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, C₉H₁₀ON₂Cl₂, was measured at 393 K using a Tian–Calvet calorimeter. The corrected experimental value Δ_subH_m^ο (298K) = 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_subH_m^ο (298K) and E_k = 152.6 kJmol⁻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-C₁₈H₃₈), hexanedioic acid (C₆H₁₀O₄), diphenylmethanone (C₁₃H₁₀O) and anthracene (C₁₄H₁₀), the agreement is very good, about 3%.