Quantitative chemically induced nuclear polarization (CIDNP) study of the kinetics of the photolysis of pivalophenone in various solutions

Abstract

A simple mechanistic study of the photolysis of pivalophenone in a series of solvents showed that under suitable conditions a unique reaction mechanism obtains. In all the solvents investigated benzoyl and t-butyl radicals are produced which undergo two competitive cage reactions, recombination and disproportionation. In chloroform at 310 K 15 % of the radicals recombine and 85 % disproportionate; in the fluorocarbon PP9 with added tetrachloromethane the figures at 310 K are 17 % and 83 % respectively and the activation energies of the two processes differ by 16.3 ± 2.0 kJ mol^–1. This observation is used to rationalise unusual behaviour in the electron polarization phenomena of the radicals.

The overall pseudo first-order rate constant for the scavenging of t-butyl radicals in neat chloroform was measured directly by flash-photolysis e.s.r. techniques as 5.44 ± 0.13 × 10³ s^–1; CIDNP exposed that two scavenging routes occur: hydrogen abstraction with a rate constant k^*_H= 2.54 ± 0.07 × 10² dm³ mol^–1 s^–1 and chlorine abstraction with a rate constant k^*_C1= 1.84 ± 0.07 × 10² dm³ mol^–1 s^–1. In PP9 the second order rate constant for reaction of t-butyl radicals with tetrachloromethane is 4.9 × 10⁴ dm³ mol^–1 s^–1, with an activation energy of 13.8 ± 4.2 kJ mol^–1. These kinetic results are compared with literature values and shown to be consistent with previous observations of the t-butyl radical.

The spin-lattice relaxation time of the protons in this radical is 2.2 ± 0.7 × 10^–3 s in chloroform solution and its diffusional correlation time is 4 ± 1 × 10^–11 s; this is consistent with theoretical predictions of the time required to produce polarization in a solution having the viscosity of chloroform. The whole provides a stringent test of kinetic CIDNP theory as applied to radical reactions in solution.