RAMAN SPECTRA OF CRYSTALLINE TARTRATES

Transcription

1 RAMAN SPECTRA OF CRYSTALLINE TARTRATES Part III. Tartaric Acid BY V. M. PADMANABHAN (From the Department of Physics, Indian Institute of Science, Bangalore) Received January 20, 1950 (Communicated by Prof. R. S. Krishnan, F.A.sc.) 1. INTRODUCTION IN Parts I and II of this series, the author (Padmanabhan, 1948, 1950) described the results obtained from studies on the Raman spectra of crystalline sodium tartrate, potassium tartrate, ammonium tartrate and Rochelle salt, using A 2536 mercury resonance radiation as exciter. This paper gives an account of a similar investigation carried out with tartaric acid in the form of a single crystal. Its Raman spectrum was photographed simultaneously by Canals and Peyrot (1938) and by Gupta (1938). In both these investigations the visible radiation of the mercury arc was employed for exciting the Raman spectrum. Canals and Peyrot, who used the powder technique, reported the existence of 13 Raman lines with frequency shifts 737, 891, 993, 1094, 1260, 1309, 1364, 1691, 1745, 2931, 2964, 3331 and 3422 cm.-1 Gupta, on the other hand, employed a single crystal, and identified eleven Raman lines in the spectrum with frequency shifts 37, 80, 103, 125, 731, 848, 895, 1140, 1745, 2934 and 2967 cm.-' It is indeed surprising to note that these authors had in common observed only five frequency shifts. A detailed investigation of the Raman effect in crystalline tartaric acid is therefore called for. 2. EXPERIMENTAL DETAILS AND RESULTS Crystals of tartaric acid were grown by the method of slow evaporation from aqueous solutions of the pure substance. The used for photographing the Raman spectrum had the size 2 x 1 x 1 cm. The direction of illumination was roughly normal to the face (100) and the direction of observation was nearly normal to the (001) face. An enlarged photograph of its Raman spectrum taken with a Hilger medium quartz spectrograph with a slit-width of mm. and an exposure of the order of a day is reproduced in Fig. 1. The positions and frequency shifts are marked in the figure. The frequencies are listed in Table 1, along with the 19 frequency shifts already observed by earlier workers. The figures in brackets represent visual estimates of the relative intensities of the lines. The frequency shifts 167

2 168 V. M. Padmanabhan observed by Edsall (1937) in the spectrum of tartaric acid solution are also included in the same table. The Raman spectrum of tartaric acid exhibits thirty-one lines which include the line at 1745 cm. -' reported by Canals G m N ^^ o Fie. 2. Microphotometer record of the lattice spectrum of tartaric acid and Peyrot and by Gupta. This line could not be identified in the spectrogram taken by the author with A 2536 excitation as it would have fallen on the mercury triplet..,a The author has not only confirmed the existence of all the 13 frequency shifts observed by Canals and Peyrot and the 11 by Gupta, but also recorded the following twelve new frequency shifts 63, 154, 170, 194, 218, 293, 317, 538, 787, 1126, 1265, and 1451 cm.-1 In the low frequency shift region, the spectrum exhibits nine lines which are exceptionally sharp. Of these three at 37, 80 and 103 cm. -1 are very prominent, the remaining six lines although feeble in intensity are clearly discernible in the spectrogram. They are more easily seen in the microphotometer record reproduced in Fig. 2. The line with the smallest frequency shift, namely 37 cm. -1, is the most intense one.

