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.

Enantiomerization Barrier of all‐cis C‐Me‐tetrahydroxy p‐tert‐butylcalix[4]arene and Atropisomeric Equilibrium of its Tetraacetoxy Derivatives

“…Calculations using density functional theory indicate that the jump mechanism is energetically favored for 1 . Both the calculated enthalpy and Gibbs free energy of activations of the jump mechanism were in good agreement with the experimental values determined by Lang et al[15a]…”

“…An alternative explanation for the slow exchange NMR spectrum is that even at 183 K the pinched‐cone/pinched‐cone interconversion is fast, but the rate of reversal of the sense of direction of the circular array of hydrogen bonds (sometimes referred as a “flip‐flop” motion) is slow on the NMR timescale. This explanation is in better agreement with the experiment, since a chiral “frozen” conformation of C 4 time‐averaged symmetry should display two signals for the aromatic protons, but single singlets for the t Bu and OH signals, as observed experimentally.…”

“…In the case of 1 this barrier was determined in 1,1,2,2‐tetrachloroethane‐ d 2 by Lang et al by measuring the transverse 13 C nuclear spin relaxation. [15a] In the 13 C NMR spectrum, the C2/C6 and C3/C5 aromatic carbons were equivalent at 252 K, but at 221 K they split into two signals, while the C1 and C4 carbons were not affected. [15a] To the best of our knowledge, the barrier for the flip‐flop motion process of 1 has not been determined by a simple dynamic 1 H NMR experiment.…”

Hydrogen Bond Energies and Cooperativity in Substituted Calix[n]arenes (n= 4, 5)

“…40 45,46 A series of NMR relaxation measurements in nonpolar solvents were applied for studying the dynamics of the circular hydrogen bond array in CX [4]. 29 Subsequently, cooperative intramolecular hydrogen bonding in CX [4] and S-CX [4] and their t-Bu derivatives has further been investigated. A substantial frequency red shift in the O−H vibration of the phenol monomer observed in the infrared spectra of CX [4] suggested strong cooperative intramolecular hydrogen bonding, which was inferred from duplication of the number of O−H infrared bands and H-bond enthalpy.…”

Role of intramolecular hydrogen bonding in the excited-state intramolecular double proton transfer (ESIDPT) of calix[4]arene: A TDDFT study

“…Four intramolecular hydrogen bonds O-H· · · O are formed in the cone conformers of C4A, which allows C4A to act as host for a wide range of guests. Thus, C4A is an excellent model in which to study intramolecular hydrogen bonding in the excited state [32][33][34][35]. However, little information is available on electronic excited-state intramolecular hydrogen bonding due to the extremely short timescales involved.…”