Kinetic and Thermodynamic Conformational Polymorphs of Bis(p-tolyl) Ketone p-Tosylhydrazone:  The Curtin−Hammett Principle in Crystallization

Bis( p -tolyl) ketone p -tosylhydrazone ( 2 ) crystallized as three conformational polymorphs, forms 1−3, under different conditions. The crystal structure of form 1 has an N−H···O S dimer synthon, but forms 2 and 3 have no strong hydrogen bonds even though the molecule contains an SO 2 NH group. Polymorphs 1 and 3 are assigned as kinetic and thermodynamic forms on the basis of the criteria of lower energy, higher melting point, and higher density for the latter modification. Calculation of both intramolecular ( E conf ) and intermolecular energy ( U latt ) gave a crystal energy difference (Δ E total ) of 2.54 kcal mol -1 between forms 1 and 3, whereas metastable and disappearing form 2 has much a higher energy of 8.71 kcal mol -1 . Sulfonamide 2 is a rare example of a polymorph cluster wherein strong hydrogen bonds stabilize kinetic form 1 whereas thermodynamic polymorph 2 has excellent close packing but no hydrogen bonds. A Curtin−Hammett energy profile for the crystallization of conformational polymorphs 1 and 3 is proposed that is consistent with structural, thermochemical, and computational data. Hydrogen bonding in tolyl compound 2 is compared with other benzophenone hydrazones varying at the para -position (e.g., H, F, Cl, Br). The bimolecular N−H···O S dimer is present in crystal structures 3 − 6 as well as in the CH 2 Cl 2 solvate of 2 and benzene solvates of chloro 5 and bromo 6 hydrazones.