The rate coefficients exhibited a strong negative temperature dependence, reaching (4.62 ± 0.84) × 10−11 cm3 molecule−1 s−1 at 32 K, and no pressure dependence was observed at 70 K. The potential energy surface (PES) of the CN + CH2O reaction was calculated at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory, with the lowest energy channel to reaction characterized by the formation of a weakly-bound van der Waals complex, bound by 13.3 kJ mol−1, prior to two transition states with energies of −0.62 and 3.97 kJ mol−1, leading to the products HCN + HCO or HNC + HCO, respectively. For the formation of formyl cyanide, HCOCN, a large activation barrier of 32.9 kJ mol−1 was calculated. Reaction rate theory calculations were performed with the MESMER (Master Equation Solver for Multi Energy well Reactions) package on this PES to calculate rate coefficients. While this ab initio description provided good agreement with the low-temperature rate coefficients, it was not capable of describing the high-temperature experimental rate coefficients from the literature. However, increasing the energies and imaginary frequencies of both transition states allowed MESMER simulations of the rate coefficients to be in good agreement with data spanning 32–769 K. The mechanism for the reaction is the formation of a weakly-bound complex followed by quantum mechanical tunnelling through the small barrier to form HCN + HCO products. MESMER calculations showed that channel generating HNC is not important. MESMER simulated the rate coefficients from 4–1000 K which were used to recommend best-fit modified Arrhenius expressions for use in astrochemical modelling. The UMIST Rate12 (UDfa) model yielded no significant changes in the abundances of HCN, HNC, and HCO for a variety of environments upon inclusion of rate coefficients reported here. The main implication from this study is that the title reaction is not a primary formation route to the interstellar molecule formyl cyanide, HCOCN, as currently implemented in the KIDA astrochemical model.Rate coefficients for the reaction of CN with CH 2O were measured for the first time below room temperature in the range 32–103 K using a pulsed Laval nozzle apparatus together with the Pulsed Laser Photolysis–Laser-Induced Fluorescence technique.Rate coefficients for the reaction of CN with CH Theoretical calculations of these kinetics parameters require knowledge of the potential energy surface (PES), in particular the energy of transition states and intermediates along the reaction coordinate. At very low temperatures, only reactions with small or no activation barriers are likely to proceed quickly. Calculated rate coefficients are highly sensitive to small changes to the PES, which can be on the order of the uncertainties of the 3 both PESs with small barriers to reaction and/or submerged barriers to reaction can cause sharp increases in rate coefficients at low temperatures until in some cases the collision limit is reached. For the case of small barriers to reaction under low temperature reaction conditions, weakly bound complexes in the entrance channel for reaction are formed with less energy compared with higher temperatures. This causes their lifetime for dissociation back to reactants to become sufficiently long that quantum mechanical tunneling through reaction barriers to products can become competitive, leading to a dramatic increase in the rate coefficient with decrease in temperature.3 Two examples are the reaction of OH with Complex Organic Molecules (COMs) such as CH3OH,3,5–8 and the reaction of C(3P) with H2O.9 For the case of submerged barriers, an increase in the rate coefficient with a decrease in temperature is observed consistent with a negative activation energy to reaction. Two examples are the reaction of C2H with O2,10 and the reaction of O(1D) with CH4.11 If a calculated PES is used in conjunction with kinetic models to predict k(T), any uncertainty in the height or width of relatively small potential energy barriers can lead to large uncertainties in the prediction of reaction rate coefficients. Therefore, these reaction-specific PES attributes are important to closely examine alongside robust experimental data spanning a broad temperature range, especially including low temperatures.There is a desire to understand what aspects of the reaction potential energy landscape control the behavior of the reaction rate coefficient and the branching to products as a function of temperature. As discussed in a recent perspective,2O (Reaction 1), which was suggested by astrochemists to be the primary formation route to the interstellar molecule formyl cyanide, HCOCN,12 a suggestion reinforced recently by the ab initio quantum calculations of the PES and master equation calculations of the rate coefficients by Tonolo et al.13 Chemical intuition and results from past studies of similar reactions14–16 suggest, however, that CN will likely attack CH2O via a hydrogen abstraction mechanism, forming hydrogen cyanide (HCN) and the formyl radical (HCO). Both reactants and predicted hydrogen abstraction products have already been observed in several astrochemical environments, such as Asymptotic Giant Branch (AGB) stellar winds,17,18 the InterStellar Medium (ISM),19–26 and dark clouds,27–30 providing further motivation for the current study at low temperatures. Additionally, the CN + CH2O reaction is relevant to combustion processes,31 planetary atmospheres,32 and Titan's atmosphere.33,34 The inclusion of new kinetic data for neutral–neutral reactions at very low temperatures can make a significant difference