Sm Bullet Research Articles

ChemInform Abstract: Radiation Chemistry of Cyanine Dyes: Oxidation and Reduction of Merocyanine 540.

Merocyanine 540 (MC) shows promise as a treatment for certain types of leukemia. It is shown that MC readily undergoes one-electron reduction under pulse radiolytic conditions. The {pi}-radical anion, produced by reduction with hydrated electrons and 2-hydroxypropyl radicals, disproportionates rapidly (k = 1.9 {times} 10{sup 9} M{sup {minus}1}s{sup {minus}1}) under anaerobic conditions but reduces O{sub 2} to superoxide ions (k = 1.6 {times} 10{sup 9} M{sup {minus}1}s{sup {minus}1}) in aerated solution. The dye reacts with trichloromethylperoxyl radicals (k = 9 {times} 10{sup 8} M{sup {minus}1}s{sup {minus}1}) to form several products, one of which is believed to be an adduct formed by addition of CCl{sub 3}OO{sup {sm bullet}} to the bridgehead carbon atom of the benzoxazole subunit. This species decays via first-order kinetics (k = 4.0 {times} 10{sup 3} s{sup {minus}1}) under pulse radiolytic conditions to form cleavage products. A second primary product is believed to arise from addition of CCl{sub 3}O{sub 2}{sup {sm bullet}} to the polymethine chain to form an {alpha}-amino carbon-centered radical capable of reducing O{sub 2} to superoxide ions. Preliminary studies indicate that the breakdown products are cytotoxic and could be important intermediates for the known antiviral activity of MC.

ChemInform Abstract: Conformational and Substituent Effects on Spin Distributions in Porphyrin Cation Radicals

Optical, redox, EPR, and ENDOR results are presented for zinc meso-tetraphenyltetrabenzoporphyrin (ZnTPTBP). The molecule is a hybrid of the common synthetic porphyrins zinc tetraphenylporphyrin (ZnTPPP) and zinc tetrabenzoporphyrin (ZnTBP). The consequences of the additional peripheral substituents in the hybrid are evident in a previously obtained x-ray structure of ZnTPTBP that shows the macrocycle to be severely puckered to minimize steric interactions between the substituents. The cumulative effects of conformational changes and substituent addition are reflected in the red-shifted optical spectrum and ease of oxidation of the neutral compound, and in a shift of unpaired spin density from a{sub 1u} (ZnTBP) to a{sub 2u} (ZnTPP) character in the radical. Significant spin density is found to delocalize onto the phenyl substituents of ZnTPTBP{sup {sm bullet}+} and illustrates the shortening of the effective distance between donor and acceptor that may occur in biomimetic complexes used to investigate the distance and energy dependence of electron transfer.

ChemInform Abstract: Selective Photoinduced Line Broadening in Proton NMR Spectra of 4- Alkoxynitrobenzenes.

Selective photoinduced NMR line broadening as well as selective downfield shifts are observed on photoexcitation of benzene solutions of 4-alkoxynitrobenzenes and of 4-(dialkylamino)nitrobenzenes. In 4-ethoxynitrobenzene the effects are largest for the methylene and for the 3 and 5 ring protons but considerably smaller for the methyl protons. On this basis these effects are attributed to fast electron exchange of intact molecule with the cation radical NO{sub 2}C{sub 6}H{sub 4}O{sup {sm bullet}+}CH{sub 2}R formed by photoejection.

ChemInform Abstract: Perfluorobutylperoxyl Radical as an Oxidant in Various Solvents.

Perfluorobutylperoxyl radicals were produced by pulse radiolysis of aerated solutions of perfluorobutyl iodide. The rate constants for reaction of this radical with several organic reductants, chlorpromazine, trolox, hydroquinone, and several other phenols, were determined in various solvents and were found to be in the range of 10{sup 5}-10{sup 9} M{sup {minus}1} s{sup {minus}1}. By comparison with other haloalkylperoxyl radicals, C{sub 4}F{sub 9}OO{sup {sm bullet}} was found to be a much more powerful oxidant, whose reactions took place more rapidly and were less sensitive to solvent and substituent effects. The rate constants (k) for oxidation of a series of para-substituted phenols by C{sub 4}F{sub 9}OO{sup {sm bullet}} gave a good linear correlation between log k and the electrophilic substituent constant {sigma}{sup +}, with a slope of {rho}{sup +} = {minus}2.3, indicating formation of a positively charged transition state. Parallel experiments with CCl{sub 3}OO{sup {sm bullet}} were limited to the most reactive phenols and gave a higher slope, {rho}{sup +} = {minus}3.3. The rates of reaction of C{sub 4}F{sub 9}OO{sup {sm bullet}} with trolox and chlorpromazine were found to depend on solvent viscosity, but much less on solvent polarity and acid-base properties, probably because they were closer to the diffusion-controlled limit. Themore » longer chain C{sub 10}F{sub 21}OO{sup {sm bullet}} was somewhat less reactive than C{sub 4}F{sub 9}OO{sup {sm bullet}} because of geometric factors.« less

