Mechanistic and kinetic approach on methyl isocyanate (CH3NCO) with OH and Cl

Methyl isocyanate (CH3NCO) is an important agricultural and industrial precursor and emission from wildfires. In this paper, we study thermodynamic, kinetic and mechanistic approach of CH3NCO with OH radical and Cl atom using M06-2X level in combination with 6-311++G(d,p) basis set, and corrected by relatively accurate the single point CCSD(T) method. The rate constants of the title reactions are calculated in the temperature range of 200 and 400 K. The reactions exhibit a positive temperature depending on rate constants and fitted to the three-parameter Arrhenius expressions: k OH = 2.76 × 10−19 T2.65 exp[−486/T] and k Cl = 3.51 × 10−18 T2.28 exp[95/T] with Wigner’s and k OH = 4.95 × 10−19 T2.57 exp[−504/T] and k Cl = 1.20 × 10−17 T2.11 exp[63/T] with Eckart’s tunnelling corrections between 200 and 400 K. The calculated rate constants for the CH3NCO + OH/Cl reaction are in very good agreement with the previously reported experimental values. Thermochemical data (ΔH°298 K and ΔG°298 K) illustrate the feasibility and spontaneity of the title reactions. The atmospheric degradation mechanism of CH3NCO is discussed. The OH/Cl-driven tropospheric lifetimes and global warming potential (GWP) for CH3NCO were calculated. It was concluded that CH3NCO existed in the Earth’s atmosphere for a few days and its GWP is not significantly large. The ozone formation potential of CH3NCO was also reported in this work. GRAPHICAL ABSTRACT


Introduction
Methyl isocyanate (MIC) is one of the important chemicals used in the production of plastics, polyurethane foam and pesticides. It is highly volatile and explosive in nature. Therefore, it is handled very carefully and shipped as a liquid. It has also a very irritating smell and is hazardous for for our health. In December 1984, MIC was leaked from Union Carbide India Limited (UCIL) pesticide plant at Bhopal, India, which killed more than 3,800 people [1]. Most deaths were observed after the exposure for several hours [2][3][4][5] suffered initially from choking sensations and difficulty in breathing which ultimately led to death. Hence, it concludes that the lethal effects of MIC were caused by pulmonary complications [6]. MIC is the simplest and most toxic member of the isocyanate family. It can also be formed from the photochemical degradation of its isoelectronic compound methyl isothiocyanate (CH 3 NCS) [7,8]. Atmospheric degradation of N-methylformamide (CH 3 NHCHO), is also a potential source of MIC in the atmosphere [9,10]. Due to its large industrial applications an increasing amount of MIC gets into the atmosphere inevitably, where it is degraded almost by the reaction with OH radicals, thus its atmospheric degradation must be understood.
Only two experimental studies are available on the kinetics of MIC with tropospheric oxidant, OH radical to the best of our knowledge [11,12]. Lu et el. [11] measured the room temperature rate constant for the reaction OH with MIC using the relative-rate technique. Very recently, Papanastasiou et al. [12] measured the temperature dependence rate constants of the CH 3 NCO + OH reaction using the photolysis−laserinduced (PLP-LIF) fluorescence absolute technique. The available room temperature rate constants fall in the range of (4-0.14) × 10 −12 cm 3 ·molecule −1 ·s −1 . Large discrepancies between these two measurements triggered us to perform a further investigation into this reaction. It is well known that the impact of the Cl atom on volatile organic compounds (VOCs) near coastal and urban areas is quite large. Papanastasiou et al. [12] also investigated the room temperature rate constant of the Cl atom with MIC using the relative rate technique. The available room temperature rate constant data predicts that the atmospheric lifetime of MIC is approximately eighty-five days, which implies that this compound has a potential impact on the biological environment surrounding us. Although two experimental measurements were available on the studied reactions, no theoretical calculations are reported to understand the detailed mechanisms and kinetics. In addition, temperature-dependent studies on MIC + Cl reaction are lacking. The gas phase reaction is initiated via the abstraction of a hydrogen atom by OH and Cl from the CH 3 group of MIC.
