title: Interplay of Exciton Coupling and Large-Amplitude Motions in the Vibrational Circular Dichroism Spectrum of Dehydroquinidine.
abstract: A detailed analysis of the computed structure, energies, vibrational absorption (VA) and circular dichroism (VCD) spectra of 30 low-energy conformers of dehydroquinidine reveals the existence of families of pseudo-conformers, the structures of which differ mostly in the orientation of a single O-H bond. The pseudo-conformers in a family are separated by very small energy barriers (i.e., 1.0 kcal mol(-1) or smaller) and have very different VCD spectra. First, we demonstrate the unreliable character of the Boltzmann factors predicted with DFT. Then, we show that the large differences observed between the VCD spectra of the pseudo-conformers in a family are caused by large-amplitude motions involving the O-H bond, which trigger the appearance/disappearance of strong VCD exciton-coupling bands in the fingerprint region. This interplay between exciton coupling and large-amplitude-motion phenomena demonstrates that when dealing with flexible molecules with polar bonds, vibrational averaging of VCD spectra should not be neglected. In this regard, the dehydroquinidine molecule considered here is expected to be a typical example and not the exception to the rule.
authors.affiliation: Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands). V.P.Nicu@uva.nl.
authors.forename: Valentin P
authors.initials: VP
authors.lastname: Nicu
authors.affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, XH Amsterdam (The Netherlands).|Current address: Max Planck Institute for the Structure and Dynamics of Matter at the Center for Free-Electron Laser Science and The Hamburg Centre for Ultrafast Imaging, Universität Hamburg, Luruper Chaussee 149, 22671 Hamburg (Germany).
authors.forename: Sérgio R
authors.initials: SR
authors.lastname: Domingos
authors.affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, XH Amsterdam (The Netherlands).
authors.forename: Benjamin H
authors.initials: BH
authors.lastname: Strudwick
authors.affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, XH Amsterdam (The Netherlands).
authors.forename: Albert M
authors.initials: AM
authors.lastname: Brouwer
authors.affiliation: Van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, XH Amsterdam (The Netherlands).
authors.forename: Wybren J
authors.initials: WJ
authors.lastname: Buma
keywords: IR spectroscopy; alkaloids; organocatalysis; quantum chemistry; vibrational circular dichroism