in the ion trap29,30 so competing reactions are also observed in
the NO spectra (d, e, f).

1452 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Kirk et al.
Figure 3. CID spectra of isolated m/z 141 ions formed from (a) the
reaction of 4-carboxylatocyclohexyl radical anion (E) with O2, (b)
deprotonation of authentic [3,4]-epoxycyclohexanecarboxylic acid and
(c) deprotonation of 4-oxocyclohexanecarboxylic acid. All spectra are
recorded under the same conditions using a normalized collision energy
of 25 (arbitrary units).
m/z 61 in the latter spectrum, which is not observed in the
former. Furthermore, the fragment ion abundances differ
significantly between the two spectra, with the reaction product
(Figure 3a) appearing significantly more labile than the authentic
epoxide product (Figure 3b) when provided with equivalent
collision energy. These data show that while the ion population
at m/z 141 arising from the reaction of the charge labeled
cyclohexyl radical (E) with dioxygen may include 3,4-epoxy-
cyclohexanecarboxylate anions (L), this is clearly not the
exclusive product of this reaction. Given this evidence, a
contribution from the 2,4-epoxycyclohexanecarboxylate anion
(O) seems likely but the presence of this isomer could not be
established unequivocally due to the unavailability of the
requisite precursor compound. Scheme 6vi also suggests a
possible ring-opened isomer N might also be present but this is
excluded on the basis of computation data (vide infra).
Figure 2. (a) Stack plot representation of the mass spectra acquired
of the mass-selected 4-carboxylatocyclohexyl radical anion (E, m/z 126)
in the presence of oxygen at reaction times ranging from 30 to 5000
ms (MS3, m/z 86 f 126 f O). (b) CID spectrum of mass-selected m/z
158 ions formed from the ion-molecule reaction of the 4-carboxyla-
tocyclohexyl radical anion (E, m/z 126) with dioxygen (MS4, m/z 86
f 126 f 158 f O). (c) Mass spectrum acquired from the reaction of
the mass-selected 4-carboxylato-1,4-dideuterocyclohexyl radical anion
(m/z 128) in the presence of dioxygen with a reaction time of 5000 ms
(MS3, m/z 87 f 128 f 160). (d) CID spectrum of mass-selected m/z
160 formed from the ion-molecule reaction of the 4-carboxylato-1,4-
dideuteriocyclohexyl radical anion (m/z 128) with dioxygen (MS4, m/z
87 f 128 f 160 f O).

Gas Phase Reaction of the Cyclohexyl Radical with O2
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1453
According to Scheme 6, the observation of the epoxide
product ions at m/z 141 implies that 2- and 3-hydroperoxyl-
cyclohexyl radicals (M and K, respectively) are intermediates
in the reaction. The presence of these rearranged •QOOH radicals
was investigated by isolating the ion at m/z 158 in the presence
of dioxygen. Significantly, no reaction of these ions was
observed even at reaction times up to 5 s (data not shown).
The absence of product ions resulting from the further addition
of O2 or formation of the carbonate radical anion supports the
notion that carboxylatohydroperoxylcyclohexyl radical anions
such as M, K, and P (•QOOH) are not stable intermediates under
the conditions of these experiments and the ion population at
m/z 158 can be exclusively assigned to 4-carboxylatocyclo-
hexylperoxyl radical (I).
Reaction Coordinate Calculations. Calculation of the reac-
tion coordinate diagram for the reaction of the 4-carboxylato-
cyclohexyl radical anion (E) with dioxygen was undertaken at
the B3LYP/6-31+G(d) level of theory, and the results are
illustrated in Figure 4. Full details of the structure and energetics
of the stationary points depicted on this potential energy diagram
are provided as Supporting Information (Table S2). Addition
of dioxygen to the 4-carboxylatocyclohexyl radical anion (E)
can give rise to two geometric isomers with the carboxylate
anion and peroxyl radical moieties bound to the same (IZ) or
opposite (IE) faces of the cyclohexane ring. While these
diastereomeric peroxyl radicals are calculated to be within 0.2
kcal mol-1, they can result in different chemistries and different
reaction energetics as illustrated in Figure 4a,b.
states for both the 1,6-hydrogen atom transfer (-9.1 kcal mol-1)
and subsequent epoxide formation (-12.2 kcal mol-1). It should
be noted, however, that this product can only be formed from
the trans-peroxyl radical (IE, see Figure 4b) while both 2,4-
and 3,4-epoxycyclohexane carboxylates (O and L, respectively)
can be formed from both cis and trans isomers (Figure 4a,b).
