Radical Cation, Dimer Radical Cation and Neutral Radical of 2,3‐Dihydropyran – Possible Initiators of its Polymerisation?

Abstract

AbstractThe radical cation, distonic dimer radical cation, and radical of 2,3‐dihydropyran (DHP), which may play an important role in mechanism of radical polymerisations of DHP, were radiolytically generated at low temperatures in freon matrices and investigated by electron paramagnetic resonance (EPR) spectroscopy, as well as by quantum chemical methods. DHP radical cation with a half‐chair conformation and six coupling protons (0.88, 0.18, 4.60, 1.88, 1.27, 0.56 mT) was found to be stable in CF3CCl3 between 77 and 145 K. The same radical cation was observed at 77 K also in CF2ClCFCl2, but on increasing temperature, transformation into the neutral 4‐dihydropyryl radical occurred; the latter also shows the half‐chair conformation at 140 K and is characterised by six coupling protons (0.14, 2.31, 0.92, 1.42, 0.28, 1.35 mT). At a high DHP concentration in CF2ClCFCl2 a distonic DHP dimer radical cation, with strongly separated spin and charge and only two coupling protons (1.8 and 3.2 mT) was identified at 100 K, also yielding, at higher temperatures, the 4‐dihydropyryl radical.The radiolytically generated species from DHP and their observed EPR spectra.magnified imageThe radiolytically generated species from DHP and their observed EPR spectra.