Absorption Spectra of Four Aldehydes in the Near Infra-Red

Absorption Spectra of Four Aldehydes in the Near Infra-RedAuthor(s): Lotte KellnerSource: Proceedings of the Royal Society of London. Series A, Mathematical and PhysicalSciences, Vol. 157, No. 890 (Oct. 1, 1936), pp. 100-113Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/96732 .

Accessed: 03/05/2014 10:44

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

The Royal Society is collaborating with JSTOR to digitize, preserve and extend access to Proceedings of theRoyal Society of London. Series A, Mathematical and Physical Sciences.

http://www.jstor.org

This content downloaded from 62.122.77.14 on Sat, 3 May 2014 10:44:26 AMAll use subject to JSTOR Terms and Conditions

http://www.jstor.org/action/showPublisher?publisherCode=rsl

http://www.jstor.org/stable/96732?origin=JSTOR-pdf

http://www.jstor.org/page/info/about/policies/terms.jsp


100

Absorption Spectra of Four Aldehydes in the Near Infra-Red

By LOTTE KELLNER

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

INTRODUCTION

It has been common knowledge for a considerable time that the infra- red spectra of organic molecules show a close resemblance to each other. It has been possible to attribute some of the bands in these spectra to inner vibrations of certain constituents of the molecules (0-H, C-H, C O, etc.), and in recent years the analysis of the fundamental vibrations of relatiyely simple molecules has been accomplished. In the present paper an investigation of the absorption spectra of four aldehydes in the near infra-red is described. Hitherto, no attention has been given to the inten6ity of the absorption, the investigators have only been concerned with the determination of the positions of the bands. In this paper the absorption coefficient of each of the substances under investigation has been determined, in addition to the wave-lengths of the bands. Further- more, an attempt is made to interpret the absorption frequencies as overtones and combination frequencies of some fundamental vibrations.

DESCRIPTION OF APPARATUS

The spectrometer employed in the present investigation was a prism spectrometer of the type ordinarily used in infra-red spectroscopy. The silvered mirrors had a focal length of 69 cm and an aperture of 6 5 cm. The prism was of quartz of the Cornu type with a refracting angle of 590 59 0' and a base of 8 8 cm. A Wadsworth mirror served to maintain the minimum deviation of the light-beam. The light-sources employed were a gas-filled tungsten filament lamp of 1000 watts for wave-lengths between 1 and 2 y, and a Nernst filament for wave-lengths greater than 2 ,u. An iron-hydrogen resistance kept the current flowing through the Nernst filament constant. The light was focussed on the first slit of the spectrometer by a silvered mirror of 53 cm focal length and 9 6 cm aperture. The first slit was 0 15 mm wide, the second 0 14 mm. The range of wave-length entering the second slit then corresponded to a rotation of the prism through an angle of 20".



Absorption Spectra of Four Aldehydes 101

A Moll-thermopile in connexion with a Moll-galvanometer* served as receiving apparatus. The slit of the thermopile was used as the second slit of the spectrometer. As the galvanometer was placed on a niche in the tiled wall of a basement room, no disturbances occurred. A 6-volt battery of large capacity furnished the current (ca. 0 - 9 amp) necessary for the electromagnet of the galvanometer. Readings of the deflexions of the galvanometer-mirror were taken by means of a scale and a telescope. The distance between the scale and the galvanometer was 2 metres.

The slit of the thermopile was covered by a quartz window and the whole thermopile kept air-tight. This precaution was necessary as the open thermopile was affected by the adiabatic fluctuations of the atmo- spheric pressure, which gave rise to deflexions of the galvanometer corresponding to several millimetres of scale reading. In order to protect the thermopile from extraneous changes of temperature, it was enclosed in a case which was stuffed with cotton wool, and cotton wool was wound round the wires which connected thermopile and galvanometer.

The whole spectrometer, including the thermopile, was enclosed in a wooden box to protect it from stray radiation. A brass shutter, which could be opened and shut from the seat of the observer, gave the light access to the spectrometer. If the lamp was left burning for at least three hours before the beginning of observations, the deflexions of the galvanometer-mirror were found to be constant to 0 - 10 *2 mm of scale reading.

