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.

Atom Tunneling in Chemistry

“…[194] Tunneling splittings caused by double proton Scheme 3: Quadruple proton transfer in calix [4] arenes. [191][192][193] transfer in various porphycenes were observed. [195][196][197] In these cases the initial and final state are equivalent: resonant tunneling takes place, which is particularly fast.…”

“…Quadruple proton transfer in calix [4]arenes. [191][192][193] Figure 5. Schematic reaction profile of the [1,7] sigmatropic hydrogen shift of 7-methylocta-1,3(Z),5(Z)-triene to 2-methylocta-2,4(Z),6(Z)triene.…”

“…One of the most interesting findings is the simultaneous tunneling of protons in solid p ‐ tert ‐butylcalix[4]arene at low temperatures . By using NMR relaxometry it was possible to investigate the phonon‐assisted tunneling in the quadruple synchronous proton transfer of calix[4]arenes (Scheme ) . The coupling of hydrogen‐bond dynamics to large‐amplitude motion in the vicinity was studied by NMR spectroscopy …”

“…[191] Using NMR relaxometry it was possible to investigate the phonon-assisted tunneling in the quadruple synchronous proton transfer of calix [4]arenes, see Scheme 3. [192,193] The coupling of hydrogen bond dynamics to largeamplitude motion in the vicinity was studied by NMR. [194] Tunneling splittings caused by double proton Scheme 3: Quadruple proton transfer in calix [4] arenes.…”

Der Tunneleffekt von Atomen in der Chemie

“…Eines der interessantesten Ergebnisse ist das gleichzeitige Tunneln von Protonen in festem p ‐ tert ‐Butylcalix[4]aren bei tiefen Temperaturen . Mit NMR‐Relaxometrie war es möglich, diesen synchronen phononenunterstützten Tunnelprozess von Calix[4]arenen zu untersuchen (Schema ) . Ebenfalls mit Kernspinresonanz wurde die Kopplung der Dynamik von Wasserstoffbrücken zu großräumigen Bewegungen in deren Umgebung untersucht …”