Vibrational analysis of the spectra of 1,3,4-oxadiazole, 1,3,4-thiadiazole, 1,2,5-oxadiazole and 1,2,5-thiadiazole: Comparison between DFT, MP2 and HF force fields

Ab initio harmonic force fields were calculated at the corresponding optimized geometries for 1,3,4-oxadiazole, (OD3), 1,3,4-thiadiazole (TD3), 1,2,5-oxadiazole (OD2) and 1,2,5-thiadiazole (TD2) with the Becke3LYP (B3LYP) density functional theory (DFT) using the 6-31G** basis set. The DFT force fields were scaled where the experimental frequencies were assigned to the calculated frequencies similar to that previously reported for the studied molecules with the second order Moller-Plesset (MP2) and the Hartree-Fock (HF) force fields using the same basis set, 6-31G**. The calculated spectra with the DFT/6-31G** force fields were in better agreement with the experimental spectra than those calculated with the MP2/6-31G** or HF/6-31G** force fields. In addition, the scale factors obtained with DFT force fields are better correlated and less divergent than those obtained with the MP2 or HF force fields. The optimized geometries at the DFT/6-31G** level have poor prediction of the S-C and N-S bond lengths and CSC and NSN bond angles which is reflected such that the root-mean-square (rms) deviation of the calculated frequencies from the experimental frequencies are worse for TD3 and TD2 than for OD3 and OD2 but generally better than those obtained with the MP2/6-31G** or HF/6-31G** force fields. The rms deviations obtained with the DFT force fields are about half those obtained with the HF force fields. In the view of the excellent results obtained by the scaled DFT force fields for the four molecules, scaled force fields calculated with the DFT method are preferred to be used for vibrational analysis rather than with the HF or MP2 methods.