Relationship between Molecular Properties and Conformation of Chiral Alkanes

Mixtures of the diastereomers of 2,2,3,5,6-pentamethylheptane were prepared in two ways, either starting with compounds of (3R)-configuration, or from compounds of (5R)-configuration. Comparison of the GC. and optical rotatory power of the fractions of these two mixtures permitted the unambiguous assignment of the absolute configuration and molar rotatory power to the various diastereomers ([M] = + 119.1° for the (3R, 5R)- and [M] = + 79.8° for the (3R, 5S)-diastereomer). The very high molar rotatory power which was expected on the basis of the conformational analysis carried out with a rotational-isomeric-3-states model is interpreted as arising from the molecular ‘conformational rigidity’, i.e. from the presence of only few conformers. Conformational properties of these compounds were computed using a new approach, which scans the whole space of each bond (2 π) in 5° steps and calculates the conformational energy based upon semiempirical potential functions. The conformational flexibility of each bond of the two diastereomers is evaluated in terms of the a priori probability density function of that bond. This allows us to analyze in detail how configurational differences affect conformational properties. The molar rotatory power of the two diastereomers as calculated with a new method recently developed in our group is in excellent agreement with experimental data. The molar rotatory power is analyzed in terms of the contribution of the single bonds.     

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.     

Effect of dimerization and racemization processes on the electron density and the optical rotatory power of hydrogen peroxide derivatives

The variation of the electron density properties and optical rotatory power of the monomers and dimers of seven monosubstituted hydrogen peroxide derivatives, HOOX (X = CCH, CH(3), CF(3), t-Bu, CN, F, Cl), upon racemization has been studied using DFT (B3LYP/6-31+G) and MP2 (MP2/6-311+G) methods. The geometrical results have been rationalized on the basis of natural bond orbital (NBO) analysis. The atomic partition of the electron density properties within the atoms in molecules (AIM) methodology has allowed investigating the energy and charge redistribution in the different structures considered. The calculated optical rotatory power (ORP) of the dimers are, in general, twice of the values obtained for the monomers.     

Structure and optical activity of a crystalline modification of isotactic poly-(S)-4-methyl-1-hexene

The results of an x-ray and polarimetric study of a crystalline modification (form I) of isotactic poly-(S)-4-methyl-1-hexene are reported and discussed. The x-ray fiber spectra of this polymer are practically indistinguishable from those of isotactic poly-(R)-(S)-4-methyl-1-hexene. Although the crystal structure of the latter can be described on the basis of helices of different screw sense packed in a P4 space group, the crystal structure of poly-(S)-4-methyl-1-hexene is better described on the basis of a P1 space group. The conclusion of the x-ray investigation, that in the crystals of the optically active polymer an equal number of right-handed and left-handed helices must be present, is supported by the polarimetric measurements, which have shown that the polymer in the crystalline form I possesses a rather low rotatory power.     

Pair potential of liquid metallic Be from electron theory compared and contrasted with recent ab initio work on the free-space beryllium dimer

Two decades ago, Perrot and March [F. Perrot and N.H. March, Phys. Rev. A. 41, 4521 (1990)] used electron theory to derive an oscillatory pair potential between the beryllium nuclei in liquid metal beryllium. They predict a first minimum at 2.1?Å, followed by a larger repulsive hump at 2.8?Å. Here, we compare and contrast this result for liquid beryllium with the recent ab initio work by Koput and the present quantum Monte Carlo (QMC) calculation on the beryllium dimer in free space. Koput situates the minimum in the potential curve for the free-space dimer at 2.4?Å and it is quite similar in depth to that for liquid metallic beryllium. Our QMC curve is similar, with the minimum at 2.33?Å. They are tabulated in this letter.     

