Structure and conformational equilibrium of thiacalix[4]arene by density functional theory

Density functional theory calculations for the structure and conformational equilibrium of thiacalix[4]arene are reported. The conformational equilibr ium of thiacalix[4]arene, a heterocalixarene in which the phenol groups are bridged by sulphur atoms is compared to the conformational equilibrium of calix[4]arene. Thiacalix[4]arene conformational energies relative to the co ne conformer (AE's) are reduced in comparison with calix[4]arene. This conf ormational change is in qualitative agreement with recent NMR spectroscopy measurements of the conformational equilibrium for a tetraethylether of thi acalix[4]arene in a CDCl3 solution which indicates an enhanced chemical exc hange of thiacalixarene conformers in comparison with similar methylene bri dged structures. Density functional theory results for the structure of thi acalix[4]arene are in good agreement with recent Xray diffraction measureme nts. The electrostatic potentials in the cone conformers of thiacalix[4]are ne and calix[4]arene suggest that their complexation or recognition abiliti es can be significantly different. Dipole moments of the four thiacalix[4]a rene conformers are in the order: cone > 1,2-alternate > partial-cone > 1,3 -alternate. (C) 2001 Elsevier Science B.V. All rights reserved.