Inhomogeneous distribution of correlation times in two-site exchange of phenyl groups and premelting of 4-ethoxyphenyl 4-ethoxybenzoate as revealed by high-resolution13C and1H solid-state NMR spectroscopy

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Abstract
Dynamic heterogeneity of the crystalline phase of thermotropic 4-ethoxyphenyl 4-ethoxybenzoate has been investigated by high-resolution 13C and 1H solid-state NMR spectroscopy. The line splittings between 3·8 and 7·9 ppm have been observed in 13C CP/MAS spectra as a consequence of unsymmetrical neighbouring of some protonated phenyl carbons in the absence of dynamic averaging due to internal motion. 13C line shape analysis for a two-site exchange has been applied to describe progressive coalescence of these splittings between 296 and 386 K. The exchange process proceeds by the appearance of central line which is a characteristic feature of inhomogeneous distribution of correlation times. The observed distribution of correlation times is well represented by the autocorrelation function of the form exp[-(t/τp)α]. Motional inhomogeneity has been additionally probed by relaxation measurements on the large frequency scale. The changes in the phase structure observed during heating from 296 to 386 K have been characterized by 1H magic angle spinning spectroscopy. The proton spectra show the coexistance of broad and narrow lines in this temperature range. The narrow component with effectively averaged dipolar interactions grows at the expense of the broad component and represents liquid-like domains resulting from premelting phenomenon near crystal defects or surface.