An electron spin resonance study of pentenynyl and pentadiynyl radicals

Pent-2-en-4-ynyl radicals were generated from pent-1 -en-4-yne and cis- and trans-1-bromopent-2-en-4-yne and observed in the trans (4) and cis (5) conformations by e.s.r. spectroscopy. The rate constant for the isomerization (5) → (4) was determined by kinetic e.s.r. to be given by equation (i) and for the isomerization (4) → (5) the log k₁ = (13.2 ± 1.3) - (47.5 ± 8.0 kJ mol^-1/2.3RT) s^-1 (i) rate constant was found to be given by equation (ii). From the observed barriers to rotation the methane based log k_-1 = (13.0 ± 1.3) - (49.2 ± 8.0 kJ mol^-1/2.3RT) s^-1 (ii) stabilization energies of (4) and (5) were estimated to be 112 and 110 kJ mol^-1. E.s.r. spectra were also recorded for the 1-trimethylsiloxy-derivatives of (4) and (5), pent-2-en-4-ynyl radicals, 1-trimethylsiloxypent-2-en-4-ynyl radicals, and penta-1,4-diynyl radicals. The enthalpies of formation, structures, and stabilization energies of the hydrocarbon radicals were computed using the MINDO/3 method. The conformational interconversion of radicals (4) and (5) was also examined by MINDO/3.