Free energies for the coordination of ligands to the magnesium of chlorophyll-a in solvents

The coordination of bases to chlorophyll magnesium modifies spectroscopic properties in solution as well as in situ in reaction centres. We evaluate the free energies of complexation of one or two pyridine, 1-propanol, diethyl ether or water solvent molecules at 298 and 150 K to rationalise observed phenomena. Various a priori dispersion-corrected density functional theory calculations are performed as well as second-order Møller-Plesset calculations, focusing on the effects of dispersion modifying the intermolecular interactions, of dispersion modifying solvation energies, of entropy, and of basis-set superposition error. A process of particular interest is magnesium complexation in ether at low temperature that is often exploited to assign the Q-band visible absorption spectrum of chlorophyll. Recently, we demonstrated that trace water interferes with this process, but the nature of the resulting complex could not be uniquely determined; here, it is identified as ether.Chlorophyll-a.H₂O, consistent with interpretations based on our authoritative 2013 assignment.