Overtone spectroscopy of v(C=O) stretching vibration of hexafluoroacetone: Experimental and ab initio determination of peak positions, absolute intensities, and band shapes

Abstract

Infrared overtone spectra of the ν(C = O) stretching vibration (ν1) of gaseous hexafluoroacetone ((CF3)2C = O, HFA) were recorded in the spectral range of 7450-3300 cm-1 with a resolution of 0.1 cm-1. Experimental absolute IR intensities and vibrational band centers of the overtones 2ν1, 3ν1, 4ν1 of HFA were measured and compared with their ab initio counterparts, calculated by the second-order canonical vibrational perturbation theory (CVPT2). A hybrid potential energy surface (PES) was evaluated using the quantum-mechanical models MP2/cc-pVQZ for harmonic and MP2/cc-pVTZ for anharmonic parts. Cubic surfaces of dipole moment components were determined using the MP2/cc-pVTZ model. The predicted IR intensities for the first and second overtones reproduced the experimental values with a discrepancy of 6% and 9%, respectively. A weak Fermi resonance between the first overtone 2ν1 and combination tone ν1 + ν2 + ν8 was predicted. The appropriate model was employed for simulating the bands studied using: (i) the asymmetric-top vibration-rotational structure of the ν1 mode, (ii) the inhomogeneous band structure due to contributions of the vibrational states populated at the room temperature, and (iii) the homogeneous broadening of each vibration-rotation transition due to the intramolecular vibrational redistribution (IVR). The rotational and vibrational anharmonic constants were taken from the ab initio calculations, whereas the IVR data were obtained from the experimental data of Chekalin et al. (JPCA 2014, 118:955) with a time resolution of ≈100 fs. A good level of agreement of the predicted shapes of bands studied with experiment is achieved.