Crystallographic and theoretical studies of (Z)/(E)-3-phenyl-4-(arylidene)isoxazol-5(4H)-ones

► Characterisation by X-ray single crystal diffraction of (Z)- and (E)-3-phenyl-4-(arylidene)isoxazol-5(4H)-ones is described. ► Theoretical calculations on models of Z/E-pair indicate that the configurational isomers have comparable thermodynamic stability. ► The effect of π-donor methoxy substituents on the proton affinity of the nitrogen atom is reported.