Conformational polymorphs of 22-cyano-N-methyl-5-phenyl-pent-2-en-4-ynamide

Although the two polymorphic modifications, (I) and (II), of the title comp ound, C13H10N2O, crystallize in the same space group (P2(1)/c), their asymm etric units have Z' values of 1 and 2, respectively. These are conformation al polymorphs, since the molecules in phases (I) and (II) adopt different r otations of the phenyl ring with respect the central 2-cyanocarboxy-aminopr op-2-enyl fragment. Calculations of crystal packing using Cerius(2) [Molecu lar Simulations (1999). 9685 Scranton Road, San Diego, CA 92121, USA] have shown that (I) is more stable than (II), by 1.3 kcal mol(-1) for the crysta llographically determined structures and by 1.56 kcal mol(-1) for the optim ized structures (1 kcal mol(-1) = 4.184 kJ mol(-1)). This difference is mai nly attributed to the different strengths of the hydrogen bonding in the tw o forms.