Conformations and rotational barriers of 2,2 '-bithiazole and 4,4 '-dimethyl-2,2 '-bithiazole semiemperical, ab initio, and density functional theorycalculations

Conformational properties of 2,2'-bithiazole and 4,4'-dimethyl-2,2'-bithiaz ole have been studied by using AM1 and PM3 semiemperical methods and ab ini tio HF/6-311+G* and B3LYP/6-311+G* calculations. All methods agree that the planar s-trans conformation is the global minimum and the perpendicular co nformation is the transition state. Additional local minima were found usin g the Hartree-Fock (HF) and B3LYP levels for 2,2'-bithiazole while for 4,4' -dimethyl derivative the minima was located only at the MP2//B3LYP level. T he barrier heights for rotation are 1.72, 7.69, and 7.88 kcal/mol at the PM 3, HF, and B3LYP levels, respectively, and methyl substitution did not affe ct appreciably this value. Fourier expansion terms and bond orders were use d to explain the origins of the rotational barrier in terms of pi conjugati on, electrostatic interaction, and steric effects, which represent the main factors in the shape of the rotational barrier. (C) 2000 John Wiley & Sons , Inc.