G. Bauer, N. Trendafilova, Tz Mihaylov:
"DFT and AIM studies of intramolecular hydrogen bonds in dicoumarols";
Chemical Physics, 302 (2004), 1-3; S. 95 - 104.

Kurzfassung englisch:
Density functional calculations with Becke´s three parameter hybrid method using the correlation functional of Lee, Yang and
Parr (B3LYP) were carried out for 3,30-benzylidenebis(4-hydroxycoumarin) (phenyldicoumarol, PhDC), 3,30-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 A. 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 (qb) and Laplacian
(r2qb) properties, estimated by AIM calculations, showed that both O H bonds have low qb and positive r2qb values (consistent
with electrostatic character of the HBs), whereas both O-H bonds have covalent character (r2qb < 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 pdelocalization
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 m(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).

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