Furthermore, from symmetry there are four near-equivalent paths
(2 for cis and 2 for trans) for formation of the 2,4- and 3,4-
epoxycyclohexane carboxylates compared to only a single
pathway for formation of the low energy 1,4-epoxyocylcohexane
carboxylate (QE). The resulting low probability for this reaction
may account for the fact that no experimental evidence (based
on deuterium-labeling) was obtained for this otherwise energeti-
cally competitive exit channel. Finally the calculated ꢀ-scission
pathway (Scheme 6vi) for the 3-hydroperoxycyclohexyl radical
(K) was found to have a transition state significantly above the
entrance channel (+11.5 kcal mol-1, Figure 4a). This result
suggests that the ring opened aldehyde isomer is an unlikely
contributor to the ion population at m/z 141.
The carboxylate charge tag is an obvious point of difference
between the distonic radical anion studied here and the
archetypal neutral radical system. Interestingly, however,
the calculated energies for the reaction of dioxygen and the
4-carboxylatocyclohexyl radical (E) are in generally good
agreement with previous computations of the analogous neutral
cyclohexyl radical reaction.19,32 For example, addition of di-
oxygen to the charge-tagged cyclohexyl radical to form the
peroxyl radical was found to be exothermic by -37 kcal mol-1
(Figure 4), which is equal to the value for the corresponding
neutral radical reaction using a G2(MP2)’ approach.19 Further-
more, the barriers to rearrangement of the 4-carboxylatocyclo-
hexyl radical via hydrogen atom transfer range between -0.4
and -13.6 kcal mol-1 compared with -2.1 and -10.1 kcal
mol-1 for rearrangement on the neutral radical potential surface.
The relative order of the barrier heights for 1,4-, 1,5-, and 1,6-
hydrogen atom transfer and 1,4-hydrogen transfer with concerted
Computations predict that rearrangement of the nascent
peroxyl radical (I) via hydrogen atom transfer can proceed
without additional activation energy from all positions except
the R-carbon of the peroxyl moiety itself (Scheme 6ix). The
latter abstraction results in concerted formation of 4-oxocyclo-
hexanecarboxylate anion (R) via a 4-centered transition state
that lies either +4.3 or +3.1 kcal mol-1 above the entrance
channel for rearrangement from the IZ and IE isomers, respec-
tively. The presence of this barrier explains why this otherwise
exothermic product channel (-61 kcal mol-1) is not observed
experimentally. Both pathways for the observed elimination of
•
elimination of HO2 are consistent those calculated by Silke et
al., namely, 1,5-hydrogen atom transfer (to form K) < 1,6-
hydrogen atom transfer (to form P) < concerted elimination of
•
HO2 (to form J) < 1,4-hydrogen atom transfer (to form M).32
•
HO2 , either via a concerted mechanism (Scheme 6i) or via the
This differs slightly from the findings of Knepp et al. who report
intermediate 2-hydroperoxylcyclohexyl radical (M, Scheme 6iii),
•
a lower activation energy for concerted elimination of HO2 with
are found to be energetically accessible. Transition structures
respect to 1,6-hydrogen atom transfer.19 The resulting hydrop-
eroxylcyclohexyl radical isomers computed in the present study
(K, M, and P, Scheme 6) range in energy between -15.6 and
-23.1 kcal mol-1 below the entrance channel, with the lowest
of these minima being identified as the 1-carboxylato-4-
hydroperoxylcyclohexyl radical (P, Scheme 6). This contrasts
with calculations of the analogous neutral surface where the
3-hydroperoxylcyclohexyl radical and 2-hydroperoxylcyclohexyl
radical are calculated as the lowest energy •QOOH structures.19,32
This is not surprising, however, as the presence of the carboxy-
late moiety in the 1-carboxylato-4-hydroperoxylcyclohexyl
radical (P) provides for resonance delocalization that is absent
in the neutral system.
