Faraday Discuss. Chem. Soc., 1984, 78,107-1 19 Charge-transfer Processes in the Quenching of Excited Uranyl Ion by Organosulphur, Organohalogen and Organometal Species BY HANNA B. AMBROZ,~ KEVIN R. BUTTER AND TERENCE J. KEMP" Department of Chemistry, University of Warwick, Coventry CV4 7AL Received 2nd May, 1984 Strong quenching of the emission of excited uranyl ion, [U022+]*, is found on addition of the molecules RHal (Hal = Br, I), R2S and R4M (M = Si, Ge, Sn, Pb), both in terms of the intensity and lifetime of the emitting species as determined by 347 nm laser flash photolysis in acetone solution. Kinetic studies have been supplemented by quantum-yield determinations (for UlV), product analysis and, in certain cases, by e.s.r. investigation of the irradiated system at 77 K.Second-order rate constants k2 for the quenching process fall in the region 107-6x lo9 dm3 mol-' s-' and, in general, correlations can be found between log k2 and the ionisation potential of the donor for a particular series. Plots for the series R4Sn and RHal yield gradients of - 1.12 f 0.10 and - 1.6 I f 0.07 eV-', respectively, while that for the series cyclo-R2S is markedly different (-0.69 k0.07 eV-I). (None of these gradients conforms to the figure of - 16.9 1 eV-' expected from the linear sloping section of a so-called Weller plot.) +(U'") is generally rather low (< 0.07) and in the case of RHal extremely so, although U'" yields could not always be determined because of precipitation of photochemical products. Organic products were identified by gas-chromatography-mass-spectrometry. E.s.r.examin- ation indicated that one-electron transfer from R2S to [U02"]* takes place to yield the species (R2S)2'+ as the identifiable radical product, although a monomeric radical is found under certain conditions. (R2S)2'+ was also observed as a transient species during laser flash photolysis of tetrahydrothiophene in the presence of U02'+ ion, while Me2S, yielded Me2S2'+. Mechanisms of the quenching processes are discussed in terms of fast, largely reversible electron transfer and the intermediacy of exciplexes. Excited-state processes of the dioxouranium(vr) (or uranyl) ion, [UO,]", have been the subject of intensive study over many years for a variety of reasons, both academic and applied, covering the analysis, separation and purification of uranium, including isotopic separation, and they have been reviewed from time to These processes are marked by their extreme diversity, including lumines- cence from the monomeric dictation,'-3 from an excimer4 and from ex~iplexes,~ non-radiative processes uia both thermally activated and non-activated pathways,6 and both reversible and irreversible electron- and hydrogen-atom-transfer processes from suitable which include inorganic and covalent molecules" and saturated, unsaturated and aromatic molecules.273 Interaction with organometallic molecules such as metallocenes' I and metal carbonyls12 appears to involve principally simple electron transfer to give reasonable yields of redox products, Cp,M'+ and M(CO),'+.However, interaction with iodo- and bromo-alkanes, while being kinetically extremely efficient, involves no detectable net charge transfer, and fast, reversible exciplex formation offers the best expla- nation.I3 t On leave from the Institute of Atomic