for the abundance of key species in interstellar environments.3,35The current study focuses on the low temperature reaction of the cyano radical with formaldehyde, CN + CH2O reaction measured overall rate coefficients between 297–673 K by Yu et al.36 and 294–769 K by Chang and Wang37  using the Pulsed Laser Photolysis–Laser-Induced Fluorescence (PLP–LIF) technique in resistively-heated flow cells. In these studies, k1(T) was found to decrease slightly with decreasing temperature, from approximately 4 × 10−11 cm3 molecule−1 s−1 near 770 K to approximately 1.5 × 10−11 cm3 molecule−1 s−1 at room temperature. The uncertainties quoted for these studies were on average 2.8% for Yu et al.,36 and 1.2% for Chang and Wang.37 It was predicted that a hydrogen abstraction process, forming HCN, was likely to dominate the chemical mechanism. Indeed, a prior theoretical study (QCISD/6-31G**//UHF/6-31G**) found only a very small (∼2.7 kJ mol−1) barrier to this H atom abstraction reaction.38 In comparison, a very recent calculation by Tonolo et al.13 (CCSD(T)/CBS+CV) reports a small submerged barrier for the abstraction pathway forming HCN. Further aspects of the reaction potential energy surface were explored in this calculation, with particular attention paid to addition reaction pathways forming CN–CH2O adducts, with bonds formed between the C of formaldehyde and either end of CN. However, the high-pressure limit rate coefficient (2 × 10−10 cm3 molecule−1 s−1) calculated using transition state theory and based on this new PES13 is close to an order of magnitude larger than the previously measured room temperature studies of YCW. Exothermic product channels for the reaction of CN with CH2O include:13,39,40Previous experimental work on the CN + CH39 for et al.;40 but for et al.13 is given. In the work presented here, low-temperature reaction rate coefficients were measured for the first time below 294 K for the CN + CH2O reaction using the PLP–LIF technique in a pulsed Laval nozzle apparatus. Using CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ, an ab initio PES was calculated for CN + CH2O and utilized in the Open Source MESMER software package41 to calculate temperature-dependent rate coefficients and product branching ratios. Using a fitting procedure, the parameters within MESMER were optimized to give the best agreement with experimental data over the range 32–769 K. These MESMER rate coefficients were then used to develop a parameterisation via a modified Arrhenius (MA) equation and extrapolated to 4 K. The fits of the rate coefficients were then incorporated into astrochemical simulations of cold dense clouds, AGB stars, and hot molecular cores.Enthalpies for 22.12O were performed in a pulsed Laval nozzle apparatus using the PLP–LIF technique, as shown schematically in Rate coefficient measurements for the reaction of CN with CH14,42–47 Gaseous formaldehyde (CH2O) is a difficult reagent to work with and particular care was given to its preparation from paraformaldehyde. Preparation of mixtures with the bath gas, its handling, and quantification of its concentration are necessary for accurate kinetics studies. Generation of the CH2O reagent gas was performed by controllably heating paraformaldehyde powder  in an evacuated 500 mL glass bottle (Duran) with a heat gun  to ∼70 °C. During heating of paraformaldehyde, the nascent CH2O gas was then passed through a −10 °C cold trap. The cold trap was submerged in ethanol  which was chilled with a refrigerated immersion probe . The cold trap was utilized to trap any water or other condensable byproducts generated during the heating of paraformaldehyde. The purified CH2O was then allowed to flow into evacuated cylinders until reaching a pressure of ∼200 torr (∼26.7 kPa). The cylinders were then filled with argon  or nitrogen  to ∼6 atm (∼608 kPa) total (generating ∼4.4% mixtures of CH2O/bath gas). The gas in the cylinders was then allowed to mix for >12 hours. This method of generating CH2O is similar to methods utilized previously by our group and other groups.14,48,49 To generate the cyano radical, CN, the vapor from the solid precursor cyanogen iodide  at 298 K.50,51) was entrained in ∼2 atm (∼203 kPa) of Ar or N2 bath gas.The generation of uniform, thermalized, cold, gas flows for kinetics measurements with this apparatus was described previously in detail and will only be described briefly here.2O, ∼0.005% ICN, and ∼99% Ar or N2 bath gas with a total flow rate of ∼2–5 slm. The gases were then allowed to mix in a mixing manifold prior to being pulsed through the Laval nozzle. Since CH2O was found to slowly repolymerize, depositing solid paraformaldehyde in the gas lines and causing the MFC calibration to slowly drift, the concentration of CH2O was determined via UV absorption measurements by sampling gas from the tubing between the mixing manifold and the pulsed valves for each concentration of CH2O utilized in a kinetics experiment.14 Measurements of the concentration of CH2O were made using a custom-made 1 m path length UV/Vis absorption cell filled to ∼1.2 atm (∼122 kPa) with the gas-mixture as measured by a capacitance manometer ). The light source for absorption measurements was a UVB lamp  with continuous output around ∼290–350 nm. Absorption measurements were performed with a UV/Vis spectrometer  with 0.75 nm resolution. Absorption spectra were integrated for ∼2 seconds and 4 spectral traces were averaged. Representative averaged UV absorption spectra for CH2O are shown in Fig. S1 in the ESI.2O in an N2 bath in the cylinders was determined to be approximately 4 days.The