Isotope-specific reactions of acetonitrile (CH3CN) with trapped,translationally cold CCl+

O. A. Krohn,1, 2, a) K. J. Catani,1, 2 J. Greenberg,1, 2 S. P. Sundar,3 G. da Silva,3 and H. J. Lewandowski1, 21)Department of Physics, University of Colorado, Boulder, Colorado, USA2)JILA, National Institute of Standards and Technology and the University of Colorado, Boulder, Colorado,USA3)Department of Chemical Engineering, The University of Melbourne, Parkville 3010, Victoria,Australia

(Dated: 23 November 2020)

The gas-phase reaction of CCl+ with acetonitrile (CH3CN) is studied using a linear Paul ion trap coupledto a time-of-flight mass spectrometer. This work builds on a previous study of the reaction of CCl+ withacetylene1 and further explores the reactivity of CCl+ with organic neutral molecules. Both of the reactantspecies are relevant in observations and models of chemistry in the interstellar medium (ISM). Nitriles, inparticular, are noted for their relevance in prebiotic chemistry, such as is found in the atmosphere of Titan, oneof Saturn’s moons. This work represents one of the first studied reactions of a halogenated carbocation witha nitrile, and the first exploration of CCl+ with a nitrile. Reactant isotopologues are used to unambiguouslyassign ionic primary products from this reaction: HNCCl+ and C2H3

+. Branching ratios are measured andboth primary products are determined to be equally probable. Quantum chemical and statistical reaction ratetheory calculations illuminate pertinent information for interpreting the reaction data, including: reactionthermodynamics, a potential energy surface for the reaction, as well as rate constants and branching ratios forthe observed products. In particular, the reaction products and potential energy surface stimulate questionsregarding the strength and role of the nitrile functional group, which can be further explored with morereactions of this class.

I. INTRODUCTION

Nitriles and nitrogen-containing compounds play aprominent role in the chemical reactions thought to takeplace in the interstellar medium (ISM). These moleculespermeate space: from small cyanides such as HCN andDCN found in the Orion Nebula2,3 to larger moleculessuch as benzonitrile, whose initial discovery in the ISMwas relatively recent.4 Nitriles, defined by their C–––Nfunctional group, are of particular interest as pre-bioticmolecules and potential precursors of amino acids. Sev-eral nitriles have been identified in the atmosphere ofTitan using the Ion Neutral Mass Spectrometer on theCassini spacecraft, and are believed to be important intholin formation,5 as well as astrobiology.6

Acetonitrile (CH3CN; the neutral reactant in thisstudy) has been found abundantly throughout many re-gions of space since its initial identification in the ISMin 1971.7 It has been observed in cold dark clouds,8

low-mass protostars,9,10 and is considered an indica-tor of the presence of hot cores.11,12 CH3CN has alsobeen discovered in dust from comet Halley,13 Hale-Bopp(C/1995 O1)14 and, more recently, at the surface ofcomet 67P/Churyumov-Gerasimenko.15 These cometaryidentifications can yield critical glimpses into the pastconditions and evolutionary history of the Milky Way.Deuterated variants CD3CN and CDH2CN have beenidentified in hot cores and star-formation regions,16 and

a)Electronic mail: olivia.krohn@colorado.edu

the presence of isotopologues of CH3CN are used to studyrelative populations of hydrogen and deuterium in someregions of the ISM.17

Halogen-containing compounds have also been iden-tified in the ISM, but their role and evolution are lesswell understood. In particular, chlorine-containing com-pounds have been found in the ISM in several smallmolecules (NaCl, AlCl, KCl, HCl),18 as well as inCH3Cl

19 and H2Cl+.20,21 The only halogenated carboca-

tion to be observed thus far in the ISM is CF+,18 whereasCCl+ has been predicted to occur, although only in lowabundances.22 CCl+ can be produced from reactions ofC+ + HCl,23 and once formed, has been assumed to bepredominantly nonreactive. Specifically, CCl+ has beenshown to not react with HCN (or CO2, CO, O2, H2O,CH4, H2). However, it has been shown to react with NH3and H2CO.

24 Recent work from our group demonstratedCCl+ reacts with acetylene (C2H2), producing small fun-damental carbocations after losing neutral Cl or HCl.1

Despite this, much remains unknown about the role ofhalogenated carbocations; it is possible that they have ahitherto underestimated role in astronomical chemistry.

In contrast to CCl+, laboratory reactions of nitrileshave been much more widely studied. Ion cyclotronresonance (ICR) spectrometry has been used to mea-sure reactions with HCN and carbocations,25 whileother ion trap experiments have investigated reactionsof CH3CN with multiple carbocations.

26 Selected-ionflow-tube mass spectrometry (SIFT) experiments demon-strated reactivity of CH3CN with O

+, H+, D+, HeD+,and HeH+,27 as well as with C2H4

+,28 and C2H2+.29

However, very few measurements have reported reactions

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of halogenated carbocations with any nitrile. The onlyreported reaction of this type is the reaction of CF3

+ withCH3CN and benzonitrile, both of which were shown toproduce only the adduct.30 The reactions of CF3

+ wereexecuted in a higher pressure regime than that of thecurrent experiment, where reactive intermediates are un-able to be stabilized through collisions with backgroundgas. The reactivity of halogenated carbocations with ni-triles is in need of further exploration, particularly in acold, low-pressure environment. This work seeks to un-derstand more about this reaction class by studying thereaction of CCl+ + CH3CN in this regime.

The cold, low-pressure environment provided by us-ing a linear Paul ion trap (LIT) is excellent for elucidat-ing ion-neutral chemical reactions.31,32 This experimen-tal setup affords a significant amount of control, includ-ing the manipulation of collisional energy,33,34 nuclearspin,35 and the measurement of isomer,36,37 isotope,38,39

and quantum state40,41 dependencies. Ions of interest areco-trapped and sympathetically cooled with laser-cooledCa+, forming a mixed species Coulomb crystal, achiev-ing translationally cold, trapped ions. Furthermore, theaddition of a time-of-flight mass spectrometer (TOF-MS)provides detection of ionic reactants and products withhigh mass resolution – a powerful tool for probing reac-tion products and kinetics.

The reaction of sympathetically cooled CCl+ withCH3CN is studied using our LIT TOF-MS. This workseeks to illuminate the role and reactivity of these novelspecies in the gas phase under experimental conditionsthat are approximate to that of the ISM and planetaryatmospheres. The primary products are found to beC2H3

+ and HNCCl+, which are unambiguously assignedthrough the use of isotope substitutions. Computationalmodeling also supports these product assignments, sug-gesting a reaction pathway requiring cleavage of the C–––Nbond of CH3CN in order to form the observed products.Furthermore, the study of CCl+ + CH3CN signifies aninitial investigation in reactions of halogenated carboca-tions with nitriles.

