摘要:
1975 553Unstable Intermediates. Part CL1I.l Radicals in Thallous NitrateBy Martyn C. R. Symons,' Douglas X. West,t and James G. Wilkinson, Department of Chemistry, TheUniversity, Leicester LE1 7RHExposure of thallous nitrate to 6oCo y-rays a t 77 K gave NO3 radicals which exhibited strong charge-transferinteraction with two equivalent thallous ions and a weaker interaction with two others. Two well defined specieswere formed, one being lost preferentially on annealing above 77 K. These results are rationalised in terms ofthe crystal structure of the nitrate.Exposure at room temperature gave NO, which again exhibited strong hyperfine interaction with four thallousions, together with a second species, possibly also NO,, but in a completely different environment, with one verystrongly coupled thallous ion.The thallous hyperfine coupling was essentially isotropic, which indicates thatthe interaction stems from an electron donation from TI+ to the radical, which is the reverse of that normally associ-ated with alkali-metal radical-anion systems.WE have recently found that cations having an electronconfiguration - - * d10,s2 can act either as electron-donors or as electron-acceptors depending upon thedemand of their partners., This effect is well demon-strated by e.s.r. spectroscopy when the partner is aradical. Thus, for example, Pb2' acts as an electron-donor with NO, or NO, radicals, but as an acceptor withNO,,- radical^.^ Thallous ions would be expected to bebetter electron-donors, but poorer acceptors thanplumbous ions, and in the present studies its roleappears to be invariably that of donor.We stress thatthe difference between these roles is readily detected bye.s.r. spectroscopy since partial donation gives anincipient - - dlo,sl configuration leading to a largehyperfine coupling and no anisotropy or g-shift, whilstpartial acceptance leads to an incipient - + - dl0,s2,p1configuration with a. consequently small, strongly aniso-tropic, coupling and appreciable g-shift.We are not aware of any previous studies of radicalsin irradiated thallous salts. However, in addition toour study of lead nitrate,3 a recent report on irradiatedlead(11) salts gave evidence for the species C0,-(Pb2+),H,kO,-( Pb2-+), and C102(Pb2+) .4EXPERIMENTALThallous nitrate (B.D.H.) was used without furtherpurification in most experiments since the salt recrystallisedfrom water showed no spectral differences. The highertemperature forms were prepared by heating in unsealedtubes a t 380 K (p) and 440 K (a) for 2 h and immersiondirectly into liquid nitrogen. Samples were irradiated in aVickrad 'j0Co source, a t a nominal dose rate of 4 MCi h-1 forup to 2 11.E.s.r. spectra were measured between 77 K androom temperature, and a t 4.2 I< on a Varian E3 X-bandspectrome ter .RESULTS AND DISCUSSIONThallium has two abundant nuclei, 205Tl[I = 1/2,70.4870] and 203Tl[I = l/2, 29.52y0] with such similarmagnetic moments that the transitions are normallycoincident, as in the present work.The hyperfinecoupling for T12' ions in aqueous solution was found to7 On Sabbatical Leave from Central hlichigan University,Mount Pleasant, Michigan, U.S.A.be ca. 43 000 G,5 so that even weak charge-transferinteractions can lead to large splittings in e.s.r. spectra.This accounts for the fact that the e.s.r. features inirradiated thallous nitrate cover a range of ca. 10oO G(Figure 1). In all three studies the powder spectrademonstrate that the e s r . spectra are almost isotropic,1 3200 GI b )MJp\I I I I I I 1 1 1 I IB A CD B O C A C DI l l 1 1 I I I I IO C A C O B C A A B B I3 B C BS. P. hlishra, I(. 1;. S . Rao, and M. C. R. Symons, J . Phys.M. C. I?. Symons, D. X. West, and J . G. Wilkinson, J.C.S.Chem., 1974, 78. 576, is taken as Part CLI.Chem. Comm., 1974, 108.I uv.FIGURE 1 First derivative X-band e.s.r. spectra for thallousnitrate after exposure t o 'j0Co y-rays at 77 K, showing sets offeatures assigned t o NO, interacting with four neighbouringthallous ions : (a) the y-form showing A and B features; (b) thea-form showing the outer features for A, B, C and D features[Note the second order splitting for the inner features for .4 andB.1so that little would be gained by the use of singlecrystals, and this has not been undertaken.Identijicatioiz of RadicaZs.-Since no coupling to 14Nwas resolved in the initial spectrum obtained fromirradiated thallous nitrate, even at 4.2 K, we setA(14N) < 5 G. This means that neither norNO, can be responsible for the observed features, andM.C. R. Symons, D. X. West, and J. G. Wilkinson, J.C.S.