Stereochemical Aspects of Vibrational Optical Activity

In Depth Study on Solution-State Structure of Poly(lactic acid) by Vibrational Circular Dichroism

“…In fact, their difference VCD spectrum yielded an almost flat signature in the carbonyl region (Figure b–d). Had there been any conformational changes upon inclusion, it should have yielded a different VCD pattern because the profile of an exciton couplet is governed by the distance and the dihedral angle of two chromophores. , This result clearly suggested that the conformation of PLLA in the complex is almost the same as that of PLLA in an isolated state. The conformational identity was further supported by the close resemblance of the difference VCD spectrum and the spectrum of maltohexaose (Figure e).…”

“…This is in contrast to other molecules whose conformation and hence VCD spectrum display solvent-dependency. ,, Importantly, these VCD spectra exhibited a strong bisignate signal in the CO stretching region: the first negative peak at 1751 cm –1 and the second positive peak at 1767 cm –1 . On the basis of the coupled oscillator model, − the signs of the couplet proposed a left-handed helical structure of PLLA in which two adjacent carbonyl groups are arranged counterclockwisely (Figure , bottom). This conclusion is the same as that stated by Ho et al in a recent report that recorded the VCD spectrum of PLLA for the first time .…”

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

“…12 In particular, since the early days of VCD spectroscopy, it has been proposed that a pair of carbonyl groups may give rise to a characteristic VCD signal whose shape is indicative of the spatial relationship of the C = O groups. 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).…”

Systematic Approach to Conformational Sampling for Assigning Absolute Configuration Using Vibrational Circular Dichroism

“…Extensive work in the field of calculated VCD spectra indicates that relatively moderate basis sets combined with hybrid DFT methods are of sufficient accuracy for assigning absolute configuration. − Many of the cited studies involve complex investigations which provide close matching of sign and intensity for all spectra peaks. While this detailed approach is understandable given the recent emergence of the VCD field, a less exhaustive but more practical approach is often sufficient for routine problem solving.…”

“…Chirality is a structural feature of many marketed drugs and an important focus of drug discovery teams in the pharmaceutical industry. Owing to the difficulty in assigning absolute configuration (AC), several methods are commonly employed, including single crystal X-ray diffraction, NMR combined with chiral derivatization, electronic circular dichroism (ECD), optical rotation, and structural proofs based on synthetic transformations. − A relatively new technique for assigning absolute configuration, − vibrational circular dichroism (VCD), has gained favor in recent years. Recent perspectives on the method emphasize that VCD can play a critical role for SAR development and provide insight into the dominant conformations of a molecule in solution. , It has been emphasized that uptake of VCD into the pharmaceutical industry is still surprisingly slow…”