A Computational Study Investigating the Energetics and Kinetics of the

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A Computational Study Investigating the Energetics and Kinetics of the HNCO + (CH)NH Reaction Catalyzed by a Single Water Molecule 3

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Arathala Parandaman, Chanin B. Tangtartharakul, Manoj Kumar, Joseph S. Francisco, and Amitabha Sinha J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08657 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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The Journal of Physical Chemistry

A Computational Study Investigating the Energetics and Kinetics of the HNCO + (CH3)2NH Reaction Catalyzed by a Single Water Molecule Arathala Parandaman,1 Chanin B. Tangtartharakul,1 Manoj Kumar,2 Joseph S. Francisco,2 and Amitabha Sinha1

1

Department of Chemistry and Biochemistry, University of California─San Diego, La Jolla, California 92093 2 Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588

Abstract High level ab initio calculations are used to explore the energetics and kinetics for the formation of 1,1-dimethyl urea via the reaction of isocyanic acid (HNCO) with dimethyl amine (DMA) catalyzed by a single water molecule. Compared to the uncatalyzed HNCO+DMA reaction, the presence of a water molecule lowers the reaction barrier, defined here as the energy difference between the separated HNCO+DMA+H2O reactants and the transition state (TS), by ~26 kcal/mol. In addition to the HNCO+DMA+H2O reaction, the energetics of the analogous reactions involving respectively ammonia and methyl amine were also investigated. Comparing the barriers for these three amine addition reactions, which can be represented as: HNCO+R-NHRˊ+H2O with R and Rˊ being either -CH3 or -H, we find that the reaction barrier decreases with the degree of methylation on the amine nitrogen atom. The effective rate constants for the bimolecular reaction pathways, HNCO••H2O + DMA and HNCO••DMA + H2O were calculated using canonical variational transition state theory coupled with both small curvature and zerocurvature tunneling corrections over the 200-300 K temperature range. For comparison, we also calculated the rate constant for the HNCO+OH reaction. Our results suggest that the HNCO + H2O + DMA reaction can make a non-negligible contribution to the gas phase removal of atmospheric HNCO under conditions where the HNCO and water concentrations are high and the temperature is low.

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1. Introduction: Isocyanic acid (HNCO) is an important atmospheric species due to its potential high abundance and propensity for the aerosol formation in the Earth’s troposphere.1-9 The concentration of HNCO in ambient urban atmosphere range from approximately 100 pptv to 1.2 ppbv.1,7-9 Various anthropogenic and biogenic emissions are responsible for the release of HNCO into the Earth’s atmosphere.1-4 Its formation is especially prominent from various pyrolytic processes.5,6,10 Roberts et al.1 measured concentrations of HNCO in the laboratory at levels up to 600 ppbv from the pyrolysis/combustion of biomass. Additional sources include the release of HNCO from vehicle engines.11,12 Krocher et al.12 report that the exhaust from diesel engines contain significant HNCO; it arises as byproduct from the use of urea selective catalytic reduction systems, which are being phased in for control of diesel engines and automobile emissions. Studies on the removal processes of HNCO in the atmosphere are limited. The reaction of HNCO with OH, for example, is expected to be slow. To the best of our knowledge, the rate of this reaction has not been reported; however, extrapolation of measured high temperature data to atmospherically relevant temperatures, suggests it will be slow.13-16 The direct photolysis of HNCO in the actinic region is also not likely to be an important removal process. Even though HNCO remains a benchmark molecule for understanding both direct and indirect photo dissociation pathways,17-28 its first UV band where photolysis can occur starts at wavelengths below 280 nm and extends to wavelengths shorter than 200 nm. Thus, this absorption lies outside the tropospheric actinic window. Currently, the major removal mechanism of HNCO in the atmosphere is thought to be from various processes in the aqueous phase. Barth et al.29 reported that the atmospheric lifetime of HNCO can be a few hours or less in lower level clouds 2 ACS Paragon Plus Environment

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depending on the temperature and pH of the cloud. Belson and Strachan30 also showed the irreversible removal of HNCO by an acid-catalyzed hydrolysis reaction in the aqueous phase. Thus, the atmospheric fate of HNCO mainly depends on the partitioning of HNCO into cloud water. In the present work, we examine the possibility of additional gas phase removal pathways for atmospheric HNCO. To the best of our knowledge, the water-catalyzed reactions of HNCO with amines, specifically NHRR' with R and R' being either -H or -CH3, have not been reported in the literature, even though the concentrations of amines and water in the atmosphere are relatively high. In the present work, we examine the addition of NHRR' amines, namely ammonia (NH3), methyl amine (CH3NH2, MA) and dimethyl amine ((CH3)2NH, DMA), onto HNCO in the absence and presence of a single water molecule. These reactions occur with the attachment of the amine nitrogen onto the carbon atom of HNCO with a concomitant transfer of the amine’s hydrogen atom. Due to the presence of two electronegative atoms on HNCO, the N and O atoms, the amine’s hydrogen atom may transfer to either of these atoms, thus giving rise to the two possible reaction paths as depicted below: HNCO + NHRR' + H2O → NH2CONRR' + H2O

(1)

HNCO + NHRR' + H2O → NHC(OH)NRR' + H2O

(2)

We have examined the energetics associated with both pathways in the present work and find that for each amine investigated, the reaction 1 exhibits the lowest barrier. Further among the series of amines involving NH3, MA and DMA, the reaction 1 with DMA has the lowest barrier and leads to the formation of 1,1-dimethyl urea. Below we discuss the energetics and kinetics of



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this reaction and assess the extent to which it can be an effective removal mechanism for atmospheric HNCO in the gas phase.

