Kinetics and Mechanism of the NH (X3Σ-) + SO (X3Σ-) Reaction: A Theoretical Approach.

The reaction mechanism, product branching ratios, and relevant rate constants for the reaction of imidogen (NH) with sulfur monoxide (SO) over singlet and triplet potential energy surfaces are theoretically investigated. Various quantum chemical methods at the single-reference methods (PBE, M06-2X, MP2, GBS-QB3, G3MP2B3, and CCSD(T)) and the multi-reference methods of CASPT2 are carried out to examine the characteristics of the title reaction's potential energy surface. Eighteen chemically activated intermediates and more than 35 different reaction channels are predicted over the singlet surface, while less species and channels are distinguished over the triplet surface. The entrance channels for both surfaces appeared to be barrier-less association reactions to form pre-reaction energized intermediates of singlet or triplet HNSO or HNOS. OH and NS radicals are indicated as the major products for the title reaction on both surfaces in agreement with the reported experimental observations. The RRKM-steady state approximation method is used to calculate the rate constants and branching ratios of the main products. The obtained overall rate constant is in agreement with the available reported experimental data over the wide range of temperature from 300 to 3000 K. By considering single-reference calculations, the singlet and triplet total rate constants were found to be k(T) = 5.04 × 1010 and 2.47 × 1012 T-0.83 exp(-1.56 kJ mol-1/T), respectively. Also, the total rate constant for the consumption of reactants by inclusion of multi-reference calculations was found to be in the range of 3.86 × 1010 to 4.18 × 1010, depending on the level of calculations. In addition, our results revealed that the total rate constant for the NH + SO reaction is pressure-independent in the range of 0.1-2000 Torr.

A Theoretical Study on the Dynamics of the Reaction of CH Radicals with Water.

Quasi-classical trajectory (QCT) and RRKM-SSA calculations are carried out to gain insight into the dynamics of the title reaction. The barrier-less initiation step in this system is governed by the capture probability in the entrance channel to form an energized adduct once the centrifugal barrier is surmounted. The dynamics of the title reaction on its lowest doublet electronic state is studied at the DFT level of MPWB1K/6-31++g(2df,2p). An iteratively modified Shepard interpolation technique implemented in the GROW program suite was used to construct a global potential energy surface for the title reaction. The total and individual cross sections for the main products and corresponding reaction probabilities as a function of initial collision energy ranging from 0.4 kJ mol-1 to 52.5 kJ mol-1 are calculated. These data are used to calculate the total rate constant for the title reaction by means of collision theory. Our calculated QCT rate constant is compared with the calculated rate constant from RRKM-SSA method at the CCSD(T)/Aug-cc-pVTZ//MP2/6-31++g(2df,2p) level. The energy partitioning for the main products CH2O + H and HCOH + H and also for the reactants after nonreactive collisions as a function of initial collision energy are discussed.

Theoretical Study on the Dynamics of the Reaction of HNO((1)A') with HO2((2)A″).

We used stochastic one-dimensional chemical master equation (CME) simulation to gain insight into the dynamics of the reaction of HNO((1)A') with HO2((2)A″). The reaction takes place over a multiwell, multichannel potential energy surface that is based on the computations at the CBS-QB3 level of theory. The calculated multipath potential energy surface consists of three potential wells and three van der Waals complexes. In solving the master equation, the Lennard-Jones potential is used to model the collision between the collider gases. The fractional population of different intermediates and products in the early stages of the reaction is examined to determine the role of the energized intermediates and van der Waals complexes on the kinetics of the title reaction. The major products of the title reaction at lower temperatures are OH, HNO2, HNOH, and O2(X(3)Σg(-)). The temperature- and pressure-dependence of the reaction over a wide range of temperature (300-3000 K) and pressure (0.1-2000 Torr) are studied. No sign of pressure dependence was being observed for the title reaction over the stated range of pressure. The calculated rate constants from the CME simulation are compared with those obtained from the RRKM-SSA method that is based on strong collision assumption. Our results indicate that the strong collision assumption increases the calculated rate constant for the formation of the main products (HNO2 + OH) by a factor of 2 at 300 K and 1 atm pressure, compared to the results of CME simulation, although the results are in good agreement at higher temperatures.

Multichannel RRKM-TST and direct-dynamics CVT study of the reaction of hydrogen sulfide with ozone.

The kinetics of the reaction of ozone with hydrogen sulfide was studied theoretically. High-level ab initio calculations were carried out to build the potential energy surface. The mechanism of the title reaction was found to be much more complicated than what is reported in the literature to date. According to our results, six different chemically activated intermediates are involved along the proposed mechanism on its lowest singlet potential energy surface that play an important role in the kinetics of this system. Multichannel RRKM-TST and CVT calculations have been carried out to compute the temperature dependence of the individual rate constants for different channels and also the overall rate constant for the consumption of the reactants. The major products are sulfur dioxide and water at lower temperatures, in good agreement with experimental reports, while at higher temperatures, formation of the other products like O2, H2SO, and radicals like cis/trans-HOSO, SH, HO3, and OH also become important.

