Dixit, L

Gupta, P L

Kapoor, S K

Singh, I D

English

IACS Institutional Repository

Indian J. Phys, 52B, 116-121 (1977)Infrared spectrum and normal coordinate analysis of tungsten trioxideL, D i x t t , S .  K ,  K a p o o r , T. D S t n g t i a n d  P. 1.. G u p t aIndian Institute o f  Petroleum, Dehra />i/a7-248005An attempt has been made to establish the symmetry of tungsten trioxidc on the basis of observed absorption bands in the infra red spectra and normal coordinate analysis. Assuming sym­metry, observed fundamental bands have been assigned as : vi (A i) ^  1020 cm \ vii (A i) — 300 cm \ v.-; (E ')  =i 1045 cm“  ^ and vi (E ‘) ~  350 cm '. The results compare fairly well with other isi^cleclronic systems like1. iNFROOUmONThe structures of known trioxides which have hither^:) been investigated (Herzberg 1945, Schmil and Muller 1974) fall in two categories: those having Da h symmetry such as SO.., and other having symmetry likeMoOa. Of the first calciiory the molecules have been extensively studied and vibrational spectra and symmetry of several molecules have been estab­lished. Vibrational (i.r.) spectrum of MoO:{ has been recently studied by Hewett et al (1975) and they have assigned a symmetry to this molecule and have questioned the symmetry of WO.{, which has previously been assigned as . In view of the industrial importance ()is catalyst) of WO. ,^ considerable interest has been shown in the study of spectrostructural aspects of this molecule.Several authors have attempted the spectrostructural studies of WO^. McDevitt and Baun f l% 4 )  have studied the ir spectra of several solid metal oxides in the 700-240 cm'^ rcJiion and have reported two poorly defined bands at 355 and 315 cm *’ for WO.  ^ without any assignment. A complete infrared spectrum of solid WO^ has also been reported by Nyquist and Kogcl (1971). Weltner and McLead (1965) have studied the oxides of tungsten in Argon and Neon matrices, and have assigned a band at 1040 cm“  ^to va (E ') species of WO- (0.^^) molecule. The assigments of bands at 1024, 874 and 487 cm~^ to (WOa)^ by these authors have been quest’icmed by Hewett et al (1975). Comparing the vapour spectra of (Moa)^, where M  =  M o or W, and taking the planar ring configuration for these polymers, they have predicted that the highest frequency due to trimer or tetramer should lie in the region 950-970 enr*^ In a recent systematic studyInfrared spectrum of tungsten trioxide 17on WOo, WO;, and W04 '- * systems, Cyvin and Harciltai (1074) have dis­cussed some of the vibrational properties of these systems usme vibrational assignments from JANAI^' Table (1071) and revised die norma! coordinate analysis by Nagarajan (1966) for WOa ). These authors (Cyvin &Hargcttai 1974) have indicated that the estimated vi frequency (564 cm~^) is toil low, which is based ihe value of a WO stretching force constant. Now with the establishment of a C,,, symmetry \o which is isoelec-tronic with WO:,, and doiib's raised aNuit svmmelrv and magnitude of liequencies of WO.,, the present study was initiated to find the vibrational frequencies of WO:: and to determine whether a C.. conficuration may be adopted to explain vibration of WO., in agivJment with normal cocudinate analysis.2 . Ta n  r i m i  n i a lThe infrared spectia of WO;, have been recorded on BecKman iR-12 Iambic Beam Grating spectrophotometei in the leeion 2(K)-I2t)0 cm ’ using Nujal mull dispersed between (\SI plates 1 he sample was prepared. b\ healing Ammonium Tungstate around M)() Ihe speetropholometer h a s  been cali­brated in the whole range using polvslyrene and spectioseopically pine Tndene.2 . R i s n i  IS  AND D i s c u s s i o nThe infrared absorption spoclium of WO>. has been shown in figure I. Four fundamental absorption bands have been observed at 1045, 1020, 350 and 3tX) cm \ The band at lOd^ s cm ’ is vciy much slron,gcr than that at 1020 cm b which appears as a shtHiIdor on the strong absorption due topolymeric form of WO.,. The intensities of bands at 300 and 350 cm  ^ arc compcirable. Table I presents the frequencies of WO., with assignment together with that of MoO., reported earlier.118TableL. Dixit, S. K. Kapoor, 1. D.Singh and P.1. Vibrational frequencies (cm~*) for tungsten comparison with molybdenum trioxideL. Gupta trioxide and theirWO., K.,WO;,Fh