CONFORMATIONS AND ROTATIONAL BARRIERS OF 2,2'-BI-1H-IMIDAZOLE - SEMIEMPIRICAL, AB-INITIO, AND DENSITY-FUNCTIONAL THEORY CALCULATIONS

Conformations and rotational barriers of 2,2'-bi-1H-imidazole (1) have been investigated by using semiempirical, ab initio, and density func tional theory (DFT) calculations. All theoretical methods employed in this study agree that the trans conformation of 1 is the global minimu m, and the cis conformation is a transition state. Although semiempiri cal methods have located only these two stationary points, ab initio a nd DFT calculations have found additional local minima at a slightly s kewed cis conformation. The torsional angle between two imidazole ring planes at these local minima is calculated to be 26.3 degrees at HF/3 -21G, 45.9 degrees at HF/6-31G, and 37.8 degrees at B3LYP/6-31G*. Our best estimate for the overall rotational barrier of 1 through the cis conformation is 11.8 kcal mol(-1), which is obtained from B3LYP/6-31G calculations with the correction of zero-point vibrational energy. E stimations of this barrier by semiempirical methods are significantly lower than 8.6 kcal mol(-1) by AM1, and 10.6 kcal mol(-1) by PM3, whil e the overall rotational barriers predicted by the SCF methods (15.6 a nd 13.6 kcal mol(-1) at the HF/3-21G and HF/6-31G levels, respectivel y) are considerably higher than the B3LYP/6-31G result. In order to b etter understand the origins of the rotational barrier, we have attemp ted to analyze (1) changes of the electrostatic potential maps and the V-min(r) values, (2) Fourier expansion terms for rotational potential energy functions, and (3) the bond length change during internal rota tion. Based on these analyses, electrostatic interaction and pi-conjug ation appear to play an important role in forming the shape of the rot ational barrier.