The structure of gaseous carbon tetraiodide from electron diffraction and all carbon iodides, CIn (n = 1-4), and their dimers, C2I2n (n = 1-3) from high-level computation. Any other carbon-iodide species in the vapor?

Abstract

The geometry of a series of carbon iodides have been determined, CI4 by gas-phase electron diffraction and CI n (n = 1–4) and C2I2n (n = 1–3) by high-level quantum chemical calulations. The bond length of the tetrahedral CI4 molecule from electron diffraction is (r g):2.157(10) A. The indication of about 20% I2 in the vapor suggests partial decomposition and it has been thoroughly investigated what other carbon iodide species might be present beside CI4. There is no appreciable amount of either of the dimeric species in the vapor phase, in spite of the suggestion from thermodynamics. On the other hand, the electron diffraction data are compatible with the presence of about 18% of either of the monomeric free radicals, CI3 or CI2, beside CI4 and I2. Possible reasons for these observations are discussed. Our correlated level computations, in agreement with other high level computations, found the singlet 1A1 state to be the ground state for CI2. This is in contrast with a recent photoelectron spectroscopic study according to which the triplet state is the ground state though with a large margin of error (1 ± 3 kcal/mol energy difference). The computed singlet-triplet separation strongly depends on the level of the computation, but it is at least 9 kcal/mol. Geometrical parameters, singlet-triplet separations, and dipole moments have been calculated for the CX2 series (X = F, Cl, Br, I, H) and their variations are discussed. The thermodynamic stability of different carbon iodide species has also been investigated.