Structural, Thermodynamic, and Spectroscopic Characterization of Diphosgene and Triphosgene

Phosgene (COCl₂) is an important industrial reagent but its gaseous state and limited availability limit laboratory use. Diphosgene and triphosgene are safer surrogates, yet their solid-state structures and vibrational properties remain poorly documented. Here we report a comprehensive investigation of both compounds combining crystallography, calorimetry, spectroscopy, and quantum chemical calculations. A new polymorph of diphosgene (β-diphosgene) was discovered. Differential scanning calorimetry revealed rare cold crystallization behavior, i.e., crystallization of a supercooled melt only during subsequent heating. Solid-state DFT calculations reproduced lattice parameters and clarified the thermodynamic balance between the polymorphs. Infrared, Raman, and inelastic neutron scattering spectra of diphosgene and triphosgene were measured and fully assigned with the aid of periodic DFT calculations, providing the first complete solid-state vibrational characterization of these compounds.