ROTATIONAL ENERGY SURFACES OF AZA-1,3-BUTADIENES AND PHOSPHA-1,3-BUTADIENES - A THEORETICAL-STUDY

The rotational energy surfaces of all stereoisomers of 1-aza-, 2-aza-, 1,3-diaza-, 1,4-diaza-, and 2,3-diaza-1,3-butadienes and 1-phospha-, 2-phospha-, 1,3-diphospha-, 1,4-diphospha-, and 2,3-diphospha-1,3-butadienes were calculated at the MP2/6-31G* parallel-to HF/6-31G* level. Rotational barriers and all local minima were rigorously located and identified. For all systems except anti-1,3-diaza-1,3-butadiene, the trans conformer is the global minimum. Stable gauche isomers are present for most azabutadienes and all phosphabutadienes. Rotational barriers for the azabutadienes are between 1.4 and 8.6 kcal mol-1 and between 3.1 and 8.1 kcal mol-1 for the phosphabutadienes. The shapes and relative heights of the critical points on the rotational surfaces are discussed in terms of pi-delocalization, 1,4-steric interactions, and intramolecular hydrogen bonding. The low rotational barriers and stable gauge conformers of the phosphabutadienes indicate that these molecules should undergo electrocyclic and cycloaddition reactions that require cis-like conformations of the diene.