2. Computational methods: In order to investigate the potential energy landscape associated with the amine addition reactions, we have carried out quantum chemistry calculations using the Gaussian-09 program suite.31 All structures were fully optimized using both second order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT). The DFT calculations were performed using M06-2X method.32,33 The MP2 geometry optimizations were carried out with all orbitals active referred to as MP2=Full. To get accurate geometries, energies, harmonic vibrational frequencies and barrier heights, we adopted the large 6-311++G(3df,3pd) basis set in our calculations. The large basis set was also needed to reduce basis set superposition error, even though full (100%) counterpoise corrections often underestimate the binding energies of dimeric complexes.34-36 All the reaction transition states (TSs) reported in this work were located using the OPT=TS keyword implemented in the Gaussian-09 program. In addition, intrinsic reaction coordinate (IRC) calculations were carried out at the B3LYP level to verify that the TS connected with the desired reactants and product complexes. All reactant, products and reaction complex stationary points were confirmed to have no imaginary vibrational frequencies, while transition states were confirmed to have one imaginary frequency. The energies of the calculated stationary points were further refined by calculating single point energies using the coupled cluster single and double substitution method with a perturbative treatment of triple excitation (CCSD(T))37 and the 6-311++G(3df,3pd) basis set. The fully optimized geometries obtained at the MP2=Full/6311++G(3df,3pd) level were used for the single point CCSD(T) calculations. The calculated total electronic energies (Etotal) and zero-point energy (ZPE) corrected electronic energies [Etotal(ZPE)] 4 ACS Paragon Plus Environment

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for the reactants, products, transition states, and complexes obtained at the various levels of theory are given in the Tables S1-S2. While the geometries, vibrational frequencies and rotational constants are given in the Tables S3-S9 and S12-S14. The calculated reaction barriers for the addition of respectively NH3, MA and DMA onto to HNCO with and without water are summarized in Table 1. The corresponding schematic potential energy surfaces for the various HNCO+HNRRˊ+H2O reactions are shown in Figs. 1-3. As is clear from Table 1, among the reactions investigated the HNCO+DMA+H2O reaction with a concomitant transfer of the DMA hydrogen atom onto the N atom of HNCO has the lowest barrier.

3. Theoretical kinetic analysis In order to determine the potential impact of the amine addition reaction on atmospheric HNCO loss, we have also carried out rate coefficient calculations using the POLYRATE (2016) program.38 Because the HNCO+DMA+H2O reaction had the lowest barrier among the amine addition reactions considered, we focused attention on the rate coefficient associated with this reaction. Further, as this reaction involves three species and termolecular reactions are slow, we considered the above reaction as occurring through three possible bimolecular encounters involving the three possible dimer species as shown below: HNCO••H2O + DMA → NH2CON(CH3)2 + H2O

(3)

HNCO••DMA + H2O → NH2CON(CH3)2 + H2O

(4)

DMA••H2O + HNCO → NH2CON(CH3)2 + H2O

(5)

Of these three pathways, we did not further consider the DMA••H2O dimer pathway as an examination of the dimer structure showed that the presence of the hydrogen bonded water 5 ACS Paragon Plus Environment


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molecule blocks the amine’s nitrogen site from attaching onto the carbon atom of HNCO. With the help of the Gaussian-09 and POLYRATE programs, the rate coefficients for the reactions 3 and 4 were investigated using canonical variational transition state theory39,40 (CVT) with both small curvature41 (SCT) and zero-curvature tunneling42 (ZCT) approximations over the temperature range between 200-300 K. In order to minimize computational cost for these rate calculations, we used harmonic vibrational frequencies and rotational constants obtained at the M06-2X/6-311++G(3df,3pd) level of theory as input to the POLYRATE program to characterize the points along the reaction coordinate. The corresponding energies of these points were calculated at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory. We note that energies of the stationary points associated with the HNCO+DMA+H2O reaction computed using CCSD(T)//MP2=Full and the CCSD(T)//M06-2X method are in good agreement with one another. This is shown in Fig. S1 where the reaction potential energy profile computed using both levels of theory are compared. As noted above, the bimolecular encounters associated with the HNCO+H2O+DMA reaction is viewed as occurring through the collision of a dimer with a monomer. We model this reactive process by noting that the collision the dimer (R1) with the monomer (R2) results in the formation of a reactant complex (RC) which then proceeds, by unimolecular isomerization, to form the products as shown below:

𝑘𝑘

𝑘𝑘 = �𝑘𝑘 1 � 𝑘𝑘𝑢𝑢𝑢𝑢𝑢𝑢 = 𝐾𝐾𝑒𝑒𝑒𝑒 𝑘𝑘𝑢𝑢𝑢𝑢𝑢𝑢 −1


(7)

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In Eq. 6, reactant-1 (dimer), reactant-2 (monomer) and the reactant complex are denoted as R1, R2 and RC respectively. The equilibrium constant Keq is associated with the formation of the reactant complex (RC) from the two reactants (dimer plus monomer) in the first step as shown in Eq. 6 and was calculated using the relevant partition functions. These partition functions were calculated using vibrational frequencies and rotational constants computed at the M06-2X/6311++G(3df,3pd) level of theory. The rate constant (kuni) is calculated through POLYRATE by employing variational transition state theory39,40 (VTST) according to Eq. 8

𝑘𝑘(CVT⁄SCT) = 𝜅𝜅

k B T QGT (𝑠𝑠 ∗ ) −𝑉𝑉(𝑠𝑠∗)⁄𝑘𝑘 𝑇𝑇 𝐵𝐵 𝑒𝑒 QRC h

(8)

In Eq. 8, 𝜅𝜅 is the tunneling parameter, kB is the Boltzmann’s constant, T is the temperature in

Kelvin (K), h is Planck’s constant, 𝑠𝑠 ∗ is the value of the reaction coordinate at the energy

maximum, QGT (𝑠𝑠 ∗ ) is partition function of the transition state, QRC is partition function of

reactant complex, and V(s∗ ) is the potential energy at the barrier maximum. The bimolecular rate

constant is then obtained as given in Eq. 7 by multiplying kuni with Keq determined over the temperature range of interest. Several studies43-45 have used a similar methodology for calculating reaction rate constants. For the purposes of comparison, the bimolecular rate constant for the OH+HNCO abstraction reaction was also calculated in an analogous manner. The calculated unimolecular rate coefficients (s-1) and corresponding bimolecular rate constants over the temperatures from 200-2500 K are given in Table 2. The equilibrium constants are given in Table S10 of the supporting information.

4. Results and Discussion We have investigated the energetics of the three amine reactions involving the addition of respectively NH3, MA and DMA onto HNCO. Specifically, the reactions barriers without any 7 ACS Paragon Plus Environment


