Trimorphism of 4,4'-Di(tert. -butyl)-biphenyl: structural, thermodynamic and kinetic aspects

摘要

Two polymorphs of 4,4'-Di(tert.-butyl)-biphenyl were discovered in addition to the known triclinic modification (form I). Form II crystallizes in the monoclinic space group P2_1/c, whereas form III crystallizes in space group C2/c. Ah initio calculations of the rotational barrier of 4,4'-Di(tert.-butyl)-biphenyl yield two symmetry equivalent energy minima at dihedral angles of 42 as well as 138 deg and the molecular conformations in form I and II are very close to this energetically most favourable conformation. In contrast only two of the six independent molecules in form III adopt this conformation. The calculation of the rotational barrier in the crystal of form I and II shows that packing effects can be neglected and that form II should be more stable than form I by about 4 kJ mol~(-1). Crystallization experiments reveal that form II is the thermodynamically most stable form at room temperature. On heating form II transforms to form I, which shows that the latter should he the thermodynamic most stable form at higher temperatures. On further heating form I melts at about 402 K without any further transformation. Cooling of the melt or kinetically controlled, rapid crystallization yields always crystals of form I exclusively. On heating form III transforms also to form I. From the experiments performed there is no hint for the transformation of the monoclinic forms II and III into each other Because form III exhibits the largest density it can be assumed, that this form should he the most stable modification at lower temperatures.