Thermodynamic and structural relationships between the two polymorphs of 1,3-dimethylurea

Polymorph (I) of dimethylurea (Fdd2) is the more stable modification at room temperature, where polymorph (II) (P2₁2₁2) is metastable; the thermodynamic transition temperature lies between 253 K and room temperature.

Methylthioureas and their morpholine and dioxane adducts; hydrogen-bonding patterns

The structures of three methyl-substituted thioureas and three related adducts with morpholine or 1,4-dioxane are presented and their hydrogen-bonding patterns discussed.