Rate Constants of the CN + Toluene Reaction from 15 to 294 K and Interstellar Implications

Article abstract of DOI:10.1021/acs.jpca.0c06900

CN is known for its fast reactions with hydrocarbons at low temperatures, but relatively few studies have focused on the reactions between CN and aromatic molecules. The recent detection of benzonitrile in the interstellar medium, believed to be produced by the reaction of CN and benzene, has ignited interest in studying these reactions. Here, we report rate constants of the CN + toluene (C7H8) reaction between 15 and 294 K using a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme; reaction kinetics in uniform supersonic flow) apparatus coupled with the pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique. We also present the stationary points on the potential energy surface of this reaction to study the available reaction pathways. We find the rate constant does not change over this temperature range, with an average value of (4.1 ± 0.2) × 10-10 cm3 s-1, which is notably faster than the only previous measurement at 105 K. While the reason for this disagreement is unknown, we discuss the possibility that it is related to enhanced multiphoton effects in the previous work.

Full text of DOI:10.1021/acs.jpca.0c06900

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A: Kinetics, Dynamics, Photochemistry, and Excited States
Rate Constants of the CN + Toluene Reaction
from 15 – 294 K and Interstellar Implications
Joseph Messinger, Divita Gupta, Ilsa Rose Cooke, Mitchio Okumura, and Ian Richard Sims
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.0c06900 • Publication Date (Web): 02 Sep 2020
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Rate Constants of the CN + Toluene Reaction from
15 – 294 K and Interstellar Implications
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Joseph P. Messinger,a,b Divita Gupta,b Ilsa R. Cooke,b Mitchio Okumura,a,* Ian R. Simsb,*
a
Arthur Amos Noyes Laboratory of Chemical Physics, Division of Chemistry and Chemical
Engineering, California Institute of Technology, 1200 East California Boulevard, Pasadena,
California 91125, United States
b Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) - UMR 6251, F-35000 Rennes, France
*Corresponding authors: MO: mo@caltech.edu, IRS: ian.sims@univ-rennes1.fr
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ABSTRACT
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CN is known for its fast reactions with hydrocarbons at low temperatures, but relatively few studies
have focused on the reactions between CN and aromatic molecules. The recent detection of
benzonitrile in the interstellar medium, believed to be produced by the reaction of CN and benzene,
has ignited interest in studying these reactions. Here, we report rate constants of the CN + toluene
(C7H8) reaction between 15 and 294 K using a CRESU (Cinétique de Réaction en Ecoulement
Supersonique Uniforme; reaction kinetics in uniform supersonic flow) apparatus coupled with the
pulsed laser photolysis – laser induced fluorescence (PLP-LIF) technique. We also present the
stationary points on the potential energy surface of this reaction to study the available reaction
pathways. We find the rate constant does not change over this temperature range, with an average
value of (4.1 ± 0.2) × 10-10 cm3 s-1, which is notably faster than the only previous measurement at
105 K. While the reason for this disagreement is unknown, we discuss the possibility that it is
related to enhanced multiphoton effects in the previous work.
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Introduction
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The CN radical has long been known to be abundant in the interstellar medium (ISM), where it
was first detected in 1940s,1-2 and in the atmosphere of Titan, where it leads to the formation of
nitrile compounds, including C2H3CN and HC3N.3-5 At the low temperatures found in these
environments, reactions between CN and other compounds are known to have fast rate constants,
on the order of 10-10 cm3 s-1, and thus must be included in gas-phase models. Reactions between
CN and hydrocarbons are among the fastest of these rates, and proceed through either an
abstraction or an addition mechanism.6 A number of cyano-containing molecules linked to these
reactions have been found in ISM, including molecules as large as HC9N and (CH3)2CHCN.7,8 On
Titan, the photolysis of these nitrile compounds formed from CN reactions may contribute to the
formation of particulate matter.9
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On Titan, toluene has been detected in the upper atmosphere by the Cassini Ion and Neutral Mass
Spectrometer,10 and while it is currently undetected in the lower atmosphere, models suggest it has
a high abundance there as a product of the fast association reaction between C6H5 and CH3.11 In
addition to benzene and toluene, a large number of specific polycyclic aromatic hydrocarbons
(PAHs) have also been identified.12 These PAHs are believed to form from smaller aromatic
compounds and to be an important component of the thick haze in Titan’s atmosphere.13
On the other hand, very few specific aromatic molecules have been directly detected in the ISM,
in part due to their low dipole moments making them difficult to observe by radio astronomy.
