Cyclohexane Vibronic States: A Combined VUV Spectroscopy and Theoretical Study

In this work, we provide results from a joint experimental and theoretical study of the vibronic features of cyclohexane (C₆H₁₂) in the photon energy range of 6.8-10.8 eV (182-115 nm). The high-resolution vacuum ultraviolet (VUV) photoabsorption measurements, together with quantum chemical calculations at the time-dependent density functional theory (TDDFT) level, have helped to assign the major electronic excitations to mixed valence-Rydberg and Rydberg transitions. The C₆H₁₂ photoabsorption spectrum shows fine structure which has been assigned to CH₂ scissoring, v3'a1g, CH₂ rocking, v4'a1g, C-C stretching, v5'a1g, and CCC bending/CC torsion, v24'eg, modes. Molecular structure calculations at the DFT level for the neutral and cationic electronic ground-states have shown the relevant structural changes that are operative in the higher-lying electronic states. Photolysis lifetimes in the Earth's atmosphere are shown to be irrelevant, while the main atmospheric sink mechanism is the reaction with the ^•OH radical. Potential energy curves have been obtained at the TDDFT level of theory, showing the relevance of interchange character mainly involving the CH₂ scissoring, v3'a1g, and CH₂ rocking, v4'a1g, modes, while Jahn-Teller distortion yields weak vibronic coupling involving the non-totally symmetric CCC bending/CC torsion, v24'eg, mode.