Energy, ORiPI, Swierk, Poland.107108 QUENCHING OF EXCITED URANYL ION Table 1. Quenching of [U022+]* by alkylmetals {[U022+] = 0.08 mol drn-,, medium: acetone (no added acid)) compound (number) I/eV AG&"/kJ mol-l k,/ lo9 dm3 mol-' s-' SnMe, SnEt, SnPr", SnBU", SnPr', SnBu", Sn Et Me, SnBu"Me3 PbMe, PbEtMe, Pb Et, Me, PbEt3Me PbEt, SiMe, SiEt, SiEtMe, Si Bu'Me, GeMe, GeEt, 1 9.69' 2 8.93' 3 8.82' 4 8.76' 5 8.46' 6 8.45' 7 9. I 0' 8 9.00' 1 8.90' 2 8.65' 3 8.45 4 8.26' 5 8.13' 1 9.98' 2 9.78' 3 9.70' 4 9.34' 1 10.02' 2 9.41' ( a ) M=Sn -15.01 -70.75 -78.8 1 -83.2 1 -105.21 -89.08 -58.28 -65.6 1 ( b ) M=Pb -72.75 -9 1.28 - 105.95 -1 19.88 -129.41 ( c ) M=Si +6.25 -8.41 - 14.28 -49.68 ( d ) M=Ge +9.I9 -35.55 0.16 * 0.007 1.7 1 * 0.05 1.40 * 0.05 1.70 f 0.08 4.60 f 0.12 2.96 * 0.06 0.56 f 0.01 0.80*0.015 2.77 f 0.14 3.71 f0.17 5.54 f 0.28 5.48 * 0.17 5.66 * 0.26 < 0.002 0.0243 * 0.0077 0.0096 f 0.0005 0.059 f 0.0029 0.0 178 * 0.0009 0.0429 f 0.0022 a Calculated from AG;, = E"(D/D+) - E(A-/A) - 'AE,,,(A*) [ref. (16)], but Data neglecting the term e;/&r; 1 converted to E"(D/D+) by treatment of ref. (17). from ref. (17). Data from ref. (18). We describe here the results of quenching [UOz2+]* with a range of organometals (Group IVB alkyls), organosulphur compounds and further halogenoalkanes, aug- menting the kinetic work, which is based on 347 nm laser flash photolysis, by product and quantum-yield studies and low-temperature (77 K) e.s.r.spectra in the case of dialkyl sulphides. The results are discussed in terms of established free-energy relationships for excited-state electron transfer. EXPERIMENTAL Laser flash photolysis was carried out at 347 nm (50 ns pulse of 100 mJ energy) as described previously," using either AnalaR grade or purified acetone (which gave similar results) or 50% (v/v) acetone +water as solvents. Quantum-yield measurements were also performed as before." E.s.r. spectra were recorded at 77 K with a Bruker model ER 200 tt spectrometer. Samples were prepared by various methods, all of which gave similar results; typically the liquid thioether was agitated with (moist) crystals of uranyl perchlorate (sometimes with added HC10,) and the organic layer taken and frozen to 77 K prior to photolysis for 1-4 h with a 900 W Xe/Hg point source, the output of which was filtered through both Schott UG5 and Pyrex filters, i.e.hirr = 330-410 nm. Mass spectra were recorded on a Kratos model MS80 spectrometer coupled to a Carlo Erba gas chromatograph equipped with an S.E. 30 column operated at 80 "C.H. 9. AMBROZ, K. R. BUTTER AND T. J . KEMP 109 Table 2. Quenching of [U022+]* by organosulphur compounds ([UO,"] = 0.08 mol dmd3, [HClO,] = 0.30 mol dm-3, medium: acetone) compound (number) I">'/eV AG;,'/kJ mol-,' k2/ lo9 dm3 mol-' s-' ethylene sulphide trimethylene sulphide tetrahydrothiophene pentamethylene sulphide thiophene 2-ethylthiophene 2-propylthiophene thiophene-2-carbonitrile 1,4-dithiane 1,3-dithiane 2-acetylthiophene 3-acetylthiophene thiolacetic acid methylthiocyanate methylthioacetonitrile pro panethiol butanethiol dimethyldisulphide di-n-butylsulphide ( a ) cyclic compounds 1 8.42 - 1 10.60 2 8.65 -90.85 3 8.62 -93.42 4 8.62 -93.42 5 8.87 -7 1.95 6 8.8 -77.96 7 8.6 -95.14 8 9.83 + 10.49 9 8.75 -82.26 10 8.33 - 1 18.33 11 9.20 -43.6 1 12 9.30 -35.03 ( b ) linear compounds I 10.06 +30.24 2 9.96 +2 1.65 3 9.77 i-5:34 4 9.19 -44.47 5 9.15 -47.9 1 6 8.7 1 -85.69 7 8.40 -112.31 2.14* 0.10 2.76k0.14 1.77 f 0.16 1.69 f 0.07 1.50*0.11 1.53 f 0.05 1.91 k0.05 0.24 f 0.04 1.60 f 0.07 2.06f0.10 0.79 * 0.02 0.53 f 0.03 0.48 f 0.04 0.39 f 0.02 1.82 f 0.07 1.44 f 0.1 0 1.81 f0.10 2.83*0.10 I .41 f 0.06 a*b Data from published ref.