Benzene has been observed through infrared observations,14 and PAHs are widely believed to be
abundant in the ISM owing to observations of the strong infrared bands characteristic of these
molecules, but the broad, overlapping nature of these bands has precluded the identification of
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specific PAHs.15 Despite this lack of definitive identifications, PAHs are believed to hold a large
fraction of carbon in the ISM,16-17 and so their formation has been the subject of extensive study.18-
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The formation of the first aromatic ring is believed to be the rate limiting step in PAH
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formation,21 and so understanding the chemistry of these monocyclic aromatic compounds is
necessary.
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The recent detection of benzonitrile in the ISM22 has suggested a new route for understanding
aromatic formation. Benzonitrile is believed to be the product from the CN + benzene reaction,
which we have recently shown to be fast at low temperatures,23 with a rate constant of (4.4 ± 0.2)
× 10-10 cm3 s-1 over the 15 – 294 K range, in good agreement with earlier work by Trevitt et al.
down to 105 K.24 Observations of benzonitrile therefore can be used as a proxy for the abundance
of benzene. Other aromatic nitriles may also serve as proxies for undetected aromatic compounds
in the ISM, as the addition of the cyano moiety gives these compounds large dipole moments and
makes them visible to radio astronomy. The identification of additional specific aromatic
compounds would significantly constrain models of PAH formation in the ISM.
However, little is known about the reaction of CN with other aromatic molecules and whether
these reactions also result in the formation of nitrile compounds. Only one rate constant for the
reaction between CN and toluene, one of the simplest aromatics, has been measured, by Trevitt et
al.24 They studied this reaction at 105 K and found a rate constant of (1.3 ± 0.3) × 10-10 cm3 s-1 using
pulsed laser photolysis – laser induced fluorescence (PLP-LIF) measurements in conjunction with
a pulsed Laval nozzle. This rate constant is a factor of 3 lower than the rate constants measured
for the CN + benzene reaction measured in the same study as well as our own,23,24 suggesting that
the structure of aromatic molecules can play a large role in the reaction rates. Furthermore, they
observed non-exponential decays of CN at room temperature in the presence of toluene and were
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therefore unable to measure a rate constant, in contrast with their measurements of benzene under
the same conditions. They suggested that this could be due to dissociation of the products back to
the CN + toluene reactants, and that further studies would be necessary to better understand these
results.
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The difference between the rate constants of the CN + benzene and CN + toluene reactions would
seem to suggest that the structure of an aromatic compound can play a large role in the reaction
dynamics. This makes it questionable whether nitrile compounds may be formed from the
reactions of CN with larger, more complex aromatic compounds, and it is essential to verify the
reliability of using cyano-substituted compounds as a proxy for larger aromatic species. To that
end, we have conducted measurements of the CN + toluene rate constant between 16 and 294 K
to gain further insight into this reaction, especially at the low temperatures relevant to the ISM and
Titan. Furthermore, we have computed stationary points on the potential energy surface (PES) of
the CN + toluene reaction to better understand the possible products and mechanism of this
reaction.
Experimental Section
Rate constants were determined using the PLP-LIF technique. Temperatures down to 15 K were
achieved using the CRESU technique (Cinétique de Réaction en Ecoulement Supersonique
Uniforme; reaction kinetics in uniform supersonic flow), which has been described in detail
previously.6, 25-26 Briefly, toluene (Sigma Aldrich, 99.9%) was introduced into the gas flow with a
Controlled Evaporation and Mixing system (Bronkhorst CEM), as described in Gupta et al.27 The
toluene and ICN (Acros Organics, 98%), used as the CN precursor, were mixed in a buffer gas of
He (99.995%, Air Liquide), Ar (99.998%, Air Liquide) or N2 (99.995%, Air Liquide), depending
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on the desired CRESU conditions. Concentrations of toluene and ICN were kept < 1% of the total
density in order not to affect the uniformity of the gas flow. The mixture was flowed isentropically
from a high pressure reservoir through specifically designed convergent-divergent Laval nozzles,
into a low pressure chamber to generate a uniform supersonic flow at the appropriate temperature
with a density of 1016 – 1017 cm-3. Each nozzle was characterized with Pitot probe impact pressure
measurements prior to experiments to determine the temperature, density and uniformity of the
gas flow. For measurements at 294 K, where a supersonic expansion is not required, the pumping
speed was decreased such that the pressure in the reservoir and the chamber were equal, while
maintaining complete gas turnover for each laser shot.
