Effect of temperature on the gas-phase reaction of CH3CN with OH radicals: experimental (T = 11.7–177.5 K) and computational (T = 10–400 K) kinetic study†

3632 | Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 This journal is © the Owner Societies 2024 Cite this: Phys. Chem. Chem. Phys., 2024, 26, 3632 Effect of temperature on the gas-phase reaction of CH 3 CN with OH radicals: experimental (T= 11.7–177.5 K) and computational (T= 10–400 K) kinetic study† Daniel Gonza ´lez, ab Andre ´Canosa, c Emilio Martı ´nez-Nu ´n ˜ez, d Antonio Ferna ´ndez-Ramos, * de Bernabe ´Ballesteros, ab Marcelino Agu ´ndez, f Jose ´Cernicharo f and Elena Jime ´nez * ab Acetonitrile (CH 3 CN) is present in the interstellar medium (ISM) in a variety of environments. However, at the ultracold temperatures of the ISM, radical-molecule reactions are not widely investigated because of the experimental handicap of getting organic molecules in the gas phase by conventional techniques. The CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) technique solves this problem. For this reason, we present in this work the kinetic study of the gas-phase reaction of CH 3 CN with one of the most ubiquitous radicals, the hydroxyl (OH) radical, as a function of temperature (11.7– 177.5 K). The kinetic technique employed to investigate the CH 3 CN + OH reaction was the pulsed laser photolysis-laser induced fluorescence. The rate coefficient for this reaction k(T) has been observed to drastically increase from 177.5 K to 107.0 K (about 2 orders of magnitude), while the increase in k(T) from 107.0 K to 11.7 K was milder (around 4 times). The temperature dependent expressions for k(T) are provided in the two distinct T-ranges, excluding the upper limit obtained for k(177.5 K): k11:7135:0KðÞ¼1:50 0:12ðÞ1011 T 300 K  ð0:530:03Þ cm3s1 k135:0149:9KðÞ¼1:50 1:48ðÞ1020 T 300 K  ð26:41:5Þ cm3s1: In addition, the rate coefficients estimated by the canonical competitive unified statistical (CCUS) theory show a similar behaviour to the experimental results, when evaluated within the high-pressure limit. This is consistent with the experimentally observed independence of k(T) with total gas density at selected temperatures. Astrochemical networks, such as the KIDA database or UMIST, do not include the CH 3 CN + OH reaction as a potential depletion process for acetonitrile in the ISM because the current studies predict very low rate coefficients at IS temperatures. According to the model (T= 10 K), the impact of the titled reaction on the abundances of CH 3 CN appears to be negligible in dark molecular clouds of the ISM (B1% of the total depletion reactions included in UMIST network). With respect to the potential formation of the CH 2 CN radical in those environments, even in the most favourable scenario, where this radical could be formed in a 100% yield from the CH 3 CN + OH reaction, this route would only contribute around 2% to the current assumed formation routes by the UMIST network. a Departamento de Quı ´mica Fı ´sica, Facultad de Ciencias y Tecnologı ´as Quı ´micas, Universidad de Castilla-La Mancha (UCLM), Avda. Camilo Jose ´Cela 1B, 13071 Ciudad Real, Spain. E-mail: [email protected] b Instituto de Investigacio ´n en Combustio ´n y Contaminacio ´n Atmosfe ´rica, UCLM, Camino de Moledores s/n, 13071 Ciudad Real, Spain c Institut de Physique de Rennes-CNRS – UMR 6251, Universite ´de Rennes, F-35000 Rennes, France d Departamento de Quı ´mica Fı ´sica, Facultade de Quı ´mica, Campus Vida, Universidade de Santiago de Compostela, Avda. das Ciencias s/n, 15782, Santiago de Compostela, Spain. E-mail: [email protected] e Centro Singular de Investigacio ´n en Quı ´mica Biolo ´gica y Materiales Moleculares (CIQUS), Campus Vida, Universidade de Santiago de Compostela, C/Jenaro de la Fuente s/n, 15782, Santiago de Compostela, Spain f Molecular Astrophysics Group, Instituto de Fı ´sica Fundamental (IFF-CSIC), Consejo Superior de Investigaciones Cientı ´ficas, C/Serrano 123, 28006, Madrid, Spain †Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp04944b Received 11th October 2023, Accepted 5th January 2024 DOI: 10.1039/d3cp04944b rsc.li/pccp PCCP PAPER Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 | 3633 1. Introduction Acetonitrile, also known as methyl cyanide (CH 3 CN), is the simplest organic nitrile molecule. This N-bearing species is present in natural environments such as the Earth’s atmosphere, solar system objects, interstellar matter, and even nearby galaxies. This prolate symmetric-top molecule is wellsuited for millimetre-wave observations due to its large permanent dipole moment (B4 Debye). 1 Regarding the terrestrial atmosphere, CH 3 CN is a trace gas, essentially issued from biomass burning. 2 Its lifetime in the troposphere is about 6 months with deposition in the ocean being the dominant removal process. Therefore, it can be transported to the stratosphere where it can stay for several decades. 