High-order torsional couplings in the infrared spectrum of 3,3,3-trifluoropropene Original Research Article

A 2 MHz resolution electric-resonance optothermal spectrometer and a microwave-sideband CO2 laser have been used with microwave-infrared double resonance to investigate high-order torsional couplings in the 10 μm infrared spectrum of 3,3,3-trifluoropropene. Three normal mode vibrations are studied with band origins at 963.4, 980.2 and 1025.2 cm−1. The 963.4 cm−1 band is well characterized by an asymmetric-top Hamiltonian, except for the presence of a weak perturbation for J′ = 7, Ka′ = 2 affecting only the A-symmetry internal-rotor state. Microwave-infrared double resonance is used to study the microwave spectrum of the perturbing or ‘dark’ state. The observed dark-state K-doublet asymmetry splittings and rotational-state selection rules indicate that the perturbing state has five quanta of excitation in the torsional mode (ν21) built upon the A″ ν19 fundamental. The precise frequency determined for 5 β21 of 421(2) cm−1 leads to the first accurate determination of the barrier to CF3 internal rotation as 641(5) cm−1. In contrast to the 963.4 cm−1 vibration, the 980.2 and 1025.2 cm−1 modes show a large number of J′ and Ka′ perturbations which differentially affect the A and E symmetry internal-rotor states. The magnitude of the perturbation-induced A/E splittings indicate that the perturbing states must have at least four quanta of torsional excitation. The present results suggest that high-order vibrational interactions are important in the vibrational dynamics of molecules at low levels of overall vibrational excitation.