3 ' Raman Sfectra of Crystalline Tartru,es--ll! 169 The first six lines 37, 63, 80, 103, 124 and 154 cm.-1 exhibit a periodic variation in intensity, the absolute intensity of all the lines diminishing gradually with increasing frequency shift. Further there is nearly the same frequency difference (of the order of 45 cm. -1) between the consecutive odd lattice lines, 37, 80, 124, 170 and 218 cm.-1 TABLE I. Raman spectrum of tartaric acid crystal Tartaric acid crystal Author j Tartaric acid solution Tartaric acid crystal Previous Previous Author workers I workers acid solution 37(20) 37 (6) 991 (2) (15) (1) (5) (8) (10) (6) (3) 1250 (6) (1) 1265 (6) 194 (1) 1307 (3) (1) 1359 (4) ) J (l ) (3) (3)b (10) (12) (15) (3) (10) (8) (6) j (2) 1432 Fig. 3 represents the Raman spectra of tartaric acid, ammonium tartrate, potassium tartrate, sodium tartrate and Rochelle salt taken with the medium quartz spectrograph and enlarged to the same extent. It is evident from the figure that the spectrum of tartaric acid exhibits the minimum number of Raman lines. Regarding the sharpness of the lines, the tartrate frequencies of ammonium tartrate are sharper than those of tartaric acid, while the converse is true for the lattice lines. 3. DISCUSSION The crystal structure of tartaric acid has been analysed using the X-ray method by Astbury (1922). The structure belongs to the monoclinic class. X-ray investigations reveal that there are two molecules in a unit cell and that the crystal belongs to space group C 2 2. The two molecules in the fundamental cell are held together end to end by forces between the hydrogen atoms of adjacent hydroxyl groups. The plane in which these molecular junctions lie is the plane 100, the perfect cleavage plane of the crystal,

4 170 V. M. Padmanabhan Lattice. frequencies. On the basis of the structure given above, it is possible to understand the appearance of 9 lattice lines in the spectrum of crystalline tartaric acid, as indicated below. The elements of symmetry for the unit cell containing two molecules are (1) identity E and (2) one two-fold screw axis C 2. The full character table for this structure is given in Table II where T and R give the number of translatory and rotatory modes inside the TABLE II External C 2 2 E C, rz1 Raman Infra-red T R A active active B active active UR 32 0 Ux(s) 2 0 Us(s v) 2 0 hr (T) 3 1 hpxp' (R) 6 0 lattice. There are on the whole nine of these modes, which are both Raman and infra-red active, five of which come under class A and four under class B. For a proper assignment of the observed lattice lines, it is necessary to study the state of polarisation of the lattice lines. The specimen of tartaric acid used in the present investigation was rather small and did not have all its faces well developed. Hence the effect of crystal orientation on the intensities and state of polarisation of the lattice lines was not studied. It is proposed to take up this investigation in the near future. Internal frequencies. The twenty-two Raman lines with frequency shifts lying in the region 250 to 3500 cm. -1 arise from the internal oscillations of the tartaric acid molecule. It is natural to expect a close correspondence between these frequency shifts and those observed in the spectrum of the tartaric acid solution (see Table I). The following peculiarities may be mentioned. The line at 2936 cm. -1 observed in the spectrum of the solution is split into two with frequency shifts 2934 and 2967 cm.-i in the case of crystal. The lines at 1119 and 1232 cm.-1 in the spectrum of tartaric acid solution, which are due to C-OH group oscillation, appear as doublets with frequency shifts 1126 and 1140, 1250 and 1265 cm; 1 in the spectrum of the tartaric

5 M. Padmana6lia,e Proc. hid. Acad. Sc., A, vol. XXXI, Fl. X N E E - I II'-:

6 Raman Spectra of Crystalline Tartrates III 171 acid crystal. The two lines at 3322 and 3407 cm : 1 which are roughly equal in intensity are due to O-H oscillations. The remaining lines (arising from oscillations due to C C, C OH, and C =0 groups) show fairly one to one correspondence in the spectra of the crystal and of the solution. In conclusion, the author wishes to record his deep sense of gratitude to Prof. R. S. Krishnan for his guidance and encouragement. 4. SUMMARY The Raman effect in a single crystal of tartaric acid has been investigated using the A 2536 mercury resonance radiation as exciter. The spectrum exhibited twelve new lines in addition to the nineteen lines observed previously. The 31 frequency shifts recorded here are made up of 9 lattice lines and 22 lines due to internal oscillations. The most striking feature of the Raman spectrum of crystalline tartaric acid is the fact that the lattice lines are sharper than the lines arising from internal oscillations. They are more or less equally spaced and the alternate lines exhibit a remarkable gradation in intensity. A close correspondence is noticed between the tartrate frequencies of tartaric acid crystal and those of tartaric acid solution. REFERENCES Astbury.. Proc. Roy. Soc., 1923, 102, 506. Canals and Peyrot.. Compt. Rend., 1938, 206, Edsall.. Jour. Chem. Phy., 1937, 5, 508. Gupta.. Ind. Jour. Phy., 1938, 12, 355. Padmanabhan.. Proc. Ind. Acad. Sci., 1948, 28, 451 ; 1950, 31, 95.