Anharmonic Aspects in Vibrational Circular Dichroism Spectra from 900 to 9000 cm-1 for Methyloxirane and Methylthiirane

Experimental and theoretical aspects of magnetic circular dichroism and magnetic circularly polarized luminescence in the UV, visible and IR ranges: A review

A historical sketch of the MCD (magnetic circular dichroism) spectroscopy is reported in its experimental and theoretical aspects. MCPL (magnetic circularly polarized luminescence) is also considered. The main studies are presented encompassing porphyrinoid systems, aggregates and materials, as well as simple organic molecules useful for the advancement of the interpretation. The MCD of chiral systems is discussed with special attention to new studies of natural products with potential pharmaceutical valence, including Amaryllidaceae alkaloids and related isocarbostyrils. Finally, the vibrational form of MCD, called MVCD, which is recorded in the IR part of the spectrum is also discussed. A final brief note on perspectives is given.

Scaling-up VPT2: A feasible route to include anharmonic correction on large molecules

Vibrational analysis plays a crucial role in the investigation of molecular systems. Though methodologies like second-order vibrational perturbation theory (VPT2) have paved the way to more accurate simulations, the computational cost remains a difficult barrier to overcome when the molecular size increases. Building upon recent advances in the identification of resonances, we propose an approach making anharmonic simulations possible for large-size systems, typically unreachable by standard means. This relies on the fact that, often, only portions of the whole spectra are of actual interest. Therefore, the anharmonic corrections can be included selectively on subsets of normal modes directly related to the regions of interest. Starting from the VPT2 equations, we evaluate rigorously and systematically the impact of the truncated anharmonic treatment onto simulations. The limit and feasibility of the reduced-dimensionality approach are detailed, starting on a smaller model system. The methodology is then challenged on the IR absorption and vibrational circular dichroism spectra of an organometallic complex in three different spectral ranges.

Mid-IR and CH stretching vibrational circular dichroism spectroscopy to distinguish various sources of chirality: The case of quinophaneoxazoline based ruthenium(II) complexes

Five diastereomers of ruthenium(II) complexes based on quinolinophaneoxazoline ligands were investigated by vibrational circular dichroism (VCD) in the mid-IR and CH stretching regions. Diastereomers differ in three sources of chirality: the planar chirality of the quinolinophane moiety, the central chirality of an asymmetric carbon atom of the oxazoline ring, and the chirality of the ruthenium atom. VCD, allied to DFT calculations, has been found to be effective in disentangling the various forms of chirality. In particular, a VCD band is identified in the CH stretching region directly connected to the chirality of the metal. The analysis of the calculated VCD spectra is carried out by partitioning the complexes into fragments. The anharmonic analysis is also performed with a recently proposed reduced-dimensionality approach: such treatment is particularly important when examining spectroscopic regions highly perturbed by resonances, like the CH stretching region.

Pushing measurements and interpretation of VCD spectra in the IR, NIR and visible ranges to the detectability and computational complexity limits

(R)-Limonene VCD and IR absorption spectra for neat liquid samples are considered from 900 to 16,000 cm-1, using mostly data by Nafie et al. up to 10,000 cm-1 and from previous investigations of the Brescia group. New VCD data are recorded in the merely overtone and combination region between 1800 and 2400 cm-1 and for the Δn= 6 overtone CH-stretching region above 15,000 cm-1. The GVPT2 anharmonic DFT calculations permit satisfactory interpretation of the fundamental + overtone/combination of deformation modes in the mid-IR up to 3500 cm-1. The GVPT2 approach is also used for the first CH-stretching overtone regions together with their combination with deformation modes up to 9000 cm-1. Then the local-mode approach developed within the DFT protocol is employed in all the CH-stretching regions (fundamental + overtones) and is found to satisfactorily account for the observed spectra, justifying the constant VCD pattern observed for all overtones. On the basis of the local-mode model the components of the bisignate VCD spectrum are attributed to the stretchings of the axial and equatorial CH bonds in α-position with respect to the ring CC double bond.

Comparison of different density functional theory methods for the calculation of vibrational circular dichroism spectra

The determination of the absolute configuration (AC) of an organic molecule is still a challenging task for which the combination of spectroscopic with quantum-mechanical methods has become a promising approach. In this study, we investigated the accuracy of DFT methods (480 overall combinations of 15 functionals, 16 basis sets, and 2 solvation models) to calculate the VCD spectra of six chiral organic molecules in order to benchmark their capability to facilitate the determination of the AC.

Selective Chirality Transfer to the Bis(trifluoromethylsulfonyl)imide Anion of an Ionic Liquid

Chirality transfer from the chiral molecule (R)-1,2-propylene oxide to the achiral anion of the 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid is observed. The chiral probe selectively affects one part of the binary ionic liquid, i. e., it has previously been shown experimentally and theoretically that this particular imidazolium cation can be affected by chirality transfer, but in the present system chirality is almost exclusively transferred to the anion and not to both parts of the solvent (anion and cation). This observation is of high relevance because of its selectivity and because anion effects are usually much more important in ionic liquid research than cation effects. From ab initio molecular dynamics simulations, a conformational analysis and dissected vibrational circular dichroism spectra are obtained to study the chirality transfer. While in the neat ionic liquid two mirror imaged trans conformers of the anion occur almost equally, we observe an excess of one of these conformers in the presence of the chiral solute, causing optical activity of the anion. Although the cis conformers are not tremendously affected by the chirality transfer, they gain in total population when (R)-1,2-propylene oxide is dissolved in the ionic liquid.

Vibrational circular dichroism spectra of natural products by means of the nuclear velocity perturbation theory

We present the application of the recently implemented nuclear velocity perturbation theory, using the combined Gaussian and plane waves approach in CP2K, to the vibrational circular dichroism (VCD) spectra of a set of natural products. Even though the calculations were carried out for isolated molecules in the gas-phase limit, neglecting inter-molecular interactions and anharmonic effects, the match between simulated and experimental spectra is reasonable. We also study the influence of different density functionals on the conformational search and the resulting VCD spectra via group coupling matrices (GCMs). The GCM analysis reveals that the VCD signal can in some cases arise from moieties which are close to each other and in other cases from moieties far from each other. Differences in spectra obtained using different exchange-correlation density functionals can be attributed to interaction terms between different moieties in the molecules changing their sign.