摘要:

J. CHEM. soc. DALTON TRANS. 1988 2759The Infrared Spectra of Matrix Isolated Thorium and Uranium Tetrachlorides.Change of Shape with Matrix GasIan R. Beattie," Peter J. Jones, Keith R. Millington, and Andrew D. WillsonDepartment of Chemistry, The University, Southampton SO9 5NHWhen thorium or uranium tetrachlorides are isolated in neon matrices, the spectra can beinterpreted in terms of a Td molecule with a small site effect. Spectra obtained using sputteringtechniques to generate ThCI, in argon matrices agreed with those obtained previously usingconventional vaporisation techniques. Detailed calculations of the i.r. spectra of isotopicallyenriched and natural abundance Th35'37CI, show that in krypton ThCI, is not tetrahedral.It is usually assumed that the geometry of molecules orfragments isolated in matrices of the inert gases or nitrogen, forexample, is close to that of the gaseous molecule.' However it isalso accepted that species in crystals are affected by theirenvironment. The term 'interallogon' was coined to describe anextreme case where both square-planar and tetrahedral forms ofa nickel(l1) complex occurred in the same unit cell of a singlecrystaL2 Species isolated in matrices, if they could be examinedin terms of their detailed geometry, represent unique systems forstudying atom-molecule interactions at low temperatures.Guests likely to be strongly affected by the host matrix would bethose where the central atom is co-ordinatively unsaturated andwhere bonding forces are not highly directional in character.Such molecules would have fewer ligands than those normallypresent in stable compounds of the central element, a highdegree of ionic character, or many possible combinations ofbonding orbitals.These are precisely the types of moleculeswhich exhibit 'anomalous geometries' in matrices. Examplesinclude non-linear SrF,,3 Tho,,, pyramidal LaF,,' and T-shaped U0,.6 This raises the problem that some of thegeometries reported could arise from the presence of the matrix.Put in an alternative way, if the sequence from the inert gas Xethrough N, and CO to the conventional ligand MeNC isconsidered, for many acceptors it is not clear that there is anabrupt change at any point.In a recent paper following on from earlier work on CsUF,'it has been suggested that the molecule CsNbF, changes shapeas a function of matrix gas.Thus in weakly interacting matricessuch as neon or argon CsNbF, adopts C,, symmetry, while incarbon monoxide or nitrogen C,, symmetry is adopted. In thiscase the matrices were 'calibrated' by use of the moleculeCsCIO, which shows a bidentate interaction between thecaesium ion and the perchlorate anion.'It is also well established that the stretching frequencies ofalkali metal- halide molecules show large shifts between the gasphase and even a neon matrix." This behaviour has beenreasonably well explained on the basis of dipole-induced dipoleinteractions.' ' Matrix shifts for diatomic molecules tend toincreasein theorder:"Ne < Ar < Kr < Xe < N,.The determination of bond angles and bond lengths in matrixisolated species is difficult.At present the principal approach isthe use of isotopic data obtained from i.r. spectroscopy. Forcompounds of the heavier elements such data are normally notsensitive to bond angles. However, distortions of a Td AB,molecule away from tetrahedral symmetry, or of a D,, (or C,,,)AB, molecule so that the C, axis is removed, leads to significantchanges in the characteristic intensity and frequency patternsfor partial isotopic substitution at the ligand atom. We are onlyaware of two examples of such behaviour: UO,, and ThC1,.l2Both warrant further consideration. The data on UO, are moreextensive and compelling because of the larger isotopic shifts(160/1sO) and the more favourable frequency region for oxides.It is noteworthy that both ThIV and Uvl are dof O systems, andboth central elements occur in the same region of the PeriodicTable.The main objective of this work was to extend theprevious studies on ThCI,. In particular, because both ThCl,and UCl, show large matrix shifts between argon and carbonmonoxide for example, we felt that significant perturbationcould be brought about by the host matrix.Results and Discussion(a) The Inert Gases.