3 The reaction with the hydroxyl (OH) radical becomes the main sink. In the context of the search for the origin of life, a main concern in modern astrochemistry and astrobiology, CH 3 CN is of special interest for prebiotic evolution because of the main role played by C–N bonds in the formation of peptide structures, such as amino acids. Then, CH 3 CN has been searched and detected routinely in a large variety of extraterrestrial environments, becoming one of the most abundant complex organic molecules (COMs) ever seen in space. In the solar system, CH 3 CN has been frequently observed in comets, 4 for instance in Kohoutek, 5 Hale-Bopp, 6,7 P/Halley 8 or 67P/Churyumov–Gerasimenko. 9 It has also been identified in one of Saturn’s moons, Titan, either by ground-based observations 10 or in situ measurements. 11 In the interstellar medium (ISM), CH 3 CN is one of the few prebiotic molecules seen in all main phases of the lifetime cycle of stars from the early stages to the last remnants. Since its first detection towards SgrA and SgrB molecular clouds in 1971, 12 it has been observed in diffuse and translucent clouds, 13–15 dark clouds, 16 photodissociation regions, 17,18 circumstellar envelopes, 19,20 hot cores, 21,22 protostellar objects, 23,24 protoplanetary disks 25,26 or molecular outflows 27,28 as well as in extragalactic sources. 29,30 In addition, CH 3 CN is a good thermometer for interstellar gas 31,32 and a useful tracer of shocks 33,34 and kinematic structures. 28 Its gas-phase formation routes have been very recently discussed by Giani et al. 35 As it can be seen, CH 3 CN is a quite important molecule in the different surroundings of the ISM, including those where the ultra-low temperatures (10–100 K) prevail. For that reason, it is essential to simulate this cold environment in the laboratory to study the gasphase reactivity of CH 3 CN towards the most abundant radicals in theISM,liketheOHradicalwhichplaysasignificantroleinmany chemical processes. 36–39 Nevertheless, although the CH 3 CN + OH reaction (reaction (1)) has been quite extensively studied both experimentally at T4250 K 40–48 and theoretically at T4200 K, 49–52 it has never been investigated at the ultralow temperatures of the ISM. CH 3 CN + OH -Products k(T) (1) In summary, the experimental kinetic studies of reaction (1) were performed in a wide range of pressures (P= 1.2–760 Torr) and temperatures (T= 250–424 K) using various techniques (see Table 1). Both absolute 40–45 and relative 47,48 kinetic techniques were employed to determine the rate coefficient k(T) of reaction (1) at a temperature T. At room temperature, k(298 K) has been reported in single temperature experiments 42,43,46,48 or as part of the temperature dependence study of reaction (1). 40,41,44,45,47 Most of the studies are in agreement, except the ones by Harris et al. 40 and Andersen et al., 48 the most recent experimental study, whose k(298 K) both differ by a factor of about 2 with previous measurements. As shown in Table 1, there are several studies on the pressure dependence of k(298 K). 43–45 Kurylo and Knable 44 observed a 10% increase of the rate coefficient in the range 20–50 Torr of either argon or SF 6 , while Zetzsch 43 found, according to Hynes and Wine, 45 an increase of k(298 K) by a factor of 2.5 in the pressure range 5–100 Torr of argon. However, k(298 K) reported by Poulet et al. 41 at 1.2 Torr of He is in agreement with high-pressure results. This was confirmed Table 1 Synthetic view of the available literature for the CH 3 CN + OH reaction: experiments. The temperature dependence of the rate coefficient for reaction (1) is mathematically fitted by the modified Arrhenius expression: kTðÞ¼AT 298 K  n exp g T P/Torr Buffer gas T/K Method a k(298 K)/10 14 cm 3 s 1 A/10 13 cm 3 s 1 ng/K Products Ref. 50 Ar 297–424 FP-RF 4.94 0.6 5.86 0 756 126 H-Abst deduced Harris et al. 40 7 Ar 295 FP-RF 2.4 0.3 H-Abst assumed Fritz et al. 42 5–100 Ar 298 FP-RF 1.9 0.2 b — Zetzsch 43 1.2 He 295 DF-EPR 2.1 0.3 H-Abst deduced Poulet et al. 41 393 20; 50 Ar 250–363 FP-RF 1.94 0.37 6.28 0 1030 H-Abst deduced Kurylo and Knable 44 SF 6 50–700 N 2 N 2 256–388 PLP-LIF 2.48 0.38 11 0 1130 90 B50% H-Abst Hynes and Wine 45 30–600 He N 2 /O 2 B50% OH Ad. (298 K) He 700 Air 296 Smog chamber FTIR B50% H-Abst. d Tyndall et al. 46 B50% OH Ad. 760 Ar 306–393 FP-FTIR 1.82 18.5 0 1377 62 H-Abst deduced DeMore 47 700 Air 296 Smog chamber FTIR 4.07 1.21 c H-Abst assumed Andersen et al. 48 a Flash photolysis resonance fluorescence (FP-RF); discharge flow coupled to electron paramagnetic resonance (DF-EPR); pulsed laser photolysislaser-induced fluorescence (PLP-LIF); Fourier transform infrared (FTIR). b From Hynes and Wine 45 at about 100 Torr. c Average rate coefficients using different reference compounds. d Indirectly deduced from HC(O)CN quantification. Paper PCCP Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online 3634 | Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 This journal is © the Owner Societies 2024 later by Hynes and Wine, 45 who essentially found no pressure dependence of k(298 K) between 30 and 700 Torr, neither in N 2 nor in He. Concerning the temperature dependence of k(T), a positive activation energy has been reported between 250 and 424 K, i.e., k(T) decreases when temperature decreases. The Arrhenius parameters reported by Harris et al., 40 derived from kinetic data obtained at temperatures above room temperature, were slightly