•
for concerted elimination of HO2 from IZ and IE are located
some -4.2 and -6.6 kcal mol-1, below the entrance channel,
while the most energetic transition states for the stepwise process
are found at -0.4 and -4.2 kcal mol-1. The calculations thus
give no clear indication as to whether the concerted or stepwise
mechanism is more likely responsible for the experimental
•
observation of HO2 loss. Transition states for rearrangement
via 1,4-, 1,5-, or 1,6-hydrogen atom transfer are calculated to
lie between -0.4 and -13.6 kcal mol-1 below the entrance
channel. While all the resulting hydroperoxylcyclohexyl radicals
(K, M, and P, Scheme 6) are found to be minima on the
potential energy surface, all are further connected to energeti-
cally accessible pathways for subsequent reaction via epoxide
formation with concomitant loss of the HO• radical. The
accessibility of such pathways is consistent with the fact that
no secondary addition of O2 was observed experimentally as
Conclusion
Overall the reaction products observed for charge-tagged
4-carboxylatocyclohexyl radical anions (E) in the ion trap mass
spectrometer closely mirror those observed in previous experi-
mental studies of the corresponding cyclohexyl radicals, namely
•
might be expected if such QOOH isomers are present in the
ion population at m/z 158. Interestingly, the calculations do not
provide a clear prediction as to the most likely structure(s) of
the product ion arising from loss of HO• at m/z 141. In particular,
the 1,4-epoxycyclohexane carboxylate (QE) is found to be
energetically accessible with among the lowest energy transition
•
reactions resulting in the elimination of HO2 and HO•.19,33
Furthermore, there is a satisfying agreement between the
computed reaction coordinate diagram for the distonic radical

1454 J. Phys. Chem. A, Vol. 114, No. 3, 2010
Kirk et al.
Figure 4. Reaction coordinate diagram for the reaction of the 4-carboxylatocyclohexyl radical anion (E) with O2 where all energy values are
presented as kcal mol-1. Addition of O2 can occur from either the face of the cyclohexyl moiety resulting in cis- and trans-isomers of the
4-carboxylatocyclohexylperoxyl radical anion. The rearrangements are calculated for both the (a) the cis-isomer IZ and (b) the trans-isomer IE. All
calculations were performed at the B3LYP/6-31+G(d) level of theory and include unscaled ZPE corrections. Full structure and energies for stationary
points are provided as Supporting Information (Tables S2).

Gas Phase Reaction of the Cyclohexyl Radical with O2
J. Phys. Chem. A, Vol. 114, No. 3, 2010 1455
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anion and previous theoretical investigations of the neutral
system. The congruence in the chemistry of E and its neutral
counterpart provide further confidence in the use of the distonic
radical anions as probes of neutral reactivity. Observation of
the 4-carboxylatocyclohexylperoxyl radical anion (I) at m/z 158,
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radical by cavity ring-down spectroscopy.56 The absence of
further reactivity of this ion with dioxygen indicates the absence
of isomeric hydroperoxylcyclohexyl radicals (e.g., K, M, and
P), often referred to as •QOOH radicals, in the ion population.
This is a significant observation, as these elusive radicals are
thought to be involved in low temperature chain branching
reactions because of the ability to add a second dioxygen. If
these intermediate species are formed under the conditions of
our experiment (2.5 mTorr and 307 K), they rapidly decompose
•
via loss of HO• and HO2 . The ability to synthesize and isolate
a model alkylperoxyl radical as described here provides a novel
pathway to probe the gas phase reactivity of these otherwise
elusive species. This approach offers promise for the study of
the bimolecular reactions of peroxyl radicals as well as their
gas phase photochemistry.
Finally, the reaction of this distonic alkyl radical with
dioxygen gives rise to characteristic products: the addition of
•
O2 (+32 Da) and neutral losses of HO• (+15 Da) and HO2
(-1 Da). Such diagnostic gas phase chemistry constitutes a mass
spectral fingerprint for not only the verification of distonic
radical character but also characterization of the structure of
the radical itself. As such, this study supports other recent
suggestions that dioxygen is an effective probe reagent for
elucidating the structure of the radical ions arising from
contemporary mass spectrometric techniques (e.g., electron
transfer dissociation and photodissociation).28,57
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Acknowledgment. B.B.K. is supported through an Australian
Postgraduate Award. D.G.H. and S.J.B. acknowledge the
financial support of the Australian Research Council (DP0452849
and DP0986628) and the University of Wollongong. S.J.B. and
B.B.K. thank the Australian Partnership for Advanced Comput-
ing (ANU, Canberra) for a generous allocation of resources
under the Merit Allocation Scheme. We acknowledge Professor
Paul Wenthold and Dr. Adam Trevitt for helpful discussions
and the late Mr. Larry Hick whose technical ability and
friendship are sorely missed.
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Supporting Information Available: CID mass spectra and
lists of geometric parameters and electronic and zero-point
energies. This material is available free of charge via the Internet
at http://pubs.acs.org.
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