The Impact of Molecular Symmetry and Rigidity on the Selective Analysis of VOCs by Mid-IR Laser Spectroscopy

This study highlights the potential of mid-infrared laser absorption spectroscopy (LAS) for analyzing volatile organic compounds (VOCs) in the gas phase. Using a Vernier-type quantum cascade laser (QCL) in the 9–10 μm spectral region, over 40 different VOCs were systematically investigated with a spectral resolution better than 0.005 cm^–1. It was found that many VOCs, even with more than six heavy atoms, can exhibit significant spectral features at ambient temperature and pressures above 10 mbar, i.e., at conditions well suited for routine analysis. Our observations suggest that in addition to molecular weight and number of atoms, molecular rigidity and symmetry have a decisive impact on the spectral feature content. The high spectral feature content observed in rigid molecules is related to their small number of conformers, while higher molecular symmetry results in fewer nondegenerate vibrational modes, principal rotational axes with different moments of inertia, and a reduced number of nonidentical conformers. Overall, this leads to distinct spectral features that enable the selective and sensitive detection of large VOCs by high-resolution LAS.