lower than those obtained by Kurylo and Knable, 44 Hynes and Wine 45 and DeMore et al., 47 whose temperature dependences were essentially identical. Harris et al. 40 and Kurylo and Knable 44 interpreted the observed normal Arrhenius behaviour as a prevalence of the H-abstraction from the methyl group of acetonitrile (channel (1a)): CH 3 CN + OH -CH 2 CN + H 2 O (1a) Nevertheless, Hynes and Wine, 45 from a series of experiments in a mixture of O 2 and N 2 , found indirect evidence that the OH- addition accounted for about 50% of the total reaction process at room temperature. This branching ratio was later confirmed by Tyndall et al. 46 in smog chamber experiments at 700 Torr. CH 3 CN + OH -CH 3 C(OH)N (1b) -CH 3 CN(OH) (1c) From a theoretical point of view, the potential energy surfaces (PES) of various routes from reactant to products 49–52 were investigated by diverse methods (see Table 2), and from the obtained PES the temperature dependence of k(T) 51–53 was calculated using different variants of the general transition state theory (TST). Li and Wang 53 calculated the rate coefficient for the reaction pathway (1a), k 1a (T), in the temperature range 250–2500 K using the canonical variational TST (CVT) with a small curvature tunnelling (SCT) correction. The calculated k(298 K) was found to agree with several experiments. 41,44,45 As mentioned earlier, the contribution of the OH-addition channels to k(T) was not negligible. Thus, the OH-addition channels were theoretically investigated by Galano, 50 concluding that reaction (1c) was endothermic while reaction paths (1a) and (1b) have similar efficiency, based on close Gibbs free energy barriers. No rate coefficients were calculated, however. Additional reaction pathways were explored by Tian et al. 51 concluding that reaction channel (1a) was the dominant one whereas the two OH-addition channels were not considered as terminal in the reaction pathways. The rate coefficient k 1a (T) was calculated using TST with the Wigner correction to take tunnelling into account. At room temperature their calculation agreed well with the measurements from Harris et al. 40 and Andersen et al. 48 More recently, Alihosseini et al. 52 found 12 viable routes along the PES of the OH + CH 3 CN reaction. Only the rate coefficient k 1a (T) was calculated using TST, at atmospheric pressure over the temperature range of 200–2500 K. In this temperature range, a non-Arrhenius behaviour of k 1a (T) was predicted, however the authors fitted the data to a conventional Arrhenius equation which gives a very poor representation of individual k 1a (T) (see Fig. 5 in Alihosseini et al. 52 ). Furthermore, k 1a (298 K) is about two orders of magnitude higher (2.17 10 12 cm 3 s 1 ) than previous investigations either experimental or theoretical. Therefore, there are still matters of controversy concerning the rate coefficient k(T), the branching ratios, and the influence of pressure in the obtained measures. In addition, the extension of experimental and theoretical studies to temperatures lower than 200 K is certainly desirable, especially for the modelling of the chemistry of interstellar objects in which CH 3 CN and OH have been observed. Astrochemical networks, such as the KIDA database or UMIST, do not include reaction (1) as a potential depletion process for acetonitrile in the ISM. The present investigation aims at studying the gas-phase kinetics of the CH 3 CN + OH reaction between 11.7 K and 177.5 K from an experimental point of view using a pulsed CRESU (French acronym for Reaction Kinetics in a Uniform Supersonic Flow) reactor and from a theoretical point of view using high level quantum chemistry tools such as the density functional theory (DFT) and the competitive canonical unified theory (CCUS) in the temperature range of 10–400 K. The results reported here were used to model the IS abundance of CH 3 CN using a pure gas-phase astrochemical model and the implications of our results in the chemistry of IS molecular clouds are discussed. Table 2 Synthetic view of the available literature for the CH 3 CN + OH reaction: theory. The temperature dependence of the rate coefficient is mathematically fitted by the modified Arrhenius expression: kTðÞ¼AT 298 K  n exp g T Method T(K) k(298 K)/ 10 14 cm 3 s 1 A a / 10 13 cm 3 s 1 ng(K) Products Ref. PMP4/6-311_G(2df,2pd)//MP2/6-311G(d,p) 250–2500 2.38 a 1.4 b 2.89 b 620 b H-Abst only considered Li and Wang 53 QCISD(T)/6-311_G(2df,2pd)//MP2/6-311G(d,p) + CVTST/SCT 250–2500 2.02 a 1.3 b 2.91 b 648 b H-Abst only considered Li and Wang 53 BHandHLYP/6-311++G(2d,2p) + CCSD(T)/ 6-311++G(2d,2p) —— — ——B50% H-Abst Galano 50 B50% OH Ad. B3LYP/CBSB7 250–3000 5.14 7.05 3.045 780 7 routes obtained Tian et al. 51 TST + Wigner correction H-Abst mainly CBS-QB3 250–430 4.79 15.6 2.36 1038 7 routes obtained Tian et al. 51 TST + Wigner correction H-Abst mainly B3LYP + G3B3 or CCSD(T)/ 6-311++G(3df,3pd) + TST 200–2500 217 489 0 1277 12 routes obtained but H-Abst only considered Alihosseini et al. 52 a Calculated at 298 K (not from fit). b Obtained in the present work from the data presented in Table 5 from Li and Wang. 53 PCCP Paper Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 | 3635 2. Experimental part The kinetic study of reaction (1) has been performed at ultralow temperatures (11.7–177.5 K) achieved by a pulsed CRESU apparatus, where the gas is not continuously passing through the Laval nozzle. A detailed description of the technique and the setup can be found elsewhere. 