STRUCTURAL CHARACTERIZATION OF HYDROXYL RADICAL ADDUCTS IN AQUEOUS MEDIA Irek Janik, GNR Tripathi Notre Dame Radiation Laboratory

Typical pulse radiolytic detection of OH radical initiated transformations

Hydroxyl radical reactions H-abstraction Addition to double bonds Electron transfer

Prototype of OH adduct to heteroatom

Time Resolved Resonance Raman Setup

The nature of OH adducts to sulfides Pseudo diatomic nature Spectrum unchanged at pH 4-13 Excited state potential energy surface not very steep S-O bond harmonic frequency cm -1 Anharmonicity constant 4.76 cm -1 Bond dissociation energy, D e 0.6eV Spectroscopic parameters  e -  e  e )  -  e  e   D e =  e 2 /4  e  e where :  observed frequency  e - harmonic frequency  e  e - ancharmonicity constant  -upper state vibrational quantum number Janik I., Tripathi GNR, JCP 138 (2013) 44506

OH induced oxidation of alkyl sulfides and sulfide derivatives Studied compounds: DMS-dimethyl sulfide; MES-methyl ethyl sulfide DES-diethyl sulfide; MTPA- 3-methylthiopropylamine MTP- 3-methylthiopropanol

Competition between direct electron transfer (ET) and OH adduct mediated electron transfer Tripathi GNR, JACS, 120 (1998) 4165

Oxidation of halides and pseudo halogen ions J.Phys.Chem.A (2013) 117, 2287

Example of OH adduct to pseudo halogen thiocyanate anion, SCN - Ancharmonicity 5.9cm -1 Harmonic frequency 309.4cm -1 Dissociation energy D e 0.5eV J.Phys.Chem.A 109 (2005) 165

Hemibonding of OH and halide ion in aqueous solution Cl-OH - Br-OH - I-OH - Lambda max [nm]350 (348)360 (343)335 (324) Epsilon/10 3 [M -1 cm -1 ]3.7 (5.7)8 (7.1)4.5 (6.9) X-O stretching [cm -1 ]162, R (X-O) [A] M. Yamaguchi, J. Phys. Chem. A 2011, 115, DFT with range-separated hybrid (RSH) exchange-correlation functional

OH adduct to bromide anion, Br - Ancharmonicity 7.75cm -1 Harmonic frequency 309 cm -1 Bond dissociation energy D e 0.38eV R2 est. ~ 2.5x10 7 s -1 J.Phys.Chem.A (2013) 117, 2287

Oxidation of transition metal cations

OH adduct to thallium cation, Tl + Ancharmonicity 8.83cm -1 Harmonic frequency cm -1 Dissociation energy D e 0.57eV J.C.S. Chem.Comm. (1975) 387

OH adduct to silver cation, Ag + Ancharmonicity 3.4cm -1 Harmonic frequency cm -1 Dissociation energy D e 2.2 eV Exchange of light water with heavy water decreases fundamental frequency about 5cm -1. Spectroscopic parameters are not affected by the change of the solvent. J.Chem.Soc.,Faraday Trans.1,(1978) 74, 1820

Conclusions Hydroxyl radical adducts to sulfur containing organic compounds, halide ions as well as some metal cations have been observed for the first time using pulse radiolysis with TRRR detection technique In majority of observed adduct intermediates bond between hydroxyl radical oxygen and parent molecule has two-center three-electron character recognized by its fundamental vibration and progression of overtones The formation of OH adduct intermediates is determined by the rate of their vibrational relaxation (reorganization) competing with the rate of solvation of products formed in alternative electron transfer process Assuming a first order anharmonicity in stretching motion of studied adduct intermediates and applying Birge-Sponer extrapolation bond dissociation energies of studied adduct intermediates have been determined Observed results may additionally serve as a experimental reference for the applicability of existing computational methods in determination of spectroscopic parameters of 2c-3e bond intermediates

Acknowledgements Students Daniel Stuart Rohit Chikkaraddy Faculty Gordon Hug Ian Carmichael