Properties of the PBN and DMPO radical cations

Recently, the radical cation of PBN has been characterized by matrix spectroscopy and its reactivity has been studied by fast spectroscopic methods (Zubarev and Brede, 1994), and found to conform to the behaviour deduced from the OsCU and TBPA + studies. y-Radiolysis of PBN in a glassy matrix of isobutyl chloride or Freon-113 (CF2C1CFC12) at 77 K produced an intensely green glass containing PBN +, the epr spectrum of which had an anisotropic nitrogen coupling constant Ay = 2.75 mT and gy = 2.0037. Tlie mechanism of the radiolysis reaction is well established (Neta, 1976) and involves the formation of solvated electrons (e ), which add to the matrix species and produce chloride ion, and positive holes (h+) which eventually come to rest at the matrix component of lowest Ip (Symons, 1997), in this case PBN (see reactions (30) and (31)). 

Upon slow warming of the matrix, the colour disappeared and a new species with A = 3.24 mT and gy = 2.0038 appeared, assigned to the formation of the chloro spin adduct [12] (32) after melting of the matrix at 240 K the characteristic solution epr spectrum of [12] was recorded. By y-radiolysis of the isomeric oxirane [13], which cannot sustain spin trapping, another way of direct matrix generation of PBN + was available and thus made possible further confirmation of these results (Zubarev and Brede, 1995). 

Fast spectroscopy was also used to probe the reactivity of PBN +. The 266 nm laser excitation of peroxydisulfate ion in aqueous solution at room temperature gives the powerful oxidant SOr, which oxidizes PBN in an exergonic reaction (by about 0.8 eV, see Tables 1 and 5) with k = 3 X 109 dm3 mol-1 s 1. The pseudo-first-order rate constant for the decay of PBN + by reaction with water to give HO-PBN was 2 x 106 s 1, a relatively slow reaction (k = 3.6 x 104 dm3 mol-1 s-1 at ambient temperature). 

Laser excitation of chloranil ([14], tetrachlorobenzoquinone) at 355 nm in acetonitrile-5 % water gives its triplet state, 3[14], which is a strong oxidant [ °(3[14] /[14] ) = 2.3 V] and will oxidize PBN to PBN + with k = 8 X 109 dm3 mol-1 s 1. In this medium the water reaction of PBN + had k = 1.5 x 10s dm3 mol-1 s-1. There was also indication of a second decay pathway of PBN + [equation (33)], involving attack upon a second molecule of PBN (k = 6 x 10s dm3 mol 1 s 1), and this was suggested to decompose rapidly into the benzoyl nitroxide [9] and the imine [15], both known products of PBN photolysis. 

A study of the DMPO radical cation showed the same features the radical cation is formed at 77 K in a CFC13 matrix and reacts with added traces of water to give HO-DMPO upon warming to 270 K (Chandra and Symons, 1986). 

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