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- dim ethylurea, C9H10ON2Cl2, was measured at 393 K using a Tian-Calvet calo rimeter. The corrected experimental value Delta(sub)H(m) degrees (298 K) = 133.87 (74) kJ mol(-1) was then compared to the half-crystal ener gy E-k. This energy was calculated from structural data using the 'con sistent valence force field', which consists of a 6-12 Lennard-Jones p otential and a Coulomb term. The disagreement between Delta(sub)H(m) d egrees (298 K) and E-k = 152.6 kJ mol(-1) is about 14%. It can be part ially attributed to the inaccuracy of the atomic coordinates of the cr ystal structure and to the charges of the force field. For simple mole cules such as normal octadecane (n-C18H38), hexanedioic acid (C6H10O4) , diphenylmethanone (C13H10O) and anthracene (C14H10), the agreement i s very good, about 3%.