Hexa phase transition temperatures

Compounds undergoing solid-solid phase transitions have been used extensively for temperature calibrations in solid-state NMR, covered later in Section 4.1, but such compounds can also be used for calibrations in the liquid state. When solid, no resonances are observed, but sharp lines appear above the phase-transition temperature. Compounds that have been studied include p-dichlorobenzene (326.4 K), cyclododecane (333.8 K), 2 -hydroxy-4, 5 -dimethylacetophenone (344.2 K), hexamethylcyclohexanetrione-1,3,5 (353.3 K), p-dibromobenzene (360.7 K), p-diacetylbenzene (386.0 K), p-diiodobenzene (402.6 K), 1,2,4,5-tetrachlorobenzene (413.1 K) and hexa-methylbenzene (438.6 K). 

In discotic systems, the roles of ATu and K22 are reversed, because in such phases the bend deformations require the lowest energy [181]. Measurements of splay and bend constants in a homologuous series of discotic n-hexa(alkanoylox)truxenes [76] revealed that K22 is always smaller than A, . The splay/bend ratio approached unity at the high temperature transition to the columnar phase. Qualitatively different results have been obtained, however, by Raghunathan et al. [75], who found K22>Kn in a disco-tic nematic phase enclosed between two columnar phases. The authors interpreted this unexpected result as being a consequence of short-range columnar order. 

---