RAMAN EFFECT I N INORGANIC COMPOUNDS BY J. C. GHOSH AND S. K . DAS
Since the discovery of the Raman effect,’ which has opened up a new method of investigating ,molecular spectra in the infra-red region, several organic and inorganic substances have been studied with the object of interpreting the frequency shifts in Raman spectra in terms of their constitution. The correlation of Raman shifts with the characteristic oscillation frequencies and molecular constitution in organic compounds has chiefly been carried out by Dadieu and Kohlrausch, Venkataswaran and Ganesan, Petrikaln and Hochberg and others and the results they have obtained are of far reaching importance. The correlation of frequency shifts in inorganic compounds with the grouping of atoms and their characteristic oscillation frequencies was first brought about by the works of Rosen, Pringshaim and Carelliz on several inorganic nitrates in solution. A single shift 1046 cm-1 was obtained corresponding to the inactive frequency of the NO3- ion, assumiy the arrangement to be an equilateral triangle with the nitrogen atom a b the centre and the three oxygen atoms a t the three corners. The same results have virtually been obtained by Gerlach. Kinsley, Bell, Frederickson, Williams, Nishi, Krishnamurthi, Ramaswamy, Rao3 and others, the latter measuring electrolytic dissociation by Raman methods. In each case the inactive frequency appeared brightest and the shifts were uninfluenced by the nature of the cation. The inactive frequencies of those which do not produce any change in the electric moment of the group do not appear in the infra-red absorption and hence the discovery of the Raman effect, where such inactive frequencies appear strongly, enables an exact determination of all the fundamental frequencies of simple atomic groups. Dillon and Djckinson’s4experiments on ionised substances of allied chemical character revealed that the frequency shift is in some cases independent of the number of oxygen atoms associated with the central atom, and further, the frequency change of the line due to inactive frequency decreases with the increase of atomic number of the central atom of the anion. A further suggestion is offered to interpret the frequency shift as a measure of the stiffness of binding. The present paper deals with certain well-defined groups of inorganic compounds in solution, the Raman shifts being interpreted in terms of characteristic oscillation and molecular constitution. How far the suggestion of Dillon and Dickinson holds good in the above cases, can be observed from the results obtained. Indian J. Phys., 2, 389 (1928). Z. Ph.ysik, 51, 511 (1928). a ROC. Roy. SOC.,127A, 279 (1930). Roc. Nat. Acad. Sci., 15, 334 (1929). 1
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RAMAN EFFECT IN INORGANIC COMPOUNDS
587
Experimental Substances soluble in water were used in state of solution. The substances were all of Kahlbaum’s or Merck’s preparation (Extra Pure), further purified if necessary. A saturated solution was made with re-distilled water and was rendered free from any suspended matter by repeated filtration. The method of illumination was virtually the same as that of The solution was put in an inner tube placed inside a vertical outer jacket, through which water was circulated, snbstituted by colored solution when necessary. The whole arrangement was clamped upright and a vertical quartz mercury-lamp is placed along-side, so that the light scattered at right angles, emerges along the axis of the vertical tube. This scattered beam is totally reflected by a rectangular prism into the slit of the spectrograph thereby making the maximum illumination available. The slit of the spectrograph was shielded from direct illumination by suitable screens and practically the whole of the light that entered the spectrograph was due to scattering a t right angles. With this arrangement, using soft-gradation panchromatic plates (Backed), H I% D 2000, and a Fuess constant deviation glass spectrograph (the investigations being carried out in the visible region of the spectrum) an exposure of four to six hours was found to give satisfactory spectrogram. Appearance of fainter details required longer exposures. The plates were all measured with a Hilger travelling micrometer, and the wave-lengths of the lines calculated by using the simplified form of Hartman’s interpolation formula (X = Xo c/(n-no)) with the known mercury lines appearing on the plates as standards. The wave-numbers of the lines (in vacuo per cm.) were then calculated and the shift in wave-numbers of the Raman lines from the corresponding exciting lines determined.
