Comparative study between the X-ray, DFT and Møller-Plesset structure of solid phenoxyacetohydrazide

The structure of phenoxyacetohydrazide was investigated at the B3LYP/6-311G^**, MP2/6-311G^** and MP4(SDQ)/6-311G^** levels of theory and compared to the corresponding structure obtained from the X-ray study. At the DFT-B3LYP level, the planar Ttcp conformation of phenoxyacetohydrazide was predicted to be about 0.7 kcal/mol lower in energy than the non-planar Tgcnp structure. At the MP2 and the MP4(SDQ) levels, a reverse conformational behavior was reported where the planar form was predicted to be about 1.3 and 0.5 kcal/mol lower in energy than the non-planar form, respectively. A drastic difference in the COCC torsional angle of about 90° was predicted between the X-ray and the computed ground state structure. The X-ray structure (near-planar Ttcnp) of the molecule was then optimized at the three levels of theory. At the DFT-B3LYP level, the near-planar Ttcnp structure was predicted to turn into the planar Ttcp structure. At the two Møller-Plesset levels the Ttcnp structure was found to be in excellent agreement with the X-ray structure but being 0.5-1.3 kcal/mol higher in energy than the computed lowest energy Tgcnp form. The computed vibrational frequencies of the near-planar structure at the B3LYP/6-311G^** level of calculation were combined with experimental infrared and Raman data to provide tentative vibrational assignments of the molecule.