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Interplay of Exciton Coupling and Large‐Amplitude Motions in the Vibrational Circular Dichroism Spectrum of Dehydroquinidine
Author(s) -
Nicu Valentin P.,
Domingos Sérgio R.,
Strudwick Benjamin H.,
Brouwer Albert M.,
Buma Wybren J.
Publication year - 2016
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201503250
Subject(s) - conformational isomerism , vibrational circular dichroism , exciton , spectral line , molecular physics , molecular vibration , amplitude , coupling (piping) , chemistry , molecule , circular dichroism , crystallography , physics , materials science , condensed matter physics , quantum mechanics , organic chemistry , metallurgy
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 OH 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 OH 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.