Vibrational circular dichroism studies of exceptionally strong chirality inducers in liquid crystals

Chiral dopants are used in liquid crystal displays to introduce uniform helical alignment. VCD can provide unambiguous determination of the absolute configuration and structural details of such a dopant, while X-ray crystallography fails.

Vibrational circular dichroism spectroscopy for probing the expression of chirality in mechanically planar chiral rotaxanes

Mechanically interlocked molecules can exhibit molecular chirality that arises due to the mechanical bond rather than covalent stereogenic units. Developing applications of such systems is made challenging by the absence of techniques for assigning the absolute configuration of products and methods to probe how the mechanical stereogenic unit influences the spatial arrangements of the functional groups in solution. Here we demonstrate for the first time that Vibrational Circular Dichroism (VCD) can be used to not only discriminate between mechanical stereoisomers but also provide detailed information on their (co)conformations. The latter is particularly important as these molecules are now under investigation in catalysis and sensing, both of which rely on the solution phase shape of the interlocked structure. Detailed analysis of the VCD spectra shows that, although many of the signals arise from coupled oscillators isolated in the covalent sub-components, intercomponent coupling between the macrocycle and axle gives rise to several VCD bands.

Analysis of Vibrational Circular Dichroism Spectra of Peptides: A Generalized Coupled Oscillator Approach of a Small Peptide Model Using VCDtools

Vibrational circular dichroism (VCD) is one of the major spectroscopic tools to study peptides. Nevertheless, a full understanding of what determines the signs and intensities of VCD bands of these compounds in the amide I and amide II spectral regions is still far from complete. In the present work, we study the origin of these VCD signals using the general coupled oscillator (GCO) analysis, a novel approach that has recently been developed. We apply this approach to the ForValNHMe model peptide in both α-helix and β-sheet configurations. We show that the intense VCD signals observed in the amide I and amide II spectral regions essentially have the same underlying mechanism, namely, the through-space coupling of electric dipoles. The crucial role played by intramolecular hydrogen bonds in determining VCD intensities is also illustrated. Moreover, we find that the contributions to the rotational strengths, considered to be insignificant in standard VCD models, may have sizable magnitudes and can thus not always be neglected. In addition, the VCD robustness of the amide I and II modes has been investigated by monitoring the variation of the rotational strength and its contributing terms during linear transit scans and by performing calculations with different computational parameters. From these studies-and in particular, the decomposition of the rotational strength made possible by the GCO analysis-it becomes clear that one should be cautious when employing measures of robustness as proposed previously.

Self-Assembly of Supramolecular Polymers of N-Centered Triarylamine Trisamides in the Light of Circular Dichroism: Reaching Consensus between Electrons and Nuclei

The self-assembly of chiral supramolecular polymers is an intricate process that spans a wide range of length scales. Circular dichroism techniques are ideal to study this process as they provide information on the molecular scale but are at the same time also sensitive probes of the long-range interactions that control the growth and morphology of these polymers. As yet, Electronic Circular Dichroism that uses electronic transitions as a probe has by far been the method of choice while Vibrational Circular Dichroism, which uses vibrational transitions to probe structure, is much less employed. Here, we report experimental and theoretical studies of the self-assembly of helical supramolecular polymers of (S)-triarylamine tris-amides ((S)-TATA) in which both techniques are applied in concert. Theoretical studies based on quantum chemical calculations and on simplified models that allow for extrapolation to “infinitely” long polymers provide a solid basis for interpreting results from each of the two techniques that on their own would appear to be contradictory. In the particular case of (S)-TATA it is shown that upon equilibration the initially formed fibers undergo a conformational transition that becomes only “visible” by the combination of the two techniques. Our studies thus show that combining electronic and vibrational domains offers a unique and complementary means to probe these polymers, precisely because they are sensitive to different aspects of molecular and polymeric structure.

