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

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.

IVR Dynamics of Vibrational Levels of the ν1 Mode in (CF3)2C═C═O Molecules Excited by Resonant IR Femtosecond Radiation.

The intramolecular dynamics of vibrational levels (up to v = 5) of the ν1 mode in the (CF3)2CCO molecule that is induced by a multiphoton selective excitation of this mode by resonant femtosecond IR radiation has been studied. The times of intramolecular vibrational energy redistribution (IVR) from each vibrational level to remaining molecular modes have been determined. In accordance with theoretical predictions, a decrease in the IVR time with increasing quantum number v has been observed for the first time. A sharp decrease in the IVR time (down to 1.5 ps) at a wavelength of 2129 cm-1, corresponding to the v = 3 → v = 4 vibrational transition, is revealed. It has been shown that, with a negative chirp of a femtosecond radiation pulse, the population of high-lying vibrational levels of the ν1 mode increases significantly.

Intramolecular vibrational dynamics in free polyatomic molecules with C═O chromophore bond excited by resonant femtosecond IR laser radiation.

In nine polyatomic molecules, we have studied the intramolecular redistribution of vibrational energy from chromophore C═O group excited by a resonant femtosecond IR laser radiation at a wavelength of ∼5 µm. All experiments have been performed in the gas phase using the IR-IR pump-probe technique in combination with the spectral analysis of the probe radiation. For molecules with one C═O end group, characteristic times of intramolecular vibrational redistribution (IVR) lie in the range between 2.4 and 20 ps and correlate with the density of four-frequency Fermi resonances. The IVR times in metal carbonyl molecules are anomalously long, being ∼1.0 ns for Fe(CO)5 and ∼1.5 ns for Cr(CO)6. In the CH3(C═O)OC2H5 and H2CCH(C═O)OC2H5 molecules, it has been observed that there are two characteristic IVR times, which differ by an order of magnitude from each other; this was interpreted in terms of the developed model of "accumulating states". For the ICF2COF molecule, it has been revealed that the IVR time decreases with increasing level of the vibrational excitation of the C═O bond of the molecule.