Evaluating the\nImportance of Conformers for Understanding\nthe Vacuum-Ultraviolet Spectra of Oxiranes: Experiment and Theory

Vacuum-ultraviolet\n(VUV) absorption spectroscopy enables\nelectronic\ntransitions that offer the unambiguous identification of molecules.\nAs target molecules become more complex, multifunctional species present\na great challenge to both experimental and computational spectroscopy.\nThis research reports both experimental and theoretical studies of\noxiranes. Computationally, the nuclear ensemble approach has been\nused to accurately predict experimental spectra for a variety of molecules.\nHowever, this approach incurs great computational cost, as ensembles\ngenerally consist of thousands of geometries. The present study aims\nto drastically reduce the ensemble by evaluating the significance\nof the conformers to the predicted spectra. This approach was applied\nto 11 substituted oxiranes using the Conformer Rotamer Ensemble Sampling\nTool (CREST) of Grimme to generate an ensemble of unique conformers\ndetermined by their Boltzmann populations. Five TD-DFT functionals\n(BMK, CAM-B3LYP, M06-2X, MN15, ωB97X-D) and EOM-CCSD were used\nto simulate the spectrum of each substituted oxirane ensemble. Computed\nspectra were then compared to the experiment using both qualitative\nand quantitative metrics. Based on these metrics, it was observed\nthat certain conformers may not be necessary to characterize this\nset of oxiranes despite the temperature (323 K) of the experiment.\nA single conformer can then be used with TD-DFT and EOM-CCSD to replicate\nthe experimental spectra of these medium-sized combustion species.