Dipole moment, solution, and solid state structure of (–)-epicatechin, a monomer unit of procyanidin polymers

Abstract

The structure of (–)-epicatechin has been determined in the crystalline state. Crystals are orthorhombic, P2₁2₁2₁, a= 670.8(1), b 1 329.1 (3), c= 1 426.2(4) pm, Z= 4, D_c= 1.516 g cm^–3, R= 0.041 for 1 624 observations. Intramolecular hydrogen bonds are absent. The aliphatic hydroxy group and three of the four phenolic hydroxy groups participate in intermolecular hydrogen bond formation. The aromatic ring bonded to C(2) is in an equatorial position, but the hydroxy group bonded to C(3) adopts an axial position. Comparison with the published structure for 8-bromotetra-O-methyl-(+)-catechin reveals that epimerization at C(3) alters the fused ring system primarily at C(2) and C(3). The heterocyclic ring is more closely described as a half-chair in (–)-epicatechin than in 8-bromotetra-O-methyl-(+)-catechin. Dipole moments have been measured for (–)-epicatechin in solution. At 300 K the root-mean-square dipole moment is 3.91 ± 0.04 D, and the temperature coefficient is small and negative. The behaviour of the dipole moment is easily rationalized via a theoretical rotational isomeric state analysis based on the structure found in the crystalline state.