controlled mixing of reagent gases was accomplished through the use of Mass Flow Controllers (MFC)  such that final mixtures of gases were ∼0.1–1% CH2 bath gases. Impact pressure measurements using a Pitot tube were utilized to determine the Mach number along the flow, and via the Rayleigh equations, the density and rotational–translational temperature of the flow,4 as illustrated for temperature in Fig. S2  to be pulsed at 5 Hz into the pre-expansion region upstream of the Laval nozzle. Gas in the pre-expansion reservoir underwent a controlled expansion through a custom-made, axisymmetric, converging-diverging Laval nozzle such that the flow into the vacuum chamber at 0.3–2 torr (40–267 Pa) was a wall-less reactor. The vacuum chamber was continuously evacuated by two Roots blowers operating in parallel: a Roots blower (Leybold RUVAC 251) backed by a rotary pump (Leybold D65B) and a Roots blower (Edwards EH250) backed by a rotary pump (Edwards ED660) and the pressure in the vacuum chamber was monitored by a capacitance manometer ). Flow temperatures between 32–103 K were achieved by switching between Laval nozzles of Mach numbers between 2.49 and 5.00 and/or switching between Ar and Ng. S2 ESI.2O, the ICN precursor was photolyzed at 266 nm (∼30 mJ per pulse) using the fourth harmonic of a Nd:YAG laser (Quantel Q-Smart 850), which was directed co-linearly with the supersonic flow along the axis of the Laval nozzle  rotational line of the B2Σ − X2Σ  vibronic transition at 357.893 nm, which is generated by a Nd:YAG  pumped dye laser . This “probe” laser beam (2Σ − X2Σ) was focused through a series of lenses and through a bandpass filter at 400 nm with a FWHM of 40 nm  onto a Channel PhotoMultiplier (CPM) . The gain of the CPM was controlled with a custom-built time-dependent high voltage gating module in order to block scattered light from the pump laser at 266 nm. Digitization and integration of the CPM signal were performed on an oscilloscope , which is then transferred and saved to a computer for further analysis via a LabVIEW program. The timing of the experiment was also controlled via LabVIEW communication with a digital delay generator , which randomly varied the order of the time delays between the pump and probe laser after each gas pulse.In order to initiate reactions of CN + CHser beam  crossed 2.22O reaction in order to further explore the reaction mechanisms responsible for the behavior of the reaction rate coefficients as a function of temperature. Geometric structures of stationary points ) were first optimized at the BHandHLYP/aug-cc-pVDZ level of theory55–58 and further refined using M06-2X/aug-cc-pVTZ.59 Higher-level single-point energy calculations were performed at the CCSD(T)/aug-cc-pVTZ level60,61 to obtain more accurate energies. Vibrational frequency calculations were performed to evaluate zero-point vibrational energies (ZPVE), where TSs were found to have only one imaginary vibrational frequency. The vibrational frequency scaling factors for BHandHLYP/aug-cc-pVDZ and M06-2X/aug-cc-pVTZ are taken to be 0.9589 and 0.956, respectively.62,63 Intrinsic reaction coordinate (IRC) calculations were carried out for all TSs located during the PES search, unless otherwise specified, to verify that they are indeed saddle points on the minimum energy pathways connecting the respective local minima. In order to further explore the long-range reaction PES as the two reactants approach each other, relaxed scans were performed along the reaction entrance channels at the BHandHLYP/aug-cc-pVDZ and M06-2X/aug-cc-pVTZ levels of theory. For a relaxed scan, one of the geometric parameters is selected as the scan coordinate while all the other geometric parameters, unless being specifically frozen , are allowed to be optimised to give the minimum energy geometry. All electronic structure calculations were carried out using the Gaussian 09 program.64Theoretical approaches were used in this work to calculate the PES of the CN + CH41 in order to obtain the rate coefficients of the system. The stationary points of the ab initio calculations provide the energies, rotational constants, and vibrational constants required by the MESMER input file. The energy wells along the PES are divided into energy grains, where each grain couples the reactant, intermediate, and product species to one another via the microcanonical rate coefficients, k(E). The individual grains can be populated/depopulated by exchange with other grains via collisional energy transfer with the buffer gas. The microcanonical rate coefficients were calculated with either Rice, Ramsperger, Kassel, and Marcus (RRKM) theory65 for reactions involving a defined transition state or the inverse Laplace transformation (ILT) method66 for barrierless reactions. Collisional energy transfer probabilities were described using the exponential-down model.67 Corrections for quantum mechanical tunneling were also included using the Eckart expression.68 The set of coupled differential equations that describe each of the energy grains is known as the energy grained master equation (EGME) and can be described by:p is the population density vector containing populations of each grain from each well, and M is the transition matrix that describes the population evolution due to collision energy transfer and reaction. The solution to p(0) contains the initial conditions for each grain, U is the matrix of eigenvectors obtained from the diagonalization of M, and Λ is the diagonal matrix of corresponding eigenvalues, where the smallest are the chemically significant eigenvalues (CSE). MESMER solves the EGME and obtains the phenomenological rate coefficients from the CSE using the procedure described by Bartis and Widom.69 Using the time dependence of the concentration of all species calculated by MESMER, the branching yield of different products can be determined. In addition, MESMER has a built-in fitting feature using available experimentally measured rate coefficients, whereby input parameters, e.g. the energies of the stationary points, can be adjusted to best fit to the experimental data. The input file for MESMER simulations used in this work are included in the online ESI.