Solvent effects on oxygen-17 chemical shifts in amides. Quantitative linear solvation shift relationships
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Diez, E.
(EN)
Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας
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Diez, E.
(EN)
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 O-17 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., -22 pi*. The influence of the solvent hydrogen-bond-donor (HBD) acidities is slightly larger for the acetamides NMA and DMA, i.e., -48 alpha, than for the formamides NMF and DMF, i.e., -42 alpha. 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., -9 beta. 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. F. St, Amour, and D. Fiat [J. Phys. Chem. 85, 502(1981)] is critically evaluated. O-17 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 O-17 shift change due to the four hydrogen bonds, -83.2 ppm, is smaller than the empirical hydration shift, -100 ppm. The O-17 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. (C) 1997 Academic Press.
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