| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 96 === Variable-Teperature Nuclear Magnetic Resonance (VT-NMR) spectroscopy tecnique has been performed on 1,4-hydroquinone-derived atropisomers to investigate the stereodynamics of different restricted rotations in 4-acetoxy-2-benzoyl-3-(1-methyl-2-oxopropyl)naphthalenol (A),4-acetoxy-2-benzoyl-5,6-dimethyl-3-(1-methyl-2-oxopropyl)phenol(B),and 4-acetoxy-2-benzoyl-5,6-dimethyl-3-(1-butyl-2-oxopropyl)phenol (C) in several solvents such as chloroform, toluene, dimethylforamide, tetrahydrofuran, and methanol within the accessible temperature range of 193-363 K. By means of VT-NMR experiments, the corresponding spectra at different temperature were obtained. The rate constants of the restricted rotations were obtained via line shape analysis and followed by evaluation of kinetic parameters, ex: activation energy (Ea), the Gibbs energy of activation (ΔG*), enthalpy of activation (ΔH*), and entropy of activation (ΔS*), using Arrehenius equation and Eyring equation. The proposed restricted-rotation mechanism resulting in the splitting of spectra at lower temperatures could be also confirmed by the 2D-NOE spectra.
Analysis of experimental results manifest that the molecules changed restricted rotation to free rotation because of that the molecules obtained enough energy to overcome the rotational barriers and unfrozen step by step in the process of temperature increasing. In protic solvent such as methanol, the rupture of the intramolecular hydrogen bond between the –OH and the carbonyl group was observed with increasing temperature and meanwhile the temperature factor (���飣/�幅) for the corresponding H in the –OH showed a relatively large value (0.5 ~ 6.2 Hz/K) due to the intramolecular H-bond breaking.
On the basis of the obtained activation parameters, in less polar solvents (toluene, chloroform) steric hindrance (large ΔH*) of rotating substituent moiety dominates the rotation barrier (enthalpic effect), whereas in the case of polar solvent methanol, solvent entropic effect (large ΔS*) becomes the prevalling energetic contribution to the barrier to the restricted rotation of those nonpolar substituentts. The high merging temperature (Tc, 310 K) and large ΔG* value (62.8 kJ/mol) for the n-butyl in compound C is ascribed to the large steric hindrance because of relatively large van der Waals radius of butyl group.
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