Reactions of the CN radical with benzene and toluene: Product detection and low-temperature kinetics

Article abstract of DOI:10.1021/jp909633a

Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range, (3.9-4.9) × 10^-10 cm³ molecule^-1 s^-1. These rapid kinetics, along with the observed negligible temperature dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 × 10 ^-10 cm³ molecule^-1 s^-1 at 105 K. At room, temperature, nonexponential decay profiles are observed for this reaction that may suggest significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene, cyanobenzene (C₆H₅CN) is the only product recorded with no detectable evidence for a C₆H₅ + HCN product channel. In the case of CN + toluene, cyanotoluene (NCC ₆H₄CH₃) constitutes the only detected product. It is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their similar ionization energies and the ～40 me V photon energy resolution of the experiment. There is no significant detection of benzyl radicals (C₆H₅CH₂) that would suggest a H-abstraction or a HCN elimination channel is prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn's moon Titan (～400 K) and are also likely to proceed at the temperature of interstellar clouds (10-20 K).

Full text of DOI:10.1021/jp909633a

J. Phys. Chem. A 2010, 114, 1749–1755
1749
Reactions of the CN Radical with Benzene and Toluene: Product Detection and
Low-Temperature Kinetics
Adam J. Trevitt,*,†,§ Fabien Goulay,‡ Craig A. Taatjes,‡ David L. Osborn,‡ and
Stephen R. Leone*,†,#
Departments of Chemistry and Physics, and Lawrence Berkeley National Laboratory, UniVersity of California,
Berkeley, California 94720, and Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories,
LiVermore, California, LiVermore, California 94551-0969
ReceiVed: October 8, 2009; ReVised Manuscript ReceiVed: December 3, 2009
Low-temperature rate coefficients are measured for the CN + benzene and CN + toluene reactions using the
pulsed Laval nozzle expansion technique coupled with laser-induced fluorescence detection. The CN + benzene
reaction rate coefficient at 105, 165, and 295 K is found to be relatively constant over this temperature range,
(3.9-4.9) × 10-10 cm3 molecule-1 s-1. These rapid kinetics, along with the observed negligible temperature
dependence, are consistent with a barrierless reaction entrance channel and reaction efficiencies approaching
unity. The CN + toluene reaction is measured to have a rate coefficient of 1.3 × 10-10 cm3 molecule-1 s-1
at 105 K. At room temperature, nonexponential decay profiles are observed for this reaction that may suggest
significant back-dissociation of intermediate complexes. In separate experiments, the products of these reactions
are probed at room temperature using synchrotron VUV photoionization mass spectrometry. For CN + benzene,
cyanobenzene (C6H5CN) is the only product recorded with no detectable evidence for a C6H5 + HCN product
channel. In the case of CN + toluene, cyanotoluene (NCC6H4CH3) constitutes the only detected product. It
is not possible to differentiate among the ortho, meta, and para isomers of cyanotoluene because of their
similar ionization energies and the ∼40 meV photon energy resolution of the experiment. There is no significant
detection of benzyl radicals (C6H5CH2) that would suggest a H-abstraction or a HCN elimination channel is
prominent at these conditions. As both reactions are measured to be rapid at 105 K, appearing to have barrierless
entrance channels, it follows that they will proceed efficiently at the temperatures of Saturn’s moon Titan
(∼100 K) and are also likely to proceed at the temperature of interstellar clouds (10-20 K).
1. Introduction
Cassini.6 The Huygens lander probe, launched from Cassini,
may have also detected evidence for benzene residing on Titan’s
Significant interest surrounds the reactions of CN radicals
with hydrocarbons in the context of planetary atmospheres,
interstellar media, and combustion, particularly concerning the
gas-phase product pathways leading to nitrile alkenes, polyaro-
matic hydrocarbon (PAH) nitriles, and polyaromatic nitrogen
heterocyclics (PANHs). The chemistrysformation and fatesof
CN-bearing molecules in the atmosphere of Saturn’s moon Titan
is one example where these reactions are thought to be
integral.1,2 The hazy atmosphere of Titan presents a fascinating
chemical system for which experimental data from the Cassini
mission continues to be collected and interpreted. Mass spectra
gathered from the ion neutral mass spectrometer (INMS) on
board Cassini reveal the presence of, and in some cases quantify,
numerous neutral hydrocarbons (including C2H2, C2H4, CH3C2H,
and C4H2) and nitrogen-bearing species (e.g., HC3N, C2N2, and
NH3).3
surface using its onboard gas chromatograph mass spectrometer.7
A mass consistent with C7H8 (toluene) is also recorded by the
INMS, but it has been suggested that quantification of toluene
is affected by possible wall-mediated chemistry within the
sampling portion of the INMS that leads to products at the
toluene mass.3,8 The C6H5 + CH3 reaction is a plausible neutral
radical-radical pathway leading to the formation of toluene in
Titan’s atmosphere.8 Complex molecules with masses up to 350
Da along with negative ions with masses up to ∼8000 Da have
been detected.5 It is proposed that these large molecules and
ions, which are postulated to be unsaturated hydrocarbons
including PAH and PANH compounds, are the precursors that
condense to form particulate matter.5 Generally, benzene is
considered pivotal in the formation of PAHs that in turn lead
to the growth of particles/aerosol that are responsible for Titan’s
hazy appearance.5 A recent study proposes sequential C2H
radical addition reactions starting with benzene as a PAH growth
mechanism.9 This centrality of benzene in the molecular weight
growth schemes is not confined to the context of Titan haze
formation; it is also commonly accepted that benzene is integral
to the formation of carbonaceous soot particles in combustion
environments.10 Schemes leading to PAH formation are emerg-
ing, but the key neutral reactions that incorporate nitrogen into
the molecular weight growth chemistry, most likely implicating
-CN species and yielding PANH scaffolds, remain unclear.