(19) and (20). ' See footnote ( a ) to table 1; E(D/D+) calculated from I values via correlation E(D+/D+) = 0.89 1-6.04 [ref. (21)]. Simple alkylmetals were obtained commercially, otherwise symmetrical tetra-alkyltin compounds were prepared by the standard reaction of anhydrous SnCl, with the correspond- ing alkyl Grignard reagent in Et20.14 Unsymmetrical compounds were prepared using the appropriate alkylchlorotin compound and the alkyl Grignard reagent.14 Bu'SiMe, was pre- pared from the appropriate alkylithium and SiCI,, followed by further reaction with the appropriate Grignard reagent." The lead alkyls were gifts from The Associated Octel Co. Ltd (Ellesmere Port), to whom we express our thanks.RESULTS LASER FLASH PHOTOLYSIS The lifetime of [U02"]* determined at 510nm in solution was systematically reduced on addition of the various quencher species. Pseudo-first-order rate con- stants were determined at ten concentrations of each quencher to give the statistically averaged values of the second-order quenching rate constant, k2, presented in tables 1-3. Radical-cation spectra were clearly apparent when relatively high concentrations of certain organosulphur compounds were flashed in the presence of aqueous U022+ ion (0.16 mol dmP3); thus tetrahydrothiophene (0.025 mol dmW3) yielded a broad spectrum with A,,, = 465 nm (7 =: 35 ps) essentially identical to that published for the dimer cation while dimethyl disulphide (0.025 rnol dm-3) yielded a similar broad spectrum, A,,, = 460 f 5 nm, attributable by comparison with Me2S2'+, a species known from pulse radiolysis (A,,, = 440 nm in water)23 and e .~ . r . ~ ~ studies.110 QUENCHING OF EXCITED URANYL ION Table 3. Quenching of [UO,,+]* by halogenoalkanes {[UO,”] = 0.08 mol drnp3, [HCIO,] = 0.30 mol dm-3, medium: acetone + H 2 0 (50: 50)) compound (number) IaYb/eV AG”,‘‘/kJ mol-l k,/ lo8 dm3 mol-l s-’ - iodomethane 1 9.54 - 14.40 5.1 1 zk0.31 1 -iodopropane 3 9.26 -38.46 11.30*0.01 2-iodopropane 4 9.18 -45.33 13.4* 0.035 1 -iodobutane 5 9.23 -4 1.04 10.60 f 0.02 1 bromoform 6 10.47 65.45 0.155 * 0.066 bromoethane 7 10.28 49.13 0.326f0.014 I -bromopropane 8 10.18 40.55 0.265 f 0.01 3 2-bromopropane 9 10.07 31.10 0.388 f 0.063 1 -bromobutane 10 10.15 37.97 0.31 *0.01 2-bromobutane I I 10.10 33.68 0.448 f 0.045 1 -iodoethane 2 9.33 -32.40 8.50 f 0.34 Data from ref.(19) and (20). See footnote (c) to table 2. Table 4. Quantum yields for UIv appearance ([UO,”] = 0.08 rnol drnp3, [HCIO,] = 0.30 mol dmP3, irradiation wavelength 401 nm) compound medium #(U‘”> ( a ) organosulphur compounds propanethiol acetone + H20 a butanethiol acetone + H20 di-n-butylsulphide acetone + H 2 0 a 1,3-dithiane acetone + H 2 0 a pentamethylene sulphide acetone + H20 a tetrahydrothiophene acetone + H 2 0 a SiEt, acetone GeMe, acetone iodomethane acetone + H20 1 -bromopropane ( b ) organometallic compounds (c) halogenoalkanes acetone + H 2 0 a 0.0 13 0.0 13 0.023 0.01 1 0.067 0.037 (0.0 1 co.01 <0.005 <0.005 a Acetone +water mixtures were 50 : 50 (v/v).QUANTUM-YIELD DETERMINATIONS These were made for selected compounds, regarded as typical, and are sum- marked in table 4. E.S.R. SPECTRA The photo-oxidation by uranyl ion at 77 K of a number of molecules R2S [R2 = Me,, Et,, Bun2, (CH,),, (CH,), and (CH),] yielded two main paramagnetic species in every case, typified particularly well by Me,S and tetrahydrothiophene. In fig. 1 are shown the radicals from Me2S ( a ) when saturated with uranyl perchlorate (either as wet crystals or dissolved in 1.0 rnol dmP3 HClO,) and ( b ) saturated withH. B. AMBROZ, K. R. BUTTER A N D T. J. KEMP 1 1 1 Fig. 1. Second-derivative e.s.r. spectra obtained on photo-oxidation of Me2S by uranyl perchlorate at 77 K: ( a ) Me$ presaturated with uranyl perchlorate either as crystals (moist) or dissolved in HClO, (1 .O mol dm-3) ; ( b ) Me2S presaturated with uranyl perchlorate dissolved in 70% HC104.Arrow signifies DPPH standard. uranyl perchlorate dissolved in 70% HClO,. Spectrum ( b ) , with a( 12H) = 0.62 mT and g,, = 2.0101 , can be readily assigned to (Me2S)2'+ by comparison with spectral data both in solution and in the solid Spectrum (a), with a(6H) = 2.24 mT and g,, = 2.0041, is regarded as being from a monomeric species. Fig. 2 illustrates the radicals obtained from tetrahydrothiophene ( a ) following saturation with moist uranyl perchlorate prior to photolysis, featuring a monomeric species with seven (?) lines and g,, = 2.0042 [one analysis indicates a(2H) = 3.72 mT and a(2H) = I .86 mT], (c) following saturation with uranyl perchlorate dissolved in 70% HClO, featuring the e s t a b l i ~ h e d ~ ~ , ~ ~ dimeric cation radical with a( 16H) = 0.605 mT and g,, = 2.0103 and ( b ) following saturation with uranyl perchlorate in HClO, (1.0 mol dm-3), when a mixture of the monomer and dimer radicals is obtained.The slightly anisotropic appearance of all spectra is due to the contribution of a minor neutral, sulphur-centred species with high g-tensor anisotropy with g, == 2.06, g,, == 2.00 and g, = 1.97. PRODUCT IDENTIFICATION After prolonged irradiation (36 h) at 364-414 nm of acetone solutions of organotin compounds (0.2 mol dm-3) in the presence of uranyl ion (0.08 mol dm-3), a fine dark green precipitate was formed. The solution was centrifuged to separate the filtrate.The precipitate was washed with acetone and dried at I10 "C. A sample was dissolved in HClO, (6.0 mol dm-3) and its u.v.-visible spectrum taken; a further portion was submitted to analysis by atomic absorption spectroscopy. The sample112 QUENCHING OF EXCITED URANYL ION Fig. 2. Second-derivative e.s.r. spectra obtained on photo-oxidation of tetrahydrothiophene by uranyl perchlorate at 77 K; tetrahydrothiophene presaturated with ( a ) (moist) uranyl perchlorate crystals, (15) uranyl perchlorate dissolved in HC104 ( 1 .O mol dm-3) and (c) uranyl perchlorate dissolved in 70% HC104. Arrow signifies DPPH standard. was found to contain 10% UIv and 45% Uvl (based on the weight of the precipitate); the atomic absorption analysis gave a zero result for Sn.The u.v.-visible spectrum of the filtrate gave only the bands of the UO;+ ion, while the g.c.