Computations of the Raman optical activity via the sum‐over‐states expansions

Simulation of Raman optical activity of multi-component monosaccharide samples

Determination of the saccharide structure in solution is a laborious process that can be significantly enhanced by chiral optical spectroscopies.

Theoretical approaches to the calculation of Raman optical activity spectra

In this article, we will give a brief account of the different approaches that have been presented in the literature for calculating Raman optical activity (ROA) spectra by ab initio methods. We will also outline the general structure of a self-consistent-field-based approach for analytic calculations of ROA spectra, including also contributions from London orbitals. The use of London orbitals ensures that the relevant ROA parameters are gauge origin independent. We will also give an outlook on the future of ab initio calculations of Raman optical activity spectra.

Spectroscopic properties of the nonplanar amide group: A computational study

Experimental studies suggest that amide bond may significantly deviate from planar arrangement even in linear peptides and proteins. In order to find out the extent to which such deviation may influence principal amide spectroscopic properties, we conducted a computational study of nonplanar N-methylacetamide (NMA) conformers. Vibrational absorption, Raman, and electronic spectra including optical activity were simulated with ab initio and density functional theory (DFT) methods. According to the results, small nonplanarity deviations may be detectable by nonpolarized spectroscopic techniques, albeit as subtle spectral changes. The optical activity methods, such as the vibrational circular dichroism (VCD), Raman optical activity (ROA), and electronic circular dichroism (CD, ECD), provide enhanced information about the amide nonplanarity, because planar amide is not optically active (chiral). For VCD, however, the inherently chiral contribution in most peptides and proteins most probably provides very weak signal in comparison with other contributions, such as the dipolar coupling. For the electronic CD, the nonplanarity contribution is relatively big and causes a strong CD couplet in the n-pi* absorption region accompanied by a red frequency shift. The pi-pi* CD region is relatively unaffected. The ROA spectroscopy appears most promising for the nonplanarity detection and the inherent chiral signal may dominate entire spectral parts. The amide I and III vibrational ROA bands are most challenging experimentally because of their relatively weak coupling to other peptide vibrations.

Theoretical Determination of the Vibrational Raman Optical Activity Signatures of Helical Polypropylene Chains

Raman and vibrational Raman optical activity (VROA) spectra of helical conformers of polypropylene chains are simulated using ab initio methods to unravel the relationships between the vibrational signatures and the primary and secondary structures of the chains. For a polypropylene chain containing three units, conformational effects are shown to lead to more acute signatures for VROA than for Raman spectra. In addition to regular polypropylene chains, which can display right and left helicities with the same probability, chirality and therefore helicity are enforced by substituting one chain end with a phenyl group. The simulations predict that the threefold helical structures, which correspond to (TG)(N) conformations of the backbone, have a specific VROA backward signature in the form of an intense couplet around 1100 cm(-1). This couplet is associated with collective wagging and twisting motions, while most of its intensity comes from the anisotropic invariants combining normal coordinate derivatives of the electric dipole-electric dipole polarizability and of the electric dipole-magnetic dipole polarizability. A similar signature has already been found in model helical polyethylene chains, whereas it is very weak in forward VROA.

Proline Zwitterion Dynamics in Solution, Glass, and Crystalline State

Raman and Raman optical activity spectra of L- and D-proline zwitterionic (PROZW) forms were recorded for H(2)O and D(2)O solutions in a wide frequency range and analyzed with respect to the motion of the proline ring and rotation of the carbonyl group. The solution spectra were additionally compared to Raman scattering of glass and crystalline powder proline. Solution and glass spectral band broadenings are similar and reveal information about the extent of internal molecular motion. Two distinct but equally populated flexible forms were found in the glass and the solution. The equal population is consistent with NMR data, temperature, and concentration dependencies. The molecular flexibility is reduced significantly in the crystal, however, where only one conformer is present. Consequently, the crystal bands are narrow and exhibit minor frequency shifts. The spectra were interpreted with the aid of density functional theory computations involving both continuum and explicit solvent. A two-dimensional potential energy surface pertaining to the five-member ring puckering coordinates was constructed and used for dynamical averaging of spectral properties. Comparison of the computed and experimental bandwidths suggests that the puckering is strongly correlated with the carbonyl rotation. An averaging over these two motions produces similar results. The interpretation of the Raman experiments with the aid of the simulation techniques also indicates that the environment modulates properties of the hydrophobic part of the molecule indirectly by interacting with the ionic group. Such behavior may be important for the reactivity and biological activity of proline-containing peptides and proteins.

Vibrational Raman optical activity as a mean for revealing the helicity of oligosilanes: A quantum chemical investigation

Using theoretical simulations based on Hartree-Fock and density-functional theory calculations, the simulated vibrational Raman optical activity spectra of helical conformers of heptasilane are shown to present signatures sensitive to the helicity. These signatures are associated with collective wagging, twisting, and rocking motions. These simulated spectra have been obtained by combining analytical and numerical differentiation procedures to evaluate the geometry derivatives of the optical tensors entering into the expressions of the vibrational Raman optical activity intensities. From an investigation of basis set and electron correlation effects, it is shown that, like for local vibrations, diffuse functions are compulsory for evaluating the vibrational Raman optical activity intensities of collective vibrational motions.