title: Effect of p-tert-butyl group substitution on the quantum tunneling rate in quadruple proton rearrangement of calix[4]arene.
abstract: Glass-transition behaviors concerning the quadruple proton/deuteron rearrangements in the crystalline p-tert-butylcalix[4]arene-toluene (BC4A·T) host-guest compound were studied by adiabatic calorimetry. The glass-transition temperatures (Tg's) of the hydrated BC4A·T-h and deuterated BC4A·T-d were found to be around 90 and 181 K, respectively. The difference of 90 K is too big to be recognized as the ordinary isotope effect, which is about 5 K in classical thermal activation processes. The temperature interval over which the enthalpy relaxation effect was observed was beyond 50 K, being too wide, in BC4A·T-h, while it was 20-30 K in BC4A·T-d; the latter is the one ordinarily observed in the classical processes. The associated equilibration rate in BC4A·T-h was evaluated from the enthalpy relaxation curve observed at 62 K to evidence the non-Arrhenius nature of the temperature dependence. The relaxation effect was detected even at 20 K. These are quite the same behaviors as observed in calix[4]arene, and the quantum tunneling rate in the quadruple proton rearrangement was concluded to be only a little affected by the substitution of the tert-butyl group at the para-position of each phenol moiety.
authors.affiliation: Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology , 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551, Japan.
authors.forename: Kouhei
authors.initials: K
authors.lastname: Ueda
authors.forename: Masaharu
authors.initials: M
authors.lastname: Oguni
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D010636:Phenols / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D013816:Thermodynamics; D014050:Toluene / Q000737:chemistry; D044366:Transition Temperature
chemical_list: D010636:Phenols; D011522:Protons; C121325:calix(4)arene; D047250:Calixarenes; D014050:Toluene