Optical Rotatory Dispersion Curves Research Articles

New Insights on Solvent-Induced Changes in Refractivity and Specific Rotation of Poly(propylene oxide) Systems Extracted from Channeled Spectra.

Investigation of chiroptical polymers in the solution phase is paramount for designing supramolecular architectures for photonic or biomedical devices. This work is devoted to the case study of poly(propylene oxide) (PPO) optical activity in several solvents: benzonitrile, carbon disulfide, chloroform, ethyl acetate, and p-dioxane. To attain information on the interactions in these systems, rheological testing was undertaken, showing distinct variations of the rheological parameters as a function of the solvent type. These aspects are also reflected in the refractive index dispersive behavior, from which linear and non-linear optical properties are extracted. To determine the circular birefringence and specific rotation of the PPO solutions, the alternative method of the channeled spectra was employed. The spectral data were correlated with the molecular modeling of the PPO structural unit in the selected solvents. Density functional theory (DFT) computational data indicated that the torsional potential energy-related to the O1-C2-C3-O4 dihedral angle from the polymer repeating unit-was hindered in solvation environments characterized by high polarity and the ability to interact via hydrogen bonding. This was in agreement with the optical characterization of the samples, which indicated a lower circular birefringence and specific rotation for the solutions of PPO in ethyl acetate and p-dioxane. Also, the shape of optical rotatory dispersion curves was slightly modified for PPO in these solvents compared with the other ones.

Chiral explosives: A theoretical investigation of structure and chiroptical properties of triacetone triperoxide and hexamethylene triperoxide diamine.

Triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) are cyclic peroxides that exhibit atropisomerism resulting from restricted rotation around three peroxide bonds. As a result, one pair of enantiomers with D3 symmetry and another pair of enantiomers with C2 symmetry can be identified. Previous studies, based on mass spectrometry data and computational results, have shown that conformations of TATP with D3 and C2 symmetry can be isolated. Assuming that enantiomer samples of TATP and HMTD can be obtained with sufficient enantiopurity, we investigated their chiroptical properties, namely, optical rotatory dispersion (ORD), vibrational circular dichroism (VCD), and Raman optical activity (VROA). ORD curves and VCD spectra are seen to be very similar for D3 - and C2 -symmetric atropisomers with the same overall helicity. Predicted VROA results, however, show significant differences between D3 - and C2 -symmetric atropisomers with the same overall helicity. The D3 -symmetric atropisomer is predicted to exhibit considerably larger magnitude vibrational optical activity signals than the C2 -symmetric atropisomer.

Method for determination of optical rotatory dispersion curve by using a polarization axis finder

We report a new, to the best of our knowledge, measurement method of optical rotatory dispersion (ORD) based on a polarization axis finder (PAF) and digital image processing. The collimated quasi-monochromatic light is output by a monochromator and passes through a polarizer and the chiral samples, and a PAF is employed to modulate the light beam spatially to form an "hourglass" intensity pattern, which is utilized to determine the polarization direction of linearly polarized light. After being recorded by a digital camera, the modulated patterns with and without samples are analyzed by our specially designed digital image processing program to calculate the optical rotation (OR) angle of the chiral substance at a certain wavelength, and the ORD curve can be obtained by fitting a series of OR angles with different wavelengths. The ORD curves of two standard quartz plates with opposite chirality and a glucose solution with different concentrations are measured experimentally, and the fitted coefficients and measurement results agree well with their theoretical values, which indicates that the presented method is an effective and accurate way to determine the ORD of chiral substances conveniently.

Optical rotation of white light

Plane-polarized monochromatic light is rotated in an optically active medium. The extent of the rotation is wavelength dependent, following an optical rotatory dispersion (ORD) curve. Typically, this phenomenon is studied by using a few discrete wavelengths. Here, we demonstrate optical rotation of white light. Corn syrup is used as the medium as large angles of optical rotation can be generated in compact containers. The Drude expression for ORD and Malus' law are used to predict the spectrum of the light transmitted as a function of the angle between polarizers located on either side of the sample. Despite the transmission spectrum of corn syrup in the absence of polarizers being unremarkable, optical rotation leads to a dramatic change in color because a “notch” is generated in the spectrum of the transmitted light. The extinction region can be translated across the spectrum by rotating the analyzer. The experimentally measured location of the region of maximum extinction and the color of the transmitted light are in excellent qualitative agreement with the predicted values. The experiment is ideal both as a lecture demonstration and for quantitative investigation in an undergraduate laboratory of the spectral distribution of light transmitted by a chiral medium.