ChemInform Abstract: Kinetics of the Reactions Between Alkyl Radicals and Molecular Oxygen in Aqueous Solution

The rate constant for the reaction R{sup {sm bullet}} + O{sub 2} {yields} ROO{sup {sm bullet}} in aqueous solution was determined for 18 alkyl radicals by laser flash photolysis. The values are all at the diffusion-controlled limit and lie in the range (1.6-4.9) {times} 10{sup 9} L mol{sup {minus}1} s{sup {minus}1}. The radicals studied are primary, substituted primary, secondary, and benzyl radicals.

Pulse radiolysis studies on the redox reactions of aqueous solutions of .gamma.-cyclodextrine/C60 complexes

Pulse radiolytical studies on the water-soluble [gamma]-cyclodextrine/C[sub 60] ([gamma]-CD/C[sub 60]) complex with primary radiolytic species, such as [sup [sm bullet]]OH, [sup [sm bullet]]H, and e[sub aq][sup [minus]] have been carried out in aqueous solution. Rare constant for the formation of the transient products are determined by following the build-up kinetics of the species at the respective absorption maxima and the bleaching of the ground state C[sub 60] at 330 nm. [sup [sm bullet]]OH- and [sup [sm bullet]]H-induced reactions lead to a transient with [lambda][sub max] at 290 nm showing identical formation kinetics as the concurrent bleaching at 330-335 nm. Studies on [gamma]-CD/C[sub 60] complexes and uncomplexed [gamma]-CD indicated that [sup [sm bullet]]OH is mainly reacting with the host ([gamma]-CD) by hydrogen abstraction (employing competition kinetics with KSCN as reference standard). The resulting radical subsequently reacts with C[sub 60] to yield a radical adduct which absorbs at 290 nm. Reaction of the Cl[sub 2][sup [minus]] with [gamma]-CD/C[sub 60] produces a transient species that shows a broad absorption maximum at 380-430 nm. This species is assigned to be the radical adduct (C[sub 60]Cl)[sup [sm bullet]]. 27 refs., 5 figs.

Consequences of Oxidation in Nonplanar Porphyrins: Molecular Structure and Diamagnetism of the .pi. Cation Radical of Copper(II) Octaethyltetraphenylporphyrin

Crystal structures are reported for the sterically crowded porphyrin Copper(II) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (Cu(OETPP), 1) and its [pi] cation radical Cu(OETPP)[sup [sm bullet]+]ClO[sub 4][sup [minus]] (2). 1 was chosen to assess the consequences of oxidation in a nonplanar porphyrin on the expectation that its multiple peripheral substituents not only induce an S4 saddle conformation on the macrocycle but should also prevent the dimerizations in the solid that have complicated several previous crystallographic studies of porphyrin [pi] cation radicals. Interest in the consequences of oxidation arises from the presence of nonplanar bacteriochlorophylls in photosynthetic reaction centers in which the chromophores lie in van der Waals contact so that even small structural changes induced by electron transfer would alter the electronic coupling between the [pi] cation and anion radicals generated by the primary photochemical charge separation. Oxidation of 1 does indeed result in further conformational changes in 2: an additional ruffling is imposed on the original saddle shape of 1 in which the pyrrole rings twist, the meso carbons move alternately up and down out of the porphyrin plane by approximately 0.2 angstroms, and the phenyl groups rotate further into that plane by more than 10[degree]. 61 refs., 11 figs., 4 tabs.

