Spin-orbit Effects on the Structure of Haloiodomethane Cations

The importance of including spin-orbit interactions for the correct description of structures and vibrational frequencies of haloiodomethanes is demonstrated by density functional theory calculations with spin-orbit relativistic effective core potentials (SO-DFT). The vibrational frequencies and the molecular geometries obtained by SO-DFT calculations do not match with the experimental results as well as for other cations without significant relativistic effects. In this sense, the present data can be considered as a guideline in the development of the relativistic quantum chemical methods. The influence of spin-orbit effects on the bending frequency of the cation could well be recognized by comparing the experimental and calculated results for CH2BrI and CH2ClI cations. Spin-orbit effects on the geometries and vibrational frequencies of CH2XI (X=F, Cl, Br, and I) neutral are negligible except that C-I bond lengths of haloiodomethane neutral is slightly increased by the inclusion of spin-orbit effects. The A' and A'' states were found in the cations of haloiodomethanes and mix due to the spin-orbit interactions and generate two 2E1/2 fine-structure states. The geometries of CH2XI + (X=F and Cl) from SO-DFT calculations are roughly in the middle of two cation geometries from DFT calculations since two cation states of CH2XI (X=F and Cl) from DFT calculations are energetically close enough to mix two cation states. The geometries of CH2XI + (X=Br and I) from SO-DFT calculations are close to that of the most stable cation from DFT calculations since two cation states of CH2XI (X=Br and I) from DFT calculations are energetically well separated near the fine-structure state minimum.