M0O3Species lorns & Present Griffiths HewettStafford work (1% 9) et al(J% 6) I.R Raman 1975Vt<A,) 1040 1020 (W) <)26 W 931 976(1021.0) (978.0)v,(A ,) — 300 (s) 322 (s) 328 (260.9)(296.7)1007 1045 tv.s) 833 (v.s) 828 922.2(1042.9) (926.1)v,(E) — 350 (s) .'22 (s) 328 (275.0)(350.7)Values in parentheses arc computedBand AssignmentThe vibrational representation of a xy;j plannar (D hj, )  molecule is represent­ed as r  ~  Ai +  2E ', whereas that of a xya pyramidal (Cav) moleculeas r  =: 2A, -f 2E. Accordingly, in a planar molecule only three freequencics (A-/' & 2E ') are i.r. active, while in a pyramidal system all the four frequencies (2A| & 2E ) should be active in the infrared. In both the systetns, the vi and v;i stretching modes arc expected to occur at higher frequencies than the va and V4 bending deformations. The appearance of two highfrequency infrared bands at 1045 and 1020 cm "’ suggests the C.iv molecular symmetry for this molecule, which is in consistence with those concluded by Hewett et al (1975) in case of VloO:i. Tse corresponding bands in MoOity have been observed at 976 and 922.2 cm '. The assignment of the high frequency fundamentals (stretching modes) can be made on the basis of their relative intensities. For slightly non-planar molecules, one expects the v;i (E ) mode to be much more intense than the vi (A ,) . On this basis we assign the band at 1020 cm ’ to be vi (A i) and that at 1045 cm ’ to be vs (E ) . An analytical expression for the intensity ratio of corresponding symmetric and asymmetric stretching modes has been proposed bv Smith (1973) to be A as /As — tan-0 which also confirms h^e above conclusion with 3 —62°. It is worth mentioning that the assignment of 976 cm ’ to vi (A ,) and 922.2 c ir r ’Infrared spectrum of tungsten trioxide 119to v.s (E ) in case of M0O3 (Hewelt el al 1975) is nol juslified on the basis of intensity considerations. However, the low frequency v- (AO and v» (E ) bands arc more difficult to assign, as no dcllnile guide-lines exist for intensity and relative magnitudes of the frequencies. In ease of MoO;,, the low- frequency bending bands have been computed, assuming a relation f  ^ 0.04 frand a constant value of f^ (0.296 mydne/A) to be 260.9 and 275 cm \ It has been observed (Wesley & Dekock 1973) that in case of pyramidal systems Vi> ( A j) < v i  (E ) . Hence following the assignment of bending modes due to Wesley and DeKt^k (1973), Lcsiecki cl al (1972) and Hauge and co-workers (1971), we have assigned v- (AO mode to 3(X) cm ‘ and v., (E ) to 350 cni"^ bands. This is in agreement with the asignment V:* (A i) :n 260.9 cm"^ and V4 (E ) =  275 enr^ in Case of MoO;,.Normal Coardinate AnalysisAssuming a planar trigonal structure of D.Ui symmcti7  for WO;; molecule, Cyvin and Hargittai (1974) have performed normal ccx)rdinalc analysis using vibrational frequencies from JA N A F Table (1971). Out of four frequencies used in these calculations only one viz. 10 40 cm“  ^ v:. (E ) has been observed (Wettner ct al 1965) in matrix-isolated infrared spectra, while frequencies vi (A l) at 564 cm~’ and V2  (A i) at 347 cni~  ^ have been estimated by Weltner and McLe<xl (1965) and another V4 (E ) at 320 cm  ^ by Dc Maria ei al (I9 6 0 ). Here wc wish to comment that the assignment of symmetry to a molecule with missing absorption bands is most unreliable. This has also been reflected in the calculation of Cyvin and Hargittai (1974) where the bond-bond interaction force constant, frr  , in case of WO.j has been reported to be —2.01 mdyne/A, which is unreasonably high in magnitude. Ih e  authors have pointed out that the value af vi (A i) 1:1; 564 cm ‘ which has been estimated using WO stretching force constant, seems to be too low and thus needs stronger experimental evidence. Based on our assignments described earlier, normal coordinate analysis using Generalized valence force held (G V FF ) model described by Nakamolo (1945) has been performed.Standard F-G matrix method of Wilson has been used to compute intra­molecular force constants within a valence force held discussed by Dixit (1975) and Sanyal and Dixit (1974). The form of kinetic energy matrix is same as described by Nakamoto (1945). The eigenvalues for Ai and E  symmetry species have been solved using Li,» O approximation, which is very well , suited