II. METHODS

A. Experimental Methods

Reaction data were collected using a LIT radially cou-pled to a TOF-MS. Detailed descriptions of the appara-tus have been outlined previously,1,42 and only a briefsummary focusing on the specific details relevant to thecurrent experiment will be given here. CCl+ was pro-duced using tetrachloroethylene (TCE, C2Cl4) seededin a pulsed supersonic expansion of rare atomic gas(1.4% C2Cl4 in ∼1000 Torr He). The skimmed molec-ular beam was overlapped with a focused beam (216 nm)from a pulsed dye laser (LIOPTEC LiopStar; 10 ns pulse,100µJ/pulse) in the center of the trap. Non-resonantmultiphoton ionization of TCE resulted in several frag-

ments, including C35Cl+, C37Cl+, 35Cl+, 37Cl+, C2+,

and small amounts of C235Cl+ (hereafter, the more

abundant isotope 35Cl will be referred to as simply Cl,while 37Cl will be specified when appropriate). Unwantedions were ejected from the trap by sweeping over res-onance frequencies of the specific mass-to-charge ratio(m/z) of undesired ions.43 This provided a clean sampleof either CCl+ or C37Cl+ with minimal impurities, asdemonstrated in Fig. 1.

After removing unwanted ionization products from thetrap, Ca+ was loaded by non-resonantly photoionizing aneffusive beam of calcium using the third harmonic of anNd:YAG (Minilite, 10 Hz, ∼ 7 mJ/pulse). The result-ing Ca+ ions were Doppler laser cooled by two externalcavity diode lasers, forming a Coulomb crystal structure,which sympathetically cooled the co-trapped CCl+ ionsvia the Coulomb interaction. Ca+ ion fluorescence wascollected using a microscope objective and focused ontoan intensified CCD camera located above the trap, al-lowing for qualitative visual monitoring of the experi-ment. The heavier “dark” CCl+ ions arrange themselvesin outer shells around the Ca+ ions, deforming the flu-orescing Coulomb crystal as seen in Fig. 1b. A typi-cal experiment utilized 150-250 CCl+ ions trapped with∼ 1000 Ca+ ions, all of which were translationally cold(∼ 10 K).

10 15 20 25 30 35 40 45 50 55 60

CCl+

Ca+

a)

b)

mass (U)

FIG. 1. a) TOF traces demonstrating before and b) aftercleaning using secular excitations. After cleaning, only Ca+

(m/z 40, m/z 42, and m/z 44) and CCl+ (m/z 47) remain inquantities greater than ∼ 5 ions. Also included on the left isa false-color CCD image of fluorescing Ca+ ions, the resultingCoulomb crystal is deformed primarily in the center sectionby the heavier CCl+ ions. The crystal appears truncatedbecause it expands slightly beyond the CCD camera frame.

3

After CCl+ and Ca+ ions were loaded, neutral CH3CN(9-10% CH3CN or CD3CN in N2) was leaked into the vac-uum chamber (2× 10−9 Torr gas pressure at 300 K) for aset duration of time using a pulsed leak-valve scheme.40,44

The measurements of gas partial pressures in the cham-ber were recorded using a Bayard-Alpert hot cathode ion-ization gauge. The opening of the leak valve (LV) definedthe zero-time point; the LV remained open for 0, 10, 30,60, 90, 120, 150, 180, 210, 240, or 330 s before ejectingthe ions into the TOF-MS. This process was repeatedabout 10 times for every time step and measured ionnumbers from each mass were averaged over each timestep. The average number of reactant and product ionswere then normalized by the initial CCl+ numbers andplotted against time, forming a reaction curve. Thesereaction curves were then used to determine the rele-vant rate constants. Reaction curves were collected inthe same manner for isotopologues C37Cl+ and CD3CN,such that all four possible combinations of isotopologueswere used. The chemical formula of each mass peak wasconfirmed by examining the shift in mass spectra as aresult of isotopologue substitution (see section III B). Inaddition, all of the ionic species were tracked via TOF-MS traces. The total number of ions were compared foreach time point to ensure that the numbers were con-stant throughout the experiment; this ruled out system-atic losses of ions from the trap. Figures illustrating con-servation of charge over each reaction are given with morecontext in the supplementary material.

B. Computational Methods

Several theoretical methods were used to explore thepotential energy surface for the reaction of CCl+ +CH3CN. In a previous study, the M06-2X/aug-cc-pVTZlevel of theory was found to produce accurate geometriesand energies for small nitrogen- and chlorine-containingcompounds,45 and was therefore chosen to determinepossible stationary points. Scans over bond lengths,angles, and dihedrals allowed identification of minimaand saddle points. Transition states were verified byvisually inspecting the single imaginary frequency andalso by using intrinsic reaction coordinate (IRC) anal-ysis. The geometries of the reactants, products, in-termediate states, and transition states were then usedas starting points for calculations at the MP2/aug-cc-pVTZ level of theory. Zero point energy (ZPE) correc-tions from calculated harmonic vibrational frequencies(MP2/aug-cc-pVTZ) were added to CCSD(T)/CBS sin-gle point energies [CCSD(T)/CBS//MP2/aug-cc-pVTZnomenclature is used in the subsequent discussions]. Ad-ditional higher order calculations were carried out atthe CCSD(T)/CBS//CCSD/aug-cc-pVTZ level of the-ory for reactants and predicted products to provide ac-curate energetics for the thermodynamic limits of the re-action within 0.04 eV. Even though 37Cl and D isotopesubstitutions were used experimentally to determine the

chemical formulas of the products, calculations accom-modating these substitutions are outside the scope of thiswork. Density functional theory (DFT) calculations andrelaxed potential energy surface scans were done usingGaussian 16,46 while the higher order MP2 and CCSDcomputations were done using Psi4 v1.3.2.47

Statistical reaction rate theory calculations were per-formed to simulate the kinetics of the CCl+ + CH3CNreaction. These calculations were carried out using a cus-tom version of the MultiWell2020 suite of programs,48–50

modified to treat bath-gas collisions using the Langevinmodel. Simulations followed a general approach that wehave used extensively to investigate ion reaction dynam-ics in a diverse range of instruments, including ion trap,51

tandem,52 and ion mobility53 mass spectrometers. Elec-tronic energies, vibrational frequencies, and momentsof inertia were from the CCSD(T)/CBS//MP2/aug-cc-pVTZ model chemistry calculations. Microscopic rateconstants were calculated via Rice-Ramsperger-Kassel-Marcus (RRKM) theory, on the basis of rigid-rotorharmonic-oscillator sums and densities of state. For bar-rierless ion-molecule reactions, association rate coeffi-cients were set at the ADO theory value, with the re-stricted Gorin model54 then applied to fit an effectivetransition state structure. Energy grained master equa-tion simulations were performed in order to predict theCCl+ + CH3CN reaction products. These calculationsfeatured energy grains of 10 cm−1 and a single exponen-tial down collisional energy transfer model, with the av-erage energy in deactivating collisions set at 200 cm−1.55

Simulations comprised 1010 trajectories, and in each casea reaction was predicted to be complete within less thanthe time required for one bath-gas collision (i.e., effec-tively collisionless). Simulations were performed at apressure of 2×10−9 Torr N2, with temperature varied be-tween 40 and 400 K in order to examine predicted ratesfrom atmospheric down to astrochemically relevant con-ditions.

