Vibrational Circular Dichroism Spectroscopy of Three Multidentate Nitrogen Donor Ligands: Conformational Flexibility and Solvent Effects

Abstract

A series of multidentate nitrogen donor ligands have been synthesized and characterized and their conformational distributions in solution have been investigated. Vibrational absorption (VA) and vibrational circular dichroism (VCD) spectroscopy, complemented with DFT calculations, have been used to probe the conformations of these important ligands in solution directly. These three ligands demonstrate very different conformational flexibility; the pyridine subunits and amine groups may adopt a number of different conformations. Experimental VA and VCD data measured in CDCl3 have been compared to the theoretical spectra of all possible most stable conformers. Solvent effects have been taken into account by using the implicit polarizable continuum model and explicit solvation model. The explicit hydrogen-bonding solvation model is important for explaining the VCD sign-reverse phenomenon in the amide I region. Good agreement has been achieved between experimental and predicted spectra for all three ligands; thus allowing detailed examination of the related conformational structures and distributions in solution.