Basis set selection for the calculation of the IR fundamental intensities for 1,1-C₂H₂F₂ and F₂CO
2012
Silva, Arnaldo F. | Soares, Daniely X. | Faria, Sergio H.D.M. | Bruns, Roy E.
The theoretical calculation of the CC, CO and the CF₂ asymmetric stretching infrared intensities of the isoelectronic 1,1-C₂H₂F₂ and F₂CO molecules has resulted in large errors varying from 41.0 to 60.5kmmol⁻¹ or 13.5% to 28.0% of their experimental intensities when using extended 6-311++G(3d,3p) basis set wave-functions at the Moller–Plesset 2 level. For this reason new theoretical intensity results are reported using wave-functions calculated with a large variety of basis sets at both the MP2 and QCISD levels. Accurate intensities were obtained with wave-functions formed from basis sets with polarization functions but without diffuse functions, 6-31G(d,p), 6-31G(2d,2p), 6-31G(3d,3p), 6-311G(d,p) and 6-311G(3d,3p) at both electron correlation levels. Best results for these bands were obtained with the 6-31G(2d,2p) wave-function at the QCISD level providing errors less than 10kmmol⁻¹ and 3.4% of the experimental values. The rms errors for all the 1,1-C₂H₂F₂ and F₂CO intensities at this level are 12.3 and 6.5kmmol⁻¹, respectively. These wave functions at both electron correlation levels also resulted in accurate values for the fundamental intensities of trans-C₂H₂F₂ and cis-C₂H₂F₂ with rms errors of less than 10kmmol⁻¹. Theoretical calculations of the infrared intensities of the 1,1-C₂HDF₂ isotopomer indicate that the experimental determination of the CH symmetric stretching intensity of 1,1-C₂H₂F₂ is probably over-estimated owing to severe band overlap and/or Fermi resonance with the ν₂+ν₃ combination band.
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