Does addition of NO2 to carbon-centered radicals yield RONO or RNO2? An investigation using distonic radical ions

Article abstract of DOI:10.1007/s13361-012-0549-x

Nitrogen dioxide is used as a radical scavenger to probe the position of carbon-centered radicals within complex radical ions in the gas phase. As with analogous neutral radical reactions, this addition results in formation of an [M + NO₂]⁺ adduct, but the structural identity of this species remains ambiguous. Specifically, the question remains: do such adducts have a nitro- (RNO₂) or nitrosoxy- (RONO) moiety, or are both isomers present in the adduct population? In order to elucidate the products of such reactions, we have prepared and isolated three distonic phenyl radical cations and observed their reactions with nitrogen dioxide in the gas phase by ion-trap mass spectrometry. In each case, stabilized [M + NO ₂]⁺ adduct ions are observed and isolated. The structure of these adducts is probed by collision-induced dissociation and ultraviolet photodissociation action spectroscopy and a comparison made to the analogous spectra of authentic nitro- and nitrosoxy-benzenes. We demonstrate unequivocally that for the phenyl radical cations studied here, all stabilized [M + NO ₂]⁺ adducts are exclusively nitrobenzenes. Electronic structure calculations support these mass spectrometric observations and suggest that, under low-pressure conditions, the nitrosoxy-isomer is unlikely to be isolated from the reaction of an alkyl or aryl radical with NO₂. The combined experimental and theoretical results lead to the prediction that stabilization of the nitrosoxy-isomer will only be possible for systems wherein the energy required for dissociation of the RO-NO bond (or other low energy fragmentation channels) rises close to, or above, the energy of the separated reactants. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13361-012-0549-x

B American Society for Mass Spectrometry, 2013
J. Am. Soc. Mass Spectrom. (2013) 24:481Y492
DOI: 10.1007/s13361-012-0549-x
FOCUS: DISTONIC IONS: RESEARCH ARTICLE
Does Addition of NO to Carbon-Centered Radicals Yield
2
RONO or RNO ? An Investigation Using Distonic Radical
2
Ions
Benjamin B. Kirk, Adam J. Trevitt, Stephen J. Blanksby
ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, School of Chemistry, University of Wollongong,
Wollongong, NSW 2522, Australia
Abstract. Nitrogen dioxide is used as a “radical scavenger” to probe the position of
carbon-centered radicals within complex radical ions in the gas phase. As with
+
analogous neutral radical reactions, this addition results in formation of an [M + NO2]
adduct, but the structural identity of this species remains ambiguous. Specifically, the
question remains: do such adducts have a nitro- (RNO ) or nitrosoxy- (RONO)
2
moiety, or are both isomers present in the adduct population? In order to elucidate the
products of such reactions, we have prepared and isolated three distonic phenyl
radical cations and observed their reactions with nitrogen dioxide in the gas phase by
+
ion-trap mass spectrometry. In each case, stabilized [M + NO ] adduct ions are
2
observed and isolated. The structure of these adducts is probed by collision-induced
dissociation and ultraviolet photodissociation action spectroscopy and a comparison made to the analogous
spectra of authentic nitro- and nitrosoxy-benzenes. We demonstrate unequivocally that for the phenyl radical
+
cations studied here, all stabilized [M + NO2] adducts are exclusively nitrobenzenes. Electronic structure
calculations support these mass spectrometric observations and suggest that, under low-pressure conditions, the
nitrosoxy-isomer is unlikely to be isolated from the reaction of an alkyl or aryl radical with NO . The combined
2
experimental and theoretical results lead to the prediction that stabilization of the nitrosoxy-isomer will only be
possible for systems wherein the energy required for dissociation of the RO-NO bond (or other low energy
fragmentation channels) rises close to, or above, the energy of the separated reactants.
Key words: Distonic radical ions, Ion-trap mass spectrometry, Ion–molecule reactions, Photodissociation, Action
spectroscopy
Received: 25 October 2012/Revised: 25 November 2012/Accepted: 26 November 2012/Published online: 23 February 2013
*
2
0
Introduction
RNO ! R þ HONO
ð2Þ
he reaction of alkyl and aryl radicals (R) with nitrogen
dioxide (NO ) has been extensively studied in the
context of combustion and atmospheric chemistries [1–3]. It
T
2
*
2
RNO ! RONO
ð3Þ
ð4Þ
is reported to proceed by barrierless radical addition to
generate a nitroalkane or nitroarene (RNO ). The nascent
2
RONO ! RO þ NO
vibrationally excited species may be (1) stabilized by
collisions, (2) eject nitrous acid (HONO), or (3) isomerize
to the corresponding nitrite (RONO) which may then go on
to (4) eject NO to form an alkoxy or aryloxy radical.
It is generally reported that the end products of alkyl
radical+NO reactions are a mixture of all four pathways,
with all reaction pathways proceeding via initial formation
of the nitroalkane [2–4], while reaction of aryl radicals with
2
*
2
M
R þ NO2 ! RNO ! RNO2
ð1Þ
NO results in reactions (1), (3) and (4) [5]. Kinetic data
2
have been reported for the reaction of a number of short
chain alkyl radicals with NO2 in flow-tube experiments
based primarily on depletion of the parent radical. Experi-
mental determination of the reaction products was difficult,
however, as the nitro and nitrite isomers are prone to
dissociative photoionization [2, 3].
Electronic supplementary material The online version of this article
doi:10.1007/s13361-012-0549-x) contains supplementary material, which
is available to authorized users.
