Detailed Assignment of the CH Chromophores in Methyl Mandelate and Mandelic Acid: A Multi-Experimental Approach Using Polarized FTIR Microspectroscopy of Sublimated Crystals

A sublimation cell for the characterization of rnicrocrystals using Fourier transform infrared (FTIR) microspectroscopy was constructed and applied to methyl mandelate and mandelic acid. It was possible to assign different CH chromophores in the FTIR spectra. Comparison to quantum chemical calculations, attenuated total reflectance infrared (ATR-IR) spectra of related compounds, and isotope labeling was used to support the assignments. Vibrational transition dipole vectors were deduced from crystal rotation and polarization experiments. The direction of these vectors can be used to constrain the absolute orientation of the molecules in the crystal as well as for the assignment of further vibrational bands.