-m.s. analysis of the photolysate from tetra-n-butyltin indicated the presence of three products, uiz. (i) those attributable to the direct photolysis of acetone solvent, (ii) octane and (iii) heptan-2-one. We could not detect butane but cannot be confident of its absence. DISCUSSION Tables 1-3 indicate highly effective quenching of [U022f]* by a variety of alkylmetals, organosulphur compounds and halogenoalkanes. In fig. 3-5 are presen- ted plots of log k, against I from which the following features are apparent. (i) All plots are linear with the partial exception of data for the organolead compounds, which do tend to a plateau value near the diffusion-controlled rate.The slopes of these plots are as follows: cyclic organosulphur compounds, -0.69 * 0.07 eV-' : organotin compounds, - 1.12 f 0.10 eV-' ; halogenoalkanes, - 1.6 1 f 0.07 eV--'. None of these figures corresponds to that of -16.9 eV-' expected for full electron transfer in the endoergonic region referring to the model devised by Rehm and Weller,16 even though some of the data relate to this region. (The proportionality between the ionisation-energy and electrode-potential scales idnot unity, and various correla- tions have been suggested and reviewed;28 the proportionality constant is in the range 0.7-0.9 depending on the class of compound.) (ii) In the highly exoergonicH. B. AMBROZ, K. R. BUTTER AND T. J. KEMP 113 9.8 9 . 5 9 . 0 8 - 5 Y 00 - 8 - 0 8 .5 7.0 2 R4 Ge 0 1 R4Si and 03 I I 1 1 8 .1 8 . 5 9.0 9 . 5 10.0 I l e V Fig. 3. Dependence of log k2 upon ionization energy ( I ) for quenching of [UOZ2+]* by alkylmetals in acetone solution: 0, lead alkyls; A, tin alkyls; +, silicon alkyls: V, germanium alkyls. Numbering as in table 1 . region the rates generally fail to achieve those expected for electron transfer. (iii) In the endoergonic region the rates generally surpass those expected for full electron transfer. The models for electron-transfer processes developed by Marcus,29 Rehm and WellerI6 and Scandola and Balzani3' have been well reviewed by Eberson2* and will not be reiterated in full here. The kinetic scheme is as follows: k2 I k32 which yields for the observed quenching rate constant, k2, kl2 k2 = k231 I4 QUENCHING OF EXCITED URANYL ION - \*OX6 3 X I I 1 1 I 1 1 I \ I 1 8-6 9 - 0 9.4 9 -8 IleV Fig.4. Dependence of log k2 upon ionization energy (I) for quenching of [UOz2’]* by organosulphur compounds in acetone solution: 0, cyclic compounds; x , linear compounds. Numbering as in table 2. For the electron-transfer step, k 2 3 , we have k23 = K O exp ( - A Gg3/ RT) k 2 3 / k32 = K23 = exp ( - AG023/ RT). (3 1 (4) Simplifying assumptions lead to eqn ( 5 ) , with k 2 1 / ~ 0 often being taken as 0.25: ( 5 ) kl2 1 +KO [exp (AG:,/RT) +exp (AG&/RT)] k, = k2 I while AG;3 is related to the calculable AGz3 by an empirical relation: where AGi3(0) is the ‘intrinsic barrier’ to electron transfer, ie. the activation free energy when AG& equals zero. AGi3(0) was taken originally’6 as 10.05 kJ mol-’, and such a value operates satisfactorily for a large number of electron-transfer systems,28 although several have emerged requiring much higher values of AGl3(0),H. B.AMBROZ, K. R. BUTTER AND T. J. KEMP 115 9 . 2 9 . 