Hole Localization or Delocalization? An Optical, Raman, and Redox Study of Lanthanide Porphyrin-Phthalocyanine Sandwich-Type Heterocomplexes

A systematic electrochemical, redox, optical absorption, and resonance Raman spectroscopic study of 22 gadolinium and cerium double- and triple-decker tetraphenylporphyrin-phthalocyanine (TPP-Pc) complexes is reported. The work is aimed at elucidating the extent of the localization or delocalization among the chromophore ligands in the mixed complexes, where there exists a charge imbalance between the metal centers and the dianion Pc/TPP ligands, as well as in different oxidation states for these types of complexes. For mixed double- and triple-decker complexes, [vert bar]Pc(M)TPP[vert bar] and [vert bar]TPP(M)TPP[vert bar] (where M = Gd[sup III], Ce[sup IV]), in which the metal cannot be oxidized, it appears that the ligand preferentially oxidized is the Pc moiety. This perfentially localized hole in the phthalocyanine results in new near-infrared transitions which show maxima around 1200-1300 nm for the double-decker complexes and shifts to 1800-2300 nm for the triple-decker derivatives. In this case, the removal of an electron from a complex redox orbital which is rich in Pc character is easily identified by the accompanying changes in the optical absorption characteristic of a Pc[sup [sm bullet][minus]] species and is also seen in FT Raman spectra (excited at 1064 nm) by a significant (>15 cm[sup [minus]1]) upshift of a redox-sensitivemore » Raman band at ca. 1500 cm[sup [minus]1]. 33 refs., 13 figs., 2 tabs.« less

Solvent Effects on Rate Constants for One-Electron Oxidation of Porphyrins

Absolute rate constants for one-electron oxidation of free base porphyrins (hematoporphyrin-IX, mesoporphyrin-IX dimethyl ester) by the trichloromethylperoxyl radical and by the radical cation of N-methylindole have been determined in a variety of solvents by the pulse radiolysis technique. All solvents were air-saturated and contained CCl[sub 4] as a source for the CCl[sub 3]O[sub 2][sup [sm bullet]] radicals. The rate constants for reactions of these radicals were measured by following the formation of the porphyrin [pi]-radical cation optical absorption at 700 nm as a function of porphyrin concentration. The rate constants for reaction of CCl[sub 3]O[sub 2][sup [sm bullet]] with the porphyrin were found to vary by an order of magnitude in the different solvents and were correlated with the solvent cohesive energy density. In the second set of experiments, N-methylindole was used in addition to CCl[sub 4] so that CCl[sub 3]O[sub 2][sup [sm bullet]] radicals oxidize this solute to its radical cation, which in turn oxidizes the porphyrin. The variations of the rate constants for oxidation of the porphyrin by the N-methylindole radical cation in the different solvents exhibited different trends than those for oxidation by CCl[sub 3]O[sub 2][sup [sm bullet]] radicals. 39 refs., 1 fig., 6 tabs.

Aspects of the Reaction Mechanism of Ethane Combustion. 2. Nature of the Intramolecular Hydrogen Transfer

A detailed molecular understanding of hydrocarbon combustion processes is recognized as an important goal. The specific system studied (via ab initio theoretical methods) in this work is the reaction between the ethyl radical (CH[sub 3]CH[sub 2][sup [sm bullet]]) and molecular oxygen (O[sub 2]). Theoretical methods used include restricted open-shell Hartree-Fock (ROHF), configuration interaction including single and double excitation (CISD), coupled cluster including single and double excitations (CCSD), and a perturbational treatment of connected triple excitations [CCSD(T)]. Basis sets up to double-[zeta] plus polarization (DZP) quality were used. Theoretical results indicate that there are two distinct but energetically proximate intramolecular hydrogen-transfer transition states on the potential energy surface. One of the structures corresponds to a [sup 2]A[double prime] electronic state (C[sub s] symmetry), the other to a [sup 2]A electronic state (no symmetry); this latter structure is derived from a [sup 2]A[prime] planar structure at which the energy Hessian possesses two imaginary vibrational frequencies. At the best level of theory used, DZP CCSD(T) including zero-point vibrational energy corrections, the [sup 2]A[double prime] transition state lies 4.5 kcal mol[sup [minus]1] above the reactants (CH[sub 3]CH[sub 2] + O[sub 2]). 44 refs., 3 figs., 7 tabs.

Regiochemistry of Radical Addition to C70

The addition of several photochemically generated organic radicals (H[sup [sm bullet]], CH[sub 3][sup [sm bullet]], C[sub 6]H[sub 5][sup [sm bullet]], CF[sub 3][sup [sm bullet]], and C[sub 2]F[sub 5][sup [sm bullet]]) to ellipsoidal C[sub 70] was studied by electron spin resonance. Three of the five possible isomers of RC[sub 70] were detected with R = H, CH[sub 3] and four with R = C[sub 6]H[sub 5], CF[sub 3], C[sub 2]F[sub 5]. Arguments based on relative abundances and on comparisons of hyperfine interactions and g factors of the RC[sub 70] isomers and of their C[sub 60] analogs allowed to place the isomers in homologous series and to identify the sites of radical addition on the C[sub 70] surface leading to each isomer. 30 refs., 4 figs., 1 tab.

