Electronic Circular Dichroism of Surface-Adsorbed Molecules by Means of Quantum Mechanics Capacitance Molecular Mechanics

Abstract

To promote a more comprehensive understanding of the influence of metal–adsorbate interaction for molecules at metallo surfaces or metallo nanoparticles in solvent environments on their electronic circular dichroism (ECD) spectra, we evaluate the application of a recently derived quantum mechanics capacitance molecular mechanics (QMCMM) model for ECD. Using helicene absorbed on gold surfaces in protic and aprotic solvents as illustration, we elucidate the detailed effects on excitation energies, transition moments, rotatory strengths, orientation dependence of ECD spectra, and the different roles of aprotic and protic solvents and the induced charge distribution patterns on the surface. These changes are decomposed in terms of surface alone, solvent alone, and combined surface–solvent influence, and furthermore into the indirect contributions by the surface-induced restructuring of the helicene. Much of the salient changes of the ECD can be rationalized to the substantial redistribution of charge at the gold surface induced by the presence of the helicene. The study indicates that through the QMCMM model the effects of a metallic surface on the circular dichroism spectra of adsorbed organic molecules can be tackled by extended QM calculations coupled to polarizability–capacitance force fields for large metallic clusters representing surfaces or nanoparticles.