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

Although the two polymorphic modifications, (I) and (II), of the title compound, C₁₃H₁₀N₂O, crystallize in the same space group (P2₁/c), their asymmetric units have Z' values of 1 and 2, respectively. These are conformational polymorphs, since the mol­ecules in phases (I) and (II) adopt different rotations of the phenyl ring with respect the central 2-cyano­carboxy­amino­prop-2-enyl fragment. Calculations of crystal packing using Cerius² [Molecular 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 crystallographically determined structures and by 1.56 kcal mol^-1 for the optimized structures (1 kcal mol^-1 = 4.184 kJ mol^-1). This difference is mainly attributed to the different strengths of the hydrogen bonding in the two forms.