Crystal Structures and Intermolecular Interactions in αα‐ and ββ‐phosgene

The crystal structures of ‐ and ‐phosgene have been elucidated on the basis of variable temperature neutron powder diffraction. Although the stable ‐phase closely mirrors the published structure (, No. 88), the new metastable ‐phase crystallises in (No. 36, at 135 K: a = 10.244042(22) Å, b = 6.280321(21) Å, c = 5.46069(4) Å). Both modifications show dipole–dipole interactions, but only ‐phosgene shows pronounced ‐hole interactions. Quantum chemical calculations in both solid state and of model dimers in gas‐phase reveal a complex interplay of intermolecular interactions. The first inelastic neutron scattering spectra of ‐phosgene shows an unusual separation of the translational and librational modes, alongside sharp bands for the internal modes. Our computational studies also show an unusual, extensive mixing of the in‐phase C–Cl stretch () and the out‐of‐plane bend () modes, which are symmetry‐forbidden from mixing in the gas‐phase.