B. E. Krisyuk

Calculation of the effect of double bond strain in 1-chloroethylene and 1,1-dichloroethylene on the rate and mechanism of their reactions with ozone

Using the 6-31+G** and aug-cc-pvDz basis sets, the ab initio (MP2, CASSCF, MRMP2) and DFT (B3LYP) calculation of the reactivity of the strained C=C bond in 1,1-dichloroethylene and 1-chloroethylene towards ozone was carried out. The concerted and unconcerted mechanisms were investigated. The strain (ɛ) was introduced into the problem by varying the C=C bond length and specifying it as the unoptimized coordinate. The data were analyzed using the previously developed approach that allows one to derive, from calculated data, an analytical relationship between the activation energy Ea and the force acting on the reaction site and to relate the strain-induced change in Ea to the length and rigidity of the initial and transition states.

The Mechanism of Ozone Addition to Acetylene

The primary stage of the reaction between ozone and acetylene was studied by the HF, MP2, and B3LYP quantum-chemical methods using the 6-31G family of basis sets and the aug-cc-pVDZ basis set. The formation of the transition state was shown to be preceded by the formation of a complex. Subsequently, the reaction could occur as concerted (the Criegee mechanism) or nonconcerted (the DeMore mechanism) addition. The geometry and energy of transition states, the entropy and enthalpy of activation, and rate constants were calculated for both reaction paths. It was shown that there was a competition of the Criegee and DeMore mechanisms, and the fraction of the reaction along the nonconcerted addition channel was 1–10%. The UB3LYP method was found to be the most suitable for solving this problem in the one-determinant approximation

Competition between the concerted and nonconcerted addition of ozone to a double bond

Quantum chemistry methods are used to investigate the mechanism of the reaction of ozone with the double bond of ethylene. It is shown that there are two possible reaction mechanism; concerted addition through a symmetrical transition state (Criegee mechanism) and nonconcerted addition through a biradical transition state (DeMore mechanism). In the single-determinant approximation, both mechanisms were described by using the QCISD, CCSD, and B3LYP methods. These methods give a reasonable ratio between the rates of the two reaction channels, with the rate constants being closer to the experiment when calculated by the CCSD and B3LYP methods. Multiconfiguration calculations are performed at the MRMP2 level. They also show the presence of both channels of the reaction and yield reasonable values of the rate constants for reaction channels and the ratio thereof. It is shown that the reaction of ethylene with ozone via the concerted addition mechanism is much faster.

Effect of chlorine atoms in chlorinated ethylene on the rate and mechanism of its reaction with ozone

The UQCISD, UB3LYP, UMP2, and MRMP2 methods in conjunction with the 6-31+G**/6-311+G** and aug-cc-PVDZ basis sets are used to study the primary reaction of ozone with chlorinated ethylene derivatives: tetrachloroethylene, trichloroethylene, 1,2-trans-dichloroethylene, 1,2-cis-dichloroethylene, 1,1-dichloroethylene, and chloroethylene. The reaction is studied for both concerted and nonconcerted ozone addition. The UB3LYP DFT method in conjunction with the 6-31+G** basis set is used to examine various modes of addition of ozone to these chlorinated ethylenes by the Criegee and DeMore mechanisms. The geometry and energy of the transition states, the enthalpy and entropy, and the rate constants and ratios thereof for all the reactions are calculated. The UB3LYP method generally satisfactorily describes the two reaction pathways and, largely correctly predicts the rate constants, in agreement with the available experimental data. At the same time, this method appears to be inapplicable to modeling the interaction of ozone with 1,1-dichloroethylene. In this case, the single-determinant approximation turns out to be unsuitable, and, therefore, MCSCF methods should be used. The MRMP2 method yields reasonable values of the rate constants for the DeMore mechanism, whereas in the case of the Criegee mechanism, the MP2 method does well. The UB3LYP/6-31+G** and UQCISD/aug-cc-PVDZ methods give similar values of the ratio between the rate constants for the two pathways, a result that demonstrates the versatility of the first one.

The reaction of ozone with hexafluoropropylene: Competition of concerted and nonconcerted addition

The initial stage of the reaction between ozone and hexafluoropropylene (HFP) was studied by the DFT/B3LYP quantum-chemical method using the family of 6-31G basis sets and the cc-pVDZ+ basis set. Two reaction paths were compared, concerted (the Criegee mechanism) and nonconcerted (the DeMore mechanism) addition. For both reaction paths, the geometry and transition state energies, entropy, and thermodynamic and electronic enthalpy were calculated and the rate constants of the reaction were estimated. It was shown that the DeMore mechanism should be given preference for the reaction of HFP with ozone. UB3LYP calculations for the DeMore mechanism give reaction rate constants close to the experimental values.

