The reaction mechanism of palladium(0)-catalyzed reaction of aryl iodides, norbrnene, and di-tert-butyldiaziridinone has been studied theoretically. Two reaction mechanisms were calculated. The reaction mechanism involving Cs2CO3 combination first followed by alkene insertion is calculated to be preferred to the one in which alkene insertion occurs first followed by Cs2CO3 combination. The calculations suggest that the reaction proceeds via C-I bond oxidative addition, CsI center dot CsCO3 cluster anion formation, alkene insertion, C-H bond activation, N-N bond oxidative addition, and two successive C(sp(2))-N bond-forming, and C(sp(3))-N bond-forming reductive elimination steps. The N-N bond oxidative addition involves the rate-determining transition state with a free energy barrier of 29.4 kcal/mol. The role of base Cs2CO3 played during CsI center dot CsCO3 cluster anion formation and N-N bond oxidative addition process has been discussed according to the formation of relatively strong Pd-O bond. The role of base K2CO3 has been calculated and compared with Cs2CO3.

Reaction mechanism of Ni(PPh3)(2)-catalyzed isomerization of N-allylamides to generate N-propenylamides was studied theoretically in detail by using density functional theory (DFT) method. The C-H bond activation, isomerization, and reductive elimination to form new C-H bond steps were involved. For C-H bond activation and isomerization steps, Ni(PPh3)(2) and Ni(PPh3) with only one PPh3 ligand were considered and found that the former was more active than the later for these two steps. Both pi-allyl and sigma-allyl mechanisms were calculated for isomerization and found that the pi-allyl mechanism was preferred kinetically to the sigma-allyl one. The rate-determining energy barrier to generate the E isomer of product is 141.8 kJ/mol, close to that of 141.1 kJ/mol to generate the Z isomer, in agreement with the experimental result that E/Z= 56/44. Considering Pd(PPh3)(2) as the catalyst active species, it is found that the rate-determining energy barriers for the formation of E and Z isomers are more than 175 kJ/mol, consistent with the experimental observation that Pd(PPh3)(4) showed no reactivity. The difference of reactivity between Ni(PPh3)(2) and Pd(PPh3)(2) can be understood from the more strong back-donation of d electrons from Ni to pi* anti-bonding orbital of allyl anion comparing with Pd. In addition, the influence of substituents in reactants on E/Z selectivity has been analyzed and found that the steric repulsion between substituents and Ph of PPh3 in the rate-determining transition states to generate E and Z isomers induces the difference of E/Z selectivity.

Herein, theoretical studies were performed on the atmospheric oxidation of sulfurous acid (H2SO3) and sulfite ions (HSO3-) by ozone (O-3) to produce sulfuric acid and hydrosulfate ions. The most favorable path for the H2SO3 + O-3 reaction has been found to be initiated from concerted H-abstraction and oxygen addition, with an overall energy barrier of 18.3 kcal mol(-1). On the other hand, the most favorable path for the HSO3- + O-3 reaction is initiated from oxygen addition, with an overall energy barrier of only 0.3 kcal mol(-1). Kinetic simulations were performed to estimate the significance of these reactions in the formation of atmospheric sulfate and destruction of the ozone layer. The results provide new insight into the missing source of atmospheric sulfate and particulate matter.

The mechanism of decomposition of sulfurous acid to sulfur dioxide and water catalyzed by formic acid, acetic acid, propionic acid and nitric acid, respectively, was investigated by ab initio calculations at the CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level of theory herein. The computational results showed that all the four acids presented catalytic effect for the decomposition of sulfurous acid. The reduction extent of the energy barrier was in the order propionic acid>acetic acid>formic acid>nitric acid. Furthermore, the energy barrier of propionic acid-catalyzed reaction decreased dramatically from 99.84 kJ/mol of naked reaction to 27.24 kJ/mol. The rate constants of the catalyzed reactions in the temperature range of 200-320 K had been calculated. In addition, the effective rate constants were also calculated in combination with the atmospheric concentrations of these acids. The results indicated that the acetic acid had the most effective catalytic effect in the atmospheric environment, and the lifetime of sulfurous acid was only 0.02 s with the appearance of acetic acid.

The reaction mechanism of Ni(COD)2catalyzed hydrodesulfurization of aryl sulfide PhSMe with HSiMe3has been predicted to have two competitive reaction pathways, with or without PhSMe spectator ligand, by using density functional theory methods.

The reaction mechanism of Ni(COD)2 catalyzed hydrodesulfurization of aryl sulfide PhSMe with HSiMe3 as the reducing agent has been studied by using density functional theory methods. Both PhSMe-coordinated pathway and “ligandless” pathway have been identified and compared. It is found that these two reaction pathways are kinetically competitive and the s-complex assisted metathesis (s-CAM) transition state is the highest point on each energy profile for both pathways. Moreover, both the singlet and triplet reaction pathways of ligand substitutions have been compared and found that both singlet and triplet reaction mechanisms are competitive for the ligand substitution of COD with PhSMe on PhSMecoordinated pathway while the triplet mechanism holds a distinct advantage over singlet one for that of COD with HSiMe3 on “ligandless” pathway.

