title: Dynamics of circular hydrogen bond array in calix[4]arene in a nonpolar solvent: a nuclear magnetic resonance study.
abstract: Hydroxyl groups on the lower rim of calix[4]arene form a circular array of four equivalent hydrogen bonds. The rate constants of reversal of the array in the temperature range of 221-304 K were determined by means of the NMR measurements of quaternary (13)C nuclear spin transverse relaxation dependence on the effective radio frequency field. The flip-flop rate constants are in the range of 1.4 x 10(2)-4.2 x 10(4) s(-1), the activation enthalpy is 36.8 kJ/mol, the activation entropy is -36 J mol(-1) K(-1). This process was found uncorrelated with conformational transition cone-inverted cone, which is about thousand times slower. Molecular tumbling of calix[4]arene measured using (13)C spin relaxation was found isotropic with correlation times lying in the range of 0.1-3 ns and with the activation energy of 21 kJ/mol. In order to assess relaxation of (13)C aromatic nuclei, their principal components of chemical shift tensor were calculated using the density functional theory approach.
authors.affiliation: Laboratory of NMR Spectroscopy, Institute of Chemical Technology, Technická 5, CZ-16628 Prague 6, Czech Republic. Jan.Lang@vscht.cz
authors.forename: Jan
authors.initials: J
authors.lastname: Lang
authors.forename: Veronika
authors.initials: V
authors.lastname: Deckerová
authors.forename: Jirí
authors.initials: J
authors.lastname: Czernek
authors.forename: Pavel
authors.initials: P
authors.lastname: Lhoták