Resonance in Formamide and Its Chalcogen Replacement Analogues:  A Natural Population Analysis/Natural Resonance Theory Viewpoint

• Published in: Journal of the American Chemical Society Vol. 119; no. 52; pp. 12940 - 12946
• Main Authors: Glendening, Eric D, Hrabal, John A
• Format: Journal Article
• Language: English
• Published: American Chemical Society 31.12.1997
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja970074j

The influence of resonance on the structure and rotation barrier of formamide and its S, Se, and Te replacements analogues is examined using the natural bond orbital methods. Calculations are performed at the RHF, B3LYP, and MP2 levels of theory with 6-31+G* basis sets and effective core potentials. At the MP2 level, the rotation barriers increase with the increasing size of the chalcogen, from 17.2 kcal mol-1 for formamide to 21.0 kcal mol-1 for telluroformamide. Natural population analysis and natural resonance theory (NRT) reveal shifts in the charge density that are consistent with the strong resonance stabilization of the equilibrium, planar geometries. NRT provides a simple, quantitative description of the amides as a resonance hybrid consisting primarily of two contributing structures, the parent Lewis structure and a secondary dipolar form. Amide resonance effects strengthen from formamide to telluroformamide as the weight of the dipolar form increases. Polarizability appears to contribute importantly, allowing the chalcogens to accommodate more charge density than anticipated on the basis of electronegativity.