ESR studies on C-60 O-x and C-60 center dot HMTTEF

The electron spin resonance (esr) of C-60.Ox and C-60.HMTTEF (hexamethylentetratellurafulvalene) has been investigated at 9.36 GHz as a function of temperature, T (298 greater than or equal to T greater than or equal to 4 K). C-60.O-x shows an esr absorption of equal 'g' value to that of C-60 exposed to O-2 and light but is more intense. The C-60 in its pure form is in a singlet state. The impurity sites introduced by O-2 produce the esr absorption. The Curie - Weiss plots of inverse esr absorption intensity versus temperature indicate an antiferromagnetic Theta approximate to 50 for air exposed C-60 and 90K for C60Ox. From the esr intensity at room temperature, the calculated number of free spins (S = 1/2) is approximate to 1/3 per mole of C60Ox. In case of C-60.HMTTEF, there is a very weak esr absorption at room temperature suggesting that the room temperature form is diamagnetic with very small charge transfer between C-60 and HMTTEF. This conclusion is consistent with the structure and magnetic susceptibility of this cocrystal. As the temperature is lowered, the equilibrium: AD <=> A(*-) + D-*+ is displaced towards the formation of free radical species, A(*-) and D-*+.