CH 3 NCO + OH → CH 2 NCO + H 2 OIn the current area, ab-initio and Density Functional Theory (DFT)-based quantum chemical computations turn out to be a powerful tool for experimentalists by elucidating the kinetics and mechanism of various gas phase atmospheric reactions which are challenging to perform experimentally. In this work, for the first time, we have theoretically investigated the temperature-dependent rate constants of CH 3 NCO with OH radical and Cl atom. First, we concentrated on the kinetics and the mechanism of the hydrogen atom abstraction of CH 3 NCO by OH radical and Cl atom. Subsequently, a thorough investigation was carried out on the atmospheric fate of the CH 2 NCO radical generated from CH 3 NCO after hydrogen abstraction.
To assess the accuracy of our results, we have compared them with the available temperature-dependent data in the literature. Furthermore, the tropospheric lifetime, GWPs and ozone formation potential of MIC are also investigated in this work.

Computational methodology
Gaussian 09 programme was used for all the quantum chemical calculations [13]. Stationary points on the potential energy surface (PES), such as reactants, transition states (TSs), products and possible reactants and product complexes, were optimised at the DFTbased correlation functional, M06-2X [14,15] with 6-311++G(d,p) basis set. This method has been established to produce the most reliable data for the kinetics and thermochemistry of atmospheric-relevant reactions [16][17][18][19][20][21][22]. The analysis of vibrational frequencies has been also performed for all the stationary points at the same level. The optimised structures have real frequencies, except transition states have a single imaginary frequency. Using Gauss view 05, all the stationary points and normal frequency modes were viewed. The zeropoint energy (ZPE) corrections were also calculated at the same level of theory. Moreover, the M06-2X level with 6-31G(d) was used to perform the intrinsic reaction coordinate (IRC) calculations to check that the TSs were properly linked to the designated minima. The minimum energy pathways (MEP) for all the reaction channels were constructed at the M06-2X/6-31G(d) level with a gradient step size of 0.01 (amu) 1/2 bohr. From IRC calculations, we also observed the formation of reaction and product complexes in both sites of each reaction channel. To improve the correctness of energy, single-point energy calculations were performed for all the optimised geometries at coupled cluster single-double and perturbative triple excitations [CCSDT(T)] combined with the 6-311++G(3d,3pd) basis set. The combination of CCSD(T) and M06-2X typically produces results that are accurate to ∼ 1-2 kcal·mol −1 . The spin contamination value < S > 2 for each stationary point was calculated and found to be negligible ( ∼ 0.75-0.85). The rate constants were computed using conventional transition state theory (vide infra section 3.2) over a temperature range of 200 and 400 K.