Benzene (C6H6) is also indentified in the Titan atmosphere
by the INMS.4,5 Further verification of the presence of benzene
in the atmosphere is reported by Coustenis et al. utilizing data
acquired from the composite infrared spectrometer on board
* To whom correspondence should be addressed. E-mail:
adamt@uow.edu.au (A.J.T.); srl@berkeley.edu (S.R.L.).
† University of California, Berkeley.
‡ Sandia National Laboratories.
§ Present address: School of Chemistry, University of Wollongong,
Wollongong, NSW, 2522, Australia.
# On appointment as a Miller Research Professor in the Miller Institute
for Basic Research in Science.
10.1021/jp909633a  2010 American Chemical Society
Published on Web 12/31/2009

1750 J. Phys. Chem. A, Vol. 114, No. 4, 2010
Trevitt et al.
A particularly salient physical parameter when considering
Titan’s atmospheric chemistry is its low temperature (100-200
K). Predicting and modeling the evolution of Titan’s atmosphere
requires chemical data in the form of rate coefficients and
reaction product branching ratios. Detailed low-temperature
product branching fractions are particularly scant. Low-tem-
perature rate coefficients for radical-neutral reactions, on the
other hand, have been accumulating in recent years primarily
due to the efforts of the groups at Rennes, Birmingham, and
Berkeley using Laval nozzle expansion techniques. Many of
these reactions have been recently reviewed.11 It is now known
that extrapolating kinetic data acquired at moderate temperatures
(300-1000 K) down to very low temperatures (<300 K) can
yield rate coefficients that disagree with measured data by as
much as several orders of magnitude. The measurement of rate
coefficients at the relevant low temperatures is therefore vital
for accurate kinetic data. Recently, low-temperature kinetic
studies have been extended to aromatic species: benzene12–14
and anthracene.15,16
2. Experimental Section
The low-temperature kinetic data are obtained using a pulsed
Laval nozzle expansion, using several nozzles designed to work
at different temperatures. Laser photolysis is employed for the
production of CN radicals and the relative concentration of these
radicals are subsequently tracked using laser-induced fluores-
cence (LIF). The experimental arrangement and characterization
of the Laval nozzle apparatus used in this work are described
in detail in previous papers,12,24 so just a brief overview of the
experiment is given herein.
A pulse of gas mixture, containing mostly N2 with traces of
CN radical precursor (cyanogen iodide, ICN) and aromatic
hydrocarbon (benzene or toluene), is expanded through a Laval
nozzle into the reaction vacuum chamber. The notable feature
of the Laval expansion is that, under optimal operating condi-
tions, a collimated supersonic gas flow is established for
approximately 15 cm into the reaction chamber. This collimated
flow has uniform temperature and density distributions in both
the radial and axial directions. In a sense, the Laval expansion
can be thought of as a wall-less flow tube where reactions can
be studied at very low temperatures without the complicating
effects of wall condensation. Nozzles are fabricated to produce
a collimated flow with a particular temperature that depends
on the nozzle geometry, the buffer gas, and the temperature of
the gas entering the nozzle. These nozzles are designed to be
interchangeable in order to perform kinetic measurements at
various temperatures. The nozzles used in this work generate
flows of 105 K (Mach 3) and 165 K (Mach 2). The pressure in
the reaction chamber is adjusted using N2 gas in order to obtain
optimal collimation in the Laval gas flow. Four milliseconds
after the initiation of the gas pulse, an ∼7 ns 40 mJ/cm2 laser
pulse (266 nm, fourth harmonic of Nd:YAG), unfocused, is
directed down the central bore of the Laval flow. This pulse
generates CN radicals from the photolysis of ICN with the initial
radical concentration estimated to be ∼1 × 1010 molecules/cm3.
The concentration of CN radicals is monitored via LIF. To
accomplish this task, a second pulsed Nd:YAG laser operating
on the third harmonic (355 nm) pumps a dye laser (using Exalite
392A dye in p-dioxane) that is tuned to excite CN radicals on
the R(4) line of the (0,0) B2Σ-X2Σ system at ∼387 nm coaxially
down the Laval expansion. Fluorescence is detected perpen-
dicular to the excitation axis, 12 cm downstream of the Laval
nozzle by a photomultiplier tube though a 10 nm wavelength
bandpass filter centered at 420 nm corresponding to the (0,1)
B2Σ-X2Σ band. The photomultiplier signal is processed in
analog fashion using a box-car integrator set to a delay so that
there was minimal interference from the LIF probe laser beam
in the integrated CN LIF signal. Measuring the CN LIF as a
function of delay between the photolysis laser and the LIF probe
laser permits the CN radical concentration to be followed as a
function of reaction time. Typically, 10 laser pulses are averaged
per point with 5-10 decay profiles subsequently summed
together for pseudo-first-order kinetic analysis. The experiment
is performed at a 10 Hz repetition rate.
The low-temperature reactivity of the CN radical has been
investigated for a range of small hydrocarbons.17 The reactions
of CN with larger hydrocarbon species including allene and
propyne were found to have rate coefficients of ∼4 × 10-10
cm3 molecule-1 s-1 over the 15-294 K range.18 Bimolecular
rates of this value are approaching the gas-kinetic limit.
Reactions of CN with unsaturated hydrocarbons that have near-
collision-limited rate coefficients at room temperature represent
examples where the kinetics depends predominantly on long-
range capture processes (i.e., are barrierless reactions) such that
it is likely the reaction rate will remain rapid down to much
lower temperatures. This notion has yet to be tested for CN
reactions with aromatic species. A rate coefficient for the CN
+ benzene reaction has been determined only at room temper-
ature to be 2.8 × 10-10 cm3 molecule-1 s-1.19 To our knowledge,
no kinetic data have been reported for the CN + toluene reaction
at any temperature.
Crossed molecular beam studies comprise a significant portion
of the product detection studies concerned with CN + hydro-
carbon reactions.20,21 Balucani et al. investigated the CN +
benzene reaction at collision energies between 19.5 and 34.4
kJ/mol and compared the results to electronic structure and
RRKM calculations.22 It was concluded that the dominant
reaction entrance channel is barrierless leading to the formation
of a CN-addition complex (C6H6CN) that subsequently dissoci-
ates to form cyanobenzene (C6H5CN) + H. The isocyanoben-
zene (C6H5NC) product channel was set to a limit of less than
2%. To our knowledge, there has not been a similar study
concerned with the CN + toluene reaction.
