Density functional theory and Monte Carlo study of macrocycle-ion interactions

Molecular cages made from macrocycles form a protective environment around guest ions or molecules. Physical and chemical properties of these host-guest systems are quite different from those of the separated components. A procedure for theoretical study of some properties of host-guest systems has been constructed, making use of the classical Monte Carlo (MC) simulations and the Density Functional Theory (DFT). Four selected applications are presented. In the first application, the electronic, optical and magnetic properties of molybdocene porphyrazines to which the guests, Cu or protons, are attached were studied by DFT and the results were compared with electrochemical, EPR and optical absorption measurements. In the second application, the electronic and structural properties of alkali metal cation cryptand[222] complexes in the gas phase were studied under DFT and MC simulated annealing, and the relationship between DFT energies and classical energies was built. In the third application, using partial charges obtained in the second application and van der Waals parameters, we performed rigid molecule MC simulations on the ionic electrolyte Li-cryptand[222]-MPSA in both gas and liquid phases, and our results agree with NMR diffusion and AC conductivity measurements. In the fourth application, MC simulations were performed on systems consisting of one cryptand (C222, C221 or C211) and two types of alkali cations (cation1-cryptand-cation2) in vacuum, and one alkali cation and two types of cryptands (cryptand1-cation-cryptand2) in water. Results from gas phase simulations on cryptand1-cation-cryptand2 were found to agree with Fourier transform ion cyclotron resonance mass spectrometry. Results from simulations on cation1-cryptand-cation2 in water gave the relative order of the complexation of cryptands in agreement with the size-match rule observed in experiments. The agreement with experiment shows that our methodology, implementation, and models are correct in general, and should be applicable to more complicated host-guest systems in supramolecular chemistry.