The molecular and electronic structures of the halogen pseudohalides, a comparison of ab initio and DFT calculated structural and vibrational results with spectroscopic data

The ground vibrational state and equilibrium structures of the halogen (F to I) azides, isocyanates and isothiocyanates have been determined, and compared with spectral and diffraction data. We present results from Möller–Plesset (MP2) and DFT (B3LYP) methodologies, with similar basis sets (TZVP and cc-pVTZ), such that detailed comparisons of each with experiment is possible. In the case for iodine, this necessitated the generation of a new basis set. The bond lengths and angles for both MP2 and B3LYP methodologies are compared, and in the majority of cases, the MP2 series is marginally closer to experiment than B3LYP. The harmonic and anharmonic frequencies have been compared with experimental vibrational spectral data using both methodologies. In this case, the B3LYP method produces anharmonic and harmonic frequencies closer to experimental data, but both methods and both series of calculated frequencies scale well against the experimental data.