Absolute IR vibrational band intensities of hexafluoroacetone: Comparison of experiment and anharmonic ab initio calculation using the second-order operator canonical Van Vleck perturbation theory

Highlights: • Absolute infrared intensities of fourteen fundamental bands of gaseous hexafluoroacetone were determined in the range of 2000–340 cm⁻¹. • A theoretical ab initio anharmonic vibrational spectrum was calculated by means of operator canonical Van Vleck perturbation theory (CVPT). • Good correspondence was achieved between experimental and theoretical values of IR frequencies and absolute integral intensities. • The ν₄, ν₇, ν₁₀ and ν₁₅ fundamental bands were reassigned, and three predicted Fermi resonances were identified in experimental spectra. Hexafluoroacetone (HFA, OC(CF₃)₂) is widely utilized in organic synthesis, chemistry of heat resistant polymers and biochemistry. In this work, infrared (IR) spectra of gaseous HFA were recorded in the range 2000–340 cm⁻¹ with a resolution of 1 cm⁻¹. For the first time, experimental absolute IR intensities of fourteen fundamental bands of HFA were measured and compared with intensities predicted by anharmonic ab initio calculations. Theoretical molecular structures, semi-diagonal quartic potential energy surfaces (PES), and cubic surfaces of dipole moment (DM) components of HFA were calculated using MP2/cc-pVTZ ab initio quantum-mechanical model. In addition, harmonic frequencies were also calculated at MP2/cc-pVQZ level of theory and used for refining the PES. The numerical-analytic implementation of the second-order operator canonical Van Vleck perturbation theory (CVPT2) was employed for predicting anharmonic IR spectra of HFA. The deviation from observations of predicted IR intensities for ten fundamentals (ν₁, ν₂, ν₈, ν₁₃–ν₂₁) was less than 25% on average. The significant difference between ab initio harmonic intensities calculated with MP2/cc-pVTZ and MP2/cc-pVQZ models for some normal modes indicates that an inaccuracy of the quantum-mechanical model is probably the major source of errors in theoretical intensities. An interpretation of the IR experimental spectra and associated literature data for HFA is accomplished; the ν₄, ν₇, ν₁₀ and ν₁₅ fundamentals were reassigned. Three predicted Fermi resonances were identified in experimental spectra, ν₃/ν₅ + ν₈, ν₁₇/ν₁₀ + ν₁₉ and ν₂₀/ν₉ + ν₂₃. An empirical fit of harmonic frequencies within anharmonic model assisted in a reliable identification of a number of overtones and binary combination tones. The fitted harmonic frequencies were compared with MP2/cc-pVQZ and a simplified complete basis set (CBS) extrapolation. It was found that MP2/cc-pVQZ frequencies were more reliable. The efficiency and convenience of the theoretical model employed and associated techniques for interpretation of complex spectra of HFA are demonstrated.