Suitability of different levels of theory for modelling of hydroxamic acids

Abstract

As the starting point for the modelling of more complex cases, we determined the ability of the semiempirical AM1 method and density functional theory (B3LYP) to describe the structural, conformational and thermodynamic properties of acetohydroxamic acid at a level comparable to MP2. We found that AM1 is unable to reproduce the intramolecular hydrogen bond of the molecule as well as the conformational behaviour. On the other hand, the structure and harmonic frequencies obtained at the DFT and MP2 levels are quite similar. However, for these two last methods the situation is dramatically different with respect to the inversion and rotation barriers. Here, the DFT results differ from the MP2 values by 50% for the nitrogen inversion and 32% for the methyl rotation, decreasing to 4% for the OH rotation. A similar behaviour is observed in the N-substituted compound CH3CONCH3OH. Determination of the anharmonic energy levels for the methyl group torsion in the acetohydroxamic acid shows clear differences between the DFT and MP2 results. This fact is important from the spectroscopic point of view, but also for the statistical computation of thermodynamic properties. Another interesting result relates to the time of calculation. Our data show that the relative time efficiency (wall time) of DFT respect to MP2 depends strongly on the RAM memory available, as a consequence of the asymptotic temporal and spatial complexity of the algorithms involved. (C) 2000 Elsevier Science B.V.