Stationary points and energetics
The potential energy surface of CH 3 NCO was scanned at M06-2X/6-311++G(d,p) level along the dihedral angle HCNC and the resulting energy diagram is shown in the supporting information ( Figure S-I). The results plotted in Figure S-I show that the only most stable conformer exists for CH 3 NCO. The minimum-energy geometries of CH 3 NCO + OH/Cl reactions are considered for our rate constant calculations. The optimised equilibrium geometries of the reactants, reaction and product complexes, transition states and products calculated at the M06-2X/6-311++G(d,p) level are shown in Figure 1. Three hydrogen atoms in methyl isocyanate molecules have the same environment and are symmetrically equivalent. Therefore, only one transition state is identified, namely, TS-OH/TS-Cl with reaction path degeneracy considered to be three. In the reaction path, we could locate a weakly bound reaction complex (RC-OH/Cl) and product complex ( Since the elongation of the breaking bonds is less than that of the forming bonds, it specifies that both transition states are reactant-like and proceed through the 'early' transition states [23]. This type of behaviour is expected for exothermic reactions. All the geometry parameters of the reactants, RCs, TSs, PCs and products obtained at M06-2X/6-311++G(d,p) level are listed in the supporting information (Table S-I). The vibrational frequencies calculated at a similar level are tabulated in Supporting Information (Table S-II). Both transition states have only one imaginary frequency signature at 867i (TS-OH) and 646i (TS-Cl). The schematic representation of the potential energy profile for H-atom abstraction from the CH 3 NCO methyl group by OH and Cl has been constructed in Figure 2. We can see that the reaction complexes (RC-OH and RC-Cl) are first formed, which are about 4.8 and 5.4 kcal mol −1 , below the energy of reactants. Then, starting from the reaction complexes, the reaction channels go through the transition states (TS-OH and TS-Cl) with energy barriers of 3.1 and 0.9 kcal·mol −1 , respectively. In the following exit routes forming the product complexes (PC-H 2 O and PC-HCl), which are about −26.0 and −9.5 kcal/mol, respectively, and more stable than the corresponding products, P-H 2 O (−22.2 kcal mol −1 ) and P-HCl (−5.3 kcal·mol −1 ). The energy barrier of TS-OH is higher than that of TS-Cl, indicating that OH reactivity with CH 3 NCO is lower than Cl. The energy of activation ( E 0 ‡ ), the entropy of activation ( S ‡ ) and reaction energies ( E r ) for all the stationary points were calculated at 298 K and 1 atmospheric pressure for all the reaction channels in the title reaction are tabulated in Table 1. The formation of alkyl isocyanate radical, CH 2 NCO by OH radical and Cl atom via H-abstraction reaction is further degraded into stable products in a nitrogen-rich environment. We have studied the subsequent chemistry originating from CH 2 NCO radical in a nitrogen-rich environment. The detailed reaction steps involved in the oxidation of the CH 2 NCO radicals are given in Table 1. We can see that the CH 2 NCO radical reacts rapidly to form an OCNCH 2 O 2 peroxy radical (INT) via a reaction complex (RC-O 2 ) which is 1.6 kcal/mol lower than the reactants followed by a transition state (TS-O 2 -add) with an energy barrier of −5.3 kcal mol −1 . The OCNCH 2 O 2 peroxy radical (INT) reacts with NO to form corresponding alkoxy radical, OCNCH 2 O and NO 2 and which are 11.5 kcal mol −1 stable than the reactants. The OCNCH 2 O radical has two possible reaction channels leading to stable products. First, unimolecular decomposition through the C-N bond cleavage to form formaldehyde, CH 2 O and an isocyanate radical, NCO radical with reaction energy ( E r ) of 15.9 kcal·mol −1 . Second, the reaction with O 2 by H-transfer reaction leads to the formation of the most stable carbonyl compound, formyl isocyanate, OCNCHO and hydroperoxy radical, HO 2 with reaction energy ( E r ) of −37.9 kcal·mol −1 via an energy barrier of 15.9 kcal·mol −1 (TS-O 2 -abs). Formyl isocyanate, OCNCHO further degraded unimolecular into NCO and formyl radical, HCO by photochemical process with reaction energy of 119.3 kcal·mol −1 .