* H. C. Starkie and hI. C. R. Symons, J.C.S. Daltoii, 1974, 731.M. C. R. Symonsand J. K. Yandell, J . Chem. SOC. ( A ) , 1971,Dalton, 1974, 2247.nc554 J.C.S. Daltonleaves NO, as being most probable. In our study oflead(r1) nitrate the e.s.r. spectrum assigned to NO,differed from that normally obtained in that the g-valuevariation was greatly reduced, all three componentsbeing close t o the free-spin value. Absence of anydetectable anisotropy for the species in thallous nitrateleads us to postulate that a similar effect is occurring inthis instance. The hyperfine coupling to 14N of ca. 3 Gexpected for NO, would be lost in the line-width evenfor the narrowest lines detected in this study.However, one of the species formed in thallous nitrateat room temperature comprises sets of triplets (Figure 2)which closely resemble features for NO, in the solid-state,',* although they are less well resolved than usual.We therefore confidently assign the major features inthis spectrum to NO,.The wide doublet of closelyspaced lines in Figure 2 is less readily assigned. If thelarge coupling is assigned to one very strongly coupledthallous ion ( A + 1100 G), the sets of lines separated byca. 20 G must stem from a mixture of interactions withthallium and nitrogen nuclei. This limits A(14N) toca. 20 or ca. 40 G, the latter being preferable in terms ofpredicted intensities. The species cannot be NOZ2- orNO,2- since these would not act as electron acceptors,and we therefore tentatively conclude that the species isa modified NO, radical, strongly interacting with onethallous ion.The NO, Centre.-This centre in the normal salt (7)gives rise to two clearly separate sets of lines, labelledA and B in Figure 1.Both sets can be analysed in termsof two pairs of equivalent Tl+ ions, four Tli ions for each32OO G* H+ 2 +1 0 -1 -2( E l (El (El (El (ElFIGURE 2 First derivative A'-band e.s.r. spectra for thallousnitrate after exposure to GOCo y-rays a t room temperature andcooling to 77 I<, showing features assigned to NO, radicals(E) interacting with four equivalent T1+ ions having Aieo =200 G and (F) outer features assigned to NO, interacting withone strongly coupled T1+ (A = 1100 G) and three weaklycoupled Tlf ions ( A + 20 G)centre.For A, the strongly coupled pair of T1' ionshave AiSo(A1) = 550 G and the weakly coupled pair have13 G. W. Chantry, A. Horsfield, J. R. Morton, and D. H.Whiffen, Mol. Phys., 1962, 5, 689.P. W. Atkins and M. C. R. Symons, J . Chem. SOG., 1962,4794.AiW(A2) = 280 G. For centre B, similarly, B, = 410 Gand 23, = 320 G. These results can be interpreted interms of the crystal structure.Thallous nitrate exists in three closely similar crjvstalforms, the normal variety, designated y, having arhombic structure, with eight molecules per unit cell.The p-form, stable above ca. 350 K has a trigonal0 - 4.35 ACrystal structure for y-thallous nitrate showing thesub-cell containing eight thallous ions together with a centralnitrate ionFIGURE 3structure, whilst the a-variety, stable above ca.420 K iscubic.g However, all forms share a sub-cell comprisingan array of eight cations with a body centred nitrate ion.We can discuss the present results in terms of this sub-structure. Since only four of the eight cations interactappreciably in either the A or B species of NO,, wesuggest that after the electron is lost, coulombic forces,which normally force NO,- to maintain the body-centred site, are lost, and in order to maximise thecharge-transfer interactions, which fall off sharply withincreasing separation, the NO, radical moves towardsone of the six cell faces. The four interacting thallousions are then differentiated by the fact that one pair willtend to interact via two oxygen atoms and the otherpair via the third (Figure 3).There are now two ways in which A and €3 can begenerated, one being through alternative orientationwithin a given face, and the other being the choice ofalternative faces, with one specific orientation of NO,dominating for each face.These alternatives can be differentiated by consider-ing the thermal modifications.Three significant factsemerge : (i) all hyperfine coupling constants are markedlydependent upon temperature (Figure 4), and (ii) theline-widths show selective broadening (especially forA), and (iii) the lines broaden beyond detection for Xin the region of 120-130 K. (Unfortunately, A is lost,irreversibly, in the region of 160 K and hence it is im-possible to achieve a line-narrowing on increasing thetemperature.) The features for B broaden in the170-180 K region, but narrow lines, presumablyassignable to B, reappear in the ZOO+ K range, although* J.H. Sharp and M. C. R. Symons, J . Chem. SOG. ( A ) , 1970,3076.9 F. Wallerant, B1111. SOG. Fvaizcr Jlineral, 1905, 28, 3111975 555they are rapidly and irreversibly decaying at thosetemperatures.We conclude that A and B are not connected by asimple rotatory motion which would make all fourthallous ions equivalent. This eliminates the firstalternative set of structures indicated above. Theresults are, however, quite compatible with the secondmodel, which we adopt. We suggest that species A,having the two most strongly coupled cations, com-prises NO, in either of the two faces sharing the twolonger sides (ca.4.35 A