Recently, we reported on the room temperature reactions of
CN + ethene and CN + propene reactions utilizing synchrotron
VUV multiplexed photoionization mass spectrometry
(MPIMS).23 Product species were identified using adiabatic
ionization energies (AIEs) and characteristic photoionization
efficiency (PIE) curves. These data were analyzed with either
known or approximated photoionization cross sections to
ultimately assign product branching fractions.
All gas flows are controlled using calibrated mass-flow
controllers. The aromatic reactant, benzene (purity >99.5%) or
toluene (purity >99.8%), is entrained in the gas flow by bubbling
a fixed flow of nitrogen (15 sccm) through a liquid sample of
either benzene or toluene. The concentration of the aromatic
reactant is varied by changing the pressure in the bubbler using
the needle valve on the output side of the bubbler. The
temperature of the bubbler was maintained below room tem-
perature, typically at 15 °C, using a water bath chiller. Varying
the bubbler reservoir temperature showed no measurable change
In this paper, we report the low-temperature rate coefficients
measured for the CN + benzene (105-295 K) and the CN +
toluene (105 K) reactions conducted using a pulsed Laval nozzle
expansion coupled with laser-induced fluorescence detection.
We also report product detection experiments for both reactions
conducted at room temperature using synchrotron MPIMS to
elucidate the dominant reaction products of these reactions.

CN + Benzene and CN + Toluene Reactions
J. Phys. Chem. A, Vol. 114, No. 4, 2010 1751
in the kinetic measurements over 10-20 °C. With knowledge
of the temperature of the liquid in the bubbler and pressure
above the liquid sample in the bubbler, it is possible to calculate
the density of either benzene or toluene entrained in the gas
flow. To ensure thorough mixing of the N2 and benzene or
toluene vapor, N2 is supplied to the bubbler through a glass
frit. After the bubbler, a mixing volume of 500 mL is in place.
All tubing after the bubbler/chiller is at room temperature. The
system is given ample time to equilibrate between acquisitions
and this is confirmed by consecutive concordant measurements.
The temperature-dependent vapor pressures for both benzene
and toluene are calculated from their known Antoine param-
eters.25
Figure 1. Measured CN + C2H4 bimolecular rate coefficient deter-
mined in this work at 105 K using ICN as the CN radical photolysis
precursor (triangle) compared to the data taken from Sims et al.17 where
NCNO was used as the CN photolysis precursor (squares).
Product detection experiments are conducted on the multi-
plexed photoionization mass spectrometer at the Chemical
Dynamics Beamline at the Advanced Light Source, Lawrence
Berkeley National Laboratory. Products of the CN + benzene
and the CN + toluene reaction are measured by using tunable
synchrotron VUV photoionization mass spectrometry. The
experimental apparatus is described comprehensively in recent
literature,26 and we recently reported product detection studies
of CN reacting with ethene and propene using this configura-
tion.23 Briefly, the experiment couples a reaction flow tube to a
magnetic sector mass spectrometer utilizing the tunable, syn-
chrotron VUV photons to selectively ionize neutral species. The
gas in the reaction flow tube is maintained at 4 Torr, made up
of the radical precursor (ICN), a known concentration of
coreactant(benzeneortoluene)andHebuffergas.Radical-neutral
reactions are initiated by an excimer laser pulse (KrF, 248 nm)
that photolyzes ICN to form CN radicals uniformly down the
reaction flow tube. A portion of the gas flow in the reaction
tube escapes through a 650 µm pinhole, situated 350 mm down
the flow tube, into a differentially pumped vacuum chamber.
This effusive gas, containing reactants, products, and buffer gas,
is sampled by a skimmer forming a molecular beam that is
directed into a differentially pumped chamber. Here, the
molecular beam is intersected with the VUV synchrotron photon
beam and photoions are then extracted and separated according
to their mass-to-charge (m/z) ratios using a compact magnetic-
sector mass spectrometer. In this configuration ions are time-
tagged relative to the pulsing of the excimer laser allowing
individual m/z channels to be monitored as a function of kinetic
time. Reactant, intermediate and product species can then be
distinguished on the basis of their temporal profiles. In an
additional dimension, scanning the VUV photon energy allows
the photoionization efficiency (PIE) of the m/z channels to be
recorded. These PIE curves are then used to distinguish
molecular isomers on the basis of their adiabatic ionization
energies (AIEs) and characteristic PIE responses as described
later in this report.
Figure 2. Decay profiles of the CN radical measured at 105 K for the
(a) CN + benzene reaction with a benzene density of 4.83 × 1013 cm-3
and the (b) CN + toluene reaction with a toluene density of 4.63 ×
1013 cm-3. Both are fitted to single-exponential decays (solid line). The
delay is the timing delay between the photolysis laser pulse and the
laser-induced fluorescence probe laser pulse.
mentioning, due to an apparent reduced commercial availability
of ICN, that BrCN (photolyzed at 248 nm) was found to be an
unsuitable precursor for low-temperature CN radical kinetics
studies in an N2 buffer gas. Substantial rotational excitation
imparted to the CN fragment was observed in measured LIF
excitation spectra, as has been reported,29 and thermalization
was found to be relatively slow. This effect manifests itself as
a reduction of the observed CN decay rates because the removal
of CN via reaction is compensated to a small but measurable
extent by quenching of high rotational energy levels that
repopulate the low rotational state probed using the LIF scheme.
In contrast, a LIF excitation spectrum acquired at a 40 µs delay
after pulsing the photolysis laser confirms that CN radicals
generated from ICN photolysis are rotationally thermalized to
105 and 165 K, for the respective Laval nozzle, and the
population almost entirely occupies the vibrational ground state.
