INDIAN J. CHEM., VOL. 15A, DECEMBER 1977

excited state of a molecule is lowered to a greaterextent as compared to the ground state in suchsolvents, the red shift is understandable. In (1;-hydroxyketones this kind of red shift has been ob-served". The spectra in Fig. 1 show that the effectdue to stability of polar states dominates in thepresent system and the red shift increases with thedielectric constant of the medium. A detailedanalysis is, however, not possible, since the chargedistribution is expected to be affected by evenhydrogen bonding and if it leads to increased dipolemoment, the expected blue shift due to hydrogenbonding may get cancelled-". The contributionof the polar structure is further apparent from theresults presented in Table 1. The oscillator strength(1) continuously increases with the dielectric. Alinear relationship is observed between the transitionenergies and the dielectric constant of the medium.With increase in polarity, the separation betweenthe two concerned electronic states decreases. Interms of Franck-Condon principle excitation froma solvated ground state leads not to the energeticallymost stable solvated cor-formation of the excitedstate, but to a conformation geometrically identicalto the solvated ground statel" Thus in the transi-tion a change in the solvation energy can resultleading to decreased separation between the groundand the excited state as observed.

Now band-II can be assigned to pi*~pi transition.I t undergoes a red shift with the increase in polarityof the medium; it is of higher energy than pi*~ntransition and it is of higher intensity than band-I(as expected-t) when solute-solvent interactionsare minimum.

Another interesting feature is that the (1) valuefor pi*~n transition is of the order of 10-2, whilein normal cases it is of the order of 10-3 to 10-4. Thiscould be due to the symmetry of various states beingmodified by vibrational interactions particularlybecause of the allowed and forbidden excited states

TABLE 1 - ENERGIES AND OSCILLATORSTRENGTH(f) OFABSORPTIONBANDs(a)

Dielectricconstant(b)

Band-I Band-II

Energy (f) (c)kcal X 102

mole"!

(f)X 10'

Energykcal

mole'?

CYCLOHEXANE2·02 132·9 4·537116·7 2·975

METHANOL32·63 4·062 132-6 (d)114·6

METHYL CYANIDE36·70 125·9 2-511114·0 4-441

80·00'VATER

6·479 127·6 7·929112·1

(a) Spectra were recorded at 25° ± 2°. (b) Values of. thedielectric constants were taken from Handbook of chemistry(Longmans, New York), 1957. (c) Oscillator s~ren&ths arecalculated from the absorption bands and their WIdths athalf height. (d) Diffused band.

1096

being closel". The pi*~n transition in the presentcase is only ,.....,15kcal below pi*~pi transition andthe (1) value for band-II is lower than that for band-Iin methylcyanide (Table 1). Furthermore, thebands are closer in this solvent which demonstratesthe possible contribution of polar states and mixingof states.

We are thankful to the CSIR, New Delhi, forfinancial assistance to two of us (H.C. and J.S.).References

1. LUMMA(Jr), W. C. & BERCHTOLD,G. A., J. org. Chem.,34 (1969), 1566.

2. FEHNAL, E. A. & CARMACK,M., J. Am. chem. Soc., 71(1949), 84.

3. BARRETT, J. & HITCH, M. J., Spectrochim. Acta, 24A(1968), 265.

4. BARRETT, J. & HITCH, M. J., Spectrochim: Acta, 25A(1969), 407.

5. PORTER, G. & SUPPAN,P., Trans. Faraday Soc., 61 (1965),1664.

6. CHALLENGER,F., MASON, E. A., HOLDSWORTH,E. C. &EMMOT,R, J. chem. Soc., (1953),292.

7. AGGRAWAL,U. & BHASKER RAO, P., Inorg. nucl. chem.Lett., 3(6) (1967), 205.

8. UNGANADE,H. E., J. Am. chem. Soc., 75 (1953), 432.9. DUBOIS, J. B., Spectrochim: Acta, 20 (1964), 1815.

10. ORCHIN, M. & JAFFE, H. H., The importance of anti-bonding orbitals (Hougon Mifflin, Boston, Massachu-setts), 1970, 63.

11. MULLIKEN, R S., J. chem. Phys., 3 (1935), 564.12. KASHA, M., The nature and significance of pi·

a composition of about 40 mlo Gd203• The cellparameters of U02-Gd203 fluorite type cubic solidsolutions vary linearly with composition. Gd203 isknown to exist in two forms>".

Many investigators! have studied the high tem-perature physical properties of U02. The hightemperature thermal expansion properties of pureU02 are also well known+", However, limiteddata2,7 exist in the literature on the thermal ex-pansion properties of pure Gd203 and U02-Gd203solid solutions.

The present investigation deals with the thermalexpansion measurements of pure U02, Gd20a (C-type)and U02-1·5 wlo Gd203 solid solution, employinghigh temperature X-ray diffractometry in the range298-1700K in pure helium atmosphere, and reportsthe cell parameters in the range up to 1700K in thecase of U02 and U02-1·5 w]» Gd203 solid solutionand 1575K in case of pure Gd20a (C-type).

Natural U02 and natural U02-1'5 w!o Gd203 ofnuclear purity were obtained from the Atomic FuelsDivision (BARC). Pure Gd20a (C-type) was obtainedfrom the Pure Materials Section of this Division.

MRC model X-86-N3 high temperature X-raydiffractometer attachment mounted on a Philipswide angle goniometer was used. The details of thishave been described elsewhere'',

The cell parameters of all the three substancesunder study vary linearly with temperature (Fig. 1).

The per cent linear thermal expansion wasevaluated from cell parameter measurements. Thelinear variation of cell parameters and of per centlinear thermal expansion could be expressed by thefollowing least square fitted equations.

U02 (temperature range 298-1700K):

ar = 0·5452± 0·00015+5·5402 xlO-6T ... (1)

0·558.--------------------,

Ec

0·556

0'554

0·552

0·550~ r" yI"~;