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

Bis(<i>p</i>-tolyl) ketone <i>p</i>-tosylhydrazone (<b>2</b>) 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 SO₂NH 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 (<i>E</i>_conf) and intermolecular energy (<i>U</i>_latt) gave a crystal energy difference (Δ<i>E</i>_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 <b>2</b> 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 <b>2</b> is compared with other benzophenone hydrazones varying at the <i>para</i>-position (e.g., H, F, Cl, Br). The bimolecular N−H···OS dimer is present in crystal structures <b>3</b><b>−</b><b>6</b> as well as in the CH₂Cl₂ solvate of <b>2</b> and benzene solvates of chloro <b>5</b> and bromo <b>6</b> hydrazones.