EXPERIMENTAL PROCEDURE

The prism was adjusted to give minimum deviation for the light of the yellow sodium lines and fixed in this position. A divided circle rotated together with the prism, and a vernier allowed the angle of rotation to be read with an accuracy of 20". The observations of this angle were made from outside the box with the help of a totally reflecting prism mounted on the vernier, and a telescope. The angles were measured relative to the position of the prism when the unresolved yellow sodium lines coincided with the middle of the thermopile-slit. This position was noted before and after each set of measurements. The wave-lengths corresponding to the measured angles were determined in the following way. If a is the angle through which the light is deflected and b the refracting angle of the prism, we have for the refractive index n:

sin( a (1)

* 'Proc. phys. Soc.,' vol. 35, p. 253 (1923).



102 L. Kellner

Taking the position of the yellow sodium lines as zero point, the value of 8 for any other line is obtained by adding twice the angle through which the prism muist be turned to bring that line to the second slit. According to Schdnrock,* the refractive index of quartz is given by the formula (where X is measured in ,u):

n 3 5968913 + 0 01064379 138 -20519 (2) ;k2O-.O0106291 III1.45202- ?X2 (2

This formula is valid over the range of X here investigated (? = 0 71 - 24 4 ). Using the equations (1) and (2), the values of a were plotted against the values of X at intervals of 0 001 p. In Table I the refractive indices of quartz are given together with the dispersive power, resolving power, and purity of the prism. If Rt is the resolving power for an infinitely narrow slit, P is the resolving power for a finite slit-width (the purity), and D is the dispersive power, then

dn R = pd (3)

P= X - i R, (4)

D da= R, 5 D ~~~~~~~~~~~~~(5)

where p is the length of the base of the prism (= 8 8 cm), 4 is the angle subtended by the diameter of the collimating mirror at the slit, d is the width of the slit, and a is the breadth of the beam leaving the prism.

TABLE I

S in 0t n R P D (in V per 20")

1.0973 1 53 366 1186 91 0 0053 1b3070 1-53 090 1140 105 0 0053 14972 1.52 842 1184 121 0 0053 19000 1.52 263 1394 200 0.0045 2 1000 1-51 927 1534 211 0.0041

In order to verify the method of calibration mentioned above, an observation of the absorption curve of water was made. The following, values were found for the positions of the maxima of absorption (Table TI1

* ' Z. InstrumKde,' vol. 48, p. 275 (1928).

t Baly, " Spectroscopy," vol. 1.




The agreement of the present observations with those of other authors is satisfactory. The values given in Table II lie between the greatest and smallest wave-lengths which have been hitherto recorded.

TABLE II

Values Previous Mean values of found observations previous observations

B in V. Bin ,u Bin v

0 988 0 97 Collins* 0985 0 99 Ellist 0 995 Dreisch

1 200 1 20 Collins 1 198 1 185 Ellis 1 t21 Dreisch

1.470 1 44 Collins 1 463 1*44 Ellis 1I475 Dreisch 1 48 Grantham? 1 48 Coblentzl

1957 2 00 Collins 1-972, 1h96 Ellis 1 97 Dreisch 1-98 Grantham 1 95 Coblentz

* 'Phys. Rev.,' vol. 36, p. 305 (1930). t 'J. opt. Soc. Amer.,' vol. 8, p. 1 (1924). * Z. Physik,' vol. 30, p. 200 (1924). ? 'Phys. Rev.,' vol. 18, p. 339 (1921). il "Investigation of Infra-Red Spectra," Washington (1905-10).

The absorption of four aldehydes in the liquid state was investigated, viz., propionic aldehyde, butyric aldehyde, valeric aldehyde, iso-valeric aldehyde. The absorption cells were rectangular frames of brass, 2 x 4 cm and of varying thickness. Two quartz windows, which were twice as long as the frame, were sealed to the two sides of a cell, so as to enclose the liquid. The cell was mounted in a holder, which was placed inside the box immediately before the first slit. This holder was free to slide on two rails so that either the cell containing the liquid or the projecting empty ends of the windows was before the slit: two stops kept it in position. Using a micrometer, the thickness of the quartz windows was measured first and the thickness of the whole filled cell determined after each set of



104 L. Kellner

readings. The difference between these two widths gave the thickness of the absorbing liquid.

The aldehydes under investigation were obtained from the British Drug Houses, Ltd., with the exception of the iso-valeric aldehyde which came from the Eastman Kodak Company, Rochester, U.S.A.