(
Correspondence to: Stephen J. Blanksby; e-mail: blanksby@uow.edu.au

4
82
B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
2 2
Xu and Lin proposed that NO could add to the phenyl
with oxygen (O ) on a 50 mL scale in a gas-tight syringe, as
2
2
radical via oxygen to directly generate nitrosoxybenzene
4]. It is unclear, however, whether this proceeds without
barrier, as the unpaired electron in NO formally resides
described previously by Mattson [15]. Nitric oxide (NO) was
purchased from Sigma Aldrich (98.5 %, St. Louis, MO, USA).
[
Industrial grade oxygen (O ) was purchased from BOC
2
2
e
2
on the nitrogen in the ground X ð A1Þ state. Significant
(999.5 %, Sydney, Australia). The target distonic radical
cations A, B, and C were prepared from the precursors: N,N,
N-trimethyl-2-([2-thioxopyridin-N-yloxy]carbonyl)benzenea-
minium iodide; 3-iodo-N,N,N-trimethylbenzaminium iodide;
and 1-(4-iodophenyl)-N,N,N-trimethylmethanaminium iodide,
synthesized as described previously [16].
energy (27.8±0.2 kcal/mol [6]) is thusrequiredtoaccessthe
e 2
Að B Þ excitedstatewheretheelectronresidesonoxygensuch
2
as to facilitate a barrierless radical addition via oxygen. Ellison
et al. proposed a two-state di-abatic model to rationalize the
F+NO2 reaction surface where an avoided crossing
between the ground and excited state of NO2 lowers
the association barrier to the nitrite (FONO) from 27.8±
Synthesis of N,N,N-Trimethyl-4-Nitrobenzaminium
Iodide
0
.2 kcal/mol to 22±3 kcal/mol [7]. It is unknown
whether a similar model is applicable for reaction of
the phenyl radical with NO ; however, one would not
expect this barrier to disappear entirely.
4-Nitroaniline (200 mg, 1.45 mmol) and potassium carbon-
ate (200 mg, 1.45 mmol) were added to iodomethane
(900 μL, 14.5 mmol) and stirred at 40 °C for 24 h. The
iodomethane was removed in vacuo and 1 mg of the
resulting yellow powder was partitioned between water
(1 mL) and CHCl (1 mL). The CHCl layer was discarded
and the water layer diluted in MeOH. While the preceding
methylation did not produce an isolable product, there was
sufficient N,N,N-trimethyl-4-nitrobenzaminium iodide gen-
erated in situ for characterization by tandem mass spectrom-
etry (vide infra).
2
In a recent mass spectrometric study, Barlow and co-
workers used NO as a “radical scavenger” [8] to probe the
2
location of a carbon-centered radical (R) generated during
collision-induced dissociation (CID) of ionized peptide
nitrate esters (i.e., R-CH ONO → R+CH O+NO ) [9].
3
3
2
2
2
2
Reaction of the nascent peptidyl radical cation was reported
+
to form an [M+NO2] adduct. Subsequent CID of this
adduct resulted in loss of HONO, in addition to fragment
ions consistent with ejection of NO and concomitant β-
scission of the nascent alkoxyl radical (i.e., an [M+NO –
2
+
NO] fragment ion was not directly observed). While this
Mass Spectrometry
CID behavior might be considered as evidence for the
nitrosoxy (RONO) structure in the adduct ion, activation of
Methanolic solutions of precursor compounds were prepared
at 10–50 μM concentrations and infused at a rate of 3–5 μL/
min into the electrospray ionization source of a Thermo
Fisher Scientific LTQ linear quadrupole ion-trap mass
spectrometer (San Jose, CA, USA). The linear ion-trap mass
spectrometer has previously been modified to perform both
photodissociation [17, 18] and ion–molecule reactions [19].
Typical source parameters were: spray voltage +3.0 to
4.5 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 experiments, ions were mass-selected with a
window of 1–4 Th and fragmented using default instrument
parameters (i.e., 0.250 activation Q, 30 ms excitation time,
unless otherwise noted. Normalized collision energies were
optimized for each CID experiment and are noted in relevant
spectra. All spectra presented are an average of at least 50
scans.
nitro (RNO ) compounds has also been shown to facilitate
isomerization to the nitrite with analogous ejection of NO
2
[
10–14]. As such, loss of NO (or related fragments) is not an
unambiguous diagnostic for the presence of a nitrosoxy
moiety. In this radical ion study therefore, as found in
previous neutral radical experiments [2, 3], the unequivocal
+
assignment of the [M+NO ] adduct to either the nitro or
2
nitrite could not be made.
With this in mind, we have investigated the reaction of
NO with simple distonic phenyl radical cations to elucidate
2
the products of such reactions. In these experiments, we (i)
observe the reaction of distonic phenyl radical cations with
+
NO ; (ii) probe the structure the resulting [M+NO ] adduct
2
2
by CID and ultraviolet photodissociation (UVPD) action
spectroscopy; and (iii) compare these spectra to those
obtained from the authentic nitrobenzene and nitrosoxyben-
zene. We demonstrate unequivocally that reaction of these
distonic phenyl radicals with NO2 generates the charge-
tagged nitrobenzene.
Photodissociation
The linear ion-trap mass spectrometer has been modified as
described elsewhere [17, 18, 20]. Briefly, a 2.75 in. quartz
viewport (MDC Vacuum Products, Hayward, CA, USA) is
affixed to the back plate of the spectrometer vacuum housing
with a CF flange to allow irradiation of trapped ions with a
laser pulse. Two laser systems are utilized: (i) a fixed
frequency 266 nm laser for generation of radicals, and (ii) a
Experimental
Materials
4
-Nitroaniline and HPLC grade methanol were purchased
from Ajax Finechem (Sydney, Australia). Nitrogen dioxide
NO ) was generated from the reaction of nitric oxide (NO)
(
2

B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
483
2
2
tunable OPO laser (215–320 nm) system used when
measuring ultraviolet photodissociation (UVPD) action
spectra [21].
is adjusted to set an ion 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
therein [19, 22]. Reaction times of 30–10000 ms were set
using the excitation time parameter within the control
software using a normalized collision energy of 0 (arbitrary
units).
