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

Abstract

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 (σσ*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 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, H2O−Ċl(H2O) 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 H2O−Ċl(H2O) 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.

Hydrophobicity with atomic resolution: Steady-state and ultrafast X-ray absorption and molecular dynamics studies

“…After 70 fs there is a clear interaction with density being donated to the iodine from the water molecule. This interaction has been previously described as a three-electron bond [15], as predicted for Cl 0 and Br 0 [25,26]. Using a Mayer bond order analysis [27], we find that this initial complex has a bond order of 0.32, which is consistent with a three-electron bond [28].…”

“…4). The binding energy of the complex is small (50 meV), and its formation is due to a three-electron bond between the I 0 and the H 2 O, as was already reported for Cl 0 [25] and Br 0 [26]. For the I-H RDF, there is a prompt expansion within about 200 fs.…”

Probing the Transition from Hydrophilic to Hydrophobic Solvation with Atomic Scale Resolution

“…Indeed, formation of a stable Cl 0 (OH 2 ) complex was previously reported in electron spin resonance studies, which were supported by quantum-chemical calculations, pointing to a three-electron (σ 2 σ 1 *) bonding. 61 The presence of a Br 0 (OH 2 ) complex was also found in Monte Carlo simulations of Br 0 in water. 36 In the case of I 0 , the complex is weakly bound ( Figure S2), with a typical well depth of ∼50 meV.…”

What can we learn from two‐center three‐electron bonding with the topological analysis of ELF?

“…phases [16], biochemistry [17][18][19][20][21][22][23], organic reactions [24][25][26], radiation studies [27][28][29][30], intrazeolite chemistry [31,32], bioinorganic enzymology [33][34][35], and are preferentially observed in cations. The most extensively studied group is that of sulfur-centered radical cations, but they can also be found in anions [7,21,25,26,29] or neutrals adducts [24,27,28,30,36].…”