SPECIATION OF 1-ALKYL-4-CYANOPYRIDINIUM IODIDE

The intensity of the charge-transfer band of 1-alkyl-4-cyanopyridinium iodides was used to estimate the contact ion pair (CIP) concentration in a number of solvents. In several nonhydroxylic solvents with diele ctric constant between 4 and 6 the transition dipole moment is very si milar, and these values have been averaged. This average value was the n used to estimate the intrinsic molar intensity of the CIP in other s olvents. The concentration dependence of the intensity of the charge-t ransfer band was used in a conventional equilibrium formulation to est imate the total ion pair concentration. The solvent-separated ion pair (SSIP) concentration was determined by difference. The results were t ested by comparison with conductance measurements. In 2-propanol and i n acetonitrile the agreement of ion pair formation constants is within experimental uncertainty. However the agreement is poorer in triethyl phosphate and 2-butanol, with a discrepancy of a factor of 3 in the l ast solvent. CIP:SSIP concentration ratios of 2:3 in trimethyl phospha te, 1:1 in acetonitrile, 3:2 in 1-propanol, 7:3 in 2-propanol, 2-butan ol, and triethyl phosphate, 4:1 in tributyl phosphate, and still highe r in the other solvents were estimated. The CIP:SSIP ratio is in gener al agreement with that determined by Arnold et al. (Arnold, B. R.; et al. J. Am. Chem. Sec. 1996, 118, 5482) for radical ion pairs. However, the CIP:SSIP ratio is an order of magnitude higher than that estimate d by Peters and Li (Peters, K. S.; Li, B. J. Phys. Chem. 1994, 98, 401 ) for diphenylcarbonium chloride in acetonitrile. The difference is at tributed to a difference in operational definitions of ''CIP''.