Rate coefficients for the CN + benzene reaction were
measured at 105, 165 and 295 K. Pseudo-first-order decay
profiles (Figure 2a) were recorded over a range of benzene
densities. In the case of CN + toluene, the rate coefficient for
this reaction at the Titan relevant temperature of 105 K is
measured to be 1.3 × 10-10 cm3 molecule-1 s-1. Nonexponential
decay profiles were observed for this reaction at room temper-
ature. A representative decay profile at 105 K is depicted in
Figure 2b. For both reactions, single-exponential decay profiles
are fitted for delay times >40 µs, ensuring good thermalization
of the CN radicals. Bimolecular rate coefficients are subse-
quently obtained from the slopes of the straight-line fit from
3. Results
3.1. Low-Temperature Kinetics. Previous low-temperature
kinetic studies concerned with CN reactivity predominantly used
NCNO as a CN radical precursor photolyzed by either 532 or
583 nm laser irradiation.17,27,28 In the current study, the 266 nm
laser photolysis of ICN is employed to generate CN radicals.
To check for consistency, the kinetics of the CN + C2H4 reaction
was measured at 105 K and compared with data from Sims et
al.17 who studied the reaction kinetics over 25-295 K with
NCNO as the CN radical precursor. A rate coefficient of 3.6 ×
10-10 cm3 molecule-1 s-1 is measured for this reaction using
ICN as the CN photolysis precursor and fits well with the trend
measured by Sims et al. (Figure 1). It is perhaps worth

1752 J. Phys. Chem. A, Vol. 114, No. 4, 2010
Trevitt et al.
reactions. For CN + benzene, one product is detected from this
reaction occurring at m/z ) 103. Examining the PIE spectrum
of this mass reveals an onset of signal at 9.70 ( 0.05 eV. The
PIE curve is compared to a photoelectron spectrum of cy-
anobenzene33 that has been digitized and integrated over energy,
revealing a good match (Figure 5). The AIE of cyanobenzene
has been determined by ZEKE photoelectron spectroscopy to
be 9.7315 eV.34 The agreement between the onset of the ion
signal in the measured PIE and the integrated photoelectron
spectrum of cyanobenzene is consistent with cyanobenzene
being the major product for this mass channel. The vertical IE
of isocyanobenzene (C6H5NC) has been measured to have a
value of 9.5 eV,35 and we see no apparent onset of signal around
this vicinity in the measured PIE spectrum of m/z ) 103; an
estimate of <10% branching fraction for the isocyanobenzene
channel is made assuming the -NC and -CN isomers have
similar photoionization cross sections. In considering alternative
products, it is conceivable that the CN radical may directly
abstract a hydrogen atom from benzene forming HCN and the
phenyl radical (C6H5); other indirect pathways could also lead
to the elimination of HCN. We checked for these reaction
pathways by summing consecutive mass spectra at a photon
energy of 9 eV. This photon energy is below the AIE of benzene,
such that interference from the benzene signal is minimized,
but is well above the reported AIE of the phenyl radical
(8.3-8.67 eV, NIST Chemistry Database). No ion signal
attributable to phenyl was recorded under these experimental
conditions and a limit is established of <15% product fraction
using a phenyl ionization cross section of 3 Mb.36
Figure 3. Pseudo-first-order plots measured at 105 K for the CN +
benzene (diamonds) and CN + toluene (circles) reactions.
Figure 4. Measured bimolecular rate coefficients for the CN + benzene
reaction (circles) compared with computationally predicted values from
Woon31 (triangles) and the CN + toluene bimolecular rate coefficient
measured at 105 K (diamond).
the pseudo-first-order rate coefficients plotted as a function of
benzene or toluene density (Figure 3). The nonzero pseudo-
first-order rate coefficients recorded for CN radicals at zero
concentration of aromatic coreactant is likely due to diffusion
of CN species out of the laser photolysis/LIF region. Interference
from the CN + ICN reaction is not expected to be kinetically
competitive based on CN + BrCN and CN + ClCN kinetic
studies that obtained rate coefficients of 9 × 10-14 and 2.1 ×
10-13 cm3 molecule-1 s-1, respectively.30
The determined rate coefficients vs temperature are presented
in Figure 4 and these data, along with experimental conditions,
are displayed in Table 1. Within the uncertainty of the
experiment, the rate coefficient for the CN + benzene reaction
remains essentially unchanged over the 105-295 K temperature
range. For comparison, Figure 4 also displays data from Woon,31
who investigated the kinetics of the CN + benzene reaction
computationally over this temperature range.
The product detection experiment of CN + toluene reveals
only one laser-dependent product for this reaction, observed at
m/z ) 117 corresponding to C8H7N. The PIE curve correspond-
ing to m/z ) 117 is depicted in Figure 6. The most likely species
responsible for this signal are the cyanotoluene isomers: ortho,
meta, and para. The S/N of the product signal in the CN +
toluene experiment was noticeably inferior to the CN + benzene
reaction at similar conditions and it was not possible to extract
any kinetic information for the product signal. This low product
yield may be due to a competitive back-reaction process leading
to a regeneration of CN that is suggested by the nonexponential
profiles in the room temperature kinetic measurements. This
observation is not explored in detail here but it is clear that
further studies are required in order to understand the kinetic
behavior of the CN + toluene reaction at various temperatures.
The PIE plot depicts the onset of ion signal on the m/z )
117 channel at about 9.35 eV. The AIEs of the ortho, meta and
para isomers of cyanotoluene are known from photoelectron
experiments to be 9.391 eV, 9.405 and 9.318 eV, respectively.37
The energy resolution of this experiment is ∼40 meV and, along
with the low signal-to-noise of the m/z ) 117 signal, it is difficult
to distinguish these isomers. Other possible species of the C8H7N
formula include bicyclic compounds indole (AIE ) 7.76 eV)
and indolizine (IEvertical ) 7.24) in addition to the -CN and
-NC conformations of benzyl cyanide. The presence of the
bicyclic species can be ruled out on the basis of their low
ionization energies. The benzyl cyanide and benzyl isocyanide,
although having reported ionization energies close to the onset
region measured in Figure 6, are unlikely to be formed from
the CN + toluene reaction on mechanistic grounds.