The procedure of observation was as follows. After the prism had been set for the desired wave-length, several readings of the deflexions of the galvanometer-mirror were taken alternately with the absorption cell before the slit and then with the quartz windows in the same position. In this way, error due to the loss of radiation through reflexion in the quartz was eliminated, as the light passed both times through the windows. If the average deflexion is D1 for the cell before the slit and D2 for the windows bcfore the slit, the transmission T of the absorbing layer is defined as:

TD, T_ D2 (6)

T defined in this way is independent of the intensity of the incident light for the values obtained in this investigation.

The absorption of the aldehydes was investigated between 1 1 and 2h 1 t. Each spectrum was taken with three cells of different thicknesses (10,7, and5inmupto 1 6 ,u and3,2,and I mmfrom l*6to2 1 4u). The maxima of absorption, as indicated by the agreement of the results with different thicknesses of liquids, could be determined with an accuracy of + 0 005 ,u. From the values for the transmission T, the molecular absorption coefficient a was calculated according to the formula:

T ee-id (7)

where d is the thickness of the absorbing layer in cm and M the molecular weight of the substance under investigation. Since the densities of the liquids examined were almost identical, the molecular absorption coefficient gives the absorption per equal number of molecules. No correction was made for the losses through reflexion at the surfaces of the liquid as no data are available for the refractive indices of the aldehydes in this spectral region. The limits of accuracy of the absorption coefficient were + 7O% for the maxima of absorption and ? 10% for the minima. It follows that the determinations of the shapes of the bands are not very exact.

EXPERIMENTAL RESULTS

In Table III the positions in wave-numbers v of the maxima of the bands are given, together with the data of previous investigations and also the corresponding absorption coefficients.




en- n 0000 0

oo 00 e- t- "- -r e q O N r~~~~~~~~~~~~ =.2 N t- ^^

8 n E ~~~~~~~~t- N% N% tn t-e On tn 1.

