Raman Spectra of Thallium(x) Nitrate Solutionsin Liquid AmmoniaBY DEREK J. GARDXNER"Newcastle upon Tyne NEl 8STALI H. HAJI AND BRIAN P. STRAUGHANDepartment of Inorganic Chemistry, University of Newcastle upon Tyae,Newcastle upon Tyne NE1 7RUDepartment of Chemistry, Newcastle Upon Tyne Polytechnic,ANDReceived 17th April, 1975Raman spectra are reported for T1NO3 solutions in liquid ammonia, ranging in composition fromT1N0,/3NH3 to T1N03/26.7NH3 mole ratios. All the spectra were obtained from solutions atambient temperatures. Intensity changes in the N-H stretching region are correlated with theeffects of the electrolyte on the solvent structure. An intense broad band at -1110 cm-' has beenassigned to the symmetric deformation mode of NH3 engaged in strong a-interactions with T1+ ions.The DSh selection rules for the unperturbed NO, anion break down in concentrated solutions ofTINOJ in NH3 and spectral evidence is presented to support a new n-interaction model for TI+-NO; ion-pairs.Raman studies on aqueous solutions of electrolytes have provided much informa-tion on the nature of these solutions.1 Recently the technique has been applied toliquid ammonia solutions of electrolytes.2-5 The results have been interpretedgenerally by analogy with the well-documented aqueous systems. Salts of singlycharged cations and singly charged polarisable anions are usually readily soluble inliquid ammonia and hence the use of nitrates, thiocyanates and iodides in this typeof study. Raman studies on molten thallium(1) nitrate 6 * ' have revealed anomalouslystrong ion-pair interactions. Thallium(1) nitrate is only sparingly soluble in wateryet very concentrated solutions can be prepared in liquid ammonia.This differencein solubility further highlights the anomalous behaviour of thallium(r) ions and thepurpose of this work is to determine the nature of the interactions present in thallium(I)nitrate solutions in liquid ammonia.EXPERIMENTALRaman spectra were recorded on a Cary 81 laser modified Raman spectrometer. The530.8 nm line from a Coherent Radiation model 52 krypton ion laser was used for excitation.The polarisation data were measured on a Spex Ramalog (4) instrument. The spectra wererecorded at ambient temperatures and they were calibrated against carbon tetrachloride andindene.B.D.H. thallium nitrate and K. and K. Laboratories Inc. thallium thiocyanate wereused without further purification. Each sample was oven dried and checked for dryness byinfrared spectroscopy. Air Products Ltd. 99.98 % anhydrous ammonia was used withoutfurther purification.Samples were prepared in -2 mm diameter Pyrex capilliary tubes having a flat end toreceive the laser beam axially. The scattering geometry was 180". All samples were handledon a vacuum line or in a nitrogen filled glove bag to preserve their anhydrous condition.994 RAMAN SPECTRA OF T1NO3RESULTSRaman spectra were obtained from thallium(1) nitrate solutions in liquid ammoniaranging in composition from 1/3 to 1/26.7 TlN03/NH3 mole ratios.A TlSCN/6.5 NH3 solution of thallous thiocyanate was studied also. The spectra may bediscussed most easily in terms of bands arising from NH3 and NO: ion vibrations.Any additional features may result from interaction processes in the solutions.NH3 VIBRATIONSThe N-H stretching region, 3500-3100 cm-l, consists principally of three bandsat - 3400, -3300 and -3200 cm-l. It is apparent from fig. 1 that the band at - 3200 cm-1 becomes markedly less intense whilst the - 3400 cm-i band increasedvery slightly in intensity as the TlN03 concentration increases. There are no markedfrequency changes in this region as a function of concentration. Table 1 includes thepositions of these bands and fig. 1 shows the N-€3 stretchmg region at the twoextremes of the concentration range examined.TABLE L-RAMAN BAND POSITIONS (F Cm-') AND ASSIGNMENTS FOR THE 03SERVED BANDS OFTlNOj IN LIQUD NH3 SOLUTIONS FOR A RANGE OF CONCENTRATIONS AT AMBIENT TEMPERATURENO: NH3mole ratio v 1 v2 v3 v4 2v2 V l v2 v3 v 4 + 2v41:3.1 1043 828 1374 709 1659 3297 1096 3387 1635 32261:3.9 1043 830 1370 710 1658 3299 1096 3387 1637 32221:4.5 1045 830 1363 710 1660 3302 1096 3389 1630 32251:4.7 1044 829 1368 709 1659 3300 1098 3388 1636 32251: 5.0 1044 830 1368 710 