The statistical uncertainty for the data displayed in Table 1
is 2σ < 10%, where σ is obtained from the least-squares fit to
the pseudo-first-order decay profiles with each rate coefficient,
measured in duplicate. Any systematic uncertainty in the rate
coefficients is primarily attributed to the uncertainty in the
concentration of the aromatic coreactant, which depends on the
pressure over the liquid the bubbler, the temperature of the
bubbler, and the flow rate of gas through the bubbler. Uncer-
tainty in the measurements of the temperature and pressure
within the bubbler affect the calculated density of aromatic vapor
in the flow. In all, error bars of 20% are ascribed to the measured
rate coefficients to take into account statistical and systematic
uncertainties. Variation in the aromatic number density due to
photofragmentation from the 266 nm laser pulse should be <1%
because the absorption cross sections for both benzene and
Another plausible reaction pathway for the CN + toluene
reaction is the ipso addition channel that has the CN radical
binding to the ring carbon of toluene bearing the methyl group.
From here, cyanobenzene can be formed from the subsequent
elimination of the CH3 radical. The ipso addition to toluene
toluene at 266 nm have been measured at <2 × 10-19 cm-2 32
.
3.2. Product Detection. The product detection experiments
at the Advanced Light Source (ALS) synchrotron are performed
at room temperature for the CN + benzene and CN + toluene

CN + Benzene and CN + Toluene Reactions
J. Phys. Chem. A, Vol. 114, No. 4, 2010 1753
TABLE 1: Rate Coefficients for the CN + Benzene and CN + Toluene Reactions Obtained with the Laval Nozzle Apparatus
a
T (K)
total flow density (1016 cm-3
)
aromatic reactant density (1012 cm-3
)
rate coefficient (10-11 cm3 s-1
)
CN + benzene
CN + toluene
105 ( 5
165 ( 15
295 ( 2
105 ( 5
2.4
4.7
6.7
2.4
0-50
0-53
0-62
0-50
43 ( 9
39 ( 8
49 ( 10
13 ( 3
a Errors in the measured rate coefficient are reported with 20% limits of uncertainty (see text).
proceeds without any positive energy barrier along the entrance
channel and is also consistent with the formation of an addition
complex. Carty et al. have reported similar temperature trends
in the rate coefficients for the CN + allene and CN + propyne
reactions measured over 15-294 K.18 The reaction of C2H +
benzene has also been shown to have a similar large rate
coefficient that remains essentially unaffected by temperature
over the 105-295 K temperature range.12 These rate coefficients
are very close to the corresponding hard-sphere collision limit,
implying reaction efficiencies close to unity.
Figure 5. Photoionization efficiency (PIE) curve of the m/z ) 103
mass channel measured from the room temperature CN + benzene
reaction (diamonds) compared to the integrated photoelectron spectrum
of cyanobenzene from ref 33.
The measured CN + benzene reaction rates are in good
agreement with the recent theoretical predictions of Woon who
used quantum chemical calculations with RRKM and master-
equation methods to predict bimolecular rates for the CN +
benzene reaction over 50-300 K.31 These data are compared
to the experimental data acquired in this current study in Figure
4. The propensity for the C6H6-CN intermediate complex to
back-dissociate to the reactants was explored in Woon’s study
over a range of pressures and temperatures. The computed rates,
with zero contribution from back-reaction, were determined to
fall within (2.8-3.6) × 10-10 cm3 molecule-1 s-1 over 50-300
K. The magnitudes of the rate coefficients measured in the
current study suggest that back-dissociation of reaction inter-
mediate complexes is negligible under these conditions. It is
worth noting that in the same Woon study, rate coefficients for
C2H + benzene reaction are also reported and compared to
experimentally determined values measured previously by our
group. The measured rate coefficients are similar to CN +
benzene, between 3 × 10-10 and 4 × 10-10 cm3 molecule-1 s-1
over 105-295 K, compared to the Woon prediction of (2-3)
× 10-10 cm3 molecule-1 s-1 over the similar temperature range.
The computational predictions compare well in both cases, CN
+ benzene and C2H + benzene, but with the computed rate
coefficients being systematically less.
Figure 6. Photoionization efficiency (PIE) curve of the m/z ) 117
mass channel measured from the room temperature CN + toluene
reaction (diamonds). The arrows indicate the adiabatic ionization
energies (AIEs) of (from left to right) the para (9.318 eV), ortho (9.391
eV), and meta (9.405 eV) isomers of cyanotoluene (from ref 37).
followed by CH3 elimination has been investigated for a range
of open-shell species including F, Cl, O(3P), H, and OH.38,39
Under the conditions of this experiment we detect no evidence
for the formation of cyanobenzene that would indicate a
significant ipso elimination channel. Based on the S/N ratio of
the experiment and assuming cyanobenzene and cyanotoluene
have similar photoionization cross sections we estimate a limit
of <15% for the CH3 elimination channel.
We also checked for the formation of benzyl radical
(C6H5CH2) that could be the product of either a direct H-
abstraction channel or a complex HCN elimination pathway.
The benzyl radical has a known AIE of 7.24 eV.40,41 Integrating
extensively at 8.5 eV photon energy resulted in only a few ion
counts on the benzyl radical mass channel. The photoionization
cross section of the benzyl radical is not known at this photon
energy and we ascribe to the benzyl product channel a limit of
<10% based on an estimated ionization cross section of 10 Mb.
Overall, under the conditions of these product experiments, the
most significant product channel of CN + toluene occurs by
radical addition to the ring followed by elimination of a
hydrogen atom forming cyanotoluene.