54–61 Hence, a schematic diagram with the experimental novelties of the present work is provided in Scheme 1 and only a brief description is given below. Seven different Laval nozzles and a total of 15 operational conditions were used in this work to fill up the temperature range. The isentropic expansion from a relative high-pressure reservoir (P res = 9.97–366.48 mbar) to a low-pressure chamber (0.12–6.00 mbar) through the nozzle results in a uniform supersonic cold jet ranging from 11.7 and 177.5 K. A rotary disk with one or two symmetrical apertures was employed to pulse the gas at 5 Hz for T= 11.7 K or 10 Hz for the rest of operational conditions, respectively. The kinetic technique pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) was also described previously. 54,55,59,62–68 The production of OH radicals along the supersonic jet was achieved by PLP of gaseous H 2 O 2 or tert-butyl hydroperoxide (t-BuOOH) at 248 nm, radiation generated by a KrF excimer laser (Coherent, model ExciStar XS 200). Both precursors were introduced in the reservoir by flowing a small portion of the buffer gas (He, N 2 , or Ar) through the bubbler containing the concentrated solution of the OH precursor (see Scheme 1). Gaseous CH 3 CN was introduced in the reservoir by flowing diluted mixtures in the buffer gas through a calibrated mass flow controller (method 1) or by bubbling liquid samples of CH 3 CN with a small portion of the buffer gas (method 2). More details of both methods are given in the ESI.†Once the OH radicals were generated in the electronic ground state, they were excited at ca. 282 nm, radiation achieved with a frequency-doubled dye laser (Lambda Physik, model Scanmate) pumped by the second harmonic of a Nd:YAG laser (Continuum, model Surelite III). Finally, the temporal evolution of OHradicalsinthepresenceandabsenceofacetonitrilewas monitored by LIF (I LIF )atca. 310 nm, using a photomultiplier tube (Electron Tube, model 9813B) with a bandpass filter centred at 310 nm with a full width at half maximum of 10 nm (Andover Co., model 310FS10-25). 2.1. Kinetic analysis The kinetic experiments were performed under pseudo-first order conditions, i.e., in large excess of acetonitrile with respect to the initial concentration of OH radicals ([CH 3 CN] c[OH] 0 ). In such a situation, after an increase due to rotational relaxation (see ESI†and already explained), 54,57 the temporal profile of I LIF results in an exponential decay primarily caused by the reaction of OH with CH 3 CN, but also by the reaction with the OH- precursor (H 2 O 2 or t-BuOOH) or diffusion out of the detection zone. Some examples of the temporal profile of I LIF recorded in the absence and presence of CH 3 CN at three selected temperatures are shown in Fig. S1 of ESI.†From the analysis of the I LIF decays, the pseudo-first order rate coefficient, k0(or k0 0in the absence of CH 3 CN) was obtained. In these experimental conditions, k0is proportional to [CH 3 CN] at a single temperature, according to the relationship given by eqn (E1): k0¼kðTÞCH3CN½þk0 0(E1) Scheme 1 Experimental set-up scheme including both methods to introduce CH 3 CN into the reservoir. MFC: mass flow controller. Paper PCCP Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online 3636 | Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 This journal is © the Owner Societies 2024 where k(T) is the second-order rate coefficient at each temperature and gas density obtained from the slope of the k0k0 0vs. [CH 3 CN] plots. Reliable knowledge of [CH 3 CN] in the jet is essential to obtain accurate k(T) at each temperature. These concentrations were determined from flow rate or vapor pressure measurements (see ESI†), according to the way CH 3 CN is introduced in the reservoir. As shown in Fig. S2 (ESI†), [CH 3 CN] determined by both methods yield k(T) at 36.2 K in excellent agreement. This linear relationship is not accomplished at high [CH 3 CN] and the bimolecular plots display a curvature. The observed downward curvature is associated with the dimerization process of CH 3 CN, as previously reported for other OH- reactions with COMs (Fig. S2, ESI†). 54–60,62,63,68–70 As can be seen in Fig. S3 (ESI†), the (CH 3 CN) 2 dimer formation is especially enhanced at low temperatures. Then, the onset of such a dimerization, i.e., [CH 3 CN] from which the bimolecular plots are not linear, increases with temperature for a total gas density of the same order of magnitude 10 17 cm 3 (e.g. [CH 3 CN] = 1.30 10 14 cm 3 at 21.7 K, 1.96 10 14 cm 3 at 76.0 K and 5.30 10 14 cm 3 at 135.0 K). At T4140 K, k0k0 0vs. [CH 3 CN] plots are linear in the entire explored [CH 3 CN] range, even reaching 1% of the total gas density. Acetonitrile concentration was not increased beyond 1% of the total gas density since it may affect the uniformity of the flow and the jet temperature. In Table 3, [CH 3 CN] range within which eqn (E1) is valid are listed for every temperature and gas density. Four examples of the k0k0 0vs. [CH 3 CN] plots are shown in Fig. 1 at selected temperatures. As shown, the slope of these plots increases when the temperature decreases, implying that the OH- reactivity of CH 3 CN is enhanced at lower temperatures. 