| Popis: |
A multiple-linear-regression analysis (MLRA) has been carried out using the Kamlet–Abboud–Taft (KAT) solvatochromic parameters in order to elucidate and quantify the solvent effects on the 17 O chemical shifts of N -methylformamide (NMF), N , N -dimethylformamide (DMF), N -methylacetamide (NMA), and N , N -dimethylacetamide (DMA). The chemical shifts of the four molecules show the same dependence (in ppm) on the solvent polarity–polarizability, i.e., −22π*. The influence of the solvent hydrogen-bond-donor (HBD) acidities is slightly larger for the acetamides NMA and DMA, i.e., −48α, than for the formamides NMF and DMF, i.e., −42α. The influence of the solvent hydrogen-bond-acceptor (HBA) basicities is negligible for the nonprotic molecules DMF and DMA but significant for the protic molecules NMF and NMA, i.e., −9β. The effect of substituting the N–H hydrogen by a methyl group amounts to −5.9 ppm in NMF and 5.4 ppm in NMA. The effect of substituting the O=C–H hydrogen amounts to 5.5 ppm in NMF and 16.8 ppm in DMF. The model of specific hydration sites of amides by I. P. Gerothanassis and C. Vakka [ J. Org. Chem. 59, 2341 (1994)] is settled in a more quantitative basis and the model by M. I. Burgar, T. E. St. Amour, and D. Fiat [ J. Phys. Chem. 85, 502 (1981)] is critically evaluated. 17 O hydration shifts have been calculated for formamide (FOR) by the ab initio LORG method at the 6-31G* level. For a formamide surrounded by the four in-plane molecules of water in the first hydration shell, the calculated 17 O shift change due to the four hydrogen bonds, −83.2 ppm, is smaller than the empirical hydration shift, −100 ppm. The 17 O shift change from each out-of-plane water molecule hydrogen-bonded to the amide oxygen is −18.0 ppm. These LORG results support the conclusion that no more than four water molecules are hydrogen-bonded to the amide oxygen in formamide. |
Full Text from ScienceDirect