Dwivedi, R. N.

Narang, K. K.

Nath, P.

English

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NotesPhotochemical Investigations of Ce(III)-V(V)System in GlassesR. N. DWIVEDI, K. K. NARANG* & P. NATHDepartment of Ceramic EngineeringInstitute of Technology, Banaras Hindu UniversityVaranasi 221005Received 17 February 1977; accepted 11 July 1977Ce(III)- V(V) system in glasses is photosensitive andthe electrons loosened from Ce(III) by the UV light aretrapped in the vacant sites provided by the irregularfive-coordinate VO:- ion. The trapped electrons formmetastable colour centres which absorb around19230crrr! and are independent of the oxidation statesof vanadium in the photo redox process.CONSIDERABLE interest has been assigned inthe photosensitivity of glasses containing ceriumoxide alongwith transition and non-transition metaloxidesH. Leffler" and Weyl" investigated V20a-Cea02 couple while Kum~r and Sen" studied t~eoptical spectra of the solarized Mn(III) and V(II) Inglasses and claimed the photoreduction of V(III) toV(II). Since there is ~? previous rep~rt on thepreparation of photosensitive glass containing Cc(III)-V(V) system under oxidizing conditions, the pre-paration and spectral study of such a glass has beenundertaken because Ce(III)-V(V) system should beeven more susceptible to photo oxidation-reductionthan Ce(III)-V(III) due to the presence of V(V)which is a stronger electron acceptor thaI! V(III).Preparation of the glass - All the chemicals usedwere of BDH (AR) quality. In a base glasscontaining (wt %) Na20 18, MgO 8, and Si02 74,Ce02 0·30% and V20S 0·15% were ad.d~d: NaN03(5%) was introduced to keep the OXIdIZing atmo-sphere in the melt. The ba.tch was thoroug.hlymixed and melted for 4 hr In a Pt-Rh cruciblein an electric furnace at 1450°. The molten glasswas poured, cooled, crushed and remelted for ar:other2 hr for attaining maximum homogeneity. Finally,the molten glass was cast in a mould and annealedat 560°. It was then cut into pieces, ground andpolished up to 2 mm thickness. The glass pieceswere exposed to UV radiations. from a 'V:0tan'high pressure mercury lamp at a distance of 9 Inchesfor 1 or 2 hr each with and without blue filter.Both the surfaces of the glass pieces were uniformlyexposed to the UV radiations. A?sorption sp~ctraof the non-irradiated and the radiated glass pieceswere recorded on a Cary 14 model in the 300-750mIL range. . .Since the above glass containmg Ce(III)-V(V) hasbeen prepared under strongly oxidizing cor"ditio.ns,it was colourIess, transparent, strongly absorbingin the UV region but showing IiO absorption in the*Present address: Department of Chemistry, BanarasHindu University, Varanasi 221005.1092visible region. It, therefore, contained V(V) whichis either colourIess or faintly yellow because of theabsence of electrons in the d-Ievel. When the glasswas subjected to the UV radiations it turned purple.Besides showing a strong absorption in the UVregion, it showed a distinct absorption band at19230 cm< in the visible region, the intensity ofwhich increased with increasing duration of exposurewithout any change in wavelength.In solution, stepwise reduction from V(V) to V(II)takes place with mild to strong reducing ag~ntsand the various oxidation states can be stabilizedin complexesv? or crystals'P+'. In glasses, howev:er,such conversions from higher to lower OXIdatIOnstates are a matter of controversy because it is asolid rigid lattice. Photoreduction of vanadium iseven more complicated to explain because of thesimultaneous existence'> of V(III), V(IV), V(V) orV(IV), V(V). Although, Kumar and Sen? claimedto have photoreduced V(III) to V(II) in Ce(III)-V(III) system but the authors failed to satisfactorilyexplain the development of a new band at 19230crrr? which is at much shorter