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CN radicals were generated by the 248 nm photolysis of ICN using a KrF excimer laser (Coherent
LPXPro 210) operating at 10 Hz, with a laser fluence of 25 mJ cm-2. The third harmonic of a
Nd:YAG laser (Continuum, Powerlite Precision II), also operating at 10 Hz, was used to pump a
dye laser (Laser Analytical Systems, LDL 20505) containing Exalite 389 (Exciton) in 1,4-dioxane
(Sigma Aldrich, 99.8%) to produce ∼389 nm light to excite the CN B2Σ+ - X2Σ+ (0,0) transition.
Fluorescence was detected from the CN (0,1) transition at ∼420 nm by a photomultiplier tube
(Thorn EMI 6723) preceded by a 420 nm bandpass filter (Ealing Optics). The delay time between
the excimer and the Nd:YAG pump laser was varied from -5 to 200 microseconds to record the
time dependence of the CN signal. The LIF signal was recorded by a gated integrator for 400
evenly spaced points and averaged 5 times. The resulting kinetic trace was fit to an exponential
decay starting ≥ 10 μs after photolysis to allow for rotational thermalization of CN.
Kinetic measurements were taken under pseudo-first order conditions with [toluene] >> [CN].
Typical toluene concentrations were on the order of 1012 – 1013 cm-3, while we estimate the CN
concentration to be roughly 1010 cm-3 based on the ICN concentration (∼1012 cm-3) and 248 nm
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absorption cross section of 4.7 × 10-19 cm2.28 More than 90% of the CN radicals from the photolysis
of ICN at 248 nm are in the ground vibrational state29 and we do not observe any influence of the
relaxation of excited vibrational states on our kinetic measurements.
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Computational Details
Investigation of possible channels for the reaction between CN and toluene, with identification of
all the stationary points (minima, complexes, and transition states), was done using Gaussian 09
software30; all included channels can be seen in Figure 1. All the species, including the reaction
complexes and transition states were optimized at (U)M06-2X/aug-cc-pVTZ level31-33 and zero-
point corrected energies were calculated for each. In addition, intrinsic reaction coordinate (IRC)
calculations were performed at (U)M06-2X/6-311G to determine the minimum energy path that
the transition states followed to confirm the connection between the appropriate reactants and
products. Gibbs energies (∆rGº (298 K)) for all included channels were also calculated at (U)M06-
2X/aug-cc-pVTZ method. Both addition-elimination channels, leading to nitrile formation, and
abstraction channels, leading to HCN formation, are considered. While reactions involving the CN
radical may produce both cyano- and isocyano- compounds, only the former pathways are
considered in these calculations. Previous work on the CN + benzene reaction34 showed a
significant barrier (28 kJ mol-1) to isocyano products, which suggests that this pathway will not be
relevant in the ISM.
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Figure 1: The reaction pathways of the CN + toluene reaction and possible products of the
abstraction (R1-R4) and CN-addition (R5-R8) channels that are considered in the theoretical
calculations.
Results
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Figure 2: Typical experimental kinetics of the CN radical measured using PLP-LIF, showing the
decay of the CN signal and resulting second-order plot at 83 K.
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Figure 3: Typical experimental kinetics of the CN radical measured using PLP-LIF, showing the
decay of the CN signal and resulting second-order plot at 294 K.
Typical LIF decays of CN at 83 K and 294 K and the second-order plots can be seen in Figures 2
and 3, respectively. The non-zero intercepts seen on the second-order plots arise from the loss of
CN via side chemistry and diffusion out of the region probed by LIF. From experiments at room
temperature, using N2 as a buffer gas and varying the total density of the gas flow, we found that
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the rate constants had no pressure dependence, implying that the reaction is either a bimolecular
reaction, or a termolecular reaction in the high pressure limit in our experimental conditions.
Unlike the room temperature measurements of Trevitt et al., we see no evidence for non-
exponential decays at any toluene concentration or total gas density used in these experiments, and
the measured rate constants are in good agreement with our values at all other temperatures.