2.2. Chemicals Buffer gases (He, N 2 , and Ar), all of them with 99.999% purity (Nippon Gases Europe), were used as supplied. Liquid acetonitrile (Z99.9%, Sigma-Aldrich) was degassed by repeated freepump-thaw cycles prior to use for the experiments in which the storage bulb was used. The aqueous solution of hydrogen peroxide (50% w/w, Sharlab) was preconcentrated as previously reported, 66 whereas the t-BuOOH solution (70%, Sigma- Aldrich) was used directly without pre-concentration, because its vapour pressure is higher than that of H 2 O. When no use, both precursor bubblers were stored in a refrigerator at 2 1Cto prevent from thermal decomposition. 3. Computational methodology All geometry optimizations were performed by the M08-HX Density Functional Theory (DFT) method 71 with the MG3S basis set, 72 as for the recently studied OH + CH 3 NH 2 reaction. 68 All zero-point energies (ZPEs) were scaled by the recommended factor of 0.973. 73 The two computed reactions are the hydrogen abstraction from the methyl group by the OH radical (reaction (1a)) and the OH-addition to the C of cyano group (reaction (1b)). It is worth noting that reaction (1a) is part of a much larger reaction network recently studied by some of us. 74 The OH-addition to the nitrogen atom, as well as other 4 additional channels previously investigated by Tian et al. 51 have very high barriers and are not competitive with reactions (1a) and (1b). Additional geometry optimizations were also carried out at oB97X-D 75 with the def2-TZVP basis set. 76 All ZPEs were scaled by the recommended factor of 0.975. 73 The geometries obtained by both electronic structure levels, that is, M08-HX/MG3S and oB97X-D/def-2TZVP, were employed as input for the CCSD(T)- F12/cc-pVTZ-F12 single-point calculations 77,78 to compare the result of both functionals on an accurate and common ground. The thermal rate coefficients were evaluated employing the competitive canonical unified theory (CCUS). In this formalism, the rate coefficient for each reaction channel, k CCUS i (where i = 1a or 1b), is given by eqn (E2), 79 k CCUS i =k CCUS a i (E2) kCCUS ¼1 ka þ1 k1  1 (E3) where k a is the rate coefficient for the association reaction and k 1 =k 1a +k 1b is the total rate coefficient for the OH + CH 3 CN reaction calculated as the forward flux from reactants through transition states TS1a (H-abstraction) and TS1b (OH-addition). The ratio between the individual rate coefficients for the transition state passage through k i and the total rate coefficient k 1 is given by a i . Because the association reaction is the same for both competitive reactions, the products branching ratio in percentage is given by 100a i . The rate coefficient for the association reaction is given by the following expression: 80 Fig. 1 Examples of k0k0 0versus [CH 3 CN] plots at four different temperature conditions. Uncertainties in [CH 3 CN] are cautiously considered as 10% and uncertainties in k0k0 0are the standard deviation obtained from the analysis of the I LIF decays. PCCP Paper Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 | 3637 k a =Cm 1/2 (d OH d CH 3 CN ) 2/3 T 1/6 (E4) where Tis the temperature, C= 1.83 10 9 is a constant that provides the rate coefficient in cm 3 s 1 ,mis the reduced mass (in uma), and d CH 3 CN = 4.02 D and d OH = 1.66 D are the dipole moments of acetonitrile and the OH radical, respectively. The rate coefficients k i were calculated employing canonical variational transition state theory with small-curvature tunnelling corrections (CVT/SCT), 81 according to the following equation: k CVT/SCT i =G CVT i k CVT/SCT i k TST i (E5) where k TST i is the conventional transition state theory (TST) rate coefficient, G CVT i is the variational coefficient that accounts for recrossing (0 rG CVT i r1), and k CVT/SCT i is the tunnelling transmission coefficient (k CVT/SCT i Z1) calculated within the SCT approximation. The evaluation of these two parameters requires the calculation of the minimum energy path (MEP), which was evaluated using the Page-McIver algorithm in massscaled coordinates. 82 The MEP for the H-abstraction and OH- addition reactions was followed employing a step size of 0.005 Bohr and Hessian calculations every ten steps. In the case of the reaction through TS1b, we corrected the energies along the MEP to match the coupled-cluster (CC) energy by employing the interpolated single-point energies algorithm. 83 The CVT/SCT calculations were performed with Pilgrim. 84 The CCUS prescription allows calculating the thermal rate coefficients in the low-pressure (LPL) and high-pressure (HPL) limits. The only difference between the two regimes is that in the LPL the lowest energy for tunnelling is the energy of reactants plus its ZPE, whereas in the HPL the lowest energy that allows tunnelling is the energy of the pre-reactive complex plus its ZPE. For the latter, it is assumed that the collisions with other molecules will stabilize the complex, a situation that cannot occur at very low pressures. Therefore, the only factor affected by pressure in eqn (E5) is the tunnelling transmission coefficient (see Section 4.2). 