A ptevrous report that distinguishable ESR spectra can be obtained from CIS-and truns-Isomers of the 9-decalyl radical m liquid solution at low temperature is shown to be incorrect. Only one spectrum, assigned to the rrans-radtcal, is observable from etther CIS-or rruns-precursors In 1985 Lloyd and Wrlhams' clatmed that two different stereoisomers of the 9-decalyl radtcal could be detected in soluuon by ESR spectroscopy at ca. 190 K. The ESR spectra were observed dunng contmuous UV photolysis of dt-t-butyl peroxide (DTBP) in the presence of cis-or rrans-decalm and were ascribed to the ccs-or rrans-9-decalyl radmals (D;: or D;), respecttvely Two years later, the same authors' reported molecular mechanics and semtemprncal molecular orbital (UNDO) calculations m support of their conclusrons and estimated that DC is less stable than D; by about 4 kJ mol ', However, much earlier non-spectroscopic studies',' of 9-decalyl radrcals derived from CIS-and frans-precursors provide strong evidence that, even at very low temperatures, DC tsomenzes to 4: much too rapidly to permit the observation of different ESR spectra from the two sources Bartlett et al.' reported that at 323 K the value of (k,,/k,) is 0.3 dm3 mol.', where k,, is the rate coefficient for reaction of DC with oxygen to give DcO;
If we take' k,, to be cu. 5 x lo9 dn? moY' s" and assume an Arrhenius A-factor of 10" s' for the tsomerisatton, the value of k, at 190 K would be ca. lC? s-' and the corresponding activation energy [E,(l)] would be ca. 17 k.J mot-'. Greene and Lowry4 studied the competition between the conversion of D;: to 4: and trapping of the former radical by 9decalylmethyl hypochlorite or by tm hydrides and concluded that the isomerisation was extremely rapid with E,(l) probably m the range 12-25 kJ mol".
For alkyl radicals R' that are generated contmuously and removed only by self-reaction (rate coefficient 2kJ, the average hfettme of R' ~111 be equal to 1/2k[R 1. Self-reaction of decalyl radicals will probably be dtffuston-5385