Additional experiments at room temperature demonstrate that changing the buffer gas from N2 to
He does not affect our measured rate constants.
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Results of the experiments between 15 and 294 K are shown in Table 1 and Figure 4. At least nine
points with varying toluene concentrations were taken for each measurement under pseudo-first
order conditions, with toluene in excess. At high reactant concentrations, the formation of toluene
dimers causes nonlinear behavior in the second-order plots at the lowest temperatures. This
therefore imposes an upper limit on the toluene concentration used in experiments in order to
minimize any effect of the reaction between CN and toluene dimers on our measurements.
Table 1: Rate coefficients determined for the CN + toluene reaction between 15 and 294 K, along
with experimental parameters for each measurement. Uncertainties in the rate constant are the 95%
confidence interval from the appropriate Student’s t test combined in quadrature with a 10%
systematic error. Bolded values represent the weighted average and uncertainty for temperatures
with multiple measurements.
Temperature Buffer Total Density (1016 [Toluene] (1012 Number of Rate Constant (10-
(K)
15
24
36
36
Gas
He
He
He
He
cm-3)
5.04
4.83
5.27
5.32
cm-3)
Points
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14
10 cm3 s-1)
4.4 ± 0.8
4.7 ± 0.6
5.9 ± 0.8
4.9 ± 1.0
5.7 ± 0.7
4.4 ± 0.7
1.87 – 10.4
1.76 – 19.5
1.25 – 17.7
1.28 – 12.7
9
70
He
6.00
2.54 – 15.2
11
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70
He
6.09
1.34 – 18.7
13
4.2 ± 0.7
4.3 ± 0.6
3.9 ± 0.5
3.8 ± 0.4
3.9 ± 0.4
3.3 ± 0.5
3.7 ± 0.6
3.7 ± 0.6
4.3 ± 0.6
4.3 ± 0.5
4.5 ± 0.5
4.3 ± 0.5
83
83
N2
N2
4.63
4.63
3.48 – 48.8
1.72 – 31.3
14
14
110
197
294
294
294
294
Ar
N2
N2
N2
N2
He
2.71
5.32
10.5
3.75
8.20
9.41
1.28 – 14.1
2.13 – 21.4
15.4 – 92.9
9.50 – 47.6
17.7 – 78.9
18.0 – 54.0
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To test whether the photolysis of toluene at 248 nm affected our measurements, we also conducted
experiments varying the power of the excimer laser at 110 K, with [Toluene] = 9 × 1012 cm-3. We
found no significant change in the measured k1st as a function of our laser power. With the excimer
laser fluence of 25 mJ cm-2 and the toluene absorption cross section at 248 nm of 2.9 × 10-19 cm2,35
we expect roughly 1% of the toluene to photolyze if the photolysis quantum yield is 1, which
should not measurably affect the observed rate constants. It has also been suggested that two-
photon absorption at 248 nm can photolyze toluene to form H atoms among other potential
processes,36 with the total absorption cross section for the second photon experimentally
determined to be 1.7 × 10-17 cm2. For the highest toluene concentrations used (~1 × 1014 cm-3), we
estimate at most 4.8 × 1011 cm-3 of toluene undergoes two-photon absorption, though it is likely
much less than that, as discussed in greater detail in the following section, and is unlikely to affect
the rate constants measured.
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Figure 4: The experimental measurements for the CN + toluene rate constant from this work
(circles) and Trevitt et al. (triangle); the weighted average value of the measurements presented in
this work, 4.1 × 10-10 cm3 s-1, is also plotted.
Table 2: Zero-point corrected reaction energies and Gibbs energies of the reaction products
calculated in this work. Note that some reaction pathways result in the same products.
