Experimental studies of the 13C NMR of iodoalkynes in Lewis-basic solvents

The C-13 NMR spectra of two different iodoalkynes, 1-iodo-1-hexyne (1) and diiodoethyne (2), exhibit a strong solvent dependence. Comparisons of the data with several common empirical models, including Gutmann's Donor numbers, Reichardt's E-T(N), and Taft and Kamlet's beta and pi*, demonstrate that this solvent effect arises from a specific acid-base interaction. Solvent basicity measures such as Donor numbers and beta values correlate well with the alpha-carbon chemical shift of 1, but polarity measures such as E-T(N) and pi* do not correlate. The similarity of the solvent effect for 1 and 2 suggests that carbon-carbon bond polarization may not play a role in the change in chemical shift, as previously hypothesized.