No abstract

Ring closure following flash photolysis in alkane solvents has been detected for several complexes in the series (η5-C5H4R)Mn(CO)3 where R = COCH3 (1), COCH2SCH3 (2), CO(CH2)2SCH3 (3), COCH2OCH3 (4), (CH2)2CO2CH3 (5), and CH2CO2CH3 (6). In each case where ring closure occurs, a metal CO is ultimately substituted by the side chain functional group. Photoacoustic calorimetry studies reveal that ring closures occur with rate constants faster than 107 s-1 or between 106 and 107 s-1, or in some cases the ring closure is biphasic with rate constants in both ranges. The enthalpies of CO dissociation followed by ring closure for 2 and 3 are the same (12 kcal/mol) and more favorable than those for 4−6 (25−15 kcal/mol). Studies of 1−3 by transient picosecond to microsecond infrared spectroscopy confirm biphasic dynamics for 2 and 3:  ring closure occurs immediately (k > 5 × 109 s-1) and at slower rates (k = 108−106 s-1). We propose that some ring closure occurs before solvent coordination and that the remaining ring closure, resulting in the displacement of solvent, is much slower. The relationships of the rates and energetics of ring closure to structure and quantum yields are discussed.

The velocity coefficients ks of reactions of polymer radicals wit'i a number of substrates have been discussed in terms of the general reactivities of the radicals and polar resoname forms in the transition state. The velocity coefficient of chain transfer to toluene at 60°C, k3,=, is used as an experimental measure of the general reactivity. Plots of log k, against log k3,= are of three types : a straight line with slope close to unity, or one of two characteristic patterns bearing an inverse relationship to each other. The first case occurs with " non-polar " reactions (relative to toluene as standard) while the second and third correspond to transition states Folarized in opposite senses.Quantitative relationships are given which allow the " polarity parameter " pR. for a radical in a given reaction and the " intrinsic polarity " for reactions with a given substrate to be estimated. There is a correlation between pR. values and the Hammett CJ functions of the radicals concerned. These results enable the velocity coefficient of a reaction to be calculated from the general reactivity and 0 function of the radical together with two parameters characteristic of the substrate. The calculated figures appear generally reliable to within a factor of 3, and often to within 20 %.The reactions of free radicals are generally transfer or addition processes. The former group includes both atom (less likely group) and electron transfers and may be represented by the equations :