Identification of Radical Cations of n-Alkanes in Irradiated Solutions by Time-Resolved Magnetic Field Effect Methods

Alkane radical cations are the key transient species determining the composition of the products of radiolysis of alkane solutions. As probed by pulse radiolysis, the lifetimes of n -alkane radical cations under common conditions range from several nanoseconds to several tens of nanoseconds [1]. The same lifetime range for these species was obtained in studying stationary magnetic field effects by radioluminescence methods [2]. Short lifetimes of n -alkane radical cations render routine ESR methods impracticable for studying these species in solutions. Only such a highly sensitive ESR technique as optically detected ESR of spin-correlated radical ion pairs made it possible to observe a line with a width of 0.27 mT upon irradiation of pentadecane; it was assigned to the pentadecane radical cation [3]. No signals were observed for other alkanes [4]. We pioneered in obtaining ESR spectra of some n -alkane radical cations in solution by means of another method applicable to investigation into spin chemistry, namely, the method of magnetic field effects in recombination fluorescence of spin-correlated radical ion pairs [5, 6]. In nonpolar alkane solutions, fluorescence decay caused by recombination of radical ion pairs is determined by the relationship