title: 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.
authors.affiliation: Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands. v.p.nicu@vu.nl
authors.forename: Valentin Paul
authors.initials: VP
authors.lastname: Nicu
authors.forename: Mojgan
authors.initials: M
authors.lastname: Heshmat
authors.forename: Evert Jan
authors.initials: EJ
authors.lastname: Baerends