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catalysis as well as with catalysis by a single water molecule was examined. Each of these addition reactions can occur via two possible pathways with the nitrogen atom of the amine, denoted generically as NHRR' (with R and Rˊ being either H or CH3), adding on to the carbon atom of HNCO accompanied by the transfer of a H atom from the amine to either the nitrogen or the oxygen atom of HNCO. Further, we point out that with the presence of a water molecule as catalyst makes these reactions involve three species, HNCO, NHRRˊ and H2O. Since termolecular reactions are slow, we have modeled these 3 species reactions as occurring through a bimolecular encounter where a dimer is first formed between two of the reagents and then this dimer collides with the remaining monomer species. The energetics of these bimolecular reactions pathways are illustrated for the water-catalyzed reactions involving NH3, MA and DMA respectively in Figs. 1-3. In these figures, the starting reactants, the dimer plus monomer reagent, are shown in the middle of the figure and the energy profile to the right is associated with the pathway involving a transfer of the amine H atom on to the nitrogen atom of HNCO. By contrast, the energy profile to the left of the reactants in these figures is associated with transfer of the H atom onto the HNCO oxygen atom. From examining these figures, certain trends become clear. First, we find that for all three amines, the reaction channel involving transfer of H atom onto the HNCO nitrogen has a lower barrier compared to that associated with the transfer of the hydrogen atom onto the oxygen of HNCO. Further, we see that the barriers for both these reaction pathways decreases with increased methylation of the amine; so, the reaction barriers decrease along the series NH3, MA and DMA. This trend in barrier heights, has also been seen in other reactions involving these amines.46-48 The trends for the present reactions are summarized in Table-1 and schematically illustrated in Fig. 4. The impact of water catalysis is also indicated in Table-1. We find that for these amine addition reactions, the presence of even a single water


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molecule reduces the barrier anywhere from ~24-26 kcal/mol at the DFT level. Apparently, the presence of a H2O molecule allows for the formation of a larger six-membered ring transition state which reduces ring strain and thus, lowers the reaction barrier relative to that without the water molecule. The calculations also suggest that among the amine reactions considered, the DMA+HNCO+H2O reaction, where the amine adds on to HNCO while the amine’s hydrogen atom is transferred onto the HNCO nitrogen, thus resulting in the formation of 1,1-dimethyl urea, will be the main loss process for HNCO. For this reaction, the transition state (TS5) lies 4.3 kcal/mol below the zero of the reaction, measured as the total energy of the separated DMA, HNCO, and H2O. To confirm that this is indeed the lowest energy pathway available for this reactive system, we have also explored the energetics associated with the carbamic acid formation pathway from the DMA+HNCO+H2O reactants. In this hydration reaction pathway, H2O adds onto HNCO with DMA acting as the catalyst as shown below: HNCO + NH(CH3)2 + H2O → NH2CO(OH) + NH(CH3)2 The

energetics

of

this

hydration

reaction

was

explored

(9) at

the

CCSD(T)/6-

311++G(3df,3pd)//MP2=Full/6-311++G(3df,3pd) level and found to have a barrier of 9.0 kcal/mol measured relative to the HNCO+DMA+H2O separated reactants. The reaction potential energy profile is shown in Fig. S2. This additional check confirmed that the formation of 1,1dimethyl urea is indeed the lowest energy pathway and will be favored over the carbamic acid formation. Given that the reaction of HNCO+DMA+H2O to form 1,1-dimethyl urea is the lowest energy pathway, we next examined the rate of this reaction in order to assess its potential



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atmospheric impact. As discussed in the previous section describing the rate calculations, we assumed that this reaction, involving three reagents, occurs via a bimolecular collision where a dimer molecule formed from two of the three species collides with the remaining monomer reagent. This leads to three possible dimer reaction pathways involving HNCO••H2O, DMA••H2O and HNCO••DMA. As discussed earlier in the theoretical kinetic analysis section, we did not consider DMA••H2O dimer pathway for rate coefficient calculation. For the other two dimer pathways, the bimolecular collision between the dimer and the monomer results in the formation of a reactant complex (RC) which then goes on to form the product via a unimolecular reaction as shown in Eq. 6. Table 3 gives the calculated unimolecular and bimolecular rate constants for the two possible elementary dimer reactions pathway. The bimolecular rate constants for the HNCO••DMA + H2O reaction are 2-4 orders of magnitude larger than the rate coefficients for the other pathway. For example, the rate coefficients for the HNCO••H2O + DMA and HNCO••DMA + H2O reactions at 250 K are 1.5х10-16 and 9.7х10-14 cm3 molecule-1 s-1 respectively. This result is consistent with the barrier for the HNCO••H2O + DMA pathway being slightly higher than for the HNCO••DMA + H2O pathway, as shown in Fig. 3. For the purpose of comparison, we also calculated the rate of removal of HNCO through the OH+HNCO reaction. Unfortunately, the kinetics of the OH+HNCO reaction has only been studied at high temperatures (> 500 K)14-16 and there are no experimental or theoretical data available on the kinetics of this reaction in the lower temperature range (200-300 K) relevant for atmospheric chemistry. One can, however, extend the Arrhenius plot from the high temperature data to lower temperatures to get an estimate of the expected rates. In order to confirm that these extrapolated rates were indeed reasonable, we also carried out of CVT/SCT and CVT/ZCT rate coefficient calculations for the OH+HNCO reaction over the 200-2500 K temperature range. The


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calculated rates were then compared with the results from the extrapolated Arrhenius plot. As with the other rate calculations, the potential energy diagram for the OH+HNCO reaction, shown in Fig. 5, was calculated at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory. This hydrogen atom abstraction reaction has a barrier height of 5.7 kcal/mol measured between the TS and the separated reactants. The rate coefficients calculated using the CVT/SCT and CVT/ZCT methods are plotted in the Arrhenius plot over the 200-2500 K temperature range in Fig. 6. The Arrhenius plot confirms that our calculated CVT/ZCT rate coefficients are in good agreement with the extrapolated data from the Tsang study.14 The CVT/SCT rate coefficients agree well with the extrapolated rate coefficients from 400-2500 K but are 2-6 times higher than the extrapolated rate coefficients over the temperature range between 200-400 K. Further, the rate coefficient calculations suggest that tunneling plays an important role in the HNCO+OH reaction as it enhances the rate by 1-3 orders of magnitude over the temperature range between 200-300 K. At 200 K, the bimolecular rate coefficient k(CVT) for the HNCO+OH reaction is 7.8 × 10-19 cm3 molecule-1 s-1, which is increased to 4.3 × 10-16 cm3 molecule-1 s-1 (CVT/SCT) upon the inclusion of the effects of tunneling. The effect of tunneling on the HNCO••H2O + DMA and HNCO••DMA + H2O reactions was also calculated. The bimolecular rate coefficients k(CVT/SCT) for these two reactions at 200 K is 6.3 × 10-16 and 1.8 × 10-12 cm3 molecule-1 s-1, which is roughly an order of magnitude larger than the k(CVT) rate constants of respectively 2.2 × 10-17 and 6.5 × 10-14 cm3 molecule-1s-1. The larger impact of tunneling for the OH+HNCO reaction can be explained by comparing the potential energy diagrams for the OH+HNCO and HNCO+DMA+H2O reactions. This is shown in Fig. S3 of the supporting information. The figure indicates that the shape of the HNCO+OH potential is slightly narrower over the region of the transition state compared to that associated with the HNCO+DMA+H2O



