Laboratory of Molecular Spectroscopy, Institute of Chemistry, Eötvös University, PO Box 32, H-1518, Budapest 112, Hungary.
Phys Chem Chem Phys. 2011 Aug 21;13(31):13972-84. doi: 10.1039/c1cp20797k. Epub 2011 Jun 23.
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).
对 S-(-)和 R-(+)-2-氯丙酸的振动圆二色性（VCD）光谱进行了彻底分析。除了单体的 VCD 光谱外，还通过实验（在溶液中和低温 Ar 基质中）和量子化学计算研究了二聚体（通过强氢键稳定）和 2-氯丙酸-CHCl(3)配合物（通过弱氢键稳定）。结果表明，二聚体的形成会剧烈改变，甚至弱配合物的形成也会对 VCD 光谱的整体形状产生实质性影响。目前和以前的结果可以推广到通过 VCD 光谱学测定羧酸的绝对构型的实践中。对于这些测量，如果体积较大的基团不阻止二聚体的形成，建议将计算出的二聚体光谱与在相对较浓（约 0.1 mol dm(-3))溶液中记录的实验光谱进行比较。我们的研究还表明，由于单体的稳定化和/或弱配合物的形成，在 CHCl(3)中记录的 VCD 光谱更加复杂，并且与本案例一样，其强度可能低于在 CCl(4)中记录的光谱。因此，如果溶解度允许，CCl(4)是比 CHCl(3)更优选的溶剂。