III. RESULTS & DISCUSSION

For the sake of clarity, the reaction thermodynamicswill be discussed with the concluded chemical formula as-signments in Section III A, followed by experimental sup-port in Section III B. Finally, in Section III C the mod-eled potential energy surface, branching ratios, and rateconstants of the reaction are discussed.

A. Reaction thermodynamics

Overall, the reaction of CCl+ + CH3CN forms the pri-mary ionic products C2H3

+ and HNCCl+, which proceedto react with excess CH3CN to form the secondary prod-uct protonated acetonitrile (CH3CNH

+). This model isillustrated in Fig. 2.

4

FIG. 2. Reaction model for CCl++ CH3CN, noting the re-action order and identity of ions. Each arrow represents areaction with a neutral CH3CN molecule. Red number belowthe molecule denotes m/z ratio. The molecular ions are de-picted above, with black indicating carbon, blue for nitrogen,white for hydrogen, and green for chlorine.

Neutral CH3CN was introduced into the vacuumchamber as a room temperature gas (300 K). There-fore, when reacting with translationally cold CCl+ (∼10 K), the calculated collision energy for the reactionis ∼ 15 meV (160 K). This provides a narrow upperlimit to the reaction energetics. The observed productsare all significantly exothermic and well below the up-per limit provided by the calculated collision energy, asshown by Equations 1-4 [CCSD(T)/CBS//CCSD/aug-cc-pVTZ; accurate within 0.04 eV].

Primary products:

CCl+ + CH3CN −−→ C2H3+ + NCCl∆E = −1.17 eV

(1)

CCl+ + CH3CN −−→ HNCCl+ + C2H2∆E = −2.09 eV

(2)

Secondary products:

C2H3+ + CH3CN −−→ CH3CNH+ + C2H2

∆E = −1.41 eV(3)

HNCCl+ + CH3CN −−→ CH3CNH+ + NCCl∆E = −0.48 eV

(4)

These calculated limits assume the lowest energy iso-mers. For example, in Eqns. 2 and 3, the C2H3

+ energyrefers to that of the non-classical “bridge” isomer (seeFig. 2 or PRD2 in Fig. 4). This non-classical isomer iswhere the third H hovers between the two carbons, as op-posed to the “classical” or “Y” structure (H2C2H

+, seePRD3). Other possible isomeric products are discussedin Section III C.

B. Reaction measurements

Curves that are produced from the reaction of CCl+ +CH3CN are shown in Fig. 3. Here, CCl

+ (m/z 47; blue)reacts to form two primary products: C2H3

+ (m/z 27;green) and HNCCl+ (m/z 62; black). The reduction ofthe CCl+ population (blue) is concurrent with the growthof C2H3

+ (green) and HNCCl+ (black). Both of the pri-mary product populations then reduce over time as thesecondary product CH3CNH

+ (m/z 42; red) populationgrows from reactions with excess CH3CN. CH3CNH

+ isconfirmed as a second order product because its maxi-mum slope coincides with the maximum number of pri-mary products. Experimental reaction rates are deter-mined by fitting the reaction data to a pseudo-first ordermodel. These curve fits are shown as lines in Fig. 3.Details of these fits are provided in the supplementarymaterial.

0 50 100 150 200 250 300time (s)

0

0.2

0.4

0.6

0.8

1

1.2

norm

aliz

ed io

n nu

mbe

rCCl+

C2H

3+

HNCCl+

CH3CNH+

FIG. 3. Rate reaction data (points) and fits (curves) forpseudo-first order reaction of CCl+ +CH3CN. CCl

+ (blue×)reacts with excess CH3CN resulting in first order productsC2H3

+ (green◦) and HNCCl+ (black∗). Each of theseprimary products then reacts with excess CH3CN to formCH3CNH

+ (red2).

The primary product mass assignments, namelyC2H3

+ and HNCCl+, given by the initial reaction ofCCl+ + CH3CN were verified by using different combina-tions of isotopologues. Specifically C37Cl+ (m/z 49) andCD3CN (m/z 44) were used to form four possible combi-nations of reactants. Reaction curves were measured foreach of the four unique pairs and mass peak shifts wererecorded for each case. Specifically, when the reactionproceeded with C37Cl+ + CH3CN, only one of the pri-mary products shifted, m/z 62→ 64 (HNC37Cl+), iden-tifying it as the only chlorine-containing product. In thecase of CCl+ + CD3CN, both primary products shifted:m/z 27 → 30 (C2D3+), and m/z 62 → 63 (DNCCl+).Furthermore, the secondary product shifted, m/z 42 →46 (CD3CND

+). In the final case, C37Cl+ + CD3CN,the mass shifts were consistent with the aforementioned

5

TABLE I. Rate constants for isotopological variations ofCCl+ + CH3CN primary products. ‘X’ represents a hydro-gen or deuterium from acetonitrile, and corresponds to theisotopologue used. Rates are in units of ×10−9 cm3/s, andreported statistical uncertainty is the calculated 90% confi-dence interval.

Reactants C2X3+ XNCCl+ total

CCl+ + CH3CN 1.6 ± 0.5 2.2 ± 0.5 3.8 ± 0.4C37Cl+ + CH3CN 2.9 ± 0.7 3.0 ± 0.7 5.9 ± 0.3CCl+ + CD3CN 2.4 ± 0.5 3.0 ± 0.5 5.4 ± 0.3C37Cl+ + CD3CN 2.9 ± 0.8 3.4 ± 0.8 6.3 ± 0.3

products. An additional process occurs in reactions in-volving CD3CN, which produces a small amount of atertiary product m/z 45, assigned to CD3CNH

+. Thistertiary process occurs possibly by either from H-D swap-ping or from contributions from a small number of con-taminant ions remaining from the initial ion loadingscheme (any given contaminant constitutes ≤ 5% of 150-250 initial CCl+ numbers). The isotopologue reactioncurves are plotted in the supplementary material. Ex-trapolated rate constants and branching ratios from thesereaction curves are provided in Tables I-III.

The measured rate constants for primary products ofCCl+ + CH3CN are reported in Table I. The Langevincapture model is a natural starting place for the analysisof experimental reaction rate constants, as it is the sim-plest and most general approach for predicting rate con-stants in this regime. Notably temperature-independent,this theory estimates the likelihood of collisions betweenan ion and a neutral nonpolar molecule. The Langevinrate constant was found to be k = 1.11 × 10−9 cm3/s,3-6 times smaller than the total reaction rate constant.This underestimation is most likely due to the polar na-ture of neutral CH3CN, which is not accounted for inLangevin theory. Average dipole orientation (ADO) the-ory expands on Langevin theory to account for the polar-ity of the neutral reactant and should show closer agree-ment with the measured total reaction rate constant.56

This is reflected in the fact that CH3CN has a ratherlarge dipole-locking constant (c) of ∼0.25, leading tokADO,unsub = 3.74 × 10−9 cm3/s (calculated with thereduced mass of unsubstituted reactants). Our mea-sured total reaction rate constant for CCl+ + CH3CN,3.8± 0.7× 10−9 cm3/s (see Table I), reflects good agree-ment with ADO theory. This agreement testifies to thehigh degree of efficiency of the CCl+ + CH3CN reaction,where effectively every ion-molecule collision results inthe formation of new reaction products, with little refor-mation of the reactants (vide infra). The high reactivityof CCl+ toward acetonitrile stands in stark contrast tomuch of the previous work on the reaction kinetics of thision with neutral molecules.