--For Td MCI, where ~ 3 ( t 2 ) is ofappreciably higher frequency than v3(al) the chlorine isotopesgive a characteristic regularly spaced five-line pattern for v3 inthe i.r. spectrum.'2" Each of the three central components of thispattern corresponds to the vibration of one isotopomer.Thesevibrations have the same symmetry as those of each of the threecentral components of v1 which derive from the same iso-topomers (but in reverse frequency order to those of v 3 ) . Thus asv3 and v1 approach one another the three central components ofv3 are pushed towards the all-35C1 position (for v,). At the sametime the central components of v1 gain in i.r. intensity from thisinteraction. Such behaviour can be seen in the spectrum ofHfCl," where v3 - v1 is less than 10 cm-'. Because of suchinteraction no totally symmetric vibration of the mixed iso-topomer is pure v3 or pure v1 in origin. In the case wherev1 z v3 all of the components lie under the all-35C1 or all-37C1positions, leading to a two-line spectrum.This is essentiallywhat is observed in our spectra of ThC14 isolated in a neonmatrix (Figure 1).ThC1, is a dof system and as such is expected to be regulartetrahedral. The spectra in Figure 1 can reasonably be inter-preted in terms of a Td molecule with satellite bands due to someoccupancy of a second site showing a shift of approximately 7cm-' to lower frequency. This result is at variance with spectraobserved in argon, krypton, and xenon. In particular Arthersand Beattie 2a interpreted their i.r. spectra of ThCl, in kryptonin terms of a C,, molecule. In view of the result for neon itappeared important to consider the results in the other matrixgases in more detail.Possible sources of error in such work include: that the ThCI,is not monomeric in certain matrices, that some chloride otherthan ThCI, has been isolated, that several matrix 'sites' canaccount for the observed spectra, and that an impurity is presentin Ar, Kr, and Xe but not in Ne.We find that sputtering thorium foil in an argon carrier gasdoped with a small amount of chlorine gives an identical i.r.spectrum to that obtained by vapourising ThC1, into an argonmatrix, emphasising the monomeric nature of the isolatedmolecule (Figure 2). Further, in all our ThC1, vapourisationexperiments the inevitable deposits on cooler parts of th2760 J.CHEM. SOC. DALTON TRANS. 1988340 330 32 0v t cm-’8070aJ u c04- c .- 6 802cI-5cFigure 1. The i.r. spectrum of thorium tetrachloride isolated in neonmatrices at several 35Cl : 37Cl ratios: ( a ) 1 : 20, (6) natural abundance,and (c) calculated natural abundance spectrum with v1 and v3 co-incidentapparatus were always colourless, in agreement with a +4oxidation state for the thorium.We thus conclude that ourspectra refer to monomeric ThCl,.Returning now to the results of Arthers and Beattie 12’ forkrypton matrices, the spectra of the ‘all-37Cl’ or ‘all-35Cl’molecules each show four main bands. Four additional bandsoccur in molecules containing both 35Cl and 37Cl. These do notlie in the central positions between the appropriate ‘all-35Cl andall-37Cl’ isotopomers. Their frequencies are: 342.6 (+ 2.0), 330.6(-lS), 321.0 (+1.3), and 311.6 (-1.7) cm-’ [values inparentheses indicate by how much the bands have been pushedup (+) to higher frequencies or pushed down (-) to lowerfrequencies relative to the central position].Several deductions can be made from the krypton spectra. (a)They cannot arise from (four) Td molecules in four different sites.On the basis of frequency and intensity calculations it is notpossible to predict spectra with only one 35C1/37C1 bandbetween each of the all-35Cl and all-37Cl pairs.Further, the factthat two bands are pushed up in frequency while two arepushed down suggests that at most two parent molecules of non-Td symmetry are present. (b) They cannot arise from one D 2 dmolecule, which would give rise to only two all-35Cl or all-37Clbands. In no case can we match the spectra using two D2dmolecules in different sites using realistic values for vl.