GUI Implementation of VCDtools, A Program to Analyze Computed Vibrational Circular Dichroism Spectra

As computing power increases, vibrational circular dichroism (VCD) calculations on molecules of larger sizes and complexities become possible. At the same time, the spectra resulting from these computations become increasingly more cumbersome to analyze. Here, we describe the GUI implementation into the Amsterdam Density Functional (ADF) software package of VCDtools, a toolbox that provides a user-friendly means to analyze VCD spectra. Key features are the use of the generalized coupled oscillator analysis methods, as well as an easy visualization of the atomic electric and magnetic transition dipole moments which together provide detailed insight in the origin of the VCD intensity. Using several prototypical examples we demonstrate the functionalities of the program. In particular, we show how the spectra can be analyzed to detect differences between theory and experiment arising from large-amplitude motions or incorrect molecular structures and, most importantly, how the program can be used to prevent incorrect enantiomeric assignments.

A Tunable, Fullerene-Based Molecular Amplifier for Vibrational Circular Dichroism

Vibrational circular dichroism (VCD) studies are reported on a chiral compound in which a fullerene C60 moiety is used as an electron acceptor and local VCD amplifier for an alanine-based peptide chain. Four redox states are investigated in this study, of which three are reduced species that possess low-lying electronic states as confirmed by UV/Vis spectroelectrochemistry. VCD measurements in combination with (TD)DFT calculations are used to investigate (i) how the low-lying electronic states of the reduced species modulate the amplification of VCD signals, (ii) how this amplification depends on the distance between oscillator and amplifier, and (iii) how the spatial extent of the amplifier influences amplification. These results pave the way for further development of tailored molecular VCD amplifiers.

Taming conformational heterogeneity in and with vibrational circular dichroism spectroscopy

Genetic fitting algorithm accounting for the uncertainty in computed energies allows a significantly more reliable assignment of stereochemistry and conformational heterogeneity of chiral compounds using vibrational circular dichroism.

The flexibility of a molecule has important consequences on its function and application. Vibrational Circular Dichroism (VCD) is intrinsically an excellent experimental technique to get a hold on this flexibility as it is highly sensitive to key conformational details and able to distinguish rapidly interconverting conformers. One of the major challenges in analyzing the spectra by comparison to theoretical predictions is the uncertainty in the computed energies of the multitude of conformations. This uncertainty also affects the reliability of the stereochemical assignment it is normally used for. We present here a novel approach that explicitly takes the energy uncertainties into account in a genetic algorithm based method that fits calculated to the experimental spectra. We show that this approach leads to significant improvements over previously used methodologies. Importantly, statistical validation studies provide quantitative measures for the reliability of relevant parameters used such as the energy uncertainty and the extent to which conformational heterogeneity can be determined. Similarly, quantitative measures can be obtained for the possibility that the flexibility that is introduced in the fit might lead to an incorrect assignment of the stereochemistry. These results break new ground for different techniques based on VCD to elucidate conformational flexibility.

Use of Density Functional Based Tight Binding Methods in Vibrational Circular Dichroism

Vibrational circular dichroism (VCD) is a spectroscopic technique used to resolve the absolute configuration of chiral systems. Obtaining a theoretical VCD spectrum requires computing atomic polar and axial tensors on top of the computationally demanding construction of the force constant matrix. In this study we evaluated a VCD model in which all necessary quantities are obtained with density functional based tight binding (DFTB) theory. The analyzed DFTB parametrizations fail at providing accurate vibrational frequencies and electric dipole gradients but yield reasonable normal modes at a fraction of the computational cost of density functional theory (DFT). Thus, by applying DFTB in composite methods along with DFT, we show that it is possible to obtain accurate VCD spectra at a much lower computational demand.

Regional Susceptibility in VCD Spectra to Dynamic Molecular Motions: The Case of a Benzyl α-Hydroxysilane

Experimental and theoretical studies of the vibrational circular dichroism (VCD) spectrum of 3-methyl-1-(methyldiphenlsilyl)-1-phenylbutan-1-ol, whose absolute configuration is key to elucidating the Brook rearrangement of tertiary benzylic α-hydroxylsilanes, are presented. It is found that the entire OH-bending region in this spectrum-a region that provides important marker bands-cannot be reproduced at all by standard theoretical approaches even though other regions are well described. Using a novel approach to disentangle contributions to the rotational strength of these bands, internal coordinates are identified that critically influence the appearance of this part of the spectrum. We show that the agreement between experiment and theory is greatly improved when structural dynamics along these coordinates are explicitly taken into account. The general applicability of the approach underlines its usefulness for structurally flexible chiral systems, a situation that is more the rule rather than the exception.