The isotope substituted total reaction rate constants(also in Table I) agree fairly well with the measured rateconstant for CCl+ + CH3CN, but do trend faster, be-tween 5.4 − 6.4 × 10−9cm3/s, compared to the unsub-

stituted total reaction rate constant. This trend is notprecisely captured by ADO theory, which predicts a verysmall (≤ 5%) reduction in the rate constant for bothC37Cl+ and CD3CN substitutions. There is precedencefor the trend of increased rate constant upon isotope sub-stitution. Indeed, recently, this inverse kinetic isotopeeffect has been observed using a similar apparatus andCoulomb crystal environment by monitoring the chargeexchange reaction between Xe+ and NH3 or ND3. Thiseffect, which was suggested to be due to intramolecu-lar vibrational redistribution (IVR) occurring at a fasterrate, and to a higher density of states in the deuteratedammonia.39 It is possible that we are observing a simi-lar effect here. It should be emphasized that we use aBayard-Alpert hot cathode ionization gauge to measurethe partial pressure of CH3CN gas in the chamber. Whilesensitivity factors for the gases used in this study havebeen previously measured, they are not well character-ized at pressures of 10−9 − 10−10 Torr (current regime).This systematic uncertainty is difficult to quantify, andis not reflected in our reported uncertainties. For thisreason, we do not make a definitive assessment as towhether we are observing an inverse kinetic isotope ef-fect. Instead, more significance is placed on the determi-nation of branching ratios (see Table II) and assignmentsof chemical formulas and structures of observed reactionproducts, rather than to individual rate constant mea-surements.

TABLE II. Branching ratios for primary products by isotopo-logical variations of CCl+ + CH3CN reaction. The calcu-lated branching ratio represents the fraction of protonatedacetylene rate constant, divided by the total CCl+ decay rateconstant. ‘X’ represents a hydrogen or deuterium, and corre-sponds to neutral reactant.

Branching RatioReactants (k(C2X3

+)/ktotal)CCl+ + CH3CN 0.43 ± 0.16C37Cl+ + CH3CN 0.50 ± 0.17CCl+ + CD3CN 0.44 ± 0.11CCl+ + CD3CN 0.46 ± 0.17

The branching ratios shown in Table II are nearly 50%for each of the primary products; here reported as therate of the C2H3

+ production over the sum of both pri-mary product rate constants. If all products branchedfrom the same final step of the potential energy surface(see Fig. 4), the more exothermic product, HNCCl+,might be expected to be favored. However, as will bediscussed in section III C, the potential energy surfaceis much more complex, with the existence of branchingpathways, as well as multiple isomers of products. Thisnecessitates an energy grained master equation approachto obtain quantitative branching ratio predictions.

Secondary reactions with excess CH3CN are comprisedof a proton transfer from either C2H3

+ or HNCCl+ form-ing CH3CNH

+. Analysis of the kinetics for these reac-tions is more straightforward, and the relative proton

6

affinities of the neutral molecules guide our expectationsfor the stability of the products. CH3CN has a largerproton affinity than either NCCl or C2H2 (see supple-mentary material for calculated values), and thus bothprimary products transfer a proton to neutral CH3CNto form the secondary product CH3CNH

+. Reaction dy-namics predicted by relative proton affinities has prece-dence in ion-neutral gas-phase chemistry, and boundson proton affinities have been determined by examiningwhich proton transfers do or do not take place.57 In ad-dition, these reactions are both energetically favorable,as per the reaction thermodynamics reported in Eqns.3-4. As for the relative rate constants calculated for thesecond order reactions, ADO theory predicts a slightlylarger rate constant for the C2H3

+ + CH3CN reaction(4.3 × 10−9 cm3/s) due to its smaller reduced mass ascompared to HNCCl+ +CH3CN (3.5×10−9 cm3/s). Thistrend is consistent with the reported experimental reac-tion rate constants in Table III. Overall, there is rea-sonable agreement within the experimental uncertaintybetween the ADO calculated rate constants and thosemeasured experimentally.

TABLE III. Rate constants for isotope variations of CCl+ +CH3CN secondary products. ‘X’ represents a hydrogen ordeuterium from CH3CN, and corresponds to the isotopologueused. Rates are in units of ×10−9 cm3/s, and reported statis-tical uncertainty is the calculated 90% confidence interval.

Reactants CX3CNX+

C2H3+ + CH3CN 4.2 ± 1.7

HNCCl+ + CH3CN 4.1 ± 1.2

C2H3+ + CH3CN 6.2 ± 2.0

HNC37Cl+ + CH3CN 3.8 ± 1.1

C2D3+ + CD3CN 6.0 ± 1.5

DNCCl+ + CD3CN 4.4 ± 0.9

C2D3+ + CD3CN 6.2 ± 2.3

DNC37Cl+ + CD3CN 5.9 ± 1.9

C. Modelling the CCl+ + CH3CN reaction

The potential energy surface shown in Fig. 4 representsa few plausible reaction pathways of the CCl+ + CH3CNreaction. It is a result of quantum chemical calculationsand is comprised of equilibrium structures that bridge thereactants and the observed products. The experimentalconditions are cold and very low pressure, which thereforemeans that there is no quenching of the internal energy ofany of the intermediate low energy structures. Further-more, the stationary points along this reaction pathwayare all exothermic with respect to the reactants, such thatthe reaction complex can sample all these intermediarystates until it leaves the surface irreversibly. It is useful toconsider the potential energy surface not only because it

is an accessible way to explore the pathways to eventualexothermic products presented, but also because it pro-vides a basis for the quantitative master equation-basedkinetic modeling presented below. For clarity, the non-hydrogen atoms will be numbered C1, C2, N3, C4, Cl5,as marked on INT1 in Fig. 4.

In the presented potential energy surface, CCl+ andCH3CN initially form the adduct INT1 as a bond isformed between N3 and C4. This structure then un-dergoes various changes in its bond lengths and anglesisomerizing into the lower energy INT2 structure. INT2can isomerize into INT4, which can dissociate without abarrier into PRD1 (HNCCl+ + HC2H), PRD2 (C2H3

+ +NCCl; where C2H3

+ is the non-classical bridge struc-ture), or PRD4 (HNCCl+ + H2C2; where H2C2 is thevinylidene isomer of C2H2). Determining the exact chem-ical identity of the C2H2 isomer is beyond the scope ofthis study: while the m/z of ionic products is knownbased on the mass spectra, neutral products are specula-tive since they cannot be observed experimentally.

