PH Dependent Chiroptical Properties of (1R,2R)- and (1S,2S)-trans-Cyclohexane Diesters and Diamides from VCD, ECD, and CPL Spectroscopy

Diesters of (1R,2R)- and (1S,2S)-cyclohexanediols and diamides of (1R,2R)- and (1S,2S)-diaminocyclohexane with p-hydroxycinnamic acid have been
known for some time to exhibit intense bisignate electronic circular dichroism (ECD) spectra in CH3OH. It was also known that added NaOH causes a bathochromic shift
of ∼50 nm in CH3OH, and an even higher one in DMSO. We have measured vibrational circular dichroism (VCD) spectra both for neutral compounds and in the
presence of NaOH and other bases. The VCD and IR spectra in the mid-IR region for CD3OD and DMSO-d6 solution exhibit high sensitivity to the charged state for the
diesters. They possess two strong bisignate features in the presence of bases in the mid-IR, which are interpreted in terms of vibrational exciton couplets, while this
phenomenon is less evident in diamides. VCD allied to density functional theory (DFT) calculations allows one to shed some light on the spectral differences of
diesters and diamides by studying their conformational properties. Optical rotatory dispersion (ORD) curves confirm the ECD data. Circularly polarized luminescence
(CPL) data have been also acquired, which are rather intense in basified solution: the CPL spectra are monosignate and are as intense in the diester and in the diamide case.