The fixed frequency 266 nm laser pulse (fluence035 mJ/
2
cm ) is generated by a flashlamp-pumped Nd:YAG laser
(
Minilite II; Continuum, Santa Clara, CA, USA). Two right-
angle prisms direct the laser pulse into the ion trap and are
adjusted to optimize overlap with the ion cloud. A TTL
n
pulse generated at the beginning of the MS activation step
triggers the laser flashlamp such that only a single laser
pulse is generated per duty cycle.
Electronic Structure Calculations
Tunable UV radiation (213–354 nm) is generated by an
OPO laser system fitted with a frequency-doubling unit
All calculations were undertaken using the hybrid density
functional theory B3LYP method [23, 24] and the aug-cc-
pVDZ basis set [25] within the GAUSSIAN09 suite of
programs [26]. 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. Minima between transition states were con-
firmed by calculation of the intrinsic reaction coordinate
(
versaScan and uvScan; Spectra-Physics, Santa Clara, CA,
USA). The OPO is pumped by the third harmonic (355 nm)
of a Quanta-Ray INDI Nd:YAG laser (Spectra-Physics;
Santa Clara, CA, USA). The OPO-generated UV laser pulse
is noticeably divergent in one dimension and this is
corrected using a single cylindrical quartz lens. Two quartz
prisms are adjusted to ensure good overlap between the UV
laser pulse and the ion cloud. The laser is operated
continuously at 10 Hz. To ensure that only a single laser-
pulse irradiates ions in the trap at the desired MS interval, a
mechanical shutter is placed before the LTQ window and is
synchronized to open for ~100 ms using a TTL pulse
(IRC) [27–29]. Calculated energies (ΔH ) include unscaled
0
zero-point energy, while Gibbs free energies (ΔG298) include
free energy corrections calculated at a temperature of
298.15 K and pressure of 1.0 atm.
n
generated during the activation step of an MS cycle [20].
Results
The normalized collision energy during this activation step
is maintained at 0 (arbitrary units) such that product ions
measured are generated solely by UVPD. Photoproduct
yields are calculated using: product yield0(Σproduct /TIC ),
Preparation of 4-(N,N,N-Trimethylammonium)
Phenyl Radical Cation (A)
λ
λ
N,N,N-trimethyl-2-([2-thioxopyridin-N-yloxy]carbonyl)ben-
zeneaminium iodide (1) was infused via positive electro-
spray ionization to generate the N,N,N-trimethyl-2-([2-
thioxopyridin-N-yloxy]carbonyl)benzeneaminium cation at
m/z 289. Subjecting this cation to a single laser shot
where Σproduct is the abundance of all photofragments, TIC
the total ion count at wavelength λ. The laser power
spectrum is measured using a thermopile power meter and
is typically between 0.2 and 1.1 mJ/pulse. A UVPD action
spectrum is obtained by plotting the UVPD product yield
against wavelength. It is important to note that the OPO
beam intensity spatial profile may vary over this wavelength
range and it is difficult to account for the overlap efficiency
between the ion ensemble and the laser beam. Therefore, we
correct the photoproduct yield for the laser power profile
over the wavelength range but not for absolute laser power.
Experiments herein were undertaken consecutively to gen-
erate comparable data under these conditions. This allows us
to confidently compare photoproduct yields between action
spectra while minimizing the effects of temporal fluctuations
in the laser power or beam profile.
(
(
266 nm, 2.4 mJ/pulse) or collision-induced dissociation
CID) resulted in formation 4-(N,N,N-trimethylammonium)-
phenyl radical cation (A) at m/z 135 (Scheme 1). Previous
experiments within our laboratory have demonstrated that
both PD and CID of 1 generate A [16, 30]. For all
experiments subsequently described, we have used CID
exclusively to generate A.
NMe3I
NMe3
NMe3
CID
ESI
-
C H NO S
6
4
2
N
N
O
O
O
O
m/z 135
Ion–Molecule Reactions
A
S
S
1
m/z 289
Modifications to the linear ion-trap mass spectrometer that
allow the introduction of neutral gases into the ion-trap
region of the instrument have been described previously
Scheme 1. Electrospray ionization (ESI) of N,N,N-trimethyl-
-([2-thioxopyridin-N-yloxy]carbonyl)benzeneaminium iodide
1) and CID of the resulting N,N,N-trimethyl-2-([2-thioxopyr-
2
(
[
19]. Briefly, reagent gases in a syringe are introduced into a
flow of Ultra High Purity (UHP) helium (3–5 psi) via a
heated septum inlet (25–250 °C). The gas mixture is
introduced into the ion trap via a variable leak valve, which
idin-N-yloxy]carbonyl)benzeneaminium cation at m/z 289
results in generation of the 4-(N,N,N-trimethylammonium)-
phenyl radical at m/z 135 (A)

4
84
B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
2 2
NMe3
Reaction of Charge-Tagged Phenyl Radicals
with NO2
[
M + 46]+
(
i)
+ NO2
The 4-(N,N,N-trimethylammonium)phenyl radical cation (A)
m/z 181
was allowed to react with NO that was seeded into the helium
buffer gas of the mass spectrometer. A typical mass spectrum
A
2
m/z 135
obtained from this reaction is shown in Figure 1 and reveals
+
NMe3I
NMe3
an abundant [M+NO ] adduct ion at m/z 181 (Scheme 2i).