INT2 can also isomerize to INT3, which leads to thebarrierless dissociation into PRD3, the classical “Y”C2H3

+ structure and NCCl. The isomerization barrierbetween the two isomers of C2H3

+ has been the sub-ject of rigorous computational and experimental studies,and was found to be 4.8 meV as calculated at the CBS-APNO level of theory.58–60 Regardless of which isomer isproduced in this reaction, both isomers are energeticallyallowed, with exothermicity larger than the isomeriza-tion barrier. Therefore, either C2H3

+ isomer may be theexperimentally observed cation.

All of the outlined products are exothermic with re-spect to the reactants and there are only submerged bar-riers in the potential energy surface. This indicates thatboth products are likely to form, which is perhaps re-flected in the experimentally observed branching ratiosbeing equal. This observation is tested below throughRRKM theory/master equation kinetic modeling.

To the best of our knowledge, there are no previousmeasurements for reactions of CCl+ with any nitrileswith which to compare the current results. It does ap-pear to be significant that the elucidated potential energysurface requires cleaving of the C–––N bond of CH3CN.However, this is perhaps unsurprising given that once abond is formed between the two reactants, more electrondensity will be pulled toward the more electronegativechlorine group. This is demonstrated in the first stepof the PES, when INT1 (see Fig. 4) is formed. TwoC-N bonds are of importance to this discussion: the C2-N3 bond, which originated from CH3CN, and the C4-N3bond, where the carbon from CCl+ attaches to the ter-minal nitrogen of CH3CN. The shift of electron densityfrom the C2-N3 bond to the C4-N3 and C4-Cl5 bondsoccurs in this first steps of this potential energy surface.On this surface, the shift of electron density between sta-tionary points INT1 and TS1 (Fig. 4) suggests the C–––Nfunctional group pairs with Cl over CH3, stabilizing thecomplex with respect to the reactants. This is perhaps

7

FIG. 4. Potential energy surface for CCl+ + CH3CN, depicting equilibrium geometries connecting the reactants (REA) tothe products (PRD1, PRD2, PRD3, and PRD4). In REA, PRD1, PRD2, PRD3, and PRD4, the bare ‘+’ denotes infinitedistance between the ion-neutral pair, while the + symbol indicates the ion of the ion-neutral pair. Geometries were calculatedat MP2/aug-cc-pVTZ level, with CCSD(T)/CBS//MP2/aug-cc-pVTZ energies. ‘INT’ refers to intermediate states, while ‘TS’indicates transition states. Asterisk denotes a step with a very shallow well (depending on the level of theory), which is discussedin detail in the supplementary material.

intuitive, as the highly electronegative Cl atom pulls elec-tron density towards itself, forming a strong bond, fur-ther assisted by the electron donating methyl group ofCH3CN.

All products that are observed in this study are pos-sibly a result of this shift and subsequent cleavage. Us-ing the 13CH3

13CN isotopologue as the neutral reactantcould possibly provide more convincing experimental ev-idence of the C–––N bond cleaving mechanism, however,the cost of the reagent was prohibitive. While unsuccess-ful attempts were made to find a reaction pathway thatdid not cleave this C–––N bond, this did not constitute anexhaustive search of the PES. Regardless of whether areaction pathway without cleavage of the C–––N bond ex-ists, this theoretical mechanism is interesting in its ownright.

To gain further insight into the CCl+ + CH3CN reac-tion, RRKM theory / master equation simulations wereconducted on the basis of the potential energy surface re-ported in Fig. 4 (with PRD4 excluded). Predicted rateconstants are plotted in Fig. 5 for the overall reactionand for formation of the PRD1 - PRD3 products as afunction of temperature. Here, the overall rate constants

reflect the ADO theory rates less any reverse dissocia-tion of the ion-molecule complex back to the reactants.Also included in Fig. 5 is the experimental measurementmade here and the ADO theory capture rate constants.

Fig. 5 indicates that the total rate constant is in goodagreement with the experimental value, which in turnis similar to the ADO capture value. This reflects thehigh efficiency of the CCl+ + CH3CN reaction, whichleads almost exclusively to new products. This is in turnattributed to both the low barriers for CH3CNCCl

+ iso-merization and the availability of dissociation channelsfor the subsequent isomers at below the reactant energy.Only at temperatures of around 300 K and above is thereverse dissociation channel significant, resulting in thepredicted rate coefficients to fall below the upper limitset by ADO theory.

Branching between the C2H3+ and HNCCl+ product

ions is approximately 50:50, again in accord with theexperiments. Interestingly, product PRD3 is predictedto be the dominant pathway to C2H3

+, suggesting thatit is formed in the classical, yet slightly higher-energy,vinylium form. This result is attributed to transitionstates TS2 and TS3 throttling the reaction flux from

8

FIG. 5. Theoretical (RRKM/ME) rate constants for the CCl+ + CH3CN reaction as a function of temperature. Values areincluded for the overall reaction (total) and for the formation of product ions HNCCl+ (PRD1) and C2H3

+ (PRD2 + PRD3).Included for comparison are the experimental measurements (at the effective temperature of 160 K) and the ADO theorycapture rate constants.

INT2 to a similar extent. Once TS2 is overcome, dissoci-ation to PRD1 outcompetes all other channels (includingPRD2), due to its low energy and high entropy. Follow-ing TS3, INT3 prefers to dissociate further to PRD3 thanto isomerize back to INT2, presumably due to the looseforward dissociation being highly favored in terms of en-tropy.

IV. CONCLUSION AND OUTLOOK

The gas-phase reaction of CCl+ +CH3CN is presented,with primary products C2H3

+ and HNCCl+ formed inapproximately equal yields, and both channels produc-ing a CH3CNH

+ secondary product. The LIT TOF-MSused in this study enables experimental conditions of lowpressures and collisional energies, limiting the reactiondynamics to exothermic pathways without quenching theinternal energy of the reaction complex. In addition,the high mass resolution afforded by the TOF-MS yieldsmethodical product identification that is supported byisotope substitution and quantum chemical calculations.The presented potential energy surface pathways indicatea series of equilibrium structures shifting electron densityfrom the original CH3CN C–––N bond to the new C–––Nbond formed with the carbon of CCl+. The experimentalrate constants were reported and compared to Langevinand ADO theory capture rates, as well as to detailedmaster equation / RRKM theory-based simulations ofthe reaction kinetics on a multiple-channel multiple-wellpotential energy surface. ADO theory, which includesthe polarity of the neutral reactant, is in good agree-ment with the observed experimental primary productrate constants. The master equation modeling indicates

that reaction is highly efficient, with the total rate con-stant predicted to approach the capture rate constant.Moreover, these calculations reproduce the experimen-tally observed branching fractions between the primaryionic products C2H3

+ and HNCCl+. Although CCl+ hasbeen predicted to not react with several neutrals, here, wesee this is not the case, which is consistent with the previ-ously observed reactions with C2H2.