Multichannel RRKM-TST and CVT rate constant calculations for reactions of CH2OH or CH3O with HO2.

The kinetics and mechanism of the gas-phase reactions between hydroxy methyl radical (CH(2)OH) or methoxy radical (CH(3)O) with hydroproxy radical (HO(2)) have been theoretically investigated on their lowest singlet and triplet surfaces. Our investigations indicate the presence of one deep potential well on the singlet surface of each of these systems that play crucial roles on their kinetics. We have shown that the major products of CH(2)OH + HO(2) system are HCOOH, H(2)O, H(2)O(2), and CH(2)O and for CH(3)O + HO(2) system are CH(3)OH and O(2). Multichannel RRKM-TST calculations have been carried out to calculate the individual rate constants for those channels proceed through the formation of activated adducts on the singlet surfaces. The rate constants for direct hydrogen abstraction reactions on the singlet and triplet surfaces were calculated by means of direct-dynamics canonical variational transition-state theory with small curvature approximation for the tunneling.

A theoretical investigation on the kinetics and mechanism of the reaction of amidogen with hydroxyl radical.

The kinetics and mechanism of the reaction between amidogen radical and hydroxyl radical have been theoretically investigated on the lowest singlet and triplet surfaces. The singlet surface consists of two long-lived chemically activated NH(2)OH* and NH(3)O* intermediates with 10 different channels. A hydrogen abstraction channel on the triplet surface proceeds through van der Waals complex in both reactant side and product side to produce NH(3) + O((3)P). The effect of multiple reflections of the van der Waals complexes on the rate constant is investigated. Multichannel RRKM-TST calculations have been carried out to calculate the individual rate constants for the formation of those products that proceed through activated adducts on the singlet surface. The rate constants for direct hydrogen abstraction reactions were calculated by using direct-dynamics canonical variational transition-state theory with small curvature approximation for tunneling.

Multichannel RRKM-TST and direct-dynamics VTST study of the reaction of hydroxyl radical with furan.

The kinetics and mechanism of the reaction of OH with furan have been theoretically studied. The potential energy surface for each possible pathway has been investigated by employing DFT, G3MP2, and CCSD methods. The potential energy surface consists of one hydrogen-bonded complex and two energized intermediates. Three different pathways are suggested to be possible for the title reaction. The most probable channel is the hydroxyl radical addition to the C(2) position on the furan ring to cause the ring-opening process. The two other pathways are hydrogen abstraction from one of the C(2) or C(3) position on furan and hydroxyl radical substitution at the C(2) or C(3) position on furan. Abstraction and substitution channels are minor paths at low temperature, but they become comparable with addition channels at high temperature. Addition and substitution reactions proceed via formation of two energized intermediates, Int(1) and Int(2). Multichannel RRKM-TST calculations have been carried out to calculate the individual and overall rate constants for addition and substitution reactions. Direct-dynamics canonical variational transition-state theory calculations with small curvature approximation for tunneling were carried out to study hydrogen abstraction pathways.

Direct-dynamics VTST study of the [1,7] hydrogen shift in 7-methylocta-1,3(Z),5(Z)-triene. A model system for the hydrogen transfer reaction in previtamin D3.

Direct-dynamics canonical variational transition-state theory calculations with microcanonically optimized multidimensional transmission coefficient (CVT/muOMT) for tunneling were carried out at the MPWB1K/6-31+G(d,p) level to study the [1,7] sigmatropic hydrogen rearrangement in 7-methylocta-1,3(Z),5(Z)-triene. This compound has seven conformers, of which only one leads to products, although all of them have to be included in the theoretical treatment. The calculated CVT/muOMT rate constants are in good agreement with the available experimental data. To try to understand the role of tunneling in the hydrogen shift reaction, we have also calculated the thermal rate constants for the monodeuterated compound in the interval T = 333.2-388.2 K. This allowed us to evaluate primary kinetic isotope effects (KIEs) and make a direct comparison with the experiment. Our calculations show that both the large measured KIE and the large measured difference in the activation energies between the deuterated and root compounds are due to the quantum tunneling. The tunneling contribution to the KIE becomes noticeable only when the coupling between the reaction coordinate and the transverse modes is taken into account. Our results confirm previous experimental and theoretical works, which guessed that the obtained kinetic parameters pointed to a reaction with an important contribution due to tunneling. The above conclusion would be essentially valid for the case of the [1,7] hydrogen shift in previtamin D3 because of the similarity to the studied model system.