2
Stabilization of either charge-tagged nitro- (A-NO ) or
nitrosoxybenzene (A-ONO) could account for this signal
and, thus, the identity of this ion remains to be established
ESI
I-
2
(ii)
-
N
N
O O
A-NO2
(
[
1
vide infra). Additional ions present in this spectrum include:
O
O
+
+
+
+
M+1] , [M+16] , [M+30] , and [M+32] ions at m/z 136,
51, 165, and 167, respectively, which arise due to reaction of
A with background O or unreacted NO (present in the
m/z 181
2
NMe3
NMe3
NMe3
+
+
synthetic NO ). The [M+16] and [M+32] ions have
2
previously been observed from reaction of A with O and
2
(
iii)
+ O2
+ NO
correspond to the phenoxyl and phenylperoxyl radicals,
+
respectively [16, 30]. The [M+30] ion at m/z 165 suggests
m/z 135
O
O
O
+
the presence of unreacted NO and is assigned as an [M+NO]
A
N
adduct. Interestingly, the phenoxyl radical at m/z 151 ion is
significantly enhanced compared with previous observa-
A-O
m/z 151
A-ONO
m/z 181
tions of the A+O reaction in the absence of NO [16, 30].
This enhancement may arise due to a reaction between the
2
x
Scheme 2. Gas-phase synthesis of isomeric m/z 181 ions
including: (i) the [M+46] ion arising from reaction of 4-(N,N,N-
+
peroxyl radical at m/z 167 and NO or NO by analogy with
2
trimethylammonium)phenyl radical cation with NO ; the
2
the well-established neutral reactivity [31]. Alternatively,
generation of authentic (ii) N,N,N-trimethyl-4-nitrobenzami-
nium cation; and (iii) N,N,N-trimethyl-4-nitrosoxybenzaminium
cation (A-ONO) by reaction of 4-(N,N,N-trimethylammonium)-
phenoxyl radical cation (A-O) with NO
m/z 151 could arise directly by NO ejection from energetic
+
[
M+NO ] adducts (vide infra) [1, 4]. Exothermic forma-
2
tion of the phenoxyl radical by either mechanism could
+
drive subsequent CH loss, therefore, the [M+1] ion is
3
+
assigned as the [M+16 – CH ] ion.
3
Previous studies by Yu et al. [32], Berho et al. [33] and Platz
et al. [34] have shown that reaction of phenoxyl radical with
NO generates nitrosoxybenzene. The reaction was employed
here to generate the charge-tagged nitrosoxybenzene analogue.
In this synthesis, the 4-(N,N,N-trimethylammonium)phenyl
radical cation (A) was allowed to react in the presence of
Independent Synthesis of Nitrobenzene (A-NO2)
and Nitrosoxybenzene (A-ONO) Analogues
in the Gas Phase
A solution containing N,N,N-trimethyl-4-nitrobenzami-
nium iodide was infused via positive ion electrospray
ionization to yield an abundant ion at m/z 181 consistent
with the target N,N,N-trimethyl-4-nitrobenzaminium cat-
NO (seeded into the helium buffer gas) and O (present in
2
background concentrations), resulting in formation of a
number of ions, including the 4-(N,N,N-trimethylammo-
nium)phenoxyl radical cation (A-O) at m/z 151. The
ion (A-NO ) (Scheme 2ii).
2
Figure 1. A mass spectrum obtained following the gas-phase reaction of the 4-(N,N,N-trimethylammonium)phenyl radical
+
2 2
cation (A) with NO . A [M+46] adduct at is observed at m/z 181 consistent with addition of NO to the phenyl radical

B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
485
2
2
charge-tagged phenoxyl radical (A-O) was then isolated and
allowed to react further in the same gas atmosphere. A mass
spectrum acquired under these conditions is shown in
Figure 2 and reveals formation of an [M+30] adduct at
m/z 181 assigned as the N,N,N-trimethyl-4-nitrosoxybenza-
minium cation (A-ONO) (Scheme 2iii).
radical at m/z 135 regenerated upon activation (c.f. Figure 1).
There was no [M – 30] ion in the CID spectrum measured
+
of authentic A-NO without NO present in the helium buffer
2
+
gas (Supporting Information, Figure S2), which suggests the
+
ion at m/z 151 arises due to (ii). However, the [M – 45] ion
at m/z 136 remained at a similar abundance to that observed
in Figure 3b, suggesting this ion arises due to (i), i.e.,
isomerization of A-NO to A-ONO followed by loss of NO
2
Comparison of CID and UVPD Spectra
and subsequent CH ejection, or alternatively, loss of CH
3
3
Figure 3a–c depict CID spectra of the three m/z 181 ions
generated by the pathways outlined in Scheme 2, namely: (i)
reaction of the 4-(N,N,N-trimethylammonium)phenyl radical
followed by subsequent NO ejection.
Representative UVPD mass spectra recorded at 225 nm
for all three isomeric m/z 181 ions are presented in
Figure 4a–c. Fragment ions featured in Figure 4a and b are
similar to those present in the corresponding CID spectra
(c.f. Figure 3a and b, respectively). The UVPD mass
spectrum shown in Figure 4c measured after PD of A-
cation (A) with NO , (ii) genuine 4-nitro-N,N,N-trimethyl-
2
benzaminium cation (A-NO ), and (iii) N,N,N-trimethyl-4-
2
nitrosoxybenzaminium cation (A-ONO). A typical CID
+
spectrum of the [M+NO ] adduct is depicted in Figure 3a
2
+
+
+
+
+
and features [M – 15] , [M – 16] , [M – 30] , [M – 45] ,
ONO reveals an [M – 30] ion at m/z 151 assigned as the
+
+
+
and [M – 36] ions assigned as [M – CH ] , [M – O] , [M –
charge-tagged phenoxyl radical arising due to ejection of
3
+
+
+
+
NO] , [M – NO – CH ] , and [M – NO ] ions, respectively.