Molecular Exciton Approach to Anisotropic Absorption and Circular Dichroism II. The Partial Optic Axis and its Application in Molecular Exciton Theory

The first part of the present communication develops the general theory of the partial optic axis, which is an excitation specific structural feature first proposed as an aid to the choice of chromophoric reference points in a molecular exciton approach to optical rotatory power, and discusses its applicability to symmetry analyses in the light of the distinction between the three categories for anisotropic optical rotatory power, i.e. chiral, achiral optically active, and achiral optically inactive molecular structures. The second part of the communication discusses the special role played by the concept of the partial optic axis in the evaluation of the anisotropic chromophoric intensity contributions in a molecular exciton approach, in particular in relation to the use of the chromophoric symmetry for the systematic selection and characterization of the intensity contributions.     

Molecular Exciton Approach to Anisotropic Absorption and Circular Dichroism II. The Partial Optic Axis and its Application in Molecular Exciton Theory

Summary. The first part of the present communication develops the general theory of the partial optic axis, which is an excitation specific structural feature first proposed as an aid to the choice of chromophoric reference points in a molecular exciton approach to optical rotatory power, and discusses its applicability to symmetry analyses in the light of the distinction between the three categories for anisotropic optical rotatory power, i.e. chiral, achiral optically active, and achiral optically inactive molecular structures. The second part of the communication discusses the special role played by the concept of the partial optic axis in the evaluation of the anisotropic chromophoric intensity contributions in a molecular exciton approach, in particular in relation to the use of the chromophoric symmetry for the systematic selection and characterization of the intensity contributions.Permanent address: Holmebjerg 5, DK-2950 Vedbæk, Denmark     

Atomic partition of the optical rotatory power of methylhydroperoxide

We applied a methodology capable of resolving the optical rotatory power into atomic contributions. The individual atomic contributions to the optical rotatory power and molecular chirality of the methylhydroperoxide are obtained via a canonical transformation of the Hamiltonian by which the electric dipolar moment operator is transformed to the acceleration gauge formalism and the magnetic dipolar moment operator to the torque formalism. The gross atomic isotropic contributions have been evaluated for the carbon, the nonequivalent oxygen, and the nonequivalent hydrogen atoms of methylhydroperoxide, employing a very large Gaussian basis set which is close to the Hartree-Fock limit.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Exact solution of multidimensional hyper‐radial Schrödinger equation for many‐electron quantum systems

In quantum theory, solving Schrödinger equation analytically for larger atomic and molecular systems with cluster of electrons and nuclei persists to be a tortuous challenge. Here, we consider, Schrödinger equation in arbitrary N‐dimensional space corresponding to inverse‐power law potential function originating from a multitude of interactions participating in a many‐electron quantum system for exact solution within the framework of Frobenius method via the formulation of an ansatz to the hyper‐radial wave function. Analytical expressions for energy spectra, and hyper‐radial wave functions in terms of known coefficients of inverse‐power potential function, and wave function parameters have been obtained. A generalized two‐term recurrence relation for power series expansion coefficients has been established. © 2016 Wiley Periodicals, Inc.     

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.     

Optical activity of chiral thin film and liquid crystal hybrids

Using the glancing angle deposition (GLAD) technique, we have fabricated porous, chiral thin films with optically anisotropic helical microstructures that exhibit optical phenomena such as wavelength specific rotation of linearly polarized light. Initial research has shown that the porosity of the films allows for the addition of nematic liquid crystals (NLCs) to the films for promising applications in dynamically switchable devices, while simultaneously enhancing the optical properties of the film. This study describes the fundamental optical behaviour of LC-filled chiral thin films in relation to material, porosity, structure and thickness. It was found that for SiO₂ films, the addition of NLCs increased the effective rotatory power by two-fold when compared with results from the film without added LCs. The rotatory power of Al₂O₃ and MgF₂ films, while being similarly increased by the addition of LCs, exhibited a reversal in sign, or direction of rotation, for the visible wavelength spectrum investigated. The effects of film porosity and structure were studied by varying the angle of incidence from 83° to 86°; it was found that the greater porosity of the films deposited at larger angles allowed for more filling by the LCs and thus a larger increase in rotatory power. Finally, the addition of LCs was observed to shift the wavelength of peak rotation towards smaller values.     

