Anharmonic vibrational properties of CH2F2: a comparison of theory and experiment

A b initio theoretical chemistry is used to provide a complete understanding of the infrared spectroscopy of CH2F2. Second-order Mo/ller–Plesset perturbation theory (MP2) with a 631G extended basis set is used to provide a quartic expansion of the potential energy surface and a cubic expansion of the dipole surface. Standard perturbation theory is then used to determine effective vibrational and rotational Hamiltonians for fundamentals, selected overtones, and combination bands. Effects of Fermi resonance, Darling–Dennison resonance, and Coriolis resonance are included by matrix diagonalization. Empirical (x,K) relations are used to demonstrate that the anharmonic constants for C–H are in good agreement with those determined from CH2Cl2. The local mode nature of the CH overtones is demonstrated. Important resonances are found to be (ν3,2ν4), (ν8,ν4+ν9), and (ν1,2ν2,2ν8,ν4+ν8+ν9, 2ν4+2ν9,ν3+2ν9). Rotational constants, quartic and sextic centrifugal distortion constants, vibration rotation interaction constants, and Coriolis constants are all in good agreement with the mass of experimental data. The signs of the dipole moment derivatives are in agreement with those deduced from experiment. The separate contributions to the infrared intensities from electrical, mechanical, and mixed anharmonicity are examined for fundamentals and overtones, but by far the most important effect arises from corrections due to resonant Fermi and Darling–Dennison interactions. In this way, the 2ν8, ν1 and ν6 experimental bands and their intensities are explained by assigning ν1 and ν6 as (ν1,ν4+ν8+ν9) and (ν6,2ν2) doublets, respectively. This paper therefore demonstrates that state of the art quantum chemistry can provide a complete interpretation of such spectroscopic data.