Solvation of Amides: Correlation of 17O NMR Chemical Shifts with IR ν(CO) Stretching Vibration

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A model of specific hydration sites of amides and peptides based on 17O NMR chemical shifts [1–3] and an excellent correlation of δ(17O) vs, ν(CO) (the amide I stretching vibration frequency) [3] is presented. Solvation of the amide hydrogen (NH) is shown to induce a very small modification of the shielding of the amide oxygen and thus can be neglected. On the contrary, long range dipole-dipole interactions and specific hydration at the amide oxygen are demonstrated to induce large and specific modifications of the 17O shielding constants (Table 1). Further, alkyl substitution of the amide nitrogen has a minor effect on δ(17O) of amides. Excellent linear correlation between δ(17O) and ν(CO) (Figure 1) was found for different solvents which have varying dielectric constants and solvation abilities. This demonstrates that both IR and 17O NMR spectroscopy appear to be reflecting a similar type of electronic perturbation, i.e., hydrogen bonding and dipole-dipole solute-solvent interactions. δ(17O) appears to be more sensitive to electronic perturbations, compared to ν(CO), as reflected by the values of the slopes of the resulting correlations. However, on the NMR time scale, the various solvation species are in fast exchange and only contribute to time-averaged features [3]. This contrasts to vibrational spectroscopy in which discrete solvates can be detected due to non-hydrogen bonded, monosolvated and disolvated species [4].