Search

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.

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.

We have prepared Au25(PET)18–2x((R)-FBINAS)x(PET = 2-phenylethylthiol, (R)-FBINAS = (R)-5,5′,6,6′,7,7′,8,8′-octafluoro-[1,1′-binaphthalene]-2,2′-dithiol) mixed ligand shell clusters via ligand exchange. Two fractions with different composition of the ligand shell were separated using size-exclusion chromatography and studied by infrared (IR) spectroscopy and vibrational circular dichroism (VCD). Both IR and VCD spectra are dominated by FBINAS vibrations although PET is more abundant on the cluster. Calculated VCD spectra on a model cluster Au25(SH)16(FBINAS)1depend on the charge state of the cluster, and the anion is in excellent agreement with the experimental spectra of the Au25(PET)18–2x(FBINAS)xsamples. Calculations of Au25(SH)14(FBINAS)2furthermore show very similar spectra for different adsorption isomers with one exception, where the two ligands share a SR–Au–SR–Au–SR staple motif. Model calculations show that it is not necessary to consider the entire cluster for obtaining reliable VCD and IR spectra, but the staple motifs cannot be neglected. A band that is assigned to PET shows significant vibrational optical activity, and an anisotropy factor that depends on the composition of the ligand shell. This shows that the FBINAS molecules can transfer chirality to achiral PET within the ligand shell of the cluster.

Vibrational modes in an achiral molecule may acquire rotational strength by complexation to a chiral molecule, as happens for achiral solvent molecules complexed to a chiral solute. We investigate this transfer of chirality in vibrational circular dichroism for the pulegone molecule in CDCl3solvent from the point of view of the robustness concept introduced recently. It turns out that the transfer of chirality yields nonrobust modes, which means that, although they are observed in vibrational circular dichroism (VCD) experiments, the sign of these modes cannot be predicted reliably with standard (Density Functional Theory) VCD calculations. This limits the usefulness of the induced chirality phenomenon for obtaining information on the intermolecular interactions that give rise to it. Chirality 21:E287–E297, 2009. © 2010 Wiley-Liss, Inc.