Calculating Optical Rotatory Dispersion Spectra in Solution Using a Smooth Dielectric Model.

The calculation of specific rotation of molecules in solution is probed at the coupled cluster (CC) level utilizing a continuum dielectric model based on a definition of the dielectric permittivity as a smooth function of electron density. Solvation effects are captured through polarization of Hartree-Fock (HF) molecular orbitals before subsequent calculations with the coupled cluster singles and doubles (CCSD) method. For the challenging ( S)-methyloxirane molecule, CCSD specific rotations yield an incorrect sign for the rotation in water, and the continuum model is unable to predict the wide variations in the optical rotatory dispersion (ORD) curves seen for nonpolar solvents of similar dielectric constant. In two molecules, (1 S,4 S)-norbornenone and ( S)-2-chloropropionitrile, specific rotations computed with CCSD in conjunction with implicit solvent fail to provide solvent shifts of the correct order of magnitude, indicating that the solvent response is a major contribution to the overall solvation effect.

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.

Combined use of three forms of chiroptical spectroscopies in the study of the absolute configuration and conformational properties of 3-phenylcyclopentanone, 3-phenylcyclohexanone, and 3-phenylcycloheptanone

Three forms of chiroptical spectroscopies, electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and optical rotatory dispersion (ORD) have been employed to study the configuration and conformational properties of the three molecules: (S)-3-phenylcyclopentanone, (S)-3-phenylcyclohexanone, and (S)-3-phenylcycloheptanone (including (S)-3-phenylcyclopentanone-2,2,5,5-d4 and (S)-3-phenylcyclohexanone-2,2,6,6-d4). ECD and VCD spectra in the mid-IR for the three molecular systems are marginally dependent on fine conformational details, as interpreted in terms of standard DFT computational methods, with common spectroscopic features to the three systems clearly identified. Accounting for vibronic coupling mechanisms reproduces the structuring of ECD n→π∗ band. The ORD curves are quite similar for the three types of molecules, but their interpretation highlights a crucial role played by conformations of the cycloalkanone ring in the case of (S)-3-phenylcycloheptanone. The same conclusions are reached by considering the VCD spectra in the CH-stretching region.

The optical rotatory dispersion of some N-decyl-N-N-dimethylalanine salts and their critical micelle concentrations.

Abstract The optically active salts of N-decyl-NN-dimethylalanine are shown to have anomalous optical rotatory dispersion curves and to not be fully ionized in aqueous solution. The position of the peak in the 225 to 230 mμ region varies with concentration of the surfactant as does the molecular rotation at a given wavelength. The positions of the peaks and the molecular rotations at these low wavelengths are pH sensitive. The change in molecular rotation with concentration enables the critical micelle concentrations of L and D N-decyl-NN-dimethylalanine hydrobromides to be determined by optical rotatory dispersion measurements, the results agreeing well with the values calculated from the literature. The change in rotation on micelle formation appears to be due to a change in the degree of ionization of these weak acids, a fact which is confirmed by pH measurements on these solutions.

Development of a Variable-wavelength Optical Rotation Detector in the Ultraviolet Region

A new type of optical rotation (OR) detector based on the phase retardation of the first-order diffraction beam has been constructed. The proposed detector responded to only OR properties by eliminating the circular dichroism (CD) effect. Thus, it could measure the optical rotatory dispersion curves in the Cotton band. Although the optical system requires the incident beam in which the phase retardation is 0.5π, the actual retardation was estimated to be around 0.3π. It means that the OR signal intensity can be doubled. Since the proposed detector works over a range of UV wavelengths, there was a compound whose OR signal intensity was higher than the CD signal intensity. The OR detection was superior to CD in stability and in defining the baseline.

