The vibrational circular dichroism exciton coupling in single-bond C–O stretching vibrations of p-menthene derivatives

Abstract

The exciton coupling (EC) phenomenon becomes a valuable tool for the absolute configuration (AC) determination of organic molecules when an interaction between two strategically located chromophores in a molecule is evidenced by circularly polarized radiation. Besides the widely used exciton chirality method in electronic circular dichroism, the technique has been also developed for vibrational circular dichroism (VCD). Notably, the information provided by CO stretching vibrations (1680−1800 cm⁻¹) in VCD have demonstrated high applicability. However, reported theoretical approaches have warned about the adequate interpretations of the EC phenomenon since analyzed bisignate bands could be associated with other vibrations. The current paper experimentally addresses the problem to expand the criteria for using EC data in AC determinations. Herein, the VCD exciton coupling (VCDEC) methodology is extended to the analysis of C–O stretching vibrations (1100–1300 cm⁻¹) to uncover their possible use in AC determinations, as well as to reveal the specific contribution of the EC related to C–O vibrations when experimental parameters are considered. To this aim, naturally occurring p-menthene alcohols 1–4 isolated from Ageratina glabrata were previously used to prepare acetates 5 and 6, and acetonides 7 and 8, and now, to prepare carbonates 9–11. Density functional theory (DFT) calculations, including the IR and VCD spectra of the seven derivatives (5–11), were carried out to reinforce the conformational and configurational analysis, while the experimental VCD spectra of 5–7, 9, and 11 demonstrated the presence of C–O couplets. A systematic analysis of these bands, including the Δε value at alpha and beta state (Δε_α, Δε_β, respectively), the amplitude of EC (A = $\sum ǀ\Delta \epsilon ǀ$) value, the couplet period (L = $\bar{v}\Delta {\epsilon }_{\beta }$ − $\bar{v}$ Δε_α) value, the specific area of C–O at couplet (S = Σf(x)(u²)), and the percentage of area related to EC from C–O (S_%) were considered. These experimental results were directly related to dihedral angles (θ) from simplistic models to establish the AC. The results suggested that A, Cp, S, and S_% parameters are deeply related to the nature of the chromophores. Thereby, adequate molecular structural moieties should be chosen for AC determination when using the EC methodology.