In order to calculate the spontaneity and feasibility of the reaction channels, the thermochemical properties, such as Gibbs free energy ( G 0 ), standard enthalpy ( H 0 ) and entropy ( S 0 ) changes, were computed at 1 atmospheric pressure and 298 K for all the reaction steps are listed in Table 1. We observed that the H 0 and G 0 of the H-abstraction from CH 3 NCO methyl group by OH radical are found to be −22.5 and −24.4 kcal·mol −1 while for Cl, the values are −7.0 and −10.1 kcal·mol −1 , respectively. These results indicate that H-abstraction channels are exothermic ( H 0 < 0), exergonic ( G 0 < 0) and spontaneous in nature. Moreover, it also suggests that H-abstraction by OH radical is thermodynamically more feasible than that of Cl. The oxidation channel of CH 2 NCO radical is also exothermic (−26.7 kcal·mol −1 ) and endergonic (−15.1 kcal·mol −1 ) in nature for the formation of peroxy radical. H 0 and G 0 of the NO oxidation of peroxy radical to alkoxy radicals are −12.4 and −14.1 kcal·mol −1 suggesting    Reaction steps the exothermic and exergonic reaction, respectively. However, the thermal unimolecular decomposition of alkoxy radical to formaldehyde and isocyanate radical is highly endothermic (20.7 kcal·mol −1 ) and endergonic (11.8 kcal·mol −1 ) in nature and indicate that the reaction is non-spontaneous. Alkoxy radicals are also oxidised to formyl isocyanate and HO 2 radicals through a highly exothermic step and released a huge amount of energy.
Formyl isocyanate is further degraded into isocyanate and formyl radical by absorbing photochemical energy.

Rate constant calculations
To the best of our knowledge, only two kinetic studies are available in the literature based on experimental measurements [11,12]. However, so far no temperature-dependent investigations based on theoretical calculations are reported. In our study, the rate constant calculations were carried out in the temperature range between 200 and 400 K with an interval of 25 K and 1 atm pressure. To check the variational effects, classical potential energy (V MEP ), vibrational ground-state adiabatic potential energy (V a G ) and the zero-point energy (ZPE) for the H-abstraction reactions were plotted as a function of the intrinsic reaction coordinate at the M06-2X/6-31G * level of theory, where V a G = V MEP + ZPE (Figure S-II and S-III). As seen in the figures, there is a shallow flat minimum on the ZPE curve so that the location of the maxima of the V MEP and V a G curves exactly coincides with each other, indicating the variational effects to be completely negligible. The rate constants data of the CH 3 NCO + OH/Cl reaction were estimated from Equation (1) where E = 0 is the zero-point-corrected energy difference between the transition state and the reactants Q = (TS) and Q R (T) denote the partition function for the transition state and reactants, respectively at temperature T, σ is the reaction path degeneracy of each reaction channel, k B is the Boltzmann constant, h is the Planck's constant, (T) is a tunnelling correction factor taken into account. (T) was calculated using Wigner's and Eckart's methods. The electronic partition function of OH radical Q E (OH)was evaluated by taking the splitting of the 2 ground state into two closely spaced energy levels 2 3/2 and 2 1/2 by 139.7 cm −1 , into account [24]. Similarly, the two spin-orbit (SO) states 2 p 3/2 (lowest) and 2 p 1/2 of Cl having degeneracies of 4 and 2, respectively, and separated by 882.4 cm −1 were included in the electronic partition function of Cl atom calculations [24].
where c 0 is the velocity of light in a vacuum. The partition functions were estimated by taking the harmonic vibrational frequencies and rotational constants of the reactants and transition states obtained at the M06-2X level. We have considered the anharmonicity of internal -CH 3 torsional modes using hindered rotor approximation. Low vibrational frequencies are taken into account for internal rotations and the hindered rotor partition functions were calculated using the method proposed by Chuang and Truhlar [25]. Lower vibrational frequencies and the hindered rotor partition functions at the studied temperatures are listed in the supporting information (Table S-III). The energy barrier obtained at the CCSD(T)/6-311++G(3d,3pd) level was used for rate constant calculations. It should be noted here that reactants and product complexes were taken into consideration while computing the rate constants. These complexes contribute to tunnelling corrections and also change the size and shape of the energy barrier. Hence, the effective forward barrier from the reaction complex to the transition state and backward barrier from the product complex to the transition state was used in Eckart's tunnelling factor, (T), whereas the imaginary frequency associated with the transition state was used for Wigner's tunnelling factor, (T). The calculated rate constants (in cm 3 ·molecule −1 ·s −1 ) for the H-atom abstraction from CH 3 NCO by OH and Cl in the temperature range of 200-400 K along with the available reported values are given in Table 2. The tunnelling contribution, indicated in Table 2, is small at lower temperatures and is found to be decreased and negligible as the temperature increases. The tunnelling factor (T) varies from 2.9 to 1.4 for H-abstraction by OH radical and from 2.3 to 1.2 for H-abstraction by Cl atom in the temperature range of 200-400 K.
