Structural study of two reactive diacetylenes

The unit cell dimensions of two diacetylenes 3BCMU and 4BCMU are measured from low temperature to 300 K. The complete crystal structures of the two phases of 3BCMU are determined at 110 K by X-ray diffraction and at 270 K by neutron diffraction helped by calculated geometrical optimization used as a starting structure. Neutron results allow to discuss the hydrogen bond strength and to determine the thermal expansion coefficients.