Circular Intensity Difference Research Articles

Comparison of Experimental and Calculated Chiroptical Spectra for Chiral Molecular Structure Determination

For three different chiroptical spectroscopic methods, namely, vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and Raman optical activity (ROA), the measures of similarity of the experimental spectra to the corresponding spectra predicted using quantum chemical theories are summarized. In determining the absolute configuration and/or predominant conformations of chiral molecules, these similarity measures provide numerical estimates of agreement between experimental observations and theoretical predictions. Selected applications illustrating the similarity measures for absorption, circular dichroism, and corresponding dissymmetry factor (DF) spectra, in the case of VCD and ECD, and for Raman, ROA, and circular intensity differential (CID) spectra in the case of ROA, are presented. The analysis of similarity in DF or CID spectra is considered to be much more discerning and accurate than that in absorption (or Raman) and circular dichroism (or ROA) spectra, undertaken individually.

Remotely excited Raman optical activity using chiral plasmon propagation in Ag nanowires

We experimentally investigated remotely excited Raman optical activity (ROA) using propagating surface plasmons in chiral Ag nanowires. Using chiral fmoc-glycyl-glycine-OH (FGGO) molecules, we first studied the local surface plasmon-enhanced ROA. We found that the Raman intensity can be excited by left- and right-circularly polarized lights and that the circular intensity difference (CID) can be significantly enhanced. Second, by selecting vibrational modes with large Raman and ROA intensities that are not influenced by chemical enhancements, we studied remotely excited ROA imaging and the CID of FGGO molecules by propagating a plasmonic waveguide using Ag chiral nanostructures. When laser light was radiated on one of the Ag terminals, the measured CID of the FGG at the other terminal showed little change compared to the local excited CID. Meanwhile, when the laser light was radiated on the Ag nanowires (not on the terminals) and was coupled to the nearby nanoantenna, the CID of the ROA could be manipulated by altering the coupling angle between the Ag nanowires. To directly demonstrate the propagation of ROA along the nanowire and its remote detection, we also measured the remotely excited ROA spectra. Our experimental method has the potential to remotely determine the chirality of molecular structures and the absolute configuration or conformation of a chiral live cell.

Ab initio study of the circular intensity difference in electric-field-induced second harmonic generation of chiral natural amino acids

We present a systematic computational study of circular intensity difference (CID) in electric-field-induced second harmonic generation (EFISHG) of some representative chiral natural amino acids (Alanine, Arginine, Aspartic Acid, Cysteine and Tryptophan), taking into account the electric-dipole, electric-quadrupole and magnetic-dipole interactions. The calculations are performed by applying cubic response theory at both Hartree-Fock and Density Functional theory levels, the latter with the popular Becke-three parameters, Lee, Yang and Parr (B3LYP) functional. Special focus is given to the basis set, electron correlation and origin dependence of the properties. The full set of molecular parameters defined by Lam and Thirunamachandran in their reference theoretical paper published in 1982 [J. Chem. Phys., 1982, 77, 3810] is obtained and discussed. This permits the prediction of the CID observable for different possible experimental setups.

Theoretical Modeling of the Surface-Enhanced Raman Optical Activity.

Surface-enhanced Raman optical activity (SEROA) is a new technique combining the sensitivity of the surface-enhanced Raman scattering (SERS) with the detailed information about molecular structure provided by the chiral spectroscopies. So far, experimental SEROA spectra have been reported in several studies, but the interpretation and theoretical background are rather limited. In this work, general expressions for the electromagnetic contribution to SEROA are derived using the matrix polarization theory and used to investigate the enhancement in model systems. The results not only reveal a strong dependence of the enhancement on the distance between the molecule and a metal part but also the dependence of the ratio of ROA and Raman intensities (circular intensity difference, CID) on the distance and rotational averaging. For a ribose model, an optimal molecule-colloid distance was predicted which provided the highest CIDs. However, the CID maximum disappeared after a rotational averaging. For cysteine zwitterion, the simulated SEROA and SERS spectra provided a qualitative agreement with previous experiments.

