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?

The geometry of a series of carbon iodides have been determined, CI4 by gas -phase electron diffraction and CIn. (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) Angstrom. The indica tion of about 20% I-2 in the vapor suggests partial decomposition and it ha s been thoroughly investigated what other carbon iodide species might be pr esent beside CI4. There is no appreciable amount of either of the dimeric s pecies in the vapor phase, in spite of the suggestion from thermodynamics. On the other hand, the electron diffraction data are compatible with the pr esence of about 18% of either of the monomeric free radicals, CI3 or CI2, b eside CI4 and I-2. Possible reasons for these observations are discussed. O ur correlated level computations, in agreement with other high level comput ations, found the singlet (1)A(1) state to be the ground state for CI2. Thi s is in contrast with a recent photoelectron spectroscopic study according to which the triplet state is the ground state though with a large margin o f error (1 +/- 3 kcal/mol energy difference). The computed singlet-triplet separation strongly depends on the level of the computation, but it is at l east 9 kcal/mol. Geometrical parameters, singlet-triplet separations, and d ipole moments have been calculated for the CX2 Series (X = F, Cl, Br, I, H) and their variations are discussed. The thermodynamic stability of differe nt carbon iodide species has also been investigated.