Flip–flop Motion of Circular Hydrogen Bond Array in Thiacalix[4]arene

Abstract

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 (38.7 kJ mol^− 1) and of the activation entropy ( − 15 J mol^− 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.

Keywords:

• Flip–flop motion
• Hydrogen bond
• Enthalpy
• Entropy
• NMR, ab initio calculation
• Calix[4]arene

Acknowledgements

Financial supports from the Czech Science Foundation (Grant. No 203/04/P168 to JL) and from the Grant Agency of the Czech Academy of Sciences (Grant No. KJB5040311 to JC and JL) are gratefully acknowledged. This work is a part of the research plan MS 0021620835 that is financed by the Ministry of Education of the Czech Republic. J.C. appreciates generous allotment of computer time in the Czech Academic Supercomputer Centre and in the Mississippi Center for Supercomputing Research. J.L. is indebted to Dr Sergei Dvinskikh from Stockholm University for his kind assistance in operating the solid state NMR spectrometer.