1 This study presentsthe first example of this class of gas-phase reactions tobe studied in a regime more closely comparable to thatof the ISM (namely low pressure and temperature), andshould aid in predicting the behavior of halogenated car-bocations and nitriles in this region.

Future studies could further characterize CH3CN withanalogous reactions of various halogenated carbocationssuch as the astrochemically relevant ion CF+. In the-ory, a reaction of CF+ with CH3CN would behave sim-ilarly, and the even more electronegative fluorine mightbe expected to reproduce chlorine’s behavior here. Thiswould be particularly relevant to verify, as the presenceCF+ in the ISM is more firmly established. It wouldalso be interesting to study the effects of various func-tional groups (possibly more electron donating or with-drawing) attached to the C–––N in lieu of the methyl ofCH3CN. For example, benzonitrile C6H5(CN) with itsattached phenyl group could help stabilize intermediatesor primary products and thus possibly shift the observedreaction rates. Studying the reaction of CCl+ with var-ious substituted nitriles might help elucidate a a trendin nitrile reactivity in this low pressure and cold regime.Overall, probing the relative C–––N bond strength acrossnitriles might contribute to the understanding and pre-dictions of the formation and reactivity of the nitrilespresent throughout the ISM. Although further isotope

9

tagging is necessary to absolutely verify the experimen-tal reaction mechanism, the computational results aresuggestive, and open questions for the role and reactivityof the C–––N bond in nitriles.

For the LIT-TOFMS apparatus, future directionsalso include the integration of a traveling wave Starkdecelerator61,62 to expand control over the internal andexternal energies of polar neutral molecules. The abilityto slow molecules down into the millikelvin regime allowsthe elucidation of whether quantum mechanical effects toplay a greater role ion-neutral chemical dynamics. In thisway, it presents an opportunity to both understand thisclass of reactions at a fundamental level, as well as fur-ther our understanding of ISM chemistry.

SUPPLEMENTARY MATERIAL

See supplementary material for expanded experimen-tal results, including: plots of averaged total ions overreaction times, details of reaction curve fits, and re-action data, as well as curves for isotopologue substi-tuted reactions. See also for computational results inmore detail: the full potential energy surface, geometriesfor stationary points at MP2/aug-cc-pVTZ level of the-ory, and geometries and energies for reaction limits atCCSD(T)/CBS//CCSD/aug-cc-pVTZ level of theory.

ACKNOWLEDGMENTS

This work was supported by the National ScienceFoundation (PHY-1734006, CHE-1900294) and the AirForce Office of Scientific Research (FA9550-16-1-0117).GdS is supported by an Australian Research Council Fu-ture Fellowship (FT130101340).

DATA AVAILABILITY

The data that support the findings of this study areavailable in the supplementary material and from the cor-responding author upon reasonable request.

REFERENCES

1K. J. Catani, J. Greenberg, B. V. Saarel, and H. J. Lewandowski,J. Chem. Phys. 152, 234310 (2020).

2L. E. Snyder and D. Buhl, Astrophys. J. Lett. 163, L47 (1971).3K. B. Jefferts, A. A. Penzias, and R. W. Wilson, Astrophys. J.Lett. 179, L57 (1973).

4B. A. McGuire, A. M. Burkhardt, S. Kalenskii, C. N. Shin-gledecker, A. J. Remijan, E. Herbst, and M. C. McCarthy, Sci-ence 359, 202 (2018).

5J. H. Waite, D. T. Young, T. E. Cravens, A. J. Coates, F. J.Crary, B. Magee, and J. Westlake, Science 316, 870 (2007).

6A. Ali, E. C. Sittler, D. Chornay, B. R. Rowe, and C. Puzzarini,Planet. Space Sci. 109-110, 46 (2015).

7P. M. Solomon, K. B. Jefferts, A. A. Penzias, and R. W. Wilson,Astrophys. J. 168, L107 (1971).

8H. E. Matthews and T. J. Sears, Astrophys. J. Lett. 267, L53(1983).

9C. Codella, M. Benedettini, M. T. Beltrán, F. Gueth, S. Viti,R. Bachiller, M. Tafalla, S. Cabrit, A. Fuente, and B. Lefloch,Astron. Astrophys. 507, L25 (2009).

10S. E. Bisschop, J. K. Jørgensen, T. L. Bourke, S. Bottinelli, andE. F. van Dishoeck, Astron. Astrophys. 488, 959 (2008).

11S. Cazaux, A. G. G. M. Tielens, C. Ceccarelli, A. Castets,V. Wakelam, E. Caux, B. Parise, and D. Teyssier, Astrophys. J.593, L51 (2003).

12S. E. Bisschop, J. K. Jørgensen, E. F. van Dishoeck, and E. B. M.de Wachter, Astron. Astrophys. 465, 913 (2007).

13J. Kissel and F. R. Krueger, Nature 326, 755 (1987).14N. Biver, D. Bockelée-Morvan, P. Colom, J. Crovisier, J. K.

Davies, W. R. F. Dent, D. Despois, E. Gérard, E. Lellouch,H. Rauer, R. Moreno, and G. Paubert, Science 275, 1915 (1997).

15A. D. Morse and Q. H. S. Chan, ACS Earth Space Chem. 3, 1773(2019).

16M. Gerin, F. Combes, G. Wlodarczak, T. Jacq, M. Guelin, P. En-crenaz, and C. Laurent, Astron. Astrophys. 259, L35 (1992).

17A. Belloche, H. S. P. Müller, R. T. Garrod , and K. M. Menten,Astron. Astrophys. 587, A91 (2016).

18B. A. McGuire, Astrophys. J., Suppl. Ser. 239, 17 (2018),1809.09132.

19E. C. Fayolle, K. I. Öberg, J. K. Jørgensen, K. Altwegg, H. Cal-cutt, H. S. P. Müller, M. Rubin, M. H. D. van der Wiel,P. Bjerkeli, T. L. Bourke, A. Coutens, E. F. van Dishoeck, M. N.Drozdovskaya, R. T. Garrod, N. F. W. Ligterink, M. V. Pers-son, S. F. Wampfler, H. Balsiger, J. J. Berthelier, J. De Keyser,B. Fiethe, S. A. Fuselier, S. Gasc, T. I. Gombosi, T. Sémon, C. Y.Tzou, and t. R. Team, Nat. Astro. 1, 703 (2017).

