Effect of Space-Orientation Factors on Kinetics of Low-Temperature Carbene Reactions

Abstract

The character of nonexponential kinetics of the low-temperature decay of triplet carbenes in polymers has been given consideration. The temperature dependencies of the decay rate and the kinetic isotope effect were found at 77-180 K. The controlling factor in the kinetic nonequivalence of carbenes is the distribution of distances of the hydrogen atom tunneling from the matrix molecules. The effect of magnetic field on the rate of the carbene decay has been seen at 77 K. On the basis of the magnetic dependence view, the mechanism of the carbene decay involving spin and chemical dynamics of intermediate short-lived radical pairs has been advanced. The nature of magnetic effect in polymers shows the configuration inhomogeneity of radical pairs.

Application of radical cage reactions in studies of microstructures of polymers and composites

“…Numerous literature data (see, e.g., Refs 70 ± 72) demonstrate the possibility of using this effect for the establishment of a detailed mechanism of processes involving RP. The application of a magnetic field was shown 46,67,69 to influence noticeably the rate of low-temperature decay of BCHC in PMMA and CTA (Fig. 2); however, the character of this influence is different.…”

“…Thus, the effective potential barrier decreases as the temperature increases, and the rate of the process increases accordingly. Vorotnikov et al 61,66,67 determined the values of parameters for the tunnelling of H atom to CHC and DPC in the same polypiperylene matrix. The height U and the width x of the potential barrier as well as the equilibrium distance (R 0 ) between carbon atoms of the active centre of carbene and the C7H groups of polypiperylene in the carbene ± polypiperylene complex were determined from the kinetic data obtained at 77 ± 133 K. The boundary values of these parameters, which correspond to the maximum and minimum rate constants of carbene decay, are presented in Table 1.…”

“…Analysis of KIE in the reaction of BCHC decay in PMMA 66,67 was performed using relations (11), (12) and the condition T& hMO 2 /mo C7H k, under which the linear temperature dependence is transformed into a T 72 dependence. The derived values of characteristic frequencies of intermolecular vibrations (O & 53 cm 71 & 10 13 s 71 ) correlate with the Debye frequencies for molecular crystals.…”