ESTIMATE OF THE IODINE IODINE 2-CENTER 3-ELECTRON BOND-ENERGY IN [CH3-I-I-CH3]+

The gas-phase ion-molecule association reaction CH3I+ + CH3I reversible [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 2-sigma/1-sigma* bond. The bond energy was estimated from DELTA-G-degrees of reaction, which was in turn determined from equilibrium experiments. Assuming a value for DELTA-S-degrees 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+, [C2H5I2]+, and [CH3-I-I-CH3]+ is discussed.