Ortho-positronium Quenching in Benzene Solutions of Iron(III) Dithiocarbamates

Abstract

Lifetimes of the long-lived component of positronium, orthopositronium (o-Ps) were measured in benzene solutions of sixcoordinate iron(III) dithiocarbamates. The lifetime decreased with increasing solute concentration. The lifetime was strongly affected by the substituent group attached to the nitrogen atom in the ligand, and was related to the redox potential of the compound; highly reducible tris(pyrrolidinyl-N-carbodithioato) iron(III), Fe(Pyrrdtc) 3 , gave the greatest rate constant, 14.9X 10' M" 1 s" 1 , whereas the least reducible of the compounds examined, tris(N, N-dicyclohexyldithiocarbamato) iron (III), Fe(c-Hx 2 dtc) 3 , gave the smallest, 4.5 X 10' M" 1 s~l. The fivecoordinate halogenobis(diethyldithiocarbamato) iron (III) complexes, which are paramagnetic and hardly reducible, were also tested for the effect on the o-Ps quenching. Unlike the six-coordinate iron(III) dithiocarbamates, they only slightly affected the o-Ps lifetime, indicating that the spin-conversion process of o-Ps quenching is much less important than the oxidation process.Positronium, which is the bound state of a positron with an electron, has attracted increasing attention from many researchers and the positron annihilation as a nuclear probe has been widely applied to the study of the chemical [ 1 -6 ], miceller [ 7 -9 ], and biological systems [10][11][12]. The variation of the intensity of the long-lived component of positronium against the inhibitor concentration in systems of organic solutions has recently been successfully explained in terms of the spur model [13][14][15]. On the other hand, the changes of the lifetime by inorganic salts have been investigated mainly in aqueous solutions and several attempts have been made to estimate the redox potential of e + /Ps couple [2,6]. In aqueous solutions, the rate constants for the reaction of o-Ps with inorganic salts have been reported to vary over a wide range; i. e., (18.0-29.0) X 10' M' 1 s" 1 for KMnO, [4,16], and nearly zero for NaCl, ZnCl 2 , KCl, etc. [17].Although the reactions of o-Ps have been comparatively well examined for simple inorganic salts, few systematic studies have been conducted on the reactions with coordination compounds. Previously, we have examined the reactions of o-Ps with various kinds of (3-diketonates in benzene, and found that the acceleration of o-Ps decay is mainly attributable to the oxidation of o-Ps by the compounds [18][19][20][21]. As an extension of our previous work, we investigated the reactions of o-Ps with a series of substituted dithiocarbamatoiron(III) complexes in benzene. As the values of the ligand field splitting parameter, Δ, of the dithiocarbamatoiron(III) complexes are close to the spin-pairing energy, the complexes show spin crossover properties, depending on the kind of the substituents in the legating dithiocarbamate. The equilibrium between the high-spin and low-spin states ( 6 A, ^ J T 2 ) may be affected by the inductive, mesomeric, and steric effects of the substituents attached to the nitrogen atom [22][23][24]. At a ...