1659 3302 1100 3388 1633 32211:5.2 1044 830 1367 710 1658 3300 1098 3388 1638 32231: 5.4 1044 830 1367 710 1660 3300 1097 3388 1637 32241: 5.9 1045 830 1367 710 1660 3300 1101 3388 1638 32221: 8.4 1045 830 1364 710 1660 3301 1101 3388 1636 32211 : 11.1 1045 832 1370 710 1665 3304 1100 3388 1636 32221 : 17.7 1045 - 1360 710 1660 3305 1097 3388 1638 32201 :26.7 1045 - 1360 710 1660 3303 1098 3388 1630 3220NH3 3306 1046 3389 1640 3221* Approximate values.Fig.2 shows the lower frequency region for the T1NO3/3NH3 mole ratio solutionand a broad and intense feature at - 1100 cm-' is observed. The intensity of thisband is concentration dependent as the graph of fig. 3 shows. Here the - 1100 cm-lband intensity, normalised by ratio with the intensity of a known NO; vibration at-710 cm-l, is plotted against the solution mole ratio. This feature appears also inthe Raman spectrum of the TlSCN solution which we studied. One further bandassociated with the NH3 vibrations appears as a weak feature at - 1630 cm-l.Table 1 includes the observed frequencies of all of these bands.No; VIBRATIONSBands arising from the NO, ion vibrations appear at -710, 830, 1045, 1360 and1660 cm-l.The latter band appears as a sharp but weak feature in all of the spectraand shows little variation in frequency. The band at 1045 cm-l is very intense andsharp, its frequency does not vary but its bandwidth increases with concentrationfrom - 5 to -7 cm-' across the range of solutions studied. The -710 cm-l banD . J . GARDINER, A . H . HAJI AND B . P. STRAUGHAN 95/, I i I t J3400 3300 3200i/crn-'FIG. I.-The N-H stretching region for TIN03 in liquid ammonia at two different concentrations.;/crn-'FIG. 2.-Raman spectrum (200-1500 cm-l) of a T1NO3/3NH3 mole ratio solution.1 3 5 7 9 I I 13 I5 17 19 21 23mole ratio NH3/TINO3FIG.3.-Plot of the band intensity ratio v2(NH3)/v4(NOJ) against the solution mole ratio96 RAMAN SPECTRA OF TlNO3behaves similarly in that it shows very little change in frequency and remains sym-metrical in shape in all of the solutions. In the 1360 em-' region a band is presentwhich is clearly asymmetric in shape (see fig. 2) and at least two components areanticipated. The frequency separation of the components appears to be concentra-tion dependent. Complete resolution of the components was not possible due totheir inherent band-widths. We have measured instead the total half band widthof this feature as a function of concentration and expect this to reflect any variation0.23-0.210.1 9 -n 0.17- 0" zP u 0.15---- Na 0 .1 3 -0.11.0.09--1 1I 3 5 7 9 Itmole ratio NH3 /TIN03FIG. 4.-Plot of the band intensity ratio uz(NO~)/u4(NO~) against the solution mole ratio.1 860 840 820 800v/cm-lFIG. 5.--Rarnan spectra showing the polarisation of the vZ(NO;) mode for a TIN03/3NH3 moleratio solution.in frequency separation of the components. The results of these measurements showthat the half band width increases by 35 cm-l over the total concentration rangeexamined.In the more concentrated solutions a band was observed at 830 cm-l (see fig. 2).The graph of fig. 4 shows that this band increases in intensity as the concentrationincreases. Furthermore, polarisation studies showed this band to be polarised, andan example of this behaviour is given in fig.5 for the TlN03/3NH3 solutionD. J . GARDINER, A . €3. HAJI AND B. P. STRAUGHAN 97The only other notable feature in these spectra was a very broad band in the300-500 cm-1 region. This was observed in all the spectra and changed little in shapeor position with concentration.DISCUSSIONThe unperturbed NH3 molecule has CJV symmetry and hence gives rise to a vibra-tional representation : rvib = 2A1 +2E with all modes infrared and Raman active.A band appearing at 3200 cm-l has been recognised as 2v4 in Fermi resonance withvl.