Isotope effect of optical activity measurements on L-α-alanine

If an optically active organic substance is labelled in the chirality center with another isotopic species (such as¹⁵N for¹⁴N) a pronounced variation of rotatory power is predicted. We tried to varify this idea experimentally on L-α-alanine and found an isotope effect of ORD (optical rotatory dispersion). The magnitude of the rotation is mainly dependent on the pH of the solvent. The ratio of the optical activity alanine-¹⁴N/alanine-¹⁵N is about 1.02. The molecular rotations show a consistently lower ratio but it can be seen that the isotope effect is not only a mass effect.     

A study on the effect of the source line profile in atomic magneto-optical rotation spectroscopy (AMORS), using a Zeeman-shifted hollow cathode lamp

Summary Using a magnetized hollow cathode lamp as a Zeeman-shifted light source, the effects of the source line profile on the intensity of rotatory radiation and the calibration curve were studied for AMORS (atomic magneto-optical rotation spectroscopy). With the alteration of the source line profile, an increase in the intensity of rotatory radiation by about max. 40% was found. The rollover of the calibration curve could be avoided by selecting the strengths of the magnetic fields applied to the atomizer and the hollow cathode lamp.

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Untersuchung der Wirkung des Linienprofils der Lichtquelle in der magneto-optischen Atomrotations-Spektroskopie (AMORS) mit Hilfe einer Hohlkathodenlampe mit Zeeman-Verschiebung

Zusammenfassung Mit Hilfe einer magnetisierten Hohlkathodenlampe als Lichtquelle mit Zeeman-Verschiebung wurden die Wirkungen des Linienprofils der Lichtquelle auf die Intensität der Rotationsstrahlung und auf die Eichkurve untersucht. Mit Änderung des Linienprofils ergab sich eine Intensitätssteigerung der Rotationsstrahlung bis zu 40%. Abweichungen der Eichkurve konnten durch geeignete Wahl der magnetischen Feldstärken am Atomizer und der Hohlkathodenlampe vermieden werden.

     

Optical rotatory dispersion (ORD) of glycogen

The ORD spectra of three types of glycogen in salt solutions at different concentrations, pH and ionic strength values were determined to investigate the influence of these variables on the rotatory power.The ORD curves obtained with the samples recently prepared correspond to positive monotone dispersion curves, similar to available data for the other polysaccharides.Once the mutarotation equilibrium is attained a maximum is observed at 210±10 nm. The rotatory power decreases with the elapsed time and in some cases, it is possible to find a change in the sign of these curves. Such a phenomenon may be attributed to conformational changes in the monomer units constituting the macromolecule.A considerable increase in the rotatory power is found at pH extreme values.The dispersion constant, _c , was calculated by means of a modified Drude equation and results indicate that the _c value is higher for muscle glycogen in the mussel species, suggesting a higher degree of asymmetry in this macromolecule.     

旋光性高分子的旋光能力（下）

在（上）中，我们讨论了结构单元及其远程作用对旋光性高分子旋光能力的影响。由于高分子具有特殊的长链结构，因而链形态对旋光能力的贡献尤其引人注目。     

SCF ab-initio ground state potential energy surfaces for HCN and HCN−

SCF computations for the ground state potential surfaces of HCN and HCN^? are performed. These calculations predict that the ground state geometry of the radical anion is R_e(CH) = 2.12 bohr, R_e(CN) = 2.33 bohr and the bond angle θ = 121.7°. The calculations also show that the CH bond in HCN^? is much weaker than in HCN and is similar to the CH bond in HCO. The computed electron affinity is ?1.95 eV. Since the minimum on the potential energy curve for the anion is above the neutral curve rapid auto-ionization should occur to HCN and an electron.     

Some aspects of vibronic coupling in circular dichroism

After a review of the quantum mechanical formulation of vibrational-electronic coupling, the adiabatic approximations for ordinary absorption dipole strength and circular dichroic absorption rotatory strength are presented and interpreted. The expressions include the effect of two vibrational quantum changes coupled to electronic excitation in addition to the more familiar concept of coupling by a one quantum change. A polarizability theory of vibronically coupled rotatory strength is presented which is comparable to the polarizability theory of rotatory strength without regard to vibration.