Theoretical study of spectroscopic constants and anharmonic force field of formaldehyde

The equilibrium geometries of formaldehyde are optimized with B3LYP, B3PW91 and MP2 methods employing three basis sets 6-311++G(2d,2p), aug-cc-pVTZ and cc-pVTZ, respectively, which agree well with the corresponding experimental and previous theoretical data. The best optimized geometries are obtained at the theoretical level B3LYP/6-311++G(2d,2p) basis set. Basing on the calculated equilibrium geometries, the spectroscopic constants and anharmonic force field of H2CO are investigated. The results show that DFT method is superior to MP2 method at the calculation of spectroscopic constants and force constants of H2CO. The vibration-rotation interaction constants and fundamental vibrational wave numbers of H2CO are firstly theoretically calculated. The Coriolis coupling constants, cubic force constants and most of quartic force constants are firstly theoretically predicted.