0- I- e0 0000 > o : *E n E ? F F m ^ X N~~- kn N 0 t-

Z~~~~~~~~~o 00 t- 'I I) W) W) o tX

ON

Oi % 0I I' I'l N%O 0 z M^F 0 I' N en en o

>0 ~ ~ ~ ~ ~ )~O 0< *' 'V0

0~~~~~~~~~~~~~ 0.0~~~~~~~~~~~~~~~~~~~0

S t * co W $jjjgE

U) ?n t- 00

- , oo b. t. m t. m m e *, 0,

.?E o ~~~~~~~l Wen N%0 N

.E l fl ~~ >X mo I t I I i

O~~n I I

o~~~~~ o

- *-

0000 1W )* *1

v ^ ?

~~~~~~~~~~N ON 0 o o

< oo ~~~~~~oo r- k tn t t

~~ 0 (~~1C q 0

o ~~~~~ ~ ~~~ ~~~.enO0 00 Ille

0 S~~~~~~~~~0 o 0 ~~~~~~~0 0tnk

I ~~~~enV- C r10 1 n 00 00 'I t.0 0n ON - -00~-

00 00 t- W) Wfl!



106 L. Kellner

For the first two aldehydes the figures in column I represent the results of the present paper, and those in column II the results obtained by other authors. Valeric and iso-valeric aldehydes have not been investigated before. The data given by Ellis and Sappenfield on propionic and butyric aldehyde agree satisfactorily.with the present results. The only serious discrepancy occurs with butyric aldehyde in the region 8000-9000. Here Ellis and Sappenfield found two bands, while one only could be found in the present investigation, though the region has been gone over nine times with two different samples of butyric aldehyde. Sappenfield found

9000 8500 8000 700 7000 6500 6250 V at I I I

70-

60

50

40 -I

30

20-

10

0 1 10 1 20 1 30 1X40 150 1 60 1 65, A

FIG. 1-Molecular absorption of propionic and butyric aldehydes between 1 10 and 1 * 65 V.. x propionic aldehyde, curve I. * butyric aldehyde, curve II.

that the band at 8370 is far stronger than the band at 7855, as occurs with propionic and valeric aldehyde (Table III). However, in the present investigation the band of butyric and iso-valeric aldehyde in the region 8000-8100 presumably corresponding to Sappenfield's 7855; was found to be much more intense than the corresponding band of propionic and valeric aldehyde and of the same order of intensity as the band 8600. It is not possible to give an explanation for this dissimilarity.

Ellis was able to resolve the broad band in the region 7200 to 6800 into two components for butyric aldehyde. In the present investigation only iso-valeric aldehyde showed a (probably incomplete) separation of this band, but it is very likely that with higher resolving power it would be




separated into several components. A glance at figs. 1 and 2 shows indications that this band is complex.

The very broad and intense band at 1 74 p (between v 6000 and v 5600) has been resolved into three components, though the resolution is by no means complete. Sappenfield gives here only one broad band, though his spectrometer had a resolving power approximately three times as great. But as his apparatus was provided with an automatically registering device, it is possible that the spectrum was moved too quickly

a 9000 8500 8000 7500 7000 6500 6250 v I I l I I I I l too

80-

600

40-

20

10 120 130 140 150 160 165', h

FIG. 2-Molecular absorption of valeric and iso-valeric aldehydes between 1 10 and 1 * 65 [. 0 valeric aldehyde, curve III. 0 iso-valeric aldehyde, curve IV.

across the slit of the thermopile, so that the components could not be properly separated.

In figs. 1-4 the absorption coefficients have been plotted against the wave-lengths for the various aldehydes. On the upper axes of the dia- grams the wave-numbers v are given.

Fig. 1 shows the absorption of propionic aldehyde (CH3 . CH2 . CHO) and butyric aldehyde (CH3 . CH2. CH2. CHO) between 1 1 and 1*6 ,6 and fig. 2 that of valeric aldehyde (CH3. CH2 . CH2. CH2. CHO) and iso-valeric aldehyde (CH3 . CH3 . CH . CH2. CHO) in the same region. The absorption curve of butyric aldehyde shows a stronger background than that of propionic aldehyde, while the peak at 1 4 p reaches nearly



108 L. Kellner

the same height. Comparison of the absorption below 1-3 [L would lead to no definite conclusions in consequence of the differences mentioned above. From a comparison of curve II, fig. 1, and curve III, fig. 2, it will be seen that valeric aldehyde shows the same background as butyric aldehyde, but that the maximum absorption at IP4 p is greater. Iso-valeric aldehyde (curve IV, fig. 2) has on the whole a weaker absorption than the normal compound, with the exception of the band at 1 * 23 P..

6250 000 5760 5500 5250 6000 4750 v

at

!00 -

250 ;. I

1 60 170 180 1.90 200 2J, A

FIG, 3-Molecular absorption of propionic and butyric aldehydes between 160 and 2-15 t. x propionic aldehyde, curve I. * butyric aldehyde, curve TI.

The intensity of the 1'4-,u band decreases very rapidly towards larger frequencies.

In figs. 3 and 4 similar curves are given for the region between 1 6 and 2 1 p. Butyric aldehyde has a greater absorption coefficient than propionic aldehyde below 2 07 t (fig. 3). The intensity of the bands at 1 *7 P. is nearly the same for butyric aldehyde as for valeric aldehyde, but the latter shows smaller absorption for the region between 1 8 and 2 0- and greater intensity above this wave-length. It follows from a comparison of curves III and IV, figs. 2 and 4, that an alteration in the molecular