wavelength ascompared to the one 17850 cm! band in thehydrated crystals of V(II). Moreover, the photo-reduced V(II) in glass does not show another bandat 12190 crrr+ observed in the hydrated crystalslO,ll.When the purple colour and the absorption bandsof the photoreduced glass containing Ce(III)- V(III)system are compared with the photoreduc.ed gl,:!sscontaining Ce(III)-V(V) system, marked similarityof the colour and absorption bands are observed.V(II) species cannot be present in Ce(III)-V(V)system because in a photochemical reaction,. theelectron donor species give one electron at a time.Even the light radiations excite only one electronso that the UV light must have excited only oneelectron in the Ce(III)-V(V) system. Moreover, theratio of Ce(III) to V(V) in the glass is 2: 1 in thepresent case. Even if it is assumed that allthe Ce(III) ions lose one electron each and all theelectrons are transferred to V(V) ions, the V(V)ions should first be reduced to V(IV) and then toV(III) state. All attempts to observe sych abehaviour have failed. Further, the behaviour ofthe photoreduced glass towards heat treatment issimilar to that in metastable V(II) developed fromV(III). In metastable V(II), the colour. changesfrom green to red-violet on photoreduction. Byheating the glass to 250-300°, the loose electronsreturn to the Ce(IV) ion with a simultaneous returnto the green colour of V(III) and emission of aquantum of lightla. The original colour of ~heglass, in the present case, is also restored on heatingaround 400°C. This observation confirms the pre-sence of metastable V(IV) or [V(V)+e] in theCe(III)- V (V) glass. . .In KVOa.H20 and V205, the vanadium IOns. haveirregular type of five-coordination which is trigonalbipyramidal-+, In glasses cerium (III) vanade+e(CeV04) may provide several vacant sites which onphotoexcitation are occupied by the electrons. Thetrapped electrons, thus held up in the state ofsuspension by strongly electronegative oxide ionssurrounding the V(V), are not able to reach thed-Ievels of vanadium(V) and, therefore, do notcause the reduction of V(V) to V(IV). Further,with increasing duration of exposure, the increasein the intensity of the 19230 crrr+ band, is anotherevidence of the trapped electror.s rather than thephotoreduction of V(V) to V(IV). Thermal agitationreverses the direction of electron mohility and theelectrons are regained by Ce(IV), consequently,restoring the original colour of the glass.References1. BADGER, A. E. & OTTOSON, A. C.,I Am. ceram. Soc.,25 (1942), 104.2. GAGIN, L. V., BACHMAN, G. S. & BADGER, A. E., GlassInd., 27 (1946), 500.3. WHITE, J. F. & SILVERMANN, W. B., J. Am. ceram,Soc., 33 (1950), 252.4. VVEYL, W. A. Coloured glasses (The Society of GlassTechnology, Sheffield, UK), 1967, 505.5. LOFFLER, R., as referred by Weyl, W. A., in ref. 4,p. 506.6. WEYL, W. A., PINCUS, A. G. & BADGER, A. E., I Am.ceram. Soc., 22 (1939), 374.7. KUMAR, S. & SEN, P., Phys. chem. Glasses, 1 (1960),175.8. CLARK, R. J. R., The chemistry of titanium and vanadium(Elsevier, New York), 1969, 170.9. JAFFE, R. R. & ORCHIN, M., Theory and application ofultraviolet spectroscopy (John Wiley, New York), 1962,518.10. J</>RGENSEN, C. K., Acta chem, Scand., 9 (1955), 116.11. ORGEL, L. E., I chem. Phys., 23 (1955), 1004.12. KAKABADSE, G. J., MANOHIN, B. & VASSILIOU, E.,Proceedings of the international conference, Marchuood(Butterworths, London), 1963, 254.13. FANDERLIK, M., Czechoslovak Scientific Society for theSilicate Industry (Prague, Czechoslovakia), 1965, 20.14. WELLS, A. F., Structural inorganic chemistry (ClarendonPress, Oxford), 1962, 687.Calculation of 1t-Electronic Spectra ofBenzonitrile, Nitrobenzene, Dinitrobenzenes &NitrophenolsA. MARTIN·Department of Chemistry, University of DelhiDelhi 11007Received 14 