We demonstrate that molecular vibrations with originally low or zero intensity in a vibrational circular dichroism (VCD) spectrum attain chirality in molecular crystals by coordinated motion of the atoms. Ab initio molecular dynamics simulations of anharmonic solid-state VCD spectra of l-alanine crystals reveal how coherent vibrational modes exploit the space group's chirality, leading to non-local, enhanced VCD features, most significantly in the carbonyl region of the spectrum. The VCD-enhanced signal is ascribed to a helical arrangement of the oscillators in the crystal layers. No structural irregularities need to be considered to explain the amplification, but a crucial point lies in the polarization of charge, which requires an accurate description of the electronic structure. Delivering a quantitative atomic conception of supramolecular chirality induction, our ab initio scheme is applicable well beyond molecular crystals, for example, to address VCD in proteins and related compounds.

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. As 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 relies on the velocity representation of VCD and the time-correlation function, whose symmetry properties are exploited to reconnect the periodic model with the finite physical system. It restores the gauge freedom of finite models, but still allows fully accounting for non-local spatial coupling terms 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. While the general workflow to compute solid-state VCD spectra relies on the interplay of experimental data and theoretical simulation, expressions of VCD in liquid state and for isolated systems can be derived as simplification of the general equations.

Solvation effects are essential for defining the shape of vibrational circular dichroism (VCD) spectra. Several approaches have been proposed to include them into computational models for calculating VCD signals, in...

We investigated the red absorbing, dark stable state (Pr state) of the second GAF domain of the cyanobacteriochrome AnPixJ (AnPixJg2) by a molecular dynamics simulation of 1 μs duration. Our results reveal two distinct conformational isoforms of the chromophore, from which only one was known from crystallographic experiments. The interconversion between both isoforms is accompanied by alterations in the hydrogen bond pattern between the chromophore and the protein and the solvation structure of the chromophore binding pocket. The existence of sub-states in the Pr form of AnPixJg2 is supported by the results from experimental C MAS NMR spectroscopy. Our finding is consistent with the observation of structural heterogeneity in other cyanobacteriochromes and phytochromes.

The established quantum cluster equilibrium (QCE) approach is refined and applied to Nmethylformamide (NMF) and its aqueous solution. The QCE method is split into two iterative cycles: one which converges to the liquid phase solution of the QCE equations and another which yields the gas phase. By comparing Gibbs energies, the thermodynamically stable phase at a given temperature and pressure is then chosen. The new methodology avoids metastable solutions and allows a different treatment of the mean-field interactions within the gas and liquid phases. These changes are of crucial importance for the treatment of binary mixtures. For the first time in a QCE study, the cis-trans-isomerism of a species (NMF) is explicitly considered. Cluster geometries and frequencies are calculated using density functional theory (DFT) and complementary coupled cluster single point energies are used to benchmark the DFT results. Independent of the selected quantum-chemical method, a large set of clusters is required for an accurate thermodynamic description of the binary mixture. The liquid phase of neat NMF is found to be dominated by the cyclic trans-NMF pentamer, which can be interpreted as a linear trimer that is stabilized by explicit solvation of two further NMF molecules. This cluster reflects the known hydrogen bond network preferences of neat NMF. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Solid‐State Vibrational Circular Dichroism (VCD) can be used to determine the absolute structure of chiral crystals, but its interpretation remains a challenge in modern spectroscopy. In this work, we investigate the effect of a twofold screw axis on the solid‐state VCD spectrum in a combined experimental and theoretical analysis of P21 crystals of (S)‐(+)‐1‐indanol. Even though the space group is achiral, a single proper symmetry operation has an important impact on the VCD spectrum, which reflects the supramolecular chirality of the crystal. Distinguishing between contributions originating from molecular chirality and from chiral crystal packing, we find that while IR absorption hardly depends on the symmetry of the space group, the situation is different for VCD, where completely new non‐local patterns emerge. Understanding the two underlying mechanisms, namely gauge transport and direct coupling, will help to use VCD to distinguish polymorphic forms.

We present a computational study of vibrational circular dichroism (VCD) in solutions of (S)-lactic acid, relying on ab initio molecular dynamics (AIMD) and full solvation with bulk water. We discuss...

The vibrational spectrum of the alanine amino acid was computationally determined in the infrared range of 1000-2000 cm−1, under various environments encompassing the gas, hydrated, and crystalline phases, by means of classical molecular dynamics trajectories carried out with the AMOEBA polarizable force field. An effective mode analysis was performed in which the spectra are optimally decomposed into different absorption bands arising from well-defined internal modes. In the gas phase, this analysis allows to unravel the significant differences between the spectra obtained for the neutral and zwitterionic forms of the amino acid. In condensed phases, the method provides invaluable insight into the molecular origins of the vibrational bands and further shows that peaks with similar positions can be traced to rather different molecular motions.