Vector part of optical activity probed with x-rays in hexagonal ZnO

We discuss how measurements of x-ray circular intensity differentials (XCIDs) could be used to probethe Voigt–Fedorov vector part of optical activity (OA) due to electric dipole–electric quadrupole(E 1E 2) interferences. The first experiment was carried out on a single crystal of zincite(hexagonal ZnO: class 6mm). XCID spectra were recorded in the x-ray resonantdiffraction regime near the Zn K edge using the (300) reflection at Bragg angles near45°. In the x-ray range, the effective operator responsible for the vector part of OA can beassigned to the vector product , in which L and ΩL are time-reversal odd operators associated with the orbitalangular momentum and the orbital anapole respectively, whereasn is a true time-reversal even electric dipole. This is consistent with the pyroelectricproperties of zincite crystals.

Surface-Enhanced Raman Optical Activity on Adenine in Silver Colloidal Solution

We report the collection of Raman optical activity (ROA) spectra of adenine in silver colloidal solution, that is, surface-enhanced Raman optical activity (SEROA) using considerably shorter data acquisition times, reduced excitation power, and lower concentration, as compared to classical ROA measurements on molecules of biological interest so far reported in the literature. These improvements in experimental parameters for ROA measurements can be explained by enhanced Raman signals in the local optical fields of the silver nanoparticles and by at least 1 order of magnitude higher values for circular intensity differences (CIDs), as compared to classical ROA that has been suggested before and theoretically discussed in terms of large field gradients near a metal surface. The measured ROA effect for adenine can be understood in terms of adsorption-induced chirality in the prochiral molecules on the silver nanoparticles. Surface-enhanced Raman optical activity offers potential capabilities for sensitive, rapid, stereochemical characterization of basic building blocks of biopolymers, such as amino acids and nucleosides, as well as biologically active molecules, in particular, also for probing organization and self-assembling of such molecules on metal surfaces.

Gauge-Origin Independent Density-Functional Theory Calculations of Vibrational Raman Optical Activity

We present density-functional theory calculations of vibrational Raman optical activity. Gauge-origin independence of the results is ensured by using London atomic orbitals, and the frequency dependence is included by using analytical response theory. The Raman optical activity circular intensity differences are obtained by combining analytical calculations of linear response functions with numerical differentiation with respect to nuclear distortions. The results are compared with experiment, previous Hartree -Fock calculations, and a more recent approximate sum-over-states approach.

Time-resolved vibrational optical activity measurement by the infrared-visible sum-frequency-generation with circularly polarized infrared light

A theoretical description of the time-resolved infrared-visible sum-frequency-generation (IV-SFG) process when the incident infrared pulsed beam is either left- or right-circularly-polarized (CP) is presented. Even for isotropic chiral molecular liquids, the CP IV-SFG polarization does not vanish because the chiral component of the associated response function is nonzero due to the breakdown of the Born–Oppenheimer approximation as well as to the finite polarizability-electric-quadrupole response function, which is a fourth-rank tensor. For a specific perpendicular detection scheme, it is shown that the three different contributions, originated from (1) all-electric-dipole-allowed polarization, (2) polarizability-quadrupole-induced polarization, and (3) interference between the above two polarizations, to the CP IV-SFG signal can be sperately measured. Also, the circular intensity difference, which is the difference between the left-CP IV-SFG intensity and the right-CP IV-SFG intensity, is theoretically investigated and found to be solely determined by the interference between the all-electric-dipole-allowed polarization and the polarizability-quadrupole-induced polarization. Thus, it is shown that the time-resolved CP IV-SFG methods are useful for the investigation of the molecular chirality of liquids and vibrational optical activity.

Large local optical activity in fractal aggregates of nanoparticles

Large local optical activity in fractal aggregates of silver nanoparticles has been observed by means of photon scanning tunneling microscopy. The effect occurs because resonant plasmon modes in random fractals can have handedness in spatial distribution of their amplitudes. In agreement with experimental observations, numerical simulations show dramatic difference in dipole-moment distributions for right- and left-circularly polarized incident light when the cluster size is comparable with or larger than the wavelength. Variations in the local parameter describing the circular intensity difference of scattered light show that fractal aggregates are characterized by broad and random distributions of chiral plasmon modes.

