Speciation of 1-Alkyl-4-cyanopyridinium Iodide

Abstract

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 dielectric constant between 4 and 6 the transition dipole moment is very similar, and these values have been averaged. This average value was then used to estimate the intrinsic molar intensity of the CIP in other solvents. The concentration dependence of the intensity of the charge-transfer band was used in a conventional equilibrium formulation to estimate the total ion pair concentration. The solvent-separated ion pair (SSIP) concentration was determined by difference. The results were tested by comparison with conductance measurements. In 2-propanol and in 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 last solvent. CIP:SSIP concentration ratios of 2:3 in trimethyl phosphate, 1:1 in acetonitrile, 3:2 in 1-propanol, 7:3 in 2-propanol, 2-butanol, and triethyl phosphate, 4:1 in tributyl phosphate, and still higher in the other solvents were estimated. The CIP:SSIP ratio is in general agreement with that determined by Arnold et al. (Arnold, B. R.; et al. J. Am. Chem. Soc. 1996, 118, 5482) for radical ion pairs. However, the CIP:SSIP ratio is an order of magnitude higher than that estimated by Peters and Li (Peters, K. S.; Li, B. J. Phys. Chem. 1994, 98, 401) for diphenylcarbonium chloride in acetonitrile. The difference is attributed to a difference in operational definitions of “CIP”.