Conformational and Crystallographic Effects on Solid-State CP/MAS ¹³C NMR Spectra of Thermotropic Phenyl Benzoates

Abstract

The crystalline structures of thermotropic ester compounds, 4-cyanophenyl 4-pentoxybenzoate (CPPOB), 4-cyanophenyl-4-heptylbenzoate (CPHB), 4-cyanophenyl 4-pentylbenzoate (CPPB), and 4-nitrophenyl 4-octyloxybenzoate (NPOOB) have been examined by high-resolution CP/MAS (cross polarization/ magic angle spinning) solid-state ¹³C NMR. The spectral features were correlated with the crystalline structures. Conformational and crystallographic effects were observed in the solid-state spectra. The resonances of aromatic carbons showed more distinct conformational effects when the molecule adopted a high degree of symmetry, that is, the exact C, symmetry for CPPOB. As for aliphatic end groups, conformational effects were clearly observed for longer alkyl chains, that is, the heptyl group of CPHB and the octyloxy group of NPOOB. It is assumed that these differences are due to different features of crystal packings.