Lu et al. [11] experimentally measured the rate constant of OH radical reaction with methyl isocyanate at 293 K using the relative-rate technique in a 100 L Tedlar bag reactor with off-line Gas Chromatography-Mass Spectrometry (GC−MS) by monitoring the loss of methyl isocyanate and reference compounds (toluene and m-xylene). The rate constant reported for the reaction of OH with methyl isocyanate by Lu et al. [11] to be 3.62 × 10 −12 cm 3 ·molecule −1 ·s −1 at 293 K, which is more than one order of magnitude larger than the recently reported rate constant by Papanastasiou et al. [12] at 295 K. They used photolysis-laser-induced (PLP-LIF) fluorescence absolute rate measurement technique to investigate the kinetics of CH 3 NCO + OH reaction and they reported a rate constant of (1.36 ± 0.04) × 10 −13 cm 3 ·molecule −1 ·s −1 at 295 K. The large discrepancies between these results are due to the interference of secondary NOx chemistry with the loss of methyl isocyanate and the reference compound. In the NOx environment, Papanastasiou et al. [12]'s absolute PLP-LIF measurement rate constant are ten times larger than NOx free environment and close to Lu et al. [11] value. At 295 K, our calculated rate constant value is (1.8-2.0) × 10 −13 cm 3 ·molecule −1 ·s −1 , which is in very good agreement with Papanastasiou et al. [12]'s value. They also experimentally measured the temperaturedependent rate constants of OH radical reaction with  Papanastasiou et al. [12] methyl isocyanate between 295 and 375 K. They observed a positive temperature dependence on rate constants over the studied temperature range. In our study, we observed a similar trend in the studied temperature range. Figure 3 displays the Arrhenius plot of the computed rate constants of CH 3 NCO + OH reaction in the temperature range of 200-400 K along with the available experimental results. In this context, we can say that the current temperature-dependent theoretical study is in very good agreement with the experimental values above room temperature and give some light on the rate constants below room temperature. Our calculation also indicates the variation of rate constants with temperature being slow in the lower-temperature region. Rate constants values increase with an increase in temperature. Papanastasiou et al. [12] also reported the room temperature rate constants of Cl atom reaction with methyl isocyanate using the relative rate technique. Our calculated rate constant of the Cl atom with methyl isocyanate is (2.1-2.5) × 10 −12 cm 3 ·molecule −1 ·s −1 at 296 K, which is slightly lower than the experimentally measured value, 5.6 × 10 −12 cm 3 ·molecule −1 ·s −1 at the same temperature. However, no temperature-dependent studies are available so far in the literature. We have calculated the temperature dependence rate constant between 200 and 400 K. Our calculation indicates a positive temperature dependence on rate constants between 200 and 400 K. Figure 3 also displays the Arrhenius plot of the computed rate constants of CH 3 NCO + Cl reaction in the studied temperature range along with the only available experimental result at 296 K. The discrepancies between experimental and theoretical results are due to the uncertainty associated with the measured and calculated rate constants. Papanastasiou et al. 12 reported ∼ 9-15% total uncertainties in their measured rate constants for the CH 3 NCO + OH/Cl reaction, which is obtained from the measurement uncertainties in the instruments used to obtain the pressure, temperature and reactant concentrations. In addition, the uncertainty associated with previously measured reference reaction rate constant in their measurements and the estimated contribution from NCO radical secondary chemistry. In our calculations, uncertainties are associated with the estimation of energies for the complexes, transition states and products at the studied level of theory. We have added ±1 kcal·mol −1 to the barrier heights and calculated the lower and upper bounds rate constants for both H-atom abstraction reactions in the temperature range of 200-400 K which are given in the supporting information (Table S-IV  and S-V). Table 2 also shows the Arrhenius parameters obtained for the reaction of CH 3 NCO with OH radical and Cl atom along with the available experimental values.

