SPIN AND REACTION DYNAMICS IN FLEXIBLE POLYMETHYLENE BIRADICALS AS STUDIED BY EPR, NMR, AND OPTICAL SPECTROSCOPY AND MAGNETIC-FIELD EFFECTS - MEASUREMENTS AND MECHANISMS OF SCALAR ELECTRON-SPIN SPIN COUPLING

A series of spectroscopic measurements on flexible polymethylene biradicals in liquid solution is reported. Time resolved electron paramagnetic resonance, chemically induced nuclear spin polarization, transient optical absorption, and external magnetic field effects on lifetimes are used to unravel the kinetics of two types of biradical, acyl-alkyl and bisalkyl, obtained from the photoinduced Norrish type I alpha-cleavage reaction of permethylated cycloalkanones. A kinetic picture for the biradical exit channels is presented which includes rate constants for spin-orbit coupling, decarbonylation, spin relaxation, and end-to-end contact of the chains. The relative merit of each of the various experimental techniques is discussed, and the close relationship between spin and chain dynamics in the biradical kinetics is clearly illustrated. Direct and indirect measurements of the singlet-triplet splitting (electronic spin-spin coupling) are made and compared for different chain lengths. The mechanism of the coupling as well as its temperature dependence is discussed in terms of through-bond and through-space models. The acyl-alkyl biradicals are shown to have kinetics dominated by spin-orbit coupling, whereas the bisalkyl biradical spectra decay on a slower time scale via spin relaxation. Optical measurement of the decarbonylation rate of the pivaloyl monoradical is also reported.