A very early kinetic experimental study19 of various CN +
hydrocarbon reactions reported the room temperature rate
coefficient for the CN + benzene reaction at 2.8 × 10-10 cm3
molecule-1 s-1 compared with the result of 4.9 × 10-10 cm3
molecule-1 s-1 reported here. No experimental data for the
reaction with benzene appears in that paper so it is difficult to
comment on the discrepancy. The authors do mention, however,
in the context of a series of CN radical kinetic measurements
with various small hydrocarbons, that the results appear in most
cases to be close to the gas-kinetic limit and that their CN +
benzene result goes against the trend of increased rate with
increased hydrocarbon size. That paper also describes the CN
+ benzene reaction occurring via H-abstraction forming phenyl
+ HCN. This conclusion is at odds with subsequent studies
and the product detection results in this paper that are consistent
with an addition-elimination mechanism.
We therefore propose that the reaction of CN + benzene
proceeds by radical addition onto any of the benzene carbons,
forming an intermediate C6H6-CN radical species. The study
of Woon and the combined computational and molecular crossed
beam experiments of Balucani et al. both conclude that the
4. Discussion
The apparent lack of temperature response depicted in Figure
4 for the CN + benzene reaction is an indication that the reaction

1754 J. Phys. Chem. A, Vol. 114, No. 4, 2010
Trevitt et al.
reaction is without barrier and that the radical attack is
dominated by the carbon side of the CN radical. Following the
CN addition, the C6H6-CN intermediate complex stabilizes by
H atom elimination leaving cyanobenzene as the sole coproduct.
Computational investigations22,31 have examined the isocyano
(-NC) adduct channel and found that, similar to the -CN
adduct channel, there is also no positive energy barrier for -NC
adduct formation. Noteworthy, however, along the -NC addi-
tion pathway is an initial prereactive complex C6H6 · · ·NC
situated at -17.2 kJ mol-1 with respect to the reactants. From
here, an ∼+6 kJ mol-1 barrier separates the prereactive complex
from the bound adduct. This intermediate adduct complex
(C6H6-NC) is calculated to be about 87 kJ mol-1 greater in
energy than the -CN adduct counterpart; i.e. the -CN adduct
complex sits lower in energy (-165.4 kJ mol-1) than the -NC
adduct (-77.8 kJ mol-1).22 Crucially for the -NC channel, a
+29.5 kJ mol-1 transition state, relative to the energy of the
reactants, separates the C6H6-NC complex and the C6H5NC +
H products. This result suggests that the channel leading to NC
products (that is, the formation of isocyanbenzene) would be
unfavorable at low temperatures. The rapid rate coefficients
measured here vary negligibly over 105-295 K, suggesting that
there are no bound prereactive complexes or any significant
back-dissociation of intermediate complexes. Stable prereactive
complexes, whose formation implies “submerged-reef” barriers,
have been identified in systems including OH + isoprene,42 OH
+ ethylene,43 and CN + C2H6.44 These reactions exhibit
pronounced negative temperature dependencies down to low
temperatures (<100 K). The temperature dependence of the
measured rate data appears to rule out any significant presence
of a prereactive complex. Product detection studies are not
possible in the current Laval nozzle experiment. The room
temperature product detection studies of this reaction support
the conclusion that the isocyanobenzene product is not formed
in any significant quantities and that cyanobenzene + H is the
dominant product channel for the CN + benzene reaction.
The 105 K rate coefficient for the CN + toluene reaction is
compared to the CN + benzene rate in Figure 4. The measured
rate coefficient for CN + toluene is about a factor of 3 less
than the CN + benzene reaction. The aforementioned conclusion
for CN + benzene is that the reaction is rapid with close to
unity reaction efficiencies occurring Via CN addition channels.
For CN + toluene the room temperature product detection
results are consistent with CN addition to the CH carbons on
the ring of toluene as the main product channel. Although the
photoionization data cannot rule out production of benzyl
cyanide, this product seems mechanistically unlikely. At the
lower temperature of 105 K the barrierless addition of CN onto
the phenyl ring of toluene is likely to remain dominant. In order
to explain the lower rate coefficient compared to the CN +
benzene case, an interaction between CN and the toluene -CH3
group needs to be considered. No signal from benzyl radical
resulting from a H-abstraction process was recorded in the room
temperature product detection studies. At lower temperatures,
it is possible that a proportion of CN radicals can interact with
the -CH3 group of toluene where reaction may be less efficient
compared to ring addition. Computational studies exploring the
interaction of the CN radical and toluene would be informative
particularly with regard to interaction at the terminal methyl
group. For the CN + C2H6 reaction it was shown that a loosely
bound van der Waals complex along the H-abstraction channel
affected the kinetics down to as low as ∼150 K.44 In the case
of toluene, a dipole-dipole interaction may have a small effect
drawing the CN radical to the -CH3 group region where a van
der Waals complex could form but a barrier to H-abstraction
renders any reaction to be inefficient such that the complex
would eventually fall apart. It is also a possibility that a small
fraction of back dissociation of the intermediate adduct com-
plexes could contribute to reducing the observed rates. Whether
this reduction is due to dissociation from CN adducts at the
ipso position or interactions with -CH3 remains to be deter-
mined. These factors may contribute to the factor of ∼3 slower
rate coefficient at 105 K compared to the CN + benzene case
but more studies are required to reveal the dominant cause.
Smith et al. have proposed that, for a radical-neutral reaction,
the difference between the ionization energy (IE) of the neutral
closed-shell molecule and the electron affinity (EA) of the
radical coreactant (IE - EA) provides predictive insight into
the low-temperature reactivity of the reaction pair.45,46 For (IE
- EA) > 8.75 eV the reaction is likely to encounter a transition
state barrier with a positive activation energy (and thus be
negligibly slow at low temperatures), but in the case where (IE
- EA) < 8.75 eV this barrier is “submerged” with respect to
the energy of the reactants such that the reaction rate is governed
by long-range “capture” processes. In this second case the
reaction should remain rapid down to low temperatures (20 K).
In the two reactions of concern here, the high EA of the CN
radical (3.86 eV) and the comparatively low IEs of benzene
(9.24 eV) and toluene (8.83 eV) result in (IE - EA) , 8.75
eV. The fast rate coefficients measured in this work at 105 K
for both reactions conform to the predictions of Smith et al.
that radical-neutral reactions with (IE - EA) < 8.75 eV will
have rates governed by long-range, capture processes. Both
reactions are likely to remain rapid down to 20 K.
