NMR chemical shielding surface ofN-Acetyl-N?-Methylalaninamide: A density functional study
✍ Scribed by Sulzbach, Horst M.; Vacek, George; Schreiner, Peter R.; Galbraith, John Morrison; Schleyer, Paul von R.; Schaefer, Henry F.
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 418 KB
- Volume
- 18
- Category
- Article
- ISSN
- 0192-8651
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✦ Synopsis
The five energetically lowest minima on the potential energy surface of Nacetyl-NЈ-methylalaninamide were optimized at the Becke3LYPrDZ d level of theory to compare these density functional theory results with the literature findings at restricted Hartree᎐Fockr3᎐21G. While the relative energies are very similar, the amide moiety is predicted to be much more flexible at Becke3LYPrDZ d. As a consequence, the three minima that favor a nonplanar amide group differ by up to 14Њ in their and values between the two levels. To compare the change in the density functional NMR chemical shifts with respect to and with experimental results, Becke3LYPrDZ d was employed to optimize a structure for N-acetyl-NЈ-methylalaninamide at each 30Њ interval Ž . on the , surface in the regions that correspond to the ␣ helix and the -pleated sheet and at each 60Њ interval elsewhere. The corresponding NMR chemical shielding surface was computed with the density functional program deMon. The resultant NMR chemical shielding surfaces for N and C  are in good agreement with the experiment, while the change in the NMR chemical shielding of CЈ and C ␣ cannot be described only in terms of and . The chemical shifts for those atoms also depend on the nonplanarity of the amide moiety. We evaluated this dependence for N-methylacetamide as a model system. Estimates of the parameters derived from N-methyl-acetamide allowed the NMR-shielding surfaces of CЈ and C ␣ to be corrected for the nonplanar nitrogen influence. Although the effect is less pronounced with lower level theoretical geometries, due to the smaller degree of pyramidalization of the Ž . amide nitrogen, the , NMR chemical shielding surfaces will need to be