6 10-0 10.4 IleV Fig. 5. Dependence of log k2 upon ionization energy ( I ) for quenching of [U0,2']* by halogenoalkanes in acetone +water solution. Numbering as in table 3. e.g. the oxidation of aliphatic amines in MeCN by excited tris(2,2'-bipyridine) complexes of CrIII, when AGi3(0) = 20 kJ m ~ l - ' , ~ ~ , ~ ' and the fluorescence quenching of various aromatic ion radicals, when values > 80 kJ mol-' appear necessary.32 Another system showing strongly deviant behaviour is the quenching of aromatic triplet states by Eu3+, when the maximum or 'plateau' rate in the exoergonic region is ca.lo6 dm3 mol-' s-I, a shortfall attributed to an anomalously small transmission ~oefficient.~~ While the Rehm-Weller treatment predicts a linear region in the plot of log k2 against AG;3 of slope -1/2.303RT when AG023 is highly endoergonic, there are reports of linearity over a much wider range of AG;3, e.g. from -90 to 90 kJ mol-' in the fluorescence quenching of various aromatic molecules by inorganic anions34 and from -25 to 90 kJ mol-' in the quenching of triplet ketones by inorganic anions.35 Linear plots covering a range in AG& of 90 kJ mol-' are also reported by Kuzmin et al. for quenching of 9,lO-dicyanoanthracene in heptane and of exciplexes in benzene.36 These various groups employ the Polanyi equation:" AGi3 = aAGZ3 +p.(7) p is equivalent to AGi3(0), and one may simplify eqn (1) by assuming that k30 >> k 3 2 and kZ1 >> k23, yielding eqn (8): k2 = k 1 2 k 2 3 / k 2 1 - (8)116 QUENCHING OF EXCITED URANYL ION Table 5. Values of a and p from eqn (7) for quenching of [UO,'+]* and other systems system CY P/kJ mol-' [ U 0 22 '1 *-S n R4 1 1.66 f 0.72 [U02*+]*-cyclic R2S 0.046 f 0.004 8.46 f 0.40 [U022+]*-halogenoalkanes 0.1 10 f 0.004 9.69 f 0.20 transition metal organometallic molecules 0.120 f 0.003 26.0 1 f 0.40 ['ArH]*-inorganic anions a 0.1 14 11.6 [3Ar2CO]-inorganic anions 0.207 10.3 0.088 f 0.008 Data from ref. (38); ' data from ref. (35). This may be rewritten as and again as k2=(1 ~ 1 0 ~ ~ ) e x p ( - A G ~ , / R T ) .(10) Plots of our various data in the form log k2 against AG;, (fig. 6) indicate a strong discrepancy with the Weller equation using AG:,(O) = 10.05 kJ mol-'. The pronoun- ced linearities suggested an attempted fit to the Polanyi equation in the form of plots of AG:,, calculated from eqn (lo), against AG;,, which should be linear according to eqn (7). These are shown in fig. 7, together with data for the quenching by metal carbonyls]' and metallocenes" of [U022+]*, and yield values for a and p listed in table 5. The data utilised in fig. 7 cover a wide variation in AG;, and confirm the applicability of the Polanyi equation to luminescence quenching of [Uo2"]*. The values of &,dox in these systems are always low (exceedingly so in the case of the halogenoalkanes), implying either highly efficient back electron transfer between the ion radicals, RHal'+ and Uv or a mechanism based on an exciplex involving weak charge transfer from a lone pair on the halogen atom to U022'. While &.dox as measured by 4(U'") is very low for tetra-alkylmetals, in the case of tetra-n-butyltin both octane and heptan-2-one were found, indicating path- ways (1 1)-( 14): Bun4Sn + [UO,"]* + Bu",Sn" + Uv (1 1) Bu",Sn'+ --+ Bu",Sn' +Bun' (12) 2Bu"' --* n-CgHig (13) (14) Bun' +CH,COCH, (ex solvent) --+ n-CSHI ]COCH3.In the case of the sulphur compounds, @(U'") is appreciably greater, and in a couple of instances the sulphur-centred cation radical was observed directly following the laser pulse. This process was confirmed in other