Aggregation of lactic acid in cold rare-gas matrices and the link to solution: a matrix isolation-vibrational circular dichroism study

Abstract

The matrix isolation (MI) technique has been utilized with vibrational circular dichroism (VCD) spectroscopy to obtain MI-VCD spectra of lactic acid (LA) in cold argon matrices, in addition to their MI-IR spectra. The experiments have been done at three different deposition temperatures (10 K, 16 K and 24 K) under different Ar flow rates so that different degrees of LA self-aggregation occur. The structural and spectral investigations of the LA monomer and the larger (LA)2,3,4 aggregates have been undertaken at three levels of theory (B3LYP/6-311++G(2d,p), B3LYP-D3BJ/6-311++G(2d,p) and B3LYP-D3BJ/def2-TZVPD) to evaluate the effects of dispersion correction and basis sets on optimized structures, relative conformer energies, and IR/VCD spectral features. Interestingly, the relative conformer energies vary considerably with and without dispersion correction, especially when the molecule gets larger and when it is placed in solution. Such uncertainties in the relative energies and in the vibrational band positions and IR/VCD intensities highlight the challenges in interpreting experimental spectroscopic data, especially those obtained in solution. With the narrow MI-IR band width and highly characteristic MI-VCD spectral features and the trend observed at three temperatures, we have been able to correlate the spectral features confidently to those of the LA monomer and the larger (LA)2,3,4 aggregates, with the aid of theoretical modeling. Finally, by noting the similarity of MI-IR and especially MI-VCD features obtained at 24 K with those of the 0.2 M solution, and with the aid of spectral simulation at the B3LYP-D3BJ/def2-TZVPD level, a composition of LA aggregates dominated by the LA tetramer and trimer has been identified. This conclusion differs from the previous reports where the LA dimer was identified as the main species at even higher concentration in CDCl3. The present work showcases the power of MI-VCD spectroscopy in aiding solution spectral assignment and in providing insight into the complex self-aggregation behavior of LA in solution.

Matrix Isolation‐Vibrational Circular Dichroism Spectroscopic Study of Conformations and Non‐Covalent Interactions of Tetrahydro‐2‐Furoic Acid

“…It is interesting to comment on the very different aggregation behavior of lactic acid which was also studied under cryogenic matrix conditions using MI–IR and VCD spectroscopy [17] . In the lactic acid case, the cis ‐COOH configuration is strongly favored in its monomeric form.…”

“…Interestingly, the dominant dimeric structures identified are very different from those in solution [10] and have not been observed experimentally before for THFA. The current results are also compared to a previous MI–VCD study of lactic acid [17] to highlight the diverse aggregation behavior of lactic acid and THFA in matrices under cryogenic conditions.…”

Competition between inter and intramolecular hydrogen bond evidenced by vibrational circular dichroism spectroscopy: The case of (1S,2R)‐(−)‐cis‐1‐amino‐2‐indanol

“…H-bonding in particular strongly impinges the shape of VCD spectra. 17 The influence of H-bond formation is observed in the solid state where strong intermolecular interactions take place [18][19][20][21] and also in matrix isolation spectroscopy [22][23][24][25][26] or in solution studies. [27][28][29] The VCD spectrum of rigid systems in solution can be reproduced by the contribution of a limited number of structures.…”

Computation of Solid-State Vibrational Circular Dichroism in the Periodic Gauge

“…23 We recently published a survey on different computational tools to describe conformational sampling. 24 Typically, VCD is measured in liquid state or in or in isolated conditions such as noble gas matrices, [26][27][28][29][30] which is a largely standardised procedure, but requires sample solubility and may exhibit spurious solvent effects. However, in analogy to IR absorption, VCD can also be measured conveniently in solid state.…”

“…We recently published a survey on different computational tools for the description of conformational sampling . Typically, VCD is measured in liquid state or in isolated conditions such as noble gas matrices, − which is a largely standardized procedure. However, the measurements in solution are limited by sample solubility and may exhibit spurious solvent effects.…”