Vibrational Raman optical activity of diacetyl L-tartaric acid and corresponding surfactants: Sodium salts and shorter analogs of surfactants simplify the interpretations

Experimental vibrational Raman and Raman optical activity (ROA) spectra for diacetyl L-tartaric acid (DAT), two of its esters, namely, monomethyl and lauryl esters (T1OH and T12OH), and corresponding sodium salts (DATNa, T1ONa, and T12ONa), are measured. T12OH and T12ONa represent the first chiral surfactants investigated using ROA spectroscopy. The quantum chemical (QC) predictions using B3LYP functional and 6-311++G(2d,2p) basis set are used to interpret the ROA spectra for DAT, DATNa, T1OH, and T1ONa. It is found that the use of implicit solvation, as represented in polarizable continuum model (PCM), for predicting the experimental ROA spectra in aqueous solutions is inadequate for DAT and T1OH. However, the same PCM predicts the experimental ROA spectra satisfactorily for the DATNa and T1ONa. This favorable observation for the latter is attributed to the absence of intra- and inter-molecular hydrogen bonding interactions for sodium salts in aqueous solutions. The overwhelming number of conformations resulting from 12-carbon alkyl chain, in T12OH and T12ONa, makes it impractical to undertake QC predictions for them. Nevertheless, it is found that the predictions made for shorter alkyl chain analogs, namely, T1OH and T1ONa, may be used to explain the experimental ROA spectra of T12OH and T12ONa. The current work highlights the importance of converting carboxylic acids to corresponding sodium salts and of QC predictions for shorter achiral alkyl chain analogs to interpret the ROA spectra of chiral surfactants that contain long achiral alkyl chains.

Automated summary

Experimental vibrational Raman and Raman optical activity (ROA) spectra were measured for diacetyl L-tartaric acid (DAT), its esters T1OH and T12OH, and corresponding sodium salts DATNa, T1ONa, and T12ONa. Quantum chemical (QC) predictions using the B3LYP functional and 6-311++G(2d,2p) basis set were used to interpret the ROA spectra. Implicit solvation model (PCM) was inadequate for predicting the ROA spectra of DAT and T1OH in aqueous solutions. However, it satisfactorily predicted the ROA spectra for DATNa and T1ONa. The overwhelming number of conformations for T12OH and T12ONa hindered QC predictions, but predictions made for T1OH and T1ONa can be applied to explain the experimental ROA spectra of T12OH and T12ONa. The study highlights the importance of converting carboxylic acids to sodium salts and QC predictions for shorter alkyl chain analogs in interpreting the ROA spectra of chiral surfactants.