Interaction of the Chlorine Atom with Water: ESR and ab Initio MO Evidence for Three-Electron (σ2σ*) Bonding

The specific interaction of chlorine atoms with water has been investigated by electron spin resonance spectroscopy and molecular orbital theory. Chlorine atoms are formed by attack of hydroxyl radicals on chloride ions in frozen aqueous solutions at low temperatures. A variety of frozen aqueous systems were irradiated at 77 K and investigated by ESR spectroscopy, and results obtained suggest a localized three-electron bond (σ 2 σ* 1 ) between • Cl and H 2 O or less likely with OH - . Chlorine atom interactions with both species were investigated by both ab initio and semiempirical molecular orbital calculations. A series 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, H 2 O−Ċl(H 2 O) n . Full optimization of • OH−Cl - aquated systems shows that energetic ion dipole forces overcome weaker σσ* interactions and result in full spin localization on the hydroxyl radical. Poor agreement with experiment is found even when the Cl • OH - structure is held in position to promote σσ* bonding. However, for H 2 O−Ċl(H 2 O) n ( n = 0, 2 and 5 considered) a comparison of the experimental hyperfine couplings and spin densities suggested from experiment, i.e., 60% spin on the chlorine atom, with the results found from ab initio calculations, gives 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 between the bonding entities.