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Abstract:
In this article, we present quantum chemical density functional
theory (DFT) calculations of the NMR 13C chemical shift (CS)
tensors in 2,3,6,7,10,11−hexahexylthiotriphenylene (HHTT). The DFT
calculations are performed on a smaller model molecule where the hexyl
chains were reduced to methyl groups (HMTT). These tensors are
compared with our previously reported experimental results carried out
under magic−angle spinning (MAS) conditions. The phase diagram of
HHTT is K T H T Dhd T I, where H is a helical phase and Dhd is a
columnar liquid crystal. The motivation for the present study was to explain
experimentally observed and puzzling thermal history effects, which resulted
in different behavior in the helical phase upon cooling and heating. In
particular, the CS tensors for the aromatic carbons measured in the helical
phase upon heating from the solid phase were essentially unaffected, while
the cooling from the columnar liquid crystal resulted in a significant averaging. We investigate the effect on the CS tensors of (i)
conformational transitions, and (ii) relative molecular orientations within the columns for dimer and trimer configurations. Finally a
motional wobbling (PIZZA) model for the dynamic averaging of the CS tensor in the helical phase is suggested