Reactivity of a 17e Organometallic Radical in Aqueous Solution

Laser flash photolysis of [(C[sub 5]H[sub 4]CO[sub 2])W(CO)[sub 3]][sub 2] in aqueous solution at 460-532 nm induces homolysis of the metal-metal bond with formation of the 17e radical (C[sub 5]H[sub 4]CO[sub 2[minus]])W(CO)[sub 3]. The rate constant for its dimerization is 3.0 x 10[sup 9] L mol[sup [minus]1] s[sup [minus]1] at 23[degree]C. The radical reacts with haloacetate ions, pseudohalo- and halometal complexes by atom transfer. The reactivity trend is normal (RI > RBr > RCl). The reaction between this radical with t-BuOOH in water occurs similarly with a rate constant of 1.37 x 10[sup 6] L mol[sup [minus]1] s[sup [minus]1]. For electron-transfer reactions, the radical acts as both reductant and oxidant. It oxidizes tetramethylphenylenediamine to TMPD[sup [sm bullet]+] in aqueous solution with a rate constant of 1.49 x 10[sup 7] mol[sup [minus]1] s[sup [minus]1]. The radical reduces MV[sup 2+] and Fe(CN)[sub 6][sup 3[minus]] to MV[sup [sm bullet]+] and Fe(CN)[sub 6][sup 4[minus]] with rate constants of 2.0 x 10[sup 6] and 6.06 x 10[sup 7] L mol[sup [minus]1] s[sup [minus]1], respectively. The phosphine Ph[sub 2]PPhSO[sub 3]Na accelerates the reaction of the radical with MV[sup 2+] very mildly. This may bear on the question of whether the tungsten-phosphine adduct is a species withmore » 19 valence electrons, a slipped cyclopentadienyl ring, or a phosphoranyl radical. 32 refs., 2 figs., 4 tabs.« less

Catalysis to the disfavored product: the case of (.eta.5-C5H5)Mo(CO)3.bul.

The title compound is a 17e organometallic radical, which ordinarily recombines to the dimer in many solvents, including acetonitrile. In the presence of 1,4-(Me[sub 2]N)[sub 2]C[sub 6]H[sub 4] (TMPD), however, the radical disproportionates to ([eta][sup 5]-C[sub 5]H[sub 5])Mo(CO)[sub 3[sup [minus]]] and ([eta][sup 5]-C[sub 5]H[sub 5])Mo(CO)[sub 3]-NCCH[sub 3[sup +]]. The mechanism involves the radical oxidizing TMPD and reducing TMPD[sup [sm bullet]+], whose buildup and decay were directly monitored. 11 refs., 2 figs.

Formation and Reactivity of (Octaethyltetraazaporphyrinato)rhodium Complexes

A series of (octaethyltetraazaporphyrinato)rhodium, (OETAP)Rh, complexes including (OETAP)Rh-I, (OETAP)Rh-CH[sub 3], and [(OETAP)Rh][sub 2] were prepared for comparison with the corresponding (octaethyltetraazaporphyrinato)rhodium, (OEP)Rh, derivatives. [(OETAP)Rh][sub 2] (1) reacts like [(OEP)Rh][sub 2] (2) with CH[sub 3]I, CH[sub 3]NC, (CH[sub 3]O)[sub 3]P, and CH[sub 2]=CH[sub 2] in forming (OETAP)Rh-CH[sub 3] and (OETAP)Rh-I, (OETAP)Rh(NC)(CH[sub 3]NC), (OETAP)Rh-P(O)(OCH[sub 3])[sub 2], and (OETAP)Rh-CH[sub 2]CH[sub 2]-Rh(OETAP), respectively, but reactions of 1 are invariably much slower than those of 2. [(OETAP)Rh][sub 2] fails to react with H[sub 2], CO, CH[sub 3]CHO, and CH[sub 3]C[sub 6]H[sub 5], which participate in prominent substrate reactions for 2. Reactivity and equilibrium studies indicate that a substantially larger Rh-Rh bond dissociation enthalpy for 1 compared with 2 is primarily responsible for the slower rates and reduced scope of substrate reactions for [(OETAP)Rh][sub 2]. Dissolution of 1 in pyridine results in formation of a persistent OETAP anion radical complex of Rh(III), (OETAP[sup [sm bullet][minus]])Rh[sup III](py)[sub 2], which contrasts with 2 where disproportionation produces the (OEP)Rh[sup I] anion and the (OEP)Rh[sup III](py)[sub 2]cation. (OETAP)Rh-CH[sub 3] (RhC[sub 33]H[sub 43]N[sub 8]) crystallizes in the monoclinic system, space group P2[sub 1]/c. The X-ray crystal and molecular structure determination for (OETAP)Rh-CH[sub 3] reveals a smaller hole size and corresponding shortermore » Rh-N distances for the (OETAP)Rh derivative but virtually identical Rh-CH[sub 3] bond distances in comparison with (OEP)Rh-CH[sub 3]. Differences in the structures and reactivities of (OETAP)Rh and (OEP)Rh complexes are discussed in terms of the smaller hole size, lower energy of the [pi]* orbitals, and the doming of the OETAP ligand.« less

