Ab initio study of 2,4-dihalosubstituted malonaldehyde and 2-halo-phenols in gas phase and solution

All the possible enol and keto conformations of malonaldehyde and its 2,4-difluoro-, 2,4-dichloro-, and 2,4-dibromo-derivatives were studied at the ab initio level, using the 6-31G** basis set. The correlation and Zero Point Vibration energies were evaluated by means of the MP2 approach and the DFT-B3LYP functional. Calculations were performed also in water and CCl4 solutions, following the SCRF-PCM method. Differently from malonaldehyde, the keto conformations of the halo-derivatives are more stable than the hydrogen-bonded enol ones and very close in energy to each other. The strength of the moderately strong O-H (...) O (E-I rotamer) and of the weak O-H (...) X (E-III rotamer) bridges, evaluated according to the classic method and from the OH internal rotation barriers, is discussed and compared with that of the corresponding bridges of malonaldehyde and ortho-halo-phenols, respectively. For these latter compounds the B3LYP/6-31 I ++G(d,p) basis too was used. (C) 2002 Elsevier Science B.V. All rights reserved.