Asymmetric radiation-induced inclusion polymerization of 3-methyl-1,4-pentadiene in deoxycholic acid

The radiation-induced polymerization of 3-methyl-1,3-pentadiene (3MPD) as an inclusion complex in deoxycholic acid (DOCA) has produced in good yield the optically active polymer poly(3-methyl-1,4-pentadiene) (P3MPD) whose structure and properties were studied by FT-IR spectroscopy and thermal analysis (TGA, DTG and DTA). The data show that the polymer is essentially trans-1,4-P3MPD as expected for the polymerization in constrained media. Trans-1,4-P3MPD is optically active with [ α] D values comprised between +4.3 and +5.6. The optical rotatory dispersion curve of the P3MPD is completely different from that of DOCA as expected.

Determination of Absolute Configurations of 4-Hydroxyequilenin-Cytosine and -Adenine Adducts by Optical Rotatory Dispersion, Electronic Circular Dichroism, Density Functional Theory Calculations, and Mass Spectrometry

Estrogen components of some hormone replacement formulations have been implicated in the initiation of breast cancer. Some of these formulations contain equine estrogens such as equilin and equilenin that are metabolized to the genotoxic catechol 4-hydroxyequilenin (4-OHEN). Auto-oxidation generates the o-quinone form that reacts with dC and dA in oligodeoxynucleotides to form unusual stable cyclic bulky adducts, with four different stereoisomers identified for each base adduct. The dC and dA adducts have the same unsaturated bicyclo[3.3.1]nonane type linkage site with identical stereochemical characteristics. Stereochemical effects may play an important part in the biological consequences of the formation of 4-OHEN-DNA adducts, and the assignment of the absolute configurations of the stereoisomeric 4-OHEN-dC and -dA adducts is therefore needed to understand structure-function relationships. We used density functional theory (DFT) to compute the specific optical rotations and electronic circular dichroism (ECD) spectra of the four 4-OHEN-C stereoisomers, and the results were compared with experimentally measured optical rotatory dispersion (ORD) and ECD spectra. The predicted ORD curves for the four stereoisomeric base adducts reproduced the shapes and signs of experimental spectra in the transparent spectral region. The stereochemistry of the C3' atom was determined by comparison of the calculated and experimental ORD and ECD spectra, and the stereochemistry of C2' was determined by mass spectrometric methods. Combining the ORD and mass spectrometry data, the absolute configurations of the four 4-OHEN-C and the stereochemically identical -dC adducts have been identified. The molecular architecture of the linkage site at the 4-OHEN-C/A and 4-OHEN-dC/dA is identical, and it is shown that the deoxyribose group does not substantially contribute to the optical activities. The absolute configurations of the 4-OHEN-dA adducts were thus deduced by comparing the experimental ORD with computed ORD values of 4-OHEN-A adducts.

Fast generation of nonresonant and resonant optical rotatory dispersion curves with the help of circular dichroism calculations and Kramers‐Kronig transformations

It can be computationally expensive to compute smooth, well resolved, optical rotation (OR) dispersion (ORD) curves from first principles theory. Instead of computing the OR at a large number of frequency points, similar results can be obtained by combined use of a computed circular dichroism (CD) spectrum along with a few OR calculations by using subtractive Kramers-Kronig transformations. We have tested various subtractive schemes for simulated (analytical) CD/ORD and for time-dependent density functional computations for dimethyloxirane, fenchone, and [4]triangulane. Nonresonant ORD can be obtained with as few as two OR and one CD calculation. For resonant ORD we found that between 7 and 15 OR computations plus the CD spectrum were typically sufficient, depending on the number of excitations within the frequency window of interest.

