Application of two different aspherical-atom formalisms to low-temperature diffraction data on pyridinium-1-dicyanomethylide (C₈D₅N₃)

Low-temperature (118 K) X-ray data on pyridinium-1-dicyanomethylide have been refined with the aspherical-atom formalism of Hirshfeld [Isr. J. Chem. (1977), 16, 226-230] and the related spherical harmonic expansion proposed by Stewart [Acta Cryst. (1976), A32, 565-575] as modified by Hansen & Coppens [Acta Cryst. (1978), A34, 909-921]. The results of the two methods are very similar, except when pseudostatic deformation maps are compared. The triple bond is clearly differentiated from the other bonds, while the ylide carbon atom is neutral. The molecular dipole moment from the multipole formalism is in close agreement with the moment obtained by direct-space integration of the electron density and with the value from solution measurements. On the other hand, the refinement with the Hirshfeld functions leads to a much larger molecular dipole moment. The difference between the two methods in this respect appears to be related to the presence of diffuse monopolar functions in the Hirshfeld expansion.