20D. C. Lis, J. C. Pearson, D. A. Neufeld, P. Schilke, H. S. P.Müller, H. Gupta, T. A. Bell, C. Comito, T. G. Phillips, E. A.Bergin, C. Ceccarelli, P. F. Goldsmith, G. A. Blake, A. Bac-mann, A. Baudry, M. Benedettini, A. Benz, J. Black, A. Boogert,S. Bottinelli, S. Cabrit, P. Caselli, A. Castets, E. Caux, J. Cer-nicharo, C. Codella, A. Coutens, N. Crimier, N. R. Crockett,F. Daniel, K. Demyk, C. Dominic, M. L. Dubernet, M. Em-prechtinger, P. Encrenaz, E. Falgarone, A. Fuente, M. Gerin,T. F. Giesen, J. R. Goicoechea, F. Helmich, P. Hennebelle,T. Henning, E. Herbst, P. Hily-Blant, A. Hjalmarson, D. Hol-lenbach, T. Jack, C. Joblin, D. Johnstone, C. Kahane, M. Kama,M. Kaufman, A. Klotz, W. D. Langer, B. Larsson, J. Le Bourlot,B. Lefloch, F. Le Petit, D. Li, R. Liseau, S. D. Lord, A. Lorenzani,S. Maret, P. G. Martin, G. J. Melnick, Menten, K. M., P. Mor-ris, J. A. Murphy, Z. Nagy, B. Nisini, V. Ossenkopf, S. Pacheco,L. Pagani, B. Parise, M. Pérault, R. Plume, S.-L. Qin, E. Roueff,M. Salez, A. Sandqvist, P. Saraceno, S. Schlemmer, K. Schus-ter, R. Snell, J. Stutzki, A. Tielens, N. Trappe, F. F. S. van derTak, M. H. D. van der Wiel, E. van Dishoeck, C. Vastel, S. Viti,V. Wakelam, A. Walters, S. Wang, F. Wyrowski, H. W. Yorke,S. Yu, J. Zmuidzinas, Y. Delorme, J.-P. Desbat, R. Güsten, J.-M.Krieg, and B. Delforge, Astron. Astrophys. 521, L9 (2010).

21D. A. Neufeld, E. Roueff, R. L. Snell, D. Lis, A. O. Benz, S. Brud-erer, J. H. Black, M. D. Luca, M. Gerin, P. F. Goldsmith,H. Gupta, N. Indriolo, J. L. Bourlot, F. L. Petit, B. Larsson,G. J. Melnick, K. M. Menten, R. Monje, Z. Nagy, T. G. Phillips,A. Sandqvist, P. Sonnentrucker, F. van der Tak, and M. G.Wolfire, Astrophys. J. 748, 37 (2012).

22D. A. Neufeld and M. G. Wolfire, Astrophys. J. 706, 1594 (2009).23J. Glosik, D. Smith, P. Španěl, W. Freysinger, and W. Lindinger,

Int. J. Mass Spectrom. Ion Processes 129, 131 (1993).24G. A. Blake, V. G. Anicich, and W. T. Huntress, Astrophys. J.300, 415 (1986).

25V. G. Anicich, W. T. Huntress, and M. J. McEwan, J. Phys.Chem. 90, 2446 (1986).

26A. S. Blair and A. G. Harrison, Can. J. Chem. 51, 1645 (1973).

http://dx.doi.org/10.1086/180664http://dx.doi.org/10.1086/181116http://dx.doi.org/10.1086/181116http://dx.doi.org/10.1126/science.aao4890http://dx.doi.org/10.1126/science.aao4890http://dx.doi.org/ 10.1016/j.pss.2015.01.015http://dx.doi.org/10.1086/180794http://dx.doi.org/10.1086/184001http://dx.doi.org/10.1086/184001http://dx.doi.org/10.1051/0004-6361/200913340http://dx.doi.org/10.1051/0004-6361:200809673http://dx.doi.org/10.1086/378038http://dx.doi.org/10.1086/378038http://dx.doi.org/10.1051/0004-6361:20065963http://dx.doi.org/10.1038/326755a0http://dx.doi.org/ 10.1126/science.275.5308.1915http://dx.doi.org/10.1021/acsearthspacechem.9b00129http://dx.doi.org/10.1021/acsearthspacechem.9b00129http://dx.doi.org/10.1051/0004-6361/201527268http://dx.doi.org/10.3847/1538-4365/aae5d2http://arxiv.org/abs/1809.09132http://dx.doi.org/ 10.1038/s41550-017-0237-7http://dx.doi.org/10.1051/0004-6361/201014959http://dx.doi.org/ 10.1088/0004-637x/748/1/37http://dx.doi.org/10.1088/0004-637X/706/2/1594http://dx.doi.org/10.1016/0168-1176(93)87037-Shttp://dx.doi.org/10.1086/163815http://dx.doi.org/10.1086/163815http://dx.doi.org/10.1021/j100402a038http://dx.doi.org/10.1021/j100402a038http://dx.doi.org/10.1139/v73-246

10

27D. Smith, P. Spanel, and C. A. Mayhew, Int. J. Mass Spectrom.Ion Processes 117, 457 (1992).

28A. Petrank, M. Iraqi, I. Dotan, and C. Lifshitz, Int. J. MassSpectrom. Ion Processes 117, 223 (1992).

29M. Iraqi, A. Petrank, M. Peres, and C. Lifshitz, Int. J. MassSpectrom. Ion Processes 100, 679 (1990).

30M. Tsuji, M. Aizawa, H. Ujita, and Y. Nishimura, Bull. Chem.Soc. Jpn. 68, 2385 (1995).

31B. R. Heazlewood, Mol. Phys. 117, 1934 (2019).32J. Toscano, H. J. Lewandowski, and B. R. Heazlewood, Phys.

Chem. Chem. Phys. 22, 9180 (2020).33P. Puri, M. Mills, I. Simbotin, J. A. Montgomery, R. Côté,

C. Schneider, A. G. Suits, and E. R. Hudson, Nat. Chem. 11,615 (2019).

34K. Okada, Y. Takada, N. Kimura, M. Wada, and H. A.Schuessler, Rev. Sci. Instr. 88, 083106 (2017).

35A. Kilaj, H. Gao, D. Rösch, U. Rivero, J. Küpper, andS. Willitsch, Nat. Comm. 9, 1 (2018), arXiv:1804.05925.

36Y.-P. Chang, K. D lugo lȩcki, J. Küpper, D. Rösch, D. Wild, andS. Willitsch, Science 342, 98 (2013).

37P. C. Schmid, J. Greenberg, T. L. Nguyen, J. H. Thorpe, K. J.Catani, O. A. Krohn, M. I. Miller, J. F. Stanton, and H. J.Lewandowski, Phys. Chem. Chem. Phys. 22, 20303 (2020).

38E. Lavert-Ofir, Y. Shagam, A. B. Henson, S. Gersten, J. K los,P. S. Żuchowski, J. Narevicius, and E. Narevicius, Nat. Chem.6, 332 (2014).

39L. S. Petralia, A. Tsikritea, J. Loreau, T. P. Softley, and B. R.Heazlewood, Nat. Comm. 11, 1 (2020).

40P. C. Schmid, M. I. Miller, J. Greenberg, T. L. Nguyen, J. F.Stanton, and H. J. Lewandowski, Mol. Phys. 0, 1 (2019),https://doi.org/10.1080/00268976.2019.1622811.

41J. Greenberg, P. C. Schmid, M. Miller, J. F. Stanton, and H. J.Lewandowski, Phys. Rev. A 98, 032702 (2018).

