Deuterium NMR studies on phenyl ring dynamics of poly( l-phenylalanine)

Abstract

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 C β-C λ 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 C α-C β axis.