Radioracemization and radiation-induced chiral amplification of chiral terpenes measured by optical rotatory dispersion (ORD) spectroscopy

For the first time the radioracemization of α(+)pinene and α(−)pinene, of turpentine and of R(−)- α-phellandrene has been studied by optical rotatory dispersion (ORD) spectroscopy. For all these compounds, the radioracemization implies a shift of the ORD curves toward lower levels of specific optical rotation. The radioracemization degree ( R R) has been defined and calculated for all the compounds studied. It has been found that for radiation dose of 1 MGy the radioracemization degree is about 4.5% for the compound with the highest optical purity and reaches 7–8% for the less optically pure compounds, demonstrating that impurities can affect greatly the radioracemization. In contrast with the general radioracemization effect exerted by high-energy radiation on chiral molecules, β(−)pinene, β(+)pinene when irradiated show an increment of their specific optical rotation. This fact has been measured for the first time by ORD spectroscopy and the amplification degree of chirality can reach 1000% in the near UV. This phenomenon is due to the formation of a chiral polymer, poly- β-pinene, which forms a solution with the monomer enhancing its optical activity. The implications for the theories of the origin of life of such unexpected phenomenon are discussed briefly.

An Optical Rotatory Detector for High-Performance Liquid Chromatography Using Polarization Modulation

A sensitive and variable-wavelength optical rotatory (OR) detector for high-performance liquid chromatography is presented. This design is entirely different from that of conventional OR detectors consisting of a crossed polarizer pair. By placing a polarizing prism and a retardation plate into a commercial circular dichroism (CD) detector, the OR signal was obtained. The Mueller matrix approach was used to prove the principle of the OR signal appearance. Sugars and 4-androstene-3,17-dione were chosen as test compounds. The limit of detection was below 0.5 microg of injected sucrose at 260 nm, which was superior to that obtained with a conventional OR detector. For 4-androstene-3,17-dione, which is CD active, and shows a large anomalous OR dispersion curve, our detector gave a large OR signal with approximately half the intensity of the CD signal at 340 nm.

Comparison of Time-Dependent Density-Functional Theory and Coupled Cluster Theory for the Calculation of the Optical Rotations of Chiral Molecules

A comparison of the abilities of time-dependent density-functional theory (TDDFT) and coupled cluster (CC) theory to reproduce experimental sodium D-line specific rotations for 13 conformationally rigid organic molecules is reported. The test set includes alkanes, alkenes, and ketones with known absolute configurations. TDDFT calculations make use of gauge-including atomic orbitals and give origin-independent specific rotations. CC rotations are computed using both the origin-independent dipole-velocity and origin-dependent dipole-length representations. The mean absolute deviations of calculated and experimental rotations are of comparable magnitudes for all three methods. The origin-independent DFT and CC methods give the same sign of [alpha]D for every molecule except norbornanone. For every large-rotation ketone and alkene for which DFT and CC yield the incorrect sign as compared to liquid-phase experimental data, the corresponding optical rotatory dispersion (ORD) curve is bisignate, suggesting that the two models cannot reliably reproduce the relative excitation energies and antagonistic rotational strengths of multiple competing electronic states that contribute to the total long-wavelength rotation. Several potential sources of error in the theoretical treatments are considered, including basis set incompleteness, vibrational and temperature effects, electron correlation, and solvent effects.

The problematic case of ( S )-methylthiirane: electronic circular dichroism spectra and optical rotatory dispersion

Coupled cluster and density-functional theory calculations of the electronic circular dichroism (ECD) spectra and optical rotatory dispersion (ORD) of (S)-methyloxirane and (S)-methylthiirane have been carried out for comparison to gas-phase experimental data. Coupled cluster simulations of the ECD spectra of both molecules are in excellent agreement with experiment, both for the positions of the transitions and for the signs and magnitudes of the rotational strengths. Density-functional theory yields the correct structure of the ECD spectra, but the transition energies are shifted to lower energies by up to 0.3 eV. Density-functional ORD curves of (S)-methylthiirane exhibit incorrect dispersion due to an early onset of the Cotton pole. The coupled cluster ORD curves of (S)-methylthiirane give the correct sign of the rotations, but overestimate their magnitude by up to 50% at shorter wavelengths. Although temperature-dependent harmonic vibrational corrections improve the comparison between coupled-cluster theory and experiment at 633 nm, such corrections produce an incorrect sign of the rotation at 355 nm.

