In the title molecule, C20H21N303, the allyl substituent is rotated out of the plane of its attached phenyl ring [torsion angle 100.66 (15)]. In the crystal, C HMthphif " "OMthphn (Mthphn = methoxyphenyl) hydrogen bonds lead to the formation of (100) layers that are connected into a three-dimensional network by C H" " -7r(ring) interactions, together with 7r rr stacking interactions [centroid-to-centroid distance = 3.7318 (10) A] between parallel phenyl rings. Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H" " "H (48.7%) and H" " "C/C" " "H (23.3%) interactions. Computational chemistry reveals that the C HMthphn" "OMthphn hydrogen bond energy is 47.1 kJ mori. The theoretical structure, optimized by density functional theory (DFT) at the B3LYP/ 6-311 G(d,p) level, is compared with the experimentally determined molecular structure. The HOMO LUMO behaviour was elucidated to determine the energy gap.