+
Results and Their Discussions I t is well known that in solutions of strong electrolytes we have complete dissociation into ions and the Raman lines observed for such solution must be ascribed to the ions in the solution. I n the following pages the Raman lines of polyatomic ions in the order of their increasing molecular complexity will be discussed. The simplest elementary ions have not yet been found to give Raman shifts. A. Raman lines due to diatomzc ions:-The investigations of Pal and Sen Gupta8 show that the CN’ ion in KCX gives a single Raman line 2080 cm-I. This frequency due to the CN’ ion is much less than the frequency observed when CN’ group forms part of a more complex ion like OCN’ (Au = 2 183) or of a non-ionisable compound like CH3CK (Av = 2256). We have always failed to obtain any Raman shift for the OH’ ion though Woodward7 claims to have observed the shift, Au = 3615 + 2 5 cm.-’ Phil. Mag., 6, 729 (1928). Indian J. Phys., 5, 612 (1930). ’Woodward: Pbysik. Z., 32, 261-262(1931).
J. C. GHOSK AND S. K. DAS
588
A solution of sodium hydrogen sulphide, a strong electrolyte, obtained by passing H2S gas through caustic soda solution and removing free HzS by air, gave a Raman line 22130 cm-'. The Raman shift 2 5 7 5 cm-l is the same as has been observed in liquid H2S and in various mercaptans.8 The behaviour of SH' ion is therefore unlike that of CN' ion in that the Raman shift is independent of the manner in which the other valency of sulphur atom is saturated. B. Raman lznes due to triatomzc tons:-The Raman shifts due to sodium metaborate (NaB02) and sodium nitrite solutionsQare given in Table I.
TABLE I NaKOz NaB02
696 cm-l 253 em-'
785 cm-'
1303 cm-l I403 cm-l
I n a symmetrical linear model like COS, CS,, HgC12 we generally have a strong Raman line representing the symmetrical inactive vibration of the terminal atoms against the central atom, with a faint companion. Thus CO2 has a strong line Av = 1284 cm-l with a very weak companion Av = 1392 em-', CS2 and HgC12 give strong lines Av = 655 cm-' and 312 cm-l with weak companions Av = 795 cm-l and 381 cm-l respectively. A non-linear model, however, according to BhagavantamlO should reveal three independent oscillation frequencies. I n the case of SO2 this is actually observed. The KO2' ion gives three independent oscillation frequencies Av = 696 cm-l, 785 cm-l, 1303 cm-l, in the case of BO*' ion two such frequencies have actually been observed, Av = 253 cm-' and 1403 cm-', the third line being perhaps too faint to be detected. Hence both these ions should be represented by a non-linear model. I t is interesting to note that unlike the case of SH group, the Raman frequency due to NO2group does not remain constant. Thus in nitrobenzene the Raman shift observed is I342 cm-l as against 1303 em-' in sodium nitrite solution. We may include in this category the hypophosphite ion, though it is not triatomic. The hydrogen and oxygen atoms however, in this ion, so far as Raman effect is concerned behave as two hydroxyl groups with the probable constitution P(OH)2'. This is of course, not in harmony with the accepted chemical constitution of this ion.
TABLE I1 Sodium hypophosphite fiequency Shift
Intensity
I080 2 952 I We have two lines of comparable intensity Av = 9 5 2 em-' and 1080 em-' analogous with the lines Av = 795 em-' and 1303 cm-l for the nitrite ion. Bhagavantam: Nature, 126, 502 (193o);Venkataswaran: Indian J. Phys., 5,223 (1930). 2. Physik, 51, 51 I (1928). lo Indian J. Phys., 5, 81 (1930). 8
RAMAN EFFECT I N INORGANIC COMPOUNDS
589
In agreement with the views of Dillon and Dickinson, we find that the frequency shift diminishes as the central atom in the ion becomes heavier. Thus we have for:-
BO*Frequency Shifts 1403 cm-l
NOzI303 cm-l
P(OH)z1080 cm-‘
I n the case of other triatomic ions, where the atoms are all different from one another, we have two characteristic frequency shifts due to the two valency bonds, e.g. in SCN ion, we have besides 2080 cm-l due to - C = N, another shift 740 cm-’ due to S-C bond.” C. Ions of the t y p e X03:-This type of ion has already been investigated and the general character of the Raman shifts exhaustively described.’? A few gaps however remain t o be filled up--e.g. POI’, VOa‘ and Xs03”’ (metaphosphate, metavanadate and arsenite) ions which are very similar to NO3’, being ions of oxy-acids of elements in the same group of the periodic table, do not appear to have been investigated.
