Solvent effects on 13C nuclear magnetic resonance shifts of polar aliphatic compounds. A charge separation model

Abstract

A model is proposed which explains the large solvent effects on the shielding of carbon atoms bearing polar bonds (ΔC_α) with an increased charge separation in solvents of a higher dielectric constant. Using cis-1-iodo-1-methyl-4-t-butylcyclohexane and t-butyl chloride as conformationally homogeneous solutes, one obtains for a series of solvents a linear correlation (r > 0.95) for ΔC_α in the expected range of 415 p.p.m./electron charge unit and 320 p.p.m./e, respectively. In a second set of experiments, the sensitivity of ΔC_α against solvent changes for differing C_α–X bonds is obtained, again yielding a linear correlation (r > 0.99) with 380 p.p.m./e. In addition, a ΔC_α enhancement with increasing carbon substitution at C_α is found for several alkyl iodides and rationalized in terms of an increasing contribution of the charged form to the resonance hydride.