Polymorphism of β R-substituted naphthalene derivatives—Review and comparative study

The polymorphic behaviour of seven β R-substituted naphthalene derivatives (R = H, F, OH, Cl, CH 3, SH and Br) is reviewed and complementary accurate data are given concerning particularly the structural evolution aspects. Sixteen phases have been characterized. It is shown that they can be classified into three categories, respectively labelled I, II and III. All the derivatives exhibit the high temperature form I isostructural with naphthalene ( P2 1 a , Z = 2). Form II ( P2 1 n , Z = 4) corresponds to a unidirectional substructure of form I along the c axis (C II = 2C 1) as R = Cl or Br. Form III which is encountered for R = F, OH, CH 3, SH or Br does not show any evident relations. The I–II phase transitions are of second order, all the others (I–III, II–III, III-III') belong to the first order. Form III (as far as known) is ordered and forms II and I are disordered. The II-I transition corresponds to a progressive modification of the symmetry of the molecular site. The statistical molecular entity becomes centrosymmetrical in the high temperature form. It seems that two mechanisms govern this modification. In form I, a dynamical disorder is hypothesized with two types of motion: reorientations of the molecule in its own plane and diffusion jumps of the molecule from site to site. In form II these motions persist at high tempeature but become progressively weaker as the temperature lowers so that finally the disorder is quite static.