Exciton Chirality Method in Vibrational Circular Dichroism

Abstract

ABSTRACT:The interaction of two IR chromophores yields a strong vibrational circular dichroism couplet whose sign reflects the absolute configuration of the molecule. We present a method to determine absolute configuration of a chiral molecule based on this couplet without need of theoretical calculation. Not only can this method analyze various molecules whose absolute configuration is difficult to determine by other spectroscopic methods, but also it can significantly enhance VCD signals.Chirality plays fundamental roles in numerous biological and nonbiological phenomena. The determination of absolute configuration of chiral molecules is an essential step in various research fields including pharmacological science, drug development, biosynthesis, asymmetric reaction, total synthesis and supramolecular chemistry. Chiroptical spectroscopy is the sole technique that can nonempirically determine molecular chirality without need of crystallization. One of the most widely used is the exciton chirality method using electronic circular dichroism (ECD), developed by Harada and Nakanishi, 1 for its high sensitivity and the ease of spectral interpretation; however, the requirement for two or more appropriate UV-Vis chromophores with proper orientation restricts its applicability. In the past decade, vibrational circular dichroism (VCD) spectroscopy using ab initio theoretical calculation has been established as a reliable and convenient approach. 2 Although its application to middle-sized molecules 3 and even peptides and nucleic acids 4 could be possible in successful cases, VCD technique has been hampered by the low sensitivity of vibrational absorption and by the computational demand. In exploration of a more universal, sensitive method, we envisioned the potential of an exciton coupling approach in VCD, also classically known as a coupled oscillator model. 5 So far, no study has reported its use for the assignment of absolute configuration; a stalk contrast with the well-established ECD exciton chirality method. Here we demonstrate the utility of the VCD exciton coupling approach as a versatile method to determine absolute configuration through a systematic study on small molecules.The through-space interaction of two electric transition moments yields a split-type bisignate CD signal that reflects the absolute sense of the twist of the two moments: 6 the positive twist generates a positive first Cotton effect ( 1 , lower in wavenumber) and a negative second Cotton effect ( 2 , higher in wavenumber), and vise verse (Supporting Figure S1). 1 The carbonyl functional groups are promising chromophores for the VCD exciton coupling approach because of their strong, sharp, isolated absorption band at around 1650-1800 cm -1 and because of their well-localized C=O stretching vibrational mode that gives rise to electric transition moments whose direction is virtually parallel to the C=O bond. Moreover, carbonyl groups can be routinely installed to a desired part of the molecule, e.g. by esterification of a hydroxyl group...

Studying the stereostructures of biomolecules and their analogs by vibrational circular dichroism

“…13,14 Recently, we examined this phenomenon in detail and established a method, which is tentatively referred to as a VCD exciton chirality method (named after the ECD exciton chirality method) 15,16 to determine the molecular structure without theoretical calculations. 17 When the absolute twist of the two carbonyl groups is positive (0°oθo+180°), a positive-negative (from lower to higher frequency) VCD couplet is observed; meanwhile, a negative twist ( −180°oθo0°) yields a negative-positive couplet (Figure 3a). This couplet is due to a through-bond and/or through-space interaction between the two C = O chromophores.…”

“…The interaction between carbonyl groups becomes weaker as the distance between the two carbonyl groups increases, and therefore, the contribution of two distant carbonyl groups to an observed couplet is often negligible. 17,18 Accordingly, even if the main chain of a polymer contains numerous carbonyl groups, its observed VCD couplet would be primarily governed by the interactions between pairs of adjacent carbonyl groups. Although the current application of this approach has been limited to a molecule with C = O or other functional groups, this method is applicable to large molecules that are beyond the scope of theoretical calculations.…”

Stereochemical Communication within a Chiral Ion Pair Catalyst

“…[27] Furthermore,w ea lso calculated the IR and VCD spectra of the (M)-1 cation as well as (R)-2 using density functional theory (DFT) methods. [28] Thec alculated spectra of 2 agree nicely with the experimentally observed spectra.…”

Determination of the absolute stereostructure of a cyclic azobenzene from the crystal structure of the precursor containing a heavy element

“…The non-empirical methods employed to determine the absolute configuration of a molecule include circular dichroism, exciton chirality methods [28–29], NMR spectroscopy [30–32], X-ray diffraction [33–35], etc. In addition to these methods, many researchers explored a combination of vibrational circular dichroism and quantum mechanical calculations to determine the absolute stereostructures [36–38]. Among various methods used for determining the absolute stereostructure of the molecules, the X-ray diffraction methods obtained more credibility, especially the method using a single parameter viz.…”