configuration has no influence on the positions of the bands in this spectral region. Except for the doubtful region below 1 3 ,u, the curve for iso-valeric aldehyde runs almost parallel to that for valeric aldehyde (fig. 4), and the shapes of the corresponding bands are very similar to each other. From 16 to 2 t the iso-compound has a stronger absorption than the normal compound. The shifts which occur in the positions of the bands are mostly within the limits of experimental error. Where they are greater, they do not show any marked regularities (Table III).

6250 6000 5750 5500 6250 000 4750v at I 1 1 1 600

450

400 -1v

350

300 _

250

200-

10o

I'V. 9o.

0 _X 1.60 1 70 180 190 2-00 2-10;uA?

FIG. 4-Molecular absorption of valeric and iso-valeric aldehydes between 1 * 60 and 2 15 ,. 0 valeric aldehyde, curve III. (0 iso-valeric aldehyde, curve IV.

DISCUSSION

The striking feature of the band spectra of the four aldehydes is the great similarity in the numbers and positions of the bands. This similarity suggests a corresponding similarity of the oscillating centre. The vibrations,* which are independent of the configuration of the molecule, are the oscillations lying outside the upper and lower limits of the C-C valence frequencies, i.e., vv 836-1185. For the aldehydes, these independent oscillations are the inner vibrations of the CH3 and CH2 groups respec-

* Bartholome and Teller, 'Z. phys. Chem.,' B, vol. 19, p. 366 (1932).



110 L. Kellner

tively, and the valence vibration of the C=O bond. In the region between 1 and 2 t we are dealing with the overtones and combination tones of these fundamentals. As the chain frequencies are very far off and of relatively low intensity, their overtones will not appear in our spectra. In Table IV the wave-numbers of the fundamentals are given which are necessary for the analysis of the observed spectra.

TABLE IV-WAVE-NUMBERS OF FUNDAMENTALS OF C=O AND C H

VIBRATIONS

Propionic Butyric Valeric Iso-valeric Mode of aldehyde aldehyde aldehyde aldehyde vibration

1377 Lt 1366 Lt (1390) K+ 1361 Lt 8(7)CH

1439 1445 (1448) 1445 23 (a)CH

1695 1695 (1717) 1695 Vc=o

2841 2882 (2874) 2865 VCH

The data in the first, second, and fourth columns, which are marked with " L ", are taken from Lecomte's* investigation. As no measurements of the infra-red spectrum of valeric aldehyde have been made, the data of the Raman lines are given in brackets in the third column.t In the last column the bands are assigned to different modes of vibration. The symbols a, v refer to bending and valence frequencies respectively. The index C=O or CH indicates which group is oscillating. The change of the dipole moment, whether perpendicular (a) or parallel (l) to the axis of the molecule, is also shown. The fundamental of the C-O valence vibration at 1695 is well known from the infra-red and Raman spectra of aldehydes, ketones, and fatty acids. In assigning 1360 to a (r)c. and 1440 to 2a (a)CH (the index 2 at the left side of a means that the vibration is doubly degenerate), I have followed Bartholome and Sachssel in their interpretation of the fundamentals of CH3. OH and, C2H6. In formic aldehyde (H. CHO) we have a parallel band at 1500 and a perpendicular band at 1340.? As, however, the molecule H. CHO must be considered as a whole for the interpretation of its vibrational spectrum, while in more complicated molecules the CH groups may vibrate inde- pendently of the rest, it seemed justifiable to assume that the oscillations in an aldehyde molecule resembled the oscillations of the same group in other compounds. In any case, the assignment can only be a preliminary

* 'C.R. Acad. Sci. Paris,' vol. 180, p. 1481 (1925). t Kohlrausch and Koppl, 'Z. phys. Chem.,' B, vol. 24, p. 370 (1934). I 'Z. phys. Chem.,' B, vol. 30, p. 40 (1935). ? Nielsen, 'Phys. Rev.,' vol. 46, p. 117 (1934).




one. 2l (a)cH and vOR are common to the CH3 group and the CH2 group. * In the Raman spectrum there is a second CH valence vibration at 2960 besides the one at 2880 given above (Table IV). As this vibration is not infra-red active, it has not been taken into account here. Table V repre- sents an attempt at interpreting the bands between 1 and 2 ,u as overtones and combination tones of the fundamentals given in Table IV. The interpretation of the bands at 5150, 6800, and 8100 as overtones of vC=0 has been previously proposed by Ellis.t

TABLE V-INTERPRETATION OF OBSERVED BAND SPECTRA

Assignment Propionic Butyric Valeric Iso-valeric aldehyde aldehyde aldehyde aldehyde

obs calc obs calc obs calc obs calc 2vc=o + 8 (7)CH 4748 4767 4762 4696 4764 (4824) 4713 4751 2,va + 28 (a)CH 4888 4829 4798 4775 4822 (4882) 4796 4833

3,vca-o 5155 5085 5149 5085 5131 (5151) 5149 5085 48 (Tr)cH(?) 5197 5508 5249 5464 5258 (5560) 5280 5444

2voH - 5682 5647 5764 5593 (5748) 5593 5730 VCH + 28 (a)HOT 5740 5729 5754 5772 5701 (5770) 5714 5755

428 (G)CH (5942 5756 (5956 5780 6035 (5792) 6024 .5780 4,vo ( 6780 6780 (6868) 6863 6780

2vcH + 8 ()C 7036 7059 7130 7067 (7138) '7091 2V0R? + (a)CH { 7121 7 7209 (7196) 7008 7175

528 (a)CH 7195 7225 (7240) 7225 68 70-H ?) 8051 (8262 8137(8196 817(8340) 814{8166

5Vcao (?) 1 8475 1 8475 8137 (8585) 8104 8475 3vcH 8523 (8646 (8622) 8595

2vcH + 2 28 (a)CH 8643 8560 8654 8636 (8644) - 8620 628 (a)CH 8634 (8670 1(8688) 8670

