Conformational studies of the N-(3-halobenzylidene)-3-haloaniline system. Part 2. Molecular energetics

Molecular energetics of the N-(3-halobenzylidene)-3-haloanilines have been studied, using semi-empirical molecular orbital calculations, and served as a basis for comparison the results from a gas-phase electron diffraction (GED) experiment. As the molecule contains a π-electronic system - a phenyl ring linked through a single bond to a double bond - which has proved problematic for semi-empirical methods, a number of these have been tested: AM1, MINDO/3, PM3 and MNDO in addition to molecular mechanics [CAChe-MM2 and MM2(91)], and this study serves as a critical comparison and evaluation of these methods for treating such a classic molecular electronic linkage. The computational methods that were found to be suitable for the N-benzylideneaniline molecule were AM1 and MM2(91). For two of the title derivatives there is a significant difference between the conformation found in the crystal structure and that determined by GED in concert with the calculations based on the AM1 method.