RESONANCE IN FORMAMIDE AND ITS CHALCOGEN REPLACEMENT ANALOGS - A NATURAL-POPULATION ANALYSIS NATURAL RESONANCE THEORY VIEWPOINT

The influence of resonance on the structure and rotation barrier of fo rmamide and its S, Se, and Te replacements analogues is examined using the natural bond orbital methods. Calculations are performed at the R HF, B3LYP, and MP2 levels of theory with 6-31+G basis sets and effect ive 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 populatio n analysis and natural resonance theory (NRT) reveal shifts in the cha rge density that are consistent with the strong resonance stabilizatio n of the equilibrium, planar geometries. NRT provides a simple, quanti tative description of the amides as a resonance hybrid consisting prim arily of two contributing structures, the parent Lewis structure and a secondary dipolar form. Amide resonance effects strengthen from forma mide to telluroformamide as the weight of the dipolar form increases. Polarizability appears to contribute importantly, allowing the chalcog ens to accommodate more charge density than anticipated on the basis o f electronegativity.