The overtones of vw11 show resonance degeneracy with the overtones of 28 (a)C and the combination tones of both. That accounts for the fact that the band at 7000 has approximately the same intensity as the band at 8600, though the latter should be weaker, belonging to a higher quantum number. The assignment of 5250 to 4a (7),H is somewhat doubtful as the agreement between calculated and observed values is not good. Besides, the band does not appear in the spectra of molecules containing only carbon aind hydrogen atoms.

Ellis+ tried to arrange the CH bands in a series of overtones of vC only. This internretation did not exnlain the nresence of the hand at 7000. As

* Dennison and Sutherland, ' Proc. Roy. Soc.,' A, vol. 148, p. 250 (1935). t 'J. Amer. Chem. Soc.,' vol. 51, p. 1384 (1929). + 'Phys. Rev.,' vol. 33, p. 27 (1929).



112 Absorption Spectra of Four Aldehydes

this band appears in the spectra of molecules containing carbon and hydrogen atoms only, it follows that it must be due to vibrations of these constituents.

In view of the fact, however, that the separation of the bands into their components is by no means complete, any assignment can only be regarded as tentative.

The aldehydes under investigation, with the exception of iso-valeric aldehyde, differ from one another by the number of CH2 groups. If the CH2 groups are the oscillating centres, we might expect an increase in the absorption coefficient of the bands with increasing number of CH2 groups, while a frequency caused by the vibrations of the C-O group should not be affected. This assumption would mean that the molecular absorption coefficient, as it determines the absorption for equal numbers of molecules, should increase from propionic to valeric aldehyde for vibrations in which the CH groups take part. If we look at the C=O frequencies (Tables III and V), we find that the intensity of 3v,=o (v -5150) remains of the same order with the exception of valeric aldehyde. It is not possible to compare the intensities of 4v0=o as this band has not been separated from the neighbouring CH vibrations.

If we now proceed to the CH vibrations, we find that the overtones and combination tones of 28 (a)cH and vcH are so near to each other that disturbances by resonance between the different vibrational levels are very likely to occur (Table V). The only bands which show a regular increase in intensity are 5700 and 4800 (Tables III and V). However, the increase continues to iso-valeric aldehyde, though iso-valeric aldehyde contains one CH2 group less than valeric aldehyde.

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

SUMMARY

An investigation has been made of the infra-red absorption spectra of propionic, butyric, valeric, and iso-valeric aldehydes in the liquid state between 1 1 and 24 I . The wave-numbers and absorption coefficients are tabulated and compared with previous observations.

An attempt is made to interpret the observed bands. as overtones and



The Crystal Structure of H3PW12040 . 29H20 113 combination tones of the inner vibrations of the CH3 and CH2 groups respectively and the C==O valence vibration. It is suggested that four fundamentals are sufficient to explain all observed bands.

It is not possible to find a relation between the intensity of the bands and the number of oscillating centres, as most of the vibrational levels lie so close together that resonance occurs.

The Crystal Structure of H3PW1204o . 29H20

By A. J. BRADLEY, D.Sc., Royal Society Warren Research Fellow, and J. W. ILLINGWORTH, M.Sc.

(Communicated by W. L. Bragg, F.R.S.-Received 1 May, 1936)

The special interest of this structure lies in the peculiar manner in which the anions (PW12040)-3 are linked together by the water molecules. The remarkable number (29) of water molecules associated with each anion is accounted for by symmetry considerations. Alternating with the anions on a body-centred cubic lattice, seventeen molecules of water are linked together in tetrahedral symmetry. These comprise a central molecule in contact with four molecules at the corners of a tetrahedron, and twelve molecules strung out in pairs along fourfold cube axes, to link together six anions. Midway between two blocks of seventeen waters and independent of them, six water molecules form a puckered hexagonal ring, which links together six anions, lying symmetrically about a threefold axis. For each anion there are two hexagonal rings and one block of seventeen molecules. This accounts for the total of 29 molecules per anion.

The environment of the water molecules is in striking contrast with that of the oxygen atoms in the anions. The latter are close-packed, being held together by the phosphorus and tungsten atoms. The water molecules are more open, and only six of the 29 have more than foLr nearest neighbours. The interatomic distances of the water molecules are very consistent, all lying between 2 8 A and 2 *9 A.

In conformity with the space-group 07h, the structure is composed of two interpenetrating diamond lattice-complexes. The one consists of eight anions (PWL204o)-3; the other consists of eight clusters of water

VOL. CLVII.-A. I



Documents

Absorption Spectra of Four Aldehydes in the Near Infra-Red