DFT and AIM studies of intramolecular hydrogen bonds in dicoumarols

Density functional calculations with Becke's three parameter hybrid method using the correlation functional of Lee, Yang and Parr (B3LYP) were carried out for 3,3-benzylidenebis(4-hydroxycoumarin) (phenyldicoumarol, PhDC), 3,3'-methylenebis(4-hydroxycoumarin) (dicoumarol, DC) and the parent compound, 4-hydroxycoumarin (4-HC). Different basis sets were tested in the course of the calculations: 6-31G*, 6-31+G** and 6-311G*. In full agreement with available X-ray data, B3LYP/6-31G* calculations of the lowest-energy conformer, PhDC showed two O-H...O asymmetrical intramolecular hydrogen bonds with O...O distances 2.638 and 2.696 Angstrom. The HB energies in PhDC were estimated of -55.46 and -52.32 kJ/mol, respectively. The values obtained correlated with the calculated and experimental O...O distances and predicted difference in the hydrogen bonding strengths in PhDC. The total HB energy in PhDC was calculated of -107.73 kJ/mol. At the same level of theory, both O...O intramolecular distances in DC were calculated identical (2.696 A) and thus two symmetrical hydrogen bondings were obtained. The single HB strength was estimated of -50.89 kJ/mol and the total one of -101.79 kJ/mol. The electron density (rho(b)) and Laplacian (del(2) rhob) properties, estimated by AIM calculations, showed that both O...H bonds have low rhob and positive del(2) rhob values (consistent with electrostatic character of the HBs), whereas both O-H bonds have covalent character (del(2) rhob < 0). Natural population analysis data for PhDC, DC and 4-HC were used to predict electrostatic interactions in the exocyclic rings. The calculated oxygen natural charges were found to correlate with the O...O distances in PhDC and DC. On the basis of the calculated bond ellipticity, the pi-delocalization in the exocyclic rings of PhDC and DC was estimated. The results thus obtained helped to describe the nature of the intramolecular O...H-O bonds and the forces driving their formation. Vibrational study of PhDC at B3LYP/6-31G* optimized geometries showed higher negative (red) shifts for the nu(O-H) of the O-H involved in the stronger HB (-610 cm(-1)) in comparison with that obtained for the second O-H which forms the weaker HB in PhDC (-559 cm(-1)). (C) 2004 Elsevier B.V. All rights reserved.