4. Results and discussion 4.1. Experimental kinetic study The individual rate coefficients k(T) determined in this work are listed in Table 3 as a function of the OH-precursor (H 2 O 2 or t-BuOOH), gas density (n), and temperature. As it can be seen in the examples depicted in Fig. 2, no influence of the OH-precursor on k(T) was observed. Thus, in Table 3 the weighted average of k(T) obtained using both precursors is given in bold. 4.1.1. Pressure dependence of k(T). The pressure dependence study of k(T) was carried out for some specific temperatures around 22 K (He as bath gas), 50 K (He, N 2 , or Ar as bath gas), and 106 K (N 2 or Ar as bath gas). The total gas density was explored in the range of (1.50–44.4) 10 16 cm 3 .nwas varied around 5 times at B22 K, 30 times at B50 K, and 3 times at B106 K. As shown in Fig. 3, no pressure dependence of k(T) has been observed at around 22 K, 50 K and 106 K, within the error bars, over the investigated range. This is in agreement with the observations of Sleiman et al. 86 for the reaction of CH 3 CN with CN radicals at 23 K (He as bath gas). These authors reported a strong dependence of the rate coefficient for the CN-reaction at 52 K and 132 K in Ar. The p-dependence was attributed by these authors as evidence of the adduct formation by association of the reactants (CN and CH 3 CN). Even though the gas density range investigated by Sleiman et al. 86 is higher (n= (3.48–82.1)  10 16 cm 3 ) than ours, if a pressure dependence of k(T)for reaction (1) prevails it would have been observed at the concentration level used in the present work. 4.1.2. Temperature dependence of k(T). As shown in Table 3, k(T) increases when temperature decreases, resulting in an acceleration of the reaction of CH 3 CN towards OH radicals of around 3800 times at 11.7 K and 10 times at 177.5 K with respect to k(300 K) = 2.2 10 14 cm 3 s 1 . 87 The value of k(177.5 K) reported here (2 10 13 cm 3 s 1 ) is an upper limit, since almost no change in k0with respect to k0 0was observed in the timescale of the experiment and within the [CH 3 CN] range used. This upper limit of kwas estimated as explained by Douglas et al., 85 i.e., as twice the uncertainty in k0 at the maximum CH 3 CN concentration, [CH 3 CN] max , used in the experiment, divided by that [CH 3 CN] max . In Fig. 4, the log–log plot of average k(T) from Table 3 versus T is presented between 11.7 K and 177.5 K. Clearly, two distinct regions in the plot were observed. Between 135.0 K and 177.5 K, an abrupt decrease of k(T) of a factor of around 90 was observed. In this steep region, the expression of k(T) provided by eqn (E6), where the estimated k(177.5 K) is excluded, is presented as the blue line in Fig. 4. k135:0149:9KðÞ¼1:50 1:48ðÞ 1020 T 300 K  ð26:41:5Þ cm3s1 (E6) Nevertheless, this expression can be used to extrapolate k(177.5 K). The extrapolated rate coefficient at 177.5 K is 1.56 10 14 cm 3 s 1 (empty black circle in Fig. 4), which is a reasonable value if it is compared with k(300 K) = 2.2 10 14 cm 3 s 1 . 87 In contrast, the observed increase of k(T) between 135.0 K and 11.7 K is milder (around 5 times) than that between 135.0 and 149.9 K. The temperature dependence of k(T) in the 11.7–135 K region is well described by eqn (E7) – depicted as the red line in Fig. 4. k11:7135:0KðÞ¼1:500:12ðÞ 1011 T 300 K  ð0:530:03Þ cm3s1 (E7) 4.2. Computational study 4.2.1. Energetics. The energy of the M08-HX and oB97X-D stationary points is compared in Table 4 to that of the CCSD(T)- F12 single point calculations with the cc-pVTZ-F12 basis set performed over the M08-HX and oB97X-D optimized geometries. The DFT energies are substantially different in the two methods and the stability of the two transition states is Paper PCCP Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online 3638 | Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 This journal is © the Owner Societies 2024 reversed when the ZPE is included. However, CCSD(T)-F12 over the DFT geometries produces very similar results, in particular, the CCSD(T)-F12/M08-HX predict that TS1a is 4.51 kcal mol 1 above TSb, and 4.65 kcal mol 1 when considering CCSD(T)-F12/ oB97X-D calculations. This indicates that both DFT methods supply similar geometries for the stationary points. The potential barrier heights are also in relatively good agreement with the CCSD(T)-F12//B3LYP/6-311++G(3df,3pd) calculations carried out by Alihosseini et al. 52 who reported barriers of 5.30 and 1.36 kcal mol 1 for TSa and TSb, respectively, with a difference between barriers of 3.94 kcal mol 1 . The difference between the two transition states is reduced to 1.22 and 1.23 kcal mol 1 for CCSD(T)-F12/M08-HX and CCSD(T)-F12/oB97X-D, respectively. These values are in good agreement with the results obtained by Galano, 50 where TS1a is above TS1b by 1.00 kcal mol 1 . Of the two DFT methods reported here, M08-HX is the most similar to the coupledcluster calculations. Additionally, it