NATURE OF THE THIYL PEROXYL RADICAL - ESR AND AB-INITIO MO EVIDENCE FOR INTERMOLECULAR STABILIZATION OF THE CHARGE-TRANSFER STATE, RS(-DOT-)()OO(CENTER)
Y. Razskazovskii et al., NATURE OF THE THIYL PEROXYL RADICAL - ESR AND AB-INITIO MO EVIDENCE FOR INTERMOLECULAR STABILIZATION OF THE CHARGE-TRANSFER STATE, RS(-DOT-)()OO(CENTER), Journal of physical chemistry, 99(20), 1995, pp. 7993-8001
The formation, chemistry, and nature of the thiyl peroxyl radicals, RS
OO(.), are investigated by ESR and UV-vis spectroscopy in a variety of
organic and aqueous matrices. Experimental evidence suggests that the
unusual properties of thiyl peroxyl radicals result from the specific
nucleophilic interaction of solvent which stabilizes the charge trans
fer state, RS(+)OO(.-). Anisotropic oxygen-17 coupling constants deriv
ed from ESR spectra of O-17 labeled species which are proportional to
the unpaired spin at the oxygen are used to estimate the spin density
distribution in RSOO(.) radicals. The couplings and spin density distr
ibution are found to vary with the nature of the thiol and the matrix.
For example, in freon the thiyl peroxyl O-17 couplings for the termin
al and inner oxygens ((17)O1, (17)O2) differ for primary (79, 62 G), s
econdary (84, 57 G), and tertiary alkyl thiols (96, 51 G), whereas in
aqueous systems or methanol all thiols yield RSOO(.) radicals of appro
ximately the same couplings (80, 62 G). All RSOO(.) species in polar m
edia have a visible absorption (lambda(max) = ca. 540 nm) and are foun
d to undergo photoisomerization to RSO(2)(.) and subsequent oxygen add
ition to form RSO(2)OO(.). About 5-10% of the spin density is found on
the sulfur atom. Results found in neutral or acid aqueous glasses sho
w no pH dependence of either the O-17 hyperfine couplings or visible a
bsorption maximum. The degree of charge transfer and the varying oxyge
n couplings are suggested to be a function of the ability of the mediu
m to act as an electron pair donor. For tertiary thiols solvent access
is sterically hindered in freons, which results in O-17 couplings and
spin distribution much like that found for a usual carbon-centered pe
roxyl radical. The solvent-stabilized charge transfer state, RS(+)OO(.
-), is found to be far more thermally stable than the uncomplexed stat
e, which is found to react likely by thermal isomerization to RSO(2)(.
) at 100 K. Ab initio MO calculations are found to mimic the charge tr
ansfer state by association of negative ions such as OH- or F- with th
e sulfur atom.