* Birchall and Drummond have studied spectra of NH3 and ND3 in the con-densed phase at various temperatures and suggest that the assignments of 2v4 and v1should be reversed. A similar conclusion is drawn by Schwartz and Wang lo aftersome temperature variation studies.However, Gardiner, Wester and Grossmanhave reported results supporting the original assignment. These workers showedthat in dilute solutions of ammonia in carbon tetrachloride and acetonitrile, whereNH3. . .NH3 interactions are reduced to a minimum, the intensity of the 3300 cm-lband is several times that of the 3200 cm-l band and hence assigned the former to vl.Curve resolution analysis of this complex N--I3 stretching region reveals the presenceof a broad underlying band at -3260 cm-l which Gardiner et al. assign to N-Hstretching of associated ammonia molecules. The presence of this band has beencorroborated by other investigators who assign it similarly.12 In the light of thisevidence we prefer to adopt the original assignment for o m spectra (see table 1).The lowering in intensity of 2Vq(NH3) as the electrolyte concentration increaseshas been observed previously.2 We are confident that this behaviour arises fromdisruption of the H-bonded structure of liquid ammonia and the creation of newinteractions with the electrolyte ions.This change in structum would affect theintensity of the broad band underlying the N-H stretching region and with anaccompanying reduction of v1 /2v4 resonance interaction, the observed drop in 2v4intensity can be accommodated. The slight increase in v3(NH3) intensity with con-centration indicates further restriction of NH, rotational motion.The most notable feature in the Raman spectra of NH3 in these solutions is thepresence of an intense and broad band at - 1100 cm-l.We have assigned this bandto v2(NH,) which in pure liquid ammonia appears at 1046 cm-l. Unfortunately astrong band due to nitrate ion masks the 1046 cm-1 position in the TlN03 solutionsand therefore to be sure that we were not observing an additional band, the Ramanspectrum of a TlSCN/6.5 NH3 mole solution ratio was studied. In this latter solutionthere was no evidence for a band at 1046 cm-l, but the intense band at - 1100 cm-Iwas present. In LiN03/NH3 solutions, a band at - 11 10 cm-1 has been reportedthough it was extremely weak. It seems likely that this intense band is due primarilyto NH3 interactions with Tlf ions. Furthermore, the interaction must be strongerthan, or completely different in nature to, the Li+-NH3 interaction.The Tlf ion isknown to engage in strong a-bonding and this fact has been used to account foranomalous intensities in the Raman spectra of TlN03 melts.6* This o-bondingcharacteristic together with the greater polarisability of the Tlf ion compared withthe Lit- ion will contribute to the intensity of the 1100 cm-l band. Furthermore, thelarge increase in frequency of v,(NH3) from 1046 cm-l in liquid ammonia to - 1100cm-l in the electrolyte solutions is expected from comparisons with transition-metalammine spectra.These arguments and observations lead us to assign the band at - 1100 cm-l tothe symmetric deformation mode of ammonia molecules (v,) engaged in stronga-interactions, with Tlf ions, via the nitrogen lone pair.The band due to the antisymmetric deformation mode of NH3 at - 1630 cm-I isI98 RAMAN SPECTRA OF TIN03little affected by the presence of Tlf, NO; or SCN- ions.This is in accord withearlier observations.2The free NO? ion has D3,, symmetry and gives rise to a vibrational representationr v i b = Ai(R)+A;(i.r.) +2B'(i.r. and R) where infrared and Raman activities areshown in parentheses.The antisymmetric stretching mode of NO; appears at - 1360 cm-I. In thedilute solutions there is some slight splitting of this mode evidenced by its asymmetricshape. The splitting probably increases at higher concentrations of TlN03, but thetwo components are never clearly resolvable except in the most concentrated solution.If the increase in total half band width of this feature is a measure of the extent ofthe splitting then an increase of about 35 cm-l is indicated over the concentrationrange.The splitting of this feature over a comparable concentration range inLiN03/NH3 solutions has been reported as 56 cm-l. These observations indicatea small Tl+-NOi interaction and little perturbation of the NO; ion by NH3 solution.T h i s latter conclusion is in agreement with earlier work.14Splitting of the v4(E') deformation mode in aqueous nitrate solutions has beentaken as an indication of ion pair formati~n.'