The Near Infra-Red Absorption Spectrum of Heavy WaterAuthor(s): Lotte KellnerSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 159, No. 898 (Apr. 1, 1937), pp. 410-415Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/96886 .

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The Near Infra-Red Absorption Spectrum of Heavy Water

BY LOTTE KELLNER

(Communicated by A. 0. Rankine, F.R.S.-1Received 28 November 1936)

1-INTRODUCTION

The infra-red absorption spectrum of liquid deuterium oxide ("heavy water") has been investigated by Ellis and Sorge (I934) and, in the region 2*5-9,a, by Plyler and Williams (I936). In the course of some other in- vestigations at the Imperial College the absorption spectrum of this sub- stance was remeasured between 0 9 and 2 1, It, but the results obtained did not fully confirm those of Ellis and Sorge. In view of the importance of heavy water as a solvent for absorption spectroscopy in the infra-red, it is necessary that its spectrum should be known as accurately as possible, and it therefore seems desirable to publish the new results for comparison with the earlier ones, and to reconsider the theoretical origin of the spectrum.

2-EXPERIMENTAL ARRANGEMENTS

A spectrometer of the Wadsworth type was used, which was provided with a 60? quartz prism.* The spectral slit width corresponded to approximately + 001 ,t. The heavy water, of 99.2 % D20 content, was provided by the

Norsk Hydro-Elektrisk Kvaelstofaktieselskab. As the sample had been opened some time before the present examination was carried out, it seemed possible that the heavy water had become contaminated with light water, but no traces of the presence of HDO molecules could be found in the absorption spectra. Three absorption cells of various thicknesses (10; 1*03 and 0 22 mm.) were used. As heavy water is very hygroscopic, the absorption cells were kept in dry air before they were filled, and the atmosphere inside the wooden case surrounding the spectrometer was freed from water vapour by means of CaCl2.

3-RESULTS

Fig. 1 represents the absorption of heavy water as function of the wave- length. In (a) the absorption curve of light water is given for comparison purposes. It has been arranged so that the bands are approximately above

* For a fuller description see Kellner (I936).

[ 410 ]

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Absorption Spectrum of Heavy Water 411

those belonging to the same type of vibration in the spectrum of D20 shown in (b). (See discussion of modes of vibration in ? 5.) The peaks in the absorption curve of D20 at 1*431 and 1*973,t are to be ascribed to the presence of light water, as indicated by the arrows in fig. 1.

100

80

-.0 4-- 60- F ;0 a12

? 40 -

20 _ I I I l ! I I I ; Il I IA

0 98 0 99 100 110 1-20 1 30 1 40 150 160 1-70,u

80 b

a' 60 t 4 ]2

40

?0 - j H20

20_

120 130 140 150 160 170 180 190 200 210u

FiG. 1- (a), H20; 1, 1 cm. cell; 2, 1 mm. cell; (b), D20; 3, 1 cm. cell; 4, 1-03 mm. cell; 5, 0-22 mm. cell.

4-COMPARISON WITH PREVIOUs RESULTS

In Table I the results of the present investigation are given together with the data of Ellis and Sorge.

The absorption values of Ellis and Sorge can only be considered as approxi- mate, as they were reduced to the thicknesses stated from their graph, but it is quite certain that they are definitely higher than my own over the whole spectral range under investigation. No traces could be found of the bands at 1 20 (which coincides with a band of H20) and 1 565,t, although the resolving power of the spectrometer was sufficient to show them. Fig. 1

Vol. CLIX-A. 2 E

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412 L. Kellher

shows the heavy water to be completely transparent in the region of 1U20,u. It should be remarked, however, that, since Ellis and Sorge's spectra showed stronger absorption throughout than mine, the absence of this band may be due to insufficient thickness of the absorption cell, but in view of the complete transparency of my specimen at this point, in spite of the presence

TABLE I-WAVE-LENGTHS AND INTENSITIES OF THE

ABSORPTION MAXIMA OF D20

Ellis and Sorge Present paper

Amax. in ,u Absorption in % Amax. in -K Absorption in % 2'040 904 2-021 + 0*004 66 2*015 87 0*22 mm. cell 2.002 + 0001 65 0-22 mm. cell 1*940 71) 1*973+ 0003 69) 1 655 91> 1*680 0-003 80 1-565 78 - 1*416 37 t 1 cm. cell 1*431 + 0'007 1 cm. cell

1*310 311 1*313 ? 0.002 23 1.20 31J -

of a little H20, this explanation seems unlikely. On the other hand, the band of Ellis and Sorge at 1.355,u might quite well have escaped me, as the American authors used a spectrometer of five times the resolving power. It is possible that the band at 1 5651t is a combination tone of the HDO molecule, as calculations show that the combination v1 + 2v2 of the known fundamentals, v, 3520 and v2 1480, would give the wave-length 1P54jt.

