Experimental and computational evidence for stabilising parallel, off-set π[C(=O)N(H)N=C] … π(phenyl) interactions in acetohydrazide derivatives† Sang Loon Tan, a Laura N. F. Cardoso, b Marcus V. N. de Souza, b Solange M. S. V. Wardell, c James L. Wardell *d and Edward R. T. Tiekink *a a Research Centre for Crystalline Materials, School of Medical and Life Sciences, Sunway University, 47500 Bandar Sunway, Selangor Darul Ehsan, Malaysia b Instituto de Tecnologia em Fármacos Farmanguinhos, FIOCRUZ Fundação Oswaldo Cruz, Rio de Janeiro 21041−250, Brazil c CHEMSOL, 1 Harcourt Road, Aberdeen, AB15 5NY, Scotland, UK d Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB24 3UE, Scotland, UK E-mail: j.wardell@abdn.ac.uk (JLW); edwardt@sunway.edu.my (ERTT) Abstract Parallel, off-set π[C(=O)N(H)N=C] … π(phenyl) interactions are observed in the crystal of (2- thienyl)CH2CON(H)-N=C(H)Ph, along with more conventional non-covalent interactions. All notable interactions have been analysed by the calculated Hirshfeld surfaces, NCI plots and QTAIM analysis. The π[C(=O)N(H)N=C] … π(phenyl) interactions, whereby the N(H) atom lies over the ring centroid and with the N and C atoms on either side of the N(H) atom closely overlay 1,3-carbon atoms of the phenyl ring, are shown to be attractive. Theory suggests the energy of association provided the π[C(=O)N(H)N=C] … π(phenyl) interaction to the molecular packing to be about 15 kJ/mol, a value similar to that provided by similarly orientated benzene