Generally, CN reactions with unsaturated hydrocarbons that
have rate coefficients on the order of ∼(3-4) × 10-10 cm3
molecule-1 s-1 at room temperature tend to have negligible
kinetic dependencies on temperature down to 100 K, and this
is the case of CN + benzene as shown in this study. The CN +
toluene reaction is also rapid at low temperature, but is notably
slower than CN + benzene at 105 K. This measurement is the
first low temperature rate reported for an alkyl benzene.
Computational studies as well as future experimental studies
with other radicals, including C2H + toluene and CH + toluene,
may reveal if the nature of the radical, including the dipole
magnitude and orientation, has any affect on the low-temperature
reactivity of toluene.
In the context of Titan chemistry, both reactions are rapid at
low temperature and therefore are viable under the conditions
of Titan’s atmosphere. Whether these reactions could contribute
to PANH and haze formation requires further study; there is
suggestion of a link between altitude-dependent concentrations
of nitriles and haze that would imply a chemical connection.47
With the significant quantities of benzene detected on Titan, in
addition to the presence of many nitrile species including HCN,
CH3CN, and C2N2 that can photolyze to produce CN,47 it is
plausible that cyanobenzene (mass ) 103 Da) is present.
5. Conclusion
The findings of this paper are summarized as follows. The
CN + benzene reaction has a rapid bimolecular rate coefficient
that remains relatively constant over the 105-295 K temperature
range. This result suggests that the reaction is without barrier
and highly efficient. Using synchrotron-multiplexed photoion-
ization mass spectrometry, the dominant product channel of this
reaction at room temperature leads to cyanobenzene + H with
no signal attributable to HCN formation, through either HCN
elimination or H-abstraction. The CN + toluene bimolecular

CN + Benzene and CN + Toluene Reactions
J. Phys. Chem. A, Vol. 114, No. 4, 2010 1755
(11) Smith, I. W. M. Angew. Chem. 2006, 45, 2842.
rate coefficient has been measured at 105 K. It is also fast but
approximately 3 times slower than the CN + benzene reaction
at this temperature. This difference may be due to back-
dissociation of adduct complexes or interactions with the -CH3
group. The room-temperature CN decay profiles measured with
toluene displayed a pronounced nonexponential form that
suggests back-dissociation of adduct complexes may be sig-
nificant at this temperature. The synchrotron photoionization
measurements reveal that cyanotoluene is the main product
channel with no measurable quantities of benzyl (C7H7) from a
HCN coproduct channel observed. There are definite implica-
tions for Titan chemistry, considering the extensive reports of
benzene and nitriles in the atmosphere of this body.
(12) Goulay, F.; Leone, S. R. J. Phys. Chem. A 2006, 110, 1875.
(13) Hamon, S.; Le Picard, S. D.; Canosa, A.; Rowe, B. R.; Smith,
I. W. M. J. Chem. Phys. 2000, 112, 4506.
(14) Hansmann, B.; Abel, B. ChemPhysChem 2007, 8, 343.
(15) Goulay, F.; Rebrion-Rowe, C.; Biennier, L.; Le Picard, S. D.;
Canosa, A.; Rowe, B. R. J. Phys. Chem. A 2006, 110, 3132.
(16) Goulay, F.; Rebrion-Rowe, C.; Le Garrec, J. L.; Le Picard, S. D.;
Canosa, A.; Rowe, B. R. J. Chem. Phys. 2005, 122.
(17) Sims, I. R.; Queffelec, J. L.; Travers, D.; Rowe, B. R.; Herbert,
L. B.; Karthauser, J.; Smith, I. W. M. Chem. Phys. Lett. 1993, 211, 461.
(18) Carty, D.; Le Page, V.; Sims, I. R.; Smith, I. W. M. Chem. Phys.
Lett. 2001, 344, 310.
(19) Bullock, G. E.; Cooper, R. Trans. Faraday. Soc. 1971, 67, 3258.
(20) Kaiser, R. I.; Balucani, N. Acc. Chem. Res. 2001, 34, 699.
(21) Zhang, F. T.; Kim, S.; Kaiser, R. I.; Jamal, A.; Mebel, A. M.
J. Chem. Phys. 2009, 130.
(22) Balucani, N.; Asvany, O.; Chang, A. H. H.; Lin, S. H.; Lee, Y. T.;
Kaiser, R. I.; Bettinger, H. F.; Schleyer, P. V.; Schaefer, H. F. J. Chem.
Phys. 1999, 111, 7457.
(23) Trevitt, A. J.; Goulay, F.; Meloni, G.; Osborn, D. L.; Taatjes, C. A.;
Leone, S. R. Int. J. Mass Spectrom. 2009, 280, 113.
(24) Vakhtin, A. B.; Lee, S.; Heard, D. E.; Smith, I. W. M.; Leone,
S. R. J. Phys. Chem. A 2001, 105, 7889.
(25) Yaws, C. L.; Narasimhan, P. K.; Gabbula, C. Yaws’ Handbook of
Antoine Coefficients for Vapor Pressure, 2nd electronic ed.; Knovel: New
York, 2009.
(26) Osborn, D. L.; Zou, P.; Johnsen, H.; Hayden, C. C.; Taatjes, C. A.;
Knyazev, V. D.; North, S. W.; Peterka, D. S.; Ahmed, M.; Leone, S. R.
ReV. Sci. Instrum. 2008, 79.
(27) Sims, I. R.; Smith, I. W. M. Chem. Phys. Lett. 1988, 151, 481.
(28) Sims, I. R.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 1993,
89, 1.
(29) Russell, J. A.; McLaren, I. A.; Jackson, W. M.; Halpern, J. B. J.
Phys. Chem. 1987, 91, 3248.
(30) Caballero, J. F.; Jackson, W. M.; Li, X. C.; Sayah, N. J. Photochem.
Photobiol. AsChem. 1989, 47, 41.