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reaction. This is likely because in the case of the HNCO+H2O+DMA reaction, the hydrogen atom transfer occurs over a greater distance involving multiple nuclei as compared to that for the OH+HNCO reaction. In order to compare the rate of HNCO loss through the HNCO+DMA+H2O reaction with that from the OH+HNCO reaction on an equal footing, we calculated the corresponding effective first-order rate constants for each reaction (in units of s-1). The rate for the HNCO••H2O + DMA bimolecular reaction, for example, can be written in terms of the reagent concentration and bimolecular rate constant as: ν3 = k3 [HNCO••H2O] [DMA]

(10)

Next the HNCO••H2O dimer concentration can be expressed in terms of the corresponding monomer concentrations making up the dimer and the equilibrium constant ĸ1𝑒𝑒𝑒𝑒 for dimer

formation as shown below:

ν3 = k3 ĸ1𝑒𝑒𝑒𝑒 [H2O][HNCO][DMA]

(11)

Finally, isolating the HNCO concentration and combining all other terms gives us the expression: ν3 = k3 ĸ1𝑒𝑒𝑒𝑒 [H2O][DMA][HNCO] = 𝑘𝑘3′ [HNCO]

(12)

Where 𝑘𝑘3′ is the effective first-order rate constant for the loss of HNCO by this reaction. A

similar approach was also taken for the HNCO••DMA + H2O channel using the corresponding parameters 𝑘𝑘4′ , k4, and ĸ2𝑒𝑒𝑒𝑒 with 𝑘𝑘4′ =k4 ĸ2𝑒𝑒𝑒𝑒 [H2O][DMA]. In an analogous manner, the effective

first order rate for the OH+HNCO reaction is defined as follows: ′ [HNCO] ν𝑂𝑂𝑂𝑂 = kOH [OH][HNCO] = 𝑘𝑘𝑂𝑂𝑂𝑂

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(13)

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The computed equilibrium constants associated with dimer formation are given in the supporting information (Table S11). The effective first-order rate coefficients 𝑘𝑘3′ = k3 ĸ1𝑒𝑒𝑒𝑒 [H2O][DMA] for ′ the HNCO••H2O+DMA, 𝑘𝑘4′ = k4 ĸ2𝑒𝑒𝑒𝑒 [H2O][DMA] for HNCO••DMA + H2O and 𝑘𝑘𝑂𝑂𝑂𝑂 = kOH [OH]

for the OH + HNCO reactions are given in Table 4, covering the temperature range between 200300 K. In generating this table, the concentrations of the reagents were taken from the literature as follows: [DMA] ≈ 1.9 х 108 molecules/cm3, [H2O] ≈ 3.9 х 1017 molecules/cm3 and [OH] ≈ 1.0 х 106 molecules/cm3.48-50 Fig. 7 shows how the effective first-order rates coefficients for the HNCO+OH and HNCO + DMA + H2O reactions compare over the temperature range of atmospheric relevance. The effective first-order rate for the HNCO+DMA+H2O reaction was obtained by adding the effective first-order rates for the HNCO••H2O + DMA and HNCO••DMA + H2O pathways given in Table 4. From the figure, we see that while the rate constant of the HNCO+DMA+H2O decreases with increasing temperature, that of the HNCO+OH reaction increases with temperature. This trend is due to the submerged barrier51 and the effect of tunneling on the HNCO+DMA+H2O reaction. Therefore, the resultant rate constants are decreasing with increasing temperature. We also find that for most temperatures of atmospheric interest, the OH+HNCO reaction is faster than the HNCO+DMA+H2O reaction. Only for temperatures below ~200 K is the HNCO+DMA+H2O reaction predicted to be faster than the HNCO+OH reaction. However, we note that while the HNCO+DMA+H2O reaction can occur under both day and nighttime conditions, the OH+HNCO reaction is expected to occur only during the daytime when the OH concentration is significant. We further point out that in the calculation presented above for the HNCO+DMA+H2O reaction, we have used the ambient concentration of [HNCO] ≈ 6 х 108 molecules/cm3.1 The concentration of HNCO can be significantly higher under condition of forest fires. Indeed



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Roberts et al. report concentrations of [HNCO] ≈ 2 х 1013 molecules/cm3 for environments involving biomass fires.1 If we consider these higher concentrations of HNCO under biomass fire conditions, the effective first-order rates (not rate constants) for HNCO loss from the HNCO+DMA+H2O reaction at 250 K increases to approximately 444 and 1224 molecules cm-3 s-1 using respectively the CVT/ZCT and CVT/SCT rate constants. These significantly higher loss rates suggest that the removal of HNCO through the HNCO+DMA+H2O reaction, although modest, will make a non-negligible contribution under conditions where the HNCO concentration is high. Further at night, when the OH concentration is negligible, this removal mechanism will dominate over the OH+HNCO reaction.

5. Conclusions The energetics and rates of ammonia, methyl amine and dimethyl amine addition to HNCO catalyzed by a single water molecule were investigated using ab-initio calculations. The energy barriers for reaction decrease with increased methylation on the amine nitrogen. Among the amine reactions investigated the HNCO+DMA+H2O reaction was found to have lowest barrier. The rate coefficient calculations for HNCO+DMA+H2O and HNCO+OH reactions were carried out using canonical variational transition state theory with tunneling correction over the atmospherically relevant temperatures range covering 200-300 K. The results suggest that quantum mechanical tunneling plays a greater role in the HNCO+OH reaction compared to the HNCO+DMA+H2O reaction. The results also suggest that the HNCO+DMA+H2O reaction, although having a modest rate, can be a non-negligible gas phase loss mechanism for HNCO under night time forest fire conditions when the HNCO concentration is high.


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Acknowledgement: A.S. thanks the National Science Foundation for support of this work under the Grant CHE-1566272. AS also thanks Prof. Paesani and the W. M. Keck Foundation, through computing resources at the W. M. Keck Laboratory for Integrated Biology, for allowing use of their computers.

Supporting Information: Optimized geometries of reactants, products, and transition states in terms of their Z-matrices, their vibrational frequencies and rotational constants, their calculated total electronic energies including zero-point energy corrections and imaginary frequencies of various TS as discussed in the text at different levels of theories.