NO. In addition, an [M – 45] ion featured at m/z 136 arises
3
2
In contrast, the CID spectrum of A-ONO shown in
Figure 3c features a single product ion at m/z 151 arising
due to loss of NO to form a phenoxyl radical. In addition,
the normalized collision energy used to generate this
spectrum was significantly lower at CE015 (arbitrary units)
indicating that loss of NO in this case requires significantly
less energy than fragmentation pathways observed after CID
due to subsequent CH3 loss from the nascent phenoxyl
radical. This additional fragmentation channel is presumably
driven, in addition to electronic effects, by the considerable
excess energy imparted on the parent ion during PD
(225 nm0127.2 kcal/mol), in contrast to the “slow-heating”
activation of CID [35].
+
A series of UVPD mass spectra for the [M+NO ] adduct,
2
+
of the [M+NO2] adduct, consistent with a low bond
A-NO and A-ONO were acquired over a wavelength range
2
dissociation energy for the RO–NO bond in a nitrosoxy-
benzene [4]. The CID spectrum of A-NO2 depicted in
Figure 3b is identical to the CID spectrum of the [M+NO2]
between 215 and 320 nm in increments of 1 nm between 215
and 240 nm and 5 nm between 240 and 320 nm. Plotting the
power-corrected photoproduct yield as a function of wave-
length results in the UVPD action spectra depicted in
+
adduct obtained at the same collision energy (Figure 3a).
+
When the CID spectrum of the [M+NO2] adduct is
Figure 5a–c. The UVPD action spectrum measured for the
+
measured at CE015 (arbitrary units), for comparison with
[M+NO ] adduct (Figure 5a, green diamonds) reveals a peak
2
Figure 3c (data not shown), no product ions are generated
centered at 226 nm with a long tail out to 280 nm. In contrast,
the major peak in the action spectrum measured for A-ONO
(Figure 5c, blue circles) is shifted to the red at 230 nm and
features a broad shoulder at 240 nm that continues to 320 nm
(scaled UVPD action spectrum clearly showing the positioning
of the major features of these spectra are presented in
Supplementary Materials, Figure S1). Within the uncertainty
indicating there is no contribution from A-ONO in the m/z
+
1
81 ion population. The [M – 30] ion generated in
Figure 3a and b must, therefore, arise due to either (i) a
significantly higher energy process, e.g., isomerization of the
nitrobenzene to the nitrosoxybenzene with concomitant
ejection of NO, or (ii) addition of O /NO to the phenyl
2
Figure 2. A mass spectrum obtained following the gas-phase reaction of the 4-(N,N,N-trimethylammonium)phenoxyl radical
+
cation (A-O) with NO. A [M+30] adduct is observed at m/z 181 consistent with addition of NO to A-O and is assigned as N,N,N-
trimethyl-4-nitrosoxybenzaminium cation (A-ONO)

4
86
B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
2 2
(
a)
(
b)
(
c)
+
Figure 3. Comparison of the spectra resulting from CID of the m/z 181 ion generated from (a) the [M+NO ] adduct formed
2
after reaction of 4-trimethylammoniumphenyl radical cation (A) with NO , (b) genuine 4-nitro-N,N,N-trimethylbenzaminium
2
2
cation (A-NO ), and (c) N,N,N-trimethyl-4-nitrosoxybenzaminium cation (A-ONO)
of the experiment, the action spectrum measured for the
from the nitrosoxy isomer N,N,N-trimethyl-4-nitrosoxyben-
zaminium cation (A-ONO).
authentic nitrobenzene (A-NO ) (Figure 5b, red squares) is
2
+
the same as measured for the [M+NO ] adduct. In addition,
2
the UVPD action spectra measured for A-ONO (Figure 4c)
Reaction of Two Alternative Charge-Tagged
Phenyl Radical Cations with NO2
feature significantly higher photoproduct yields than spectra of
+
both the [M+NO ] adduct and A-NO (note the relative
2
2
magnifications shown in Figure 4a and b), consistent with the
lower RO–NO bond dissociation energy of A-ONO. It is
important to note that the UVPD action spectra being compared
here were measured consecutively under identical conditions,
in order that temporal fluctuations in the laser power or beam
profile were minimized.
In the previous section, we demonstrated that the stabilized
[M+NO2] adduct formed by the reaction of 4-(N,N,N-
+
trimethylammonium)phenyl radical cation (A) with NO2
was exclusively the charge-tagged nitrobenzene (A-NO2).