The hydrolysis reaction of sulfur dioxide (SO2) to form sulfurous acid involving additional sulfurous acid (H2SO3) was investigated using high-level computational methods. With H2SO3, the reaction takes place via a double proton transfer process with a cage -like structure, which is different from the planar ring structure involved in a corresponding process with an additional water molecule (served as a catalyst). Our results show that H2SO3 is a better catalyst than water, as the barrier height for the H2SO3-catalyzed reaction is only 5.5 kcal/mol, compared to over 25.0 and 15.0 kcal/mol for the reaction without a catalyst and the H2O -catalyzed reaction, respectively. In addition, the sulfurous acid dimer from the H2SO3-catalyzed reaction is more stable than hydrated H2SO3 from the H2O-catalyzed reaction. Considering the existence of sulfurous acid in the aqueous phase and acidic aerosols, as well as the importance of SO2 and H2O in the atmosphere, our results will have potentially significant implications on the homogeneous and heterogeneous nucleation processes. (C) 2015 Elsevier B.V. All rights reserved.

The cleavage of S–N bond prefers to take place via concerted σ-bond metathesis rather than oxidative addition proposed in experiment.

The deposition and hydrolysis reaction of SO2 + H2O in small clusters of sulfuric acid and water are studied by theoretical calculations of the molecular clusters SO2-(H2SO4)n-(H2O)m (m = 1,2; n = 1,2). Sulfuric acid exhibits a dramatic catalytic effect on the hydrolysis reaction of SO2 as it lowers the energy barrier by over 20 kcal/mol. The reaction with monohydrated sulfuric acid (SO2 + H2O + H2SO4 - H2O) has the lowest energy barrier of 3.83 kcal/mol, in which the cluster H2SO4-(H2O)2 forms initially at the entrance channel. The energy barriers for the three hydrolysis reactions are in the order SO2 + (H2SO4)-H2O > SO2 + (H2SO4)2-H2O > SO2 + H2SO4-H2O. Furthermore, sulfurous acid is more strongly bonded to the hydrated sulfuric acid (or dimer) clusters than the corresponding reactant (monohydrated SO2). Consequently, sulfuric acid promotes the hydrolysis of SO2 both kinetically and thermodynamically. Kinetics simulations have been performed to study the importance of these reactions in the reduction of atmospheric SO2. The results will give a new insight on how the pre-existing aerosols catalyze the hydrolysis of SO2, leading to the formation and growth of new particles.

Effects of ammonia and water molecules on the hydrolysis of sulfur dioxide are investigated by theoretical calculations of two series of the molecular clusters SO2-(H2O)n (n = 1-5) and SO2-(H2O)n-NH3 (n = 1-3). The reaction in pure water clusters is thermodynamically unfavorable. The additional water in the clusters reduces the energy barrier for the reaction, and the effect of each water decreases with the increasing number of water molecules in the clusters. There is a considerable energy barrier for reaction in SO2-(H2O)5, 5.69 kcal/mol. With ammonia included in the cluster, SO2-(H2O)n-NH3, the energy barrier is dramatically reduced, to 1.89 kcal/mol with n = 3, and the corresponding product of hydrated ammonium bisulfate NH4HSO3-(H2O)2 is also stabilized thermodynamically. The present study shows that ammonia has larger kinetic and thermodynamic effects than water in promoting the hydrolysis reaction of SO2 in small clusters favorable in the atmosphere.

The detailed reaction mechanism for the isomerization of 1,3-conjugated dienes catalyzed by the ruthenium hydride complex RuHCl(CO)(H(2)IMes)(PCy3) has been studied with the aid of density functional theory (DFT) calculations. Both cis and trans isomers of a 1,3-conjugated diene were considered as the reactants. For each isomer, two catalytic cycles were calculated, which (respectively) generate a 1,3-hydride shift product or a 1,5-hydride shift product. Both catalytic cycles proceed via alkene migratory insertion into the Ru-H bond, sigma-allyl ruthenium isomerization, and beta-H elimination steps. Our computational study shows that the cis isomer of the model reactant reacts preferentially via the pathway leading to the 1,5-hydride shift product, consistent with the experimental results. The sigma-allyl ruthenium isomerization step is found to be crucial for reaction regioselectivity. Strong binding of the C=C bond to Ru is involved in the generation of the 1,5-hydride shift product. In addition, the steric effect of the bulky N-heterocyclic carbene ligand in ruthenium hydride RuHCl(CO)(H(2)IMes)(PCy3) was considered theoretically.

The detailed reaction mechanism for the Cu(I)-catalyzed cross-coupling of (diazomethyl)benzene with trimethylsilylethyne and tert-butylethyne was studied with the aid of density functional theory calculations. For both reactions, two catalytic cycles were considered. In one catalytic cycle, the active species reacts first with trimethylsilylethyne or tert-butylethyne, whereas, in the other one, the active species reacts first with (diazomethyl)benzene. In both catalytic cycles, the copper acetylide formation, copper carbene migratory insertion, and protonation steps are involved. The calculation results show that the protonation step is crucial for the product selectivity. In addition, the reaction of diazoethane with tert-butylethyne and the reaction of (diazomethyl)benzene with phenylacetylene were also considered theoretically.