†From the generated PES, statistical rate theory calculations were performed using the MESMER software program33.12O were obtained by first measuring the time-evolution of the relative transient LIF signal of CN as the radical reacted with an excess abundance of CH2O under pseudo-first-order conditions. The integrated LIF signal at each time delay between the photolysis laser (pump) and the dye laser (probe) was collected at least 5 times in order to obtain averaged traces of the temporal evolution of CN. Any background signal due to the probe-laser only was subtracted using the average value of data recorded prior to the pump-laser pulse.The temperature-dependent rate coefficients for the reaction of CN + CH52–54 CN into the CN  laser-probed state led to a rapid rise in the CN LIF signal (≲1 μs) which was not resolved in our experiments. Therefore, traces were analyzed after ∼5 μs following the initial rise in the signal, after which there was an exponential decay of CN due to diffusion, reaction with CH2O, and reaction with other possible species. Diffusion of CN out of the volume of supersonic flow through which the pump laser passed is given by:The initial photolytic production of CN and fast collisional relaxation of rotationally-excited2O , and potential reactions with other species  is given by:Xi represents other non-reagent species i. The observed pseudo-first-order rate coefficient for the loss of CN, kobs, is thus given by:N is the total number of species other than CH2O in the cold gas flow and kint represents the intercept of a linear fit of a plot of kobsversus [CH2O], with kint expected to be dominated by kdiff. Each CN integrated LIF decay trace, I(t), which is proportional to [CN]t, was fitted with a single exponential decay function:t is the pump–probe time delay, and I0 is the fitted value of LIF signal at t = 0 for exponential decay fits of the signal. Fits of The reaction of CN with CH2O concentration, the experiment was repeated at least five times and the kobs values obtained were averaged to give k̄obs. Second-order (bimolecular) plots of k̄obsversus [CH2O] were then generated at each temperature, examples of which are given in kint subtracted for clarity) with a linear least-squares fit of k1 from the gradient.For each CH2O] for which k̄obs is linear with [CH2O] to avoid any possible influence from CH2O dimers. Evidence for dimerization of CH2O was found in previous low-temperature studies at higher [CH2O] performed using a Laval nozzle, namely for rate coefficient determinations of the reactions OH + CH2O,15 CH + CH2O,14 and NH2 + CH2O.70 The range of values of the intercept (at [CH2O] = 0 molecule cm−3) was kint = 2200–7600 s−1, and the variation of k̄obs with [CH2O] without the subtraction of kint can be found in Fig. S3 of the ESI.2O] used to obtain kobs for a given temperature, always using less than the [CH2O] at which second-order plots have been seen to become non-linear in similar studies of radical + CH2O reactions.14,15 The values of k1(T) determined in this work for the CN + CH2O reaction, together with corresponding experimental conditions, are given in k1 was determined at two overall densities providing evidence that k1 is independent of pressure.For the linear fits of the second-order plots such as shown in k1 for CN + CH2O given in 2O reaction.14 The uncertainty quoted is a statistical error only from linear fits of the type shown in k1(T) is about 10–100 times smaller than k2OCH+CH(T) across the range of temperatures studied . Hence, for the same [CH2O] and available reaction time , the LIF signal from CN does not decay as much as for CH, resulting in a relatively larger uncertainty in the fit of kobs. The values of k1(T) determined in this work for T = 32–103 K for the CN + CH2O reaction are shown in 36,37 reported over the temperature range 294–769 K.The measurement uncertainty for k1(T) exhibits a weak positive temperature dependence above room temperature but a strong negative temperature dependence below ∼100 K, with a likely minimum somewhere between 100–200 K, suggestive of a change in reaction mechanism between the low and high temperature regimes. The values of k1(T) at low temperatures are less precise than those previously reported at higher temperatures,36,37 due to the challenges for this experiment as discussed above. It is noted though that for the previous data reported at higher temperatures, the absolute concentration of CH2O was not determined by UV absorption spectroscopy (in contrast to the low temperature work reported here), rather manometric/flow methods were used to determine [CH2O].3.22O reaction is shown in 2O entrance channel.The overall potential energy surface for the CN + CHThe geometries of the stationary points, obtained at the M06-2X/aug-cc-pVTZ level of theory, are shown in Fig. S4 ESI. The opt†−1) is identified following the approach of CN to CH2O. As seen in 3CH2 (P3) or HC(O)CN + H (P4). The formation of HCN + HCO  is accessible through the submerged barrier TS_VDW/P1 (−0.62 kJ mol−1) while the formation of HNC + HCO  involves a small positive energy barrier relative to the CN + CH2O entrance channel . The error of these two calculated barrier heights, even with the high level of theory used here, are such that they could both be either positive or submerged barriers if calculated at a different level of theory. At the CCSD(T)/aug-cc-pVTZ level of theory, the error of the energy is estimated to be 3.0–4.5 kJ mol−1 (250–450 cm−1).1A weakly bound van der Waals complex VDW (−13.3 kJ mol2O involves surmounting a large barrier TS_VDW/P3-4 (32.9 kJ mol−1), leading to the formation of the intermediate H2C–O–CN . H2C–O–CN can dissociate to form NCO + 3CH2  or undergo cyclization through TS_1/2 (−44.2 kJ mol−1) to form the cyclic intermediate Int2 (−81.2 kJ mol−1). By going through TS_2/3 (−62.5 kJ mol−1), the ring opens to form the intermediate H2C(O)CN . Breaking one of the CH bonds gives the products HC(O)CN + H . Although the IRC calculation did not converge successfully for TS_3/P4 (−41.0 kJ mol−1), judging from the vibrational mode of the imaginary frequency, it is likely that it is the TS connecting Int3 and P4. A dashed line connecting TS_3/P4 reflects the incomplete mapping of the IRC along this coordinate.The addition of CN onto the O atom of CH2O. Two different reactant approaches were investigated: the CN radical approaching from the oxygen side of CH2O, and CN approaching from the hydrogen side. For each scan, the distance between the two reactants was fixed while all other coordinates were allowed to optimize. When CN approaches from the oxygen side of CH2O, as shown in Fig. S5  reaches −5 kJ mol−1 relative to the entrance channel. In order to further explore this “flat” region of the PES, a relaxed scan from this point has been done using the O PBM data was replaced with SVG by xgml2pxml:<glyph-data id="z.dbd" format="PBM" resolution="300" x-size="8" y-size="10" xml:space="preserve">00000000000000000000000000000000111111110000000011111111000000000000000000000000</glyph-data><svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>Created by potrace 1.16, written by Peter Selinger 2001-2019</metadata><g transform="translate scale" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>C⋯N angle as the scanning parameter, as shown in In the case where CN approaches from the hydrogen side of CHg. S6 ESI, it is f2O, the potential energy first experiences a fairly flat region of the potential and then falls smoothly into the potential energy well corresponding to the van der Waals structure VDW. Thus, it is suggested that both ways of approach can eventually lead to the van der Waals structure VDW.While the CN radical rotates around CH2O to form HCN + HCO was studied in a previous calculation using the QCISD/6-31G**//UHF/6-31G** level of theory.38 This prior work suggested that the reaction mechanism involved overcoming a small positive ∼2 kJ mol−1 (emerged) barrier relative to the CN + CH2O entrance channel. The current study and a very recent study13 have not identified this pathway on the reaction coordinate, perhaps because of the lower level of theory used for geometry optimization (UHF/6-31G**) in the previous study.13 In the current study, instead of direct abstraction, an indirect channel to form HCN + HCO was found which involves the van der Waals complex VDW and a small submerged barrier TS_VDW/P1. A very recent theoretical study13 by Tonolo et al. also identified the VDW structure on the reaction PES /CBS + CV) leading to hydrogen abstraction to form HCN + HCO through a submerged barrier (−1.26 kJ mol−1). This pathway is consistent with our current work, with less than 0.5 kJ mol−1 difference in energy in the complex and barrier. Our work also details for the first time the presence of the HNC pathway from the VDW complex.A direct H abstraction mechanism of CN from CHet al. identifies several stationary points along the reaction coordinate not used in the current study. For example, Tonolo et al. claim a barrierless route to forming a C–C bond directly from the reactants, resulting in a tetrahedral intermediate which they label 1C in a deep potential well (−153 kJ mol−1). Although we agree that this structure can be formed (Int3 in −1 (CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ) higher in energy than the reactants, as shown in Fig. S7 , although we identified a new pathway to HC(O)CN + H through VDW which does not involve roaming and has a barrier (TS_VDW/P3-4).The work from Tonolo  Int3 in , we wereg. S7 ESI. It is l2O, which has been studied extensively in previous work,2,16,71–75 the results from the current work on CN + CH2O show that both systems share a similar shape of the PES in terms of the energy profile or mechanism for the H abstraction reaction. Following the approach of the two reacting species, a pre-reaction complex is formed followed by a transition state with a small barrier to form products. The difference in energy which determines whether the barrier is positive or submerged relative to the reactants energies is in the kJ mol−1 range, and hence within the uncertainty of most calculation methods, and so whether the transition state is submerged or not will depend on the level of theory used.In comparison with the PES of OH + CH2O computed with more robust methods tend to give lower values,2 decreasing the barrier from being slightly emerged to slightly submerged. The latest value reported from Machado et al.2 obtained at CCSD(T)/CBS level is approximately −5.7 kJ mol−1.For example, the energy values of the transition state for H abstraction channel of OH + CH3.3ab initio calculations were used as the inputs for the master equation solver, MESMER,41 in order to calculate the rate coefficients for CN + CH2O. From simulations over a wide range of temperatures (4–1000 K) and pressures (1015–1019 molecule cm−3) it was observed that HCN and HNC accounted for greater than 99.99% of the products, under all conditions. The reaction occurs initially via van der Waals complex (VDW) formation followed by transition states TS_VDW/P1 and TS_VDW/P2 to form HCN + HCO and HNC + HCO, respectively. The HNC channel never accounts for more than 1% yield, as shown in Fig. S8(b) (ESI−1 at the CCSD(T)/aug-cc-pVTZ level of theory1) can result in a reverse situation for which TS_VDW/P2  ESI. HoweverS_VDW/P2  can be s2O did not show a pressure dependence for k1(T), either experimentally near 1017 molecule cm−3 or from MESMER simulations between 1015–1018 molecule cm−3, the gas density was set at 1013 molecule cm−3 (so making sure in a pressure independent region) for the MESMER simulations. Above 1019 molecule cm−3 and T < 50 K, a pressure dependence was evident and the VDW species was populated. No pressure dependence was observed in the current calculations, implying that the van der Waals complex is not significantly stabilised, even at the lowest temperatures.As CN + CHTherefore, to a very good approximation, 66 was used to calculate the microcannonical rate coefficients for the barrierless formation of the VDW complex from CN + CH2O. The ILT approach overcomes the problem of explicitly assigning a transition state for a barrierless process, whose position would be varying as a function of temperature. The ILT method is especially convenient when experimental data are available, as for this study. The ILT parameters for VDW formation were assigned by:A∞ILT,VDW and n∞ILT,VDW are the ILT parameters which describe the rate coefficient for the formation of the VDW at the high pressure limit (in cm3 molecule−1 s−1 and unitless respectively). A reference temperature in the expression of 30 K was chosen so that the A factor is representative of the lowest experimental data point at 32 K, making the ILT more suited with the low temperature regime relevant to conditions in the ISM. The MESMER input file for these calculations is given at the end of the ESI.†The inverse Laplace transformation (ILT) method76 to adjust the parameters in order to minimize χ2:kcalc(T) is the MESMER calculated rate coefficient, and kexpt(T) and σ are the experimental rate coefficients and their associated error. For the MESMER fitting exercise, the present low temperature results used the experimental errors given in 36,37 . The MESMER fitting scenarios used in this work and the optimised kinetic parameters are summarised in As well as performing simulations, MESMER can adjust the important rate controlling parameters in ab initio calculated values, as shown in χ2/N is a measure of the goodness of fit, where χ2 is given by N is the number of degrees of freedom, which is equal to the number of data points minus the number of fitting parameters. A χ2/N value close to 1.0 represents a good fit, and from Initially, MESMER data fitting was carried out with energies fixed at the 36,37  is submerged, i.e. is negative with respect to the reagent energies, and a reaction mechanism proceeding via a submerged transition state is going to predict k1 to decrease with increasing temperature, which is at odds with the high temperature literature data, which requires the transition state to have a positive energy with respect to the reagents. The simplest way to account for the variation of k1 across the full range of temperatures is to increase the transition state energy so that it is positive, making it consistent with the high-temperature literature data trends and to increase the imaginary frequency of the tunnelling coordinate in the transition state so that quantum mechanical tunnelling overcomes the effect of the positive barrier, leading to an increase in k1 at lower temperatures, as is observed experimentally. When the model scenario Laval + Lit 2  from our ab initio calculated values at the CCSD(T)/aug-cc-pVTZ level of theory.1 However, this adjustment seems reasonable given it is close to the estimated uncertainty at the CCSD(T)/aug-cc-pVTZ level of theory of ∼3.0–4.5 kJ mol−1.1 Similarly, even though the imaginary frequencies has been increased significantly, from 205 cm−1 to 806 cm−1, the adjusted value of ∼800 cm−1 again is not unreasonable.5 As both transition states emanate from the same pre-reaction complex leading to H abstraction products, it seems reasonable to adjust the energies of each of these by the same amount whilst maintaining the same difference between them. The motivation for changing the initial ab initio results to the fitted results using MESMER is to show that the experimental data can be fitted well if the potential energy surface, specifically for these two transition states, is modified. Just changing the properties of the two initially formed transition states which are formed from the same pre-reaction complex is the most straightforward way to do this.The MESMER simulation model scenario Laval + Lit 2 of the fit to the data is shown in k1 occurs around 150 K, which is the point at which the controlling influences of quantum mechanical tunnelling and the 4.0 kJ mol−1 barrier for TS_VDW/P1 become balanced. Also included in k1 is approximately twice that of the Laval + Lit 2 model. This significant difference emphasizes how sensitive k1 is to the parameters used in the models when extrapolating down to very low