March 1977; accepted 5 August 1977The 1t-electronic spectra of benzonitrile, nitro-benzene, dinitrobenzenes and nitrophenols have beencalculated using Molecules in Molecules method.The two-centre repulsion integrals are calculated bythe Nishimoto-Mataga approximation. The valuesof the resonance integrals are considered as adjustableparameters. The direct charge transfer energies andthe effect of hydrogen bond have been taken intoconsideration in case of nitrophenols. The calculatedspectral transition energies are in good agreementwith the experimental ones. Starting parametersare suggested for MIM calculations of compoundshaving nitro, cyano, hydroxy groups.*On leave from Department of Physical Chemistry,Technical University, Budapest, Hungary.NOTESTHE -e-electronic spectra of benzonitrile, nitro-benzene, dinitrobenzenes and nitrophenols werecalculated by the Molecules in Molecules (MIM)method. The two-centre repulsion integrals werecalculated by the Nishimoto-Mataga approximation-in preference to the other methods=". The ~I"V reso-nance integrals were taken as variable parameters.The nitro and cyano groups were taken as acceptorsand the hydroxy group as donor compared to thebenzene ring.The various experimental and theoretical" para-meters, bond distances (rl"v), valence state ionizationpotentials (11") and electron affinities (AI")' one-centrerepulsion integrals (Y1"1") needed for the calculationare given in Table 1. The locally excited con-figurations of the subsystems were calculated by thePPP type SCF calculations using the above dataas well as ~C=N = -3·5 eV and ~NO = -3·0 eVfor resonance integrals (values found as the best forbenzonitrile and nitrobenzene). The calculated wavefunctions (~) and the experimental -e-electron tran-sition energies of the subsystems, nitromethane andacetonitrile are given below, where Xi denote theatomic orbitals. CH3N02; (Eexp\ = 6·2616 eV)3.6;~l = a:.?a3; ~2 = aiIa3' a1 = 0·7297x1+0'4835(x2+x3); a2 = 0·7071 ( -X2+X3); and aa = 0·6838x1-0·5160 (X2+X3)' CH3CN; Eexp\ = 7·4240 eV)3,6;IjJ = aiIQ(z. al = 0·6415 x1+O·7671 x2; and a2 =0·7671x2 -0·6415x2•The locally excited configurations and electrontransitions of benzene ring were taken from ourearlier work", The experimental ionization poten-tials of donor subsystems are: 1benzene = 9·245 eV8,IcH.OH = 10·83 eV9. The experimental electronaffinity of acceptor subsystems are: A benzene = -1·10eVIO, ACH,NO, = -0·56 eVll.There is lack of experimental electron affinitydata on benzonitrile. Therefore this parameter hadto be taken as a variable parameter together withthe resonance integral ~C.CN' The results are shownin Fig. 1, where the horizontal lines represent theexperimental energies. The differences between thetwo ~c.CNvalues giving the two exact energies aredrawn in the upper part of the Fig. 1. The resultsshow that calculations carried out with largerelectron affinities give better results. On the otherhand considering the cyano group as an acceptorthe upper limit of its electron affinity seems to bethat of benzene ring (-1·10 eV). For this reason,the electron affinity of acetonitrile was taken asACH,CN = -1·00 eV, corresponding to an optimumvalue of ~C-CN = -2·5 eV.TABLE 1 - CALCULATED ATOMIC PARAMETERS ANDEXPERIMENTAL BOND DISTANCESC (trlrtr ,,) 11·16N (trtrtr ,,0; nitroi) 25·73o (tr"tr"tr rr ; nitroi) 17·70C (didi rerr ; cyanoi] 11·19N (di2di rrrr ; cyanoi) 14·18o (trOtrtr ,,~; hydroxyi) 30·07*Obtained from ref.tYNO.0·038·972·470·101·6610·8314-16.11·1316·7615·2311·0912·5219·241-3971-461·21 t1·4552·6141'361093

Photochemical Investigations of Ce(III)-V(V) System in Glasses

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Photochemical Investigations of Ce(III)-V(V) System in Glasses

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