to molecule having comparatively low mass-coupling. The description of internal and symmetry coordinates has been shown in hgurc 2 . The angle between two W-O bonds has been found to be 99”45' using the relationship sin p — 2/ (3)” - sin a/2) where P is the pyramidal angle which has been taken to be 62”. in case of MoO^, a pyramidal angle of 61..S'* has been taken for 1.8 A MoO bond, and, since a bond length of 1.81 A for W-O bond hasbeen used by Cyvin and HargiUai (1974) in ease of planar WOjf", we have retained the same (approximately) value of fi h r^ WO.i as that for MoO;. under constant bond length approximation.I'abic 2 . Valence force conslanls (G .V .l .F.) in mydn/A of the trioxidcs of tungsten and MolybdCiUim in C. v^ model120 L . D ixil, S. K . K a ix )o r , I. D .S in gh  an d  P . L . G u p tai orcc constants MoO.," WO3fr 7 46 9.326frr - 0.11 -0 .0 2 5f 0 .2% 0.477U ’i — -0-.015f, < —• 0.017f'r«( — 0.023* Valence loTcc tnnstantb denved hiMii Ihc sMiimctiN loicc ctvirUinl icpur.cd b\HcwcU cl «/, \'H5Ja b lc 3. Symmetri/cd and valence loicc constants ( indync/A) of WO,systems, n 2, 3, 4, <!v: (iSystems S> nimetr> i ' l l  i ’a:; h fr. RefWOo 8 78 7.27 8.03 0.70 aWO;; ‘■ '3 V 2S 34 0 - 0 0 2 1 his vvt)i KIX  „ 3 vM> y.03 7.02 — 2.01 aT a 8.17 5.84 6.42 0 ..*i8 aWO«'> O), 6.52 3.20 4.10 0.37 b(a )  See (  y\in & HargiUai P>74.(b )  See Ahmad, D ixil  <!it S a in a l  0>74.'The symmetry force consumls for WO.; together with that of MoO;; have been reported in table 2 lable 3 presents the symmetry force constants for WOa (C;;v ) alongwith other WOn systems. From this table, it is observed that the bond strength of W-O decrease regularly increasing number of oxygen atom in WOn system, which is reflected in the form of decreasing magnitude o f fr  (W -O ). In our calculation, bond-bond interaction force constant, fr r  (W -O) comes out to be —0 023 mdync/A, and positively represents much stabilizied structure (Csv ) for WO3 in contrast to that ofInfrared spectrum of tun^ fsten trioxicie I2iCyvin and Hargitlai (1974) where f n  (W -O) in WO;i has a value —2.1 mdync/A. riiesc approximale symmetry force constants have also been used to compute the stretching and bending frequencies, which compares well with the observed ones ((able 1). A more definite .issignment of symmetry to WO;! could be made if the isotopic shift is studied using isotopes of tungsten and/or oxygen.A( KNOWl.IIXIMl NlAuhors arc thankful to Dr. P. Ratnasumy, I.I.P. for providing the sample and helpful discussion.Rl;l*hRl,N (hSllcrzberg, (i.. I'M.*! Inf fared ami Raman Speclra of RolyaUnnw molcatles I)e Van NiiislandSdimil K. II and Miillcr A . I'I7h, Cooidinalion Chem. Rev., 14, ll.‘'-17Vllcv^cli, \V I) Ji . Ncwldp I H , and VNclInci. \\ , Ji 1^175, J. Ph.\s ( hcni 79, 2640.McDeviU, N 1. and Haun W. I., IVS4, Specliochim Acta, 20, 7W.Nyqu’.st R. A. and Kagcl K. O , IV71 Injiau d Spniiu o) liioinaiiic t oiiipiiunds Acad PressW'cliiici \\. Jr. and Mcl c^od, 1). J r ,  l'X).S, J. Mol. Spcctnrsc, 17, 270 (>\iii S. J. and Hargitt'ii 1, 1V74, Acta (him. Acad. Sci Hang (lomns) 83, .t2l-32v. Stull, I) K. and I’ lophcl H (Piojcci Directors), l')7l, l,\NAI' Ihcimothenncailablcs, 2nd addition. National lJuicau ol Stiuidards, Washington DC. Nagarajan, Ci l‘K)6. Indian J. Pure AppI Phvs 4, l.‘'S.Wcsie> R 1). and Dckock C. W', l'J7.l. J. Ph>s (hem 77, 4o6 Lcsiccki, M., Nibicr, J. W. and Dckock, (' W , l')/2. .1 (hem. Ph\s 57, 13^ 2 llaugi, R. H. Haiivtic, J. W'. and Margrave, J. L ,  1V71. J. less ( omnion metals; 23, 359. De Maria G.. Burns R. P ,  Diowart, J and Inghiam M. (i 1%0, J ( hem J*hys 32, 1373. Nakamoto, K. 1945 Inlrared Sinrlra a{ iiwij’anie and eaaidinaiion lomimunds, Wiley. Dixit I.. 1975 Ph.D. Ihcsis, University of (lorakhpiir. (iorakhpur. (India)Sanyal N. K. and Dixit L. (1974) Z. Naturforseh, 29a, (v93-97.Sm,t W. M. A., 1973, J. Mol. Structure, 19, 789.Ahmad, P. Dixit L and .Sanyal M. K 1974, Indian J. Pure Appl Phys. 12, 489. loins, ’I. V. and Slafloid 1'. E. (I% 6), J. Am. (hem Soc 88, 4819.GriflHhs W. P , I%9 J. Chem. ,Soc. A 211.

Infrared Spectrum and Normal Coordinate Analysis of Tungsten Trioxide

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Infrared Spectrum and Normal Coordinate Analysis of Tungsten Trioxide

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