Vibrational circular dichroism spectra of proline in water at different pH values.

Recording VCD spectra of aqueous solution poses a particular challenge as water is a strong infrared absorber. Likewise, the computational analysis of VCD spectra by means of DFT-based spectral calculations requires the consideration of explicit solvent molecules, thus posing an even greater challenge. Several studies suggested that by modeling the solvent environment with a few water molecules in a micro-solvation approach would be sufficient to describe experimental spectra. For example, using proline at different pH values, we herein show that a change in the relative spatial orientation of a single water molecule in five-fold solvated structures strongly affects the computed VCD spectral signatures and that Boltzmann-weighted spectra do not correctly reproduce the experiment. We thus explored an approach based on molecular dynamics and subsequent DFT-calculations, in which we considered 30 water molecules (about 1.5 solvation shells). Once again, it was found that the Boltzmann-weighted spectra obtained on the basis of several hundred structures did not correctly reproduce experimental signatures, and a simple averaging scheme resulted in well-matching spectra with comparable bandwidths. The rationale behind the procedure was that sampling the configurational space of the solvent molecules is as equally important as the conformational sampling of the solute. For conformationally more flexible molecules, it is assumed that a much larger set of structures will have to be computed in order to properly sample the conformational space of both solute and solvent.

Stereochemistry Controls Dihydrogen Bonding Strengths in Chiral Amine Boranes Adducts.

The growing interest in exploiting novel concepts of non-covalent interactions in catalysts and supramolecular chemistry made us revisit a special kind of hydrogen bonding: the dihydrogen bond (DHB), formed between a classical hydrogen bond donor and a hydridic hydrogen as acceptor. Herein, we investigate how the strength of the N-Hδ+ ⋅⋅⋅δ- H-B interaction and hence the DHB-driven self-aggregation of amine-borane adducts is governed by steric effects by comparing the structures and binding enthalpies of various chiral derivatives. For a diastereomeric pair of amine-boranes prepared from a chiral secondary amine, we show that the stereochemistry at the nitrogen has significant influence on the interaction enthalpy. Based on this finding, N-chiral amine boranes can be envisioned to become interesting building blocks in supramolecular chemistry to fine-tune the formation dynamics of assemblies.