Effects of strong and weak hydrogen bond formation on VCD spectra: a case study of 2-chloropropionic acid

Abstract

The vibrational circular dichroism (VCD) spectrum of S-(-) and R-(+)-2-chloropropionic acid is thoroughly analyzed. Besides the VCD spectrum of the monomer, the dimers (stabilized by strong hydrogen bonds) and the 2-chloropropionic acid-CHCl(3) complexes (stabilized by a weak hydrogen bond) are studied both experimentally (in solution and in low-temperature Ar matrix) and by quantum chemical computations. It is shown that dimer formation drastically changes, and even weak complex formation can also substantially affect the overall shape of the VCD spectrum. The present and previous results can be generalized for the practice of absolute configuration determination of carboxylic acids by VCD spectroscopy. For these measurements, if bulky groups do not block dimer formation, comparison of the computed spectra of the dimers with the experimental spectra recorded in relatively concentrated (∼0.1 mol dm(-3)) solutions is suggested. Our study also shows that due to the stabilization of monomers and/or the formation of weak complexes, the VCD spectrum recorded in CHCl(3) is more complex and, like in the present case, can have a lower intensity than that of the spectrum recorded in CCl(4). Therefore, if solubility allows, CCl(4) is a much preferred solvent over CHCl(3).

Vibrational Circular Dichroism (VCD) Reveals Subtle Conformational Aspects and Intermolecular Interactions in the Carnitine Family

“…This motivated us to build models of D‐acetyl carnitine ( D-3 ), either surrounded by CD 3 OD explicit solvent molecules or to consider a dimer of two D-3 molecules tied by a pair of hydrogen bonds (HB), still within the PCM approximation. Numerous models have appeared in the literature dealing with this kind of system and we refer to four recent papers, adopting a similar approach as the one reported here. The results are given in Figure for one, two, and three explicit CD 3 OD solvent molecules (in the first three squares from the left) and also for a D-3 dimer (at the extreme right): since HB reduces the strength of the C=O stretching force constant, we predict also lower C=O stretching frequency associated with the COOD moiety, together with still a high C=O stretching frequency associated with the COOCH 3 moiety.…”

VCD Robustness of the Amide‐I and Amide‐II Vibrational Modes of Small Peptide Models

“…Since the rotatory strength (i.e., the intensity) of a VCD band is proportional to the scalar product of these values, angles close to 90° were supposed to indicate that even very small structural changes or small external perturbations can result in sign change for these vibrational modes. As shown later by the present authors, and Nicu and Baerends, this angle is gauge‐dependent, and, for example, this can lead to wrong classification of vibrational modes of large molecules, e.g., that of longer peptide chains. To overcome this problem an alternative gauge‐independent measure of the robustness has recently been suggested by some of the present authors, which utilizes the ratio between the rotational and the dipole strength, ζ k = R k /D k .…”

On the Complementarity of ECD and VCD Techniques

“…(As illustrated in fig. 1 in ESI, the differences observed among the VCD spectra of the conformers in the VCD family 2 are a consequence of the fact that conformers 260 and 320, unlike the rest of the conformers in this VCD family, exhibit a weak (nonclassical) intramolecular H bond . As shown in fig.…”