A density-functional-theory study of bacteriochlorophyll b

The molecular structure, electronic absorption spectrum, transition-dipole-moment directions, and rotatory strengths have been studied at density-functional-theory (DFT) levels for bacteriochlorophyll b ligated with an imidazole. In the DFT calculations, pure density functionals, i.e. functionals without Hartree-Fock exchange terms, as well as hybrid functionals were employed. The pure density functionals yielded practically identical excitation energies. However, with pure density functionals spurious states with small oscillator strengths were obtained between the Q and B bands, whereas the electronic absorption spectra calculated using the popular B3LYP hybrid functional consist of less transitions and agree well with measured excitation energies and band strengths. At the B3LYP level, the Q(y), Q(x), and B bands are obtained at 1.80 eV (1.56 eV); 2.11 eV (2.09 eV), 3.05 eV (3.03 eV); 3.34 eV, 3.36 eV, and 3.39 eV (3.33 eV), respectively. The experimental values are given in parentheses. Two weak transitions were also obtained between the Q and B bands at 2.93 eV and 3.00 eV. When the oscillator strengths are also taken into account, the B3LYP spectrum shows an almost perfect pattern, matching with the experimental spectrum; the largest discrepancy of 0.24 eV was obtained for the first excited state. Transition-moment directions and rotatory strengths calculated at the B3LYP level were also found to be in close agreement with available experimental data showing that B3LYP DFT calculations can be used for supporting the interpretation of data obtained using advanced spectroscopy on chlorophylls.