We wished to explore the effect of the charge-tag on this
reaction and to investigate whether it is possible to generate a
stabilized nitrosoxybenzene (RONO) by modification of the
substitution of the phenyl radical. For this purpose, analogous
experiments were undertaken with two additional charge-
tagged phenyl radical cations generated by 266 nm PD of their
respective iodobenzene precursors: (i) 3-(N,N,N-trimethylam-
Taken together, both CID spectra and UVPD action spectra
+
demonstrate that the [M+NO2] adduct ion generated by
reaction of the charge-tagged phenyl radical (A) with NO2
is 4-nitro-N,N,N-trimethylbenzaminium cation (A-NO ) and,
2
furthermore, there is no contribution in this ion population

B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
487
2
2
(
(
(
a)
b)
c)
+
Figure 4. Comparative UVPD (225 nm) mass spectra of (a) the [M+NO ] adduct generated during reaction of the 4-(N,N,N-
2
trimethylammonium)phenyl radical cation with NO , (b) authentic 4-nitro-N,N,N-trimethylbenzaminium cation (A-NO ), and (c) 4-
2
2
nitrosoxy-N,N,N-trimethylbenzaminium cation (A-ONO). *Ions arising due to laser ablation of the ion-trap back plate or
ionization of contaminants
monium)phenyl radical cation (B), where coupling between
similar product ions to those present after CID of the
the charge-tag and radical moiety has be altered by reposi-
corresponding para-isomer A-NO (c.f. Figure 3a and c),
2
+
+
tioning the NMe charge-tag from the para to the meta
however, the [M – NO – CH ] ion featured at m/z 136 during
3
3
position (Scheme 3i, ii); and (ii) 1-(4-(N,N,N-trimethylammo-
nium)methyl)phenyl radical cation (C), where the charged
moiety is separated from the phenyl ring by a methylene
linker (Scheme 3iii, iv). The phenyl radicals B and C were
CID of A-NO is absent. This is consistent with the location of
the nitro moiety, now at the meta-position of the phenyl ring,
where the unpaired electron on the nascent phenoxyl radical
2
will no longer be resonantly coupled to the methyl groups of
+
+
allowed to react with NO resulting in formation of [B+NO ]
the NMe charge-tag. Subjecting the authentic nitrosoxyben-
2
2
3
+
+
and [C+NO ] ions at m/z 181 and m/z 195, respectively
zene (B-ONO) to CID results only in an [M – NO] product ion
2
(
Scheme 3). The authentic nitrosoxybenzenes B-ONO and
at m/z 151 ion (shown in Figure 6b). When the collision energy
+
C-ONO were generated by the same method as A-ONO
employed for CID of the [B+NO ] adduct was reduced to
2
(
vide supra), as depicted in Scheme 3ii and iv, respectively.
CE015 (arbitrary units) in order to compare directly to the CID
spectrum of B-ONO, there were no products, demonstrating
+
Subjecting the [B+NO2] adduct to CID (shown in
+
Figure 6a) resulted in a dominant [M – CH ] ion at m/z 166,
in addition to minor [M – NO] and [M – NO ] fragment ions
there was no contribution from the nitrosoxy isomer (B-ONO)
3
+
+
+
in the [M+NO ] ion population. These data suggest reaction
2
2
at m/z 151 and 135, assigned as the charge-tagged phenoxyl
radical and phenyl radical, respectively. This spectrum reveals
of 3-(N,N,N-trimethyammonium)phenyl radical cation (B)
with NO results exclusively in the nitro isomer (B-NO ).
2
2

4
88
B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
2 2
Overall, there was no evidence for formation of a
stabilized nitrosoxybenzene isomer following reaction of
+
(
(
(
a) NMe −Ph• + NO
3
2
either B or C with NO . The absence of this isomer in the
2
+
b) NMe −PhNO
+
3
2
[M+NO2] adduct population, and the low CID energy
+
c) NMe −PhONO
required for ejection of NO from the authentic nitro-
soxybenzenes (RONO) may suggest that should the nitro-
soxybenzene form either by direct addition or isomerization
of the nitrobenzene, rapid ejection of NO may follow driven
by the large exothermicity of the radical addition reaction.
3
220
240
260
280
300
320
Wavelength (nm)
Calculation of Reaction Coordinates
Figure 5. UVPD action spectrum of (a) the m/z 181 ion
generated during reaction of the 4-(N,N,N-trimethylammo-
We have undertaken electronic structure calculations in an
effort to explain the absence of a stabilized nitrosoxybenzene
nium)phenyl radical cation with NO (green diamonds), (b) 4-
2
+
in the [M+NO ] ion population of all three systems studied
nitro-N,N,N-trimethylbenzaminium cation (A-NO
2
, red
2
squares), and (c) N,N,N-trimethyl-4-nitrosoxybenzaminium
cation (A-ONO, blue circles)
and to further elucidate the fragmentation pathways ob-
served during CID of these charge-tagged nitro- (RNO ) and
2
nitrosoxybenzenes (RONO). The reaction coordinates of the
(
i) 4-(N,N,N-trimethylammonium)phenyl radical cation (A)+
+
Subjecting the isolated [C+NO ] adduct to CID resulted
NO , and (ii) 4-((N,N,N-trimethylammonium)methyl)phenyl
2
2
in the mass spectrum depicted in Figure 6c. An array of
radical cation C+NO reactions are depicted in Figures 7
and 8, respectively. The initial addition of NO2 to A
2
+
product ions is generated, including [M – NO ] , [M –
2
+
+
+
+
NMe ] , [M – NO – CH ] , [M – NO – NMe ] , [NMe ] ,
and [CH NMe ] ions at m/z 149, 136, 134, 106, 59, and 58,
respectively. Interestingly, there is no [M – NO] ion at m/z
65, instead the majority of the product ions arise due to loss
of NO and/or the NMe charge-tag. This does not rule out
(Figure 7) proceeds without barrier to generate A-NO with
3
2
3
3
3
2
+
a reaction exothermicity of 60 kcal/mol. Isomerization to the
nitrosoxybenzene (A-ONO) requires surmounting a signifi-
cant barrier of 57 kcal/mol with a reaction exothermicity of
66 kcal/mol relative to the entrance channel. The free energy
2
2
+
1
+
2
3
‡
the isomerization channel resulting in ejection of NO, but
barrier (ΔG298 ) for this isomerization is similar in height to
+
•
‡
may instead suggest radical-driven ejection of NMe from
the internal energy barrier (ΔH ) at 56 kcal/mol, but resides
3
0
the nascent phenoxyl radical following ejection of NO
around 9 kcal/mol above the entrance channel. Should the
nitrosoxybenzene form, ejection of NO requires only
24 kcal/mol of energy suggesting that trapping significant
populations of stabilized nitrosoxybenzenes (RONO) in this
potential well following isomerization is unlikely, consistent
with these experiments.
+
(
analogous to ejection of the CH from [A-NO – NO] ).
3
2
In this case, the positive charge no longer resides with the
larger phenoxyl-containing fragment. In contrast, CID of the
nitrosoxy isomer (C-ONO), presented in Figure 6d, again
+
results only in generation of an [M – NO] ion at m/z 165.