Magnetic chiroptical effects in surface second harmonic reflection

We perform surface second harmonic reflection (SHR) to study a chiral stilbene. It exhibits a strong circular intensity difference in the second harmonic signal, but no linear intensity difference and no rotation of polarization. This SHR optical activity is characteristic of strong magnetic effects and proves that the molecule under study exhibits a one-electron chirality.

Prediction of pure electric-dipole two-photon absorption circular dichroism in lanthanide compounds

Standard theories of chiral linear and nonlinear optical effects, such as circular intensity differentials and optical activity, require interference between electric-dipole and magnetic-dipole (or higher-order) contributions to the transition moments. Recent experimental and theoretical work on second-harmonic generation processes has shown that in the resonant case, pure electric-dipole contributions to such chiral effects are possible. In this paper, we consider the implications of pure electric-dipole contributions to circular dichroism in two-photon absorption transitions within the 4 f N configuration of lanthanide ions. We show that in some lanthanide compounds, when several intermediate states are near to resonance, the pure electric-dipole mechanism can give larger contributions than the standard electric-dipole magnetic-dipole mechanism.

Surface second-order nonlinear optical activity

Following the recent observation of a large second-harmonic intensity difference from a monolayer of chiral molecules with left and right circularly polarized light, the scattering theory is generalized and extended to predict linear and circular intensity differences for the more versatile sum-frequency spectroscopy. Estimates indicate that intensity differences should be detectable for a typical experimental arrangement. The second-order nonlinear surface susceptibility tensor is given for different surface point groups in the electric dipole approximation; it is shown that nonlinear optical activity phenomena unambiguously probe molecular chirality only for molecular monolayers that are symmetric about the normal. Other surface symmetries can give rise to intensity differences from monolayers composed of achiral molecules. A water surface is predicted to show linear and nonlinear optical activity in the presence of an electric field parallel to the surface.

A Classical Theory for the Raman Optical Activity

A classical theory for the Raman optical activity (ROA) is proposed based on a picture that for a normal mode, atoms of a molecule with partial residual charges act as oscillating dipoles around their equilibrium positions, which also induce magnetic moments on the other atoms (dipoles), reciprocally. Then the resulting coupling between electric dipoles and magnetic moments of the molecule contributes to the optical activity. Despite its crude approximation, this model offers a physical essence for ROA and produces results that are of quality comparable to the complicated quantum-chemical calculation, as far as the sign of the circular intensity difference for the ROA is concerned.

Calculations of circular intensity differences in electric-field-induced second harmonic generation

Ab initio calculations of electric-field induced second-harmonic generation for chiral molecules are presented. The calculations employ cubic response theory and take into account electric-dipole, electric-quadrupole and magnetic-dipole interactions, which result in an intensity dependence on the helicity of the light. The circular intensity difference is calculated for the chiral molecules twisted ethylene, trans-1,2-dimethylcyclopropane and methyloxirane. The calculated circular intensity differences are small – being of the order of 10 −4–10 −3 – but such electric-field induced second harmonic generation intensity dependencies should be experimentally detectable.

Ellipsometric penetration of turbid media: depolarization and surface characterization

Ellipsometry can be used to reveal characteristics of solid surfaces imbedded in turbid layers. Using a photoelastic modulator (PEM) to record the linear and circular intensity differences (LID, CID) of backscattered light as a function of position across a sample cell, we have observed reflective, diffractive and absorptive surfaces immersed in optically dense aqueous suspensions of latex particles.

On the degress of circularity for various kinds of polarized light in a nonpolar liquid mixture

The explicit results of the degrees of circularity for various kinds of completely polarized light in a nonpolar binary liquid mixture composed of an optically active solute and an optically nonactive solvent are obtained with the aid of the Ornstein-Zernike form for the correlation function of density fluctuations. The results are discussed in detail far from and near the critical point and compared with circular intensity differences.

