New members of an old family: isolation of IC(O)Cl and IC(O)Br and evidence for the formation of weakly bound Br*...CO.

The photochemically induced reactions of a dihalogen, XY, with CO isolated together in an Ar matrix at about 15 K lead to the formation of carbonyl dihalide molecules XC(O)Y, where X and Y may be the same or different halogen atoms, Cl, Br, or I. In addition to the known compounds OCCl2, OCBr2, and BrC(O)Cl, the carbonyl iodide chloride, IC(O)Cl, and carbonyl iodide bromide, IC(O)Br, compounds have thus been identified for the first time as products of the reactions involving ICl and IBr, respectively. The first product to be formed in reactions with Cl2, BrCl, or ICl is the ClCO* radical, which reacts subsequently with a second halogen atom to give the corresponding carbonyl dihalide [OCCl2, BrC(O)Cl, or IC(O)Cl]. The analogous reaction with Br2 affords, in low yield, the unusually weakly bound BrCO* radical, better described as a van der Waals complex, Br*...CO. The changes have been followed and the products characterized experimentally by their infrared spectra, and the spectra have been analyzed in light of the results afforded by ab initio (Hartree-Fock and Moeller-Plesset second-order) and density functional theory calculations.