Structural Analysis of a Simple Amide
A Paradigm for Studying Resonance Theory

Methodology
MNDO/AM1/PM3 (Semiempirical MO)

Techniques Used
Building, Optimization, Analysis of
Bonding Parameters, Output Analysis


Abstract. You will optimize the structure of acetone and of acetamide using a semiempirical method. You will then analyze various differences between the molecular and electronic structures of these species. The results should be germane to the usual resonance descriptions of these molecules.


Procedure. Build and optimize acetone and acetamide. Analyze the bonding parameters, bond orders, and charge distribution in these molecules, and do torsional analyses about the CO-CH3 and CO-NH2. In the results table record the method used, and the parameters described above for these molecules. Note the energy barrier to torsion for the two alpha bonds. How well does acetamide resemble its putative zwitterionic resonance structure?


Benchmarks. Torsional energy barriers for the test molecules are given below.


Results.

 Experiment

 Computed
 Acetone Torsional Barrier

 0.2 kcal/mol
 
 Acetamide Torsional Barrier

 18 kcal/mol
 

Copyright by Paul M. Lahti, 1995. All rights reserved.