Basis set selection for the calculation of the IR fundamental intensities for 1,1-C2H2F2 and F2CO

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 60.5 km mol−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 10 km mol−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 6.5 km mol−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 10 km mol−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.

► Calculations were carried out with 22 different basis sets at MP2 and QCISD levels.
► The basis set providing the most accurate results was simple, a 6-31G(2d,2p) basis.
► At the QCISD level calculated errors for this basis were less than 10 km mol−1.
► Errors between 41 and 61 km mol−1 were reported for MP2/6-311G++(3d,3p) basis set.
► The 6-31G(2d,2p) basis set also provided accurate results for cis and trans isomers.