Raman and Raman optical activity (ROA), IR, and vibrational circular dichroism (VCD) spectra of (R)- and (S)-pantolactone have been recorded in three solvents. ROA has been employed on water and DMSO ...solutions, VCD on DMSO and CCl4 solutions. In the last solvent, monomer–dimer equilibrium is present. Due to the low conformational flexibility of the isolated molecule and to the possibility of aggregation, this compound has been used here to test different protocols for computation of the spectroscopic responses taking into account solvent effects. Molecular dynamics (MD) simulations have been carried out together with statistical clustering methods based on collective variables to extract the structures needed to calculate the spectra. Quantum mechanical DFT calculations based on PCM are compared with approaches based on different representations of the solvent shell (MM or QM level). Appropriate treatment of the solvent permits obtaining of good band-shapes, with the added advantage that the MD analysis allows one to take into account flexibility of dimeric structures justifying the broadness of observed bands and the absence of intense VCD couplets in the carbonyl and OH stretching regions.
The vibrational exciton (VE) interpretation of intense bisignated couplets in vibrational circular dichroism (VCD) spectra of a pair of atropisomeric BODIPY (boron dipyrrin) dimers is discussed. The ...role of intrinsic magnetic moments is crucial to reproduce the different behaviors of quasi-isomeric BODIPY dimers with different aryl junction.
Three helicenes based on a borylated arylisoquinoline skeleton have been prepared in their enantiopure forms and characterized with respect to their photophysical properties, including the use of ...chiroptical spectroscopies. The dyes show varying charge‐transfer characteristics and efficient emission (quantum yields between 0.13 and 0.30, in toluene), which is governed by the electron‐donor substitution (p‐MeO‐phenyl, p‐Me2N‐phenyl) at the helicene. Marked differences in the emission wavelength and Stokes shift are observed, with the dimethylamino‐substituted derivative emitting most red‐shifted (maximum at ca. 590 nm) and displaying the highest Stokes shift (ca. 6000 cm−1) in toluene. The helicenes show electronic circular dichroism (ECD) and significant circularly polarized luminescence (CPL) with dissymmetry factors of up to 3.5×10−3. The sign of the ECD band corresponding to the first transition and of the CPL spectrum depend sensibly on the electron‐donor substitution.
A new twist! The dye family of borylated arylisoquinolines (BAI dyes) has new members. Enantiopure azabora5helicenes with varying charge‐transfer character were prepared and their photophysical properties were determined. Spectrally variable and intense circularly polarized luminescence with high dissymmetry factors was observed. Further, the charge‐transfer character of the dye determines the sign pattern of the chiroptical spectra.
A new experimental-computational strategy for the determination of the absolute configuration (AC) of complex chiral molecules is proposed by combining diverse experimental spectroscopies with ...quantum-mechanical simulations well beyond the current computational practice. Key features are the conformer search and relative stability evaluation performed by a new stochastic two-level tool followed by a vis-à-vis comparison of experimental and computed spectra without any ad hoc adjustment. The entire computational procedure is embedded in the user-friendly VMS software, and its reliability is granted by the inclusion of mechanic/electric/magnetic anharmonicity as well as ro-vibrational and vibronic couplings by means of generalized perturbation theory in conjunction with double-hybrid functionals combined with empirical dispersion contributions and suitable basis sets. To test and validate the new approach, the puzzling case of diplopyrone, a fungal phytotoxic metabolite, has been chosen: the close match between new experimental and simulated infrared absorption and vibrational circular dichroism spectra has led to the unbiased evaluation of its AC.