temperatures, i.e. <10 K, as both the Laval and Laval + Lit 2 models give almost the same quality fits to the experimentally measured k1 using the Laval nozzle.From 3.4k1(T) over a very wide temperature range from 4–1000 K, as tabulated in Table S10 (ESIk1(T) together with the MESMER simulations using the Laval + Lit 2 model. As CN + CH2O did not show a pressure dependence for k1(T), neither experimentally near 1017 molecule cm−3 nor from MESMER simulations between 1015–1018 molecule cm−3, the gas density was set at 1013 molecule cm−3 for the MESMER simulations; above 1019 molecule cm−3 and T < 50 K a pressure dependence was evident and the VDW species begins to become stabilized. From 50–1000 K, a 30 cm−1 grain size was sufficient to calculate a converged rate coefficient, but it was reduced down to 2 cm−1 for the 4–50 K simulations. It was not possible to run MESMER simulations at lower temperatures. Over the range of 4–1000 K, the MESMER simulations indicate that HCN + HCO are the only significant products formed. The fractional yield of HNC is less than 0.33% for all temperatures between 4–1000 K, because the energy of TS_VDW/P2 (forming HNC) is 4.59 kJ mol−1 above that of TS_VDW/P1 (forming HCN), and the imaginary frequencies of the both transition states is 806 cm−1. The temperature dependence of the fractional yield of the HCN and HNC products is shown in Fig. S8(b)  ESI. The reshttps://udfa.ajmarkwick.net) or KIDA (https://kida.obs.u-bordeaux1.fr78) databases) to represent k(T) is a single modified Arrhenius (MA) equation:α, β and γ are best-fit parameters. Although Table S10 (ESIk1(T) at a given temperature in the range 4–1000 K, a parameterisation in the form of k1(T) using an expression traditionally used by astrochemical modellers. However, a single MA k1(T) for CN + CH2O over the full range of temperatures 4–1000 K. It is especially inadequate for the k1 values around 150 K, where the T dependence changes from negative to positive, see k1 is rapidly increasing. Running MESMER simulations down to the lowest temperatures means that no extrapolation of fits using k1(T), and Fig. S10, ESIThe equation utilized commonly as a parameterisation in several astrochemical models  for use in astrochemical modelling.In the parameterization given by . S10 ESI. Also ink1(T) gives values that are realistic at the very lowest temperatures, classical capture theory (CCT) calculations have been carried out for the CN + CH2O reaction to calculate the rate coefficient at the collision limit kcoll(T), using the molecular parameters given in Table S11  is dominated by large dipole–dipole interactions below 600 K. As shown in kcoll at ∼3 K is equal to 1.3 × 10−9 cm3 molecule−1 s−1, and from the MESMER simulations k1(T) is still calculated to be a factor of 10 or so less than kcoll(T), and does not get close nor indeed exceed kcoll(T) for all temperatures relevant to conditions used in astrochemical models.As a check of whether the parameterisation for e S11 ESI. As showe S11 ESI for the 4.k1(T)) for the reaction of CN with CH2O over a wide range of temperatures. These were a dark cloud model for a total density of 2 × 104 cm−3, at T = 5, 10 and 30 K, hot cores/corino models at higher densities and temperatures, and C-rich and O-rich AGB outflow models, further details of which are given in the following sections. The network for the model utilised the UMIST Rate12 kinetic database77 with updates to rate coefficients to include recent measurements of SiH with O279 and CH with CH2O.14 Further details are given in Section 3.4, together with the parameterisation used for k1(T).Three different astrochemical environments were considered in order to explore the impact of the newly measured and evaluated rate coefficients (4.1n(H2) = 104 cm−3 and temperatures of 5, 10 and 30 K, comparing results using the new Laval + Lit2 MA parameterisation to those obtained with the rate coefficient of the CN + CH2O reaction in the UMIST Rate12  database. The latter adopts an energy barrier of 826 K (6.9 kJ mol−1) over the range 297–2500 K and, in the lack of low-temperature information, the rate coefficient extrapolates to zero at low temperatures. We find that the inclusion of reaction (1), even at its faster low-temperature fit, makes no difference to the abundances of CN, CH2O, HCO, HCN and HNC  = 104 cm−3, T = 10 K).80 Our results indicate that, since CH2O is an abundant reactant, the total production rate of HCOCN is likely to be an order of magnitude less than the observational requirement.80The inclusion of the title reaction does, however, impact on the abundance of HCOCN which our 4.2n(H2) = 5 × 107 cm−3, T = 225 K) and the Plateau (n(H2) = 106 cm−3, T = 125 K). In these sources, initial abundances are molecular and reflect those in dust grain ice mantles which evaporate in the hot gas surrounding a massive young star. Our initial fractional abundances are taken from Doddipatla et al.81 Although these values are specific to these objects, our results are likely appropriate to other hot core/corino sources. Fig. S12 (ESI77) rate coefficient. The major reason for this is that neither CN nor CH2O are abundant species in these evaporating interstellar ice mantles nor are they produced in significant fractions in the hot gas chemistry. We note in passing that the difficulty mentioned above, in reproducing the observed HCOCN abundance in hot sources, is likely to persist until kinetic data for any new significant HCOCN production reactions are measured or calculated.We also investigated higher density, higher temperature models appropriate for hot core/corino