Estimate of the iodine-iodine two-center three-electron bond energy in [CH3-I-I-CH3]+

Article abstract of DOI:10.1021/ja00005a006

The gas-phase ion-molecule association reaction CH₃I⁺ + CH₃I ? [CH₃-I-I-CH₃]⁺ in various bath gases was studied at 503 K. The iodine-iodine bond in the association product is an example of a two-center three-electron (2c-3e) or a 2σ/1σ* bond. The bond energy was estimated from ΔG° of reaction, which was in turn determined from equilibrium experiments. Assuming a value for ΔS° of reaction of -20 to -25 cal/(mol K), a bond strength of 23-26 kcal/mol is estimated. This is the first experimental gas-phase binding energy estimate for a 2c-3e bond in an organic molecule involving an iodine-iodine interaction and one of only a few experimental studies of well-characterized gas-phase 2c-3e bonding interactions between heteroatoms in organic molecules. A study of the ion-molecule reactions occurring at low ionizing energies leading to (CH₃)₂I⁺, [C₂H₃I₂]⁺, and [CH₃-I-I-CH₃]⁺ is discussed.