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References: (1) Roberts, J. M.; Veres, P. R.; Cochran, A. K.; Warneke, C.; Burling, I. R.; Yokelson, R. J.; Lerner, B.; Gilman, J. B.; Kuster, W. C.; Fall, R.; et al. Isocyanic Acid in the Atmosphere and Its Possible Link to Smoke-Related Health Effects. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 89668971. (2) Roberts, J. M.; Veres, P.; Warneke, C.; Neuman, J. A.; Washenfelder, R. A.; Brown, S. S.; Baasandorj, M.; Burkholder, J. B.; Burling, I. R.; Johnson, T. J.; et al. Measurement of HONO, HNCO, and Other Inorganic Acids by Negative-Ion Proton-Transfer Chemical Ionization Mass Spectrometry (NI-PT-CIMS): Application to Biomass Burning Emissions. Atmos. Meas. Tech. 2010, 3, 981-990. (3) Karlsson, D.; Dalene, M.; Skarping, G.; Marand, A. Determination of Isocyanic Acid in Air. J. Environ. Monitor. 2001, 3, 432-436. (4) Veres, P.; Reberts, J. M.; Burling, I. R.; Warneke, C.; de Gouw, J.; Yokelson, R. J. Measurements of Gas-Phase Inorganic and Organic Acids from Biomass Fires by Negative-Ion Proton-Transfer Chemical Ionization Mass Spectrometry. J. Geophys. Res. Atmos. 2010, 115, D23302. (5) Hansson, K.-M.; Samuelsson, J.; Tullin, C; Åmand, L.-E. Formation of HNCO, HCN, and NH3 from the Pyrolysis of Bark and Nitrogen Containing Model Compounds. Combust. Flame 2004, 137, 265-277. (6) Burling, I. R.; Yokelson, R. J.; Griffith, D. W.; Johnson, T. J.; Veres, P.; Roberts, J. M.; Warneke, C.; Urbanski, S. P.; Reardon, J.; Weise, D. R.; et al. Laboratory Measurements of Trace Gas Emissions from Biomass Burning of Fuel Types from the Southeastern and Southwestern United States. Atmos. Chem. Phys. 2010, 10, 11115-11130. (7) Roberts, J. M.; Veres, P. R.; VandenBoer, T. C.; Warneke, C.; Graus, M.; Williams, E. J.; Lefer, B.; Brock, C. A.; Bahreini, R.; Öztürk, F.; et al. New Insights Into Atmospheric Sources and Sinks of Isocyanic Acid, HNCO, from Recent Urban and Regional Observations. J. Geophys. Res. Atmos. 2014, 119, 1060-1072. 16 ACS Paragon Plus Environment

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(8) Wentzell, J. J. B.; Liggio, J.; Li, S.-M.; Vlasenko, A.; Staebler, R.; Lu, G.; Poitras, M.-J.; Chan, T.; Brook, J. R. Measurements of Gas Phase Acids in Diesel Exhaust: A Relevant Source of HNCO?. Environ. Sci. Technol. 2013, 47, 7663-7671. (9) Woodward-Massey, R.; Taha, Y. M.; Moussa, S. G.; Osthoff, H. D. Comparison of NegativeIon Proton Transfer with Iodide ion Chemical Ionization Mass Spectrometry for Quantiﬁcation of Isocyanic Acid in Ambient Air. Atmos. Environ. 2014, 98, 693-703. (10) Nelson, P. F.; Li, C. Z.; Ledesma, E. Formation of HNCO from the Rapid Pyrolysis of Coals. Energy Fuels 1996, 10, 264-265. (11) Heeb, N. V.; Zimmerli, Y.; Czerwinski, J.; Schmid, P.; Zennegg, M.; Haag, R.; Seiler, C.; Wichser, A.; Ulrich, A.; Honegger, P.; et al. Reactive Nitrogen Compounds (RNCs) in Exhaust of Advanced PM-NOx Abatement Technologies for Future Diesel Applications. Atmos. Environ. 2011, 45, 3203-3209. (12) Kröcher, O.; Elsener, M.; Koebel, M. An Ammonia and Isocyanic Acid Measuring Method for Soot Containing Exhaust Gases. Anal. Chim. Acta 2005, 537, 393-400. (13) Tully, F. P.; Perry, R. A.; Thorne, L. R.; Allendorf, M. D. Free-Radical Oxidation of Isocyanic Acid. Symp. (Int.) Combust. [Proc.] 1989, 22, 1101-1106. (14) Tsang, W. Chemical Kinetic Data Base for Propellant Combustion. II. Reactions Involving CN, NCO, and HNCO. J. Phys. Chem. Ref. Data 1992, 21, 753-791. (15) Wooldridge, M. S.; Hanson, R. K.; Bowman, C. T. A Shock Tube Study of CO + OH → CO2 + H and HNCO + OH → Products via Simultaneous Laser Absorption Measurements of OH and CO2. Int. J. Chem. Kinet. 1996, 28, 361-372. (16) Mertens, J. D.; Chang, A. Y.; Hanson, R. K.; Bowman, C. T. A Shock Tube Study of Reactions of Atomic Oxygen with Isocyanic Acid. Int. J. Chem. Kinet. 1992, 24, 279-295. (17) Yu, S. R.; Su, S.; Dorenkamp, Y.; Wodtke, A. M.; Dai, D. X.; Yuan, K. J.; Yang, X. M. Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO. J. Phys. Chem. A 2013, 117, 11673-11678.



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(18) Dixon, R. N.; Kirby, G. H. Ultra-Violet Absorption Spectrum of Isocyanic Acid. Trans. Faraday Soc. 1968, 64, 2002-2012. (19) Rabalais, J. W.; McDonald, J. R.; McGlynn, S. P. Electronic States of HNCO, Cyanate Salts, and Organic Isocyanates. II. Absorption Studies. J. Chem. Phys. 1969, 51, 5103-5111. (20) McGlynn, S. P.; Rabalais, J. W.; McDonald, J. M.; Scherr, V. M. Electronic Spectoscopy of Isoelectronic Molecules. II. Linear Triatomic Groupings Containing Sixteen Valence Electrons. Chem. Rev. 1971, 71, 73−108. (21) Nakamura, H.; Truhlar, D. G. Extension of the Fourfold Way for Calculation of Global Diabatic Potential Energy Surfaces of Complex, Multi Arrangement, Non-Born-Oppenheimer Application to Systems: HNCO (S0, S1). J. Chem. Phys. 2003, 118, 6816-6829. (22) Zyrianov, M.; DrozGeorget, T.; Sanov, A.; Reisler, H. Competitive Photodissociation Channels in Jet Cooled HNCO: Thermochemistry and Near Threshold Predissociation. J. Chem. Phys. 1996, 105, 8111-8116. (23) Conroy, D.; Aristov, V.; Feng, L.; Sanov, A.; Reisler, H. Competitive Pathways via Nonadiabatic Transitions in Photodissociation. Acc. Chem. Res. 2001, 34, 625-632. (24) Brown, S. S.; Berghout, H. L.; Crim, F. F. Vibrational State Controlled Bond Cleavage in the Photodissociation of Isocyanic Acid (HNCO). J. Chem. Phys. 1995, 102, 8440-8447. (25) Brown, S. S.; Berghout, H. L.; Crim, F. F. The HNCO Heat of Formation and the N-H and C-N Bond Enthalpies from Initial State Selected Photodissociation. J. Chem. Phys. 1996, 105, 8103-8110. (26) Brown, S. S.; Berghout, H. L.; Crim, F. F. Internal Energy Distribution of the NCO Fragment from Near-Threshold Photolysis of Isocyanic Acid, HNCO. J. Phys. Chem. 1996, 100, 7948-7955. (27) Berghout, H. L.; Brown, S. S.; Delgado, R.; Crim, F. F. Nonadiabatic Effects in the Photodissociation of Vibrationally Excited HNCO: The Branching between Singlet (a1Δ) and Triplet (X3Σ−) NH. J. Chem. Phys. 1998, 109, 2257-2263.