Reaction energy ∆rUº
Gibbs energy ∆rGº (298 K) (kcal
Reaction channel products
(kcal mol-1)
-18.1
mol-1)
-18.1
-18.6
-18.2
-38.1
-23.6
-35.5
-23.6
-22.5
-23.8
-35.5
R1 (o-C6H4CH3 + HCN)
R2 (m-C6H4CH3 + HCN)
R3 (p-C6H4CH3 + HCN)
-18.3
-17.8
-39.1
-28.0
-35.4
-28.0
-26.8
-27.2
R4 (C6H5CH2 + HCN)
R5a ( = R5c, o-C6H4CH3CN + H)
R5b ( = R8, C6H5CN + CH3)
R5c ( = R5a, o-C6H4CH3CN + H)
R6 (m-C6H4CH3CN + H)
R7 (p-C6H4CH3CN + H)
R8 ( = R5b, C6H5CN + CH3)
-35.4
As shown in the Figure 5, both stationary points (reactants, products, intermediates, transition
states) for both the abstraction (R1-R4) and addition-elimination (R5-R8) channels were
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characterized for the reaction between CN and toluene. An additional substitution channel, leading
to the formation of benzyl cyanide, was found to be exothermic at (U)M06-2X/aug-cc-pVTZ, but
has a large barrier (~20 kcal mol-1), and hence will not be relevant under interstellar conditions and
is excluded. The relative reaction energy ∆rUº and Gibbs energy at 298 K ∆rGº for all calculated
product channels can be seen in Table 2. Intermediates formed from the addition of CN to the
aromatic ring were found to form barrierlessly, subsequently followed by submerged barriers
leading to the formation of stable nitrile products. This mechanism closely resembles the
mechanism of benzonitrile formation from the reaction of benzene and CN,34, 37 although it does
differ from the reaction between toluene and OH, which features both pre-reactive complexes and
barriers before formation of the addition product.38 The energies determined for the addition-
elimination channels are generally similar to those calculated for the CN + benzene reaction done
at G3//B3LYP and BCCSD(T)//B3LYP.34
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Abstraction pathways, shown in blue in Figure 5, were found to have slightly submerged transition
states, and therefore are possible products at low temperatures. However, higher level calculations
are needed to confirm these barrier values, as similar abstraction pathways from aromatic
compounds have been shown to possess positive barriers37-38. At the level of theory used, these
barrier values are likely within the error of the calculations. An important point to note is that no
transition state or complex could be characterized for the abstraction channels, though we do not
rule out the existence of these stationary points.
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Figure 5: The stationary points for PES for the CN + toluene reaction, performed at (U)M06-
2X/aug-cc-pVTZ including zero-point energy corrections, showing the abstraction (abs-)
pathways (blue) and addition-elimination (add- and elim-) pathways (red), the latter of which can
undergo an internal hydrogen shift (-bridge). Note that neither a barrier leading to the formation
of C6H5CH2 + HCN nor any pre-reactive complexes for the abstraction pathways are included,
though we do not preclude their existence.
Discussion
As Figure 4 demonstrates, we find that the rate constant of the CN + toluene reaction is independent
of temperature over the 15 – 294 K range, with a weighted average value of (4.1 ± 0.2) × 10-10 cm3
s-1. Our results are in contrast to the results of Trevitt et al., who measured a rate constant of (1.3
± 0.3) × 10-10 cm3 s-1 at 105 K in their LIF experiments. They used a similar LIF method to detect
CN, under similar experimental conditions of total density, and CN and toluene concentrations.
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Furthermore, this group measured CN + benzene rate constants that agree well with recent results
from our group,23 which suggests that this discrepancy is related to the toluene system.
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Trevitt et al. reported observing non-exponential decays of CN at room temperature, which they
suggested might be due to back-dissociation of adduct complexes. However, no such behavior was
observed in this work, suggesting that the discrepancy might have arisen from differences in the
photolysis step. Trevitt et al. photolyzed their sample at a wavelength of 266 nm, with a laser
fluence of 40 mJ cm-2 (5.0 ´ 1016 photons cm-2, in a probable 3-6 ns long pulse), in contrast to the
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248 nm laser beam with a fluence of 25 mJ cm-2 (3.1 ´ 1016 photons cm-2, 22 ns long pulse) used
in this work. The ICN photolysis cross sections are similar at these two wavelengths.28, 39 . At room
temperature, the toluene absorption cross section to the S1 state at 266 nm is 1.3 ´ 10-19 cm2.40 The
S1 state fluoresces with a lifetime of 86 ns when excited at 266 nm at low pressures.41 At 248 nm,
the cross section is larger (2.9 ´ 10-19 cm2)40 but the fluorescence lifetime is much shorter due to
rapid internal conversion to S0, displaying approximately equal intensity 3 ns and 26 ns
components at low pressures.42 As discussed above, multiphoton absorption at 248 nm of toluene
is known to lead to photolysis,36 and may additionally lead to photoionization, as the excited
toluene is higher in energy than the ionization onset of toluene (8.3 eV).43 Both of these processes
are also likely to occur in the 266 nm experiments.
