Comparative Study of Measured and Computed Raman Optical Activity of a Chiral Transition Metal Complex

Comparative Study of Vibrational Raman Optical Activity with Different Time-Dependent Density Functional Approximations: The VROA36 Database

A new database, VROA36, is introduced to investigate the performance of computational approaches for vibrational Raman optical activity (VROA) calculations. The database is composed of 36 molecules with known experimental VROA spectra. It includes 93 conformers. Normal modes calculated with B3LYP-D3(BJ)/def2-TZVP are used to compute the VROA spectra with four functionals, B3LYP-D3(BJ), ωB97X-D, M11, and optimally tuned LC-PBE, as well as several basis sets. SimROA indices and frequency scaling factors are used to compare calculated spectra with each other and with experimental data. The four functionals perform equally well independently of the basis set and usually achieve good agreement with the experimental data. For molecules in near- or at-resonance conditions, the inclusion of a complex (damped) linear response approach is important to obtain physically meaningful VROA intensities. The use of any of the tested functional approximations with the def2-SVPD Gaussian-type basis set, or a basis of similar flexibility, can be recommended for efficient and reliable theoretical VROA studies.

Resonance Effects in the Raman Optical Activity Spectrum of [Rh(en)3]3

Raman optical activity spectra of Λ-tris(ethylenediamine)-rhodium(III) {[Rh(en)₃]³⁺} have been calculated at 16 on-, near-, and off-resonant wavelengths between 290 and 800 nm. The resulting spectra are analyzed in detail with a focus on the observed resonance effects. Because several electronically excited states are involved, the spectra are never monosignate, as is often observed in resonance Raman optical activity spectra. Most normal modes are enhanced through these resonance effects, but in several cases, de-enhancement effects are found. The molecular origins of the Raman optical activity intensity for selected normal modes are established by means of group coupling matrices. In general, this methodology allows one to produce an intuitive explanation for the intensity behavior of a given normal mode. However, due to the complex electronic structure of [Rh(en)₃]³⁺, there are some intriguing resonance effects the origins of which could not be fully clarified in terms of group coupling effects. Therefore, simple and general rules that predict how the intensity of a specific normal mode is affected by resonance effects are difficult to devise.

Localized molecular orbitals for calculation and analysis of vibrational Raman optical activity

First calculations of vibrational Raman optical activity based on localized molecular orbitals are presented, which pave the way for novel insight into spectroscopic signatures of chiral systems.

Where does the Raman optical activity of [Rh(en)3]3+ come from? Insight from a combined experimental and theoretical approach

Raman optical activity (ROA) spectra are measured and calculated for Δ- and Λ-tris-(ethylenediamine)rhodium(iii) chloride in aqueous solution.

Theoretical investigation of Raman optical activity signatures of Tröger's base

The Raman and VROA spectra of (S,S)-Tröger's base are simulated. We mainly discuss the peaks in the 1140-1400 cm(-1) wavenumber range where an intense VROA signature is found. In this range, nearly all of the Raman-active bands belong to the irreducible representation A (C(2) point group), whereas no such observation is made for the VROA spectrum. The vibrational normal modes associated with the peaks in this range mainly consist of wagging and twisting motions of the hydrogen atoms. From the atomic contribution patterns (ACPs) and the group coupling matrices (GCMs), one finds that the VROA backward-scattering intensities mainly arise from hydrogen and carbon atoms in the vicinity of the two chiral nitrogen atoms. The VROA signatures in the 1140-1400 cm(-1) range are therefore a fingerprint of the local chirality around the two chiral nitrogen centers.