The photolysis of pentamethylcyclopentadiene: experiments with isotopically labelled material

Abstract

Photolysis of the ¹³C-labelled pentamethylcyclopentadienes (¹³CH₃)(CH₃)₄C₅H and (CH₃)₅¹³CC₄H in hexane solution gives the corresponding pentamethylcyclopentadienyl radicals with the e.s.r. parameters a(¹³C_α) 2.68 and a(¹³C_β) 3.35 G; this confirms that these are π-delocalised radicals which can be treated by π-electron theory. Photolysis of various deuterium-labelled pentamethylcyclopentadienes shows that, in the dihydrogen which is evolved, one hydrogen atom is derived from the CH of the ring and the other from a CH₃ group, rather than both from the ring. The use of Raman spectroscopy as an easy and effective method for the analysis of the isotopic composition of dihydrogen is described.