does not revert the stability of the two transition states, as is the case of the coupled-cluster calculations. Therefore, hereafter all thermal rate coefficient calculations will involve M08-HX calculations of the MEP and additional single-point energy corrections for reaction (1b), to correct the difference between this DFT method and the coupled-cluster calculations. Fig. 5 plots a schematic reaction network of the CCSD(T)-F12/M08-HX ZPE-corrected level regarding reactants, which is the basic scheme for further dynamics calculations. The pre-reactive complex (C1) and the transition states that correspond to reactions (1a) and (1b) obtained at M08-HX/ MG3S are depicted in Fig. 6. It can be observed that the C1 complex can easily react through both transition states. 4.2.2. Computed rate coefficients. The calculated rate coefficient for the association reaction k a decreases slightly with temperature and it presents a value of 1.28 10 9 cm 3 s 1 at Table 3 Summary of the experimental conditions employed in this work, individual k(T) as a function of the gas density (n) and average k(T) (in bold) as a function of temperature a ET/K T/K n/10 16 cm 3 [CH 3 CN]/10 13 cm 3 k(T)/10 11 cm 3 s 1 12 11.7 0.7 6.88 0.62 0.76–7.81 8.18 0.85 b 1.35–6.15 8.45 0.92 c 8.25 0.89 22 21.1 0.6 3.37 0.15 0.87–8.93 7.03 0.86 b 0.87–8.89 5.89 0.62 c 6.05 1.28 21.7 1.4 16.65 1.61 1.63–12.7 6.17 0.71 b 36 36.2 1.2 17.73 0.86 2.00–14.5 5.27 0.57 b 1.37–15.0 4.91 0.57 c 5.14 0.71 50 50.5 1.6 1.50 0.12 1.94–12.5 4.04 0.50 b 1.19–8.82 4.29 0.46 c 4.23 0.52 49.9 1.8 3.67 0.32 1.12–9.99 4.33 0.52 b 1.12–10.1 3.91 0.57 c 4.18 0.71 52.1 0.5 19.52 0.28 0.83–11.7 4.07 0.42 b 0.92–8.08 4.00 0.61 c 4.07 0.41 76 76.0 0.8 14.96 0.34 2.19–15.8 2.70 0.37 b 106 106.0 0.6 14.02 0.11 1.26–8.58 2.52 0.38 b 0.79–6.52 2.70 0.35 c 2.63 0.36 107.0 0.5 4.90 0.06 4.11–30.1 2.20 0.23 b 4.55–14.8 2.28 0.33 c 2.21 0.23 135 135.0 0.8 29.45 0.45 6.19–45.4 1.82 0.20 b 136.1 0.8 24.92 0.35 8.47–63.9 1.40 0.17 b 140 140.4 1.0 21.68 0.40 10.1–113 0.60 0.06 b 143.3 0.6 17.02 0.17 13.4–166 0.32 0.04 b 150 149.9 0.7 10.69 0.12 18.1–134 0.07 0.01 b 177 177.5 1.2 6.71 0.11 22.8–67.1 o0.02 bd a Uncertainties in k(T) account for statistical (2s) and 10% systematic errors. In nthe indicated errors result from the aerodynamic fluctuations in the flow (1s). b H 2 O 2 as OH-precursor. c t-BuOOH as OH-precursor. d Upper limit calculated as Douglas et al. 85 PCCP Paper Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 | 3639 T= 10 K, and a value of 9.75 10 10 cm 3 s 1 at T=50K.In Table 5, variational and tunnelling transmission coefficients are presented for selected temperatures between 50 and 300 K. A full account of these parameters and the thermal rate coefficients k 1a and k 1b is given in the ESI.† Tunnelling effects are more important for the H-abstraction than for the OH-addition reaction. This is expected as the hydrogen atom is a light particle, whereas the OH-addition involves heavy-atom tunnelling. 88 However, variational effects are also more relevant in the H-abstraction reaction, so the result of multiplying the variational coefficient by the tunnelling coefficient favours the addition reaction. Moreover, the potential barrier is higher for the H-abstraction reaction and therefore the OH-addition reaction dominates between 10 and 300 K. In fact, it contributes 100% at 10 K and 87% at 300 K. The variational effect in the H-abstraction shows the magnitude of recrossing in the reaction. In CVT/SCT, this effect is calculated by finding the maximum of the Gibbs free energy along the reaction coordinate sof the MEP (negative in the reactant site, positive in the products site and zero at the transition state), which for the abstraction reaction is located at s= 0.267 Bohr at 0 K and at s=0.250 Bohr at 400 K. Variational effects are due to the increasing value of the ZPE as we head toward reactants; it is 31.51 kcal mol 1 at the transition state and 33.17 kcal mol 1 at s=0.250 Bohr. This ZPE variation displaces themaximumofthefreeenergytowardsthereactantssite. For the evaluation of the normal-mode frequencies along the MEP, we have employed redundant internal coordinates, because it is well-known that an accurate evaluation of the ZPE along the reaction path requires the use of internal coordinates. They provide a more physical description of the normal-mode frequencies than the Cartesian coordinates (notice that the calculated normal mode frequencies in non-stationary points depend on the system of coordinates). 89 Theoretical methods based on conventional TST that incorporate tunnelling effects (for instance, through the Eckart barrier), completely ignore the recrossing, which in this case has the effect of reversing the contribution of the two channels to the branching ratios. We have also included torsional anharmonicity in the transition state structures applying the same protocol as for the Fig. 2 Examples of the bimolecular plots at 50.5 and 107.0 K using two OH-precursors. Fig. 3 Dependence of k(T) on the jet gas density (n)atca. 22, 50, and 106 K. Black triangles correspond to experiments at 22 K in He, while at 50 and 106 K the bath gas is indicated next to each point. Error bars in k(T) are statistical (2s) and 10% systematic errors and in nare only statistical (s). Fig. 4 Experimental and theoretical temperature dependence of the rate coefficient for the CH 3 CN + OH reaction. Red and blue lines represent the resulting fit to k(T)=A(T/300 K) n in the stated temperature range. Solid and dashed black curves are the computed HPL and LPL rate coefficients. Paper PCCP Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online 3640 | Phys. Chem. Chem. Phys., 2024, 26, 3632–3646 This journal is © the Owner Societies 2024 OH + methylamine system. 