On the scattering phenomena for various kinds of polarized light in a nonpolar fluid composed of chiral molecules

The explicit result of the coefficients of the Mueller matrices, which express the Stokes parameters of single and double scattered lights in term of the parameters of the corresponding incident light, are obtained for a nonpolar fluid composed of chiral molecules with the aid of the Ornstein–Zernike form for the correlation function of density fluctuations. The explicit results show that any higher order multiple scattering satisfies the principle of symmetry and law of reciprocity. From the matrices some scattering phenomena for six kinds of completely polarized light in a nonpolar achiral fluid are discussed in the cases of far from and near the critical point. While the degrees of circularity (DCs) give identical information with the circular intensity differences (CIDs) in the region far from the critical point, the critical DCs for circularly polarized lights is quite different from the critical CIDs. The theoretical result of DCs for the circularly polarized lights indicates that the DCs can be other effective experimental tools to measure the critical scattering of a fluid.

New aspects of second-harmonic optical activity from chiral surfaces and interfaces

New expressions are developed within the electric dipole approximation for the four Stokes parameters characterizing the polarization properties of coherent second-harmonic radiation generated from chiral isotropic surfaces in reflection. These are employed to derive, in addition to the three well-known incident circular polarization second-harmonic opticalactivity (SHOA) observables, thirteen new basic observables for the detection of surface SHOA which involve second-harmonic intensity difference measurements using right and left circular, and + 45 and - 45 linear, polarization modulation in the incident and second-harmonic radiation and in both simultaneously. Because of their dependence on a fully electric dipole allowed tensor these circular and linear intensity differences and the corresponding intensity sums have the same order of magnitude, so that all the dimensionless surface SHOA effects they generate are of the order of unity. The circular intensity differences (CIDs) depend on the imaginary p...

Polariton dispersion relation and stimulated Raman scattering from the 132 cm -1 E mode in α-quartz

Stimulated and spontaneous Raman scattering from the 132 cm -1 E mode was investigated along the c axis of an α-quartz crystal (length 7 cm) with parallel surfaces at low temperatures (10 K). The normalized difference of the threshold intensities of stimulated Raman scattering for right- and left-circularly-polarized pump light was large (about 20%) for long pump laser pulses (duration 12 ns) and zero within the experimental accuracy for short pulses (duration 20 ps). Measurements of the spontaneous Raman intensities and of the gain factors for stimulated Raman scattering showed that the difference between the Raman scattering cross sections for right- and left-circularly-polarized laser light is zero within the experimental accuracy. The observed large circular threshold intensity difference for the 12 ns pump pulses is explained by the peculiarities of the dispersion relation of the 132 cm -1 polariton mode in connection with the feedback provided by the surfaces of the quartz crystal.

19] Raman optical activity

Raman optical activity (ROA) is the difference in the intensity of Raman scattered light with respect to right and left circularly polarized incident or scattered radiation, or both. The form of ROA defined with circular polarization modulation of the incident laser was first described theoretically in 1971 and was first measured in 1973. As with other forms of natural optical activity, a molecular sample must be chiral to possess a nonzero ROA spectrum. The measurement of an ROA spectrum requires particular care to avoid the presence of optical artifacts since ROA intensities are small, being typically three to four orders of magnitude below those of the parent Raman spectrum. The value of ROA as a spectroscopic technique lies in its particular sensitivity to chiral molecular environments. After nearly two decades of steady development, ROA is now emerging as an important new spectroscopic tool in the study of the stereo conformations of optically active molecules, including biological macromolecules and metal complexes. Advances in three principal areas have revolutionized the measurement of ROA spectra. These are (1) the use of alternative scattering geometries including backward scattering, (2) the use of charge-coupled device (CCD) detectors as nearly ideal, low-background, large area, multichannel detectors, and (3) the introduction of new polarization modulation techniques involving the measurement of circular polarization intensity differences associated with the scattered beam.