Ab Initio Study on the Mechanism of Tropospheric Reactions of the Nitrate Radical with Haloalkenes:  Chloroethene

Abstract

A mechanism for the reaction of the NO 3 radical with vinyl chloride is proposed on the basis of B3LYP and CASSCF quantum chemistry calculations. Two initial transition states lead to the Markownikoff and contra-Markownikoff addition of NO 3 to chloroethene. From the two initial adducts different interconnected pathways start. We have found epoxichloroethane, chloroethanal, nitrous acid, acetyl chloride, nitric acid, formaldehyde, formyl chloride, and NO radical as main products. The energy of the reactants, products, and all intermediates along with the barrier heights for each reaction path has been calculated. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurface was made at the B3LYP level with a 6-31G* basis set, and some of the stationary points have been reoptimized with the 6-311+G* basis set. Later, single-point CASSCF calculations were carried out with the same basis set, building the active space with seven active electrons in eight active orbitals. It can be concluded that an equilibrium distribution of formyl chloride, formaldehyde, and the NO radical will be obtained along with small proportions of epoxychloroetane and the Markownikoff radical adduct. An estimation of the reaction rate constant has also been calculated.

Evidence of A Possible Cycloaddition Channel in the Ethene + NO3 Reaction

“…In fact, even very high level quantum chemistry methods do not agree on the symmetry of the ground state of NO 3 . To our knowledge, the only theoretical studies on the reaction of alkenes with the nitrate radical are those performed by Pérez-Casany et al for ethene, propene, methyl substituted butene, and haloalkenes , using the ROHF and the CAS(SCF) methods in the first one, and density functional theory in the others. Their mechanism is compared with results of experimental studies at low pressures and temperatures in anaerobic conditions.…”

“…The CAS(SCF) calculations on ethene + NO 3 14 did not detect any structure between the reactants and the nitroalkyl adduct, and thus the 6.17 kcal/mol reported experimental activation barrier could not be reproduced. Later work by the same authors on other alkenes, − performed using the B3LYP density functional method, yielded a transition state whose reaction vector clearly corresponds to the formation of an O−C bond. Its energy, however, lies below the energy of the reactants, in disagreement with experimental results.…”

Quantitative Assessment of the Multiplicity of Carbon–Halogen Bonds: Carbenium and Halonium Ions with F, Cl, Br, and I

“…Because halogen has a valence of 1, chemistry traditionally considers any bond multiplicity higher than 1 for halogen–carbon bonds to be less likely . However, there have been a number of studies in the literature that suggest some variation in the CX single-bond character. − …”

Are carbon—halogen double and triple bonds possible?

“…Halogens can stabilize radical intermediates via three‐electron (3e) interactions . The stabilization of carbenium ions by α‐halogen substituents is even stronger .…”