INTERACTION OF THE CHLORINE ATOM WITH WATER - ESR AND AB-INITIO MO EVIDENCE FOR 3-ELECTRON (SIGMA(2)SIGMA-ASTERISK(1)) BONDING

The specific interaction of chlorine atoms with water has been investi gated by electron spin resonance spectroscopy and molecular orbital th eory. Chlorine atoms are formed by attack of hydroxyl radicals on chlo ride ions in frozen aqueous solutions at low temperatures. A variety o f frozen aqueous systems were irradiated at 77 K and investigated by E SR spectroscopy, and results obtained suggest a localized three-electr on bond (sigma(2) sigma(1)) between Cl-. and H2O or less likely with OH-. Chlorine atom interactions with both species were investigated by both ab initio and semiempirical molecular orbital calculations. A se ries of isolated chlorine-water radical species consisting of hydrated chlorine atoms as well as chloride anions with hydroxyl radicals were considered. Best agreement with experiment is found for chlorine atom -water interactions, H2O-(C) over dot l(H2O)(n). Full optimization of (OH)-O-.-Cl- aquated systems shows that energetic ion dipole forces ov ercome weaker sigma sigma interactions and result in full spin locali zation on the hydroxyl radical. Poor agreement with experiment is foun d even when the (ClOH-)-O-. structure is held in position to promote s igma sigma bonding. However, for H2O-(C) over dot l(H2O)(n) (n = 0, 2 and 5 considered) a comparison of the experimental hyperfine coupling s and spin densities suggested from experiment, i.e., 60% spin on the chlorine atom, with the results found from ab initio calculations, giv es improved agreement as n increases, with best agreement found for n = 5. The theoretical results support the formation of a water-chlorine three-electron bond with a substantial sharing of the unpaired spin b etween the bonding entities.