68 In the case of TS1a, the torsional anharmonicity contains the rotation of the OH about the methyl group (with a harmonic frequency o= 144.2 cm 1 ). For TS1b, there are two rotors, the internal rotation of the OH (with o= 221.2 cm 1 ) and methyl (with o= 140.8 cm 1 ) groups. For the two transition states, the treatment of these motions, as one-dimensional hindered rotors, instead of harmonic oscillators has a modest effect in the rate coefficients. The ratio between the anharmonic and harmonic partition functions is always smaller than 2 between 50 and 400 K (see the ESI†). The total CCUS rate coefficients are plotted in Fig. 4 and summarized in Table 6 for selected temperatures. At the LPL, k a ck 1 and the rate-determining reaction is always k 1 , so this rate coefficient and k CCUS coincide. Above T= 150 K the thermal rate coefficients evaluated at the two pressure limits coincide, this is because at this temperature and above the molecules populate the same levels with independence of the depth of the lowest energy level. In other words, the pre-reactive complex is irrelevant for the course of the reaction. However, below T= 150 K the HPL rate coefficients start to increase fast and at TB 40 K k HPL becomes larger than the association rate coefficient, being k a the rate-determining reaction. Although the agreement between theory and experiment obtained in this work is not perfect, the HPL results capture the wild variation of the rate coefficients with an astonishing increase of about four orders of magnitude in an interval of less than 100 K. 4.3. Comparison with previous studies 4.3.1. Experimental OH-reactivity trend towards CH 3 X molecules (X = –CN, –OH, –NH 2 ,–CHO,CH 2 OH, –C(O)CH 3 ...). The OH-reactivity and its temperature dependence at very low temperatures can be analysed for CH 3 X molecules, such as CH 3 CN, CH 3 OH, CH 3 NH 2 ,CH 3 CHO, or CH 3 CH 2 OH. As it can be seen in Fig. 7, the T-dependence of k(T) for the CH 3 CN + OH reaction presents a ‘‘pipe-type’’ shape already observed for the CH 3 OH + OH 55,90,91 and CH 3 CH 2 OH + OH 54,92 reactions. In the low temperature regime, these ‘‘pipe-type’’ shape plots are characterized by a drastic change in the OH-reactivity at a certain temperature followed by a slighter increase in k(T) when the temperature approaches 10 K. The change in the trend for the T-dependence of k(T) appears between 177.5 and 250 K for the CH 3 CN + OH reaction, while the minimum k(T)was observed at B200 K for the reaction of OH with CH 3 OH 90 or CH 3 CH 2 OH. For the CH 3 C(O)CH 3 + OH reaction, the expected minimum k(T) must lie somewhere between 300 K and 146 K. 93 In contrast, for the CH 3 NH 2 +OH 68 and CH 3 CHO + OH 56 reactions, a ‘‘hammock-type’’ shape was observed, i.e. once the change in trend in the T-dependence k(T) is produced, it continuously increases at low temperatures. For CH 3 CHO, the minimum k(T) was observed at B500 K. 56 4.3.2. Experimental CH 3 CN + R reactions (R = OH, CN, C 2 H, C( 3 P) or O( 1 D)). When comparing the OH-reactivity towards CH 3 CN, with that of other radicals, such as CN, C 2 H, C( 3 P), or O( 1 D), we find scarce kinetic information. Particularly interesting is the comparison between OH and CN reactions, 86 where a similar behaviour was observed in the low temperature regime. For the CH 3 CN + CN reaction, 86 a ‘‘pipe-type’’ shape was also reported with an expected minimum k(T) between 168 and 258 K. The interpretation of the increase in k(T) when Table 4 Relative energies (DE) and ZPE-corrected energies (DH 0) with respect to reactants of the complex (C), transition states (TS), and products (P) for reactions (1a) and (1b). Optimizations at the M08-HX and oB97X-D levels and single-point calculations at the CCSD(T)-F12 level. Energies in kcal mol 1 DE M08/oB97 DE CC(M08/oB97) DH 0;M08=oB97 DH 0;CCðM08=oB97Þ Reactants 0.00/0.00 0.00/0.00 0.00/0.00 0.00/0.00 C1 5.20/4.04 3.88/4.03 4.24/3.30 2.90/3.09 TS1a 5.67/3.12 5.71/5.77 4.09/1.42 4.13/4.05 P1a 21.21/20.27 21.36/21.40 21.91/21.10 22.06/22.23 TS1b 2.18/0.03 1.20/1.12 3.89/1.69 2.91/2.82 P1b 28.98/32.31 28.46/28.42 24.90/27.90 24.38/24.01 Fig. 5 Schematic ZPE-corrected energy diagram at the CCSD(T)-F12/ M08-HX level of theory for reactions (1a) and (1b). See the last column of Table 4. Fig. 6 Pre-reactive complex (C1), TS1a, and TS1b structures with some selected distances (in Å) and bond angles (in degrees). Geometries optimized at the M08/MG3S level. PCCP Paper Open Access Article. Published on 13 January 2024. Downloaded on 12/18/2024 9:38:37 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online