~ Removal of the degeneracy fromthis mode requires ion-pair interactions to occur through 0 atoms of nitrate producinga C,, or C, species. In all of the solutions studied here, there was no evidence ofsuch a splitting.Following this interpretation we conclude either that there are noion-pairs formed or that a species of higher symmetry is produced.Evidence for the latter explanation is convincingly provided by the appearance inthe more concentrated solutions of the NO; out-of-plane deformation mode at - 830 cm-l. Furthermore, this mode, which is Raman inactive for D3h NO, belongsto a totally symmetric character species as shown by its polarisation behaviour. Amodel to explain these observations would be a nitrate ion belonging to point groupc30r v i b = 2A1(i.r. and R)+2B(i.r. and R).However, considering the possible types of interaction that may be present in thesesolutions, we feel it unlikely that sufficient energy could be provided to distort thenitrate ion from its planar nn-bonded configuration. It seems more likely that thenitrate ion in our more concentrated solutions interacts with TI+ to produce a C,,ion-pair.This can be achieved by allowing Tlf to interact with the n-cloud of NO;along the principal axis of the anion. The following schematic depicts such aninteraction and shows how solvating NH3 molecules may be accommodated in themodel to produce a C3, species. Alternatively, the solvating NH3 molecules maylower the symmetry to C 3 ; this point group still allows for the observed spectra.NH3 NH3 NH3\ I /T1+We are left, however, with the slight inconsistency that the band at 136Ocm-1retains some doublet characteristics while the degenerate band at -710 cm remainsunsplit.This observation may be accounted for if an asymmetric solvent cage orion-pair environment is present. This would cause only minor perturbations anD. J . GARDINER, A . H. HAJI AND B . P . STRAUGHAN 99would affect only the most symmetry sensitive vibrations. Force field perturbationcalculations l6 indicate that the antisymmetric stretching mode of NO; is by far themost susceptible to such situations.CONCLUSIONSOur results lead to a model for the structure of T1NO3/NH3 solutions. In thedilute systems each of the ions appears to be fully solvent separated. As the concen-tration of TlN03 increases the H-bonded structure of liquid ammonia breaks up andion-pair interactions take over.However, with Tlf ions the interaction proceeds withthe NO; ion via its z-cloud. The presence of this type of interaction though theoreti-cally quite feasible, has not been demonstrated previously for liquid NH3 or aqueoussysterns.l8 Raman spectra from molten TlN03 reveal a band at 813 cm-’ but nopolarisation data are available.” The questions must arise as to whether thissituation obtains in concentrated aqueous solutions and whether other heavy metalions interact similarly. Experiments directed towards these problems are beingcarried out.We thank Dr. H. Hallam of Swansea University for use of the Spex Ramalog (4)instrument.D. E. Irish in Zonic Interactions Vol. ZI, ed. C. Petrucci (Academic Press, London, 1971), chap. 9.D. J. Gardiner, R. E. Hester and W. E. L. Grossman, J. Chem. Phys., 1973, 59, 175.P. Gans and J. B. Gill, Chem. Coim., 1973,23,914.K. R. Plowman and J. J. Lagowski, J. Phys. Chem., 1974,78,143.A. T. Lemley and J. J. Lagowski, J. Phys. Chem., 1974,78,708.D. W. James and W. H. Leong, Trans. Faraday Soc., 1970, 66, 1948.C. A. Plint, R. M. B. Small and H. L. Welsh, C a d . J. Phys., 1954, 32, 653.T. Birchall and I. Drummond, J. Chem. SOC., 1970,1859.D. J. Gardiner, R. E. Hester and W. E. L. Grossman, J. Raman Spectr., 1973, 1, 87.’ D. W. James, R. D. Carlisle and W. H. Leong, Austral. J. Chem., 1970,23, 1779.lo M. Schwartz and C. H. Wang, J. Chem. Phys., 1973, 59, 5258.l2 A. T. Lemley, J. H. Roberts, K. R. Plowman and J. J. Lagowski, J. Phys. Chem., 1973,77,2185.l3 D. M. Adams, Metal-Ligand and ReZated Vibratiom (Arnold, London, 1967).l4 N. Smyrl and J. P. Devlin, J. Phys. Chem., 1973, 77, 3067.l5 D. E. Irish, A. R. Davis and R. A. Plane, J. Chem. Phys., 1969,50,2262.H. Britzinger and R. E. Hester, Znorg. Chem., 1966, 5,980.G. J. Janz, T. R. Kozlowski and S. C. Wait Jr., J. Chem. Phys., 1963,39, 1809.T. C. G. Chang and D. E. Irish, J. Solution Chern., 1974, 3, 161, have presented evidence for a“ roll-on ”, off axis Ag+--No, ion-pair configuration in acetonitrile