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(28) Brown, S. S.; Metz, R. B.; Berghout, H. L.; Crim, F. F. Vibrationally Mediated Photodissociation of Isocyanic Acid (HNCO): Preferential N-H Bond Fission by Excitation of the Reaction Coordinate. J. Chem. Phys. 1996, 105, 6293-6303. (29) Barth, M. C.; Cochran, A. K.; Fiddler, M. N.; Roberts, J. M.; Bililign, S. Numerical Modeling of Cloud Chemistry Effects on Isocyanic Acid (HNCO). J. Geophys. Res. Atmos. 2013, 118, 8688-8701. (30) Belson, D. J.; Strachan, A. N. Preparation and Properties of Isocyanic Acid. Chem. Soc. Rev. 1982, 11, 41-56. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision B.01; Gaussian, Inc: Wallingford, CT, 2010. (32) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215-241. (33) Zhao, Y.; Truhlar, D. G. Density Functionals with Broad Applicability in Chemistry. Acc. Chem. Res. 2008, 41, 157–167. (34) Reimann, B.; Buchhold, K.; Barth, H.-D.; Brutschy, B.; Tarakeshwar, P.; Kim, K. S. Anisole-(H2O)n (n = 1−3) Complexes: An Experimental and Theoretical Investigation of the Modulation of Optimal Structures, Binding Energies, and Vibrational Spectra in Both the Ground and First Excited States. J. Chem. Phys. 2002, 117, 8805-8822. (35) Galano, A.; Alvarez-Idaboy, J. R. A New Approach to Counterpoise Correction to BSSE. J. Comput. Chem. 2006, 27, 1203-1210. (36) Alvarez-Idaboy, J. R.; Galano, A. Counterpoise Corrected Interaction Energies Are Not Systematically Better than Uncorrected Ones: Comparison with CCSD(T) CBS Extrapolated Values. Theor. Chem. Acc. 2010, 126, 75-85.



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(37) Noga, J.; Bartlett, R. J. The Full CCSDT Model for Molecular Electronic Structure. J. Chem. Phys. 1987, 86, 7041-7050. (38) Zheng, J.; Zhang, S.; Lynch, B. J.; Corchado, J. C.; Chuang, Y.- Y.; Fast, P. L.; Hu, W.-P.; Liu, Y.-P.; Lynch, G. C.; Nguyen, K. A. et al. POLYRATE, version 2016; University of Minnesota: Minneapolis, MN, 2016. (39) Truhlar, D. G.; Garrett, B. C. Variational Transition-State Theory. Acc. Chem. Res. 1980, 13, 440−448. (40) Garrett, B. C.; Truhlar, D. G. Criterion of Minimum State Density in the Transition State Theory of Bimolecular Reactions. J. Chem. Phys. 1979, 70, 1593-1598. (41) Liu, Y. P.; Lynch, G. C.; Truong, T. N.; Lu, D. H.; Truhlar, D. G.; Garrett, B. C. Molecular Modeling of the Kinetic Isotope Effect for the [1,5]-Sigmatropic Rearrangement of cis-1,3Pentadiene. J. Am. Chem. Soc. 1993, 115, 2408-2415. (42) Chuang, Y. Y.; Truhlar, D. G. Improved Dual-Level Direct Dynamics Method for Reaction Rate Calculations with Inclusion of Multidimensional Tunneling Effects and Validation for the Reaction of H with trans-N2H2. J. Phys. Chem. A 1997, 101, 3808-3814. (43) Buszek, R. J.; Torrent-Sucarrat, M.; Anglada, J. M.; Francisco, J. S. 2012. Effects of a Single Water Molecule on the OH + H2O2 Reaction. J. Phys. Chem. A 2012, 116, 5821-5829. (44) Kumar, M.; Anglada, J. M.; Francisco, J. S. Role of Proton Tunneling and Metal Free Organocatalysis in Decomposition of Methanediol: A Theoretical Study. J. Phys. Chem. A 2017, 121, 4318-4325. (45) Anglada, J. M.; Crehuet, R.; Martins-Costa, M.; Francisco, J. S.; Ruiz-López, M. The Atmospheric Oxidation of CH3OOH by the OH Radical: The Effect of Water Vapor. Phys. Chem. Chem. Phys. 2017, 19, 12331-12342. (46) Perez, J. E.; Kumar, M.; Francisco, J. S.; Sinha, A. Oxygenate-Induced Tuning of Aldehyde-Amine Reactivity and Its Atmospheric Implications. J. Phys. Chem. A 2017, 121, 1022-1031.


