Reinvestigation of molecular structure and barrier to internal rotation of pyridinium N-phenolate betaine dye

In the present paper, the results of a systematic theoretical study of the molecular structure of 4-(1-pyridinium-1-yl)phenolate betaine are reported. The ground-state molecular structure and the barrier to internal rotation of the betaine dye molecule were calculated ab inito (with Hartree-Fock theory and the second-order of Moller-Plesset method) and with density functional theory (DFT). In order to estimate the complete basis set limit, the calculations of barriers to internal rotations were performed using correlation-consistent basis sets with a maximal cardinal number of four. It was determined that electron correlation is crucial in order to obtain reliable geometries and rotational barriers of the molecule investigated. For the sake of comparison, the results of calculations using the AM1 Hamiltonian are also presented.