The infrared (IR) and vibrational circular dichroism (VCD) spectra of 2,3-butanediol and trans-1,2-cyclohexanediol from 900 to 7500 cm–1 (including mid-IR, fundamental CH and OH stretchings, and ...near-infrared regions) have been investigated by a combined experimental and computational strategy. The computational approach is rooted in density functional theory (DFT) computations of harmonic and leading anharmonic mechanical, electrical, and magnetic contributions, followed by a generalized second-order perturbative (GVPT2) evaluation of frequencies and intensities for all the above regions without introducing any ad hoc scaling factor. After proper characterization of large-amplitude motions, all resonances plaguing frequencies and intensities are taken into proper account. Comparison of experimental and simulated spectra allows unbiased assignment and interpretation of the most interesting features. The reliability of the GVPT2 approach for OH stretching fundamentals and overtones is confirmed by the remarkable agreement with a local mode model purposely tailored for the latter two regions. Together with the specific interest of the studied molecules, our results confirm that an unbiased assignment and interpretation of vibrational spectra for flexible medium-size molecules can be achieved by means of a nearly unsupervised reliable, robust, and user-friendly DFT/GVPT2 model.
We have developed an efficient chemical resolution of racemic hydroxy substituted dithia-aza4helicenes (DTA4H)
using enantiopure acids as resolving agents. The better diastereomeric separation was ...achieved on esters prepared with (1
)-(-)-camphanic acid. Subsequent simple manipulations produced highly optically pure (≥ 99% enantiomeric excess) (
) and (
)-
in good yields. The role of the position where the chiral auxiliary is inserted (
- vs.
) and the structure of the enantiopure acid used on successful resolution are discussed.
In this work we review research activities on a few of the most relevant structural aspects of bilirubin (BR) and biliverdin (BV). Special attention is paid to the exocyclic C=C bonds being in mostly
...rather than
configurations, and to the overall conformation being essentially different for BR and BV due to the presence or absence of the double C=C bond at C-10. In both cases, racemic mixtures of each compound of either
or
configuration are present in achiral solutions; however, imbalance between the two configurations may be easily achieved. In particular, results based on chiroptical spectroscopies, both electronic and vibrational circular dichroism (ECD and VCD) methods, are presented for chirally derivatized BR and BV molecules. Finally, we review deracemization experiments monitored with ECD data from our lab for BR in the presence of serum albumin and anesthetic compounds.
Four different hexahelicenes, 5-aza-hexahelicene (1), hexahelicene (2), 2-methyl-hexahelicene (3), and 2-bromo-hexahelicene (4), were prepared and their enantiomers, which are stable at r.t., were ...separated. Vibrational circular dichroism (VCD) spectra were measured for compound 1; 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 1 is also examined from this point of view.
We present the simulation of the absorption (ABS), electronic circular dichroism (ECD), emission (EMI), and circularly polarized luminescence (CPL) spectra for the weak electronic transition between ...the ground (S0) and the lowest excited state (S1) of hexahelicene, 2-methylhexahelicene, 2-bromohexahelicene, and 5-azahexahelicene. Vibronic contributions have been computed at zero Kelvin and at room temperature in harmonic approximation including Duschinsky effects and accounting for both Franck–Condon and Herzberg–Teller contributions. Our results nicely capture the effects of the different substituents on the experimental spectra. They also show that HT effects dominate the shape of ECD and CPL spectra where they even induce changes of signs; HT effects are also relevant in ABS and EMI, tuning the relative intensities of the different vibronic bands. HT effects are the main reason for the differences in the line shapes of ABS and ECD and of EMI and CPL spectra and for the mirror-symmetry breaking between ABS and EMI and between ECD and CPL spectra. In order to check the robustness of our results, given also that few examples of calculations of vibronic CPL spectra exist, we adopted both adiabatic and vertical approaches to define the model potential energy surfaces of the (S0) and the (S1) states; moreover we expanded the electric and magnetic dipole transition moments around both the S0 and S1 equilibrium geometries.
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![Azabora[5]helicene Charge‐T...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0947-6539/small "Azabora[5]helicene Charge‐Transfer Dyes Show Efficient and Spectrally Variable Circularly Polarized Luminescence")
