Regional Susceptibility in VCD Spectra to Dynamic Molecular Motions: The Case of a Benzyl α‐Hydroxysilane

• Published in: Chemphyschem Vol. 19; no. 5; pp. 561 - 565
• Main Authors: Xia, Yiyin, Koenis, Mark A. J., Collados, Juan F., Ortiz, Pablo, Harutyunyan, Syuzanna R., Visscher, Lucas, Buma, Wybren J., Nicu, Valentin P.
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 05.03.2018
• Subjects: Band theory; configuration determination; conformations analysis; density functional theory; Dichroism; vibrational circular dichroism; vibrational spectroscopy
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201701335

Experimental and theoretical studies of the vibrational circular dichroism (VCD) spectrum of 3‐methyl‐1‐(methyldiphenlsilyl)‐1‐phenylbutan‐1‐ol, whose absolute configuration is key to elucidating the Brook rearrangement of tertiary benzylic α‐hydroxylsilanes, are presented. It is found that the entire OH‐bending region in this spectrum—a region that provides important marker bands—cannot be reproduced at all by standard theoretical approaches even though other regions are well described. Using a novel approach to disentangle contributions to the rotational strength of these bands, internal coordinates are identified that critically influence the appearance of this part of the spectrum. We show that the agreement between experiment and theory is greatly improved when structural dynamics along these coordinates are explicitly taken into account. The general applicability of the approach underlines its usefulness for structurally flexible chiral systems, a situation that is more the rule rather than the exception. Extreme sensitivity to motion: Large discrepancies are observed between the experimental and theoretical vibrational circular dichroism spectra of α‐hydroxysilanes. It is shown that small fluctuations of the C−O−H angle due to the room‐temperature dynamics are the prime reason for these differences. The prominent role of this angle in the exciton coupling mechanism then leads to an extreme susceptibility of the computed spectra to this motion.