Crystal Structures and Intermolecular Interactions in α $\alpha$ - and β $\beta$ -phosgene

The crystal structures of $\alpha$ - and $\beta$ -phosgene have been elucidated on the basis of variable temperature neutron powder diffraction. Although the stable $\alpha$ -phase closely mirrors the published structure ( $I{4}_1/a$ , No. 88), the new metastable $\beta$ -phase crystallises in $Cmc{2}_1$ (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 $\alpha$ -phosgene shows pronounced $\sigma$ -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 $\alpha$ -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 ( ${\nu }_2$ ) and the out-of-plane bend ( ${\nu }_6$ ) modes, which are symmetry-forbidden from mixing in the gas-phase.