The above considerations suggest that single photon excitation of toluene is occurring at both
photolysis wavelengths, but only multiphoton effects are likely to give rise to interferences.
Possible pathways for CN generation on the timescale of the experiment exist by either two-photon
photodissociation or two-photon ionization, particularly at 266 nm by quadrupled Nd:YAG lasers.
Such effects are likely to be significantly lower when toluene is excited at 248 nm light produced
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by an excimer, due to the longer pulse duration and rapid internal conversion of the S1 state. This
is in good agreement with the experimental measurements reported here, showing no relationship
between excimer power and k1st, and no evidence for non-exponential decays. In the experiments
of Trevitt et al. at 266 nm, however, the long lifetime of the S1 state, short photolysis pulse duration
and higher laser fluence may have caused larger amounts of multi-photon absorption to occur,
such that photodissociation or photoionization products could have affected their measurements
for toluene. Such effects would not be observed upon 266 nm excitation of benzene, however, as
266 nm lies below the absorption threshold for the benzene S1 state and so two-photon dissociation
would have to occur by a non-resonant process.
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Further work from Trevitt et al. measured branching ratios at room temperature, using slow flow
reactors in conjunction with product detection by multiplexed photoionization mass spectrometry
(MPIMS) to identify species by mass and photoionization spectrum. They found that the reaction
between CN and toluene exclusively forms tolunitrile (methylbenzonitrile), with no evidence for
the hydrogen abstraction channels or for benzonitrile formation. Due to the similarities in the
calculated photoionization spectra of the ortho-, meta-, and para- isomers of the tolunitrile, they
were unable to distinguish the precise isomers of tolunitrile formed from this reaction. They found
a similar result for the reaction of CN with benzene, with benzonitrile being the only detected
product. Lenis and Miller also measured the products of the CN + toluene reaction using the 254
nm photolysis of ICN and analyzing the resulting products with GC-MS44 and observed both
tolunitrile and a small yield (9%) of benzonitrile. While it is unclear if this benzonitrile is formed
as a result of CN + toluene or side chemistry, particularly in light of its non-detection in the MPIMS
experiments by Trevitt et al., our calculations do show potential pathways to benzonitrile formation
from ortho- or ipso- addition of CN to the aromatic ring.
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The rate constants measured here are in good agreement with the value of (4.4 ± 0.2) × 10-10 cm3
s-1, independent of temperature over 15 – 295 K range, that we determined for the reaction between
CN and benzene.23 In conjunction with the similarities in products measured by MPIMS, this
suggests that the major mechanism is the same for reactions between the CN radical and either
benzene or toluene, and results in formation of cyano-substituted aromatic compounds.
Investigation of other substituted compounds, such as xylenes or deuterium-substituted benzene,
may yield further insight into whether this mechanism is general for these reactions. This will aid
in future astronomical searches to improve our understanding of the formation of small aromatic
rings in the ISM.
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The submerged barriers found for the various channels using quantum chemical calculations
highlight the diversity of the products that could be formed from this reaction at low temperatures.
While the abstraction channels were found to have slightly submerged barriers, calculations at
higher level of theory are necessary to correctly estimate their energies. Furthermore, the transition
state(s) and/or a possible complex in the case of the hydrogen abstraction from the methyl group
pathway remain to be explored further. This will also provide the accuracy necessary for master
equation calculations, which would further elucidate the mechanism and product branching ratios
of this reaction.
On Titan, the CN radical is mainly generated from the photolysis of HCN, which is formed through
reactions of N(4S) or through ion chemistry.4-5 Once formed, CN reacts primarily with the highly
abundant CH4 to reform HCN. This cycle can be interrupted, however, by CN reactions with other
compounds, most commonly C2H2 or HC3N. While this reaction has not explicitly been included
in models, recent work has suggested that the concentrations of benzene and toluene in the Titan
atmosphere are similar, peaking at a mole fraction of 10-6 at an altitude of roughly 1000 km above
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the surface.11 Benzonitrile has not been detected on Titan and is predicted to be formed in low
quantities, largely due to CN being sequestered by reaction with CH4. Even with the larger rate
constants measured in this work, this is likely also the case for the products of the reaction between
CN and toluene, though implementation of these results into Titan models may still be beneficial
to determine if they have any influence in the atmosphere.
