Vibrational and Electronic Circular Dichroism of Dimethyl Mesobilirubins-XIIIα

The vibrational circular dichroism (VCD) spectra of (α<i>R</i>,α′<i>R</i>)-, (α<i>S</i>,α′<i>S</i>)-, (β<i>R</i>,β′<i>R</i>)-, and (β<i>S</i>,β′<i>S</i>)-dimethylmesobilirubin-XIIIα have been recorded in the range of 1800–900 cm^–1 in CDCl₃ solution and in mixed DMSO-<i>d</i>₆/CDCl₃ solutions. Ab initio density functional theory (DFT) calculations predict IR vibrational absorption (VA) and VCD spectra in excellent to good correspondence with observed data. The same calculations confirmed the ridge-tile conformation that has been known for a long time. Assignment of vibrational normal modes (NMs) sheds light on the relative importance of local moieties and groups in determining conformational properties of the molecules, as well as their interaction with solvent molecules. Time-dependent DFT (TDDFT) calculations were also performed to provide an understanding of electronic circular dichroism (ECD) spectra and confirm the well-known interpretation based on the exciton model

pH Dependent Chiroptical Properties of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-<i>trans</i>-Cyclohexane Diesters and Diamides from VCD, ECD, and CPL Spectroscopy

Diesters of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-cyclohexanediols and diamides of (1<i>R</i>,2<i>R</i>)- and (1<i>S</i>,2<i>S</i>)-diaminocyclohexane with <i>p</i>-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-<i>d</i>₆ 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

Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism, in Circularly Polarized Luminescence, and in Raman Spectra of Some Simple Optically Active Hexahelicenes

Four different hexahelicenes, 5-aza-hexahelicene (<b>1</b>), hexahelicene (<b>2</b>), 2-methyl-hexahelicene (<b>3</b>), and 2-bromo-hexahelicene (<b>4</b>), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound <b>1</b>; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound <b>1</b> is also examined from this point of view