NMe3
NMe3
NMe3
NMe3
O
N
[M + 46]+
(i)
(ii)
+ O2
+ NO
+
NO2
[
B + NO2]+
O
O
B
B
m/z 181
B-O
m/z 151
B-ONO
m/z 181
m/z 135
m/z 135
NMe3
NMe3
NMe3
NMe3
[M + 46]+ (iv)
C + NO2]+
(iii)
+ O2
+ NO
+
NO2
[
m/z 195
O
O
O
C
N
C
m/z 149
m/z 149
C-O
m/z 165
C-ONO
m/z 195
Scheme 3. Outline of the general experiment resulting in isobaric m/z 181 ions arising due to (i) the reaction of 3-(N,N,N-
trimethylammonium)phenyl radical cation and NO , and authentic (ii) N,N,N-trimethyl-3-nitrosoxybenzaminium cation (B-ONO),
and the isobaric m/z 195 ions arising due to (iii) reaction of 4-([N,N,N-trimethylammonium]methyl)phenyl radical cation (C) with
NO , and authentic (ii) 1-(4-nitrosoxyphenyl)-N,N,N-trimethylmethaminium cation (C-ONO)
2
2

B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
489
2
2
(a)
in accord with the potential energy surface where loss of
both CH and NO from A-NO are predicted to require less
3
2
2
energy than isomerization to A-ONO, with reaction endo-
thermicities of 51 kcal/mol and 60 kcal/mol, respectively.
The significant isomerization barrier to formation of the
nitrosoxybenzene (A-ONO) explains the low abundance of
+
+
the [M – NO] and [M – NO – CH ] product ions during
3
CID of A-NO and their complete absence at low collision
2
energies. Ejection of NO after preliminary loss of CH is not
3
predicted to be energetically feasible, as the R-NO to R-
2
ONO barrier of the demethylated nitrobenzene analogue
resides 35 kcal/mol above the entrance channel (calculations
reported in Supporting Information). When A-ONO is
formed after isomerization, however, the significant internal
energy (66 kcal/mol) should drive prompt ejection of NO
(b)
(
24 kcal/mol) and subsequent loss of CH (which requires
3
only an additional 26 kcal/mol). As discussed above, loss of
NO from A-ONO is the lowest energy process from this
isomer, which is consistent with the observation of exclusive
NO loss when A-ONO is subjected to CID. No additional
product ions are observed in this case, as ejection of NO will
proceed before the parent ion gains sufficient energy to
explore alternative fragmentation channels. The significant
differences in the collision energy required for fragmentation
(
(
c)
of A-NO (CE035 arbitrary units) and A-ONO (CE015
2
arbitrary units) are clearly consistent with the relative
reaction energies of these fragmentation processes.
Despite separating the charge-tag from the phenyl ring by
a methylene linker, these calculations suggest addition of
NO to C to form the nitrobenzene (C-NO ), its isomeriza-
2
2
tion to the nitrosoxybenzene (C-ONO) and ejection of NO,
are similar in energy to the A+NO reaction surfaces (shown
2
d)
in Figure 8). Noticeably, these calculations predict an
increase in reaction endothermicity for loss of CH3
(67 kcal/mol), consistent with its absence during CID of
C-NO . Instead, loss of NMe to form the 4-nitrotoluene
2
3
cation is lower in energy, at 56 kcal/mol, as is ejection of
NO to reform the phenyl radical (C). The energy required
2
for ejection of NO from C-ONO is 22 kcal/mol lower than
for other fragmentation pathways from this isomer. This is
consistent with the CID spectrum of C-ONO, where NO
loss was the solitary fragmentation channel. Isomerization of
C-NO to C-ONO proceeds with a similar barrier to that of
2
Figure 6. Mass spectra obtained following CID of the m/z
81 ion generated from (a) reaction of 3-(N,N,N-trimethylam-
monium)phenyl radical cation (B) with NO , and (b) 3-nitro-
soxy-N,N,N-trimethylbenzaminium cation (B-ONO), and the
m/z 195 ion generated from (c) reaction of 4-(N,N,N-trimethy-
A-NO at 57 kcal/mol with an overall reaction exothermicity
1
2
of 66 kcal/mol. Should ejection of NO follow, the
considerable excess internal energy of the nascent phenoxyl
radical (35 kcal/mol) will drive subsequent ejection of
2
+
•
lammonium)methylphenyl radical cation (C) with NO
-(4-nitrosoxyphenyl)-N,N,N-trimethylmethaminium cation
C-ONO)
2
, and (d)
NMe , which requires only 24 kcal/mol. This fragmenta-
3
1
(
tion channel is favored over both CH and NMe loss, which
3
3
in this case require significantly more energy at 59 kcal/mol
and 55 kcal/mol, respectively.