42P. C. Schmid, J. Greenberg, M. I. Miller, K. Loeffler, and H. J.Lewandowski, Rev. Sci. Instr. 88, 123107 (2017).

43B. Roth, P. Blythe, and S. Schiller, Phys. Rev. A 75, 023402(2007).

44C. Q. Jiao, D. R. A. Ranatunga, W. E. Vaughn, and B. S.Freiser, J. Am. Soc. Mass Spectrom. 7, 118 (1996).

45F. Castet and B. Champagne, J. Chem. Theory Comput. 8, 2044(2012).

46M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Peters-son, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino,B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian,J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng,A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski,J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara,K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima,

Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A.Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J.Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A.Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Ren-dell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M.Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L.Martin, K. Morokuma, O. Farkas, J. B. Foresman, and D. J. Fox,“Gaussian˜16 Revision C.01,” (2016), gaussian Inc. WallingfordCT.

47R. M. Parrish, L. A. Burns, D. G. A. Smith, A. C. Simmonett,A. E. DePrince, E. G. Hohenstein, U. Bozkaya, A. Y. Sokolov,R. Di Remigio, R. M. Richard, J. F. Gonthier, A. M. James, H. R.McAlexander, A. Kumar, M. Saitow, X. Wang, B. P. Pritchard,P. Verma, H. F. Schaefer, K. Patkowski, R. A. King, E. F. Valeev,F. A. Evangelista, J. M. Turney, T. D. Crawford, and C. D. Sher-rill, J. Chem. Theory Comput. 13, 3185 (2017), pMID: 28489372.

48J. R. Barker, T. L. Nguyen, J. F. Stanton, C. Aieta, M. Ceotto,F. Gabas, T. J. D. Kumar, C. G. L. Li, L. L. Lohr, A. Maranzana,N. F. Ortiz, J. M. Preses, J. M. Simmie, J. A. Sonk, and P. J. Sti-mac, “Multiwell-2020 software suite,” http://clasp-research.

engin.umich.edu/multiwell/ (2020).49J. R. Barker, Int. J. Chem. Kinet. 33, 232 (2001).50J. R. Barker, Int. J. Chem. Kinet. 41, 748 (2009).51G. da Silva, B. B. Kirk, C. Lloyd, A. J. Trevitt, and S. J.

Blanksby, J. Phys. Chem.y Lett. 3, 805 (2012).52K. J. Catani, G. Muller, G. da Silva, and E. J. Bieske, J. Chem.

Phys. 146, 044307 (2017).53J. N. Bull, M. S. Scholz, E. Carrascosa, G. da Silva, and E. J.

Bieske, Physical Review Letters 120, 223002 (2018).54G. P. Smith and D. M. Golden, Int. J. Chem. Kinet. 10, 489

(1978).55A. K. Y. Lam, C. Li, G. Khairallah, B. B. Kirk, S. J. Blanksby,

A. J. Trevitt, U. Wille, R. A. J. O’Hair, and G. da Silva, Phys.Chem. Chem. Phys. 14, 2417 (2012).

56T. Su and M. T. Bowers, J. Chem. Phys. 58, 3027 (1973).57J. A. Burt, J. L. Dunn, M. J. McEwan, M. M. Sutton, A. E.

Roche, and H. I. Schiff, J. Chem. Phys. 52, 6062 (1970).58B. T. Psciuk, V. A. Benderskii, and H. B. Schlegel, Theor. Chem.

Acc. 118, 75 (2007).59M. W. Crofton, M. Jagod, B. D. Rehfuss, and T. Oka, J. Chem.

Phys. 91, 5139 (1989).60A. R. Sharma, J. Wu, B. J. Braams, S. Carter, R. Schneider,

B. Shepler, and J. M. Bowman, J. Chem. Phys. 125, 224306(2006).

61Y. Shyur, N. J. Fitch, J. A. Bossert, T. Brown, and H. J.Lewandowski, Revi. Sci. Instr. 89, 084705 (2018).

62Y. Shyur, J. A. Bossert, and H. J. Lewandowski, J. Phys. B:At., Mol. Opt. Phys. 51, 165101 (2018).

http://dx.doi.org/https://doi.org/10.1016/0168-1176(92)80108-Dhttp://dx.doi.org/https://doi.org/10.1016/0168-1176(92)80108-Dhttp://dx.doi.org/ https://doi.org/10.1016/0168-1176(92)80096-Jhttp://dx.doi.org/ https://doi.org/10.1016/0168-1176(92)80096-Jhttp://dx.doi.org/ https://doi.org/10.1016/0168-1176(90)85102-8http://dx.doi.org/ https://doi.org/10.1016/0168-1176(90)85102-8http://dx.doi.org/ 10.1246/bcsj.68.2385http://dx.doi.org/ 10.1246/bcsj.68.2385http://dx.doi.org/10.1039/D0CP00931Hhttp://dx.doi.org/10.1039/D0CP00931Hhttp://dx.doi.org/ 10.1038/s41557-019-0264-3http://dx.doi.org/ 10.1038/s41557-019-0264-3http://dx.doi.org/10.1038/s41467-018-04483-3http://arxiv.org/abs/1804.05925http://dx.doi.org/10.1126/science.1242271http://dx.doi.org/ 10.1039/D0CP03953Ehttps://doi.org/10.1038/nchem.1857https://doi.org/10.1038/nchem.1857http://dx.doi.org/10.1038/s41467-019-13976-8http://dx.doi.org/ 10.1080/00268976.2019.1622811http://arxiv.org/abs/https://doi.org/10.1080/00268976.2019.1622811http://dx.doi.org/10.1103/PhysRevA.98.032702http://dx.doi.org/10.1063/1.4996911http://dx.doi.org/10.1103/PhysRevA.75.023402http://dx.doi.org/10.1103/PhysRevA.75.023402http://dx.doi.org/10.1021/jasms.8b00781http://dx.doi.org/10.1021/ct300174zhttp://dx.doi.org/10.1021/ct300174zhttp://dx.doi.org/10.1021/acs.jctc.7b00174http://clasp-research.engin.umich.edu/multiwell/http://clasp-research.engin.umich.edu/multiwell/http://dx.doi.org/10.1063/1.1679615http://dx.doi.org/ 10.1063/1.1672909http://dx.doi.org/10.1007/s00214-006-0242-xhttp://dx.doi.org/10.1007/s00214-006-0242-xhttp://dx.doi.org/10.1063/1.2402169http://dx.doi.org/10.1063/1.2402169http://dx.doi.org/ 10.1063/1.5040267http://dx.doi.org/10.1088/1361-6455/aad1b0http://dx.doi.org/10.1088/1361-6455/aad1b0

Isotope-specific reactions of acetonitrile (CH3CN) with trapped, translationally cold CCl+AbstractI IntroductionII MethodsA Experimental MethodsB Computational Methods

III Results & DiscussionA Reaction thermodynamicsB Reaction measurementsC Modelling the CCl+ + CH3CN reaction

IV Conclusion and outlook Supplementary Material Acknowledgments Data Availability References