Atmospheric implications
The atmospheric degradation of methyl isocyanate primarily occurs via its reactions with various tropospheric oxidants like OH and NO 3 radicals, O 3 and Cl atoms. However, the rate constants for the reactions between methyl isocyanate with NO 3 and O 3 are expected to be slower than with OH and Cl. Therefore, loss of methyl isocyanate due to reactions with NO 3 radical and O 3 is a negligible contribution to its total atmospheric lifetime. The OH-and Cl-driven tropospheric lifetime (τ ) of methyl isocyanate was estimated using the following equation.
where X = OH or Cl The global weighted-average OH concentration [26,27] (10 6 radicals cm −3 ) and CH 3 NCO with OH radicals reaction's rate constant at 277 K were used to calculate the OH-driven tropospheric lifetime (τ OH ) of methyl isocyanate. The calculated OH-driven tropospheric lifetimes of methyl isocyanate are estimated using the rate constants obtained with Wigner's and Eckart's methods, which are 83 and 75 days, respectively. These values are in very good agreement with the experimentally observed OH-driven tropospheric lifetime value by Papanastasiou et al. [12] ( ∼ 85 days). By taking the global average Cl atom concentration reported by Singh et al. [28] (10 3 atoms cm −3 ) and CH 3 NCO + Cl reaction's rate coefficient at 298 K, the Cl-driven tropospheric lifetimes of methyl isocyanate were 5431 and 4611 days at Wigner's and Eckart's methods, respectively. However, in coastal and marine boundary layers where the reported peak concentration of Cl atoms is high enough (1.3 × 10 5 atoms cm −3 ) [29], the Cl-driven tropospheric lifetimes of methyl isocyanate were 42 and 35 days at Wigner's and Eckart's methods, respectively and which are in very good agreement with the experimentally reported value by Papanastasiou et al. [12] ( ∼ 40 days). The tropospheric lifetime's data are summarised in Table 3. These data indicate that the loss of methyl isocyanate is mostly dependent on OH reaction globally and compete with Cl at the coastal and marine boundary layer.
Another important atmospheric parameter of methyl isocyanate that has to be determined in the troposphere is the Global Warming Potential (GWP). The tropospheric lifetime of methyl isocyanate may be used to determine the global warming potential (GWP). The GWP [30,31] relative to CO 2 was estimated based on vibrational frequencies (v k ) [32], intensities (A k ) of the corresponding vibrational mode k and OH-driven atmospheric lifetime  [29]), d Papanastasiou et al. [12] of methyl isocyanate where 'a' is the total instantaneous infrared radiative forcing (W/m 2 ppbv). F(v k ) is the radiative forcing function per unit cross-section per wave number (Wm −2 (cm −1 ) −1 (cm 2 ·molecule −1 ) −1 ) [33], estimated at the scaled band centre frequency v k . AGWP is an absolute global warming potential for CO 2 , used as a reference molecule. The total radiative forcing value for methyl isocyanate is 3.7 × 10 −2 Wm −2 ppbv −1 . The AGWP for CO 2 was taken as 1.92 × 10 −4 , 6.76 × 10 −4 and 2.22 × 10 −3 Wm −2 ppbv −1 for 20, 100 and 500 years' horizon of time [34]. All the frequencies, IR intensities and radiative forcing data used in this calculation are given in supporting information (Table S-VI). Using all these data, the GWPs of methyl isocyanate were computed for 20, 100 and 500 years' horizon of time and are tabulated in Table 3. Comparatively small GWPs obtained (Table 3) for isocyanate depicted a low impact on global climatic conditions. It is well known that when VOCs are degraded in the troposphere by various oxidising species generate tropospheric O 3 and it can have an adverse effect on humans, animals and plants. According to Bufailini et al. [35], carbon and hydrogen