On the use of the MNDO-d semiempirical method for the structural study of chlorophyll b and anhydrous chlorophyll b dimer

Semiempirical MNDO-d molecular orbital calculations have been used for the structural study of chlorophyll b and anhydrous chlorophyll b dimer systems in their respective ground states. A model of chlorophyll b dimer has been considered whereby the magnesium atom is five-coordinate with an oxygen atom in the fifth coordination site. Total energies, net electric charges, and electronic dipole moments have been determined for the ground states of the above-mentioned molecules. The proposed models allow for the examination of distortions within the pyrroles and porphyrin macrocycles of these magnesium complexes. They also permit the estimate of changes in bond lengths, bond angles and dihedrals produced by dimerization via the acetyl oxygen of the carbometoxy group and the central magnesium atom, and changes in the π-electronic system of the chlorin systems. The results obtained are in good agreement with the experimental findings, and they reproduce many of the observed trends. In particular, neutron diffraction crystal structure, NMR and infrared spectra, provide experimental evidence in support of these theoretical predictions. Comparison with other theoretical results, shows that there is a good correlation between the DFT/B3LYP(6-31G*) and the MNDO-d, and also that the MNDO-d performs better than the HF/6-31G* and the PM5 in general.