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.

Detailed quantitative calculations are reported for different resonant states of H2−. Results tie in quantitatively with dissociative attachment experiments and electron-scattering experiments, and qualitatively with vibrational-excitation experiments. Preliminary results on other states which have not as yet been experimentally observed are also discussed. Wavefunctions and energies are reported.

In this work, we employ semiempirical and low-level ab initio molecular orbital theories with isodesmic/ isogyric reactions to estimate the energetics of free radical reactions important to radiolytic damage to DNA. Results are presented (1) for H• and OH• addition reactions to the natural DNA bases, (2) for the addition of H• to the DNA base radical adducts so obtained, and (3) for the dehydration of the DNA base hydrates and regeneration of the undamaged bases. The technique employed sums the calculated enthalpy of an isodesmic reaction with the experimentally known reaction enthalpy of an associated reaction to estimate the enthalpies of the DNA base reactions of interest. The calculational techniques employed are the ROHF/6-31G* ab initio method and the ROHF/PM3 semiempirical method. For H• addition reactions, the ROHF ab initio method employing the 6-31G* basis set gives results that are in closest agreement with known experimental results. For the OH• addition processes, the ROHF/PM3 semiempirical technique results in predictions more in keeping with the experimental model systems. The values calculated are employed to estimate the heats of formation of various DNA base radicals and DNA base hydrates. Economical PM3 calculations of isodesmic reactions when combined with well-known experimental enthalpies of reaction are found to yield reliable thermodynamic quantities.

The study of the ESR spectra parameters of Cu(Acac)2 in different solvents shows that the anisotropy of the nuclear hyperfine interaction tensor varies with changes in solvent. The effect is most clearly exhibited by the parameter A⊥. A correlation is obtained between the spectral data and the nature of the solvent ligand. The results are in agreement with the empirical parameters DII,I of Selbin and DN of Gutmann. Our empirical parameter is also a moderately sensitive one for ranking solvents.