Conflict in the Mechanism and Kinetics of the Barrierless Reaction between SH and NO₂ Radicals

Abstract

There are some unsettled issues regarding the mechanism and kinetics of an important atmospheric reaction of NO radical with the SH radical. The existing mechanism is based on the formation of HSO and NO radicals, both of which can result only along one barrierless channel. However, the detection of NO radical has never been reported though the formation of HSO radical has been followed in some studies to determine the rate constants. The latter are mainly obtained by monitoring the SH decay, but rate constants are reported to be highly conflicting among the existing studies reporting its value ranging from 10 to 10 cm molecule sec. The present work attempts to resolve these issues by exploring various reaction pathways through the global reaction route mapping of the potential energy surface at the level of spin-unrestricted and spin-restricted coupled-cluster and density functional theories. The initial association of two radicals was found to proceed via two barrierless modes: (1) S-N association leading to HSNO and, (2) S-O association resulting in HSONO, in particular the cis-isomer. The kinetics of the barrierless pathways was investigated through rate constants computed using canonical variational transition state theory (CVTST) along with their temperature and pressure dependence investigated using the master equation. The rate constants calculated using spin-unrestricted methods are found to be in agreement with experimentally observed range of rate constant, and the formation of cis-HSONO (via mode 2) is observed to be the main contributing channel. Contrary to the results of spin-restricted calculations, the barrierless channel (mode 1) leading to the formation of HSNO is predicted to involve two bottlenecks when results using spin-unrestricted calculations were analyzed. Notably, the spin-unrestricted calculations predict a prereaction complex for the formation of S-N bond (via mode 1) which has been treated using Miller's unified transition state theory with a two transition state model. The fate of all the species involved in the reaction is critically evaluated in the present work, and the predictions made can be a subject of further experimental and theoretical studies involving radical-radical reactions.

Oxa- and Azabenzonorbornadienes as Electrophilic Partners under Photoredox/Nickel Dual Catalysis

“…Despite various attempts, the transition state could not be located for radical addition at this level of theory. Analysis of relaxed scans of the forming Ni–C bond − (see the Supporting Information for details) revealed a barrierless interconversion between 3 and 4 , while calculations optimized in the gas phase revealed a small energetic barrier (ca. 3 kcal/mol) between these two species (see the Supporting Information).…”

Experimental and Computational Studies of Unimolecular 1,1-HX (X = F, Cl) Elimination Reactions of C2D5CHFCl: Role of Carbene:HF and HCl Adducts in the Exit Channel of RCHFCl and RCHCl2 Reactions

“…Details associated with a given potential energy surface may depend on computational method, see Figure as an example for C 2 D 5 CHFCl, but the main features of the exit channel for 1,1-HCl elimination can be accepted. For cases with inner and outer transition states, the observed rate constant is given by eq ; − k HX , k DIS , and k R are elementary RRKM rate constants; k HX refers to HCl elimination, and k DIS and k R refer to dissociation and reverse reaction of the adduct: If k DIS is larger than k R , k obs , the observed rate constant, is just k HX , which is the case for the majority of 1,1-HF elimination reactions, because E 0 (1,1-HF) is larger than the enthalpy of reaction. However, for 1,1-HCl reactions, k DIS may be comparable to k R because E 0 (1,1-HCl) is less than the enthalpy, and it is necessary to have an estimate for k DIS .…”

“…Dissociation belongs to the class of barrier-less unimolecular reactions, and variation transition-state theory is needed for analysis. , For our model system, five frequencies of CH 3 (F)­C:HCl will be converted to rotational and translational motions of products upon dissociation. These transitional frequencies are four bending modes of HCl relative to CH 3 (F)­C: plus the C:--(HCl) stretch mode, which becomes the reaction coordinate.…”

Collision-induced dissociation of Na+-tagged ketohexoses: experimental and computational studies on fructose

“…We have gradually elongated the Na + -sugar distance by 0.5 Å in each step for 40 steps in total. At each of the 40 geometries, we have calculated the vibrational frequencies and the single-point energies at B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) levels, respectively, and determined the trial CVT rate constant: 58 Here, s is an index representing one of the geometries along the reaction coordinate. Note that, for saving computational costs, the vibrational partition function of the trial TS has been calculated based on frequencies at the B3LYP/6-311+G(d,p) level, while the reaction barrier has been estimated from MP2/6-311+G(d,p) level electronic energies and B3LYP/6-311+G(d,p) level ZPE.…”