(31) Woon, D. E. Chem. Phys. 2006, 331, 67.
Acknowledgment. The support of personnel (A.J.T., F.G.)
for this research by the National Aeronautics and Space
Administration (grant NAGS-13339) is gratefully acknowledged.
Sandia authors and some of the instrumentation for this work
are supported by the Division of Chemical Sciences, Geo-
sciences, and Biosciences, the Office of Basic Energy Sciences,
the U.S. Department of Energy. Sandia is a multiprogram
laboratory operated by Sandia Corp., a Lockheed Martin Co.,
for the National Nuclear Security Administration under contract
DE-AC04-94-AL85000. The Advanced Light Source and
Chemical Sciences Division (S.R.L.) are supported by the
Director, Office of Science, Office of Basic Energy Sciences
of the U.S. Department of Energy under Contract No. DE-AC02-
05CH11231 at Lawrence Berkeley National Laboratory.
References and Notes
(32) Fally, S.; Carleer, M.; Vandaele, A. C. J. Quant. Spectrosc. Radiat.
Transfer 2009, 110, 766.
(33) Klasinc, L.; Kovac, B.; Gusten, H. Pure Appl. Chem. 1983, 55,
289.
(34) Araki, M.; Sato, S.; Kimura, K. J. Phys. Chem. 1996, 100, 10542.
(35) Young, V. Y.; Cheng, K. L. J. Electron Spectrosc. Relat. Phenom.
1976, 9, 317.
(36) Sveum, N. E.; Goncher, S. J.; Neumark, D. M. Phys. Chem. Chem.
Phys. 2006, 8, 592.
(37) Suzuki, K.; Ishiuchi, S.; Sakai, M.; Fujii, M. J. Electron Spectrosc.
Relat. Phenom. 2005, 142, 215.
(38) Uc, V. H.; Alvarez-Idaboy, J. R.; Galano, A.; Vivier-Bunge, A. J.
Phys. Chem. A 2008, 112, 7608.
(39) Uc, V. H.; Hernandez-Laguna, A.; Grand, A.; Vivier-Bunge, A.
Phys. Chem. Chem. Phys. 2002, 4, 5730.
(40) Eiden, G. C.; Weisshaar, J. C. J. Phys. Chem. 1991, 95, 6194.
(41) Houle, F. A.; Beauchamp, J. L. J. Am. Chem. Soc. 1978, 100, 3290.
(42) Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein,
S. J. J. Phys. Chem. A 2007, 111, 5582.
(1) Lavvas, P. P.; Coustenis, A.; Vardavas, I. M. Planet. Space. Sci.
2008, 56, 27.
(2) Lavvas, P. P.; Coustenis, A.; Vardavas, I. M. Planet. Space. Sci.
2008, 56, 67.
(3) Cui, J.; Yelle, R. V.; Vuitton, V.; Waite, J. H.; Kasprzak, W. T.;
Gell, D. A.; Niemann, H. B.; Muller-Wodarg, I. C. F.; Borggren, N.;
Fletcher, G. G.; Patrick, E. L.; Raaen, E.; Magee, B. A. Icarus 2008, 200,
581.
(4) Waite, J. H.; Niemann, H.; Yelle, R. V.; Kasprzak, W. T.; Cravens,
T. E.; Luhmann, J. G.; McNutt, R. L.; Ip, W. H.; Gell, D.; De La Haye, V.;
Muller-Wordag, I.; Magee, B.; Borggren, N.; Ledvina, S.; Fletcher, G.;
Walter, E.; Miller, R.; Scherer, S.; Thorpe, R.; Xu, J.; Block, B.; Arnett,
K. Science 2005, 308, 982.
(5) Waite, J. H.; Young, D. T.; Cravens, T. E.; Coates, A. J.; Crary,
F. J.; Magee, B.; Westlake, J. Science 2007, 316, 870.
(6) Coustenis, A.; Achterberg, R. K.; Conrath, B. J.; Jennings, D. E.;
Marten, A.; Gautier, D.; Nixon, C. A.; Flasar, F. M.; Teanby, N. A.; Bezard,
B.; Samuelson, R. E.; Carlson, R. C.; Lellouch, E.; Bjoraker, G. L.; Romani,
P. N.; Taylor, F. W.; Irwin, P. G. J.; Fouchet, T.; Hubert, A.; Orton, G. S.;
Kunde, V. G.; Vinatier, S.; Mondellini, J.; Abbas, M. M.; Courtin, R. Icarus
2007, 189, 35.
(43) Greenwald, E. E.; North, S. W.; Georgievskii, Y.; Klippenstein,
S. J. J. Phys. Chem. A 2005, 109, 6031.
(7) Niemann, H. B.; Atreya, S. K.; Bauer, S. J.; Carignan, G. R.;
Demick, J. E.; Frost, R. L.; Gautier, D.; Haberman, J. A.; Harpold, D. N.;
Hunten, D. M.; Israel, G.; Lunine, J. I.; Kasprzak, W. T.; Owen, T. C.;
Paulkovich, M.; Raulin, F.; Raaen, E.; Way, S. H. Nature 2005, 438, 779.
(8) Vuitton, V.; Yelle, R. V.; Cui, J. J. Geophys. Res. Planets 2008,
113.
(44) Georgievskii, Y.; Klippenstein, S. J. J. Phys. Chem. A 2007, 111,
3802.
(45) Sabbah, H.; Biennier, L.; Sims, I. R.; Georgievskii, Y.; Klippenstein,
S. J.; Smith, I. W. M. Science 2007, 317, 102.
(46) Smith, I. W. M.; Sage, A. M.; Donahue, N. M.; Herbst, E.; Quan,
D. Faraday Discuss. 2006, 133, 137.
(9) Mebel, A. M.; Kislov, V. V.; Kaiser, R. I. J. Am. Chem. Soc. 2008,
130, 13618.
(47) Bezard, B. Philos. Trans. R. Soc. A 2009, 367, 683.
(10) Frenklach, M. Phys. Chem. Chem. Phys. 2002, 4, 2028.
JP909633A