The Vibrational Circular Dichroism Pattern of the ν(C=O) Bands in Isoindolinones

The IR and vibrational circular dichroism (VCD) spectra of both enantiomers of Me-, iPr-, nBu-, Ph-, and CH₂ Ph-substituted isoindolinones in solution and KBr pellets were measured and interpreted by DFT calculations. The spectra in solution revealed no important differences in the C=O stretching vibration region while the interpretation of very distinct spectra taken in pellets required determining the crystal structures. The studied compounds crystallized in the P2₁ 2₁ 2₁ (Me, iPr, CH₂ Ph), P3₁ (nBu), and P2₁ (Ph) space groups. We found that the quality of simulated spectra strongly depends on the substituent, the structure of the molecular cluster assumed, basis set, and use of the dispersion correction. The IR spectra can be reproduced well based on the simplest linear arrangement of hydrogen-bonded chains mimicking the molecular arrangement in the crystals. We found no common approach to reproduce all the registered VCD spectra in the crystal phase. For the Me and nBu isoindolinones, the VCD pattern was the best reproduced by full optimization of the selected large molecular clusters. For iPr, Ph and CH₂ Ph derivatives optimizing only the position of H-atoms in a fragment frozen as in the crystal provides the best results. Such an approach can reduce the computation time from months to one week.