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Figure captions: Figure 1. Energy level diagram for the addition of ammonia onto HNCO catalyzed by single water molecule obtained at the CCSD(T)/6-311++G(3df,3pd)//MP2=Full/6-311++G(3df,3pd) level of theory. The symbols correspond to: D0 (HNCO••H2O dimer), D1 (HNCO••NH3 dimer), RC1, RC2 (reactant complexes), TS1, TS2 (transition states), and PC1, PC2 (product complexes) respectively. The barrier heights are measured relative to HNCO+NH3+H2O separated reactants which is located at the center and corresponds to the zero of energy. Figure 2. Energy level diagram for the addition of methyl amine onto HNCO catalyzed by single water molecule obtained at the CCSD(T)/6-311++G(3df,3pd)//MP2=Full/6-311++G(3df,3pd) level of theory. The symbols correspond to: D0 (HNCO••H2O dimer), D1 (HNCO••NH2(CH3) dimer), RC3, RC4 (reactant complexes), TS3, TS4 (transition states), and PC3, PC4 (product complexes) respectively. The barrier heights are measured relative to HNCO+NH3+H2O separated reactants which is located at the center and corresponds to the zero of energy. Figure 3. Energy level diagram for the addition of dimethyl amine onto HNCO catalyzed by single

water

molecule

obtained

at

the

CCSD(T)/6-311++G(3df,3pd)//MP2=Full/6-

311++G(3df,3pd) level of theory. The symbols correspond to: D0 (HNCO••H2O dimer), D3 (HNCO••NH(CH3)2 dimer), RC5, RC6 (reactant complexes), TS5, TS6 (transition states), and PC5, PC6 (product complexes) respectively. The barrier heights are measured relative to HNCO+NH3+H2O separated reactants which is located at the center and corresponds to the zero of energy. Figure 4. Changes in reaction barrier height for the water-catalyzed HNCO + amine reaction as a function of the degree of methylation on the amine nitrogen. Results are at the CCSD(T)/622 ACS Paragon Plus Environment

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311++G(3df,3pd)//MP2=Full/6-311++G(3df,3pd) level of theory. The blue and green color dotted lines correspond respectively to the transfer of the amine H atom onto the HNCO nitrogen and oxygen atoms. Figure 5. Potential energy diagram for the HNCO + OH reaction obtained at the CCSD(T)/6311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory. The symbols RC8, TS8, and PC8 stands for reactant complex, transition state, and product complex respectively. Figure 6. Arrhenius plots for the HNCO + OH reaction over the temperature range of 200-2500 K. The blue solid and blue dotted lines are respectively the measured and extrapolated rate coefficients from the Tsang study.14 Figure 7. A comparison of the effective first order rate coefficients for the HNCO + OH reaction with that for the HNCO+DMA+H2O reaction over the temperatures between 200 and 300 K. The effective rate for the HNCO+DMA+H2O reaction is the sum of the rates associated with the HNCO••H2O+DMA and HNCO••DMA+H2O pathways. The open and closed symbols correspond to the two types of tunneling corrections considered.



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Fig. 1


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Fig. 2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 3


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Fig. 4



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 5


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Fig. 6



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 1. Reaction barrier heights (kcal mol−1) at various levels of theory for the addition of ammonia, methyl amine and dimethyl amine onto HNCO in the absence and presence of a single water molecule to form various products.a

with one water

without water

a

Transition state

M06-2X/6-311++G(3df,3pd)

MP2=Full/6-311++G(3df,3pd)

CCSD(T)/6-311++G(3df,3pd)b

HNCO••H2O••NH3 (TS1)c HNCO••H2O••NH3 (TS2)d HNCO••H2O••MA (TS3)c HNCO••H2O••MA (TS4)d HNCO••H2O••DMA (TS5)c HNCO••H2O••DMA (TS6)d HNCO••NH3 (TS)e HNCO••NH3 (TS)f HNCO••MA (TS)e HNCO•• MA (TS)f HNCO••DMA (TS)e HNCO••DMA (TS)f

5.4 10.5 -1.1 3.9 -5.1 0.9 31.7 36.8 24.8 29.5 20.4 25.0

7.8 (9.9)g 13.9 (14.9)g -0.8 (1.0)g 5.4 (7.3)g -6.7 (-4.6)g 0.5 (2.6)g

9.1 (13.6)g 14.3 (18.8)g 1.1 (5.6)g 6.5 (11.0)g -4.3 (0.2)g 2.1 (6.6)g

All barrier heights are relative to the HNCO+Amine+H2O separated reactants. bCorrected using MP2 ZPE. ctransfer of H atom onto

the HNCO nitrogen with one water molecule. dtransfer of H atom onto the HNCO oxygen with one water molecule. eTransfer of H atom onto the HNCO nitrogen in absence of water molecule; ftransfer of H atom onto the HNCO oxygen in absence of water molecule. gBarrier heights presented in parenthesis are relative to the HNCO••H2O + NH3, HNCO••H2O + MA, and HNCO••H2O + DMA as reactants.



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Table 2. Unimolecular rate coefficients (in s-1) and bimolecular rate coefficients (cm3 molecule-1 s-1) with corresponding tunneling corrections (ZCT and SCT) for the HNCO+OH reaction over the temperature range of 200-2500 K obtained at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6311++G(3df,3pd) level of theory.

T (K) 200 210 220 230 240 250 260 270 280 290 300 400 500 600 700 800 900 1000 1500 2000 2500

CVT 2.57 х 104 5.79 х 104 1.21 х 105 2.36 х 105 4.37 х 105 7.67 х 105 1.29 х 106 2.08 х 106 3.24 х 106 4.90 х 106 7.20 х 106 1.14 х 108 5.82 х 108 1.71 х 109 3.70 х 109 6.62 х 109 1.04 х 1010 1.51 х 1010 4.70 х 1010 8.55 х 1010 1.24 х 1011

kuni (in s-1) CVT/ZCT 2.54 х 106 3.76 х 106 5.45 х 106 7.75 х 106 1.08 х 107 1.48 х 107 2.00 х 107 2.65 х 107 3.46 х 107 4.46 х 107 5.68 х 107 3.65 х 108 1.23 х 109 2.86 х 109 5.38 х 109 8.76 х 109 1.30 х 1010 1.79 х 1010 4.98 х 1010 8.70 х 1010 1.21 х 1011

CVT/SCT 1.41 х 107 1.88 х 107 2.48 х 107 3.22 х 107 4.14 х 107 5.27 х 107 6.63 х 107 8.26 х 107 1.02 х 108 1.24 х 108 1.51 х 108 6.81 х 108 1.88 х 109 3.91 х 109 6.81 х 109 1.05 х 1010 1.50 х 1010 2.02 х 1010 5.27 х 1010 8.97 х 1010 1.24 х 1011