each), whilst B comprises NO,in any of the four faces having two shorter sides (3.99 A)(as depicted in Figure 3). This is based on the fact thatthe concentration of B is roughly twice that of A asrequired statistically. It is not clear why two of theI I I0 50 100 150 200 250 300T l KFIGURE 4 Temperature dependence of the hyperfine coupling(a) At about this temperature constants for centres A and B.the A features become undetectableTli- ions are more strongly coupled in .4, but thisprobably is a subtle function of the effective overlapbetween the 6s orbital on thallium and the orbital ofNO,, which is confined to the in-plane oxygen 2p-orbitals.The temperature dependence probably reflectsthe crystal expansion. However, slight movement ofNO, back towards the face-centred site is also likely asthe temperature increases, although this is never enoughto introduce a coupling to the more distant set ofthallous ions. Such a movement might facilitate jumpsbetween opposite faces at rates sufficient to cause line-broadening, and the distance involved is greater (4.35 A)for B than for A (3.99 h;) so A should broaden at a lowertemperzture than B as is observed. Various othermigrations could cause line-broadening, and may wellalso contribute to the overall effect. One probablemovement is for NO, to pivot about the two moststrongly bonded thallous ions so that the more weaklyinteracting pair are exchanged for those diagonallyopposite.This would initially lead to an apparentreversible fall in intensity, which appears to be the case.Eventually, however, the three components for the twomore weakly coupled Tl+ ions would become five,covering the same field range. However, extra lines inthe expected regions were never observed.We have endeavoured to 'freeze i n ' the a- and p-crystal modifications by quenching heated material inliquid nitrogen, but the results were relatively un-informative. In both cases A and B were again formed,though generally B dominated in these experiments.However extra features, labelled C and D in Figure l bappeared from new, well defined, centres.Althoughcomplete analysis of these new features was not possiblebecause too few lines were defined, nevertheless bothare clearly very similar to A and B, and must compriseNO, trapped in subtly different sites. We suggest thatthese are preferentially formed near to ionic vacanciesinduced by the heat treatment. This would accordwith the fact that centres C and D are far less thermallystable than A or B.The NO, Centres.-These centres decay a t roomtemperature, and hence it proved to be very difficult toaccumulate sufficient for satisfactory e.s.r. analysis.The fact that they appear during the annealing processstrongly suggests that N 0 2 - centres are initially formed,but not detected:NO,- NO, + e- (1)NO,- + e-- NO:- (2)(3) Tl+ + NO$--+ T10- + NO,We suggest that NO,,- ions have features that are tooseverely broadened to be detected in the presence of theintense NO, lines because they act as electron donorstowards thallous ions and this should lead to stronglyanisotropic features, as was observed for NO,,- coupledto "Pb2+ ions.,The major NO, centre (central features in Figure 2)exhibits strong coupling to four equivalent thallous ions( A = 200 G) at 77 K.We suggest that NO,, like NO,,nioves towards one of the faces of the parent ' cube,' inwhich case some form of rotation is necessary in order torender the four thallous ions equivalent. This is mostlikely to be about the long axis, and such a rotation isknown to be ready even at 4.2 K.7 In fact, this rotationis evident in the form of the hyperfine features as shownin Figure 2. Although the lines are broad, the form ofthe spectrum is caused by averaging the x and x com-ponents, leaving y as apparent parallel features. If thisis correct, we find ' A I ' = 60 G and ' A II ' = 55, whereasthe predicted values are 56-5 and 45 respectively. Thesmall anisotropy suggests some libration about theother two axes,* whilst the increase in Aiso, despite thedelocalisation onto four Tl+ ions, suggests an increase inthe bond angle, induced by the ' co-ordinated ' thallousions.As mentioned above, the remaining outer features aremore difficult to analyse. We favour a 14N coupling ofca. 40 G but this is by no means certain. In this casethe NO, (if correctly identified) has moved towards oneT1+ ion and is probably bonded via nitrogen. Thi556 J.C.S. Daltonwould now be expected to decrease the bond angle for There is also a weak interactionNO,, which would account for the fall in the isotropiccoupling. It is not to us why favour We thank the University of Leicester for a Fellowship totwo such different modes of interaction, but in view ofthe uncertainties in identification further speculation 1411622 Received, 2nd August, 19741is surely unwarranted.(-20 G) to three other Tl' ions.J. G. w
ISSN:1477-9226
DOI:10.1039/DT9750000553
出版商:RSC
年代:1975
数据来源: RSC