Kinetics and mechanism of ozone addition to olefins and dienes

The mechanism of the initial stage of the ozonolysis of a series of olefins and trans-1,3-butadoiene has been investigated by the B3LYP density functional theory (DFT) method, B2PLYP double hybrid method based on DFT and the MP2 approximation, and CCSD coupled cluster method. Two possible butadiene and olefin ozonolysis mechanisms are considered: concerted 1,3-cycloaddition, which yields a primary ozonide (Criegee mechanism) and stepwise ozone addition via a biradical transition state (DeMore mechanism). The geometries of the initial and transition states and the energies of the elementary steps of the reaction have been determined. The geometric structures of the stationary states determining the rate constant of the reaction have been completely optimized using the above methods and the aug-cc-pVDZ basis set. The rate constants for both reaction pathways have been calculated. For butadiene, the contribution from nonconcerted addition can reaches 25%. According to the MRMP2 method, the overall rate constant (which includes both reaction pathways) is 1778 L mol–1 s–1; according to B2PLYP, 1640 L mol–1 s–1; according to CCSD, 1424 L mol–1 s–1 (aug-cc-pVDZ basis set). These results are in good agreement with experimental data (k = 3 × 103 L mol–1 s–1 and with earlier calculations. The data calculated for olefins are also in agreement with experimental data.

Quantum chemical study of the addition of ozone to acetylene

The primary step of acetylene ozonation was studied by the B1LYP, PBE0, CASSCF, MRMP2, and CCSD methods using the 6-31+G**, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets. The study confirmed the earlier B3LYP-based conclusions that the intermediate complex, as well as the transition states of the concerted addition (Criegee’s mechanism) and unconcerted addition (DeMore’s mechanism), are involved in this reaction event. The activation enthalpy and entropy and rate constants of the reaction were calculated. Criegee’s mechanism was shown to dominate in the reaction with acetylene, but the contribution from DeMore’s mechanism is also noticeable, being 1–8%.

The influence of deformation on the reactivity of C=C bonds in reactions with ozone

The influence of strain in the reaction center containing a double bond on double bond reactivity at the first stage of the reaction with ozone was studied by the B3LYP density functional theory method and ab initio MP2, CCSD, QCISD, and MRMP2 multireference methods. The 6-31+G** and 6-311+G** basis sets were used. The reactions of ozone with ethylene and butylene were studied. Deformation (ɛ) was introduced by using the C=C bond length or the distance between extreme carbon atoms as a coordinate not subjected to optimization. Stretching of the double bond was found to activate the reaction by the mechanisms of symmetrical and nonsymmetrical addition. The sensitivities to deformation were similar in the two channels. When the butene fragment as a whole was stretched, a different picture was observed, and the reaction with ozone was decelerated. In both cases, the logarithms of rate constants linearly depended on ɛ. The calculation results were analyzed using the approach developed earlier, which allowed the calculation results to be used to obtain an analytic form of the dependence of activation energy Ea on the strength of bonds and to relate deformation-induced changes in Ea to the length and rigidity of the initial and transition states.

Quantum-Chemical Study of Stressed Polyethylene and Butadiene Rubber Chain Scission

The thermal decomposition of polyethylene and butadiene rubber chains in the presence of a tensile force acting along the axis of the molecule was simulated. The reaction of an isolated chain was considered. The chain models were the octane and 2,6-octadiene molecules. A deformation was introduced in the problem by fixing nonequilibrium distances between the terminal carbon atoms. The reaction coordinate (the middle C–C bond length R) was scanned at a fixed length of the molecule (L). That is, the potential energy surface section of the reaction was constructed at L = const. The reaction sensitivity to deformation was evaluated by B3LYP, LC-ωPBE, CCSD(T), CASSCF, and MP2 quantum-chemical calculations. All these calculations showed that the molecule elongated by ~1 Å for polyethylene, but shortened by 0.3–0.5 Å for 2,6-octadiene during chain scission. This means that the tensile deformation accelerates the decomposition of polyethylene, but decelerates the decomposition of butadiene rubber.

Reactivity of cycloalkanes in hydrogen abstraction with different acceptors

The reactions of some acceptors (∙CH3, ∙OOH, ∙CCl3, O3, and Br∙) with saturated cyclic hydrocarbons, viz., cyclohexane, cycloheptane, and cyclooctane were studied by the DFT methods based on B3LYP and PBE0 functionals, the method based on the double hybrid functional B2PLYP, a combined ONIOM approach (CCSD:B3LYP)) and the coupled-cluster method (CCSD) using the 6-31+G**, aug-cc-pVDZ, Midi-X, and SVP basis sets. A specific feature of these reactions is that their rates depend on the excess ring energy, although no ring opening occurs in all cases.