Symmetry Breaking in Chiral Ionic Liquids Evidenced by Vibrational Optical Activity

Chiral Deep Eutectic Solvents Enable Full-Color and White Circularly Polarized Luminescence from Achiral Luminophores

Herein, chiral deep eutectic solvents (DES) are prepared by lauric acid as hydrogen bond donors (HBD) and chiral menthol as hydrogen bond acceptors (HBA). When achiral fluorescent molecules are dopedin the menthol-based chiral DES, they emit circularly polarized luminescence (CPL) with handedness controlled by the molecular chirality (l or d) of menthol. Remarkably, the strategy is universal and a series of achiral fluorescent molecules can be endowed with CPL activity, showing a full-color and white CPL upon appropriate mixing, which paves the way to prepare white CPL materials. Interestingly, CPL appears only in a certain temperature range in the DES. Variable-temperature spectra and other characterization methods reveal that the H-bond network in the chiral DES plays an important role in inducing CPL. This work unveils how the interior structure as well as the hydrogen-bond network of a chiral DES can transfer its chirality to achiral luminophores for the first time and realizes a full-color and white CPL in a DES.

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.

Dual-comb optical activity spectroscopy for the analysis of vibrational optical activity induced by external magnetic field

Optical activity (OA) spectroscopy is a powerful tool to characterize molecular chirality, explore the stereo-specific structure and study the solution-state conformation of biomolecules, which is widely utilized in the fields of molecular chirality, pharmaceutics and analytical chemistry. Due to the considerably weak effect, OA spectral analysis has high demands on measurement speed and sensitivity, especially for organic biomolecules. Moreover, gas-phase OA measurements require higher resolution to resolve Doppler-limited profiles. Here, we show the unmatched potential of dual-comb spectroscopy (DCS) in magnetic optical activity spectroscopy (MOAS) of gas-phase molecules with the resolution of hundred-MHz level and the high-speed measurement of sub-millisecond level. As a demonstration, we achieved the rapid, high-precision and high-resolution MOAS measurement of the nitrogen dioxide [Formula: see text]+[Formula: see text] band and the nitric oxide overtone band, which can be used to analyze fine structure of molecules. Besides, the preliminary demonstration of liquid-phase chiroptical activity (as weak as 10^-5) has been achieved with several seconds of sampling time, which could become a routine approach enabling ultrafast dynamics analysis of chiral structural conformations.

Vibrational Circular Dichroism from DFT Molecular Dynamics: The AWV Method

The paper illustrates the Activity Weighted Velocities (AWV) methodology to compute Vibrational Circular Dichroism (VCD) anharmonic spectra from Density Functional Theory (DFT) molecular dynamics. AWV calculates the spectra by the Fourier Transform of the time correlation functions of velocities, weighted by specific observables: the Atomic Polar Tensors (APTs) and the Atomic Axial Tensors (AATs). Indeed, AWV shows to correctly reproduce the experimental spectra for systems in the gas and liquid phases, both in the case of weakly and strongly interacting systems. The comparison with the experimental spectra is striking especially in the fingerprint region, as demonstrated by the three benchmark systems discussed: (1S)-Fenchone in the gas phase, (S)-(-)-Propylene oxide in the liquid phase, and (R)-(-)-2-butanol in the liquid phase. The time evolution of APTs and AATs can be adequately described by a linear combination of the tensors of a small set of appropriate reference structures, strongly reducing the computational cost without compromising accuracy. Additionally, AWV allows the partition of the spectral signal in its molecular components without any expensive postprocessing and any localization of the charge density or the wave function.

Chemistry Dissolved in Ionic Liquids. A Theoretical Perspective

The theoretical treatment of ionic liquids must focus now on more realistic models while at the same time keeping an accurate methodology when following recent ionic liquids research trends or allowing predictability to come to the foreground. In this Perspective, we summarize in three cases of advanced ionic liquid research what methodological progress has been made and point out difficulties that need to be overcome. As particular examples to discuss we choose reactions, chirality, and radicals in ionic liquids. All these topics have in common that an explicit or accurate treatment of the electronic structure and/or intermolecular interactions is required (accurate methodology), while at the same time system size and complexity as well as simulation time (realistic model) play an important role and must be covered as well.

Computation of Solid-State Vibrational Circular Dichroism in the Periodic Gauge

We introduce a new theoretical formalism to compute solid-state vibrational circular dichroism (VCD) spectra from molecular dynamics simulations. Having solved the origin-dependence problem of the periodic magnetic gauge, we present IR and VCD spectra of (1S,2S)-trans-1,2-cyclohexanediol obtained from first-principles molecular dynamics calculations and nuclear velocity perturbation theory, along with the experimental results. Because the structure model imposes periodic boundary conditions, the common origin of the rotational strength has hitherto been ill-defined and was approximated by means of averaging multiple origins. The new formalism reconnects the periodic model with the finite physical system and restores gauge freedom. It nevertheless fully accounts for nonlocal spatial couplings from the gauge transport term. We show that even for small simulation cells the rich nature of solid-state VCD spectra found in experiments can be reproduced to a very satisfactory level.

Recognition in Chiral Ionic Liquids: The Achiral Cation Makes the Difference!

By simulating butan-2-ol dissolved in the chiral ionic liquid 1-ethyl-3-methylimidazolium (S)-alaninate, we investigate the chiral recognition of butan-2-ol in the ionic liquid. The hydrogen bonding between the chiral anion and both enantiomers of butan-2-ol is similar; however, both chiral molecules (anion and alcohol) induce an asymmetry in the achiral cation which leads to a more favorable environment for the alcohol in the heterochiral system as compared to the homochiral system and hence provides an energetic stabilization of the former.

Glucose in dry and moist ionic liquid: vibrational circular dichroism, IR, and possible mechanisms

Ionic liquids and their mixtures with water show remarkable features in cellulose processing. For this reason, understanding the behavior of carbohydrates in ionic liquids is important. In the present study, we investigated three d-glucose isomers (α, β and open-chain) in 1-ethyl-3-methylimidazolium acetate in the presence and absence of water, through ab initio molecular dynamics simulations. In the complex hydrogen bonding network of these mixtures, the most interesting observation is that upon water addition every hydrogen bond elongates, except the glucose-glucose hydrogen bond for the open-chain and the α-form which shortens, clearly showing the beginning of the crystallization process. The ring glucose rearranges from on-top to in-plane and the open form changes from a coiled to a more linear arrangement when adding water which explains the contradiction that the center of mass distances of the glucose molecules with other glucose molecules grow while the hydrogen bonds shorten. The appearance of coiled open forms indicates that the previously suggested isomerization between these forms is possible and might play a role in the solubility of the related carbohydrates. The calculated IR and VCD spectra reveal insight into the intermolecular interactions, with good to excellent agreements with experimental spectra. Investigating the role of the cation, distances between the acidic carbon atom of the cation and the glucose carbon atom where ring closure and opening occurs are found, which are way shorter than dispersion-like interactions between aliphatic hydrocarbons.

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.

Effective computational route towards vibrational optical activity spectra of chiral molecules in aqueous solution

A polarizable QM/MM approach to accurately compute the Vibrational Optical Activity (VOA) spectra of chiral systems is proposed and applied to aqueous solutions of (l)-methyl lactate and (S)-glycidol.