kOH (cm3 molecule-1 s-1) CVT CVT/ZCT CVT/SCT -19 -17 7.78 х 10 7.69 х 10 4.27 х 10-16 1.54 х 10-18 9.98 х 10-17 4.99 х 10-16 2.86 х 10-18 1.29 х 10-16 5.86 х 10-16 5.03 х 10-18 1.65 х 10-16 6.86 х 10-16 8.49 х 10-18 2.10 х 10-16 8.04 х 10-16 1.37 х 10-17 2.65 х 10-16 9.44 х 10-16 2.15 х 10-17 3.33 х 10-16 1.10 х 10-15 3.25 х 10-17 4.14 х 10-16 1.29 х 10-15 4.77 х 10-17 5.10 х 10-16 1.50 х 10-15 6.86 х 10-17 6.24 х 10-16 1.74 х 10-15 9.63 х 10-17 7.59 х 10-16 2.02 х 10-15 1.19 х 10-15 3.81 х 10-15 7.10 х 10-15 5.82 х 10-15 1.23 х 10-14 1.88 х 10-14 1.79 х 10-14 3.00 х 10-14 4.10 х 10-14 4.24 х 10-14 6.17 х 10-14 7.80 х 10-14 8.46 х 10-14 1.12 х 10-13 1.34 х 10-13 1.50 х 10-13 1.87 х 10-13 2.16 х 10-13 2.46 х 10-13 2.91 х 10-13 3.28 х 10-13 1.37 х 10-12 1.45 х 10-12 1.54 х 10-12 4.13 х 10-12 4.20 х 10-12 4.33 х 10-12 9.19 х 10-12 8.96 х 10-12 9.19 х 10-12


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Table 3. Unimolecular rate coefficients (in s-1) and bimolecular rate coefficients (cm3 molecule-1 s-1) with corresponding tunneling corrections (ZCT and SCT approximation) for the HNCO••H2O + DMA (k3), and HNCO••DMA + H2O (k4) reactions computed at the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level of theory.

T (K) 200 210 220 230 240 250 260 270 280 290 300

CVT 2.30 х 107 3.81 х 107 6.04 х 107 9.17 х 107 1.34 х 107 1.91 х 108 2.63 х 108 3.54 х 108 4.66 х 108 6.02 х 108 7.63 х 108

kuni (in s-1) CVT/ZCT 1.46 х 108 2.04 х 108 2.28 х 108 3.72 х 108 4.86 х 108 6.24 х 108 7.87 х 108 9.79 х 108 1.20 х 109 1.45 х 109 1.74 х 109

CVT/SCT 6.49 х 108 8.06 х 108 9.90 х 108 1.20 х 109 1.45 х 109 1.72 х 109 2.03 х 109 2.38 х 109 2.76 х 109 3.18 х 109 3.64 х 109

k3(cm3 molecule-1 s-1) CVT CVT/ZCT CVT/SCT -17 -16 2.23 х 10 1.42 х 10 6.30 х 10-16 -17 -16 2.10 х 10 1.13 х 10 4.45 х 10-16 1.99 х 10-17 9.17 х 10-17 3.27 х 10-16 1.89 х 10-17 7.66 х 10-17 2.47 х 10-16 1.78 х 10-17 6.46 х 10-17 1.93 х 10-16 1.71 х 10-17 5.58 х 10-17 1.54 х 10-16 1.63 х 10-17 4.86 х 10-17 1.25 х 10-16 1.55 х 10-17 4.30 х 10-17 1.04 х 10-17 1.49 х 10-17 3.83 х 10-17 8.80 х 10-17 1.46 х 10-17 3.44 х 10-17 7.54 х 10-17 1.37 х 10-17 3.11 х 10-17 6.51 х 10-17


k4(cm3 molecule-1 s-1) CVT CVT/ZCT CVT/SCT -14 -13 6.51 х 10 4.13 х 10 1.84 х 10-12 -14 -13 4.28 х 10 2.29 х 10 9.06 х 10-13 2.92 х 10-14 1.34 х 10-14 4.78 х 10-13 2.04 х 10-14 8.28 х 10-14 2.67 х 10-13 1.46 х 10-14 5.30 х 10-14 1.58 х 10-13 1.08 х 10-14 3.52 х 10-14 9.71 х 10-14 8.07 х 10-15 2.42 х 10-14 6.23 х 10-14 6.17 х 10-15 1.71 х 10-14 4.15 х 10-14 4.79 х 10-15 1.23 х 10-14 2.84 х 10-14 3.78 х 10-15 9.11 х 10-15 2.00 х 10-14 3.03 х 10-15 6.90 х 10-15 1.44 х 10-14


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Table 4. Effective first order rate coefficients (keff in s-1) for the HNCO••H2O + DMA (𝑘𝑘3′ ),

′ HNCO••DMA + H2O (𝑘𝑘4′ ) and HNCO + OH (𝑘𝑘𝑂𝑂𝑂𝑂 ) reactions over the temperatures between 200

and 300 K.

T (K) 200 210 220 230 240 250 260 270 280 290 300

HNCO••H2O + DMA (𝑘𝑘3′ ) (CVT/ZCT) (CVT/SCT) 3.48 х 10-10 1.55 х 10-9 1.62 х 10-10 6.39 х 10-10 -11 8.11 х 10 2.89 х 10-10 4.37 х 10-11 1.41 х 10-10 2.47 х 10-11 7.38 х 10-11 1.48 х 10-11 4.09 х 10-11 9.27 х 10-12 2.39 х 10-11 6.04 х 10-12 1.47 х 10-11 4.06 х 10-12 9.34 х 10-12 2.81 х 10-12 6.17 х 10-12 -12 2.01 х 10 4.20 х 10-12

HNCO••DMA + H2O (𝑘𝑘4′ ) (CVT/ZCT) (CVT/SCT) 1.73 х 10-10 7.67 х 10-10 8.02 х 10-11 3.17 х 10-10 -11 4.02 х 10 1.43 х 10-10 2.17 х 10-11 6.99 х 10-11 1.23 х 10-11 3.68 х 10-11 7.37 х 10-12 2.03 х 10-11 4.61 х 10-12 1.19 х 10-11 3.00 х 10-12 7.29 х 10-12 2.02 х 10-12 4.64 х 10-12 1.40 х 10-12 3.07 х 10-12 -12 1.00 х 10 2.10 х 10-12


′ HNCO + OH (𝑘𝑘𝑂𝑂𝑂𝑂 ) (CVT/ZCT) (CVT/SCT) 7.69 х 10-11 4.27 х 10-10 9.98 х 10-11 4.99 х 10-10 1.29 х 10-10 5.86 х 10-10 1.65 х 10-10 6.86 х 10-10 2.10 х 10-10 8.04 х 10-10 2.65 х 10-10 9.44 х 10-10 3.33 х 10-10 1.10 х 10-9 4.14 х 10-10 1.29 х 10-9 5.10 х 10-10 1.50 х 10-9 6.24 х 10-10 1.74 х 10-9 7.59 х 10-10 2.02 х 10-9

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TOC Graphic


A Computational Study Investigating the Energetics and Kinetics of the

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