Density functional studies of torsion potentials of neutral and protonated bipyridines

The conformational properties of conjugated heteroaromatic bipyridines, pyridyl-pyridiniums and bipyridinium have been investigated using density functional theory (DFT). The molecules studied were neutral 2,2'-bipyridine (2,2'-bpy), 2,3'-bipyridine (2,3'-bpy) and 3,3'-bipyridine (3,3'-bpy) and the singly protonated 3-pyridyl-2'-pyridinium (3-py-2'-pyH) and 2-pyridyl-2'-pyridinium (2-py-2'-pyH) and the doubly protonated 2,2'-bipyridinium (2,2'-bpyH), i.e. typical groups with which longer non-protonated, partially protonated or fully protonated conjugated polypyridine chains with feasible optoelectric properties can be constructed. Comparison of the DFT results including different basis sets has been performed with Hartree-Fock theory and second-order Moller-Plesset perturbation theory (MP2). The DFT (B3LYP/6-31G(d,p)) was found to be adequate to describe the torsion potentials for a force field parametrization. Conformational structures, torsion potential energy surfaces and electrostatic potential (ESP) derived atomic charges have also been calculated. These results are used to parametrize torsion potential parameters in order to improve the description of conformational properties of the molecules using force field based methods. The reparametrized force fields showed the importance of cos 4phi term which improved the description significantly for cases which failed without it. Replacing the default atomic partial charges with ESP derived charges improved the description of torsion potentials still. The results allow to gain new insight into importance of accurate force field parameters in the area of conjugated molecules. (C) 2003 Elsevier B.V. All rights reserved.