cases by e.s.r.identification of radical products on photolysis of a glassy matrix of the organosulphur compound containing U022+ perchlorate at 77 K. At relatively high acidities the product was unmistakably the dimer cation identified by its g-tensor and the proton- coupling pattern (fig. 1 and 2); at low acidity or in neutral solution another species is produced with intriguing features, namely the proton-coupling pattern and hyper- fine coupling constant expected of the monomeric species R2S'+ but with an isotropicH. B. AMBROZ, K. R. BUTTER AND T. J. KEMP 117 \ - - - - _ _ - - - \ 10.0 " . O i \ \ 1 -._ R,Pb \ R W I D-m--D._ \ \ 2 nn 9 * 0 t 8 . 0 . A - + :: \ I \ I I I I I 1 I 1 I 1 -120 -80 - 40 0 40 80 AG&/kJ mol-' Fig. 6. Plots of log k, against AG& for quenching of [U02'+]* by lead alkyls (O), tin alkyls (A), cyclic organosulphur compounds ( X ) and halogenoalkanes (0).The broken curve denotes rates calculated on the basis of the Rehm-Weller treatment, using AG&(O)= 10.05 kJ mol-'. g-value of only 2.0041 *0.0001 (for both Me,S and tetrahydrothiophene), which is far from that associated with this type of species, e.g. 2.0113 for Me2S.+.39 A carbon-centred radical seems to be excluded by the seven-line and binomial spectrum exhibited from Me,S and a five-line binomial spectrum given by Et,S ( a , = 2.04 mT, g = 2.0029), and we favour a monomeric cation-radical species experiencing a g-shift by virtue of a local environmental effect. The pronounced effect of acidity upon this type of radical is well established for sulphur compounds in solution although in the solid state matrix effects must also be significant in controlling ion-molecule reactions.The minor species found in all spectra is attributed to RS- SR2, which features similar values for the components of the g-tensor.,' Finally, the interaction of [UOz"]* with the rnetallocenes gives a high yield of charge-separated species, with +(Cp2M+) = 0.62 for M = Fe," and the anomalous behaviour of the organometallics must refer to photoelectron transfer rather than an alternative process. In summary, quenching of [UOZ2+]* by sulphur-, metal- and halogen-centred compounds proceeds, with the possible exception of the last, by electron transfer to give U'", cation radicals and, ultimately, organic products derived from these. The relations between k2 and AG;3 appears to fit the Polanyi equation rather well.118 QUENCHING OF EXCITED URANYL ION 4 I t I I I I I -200 -1 60 -120 -80 -40 0 40 80 AG';,/kJ mol-' Fig.7. Plots of AGi3 [where k2 = 1 X 10" exp (-AG:,/RT)] against AG;3 for the quenching of [UO,"]* by tin alkyls (A), cyclic organosulphur compounds ( x ), halogenoalkanes (0) and organo-transition metals (OTM, +); the latter refer to quenching by metal carbonyls" and metallocenes.' We thank British Nuclear Fuels Ltd. and the S.E.R.C. for support of K.R.B. through a CASE award and the S.E.R.C. for a Visiting Fellowship to H.B.A. Mr M. A. Shand began exploratory work on the Uv'-organosulphur systems. We thank Mr H. G. Beaton for the synthesis of a number of organometallic compounds.I E. Rabinowitch and R. L. Belford, Spectroscopy and Photochemistry of Uranyl Compounds (Per- gamon Press, London, 1964). H. D. Burrows and T. J. Kemp, Chem. SOC. Rev., 1974, 3, 139. H. 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