Regioselective Rearrangement of Bridgehead-Methyl-Substituted Radical Cations Derived from Bicyclo[2.1.0]pentanes and 2,3-Diazabicyclo[2.2.1]hept-2-enes through Photoinduced Electron Transfer and Radiolytic Oxidation: Product Distribution and Matrix ESR Studies

Cyclopentane-1,3-diyl radical cations were generated from the 1-methyl- and 1,4-dimethyl substituted bicyclo-[2.1.0]pentanes 1b,c through photoinduced electron transfer (PET) and radiolytic oxidation. The unsymmetrical bridgehead-substituted bicyclopentane 1b rearranged spontaneously and exclusively to the 3-methylcyclopentene 3b under PET conditions. ESR studies showed similarly that 3b[sup [sm bullet]+] was the only final oxidation product of 1b; the initial radical cation 1b[sup [sm bullet]+] was not detected because it rearranges rapidly and stereoselectively by a 1,2-hydrogen shift to 3b[sup [sm bullet]+], even at 80 K, and no trace of the more stable 1-methylcyclopentene radical cation 3a[sup [sm bullet]+] was observed. This contra-thermodynamic regioselectivity is rationalized in terms of essential localization of positive charge at the tertiary center as the reaction proceeds in the 1,3-diyl radical cation 1b[sup [sm bullet]+]. The symmetrical dimethyl derivative 1c rearranged much more reluctantly than 1b despite its lower oxidation potential, and this is attributed to the greater persistence of radical cation 1c[sup [sm bullet]+] through its reluctance to undergo a 1,2-H shift. The oxidation of these azoalkanes generates highly reactive transients, presumably diazenyl radical cations, which readily denitrogenate and undergo 1,2-H shifts in either a consecutive or concerted manner to form olefin radical cations. 25 refs., 6 figs., 2more » tabs.« less

Kinetics of the C6H5 + NO Association Reaction

The rate constants for the C[sub 6]H[sub 5] + NO [yields] C[sub 6]H[sub 5]NO reaction have been measured with the cavity-ring-down method between 298 and 500 K to be k = 10[sup [minus]11.35[+-]0.06] exp[(+433 [+-] 111)/T] cm[sup 3]/s. The effect of pressure was examined at 388 K by sextupling of the total pressure from 20 to 120 Torr, with no significant increase in the measured rate constants within the scatter of the data. This is consistent with the k/k[sup [infinity]] ratio predicted for 388 K using the known literature value of k[sup [infinity]] for the unimolecular decomposition of nitrosobenzene. Our result for C[sub 6]H[sub 5] + NO compares reasonably with the values for other R[sup [sm bullet]] + NO association reactions. 20 refs., 8 figs., 2 tabs.

Photoinduced electron and energy transfer in molecular pentads

A series of molecular pentads, each consisting of a porphyrin dyad (P-P) covalently linked to a carotenoid polyene (C) and a diquinone moiety (Q[sub A]-Q[sub B]), have been prepared, and the photochemical properties of these molecules have been studied using steady-state and transient absorption and emission spectroscopies. Each of the pentads undergoes photoinduced electron transfer from the C-P-[sup 1]P-Q[sub A]-Q[sub B] singlet state to yield the charge-separated state C-P-P[sup [sm bullet]+]-Q[sub A][sup [sm bullet][minus]]-Q[sub B]. Competing with charge recombination of this species are additional electron-transfer reactions operating in series and in parallel which converge on a final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state. The electron-transfer rate constants and the quantum yields of the various charge-separated species are sensitive functions of the state energies and the electronic coupling between the porphyrin and diquinone moieties. One of the pentads undergoes photoinduced electron transfer to produce the final C[sup [sm bullet]+]-P-P-Q[sub A]-Q[sub B][sup [sm bullet][minus]] state with a quantum yield of 0.83 and a lifetime of 55 [mu]s. This example of an artificial photosynthetic reaction center preserves about half of the initial excited singlet state energy as chemical potential. Other pentads have charge-separation lifetimes of several hundred microseconds. 28 refs., 10more » figs., 1 tab.« less

