Half-Crystal Energy and Enthalpy of Sublimation of Diuron: 3-(3,4-Dichlorophenyl)-1,1-dimethylurea

The enthalpy of sublimation of diuron, 3-(3,4-dichlorophenyl)-1,1-dimethylurea, C 9 H 10 ON 2 Cl 2 , was measured at 393 K using a Tian-Calvet calorimeter. The corrected experimental value Δ sub H m o (298 K) = 133.87(74) kJ mol -1 was then compared to the half-crystal energy E k . This energy was calculated from structural data using the 'consistent valence force field', which consists of a 6-12 Lennard-Jones potential and a Coulomb term. The disagreement between Δ sub H m o (298 K) and E k = 152.6 kJ mol -1 is about 14%. It can be partially attributed to the inaccuracy of the atomic coordinates of the crystal structure and to the charges of the force field. For simple molecules such as normal octadecane (n-C 18 H 38 ), hexanedioic acid (C 6 H 10 O 4 ), diphenylmethanone (C 13 H 10 O) and anthracene (C 14 H 10 ), the agreement is very good, about 3%.