atoms in a hydrocarbon molecule indicate how many ozone molecules can be formed. In methyl isocyanate, a maximum of 4 (nC + nH = 4) ozone molecules can be formed from one molecule. The average ozone formation during a 99% reaction of methyl isocyanate with OH radical was calculated to be 6-7 ppb using the following equation [34] where n is the maximum possible ozone molecules that can be formed from one molecule of methyl isocyanate, k is the rate constant of OH radical with methyl isocyanate with OH radical and (OH) is the global weighted average OH radical concentration reported by Prinn et al. [26] (10 6 radical·cm −3 ). Derwent et al. [36] and Jenkin et al. [37] developed a methodology to calculate the photochemical ozone creation potentials (POCPs) of various organic compounds based on the fundamental molecular properties using the following expressions: where E POCP is the calculated POCP, f 1 , f 2 and Z are constants with values of 111, 0.04 and 0.5, respectively, as mentioned by Wallington et al. [38] C n represents the number of carbon atoms in the methyl isocyanate, and S and R Z are the ozone formation indices, which are associated with the structure and reactivity. n A is equivalent to the number of reactive C-C and C-H bonds in the methyl isocyanate, M is the molecular mass, k OH is the rate constant of CH 3 NCO + OH reaction at 298 K and 1 atm pressure of air and k ethene OH is the rate constant (8.64 × 10 −12 cm 3 ·molecule −1 ·s −1 ) for C 2 H 4 + OH reaction at 298 K and 1 atmospheric pressure of air, as described by Jenkin et al. [37] Using the above parameters POCPs for methyl isocyanate were1.1-1.2. The calculated POCPs are relatively less than that of ethane. Hence, we conclude here that methyl isocyanate does not significantly contribute to tropospheric ozone formation.

Conclusions
Using high-level DFT and ab-initio calculations, the kinetics and mechanisms of the hydrogen atom abstraction reactions of CH 3 NCO with OH radical and Cl atom are investigated. The rate constants for the H-atom abstraction reactions of CH 3 NCO with OH radical and Cl atom are 2 × 10 −13 and 2.5 ×10 −12 cm 3 ·molecule −1 · s −1 at room temperature and atmospheric pressure, respectively, which are reasonably consistent with the experimental values. This work represents the first theoretical kinetic investigations on the CH 3 NCO + OH/Cl reactions as a function of temperature. Moreover, the thermochemical analysis of the tropospheric degradation pathways of the alkyl isocyanate radical indicates that the final products are formaldehyde, formyl isocyanate, NCO, HCO and HO 2 radicals, which are known to be harmful and further degraded into CO, CO 2 and N 2 O. However, estimated OH-driven tropospheric lifetimes of CH 3 NCO are found to be not high enough ( ∼ eighty days) and compete with the reaction of chlorine atoms in the coastal and marine boundary layer (where the chlorine concentration is high). GWPs and POCPs were also found to be small thus indicating slight damage to the Earth's atmosphere. We hope that the present theoretical studies will strengthen the chemical kinetics database and depth of understanding for further inspection of the reactions concerning analogous species.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Supporting information
Tables S-I to S-VI (Cartesian coordinates, Harmonic frequencies, partition functions, rate constants, IR intensities and radiative forcing) and Figures S-I to S-III (PES scan, variational plots)

Author contributions
MRD did all the electronic structure optimizations and chemical kinetic computations. MRD and SSM prepared the original draft of the manuscript.

Data availability
All data obtained through this computation are tabulated in Supporting Information.