Calculation of circular dichroism spectra from optical rotatory dispersion, and vice versa, as complementary tools for theoretical studies of optical activity using time-dependent density functional theory

A comparison of two theoretical methods based on time-dependent density functional theory for the calculation of the linear dispersive and absorptive properties of chiral molecules has been made. For this purpose, a recently proposed computational method for the calculation of circular dichroism (CD) spectra from the imaginary part of the optical rotation parameter has been applied to six rigid organic molecules. The results have been compared to the CD spectra obtained from the rotatory strengths and from the Kramers-Kronig transformation of optical rotatory dispersion (ORD) curves. We have also investigated a criterion based on the Kramers-Kronig integration formula to determine a number of excitations in truncated CD spectra which may yield a reasonable low frequency resonant ORD. It has been tested by calculating the ORD from the sum-over-states formula both in the nonresonant and resonant regions. Finally, we have applied these methods to model the resonant optical activity of proline at low pH.

Ab initio calculation of optical rotatory dispersion (ORD) curves: a simple and reliable approach to the assignment of the molecular absolute configuration.

In this paper, both Hartree-Fock (HF) and density functional theory (DFT) methods have been used to make ab initio calculations of the optical rotatory power of selected molecules at several wavelengths; that is, part of the optical rotatory dispersion (ORD) curve has been predicted. This approach constitutes a new, simple, and reliable method to assign the molecular absolute configuration, at least for rigid molecules such as those studied in the present work. In fact, in this way, it is possible to overcome the difficulties connected to some relevant cases, in particular that of (-)-beta-pinene, for which even a very high-level (DFT/B3LYP/6-311++G(2d,2p)) calculation affords the wrong sign of the optical rotation at 633 nm. On the contrary, the predicted ORD curve, even using small basis sets, reproduces (below 400 nm) the experimental trend well, allowing for the correct configurational assignment. This result clearly shows that to have a reliable configurational assignment the comparison between experimental and predicted rotation values must be carried out at different wavelengths and not at a single frequency. The reason for this is that working at wavelengths approaching the absorption maximum the [alpha](lambda) values become larger and their prediction becomes more reliable. Coupling the use of an inexpensive instrument (a polarimeter working at a few wavelengths) with the use of a DFT-calculation package can also allow the experimental organic chemist to arrive, quickly and reliably, at the assignment of the molecular absolute configuration.

Optical rotatory dispersion from liquid crystalline solutions and films of hydroxypropylcellulose

Abstract The chiroptical properties of (hydroxypropyl)cellulose (HPC) in methanol are measured by optical rotatory dispersion (ORD), both in isotropic solution and in the cholesteric liquid crystalline phase. The ORD spectra of HPC films cast from lyotropic solution are also examined. Isotropic solutions of HPC in methanol provide no ORD evidence for the presence of a helical conformation, or for concentration-induced changes in conformation. The ORD curve of HPC in methanol, when expressed as specific rotation, is independent of polymer concentration for isotropic solutions containing between 1 and 43 per cent polymer by weight. From the slope of Drude plots, the Cotton effect responsible for the observed ORD curve occurs at 175–180 nm. However a 45 per cent liquid crystal-line solution exhibits plain positive dispersion, and the magnitude of the specific rotation is also much greater than that found for solutions containing less than 43 per cent polymer. Results for more concentrated solutions confirm t...

Metal complexes of some asymmetric diaminodiamide ligands—II. Stabilities of complexes of nickel(II) in aqueous solution

Equilibria between a series of asymmetric diaminodiamides and Ni(II) in aqueous solution have been studied by potentiometric and spectrophotometric methods. The ligands were S,R,S- and S,S,S-N,N′-dialanylpropylenediamine (DAPN), S,S-N,N′-dialanylethylenediamine (DAEN), R-N,N′-diglycylpropylenediamine (DGPN), and N,N′-diglycylethylenediamine (DGEN). At lower values of pH the complexes NiL 2+ and in some cases NiL 2+ 2 were formed. Spectral data indicated that both of these were octahedral. At higher pH the two amide protons were lost and square planar complexes were formed. The diastereomeric DAPN ligands showed stereoselectivity in the equilibrium constants for deprotonation and formation of the square planar complexes. Comparison of optical rotatory dispersion curves and absorption spectra for the various complexes permits partial assignment of structures.