Sodium nitrate13 Sodium metaphosphate Sodium metavanadate Sodium arsenite
TABLE I11 726 cm-l 228 cm-I 144 cm-l
1048 em-’ 1 1 3 7 cm-’ 367 cm-l 203 cm-I
In sodium metaphosphate we have been able to observe only one Raman shift 1 1 3 7 cm-l. It may be observed here that viscous liquids do not as a rule give conspicuous Raman lines. We rather meet with general scattering, due perhaps, to the considerable forces, which neighbouring molecules exert on one another. The Raman shifts of vo3’ ion are similar to those of OS' ion. In fact 726 1048 23(At. KO. of V) we have the curious coincidence that the ratios of -, -,and 228 3 6 7 7(At.Xo.ofK) have approximately the same value. Comparing KO3’ with As03’” ion we 726 1048 find again that the ratios -7 __ and 3 5 w . No. of as) are approximately 144 203 7(At. No. of N) the same. Thus for similarly constituted ions containing several atoms of oxygen, the ratio of the comparable Raman shifts is inversely as the atomic number of the central atom. D. Ions of the type XO4:-1nvestigations of this type of salts have been carried out by Kri~hnamurthi’~ and Kishi.*j Krishnamurthi: Indian J. Phys., 5, 654 (1930). Leontowitsch: 2. Physik, 54, 153 (1929);Gerlach: Nature, 1930, 819;Nishi: Mem. Coll. Sci. Kyoto, 13A, 163 (1930);Krishnamurthi: Indian J. Phys., 5, 633 (1930). l 3 Ramaswamy: Indian J. Phys., 5, zoo (1930). l4 Indian J. Phys., 5, 633 (1930). Jap. J. Phys., 5, I 19 (1929). l1
l2
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We have not been able to observe any Raman line for solutions of Na3P04, but for solutions of arsenates NasAs04and orthovanadates NasV04, Raman lines have been observed. It may be mentioned here that orthovanadates are formed by the addition of alkali to metavanadates and we find accordingly a complete change in the Raman spectra by progressive addition of hydroxyl ion to metavanadate solution. Thus for metavanadate we have Raman shifts of 228 and 367 cm-l while for orthovanadate, we have Raman shifts of 880 and 907 cm.-‘ TABLE IV AS04”
vo4”’ s0~3 S20S”
349 cm4
462 cm-l 880 cm-‘ 907 cm-1
457 cm-’
617 cm-I
981 cm-I 833 cm-I
1102
cm-I
1081 cm-I
Here also we find the qualitative relationship that as the atomic number of the central atom increases the corresponding Raman shifts become less. The comparable Raman frequencies in persulphate ion have smaller values than those of sulphate ion. Ammonium molybdate and sodium tungstate solutions give a large number of Raman lines and the spectra appears to be similar in nature.
Moop“’ 256 cm-l
wok’
TABLE V 356 cm-* 879.8 cm-I 932.4 cm-I 371 cm-I 925 cm-I 1017 cm-I
1540cm-~13g4cm-i
If, however, to sodium tungstate solution we add hydrochloric acid so as to obtain a tungstic acid sol Raman lines become very faint. Several other acid sols like silicic acid, molybdic acid have been tried in this laboratory, but no Raman lines could be detected. It appears that colloidal solutions and amorphous solids cannot be easily stimulated to yield Raman radiations. E. Acids and acid salts of the t y p e &POs, NaHzP03, Na2HP03,NaHSOa:These have been investigated and show considerable similarity in behaviour. TABLE VI HsP03 NaHpPOa Na2HP03 NaHS03
672 cm-l 642 cm-l 346 cm-l
940 cm-I 925 cm-I
980 cm-I 826 cm-I
1012
cm-I
1068 cm-l 1030 cm-1 963 cm-‘
1406 cm-I
The Raman lines of phosphorous acid and sodium dihydrogen phosphite are almost identical, the lines being really due to HzP03’ ion. The ions HS03’ and HP03’, give analogous Raman lines. It may be mentioned here that the Raman spectra of liquid SO2 (Av = 526 cm-I, 1146 cm-land 1340
591
RAMAN EFFECT I N INORGANIC COMPOUNDS
cm-1) are completely different in nature from those of HSOa' ion, thus indicating a considerable change in the molecular structure of SOn gas when dissolved in alkali. I n Table VI1 are given the wave-lengths, the corresponding frequencies expressed as wave-numbers per cm. in vacuum and the notations of the exciting mercury lines. Tables VIII-XXIII show the wave-numbers in vacuum per cm. of the modified lines, their intensity and the corresponding exciting lines.
