MOLECULAR-ORBITAL CONSTRAINED ELECTRON-DIFFRACTION STUDIES - CONFORMATIONAL BEHAVIOR OF 1,2-DIMETHYLHYDRAZINE

The results of a conformational analysis of 1,2-dimethylhydrazine are described which were obtained b applying MOCED (molecular orbital constrained electron diffraction). In this procedure primary structural parameters (differences between bond distances and bond angles), which can be reliably predicted by complete ab initio force relaxations, are used as constraints needed to interprete the gas electron diffraction data of a molecule. The diffraction data can be used to describe the conformational behavior of a molecule as a means to interpret the calculated ab initio conformational energy differences, which are usually less well established than ab initio structures because of basis set effects, correlation effects, and vibrational effects. Results for complete ab initio (4-31G) force relaxations of three gauche forms of 1,2-dimethylhydrazine are given. The investigation confirms the existence of the inner-outer conformer (CNNC torsional angle about 90°) as a major conformation. Details of analysis suggest the presence of at least one other conformer which cannot be definitely determined from the diffraction data. Both inner-inner (calculated CNNC angle 50°) and outer-outer forms (calculated CNNC angle 140°) must be considered. The study demonstrates the power of hybrid theoretical and experimental procedures in conformational analyses. It also demonstrates that ab initio conformational analyses using standard geometries can be misleading, since each conformation may deviate from the standard in an unpredictable manner. © 1979, American Chemical Society. All rights reserved.