Gas-phase ion/molecule reactions of corannulene, a fullerene subunit

Corannulene is intriguing, not only because of its highly-strained bowl-like structure, but also as a subunit of C[sub 60] and other fullerenes. The carbon skeleton of corannulene appears several times in C[sub 60], and its curvature mimics the curvature of C[sub 60]. Inspired by this curiosity, and having previously investigated ion/molecule reactions of C[sub 60] cations, we began an unprecedented experimental investigation of ion/molecule reactions of corannulene. Here we report the first observations of gas-phase ion/molecule reactions with corannulene: reactions of the corannulene cation (cor[sup [sm bullet]+]) with C[sub 60] and of neutral corannulene with Ar[sup [sm bullet]+], cor[sup [sm bullet]+], C[sub 60][sup [sm bullet]+], C[sub 60][sup 2+], and C[sub 60][sup [sm bullet]3+]. Both electronic and topographical features are expected to be of consequence in many of these reactions. 22 refs., 1 tab.

Photoelectron transfer between a magnesium-free-base porphyrin heterodimer and duroquinone. Selective excitation and time-resolved EPR studies

The photoexcited triplet state of a cofacial heterodimer, [Mg-H[sub 2]], comprised of Mg and H[sub 2] porphyrins, and photoinduced electron transfer (ET) between the heterodimer and duroquinone in 1:1 mixture of CH[sub 2]Cl[sub 2] ethanol were studied by selective laser excitation combined with time-resolved CW or with pulsed EPR spectroscopies. ET originates from the photoexcited triplet or triplet radical pair states. Upon photoexcitation of the Mg subunit (580 nm), a noticeable delay time of approximately 20 ns in ET is observed, whereas with photoexcitation of the H[sub 2] part (620 nm), no delay time in ET is noticed. The dependence of ET upon excitation wavelength is interpreted in terms of the formation and participation of a charge-transfer state that is operative at 205 K. The delay time is attributed to an intradimer ET that produces the triplet radical pair state [sup 3][Mg[sup [sm bullet]+]-H[sub 2][sup [sm bullet][minus]]]. Photoexcitation at 620 nm results in ET via the lower-lying triplet of H[sub 2] without involvement of the charge-transfer state. 35 refs., 9 figs., 3 tabs.

Photoinduced hole transfer from titanium dioxide to methanol molecules in aqueous solution studied by electron paramagnetic resonance

An electron paramagnetic resonance (EPR) study is reported on the paramagnetic species formed on irradiation of aqueous TiO[sub 2] colloids in the presence of methanol. In laser (308 nm) irradiated aqueous solution of methanol at 1.9 K, the observed EPR signals are centered around g = 2 and have a line shape characteristic of the CH[sub 2]OH radical. The appearance of these radicals at the very low temperature of 1.9 K implicates charge transfer from the oxygen lattice holes to methanol molecules within a few monolayers of the surface of the TiO[sub 2] particles. Different radicals are formed when CH[sub 3]OH is chemisorbed on TiO[sub 2]. The EPR signals detected in TiO[sub 2] colloidal solution prepared in methanol and then evaporated and dissolved in water also show the formation of the CH[sub 2]OH radical due to oxidation of chemisorbed methanol on the TiO[sub 2] surface. These particles, with high laser pulse intensities, oxidize the chemisorbed methanol further to give the CHO radical. In addition, the CH[sub 3] radical is formed from reducing processes and observed at 6-50 K. In the absence of methanol, holes are trapped by surface hydroxide groups forming TI[sup IV]-O-Ti[sup IV]-O[sup [sm bullet]]. At higher temperature (150more » K) in aqueous methanol solution, the CH[sub 2]OH radical transfers an electron to TiO[sub 2], doubling the yield of formation of surface Ti[sup III] ions which are stable in air-free solution. When an electron scavenger such as Hg[sup 2+] is present in solution, the EPR signal of Ti[sup III] disappears since mercury ions are reduced. 44 refs., 7 figs.« less