TABLE VI1 In I.A. I n cm-l Sotations
43583 22938 a
TABLE VI11 Sodium Hydrogen Sulphide '
Wave-number of R (vac) per em.
Intensity
22130
5
Exciting line and frequency shift b-2jjs
TABLE IX Sodium Metaborate Wsve-number of R (vac) per cm.
23302 22685
Intensity
3 I
Exciting line and frequency shift
b-I403 a--253
TABLE X Sodium Nitrite Wave-number of R (vac) per em.
22342 22153 221635
Intensity 2
I
5b
Exciting line and frequency shift
a- 696 a- 785 a-1303
TABLE XI Sodium Hypophosphite Wave-number of R (vac) per om.
23628 21856 2 1986
Intensity
Exciting line and frequency shift
b-Ioji a-1082 a- 952
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TABLE XI1 Sodium Metaphosphate Wave-number of R (vac) per cm.
Intenaity
Exciting line and frequency shift
23570
I
2 I799
2
b-1135 a-1139
TABLE XI11 Sodium Metavanadate Wave-number of R (vac) per cm.
Intensity
22710
I
22571
I
Exciting line and frequency shift
a-228 a-367
TABLE XIV Sodium Arsenite Wave-number of R (vac) per cm.
Intensity
22794 22735
I
I
Exciting line and frequency shift
a-I44 a-203
TABLEXV Sodium Arsenate Wave-number of R (vac) per cm.
22589 22476
Intensity I
2
Exciting line and frequency shift
a-349 a-462
TABLE XV1 Sodium Orthovanadate Wave-number of R (vac) per cm.
Intensity
Exciting line 2nd frequency shift
23823 22064 23798
2b
b-882
2b
a-878 b-907
TABLE XVII Sodium Persulphate' Wave-number of R (vac) per cm.
23871 22105 23621 21857
Intensity
Exciting line and frequency shift
b- 834 a- 833 b-1084 a-io8i
*
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RAMAN EFFECT IN ISORGANIC COMPOUNDS
TABLE XVIII Ammonium Molybdate Wave-number of R (vac) per cm.
Intensity
Exciting line and frequency shift
a-930 b-93 6 a-8 79.8 a-256 a-3 56
22008 23769 220582 22682 22582
TABLE XIX Sodium Tungstate Wave-number of R (vac) per cm.
Intensity
Exciting line and frequency shift
b-153 7 a--1543 b- 925 a- 930 a-1394 a-1017 a- 371
23168 21375 23780 22008 21544 21921 a2567
TABLE XX Phosphoric Acid Exciting line and frequency shift
Wave-number of R (vac) per cm.
Intensity
23764
4 4
ba-
2
b-1012
22002
23692 21926 22265
a-1013 a- 672
2
5b
TABLE
944 936
XXI
Acid Sodium Phosphite Wave-number of R (vac) per cm.
23639 21868 22013 22296
Intensity 2
4b 2
4b
Exciting line and frequency shift
b-1066 a-1070 a- 925 a- 642
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TABLE XXII Normal Sodium Phosphite Wave-number of R (vac) per cm.
Intensity
Exciting line and frequency s h f t
b-
23727 23678 21956 21905
978
b-1027
a- 893 a-1033
TABLE XXIII Sodium Hydrogen Sulphite Wave-number of R (vac) per em. 23299 22.592 22112
21975 Chemical +ubopwy, Daeca Unzvermty, India, September 17, 2931.
Intensity
Exciting line and frequency ahift b-1406 a- 346 a- 826 a-
963
