Flip-flop motion of circular hydrogen bond array in thiacalix[4]arene

Thiacalix[4]arene contains a circular array of four equivalent hydrogen bonds on its lower rim. The array undergoes a flip-flop motion between two possible directions. The rate of this motion in the temperature range 223-313 K is assessed by means of measurements of the nuclear spin relaxation. The values of the activation enthalpy (38.7kJmol(-1)) and of the activation entropy (-15Jmol(-1) K-1) were determined. In addition, correlation times of molecular tumbling have been determined in the same temperature range. The measured properties of thiacalix[4]arene are compared to those of the "classical" calix[4]arene in order to utilize them for fine tuning of the building blocks in supramolecular chemistry.