Signatures of counter-ion association and hydrogen bonding in vibrational circular dichroism spectra

Abstract

We study the effect of counter-ion complexation on the example of Cl⁻ ions interacting with the [Co(en)₃]³⁺ complex. The H-bonding of the N–H groups of the ethylenediamine (en) ligands with the Cl⁻ ions may lead to giant enhancement of the VCD intensity for the N–H stretches, but may also lead to VCD sign changes in the finger print region of N–H wagging, twisting and scissoring motions. Such sign changes should not be mistaken for signatures of the presence of the other enantiomer. We elucidate the mechanism for the sign changes and give a recommendation on how to deal with this problem. We also show that the experimental spectrum is only in good accord with the calculations if complexation of 5 Cl⁻ ions (two axial, three equatorial) is assumed, but not with two (axial) or three (equatorial) Cl⁻ ions, thus showing the potential of VCD to be used as an experimental probe for complexation.