THE TRANSIENT CCL3+ - ITS OPTICAL-ABSORPTION AND ITS ABILITY TO FORM ION-PAIRS IN METHYLCYCLOHEXANE SOLUTION OF CCL4

The nature of the well-known 470-nm band in pulse-irradiated CCl4/ethylcyclohexane (MCH) systems is investigated. The band has been attributed to the CCl3+ within the ion pair (CCl3+/Cl-). However, from an ah initio calculation for the ground and excited states of the isolated CCl3+ there is no transition available to explain the observed 470-nm absorption. As the spectral characteristics of the band (R,,, absorption, bandwidth) are found to be strongly dependent on system parameters, such as temperature and solute concentration, the absorption is ascribed to a charge-transfer (CT) band between CCl3+ and a solvent or solute molecule. In a MCH solution Of CCl4 both CT complexes, CCl3+ <-- MCH and CCl3+ <-- CCl4, are possible. Their buildup can be observed at low temperatures. The band disappearance is due to the decay of the corresponding ion pairs {(CCl3+ <-- MCH)\Cl-} and {(CCl3+ <-- CCl4)\Cl-}. Except for the very early times, the band decay follows first-order kinetics up to 4 half-lives, which is characterized by a low activation energy and a very low preexponential factor. Since this decay is faster at higher CCB concentrations, it is obvious that the ion pair with CCl4 is less stable (more reactive) than that with MCH. From a dose effect on the decay rate, it is concluded that the ion pairs are able to react with the solvent radicals. This ion pair reactivity, together with other arguments, like the very low preexponential factor, indicate that the ion pairs are probably solvent separated.