sources. In particular, we modeled two of the hot sources in the Orion Molecular Cloud, the Orion Hot Core . Following Van de Sande et al.,82 the temperature of the outflows is parameterised as  where T* and R* are the stellar temperature and radius, respectively, and ε is the power law exponent. We assumed a value of R* = 5 × 1013 cm, T* = 2300 K and ε = 0.7. The outflow is assumed to be spherically symmetric with an expansion velocity of 15 km s−1, while varying the mass-loss rate between 10−5, 10−6, and 10−7 Msun per year. Similar to West et al.,14 we considered both an O-rich and C-rich outflow, with the abundance of parent species taken from Agúndez et al.83The AGB outflow model is based on the publicly available UMIST CSE model  are not changed. Fig. S13 and S14 , for the reaction CN + CH2O have been measured for the first time below room temperature in a pulsed Laval apparatus using the PLP–LIF technique. A negative temperature dependence of k1(T) was observed below ∼100 K, in contrast to the positive temperature dependence of k1(T) reported previously in experiments performed above 300 K. Both ab initio calculations of the potential energy surface (PES) for the reaction and MESMER rate-theory simulations making use of this PES were performed in order to explore the overall mechanism that is consistent with the observed k1(T). Two low energy pathways were found on the PES through a VDW complex (binding energy of 13.3 kJ mol−1) with either a small positive barrier (3.97 kJ mol−1) to form HNC + HCO, or a submerged barrier (−0.62 kJ mol−1) to form HCN + HCO. This calculated PES can explain the negative temperature dependence of the rate coefficients at low temperatures but not the previously measured literature rate coefficients at room temperature and above. For the formation of formyl cyanide, HCOCN, a large activation barrier of 32.9 kJ mol−1 was calculated. Increasing both the energy of the transition state for the formation of HCN + HCO products from −0.62 to +4.0 kJ mol−1 (which is within the accuracy of the theory used) and also its imaginary frequency to 806 cm−1 in the MESMER simulations, enabled the temperature dependence of k1(T) across the entire experimental data to be reproduced very well. The energy and imaginary frequency of the other transition state for formation of HNC + HCO products were changed by the same amounts so their relative values were the same, ensuring that only the HCN + HCO remains the important one for this reaction. The postulated mechanism is similar to previous reactions of OH with volatile organic compounds where the formation of a weakly bound van der Waals complex, whose lifetime is extended at low temperature, is then followed by quantum mechanical tunneling through a small activation barrier to products.3Low-temperature rate coefficients, k1(T) from 4–1000 K, and this dataset was used to recommend a parameterization from best-fit modified Arrhenius expressions for k1(T) for use in astrochemical modelling. Even at 4 K the simulated values of k1(T) were a factor of 10 less than the collision limit. This parameterization of k1(T) was used as input to astrochemical models of dark clouds, hot core/corinos, and Asymptotic Giant Star (AGB) stellar outflow environments using the UMIST Rate12 (UDfa) network of reactions. For a range of temperatures, the models yielded no significant changes in the abundances of HCN, HNC, and HCO upon inclusion of their formation using the rate coefficients reported here. It was found that the new rate coefficients for the reaction CN + CH2O do not have a significant effect on the modeled abundances of reagents and products of this reaction due to several other competing reactions with large rate coefficients. However, we note that the uncertainty at the CCSD(T)/aug-cc-pVTZ level of theory, (∼3.0–4.5 kJ mol−1) is similar to the difference in energy (4.59 kJ mol−1) calculated for the transition states forming HCN+HCO and HNC+HCO products. Hence, although HCN+HCO are the most likely products, it is possible that the HNC channel may also be the dominant one, as within the calculation errors the HNC+HCO transition state might be the lower one. In order to examine the impact of the scenario in which HNC is the dominant product of the reaction, the rate coefficient k1 was maintained at the same value, but it was assumed that HNC+HCO formed 100% of the products. For a 10 K rate coefficient and 100% formation to HNC, the title reaction provides less than 1% of the overall HNC production rate for cold environments such as molecular clouds. For the Orion Hot Core case at higher temperatures, the HNC+HCO reaction provides around 1% of the total HNC production rate. So, assuming 100% HNC production rather than very little production from the CN+CH2O reaction made a negligible difference to the modelled HNC abundance.Using the MESMER package the product branching ratio was calculated and the formation of HNC was not found to be important as a reaction channel. MESMER was then used to simulate the rate coefficients The main astrochemical implication of this paper is that the title reaction cannot contribute to the formation of formyl cyanide, HCOCN, in interstellar clouds, as has been suggested previously and currently implemented in astrochemical models.DEH and LD conceived the experiment, NAW and ER performed the experiments, LHDL and MAB performed the theoretical calculations, TJM and MVdS performed the astrochemical calculations, NAW, DEH and JHL wrote the manuscript with contributions from all authors.There is no conflict of interest to declare.CP-025-D2CP05043A-s001