( c )Finally all of the spectra for different 35Cl:37Cl ratios can besatisfactorily calculated using a C,, molecule with oppositeangles of 110 and 160’. The results of our frequency andintensity calculations are summarised in Figure 3. We have notallowed for the previously proposed site effect on the band at335.8 cm-’. However, the agreement is clearly excellent.3LO 320v t cm-’Figure 2. The i.r. spectrum of ( a ) thorium tetrachloride in an argonmatrix and ( h ) thorium foil sputtered using argon containing 0.1 mol”/,chlorineOur independent spectra on natural abundance ThCl, inkrypton or argon agreed closely with those of Arthers andBeattie (maximum difference 0.2 cm-’).However, we notethat in Table 3 of their paper under 35Cl: 37Cl of 1 : 2 ratio theband at 342.6 cm-’ should be included.We are once again forced to the conclusion that in certainmatrix gases ThCl, does not adopt a T d conformation. Thecomplexity of the vibrational problem and the assumptionsinherent in the bonddipole method for calculating intensitiesmeans that both the proposed force field and geometry shouldbe treated with caution.We also carried out very careful annealing experiments onThCl, isolated in a krypton matrix. We could not obtain anydata which showed unambiguously that the bands concerneddid not all originate from the same molecular species. Howeverafter annealing at the very high temperature of 55 K for 10 min aband was observed to grow in at 327.6 cm-’, although thestrongest bands in the original spectrum were now only about aquarter of their original intensity. Similarly codepositing ThC1,with argon at 25 K did not affect the general spectral featuresalthough some broadening of the bands was observed.(b) Nitrogen and Carbon Monoxide.-Between neon andcarbon monoxide there is a shift of approximately 40 cm-l foJ.CHEM. SOC. DALTON TRANS. .988 276 1'" 340 320 340 320v/cm-'Figure 3. Calculated (upper trace) and observed (lower trace) i.r.spectra of thorium tetrachloride in krypton at various 35Cl: 35Cl ratios:( a ) 45: I. ( b ) natural abundance, (c) 1 :2, ( d ) 1 :9 (* = impurity). Therange of transmittance values for the calculated spectra is in all cases95-45"/,,.Marked points on axes refer to 90% or 60% transmittancefor experimental spectraboth ThCI, and UCI,. This represents a decrease in theantisymmetric stretching frequency by some 12%. It is roughlyequivalent to the addition of one chloride ion. The spectrum ofThCI, in nitrogen can be modelled by assuming two matrix sitesfor a Td molecule, separated by ca. 7 cm-l. The spectra in carbonmonoxide were of poor quality.ConclusionsOur observations are summarised in the Table and in Figures 1and 2. In neon matrices the i.r. spectra of ThC1, and UCI, can beexplained on the basis of tetrahedral molecules assuming: ia)there is partial occupancy of a second site in the matrix and (6)v 1 and v3 are coincident or nearly coincident.Figure 4 showsthe effect of carrying out a D 2 d distortion on the T d ThC1,molecule. Clearly the initial steps of such a distortion would bea broadening of the bands. (A similar broadening could alsooccur if v1 and v3 are not exactly coincident.) For ThCI, andUCI, the full width at half peak height for the main 35Cl i.r.band in a neon matrix is ca. 2 cm-'. This may be compared withthat of ca. 1 cm-' found in spectroscopy on related compounds.Modelling spectra for small D 2 d distortions suggested that themaximum deviation from tetrahedral (for the angles enclosingthe S, axis) would be 1". In nitrogen (and probably carbonmonoxide, where the bands are too broad to give isotopic data)the spectra can also be modelled on a Td MCI, unit.This set of conclusions, apart from the fact that ThCl, andUCI, are tetrahedral in the gas phase and neon, may seemunlikely.However our detailed interpretation of ThCl, inkrypton gives quite excellent agreement with the experimentalTable. The i.r. spectra of thorium and uranium tetrachlorides in variousmatrix gasesCompound Matrix 3 5 ~ 1 : 37c1 Frequencies (cm-')ThCl, Neon Natural 341.0, 334.0ThCl, Neon 9: 1 341.0, 334.0ThCl, Neon 1:20 333.5, 325.8ThCI, co 9: 1 302ThCl, co 1:20 29 7UCl, Neon Natural 347.2, 339.6v /cm-'350 330I I I IFigure 4. Calculated i.r. spectrum of ThCl, for a D,, distortion. Bondangle bisected