5-INTERPRETATION OF THE SPECTRUM

Ellis and Sorge interpret their results as overtones and combination tones of the four fundamentals for liquid water, which are represented according to the notation of Mecke (I933), and Ellis and Sorge by the symbols v(o), V(ir), 8(ir), v*. Of these the 7T vibrations are symmetric with regard to the symmetry axis of the molecule, while the o vibration is antisymmetric. The first three vibrations are the same as the three fundamentals for water vapour, while the fourth is usually associated with the polymerization of the water molecules in the liquid state. Ellis and Sorge justify their inter- pretation by the observation by Ellis (I93 i) of an H20 band at 1'78/tt, which had not been previously known. The absorption of liquid H20 at 1'78,u however was remeasured by me with cells of 1 and 2 mm. thickness, and no trace of a band in this region could be found (fig. 2). The spectrum of liquid H20 shows the same features between 0 9 and 2 1,u (when the existence of

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Absorption Spectrum of Heavy Water 413

a band at 1 78,u is excluded) as the vapour spectrum. This seems to cast some doubt on the validity of Ellis and Sorge's interpretation of the vibrations. Mecke has shown that it is possible to assign the overtones and combination tones of water vapour to definite modes of vibration by analysing the rotational structure of the bands. It seems legitimate therefore to give the same interpretation to the spectrum of the liquid, in which case the spectrum would be explained entirely in terms of three fundamentals. In Table II Mecke's assignment of the overtones and combination tones of liquid H20 is compared with that of Ellis and Sorge.

040

IC' 0-30 -

E 0310 _ 2 o.zo - 01

I I I I . I I II I I I I I I H

170 172 174 176 1.78 180 182 1-84 1 86Ak

FIG. 2-Absorption spectrum of liquid H.0 (2 mm. cell).

The following remarks should be made on the table. The values of the overtones and combination tones of liquid H20 are the mean values of the data of different authors (Kellner I936, Table II). The observed wave numbers given for D20 are averages of Ellis and Sorge's and my own data. The Raman lines for P(o-), v(1r) and 2&(ir) are given, as the infra-red spectrum shows only a very broad band at v 3300. The o- vibration is much more intense than v(7r) and 8(7T), which explains the fact that the vibration v(7T) is not observed in the infra-red spectrum of liquid H20 and D20, as it is masked by the neighbouring and extremely intense absorption corre- sponding to v (o-). Weizel (I934) has shown that the odd overtones of v (O) and all combinations of v(o-), 3v(o-), 5v(o-) etc. with all multiples of v(1T) and 4(7n)

should occur in the spectrum. The appearance of 28(7T) in the Raman spectrum of liquid H20 and the infra-red spectrum of gaseous H20 may be due to resonance with v(2i). In the last column of Table II the ratios of vH20 to VD20 are given. This ratio has generally the same value for the com- bination tones as for the fundamentals. An exception to this rule is the band v 3509 of the D20 molecule, but 3509 is probably not the correct value for this band, as it is certainly overlapped by the very intense band v(o-) 3580 of H20.

A further reason for preferring the classification of Mecke to that of Ellis and Sorge is that it accords with the selection rules derived by Weizel for the overtones and combination tones of the H20 and D20 molecules, which are violated by the classification chosen by Ellis and Sorge. Besides, there seems

2 E 2

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414 L. Kellner

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Absorption Spectrum of Heavy Water 415

to be no reason why corresponding lines of the gaseous and liquid substance should be ascribed to different modes of vibration.

I am very much indebted to Prof. G. P. Thomson and Dr Herbert Dingle for placing the laboratory facilities of the Imperial College at my disposal. Furthermore, I wish to express my gratitude to the Central British Fund for German Jewry for a grant awarded to me.

6-SUMMARY

An investigation has been made of the infra-red absorption of heavy water in the region 0 9-2 1jt. Four absorption bands due to the vibrations of the D20 molecule have been found whose positions are given in Table I. The results are compared with previous observations.

It is shown that these bands can be explained as certain overtones and combination tones of the three fundamentals in the same way as the corresponding bands of light water vapour have been interpreted by Mecke.

REFERENCES

Ellis 1931 Phys. Rev. 38, 693. Ellis and Sorge I934 J. Chem. Phys. 2, 559. Kellner 1936 Proc. Roy. Soc. A, 157, 100. Mecke I933 Z. Phys. 81, 313. Plyler and Williams 1936 J. Chem. Phys. 4, 157. Weizel 1934 Z. Phys. 88, 214.

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