The crystal and molecular structures of two potent anticonvulsant compounds: 1-(n-butyl)-5-(p-fluorophenyl)-3-pyrazolidone (1) and 1-(n-propyl)-5-(p-methylphenyl)-3-pyrazolidone (2) were determined by X-ray diffraction method. The atomic net charges were calculated by CNDO/2 method. It is shown that the bond lengths of C-N and C-O of the conjugated amido groups and the atomic net charges in the five membered rings of 3-pyrazolidones are affected by the electronreleasing capabilities of R^1 groups. It is considered that a substituent being more electron-releasing at position 1 of the five membered ring of 3-pyrazolidone is favorable on enhancement of the anticonvulsant activity, and the substituent at position 5 might mainly affect the molecular steric shape and the hydrophobic property.

Key words: MOLECULAR STRUCTURE, X-RAY DIFFRACTION ANALYSIS, PYRAZOLONE P, CNDO APPROXIMATION, SUBSTITUENT EFFECT