Add time:08/30/2019         Source:sciencedirect.com

Phenyl ring motion in β-form poly(l-phenylalanine) was studied using solid-state deuterium NMR spectroscopy as a function of temperature and echo delay time. The spectra show strong temperature dependence from 173 to 417 K. The line shape is a superposition of two contributions from phenyl groups performing π-flip motion about the Cβ-Cλ axis and fast librations. The π-flipping motion is characterized by a fairly broad distribution of correlation times. The mean correlation times of the distribution vary from 3.8 × 10−3 to 3.1 × 10−8 s with increasing temperature of 173 to 338 K, respectively. The apparent activation energy of the π-flip motion is 28 kJ mol−1, showing low steric hindrance of the rings. There is no evidence of the fast large amplitude motions about the Cα-Cβ axis.

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