by S,: (a) I05 and (6) 1 I 5 O (vl fixed at 340 cm-')data. There is very strong evidence that CsNbF, changes shapebetween neon and nitrogen for example.Further, within theinert gases the greatest change in physical properties occursbetween neon and argon. Finally is should be recalled that thespectrum of HfCI, in argon and in nitrogen can be fullyexplained on the basis of a tetrahedral molecule. It is interestingto note that if we place four C1- ions on the surface of a Th4+sphere, using reasonable ionic radii and assuming all spheres arenearly in contact, this leads to a structure very close to thatproposed by Arthers and Beattie.'" It is possible that twokrypton atoms complete a highly distorted octahedron aroundthe central thorium.Clearly if these results are correct, they throw doubt onstructures derived from studies of 'co-ordinatively unsaturated'molecules in inert matrices, notably where these molecules havea high degree of ionic character and many bonding orbitals areavailable2762 J.CHEM. SOC. DALTON TRANS. 1988water cooled cathode /I I1 1 - \'anode / muff coup1 ingFigure 5. The sputtering deviceExperimentalThe conditions used to matrix isolate ThCl, and UCl, were thesame as those used previously 2a except that a modifiedcryopump (Air Products DE204SL) was used which was (just)capable of isolating samples in neon (tip temperature 8-9 K).For spectra in neon we also used a liquid-helium cryostat. Theresults obtained (Figure 1) were identical to those found using theDE204SL except that the bands were narrower. Infrared spectrawere recorded (4 O G - 1 8 0 cm-') on a Perkin-Elmer 9836 i.r.spectrometer calibrated using residual water vapour in theinstrument.The sputtering device was similar to that describedby Green and Reedy (Figure 5). The glass vacuum jacket wasattached to the water cooled metal flange carrying the hollowcathode uiu an Edwards SClO muff coupling. The platinumanode was held in the centre of the poly(tetrafluoroethy1ene)barrel of a modified greaseless tap, thus allowing the position ofthe anode to be varied. A thin silica tube was used to hold theanode centrally in the hollow cathode. The high voltage powersupply was designed to operate in the 30&-600 V, 0-50 mArange.Cu1cuZations.-The spectra were calculated using theSOTONVIB program. l4 Intensity patterns were obtained uiuthe L-vectors using the bonddipole approximation. Thediagram below defines the geometry and force constantsadopted (dyn = lop5 N).AcknowledgementsWe thank Dr.D. W. Green of Argonne National Laboratory fordetailed drawings of the sputtering source, Mr. Robert Bellfor preliminary work, Dr. T. R. Gilson and Dr. G. Rosenblatt forhelpful discussions, the S.E.R.C. and A. E. E. Winfrith forfinancial support, and Mr. A. G. Hutchins for valuable help withthe helium cryostat.References123456789101 1121314R. N. Perutz, Chem. Rev., 1985, 85, 77.B. T. Kilbourn and H. M. Powell, J. Chem. Soc. A , 1970, 1688; seealso, A. Terzis, K. N. Raymond, and T. G. Spiro, Inorg. Chem., 1970,9, 2415.V. Calder, D. E. Mann, K. S. Seshadri, M. Allevana, and D. White, J.Chem. Phys., 1969, 51, 2093.S. D. Gabelnick, G. T. Reedy, and M. G. Chasanor, J. Chem. Phys.,1974, 60, 1 167.See, for example, J. W. Hastie, R. H. Hauge, and J. L. Margrave, J.Less-Common Met., 1975, 39, 309.See, for example, S. D. Gabelnick, G. T. Reedy, and M. G. Chasanov,J. Chem. Phys., 1973,59,6397.I. R. Beattie and K. R. Millington, J. Chem. SOC., Dalton Trans., 1987,1521.S. A. Arthers and I. R. Beattie, J. Chem. SOC., Dalton Trans., 1984,71 1.I. R. Beattie and J. E. Parkinson, J. Chem. Soc., Dalton Trans., 1983,1 185.M. E. Jacox, J. Mol. Spectrosc., 1985, 113, 286.M. J. Linevsky, J. Chem. Phys., 1961, 34, 587; S. Schlick and 0.Schnepp, J. Chem. Phys., 1964, 41, 463.(u) S. A. Arthers and I. R. Beattie, J. Chem. Soc., Dalton Trans., 1984,819, and refs. therein; (b) K. H. Lau and D. L. Hildenbrand, J. Chem.Phys., 1984,80, 1312.D. W. Green and G. T. Reedy, J. Chem. Phys., 1978,69, 544.I. R. Beattie, N. Cheetham, M. Gardner, and D. E. Rogers, J. Chem.Soc. A , 1971, 2240.Received 7th April 1988; Paper 8/01444

ISSN:1477-9226    

DOI:10.1039/DT9880002759

出版商:RSC    

年代:1988

数据来源: RSC