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Astronomical searches for toluene and the tolunitrile products of this reaction would test the
robustness of using cyano-containing species as proxies for the unsubstituted hydrocarbons. While
benzene has no permanent dipole moment, toluene has a small one (0.37 Debye)45 and may be
observable via radio astronomy, thought it would have to be present in higher abundance than, say
benzonitrile, to be detectable. The use of velocity stacking, which averages the signal of multiple
transitions together to increase the signal-to-noise ratio,46-48 may assist in searching for toluene in
the ISM. While there have been no previous detections of toluene in the ISM, it has been argued
that the protonated toluene ion, C7H9+,49 and methyl-substituted PAHs50-51 are possible carriers of
the 6.2 and the 3.4 μm unidentified infrared bands, respectively. Definitive detection of toluene
and related compounds, such as these, would allow us to constrain aromatic pathways and, in
particular, could test the bottom-up mechanism for PAH formation, wherein small molecules, such
as toluene, react progressively to form large clusters.
The origin of the first aromatic ring in the interstellar medium in molecules such as benzene and
toluene remains unknown. It has been argued that the reaction between C2H and isoprene (C5H8;
2-methyl-1,3-butadiene), a barrierless reaction that produces toluene, may be a source of it at low
temperatures,52 but it is unknown whether isoprene is present in the ISM – isoprene is not included
in astrochemical databases such as the Kinetic Database for Astrochemistry53 (kida.obs.u-
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bordeaux1.fr, accessed July 2020). Other mechanisms, such as ion-neutral reactions, may also
contribute, but further investigation is necessary.
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In order to better understand the potential formation pathways of these products of this reaction in
the ISM, more accurate measurements of the product ratios are required, and specifically, the
branching ratio for the tolunitrile and potential benzonitrile products. While challenging for many
techniques due to the similarities of the isomers, recent work has coupled low temperature
supersonic uniform flows to microwave spectrometers54-55 in order to determine branching ratios
relevant for astrochemistry. As each of these compounds will have a unique rotational spectrum,
this technique is well suited for quantitatively measuring the product branching ratio of this
reaction.
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Conclusions
We have measured rate constants for the CN + toluene reaction between 15 and 294 K and find
that the rate constant is independent of temperature over this range. These results closely match
our recent study on the reaction of CN with benzene but are higher than the only previous
measurement of this rate constant at 105 K, for reasons that remain unresolved, but may be related
to multiphoton effects at the higher laser intensities and 266 nm photolysis wavelength used in that
study. This similarly suggests that the reactions between CN and simple aromatics proceed through
an analogous mechanism, which is supported by our theoretical calculations and previous product
measurements. Further work, particularly on the products formed from this reaction, would be
beneficial to determine their potential detectability in the ISM. The ability to detect and use cyano-
substituted aromatics, which have large dipole moments, as proxies for unsubstituted aromatic
compounds in the ISM would help advance our knowledge of PAH formation.
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Supporting Information
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Geometries of calculated structures for all molecules and transition states
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Acknowledgments
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The authors thank Jonathan Courbe, Jonathan Thiévin, Didier Biet, Ewen Gallou and Alexandre
Dapp for technical support. We would like to acknowledge Mayank Saraswat for helpful
discussions regarding the PES calculations. J. P. M. was supported by the National Science
Foundation Graduate Research Fellowship (NSF GRP) and the National Science Foundation
Graduate Research Opportunities Worldwide (NSF GROW) programs. J. P. M. would also like to
thank the Office of Science and Technology of the Embassy of France in the United States for a
Chateaubriand Fellowship. The authors acknowledge funding from the European Union’s Horizon
2020 research and innovation programme under the European Research Council (ERC) grant
agreement 695724-CRESUCHIRP and under the Marie Skłodowska-Curie grant agreement
845165-MIRAGE. Acknowledgment is made to the donors of The American Chemical Society
Petroleum Research Fund for partial support of this research. The authors are also grateful for
support from the European Regional Development Fund, the Region of Brittany and Rennes
Metropole. This work was supported by the French National Programme “Physique et Chimie du
Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES.
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