Polymorphism and Crystal Field Effects

In molecular crystals we assume to a first approximation that the molecules are not changed in their structure and chemical bond properties. Therefore the differences in NQR frequencies must be due to the crystal field effect. The following forces may be responsible for the stability of different crystal modifications at different temperatures or pressures  

Substance Modifi- cation v/MHz (77 °K) Assign- ment Characterization of modification Crystal field. cc, i i Remarks effect A /MHz Ref. 

6-Trichlorobenzene- I 38.587 2,6 Crystallized from Aiu(2,6) The very high crystal 126) 

4-Bromophenol 1 274.795 271.500 (79Br). 1.610 6.417 Probably very strong influence of intermolecular hydrogen bond 17) 48) 74) 

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