Overall, these calculations suggest that isomerization of
the nascent nitrobenzene to the nitrosoxybenzene during
These electronic structure calculations also provide a
clear rationale for the CID spectra of A-NO and A-ONO
reaction of the charge-tagged phenyl radical with NO is
2
2
(
Figure 4a and b, respectively). The major fragments
observed during CID of A-NO are loss of CH and NO
2
unlikely, as the free energy barrier for this reaction pathway
resides above the entrance channel. This result supports
2
3

4
90
B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
2
2
NMe3
(A)
~
2
(8.9)
+
NO2 (X A1)
NMe2
ONO
ΔG298 = (0.0)
-
3.1
ΔH0 = 0.0
+CH3
-
9.0
-9.2)
CH3
NMe2
NO2
NMe2
(
-18.2
+
NO
+
-
24.5
+
CH3
(
-24.8)
O
(
-28.3)
NMe3
(
-42.5)
43.7
(
-47.5)
-
+
NO
(
-54.6)
-
59.7
O
(A-O)
NMe3
NO2
-66.4
NMe3
(
A-NO2)
(A-ONO)
ONO
2
Figure 7. Potential energy surface of the 4-(N,N,N-trimethylammonium)phenyl radical cation (A)+NO calculated at the B3LYP/
aug-cc-pVDZ level. Solid lines are the Gibbs free energy values (ΔG298), whereas the hashed blue lines are the energies with
zero-point energy correction (ΔH ). Energies are given in kcal/mol
0
NMe2
NMe3
NMe2
NMe2
+NO
+
CH3
(
C)
+CH3
+CH3
14.5
O
NO2 (X 2A1)
~
+
(
8.3)
3.7
NO2
6.7
ONO
7.8
ΔG298 = (0.0)
ΔH0 = 0.0
(6.4)
-1.3
(5.9)
+NO
-
4.6
-4.8)
NMe3
+NMe3
(
-2.1)
-
(-3.2)
(
O
+
+NMe3
-21.1
NO2
ONO
31.2
-32.1)
-
+NMe3
(
(-34.0)
O
(
-43.5)
(
-48.0)
-44.7
NMe3
(
-54.2)
-60.3
-65.9
(C-O)
+NO
NMe3
NMe3
O
(C-NO2)
(
C-ONO)
NO2
ONO
Figure 8. Potential energy surface of the 4-([N,N,N-trimethylammonium]methyl)phenyl radical cation (C)+NO
2
calculated at the
B3LYP/aug-cc-pVDZ level. Solid lines are the Gibbs free energy values (ΔG298), whereas the hashed blue lines are the energies
with zero-point energy correction (ΔH ). Energies are given in kcal/mol
0

B. B. Kirk et al. : Does R + NO yield RONO or RNO ?
491
2
2
experiments of the charge-tagged phenyl+NO2 reaction
where X-ONO (X0A, B, C) was not detected. Furthermore,
in the case of A+NO , ejection of CH from A-NO2
proceeds at a lower energy than isomerization to the
nitrosoxybenzene (A-ONO); as such, one would expect
these product channels to be competitive with isomerization,
to the nitrosoxyalkane would result in ejection of NO and
the excess internal energy of the nascent alkoxyl radical
could then presumably drive the additional fragmentation
channels observed.
Overall, we expect stabilization of a nitrosoxyalkane or
nitrosoxyarene (RONO) to be dependent on the energy
required for NO ejection. Observation of a stabilized RONO
under low-pressure conditions would only be expected when
the energy required for ejection of NO or other low energy
fragmentation pathways from RONO rises close to, or
2
3
which is clearly not the case, as indicated by the absence of
+
an [M – CH ] ion in Figure 1. As was previously noted by
3
Glenewinkel-Meyer and Crim [36] for the analogous neutral
reaction, should nitrobenzene isomerize to nitrosoxyben-
zene, the exothermicity of this reaction would easily drive
ejection of NO, thus, it would be unlikely to observe a
stabilized nitrosoxybenzene product from this reaction.
above, the R+NO entrance channel.
2
Acknowledgments
B.B.K. was supported through an Australian Postgraduate
Award. The authors acknowledge the financial support of
the Australian Research Council (DP0986738, DP0452849),
ARC Centre of Excellence for Free Radical Chemistry and
Biotechnology (CE0561607) and the University of Wollon-
gong. The authors thank Christopher Hansen (UOW) for
assistance with the LTQ+OPO instrumentation, in addition
to Intersect (NSW, Australia) and the NCI National Facility
Conclusion
We have successfully investigated the reaction of phenyl
radicals with NO using distonic radical models. CID and
2
+
UVPD action spectroscopy of (i) the [M+46] product ion
generated during this reaction, and authentic charge-tagged
(
ii) nitrobenzene, and (iii) nitrosoxybenzene isomers, dem-
+
onstrated unequivocally that the [M+NO ] reaction product
2
was nitrobenzene, not the nitrosoxybenzene isomer. These
results compare well with the computational study of Xu and
Lin [4], where under low-pressure conditions, formation of
nitrosoxybenzene is primarily expected to dissociate to the
phenoxyl radical (R+NO2 → [RONO]* → RO+NO),
consistent with its absence during our experiments.
CID of the authentic charge-tagged nitrobenzene and
nitrosoxybenzene resulted in significantly different mass
spectra. The observation of ions arising due to NO ejection
and concomitant fragmentation processes, e.g., CH3 loss
from the charge-tag, was diagnostic for isomerization of the
nitrobenzene to the nitrosoxybenzene. Electronic structure
calculations suggest the significant internal energy of the
nascent nitrosoxybenzene formed after isomerization of the
nitrobenzene is sufficient to drive ejection of NO and
additional fragmentation pathways. This was in direct
contrast to CID of the authentic nitrosoxybenzene, where
no excess internal energy is anticipated as CID results in
slow heating of the ions and, thus, predominantly dissoci-
ation via the lowest energy fragmentation channels.
The question arises as to whether these experimental data
may help identify the ionized peptidyl radical+NO adduct
generated by Barlow et al. [9] and to elucidate the
fragmentation pathways observed therein. A recent compu-
tational study of the ethyl+NO reaction suggested that loss
of HONO, the major product ion detected during CID of the
peptidyl radical+NO adduct, may proceed via the interme-
diacy of nitroethane (RNO → R′+HONO) [5]. The barrier
for this reaction channel was calculated at 42.8 kcal/mol, in
contrast to the significant barrier to nitro-nitrosoxyethane
isomerization of 60.7 kcal/mol. In direct comparison to our
experiments (vide supra), we would hypothesize that the
(Canberra, Australia) for a generous allocation of resources
under the Merit Allocation Schemes.
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