CHLORIDE MONOHYDRATE

Transcription

1 RAMAN SPECTRUM OF DIGLYCINE BARIUM CHLORIDE MONOHYDRATE BY R. S. KRISHNAN, F.A,Sc. AND K. BALASUBRAMANIAN (Department of Physics, Indian Institute o/ Science, Banga/oee-12) Received November 8, 1963 I. INTRODUCTION CfLYCINE forms an addition compound with barium chloride. Aecording to Pfeiffer (1922), whatever be the. stoichiometric quantities in which BaCI2 and glycine are mixed, the resulting addition compound is alwaysdiglyr barium chloride monohydrate [(NHaCH2.COO)2BaCI2.HO]. Berngl (1931) who gave the preliminary crystallographic data did not refer to the presence of water of crystallisation. The crystal belongs to the orthorhombic class with space group Vn 16, Pnma. The Ba atoms are supposed to be at the centres of symmetry. Pfeiffer (1927) suggested that the inorganic salts might be co-ordinately bound to carboxyl oxygen of the amino-acids, or that amphisalts of the following character,'x- (NH3 + CHR COO-) M +, might represent 91 mode of combination. The details regarding the complete crystal structure of diglycine barium chloride are not known. The work on the Raman spectrum of diglycine barium chloride was undertaken in order to investigate the nature of the hydrogen bonds in this substance. 2. EXPERIMENTAL DETALS Single crystals of diglycine barium chloride were obtained by the slow evaporation of the aqueous solution of the mixture of glycine and barium chloride in stoichiometric proportions. The crystals obtained therefrom were in the form of thin strips of dimensions 1" 1/2 1/4". The molecular formula of this crystal has been established from density and X-ray measurements carried out in our laboratory (Vaidya and Balasubramanian, 1963). The formula is (NH3CH2.COO)z.BaCI.HO. This is in conformity with that given by Pfeiffr from chemical data. The Raman spectrum was photographed using a Hilger medium Quartz spectrograph with a slit width of mm. Exposures of the order of 6 or 7 hours were given in order to get a reasonably good spectrogram. 14

2 Raman Spectrurn of Diglycine Bariur Cbloride Monohydrate Rs Ah enlarged photograph of the Raman spectrum of diglycine barium ehloride is reproduced in Fig. 1. The microphotometer record of the spectrum is also shown in Fig. 1. The frequency shifts of the Raman lines ate marked on the microphotometer record and ate also listed in Table I. The frequencies of the Raman lines observed in the case of a-glycine (Krishnan and Balasubramanian, 1958), diglycine hydrochloride (Balasubramanian, 1961) and barium chloride dihydrate (Galy, 1962) are included in Table 1 for comparison purposes. TABLE I Raman spectrttm of diglycine barium chloride monohydrate SI. No. G2BaCI2.H20 G2HCI BaC12.2H20 a-giycine Assignments (7) Lattice oscillations 2 59 (7) 60 62, (5) 73 71,, (6) 90,, (6) ,, (15) 122,, (2) ,, (4d) NH'..O, OH.- -CI (4d) NH...O, OH...C (1) (2d) 330 C-C torsion (3) ( G is used as a shortened forro for G yeine C--CN bending NHa torsion COO- sym. bending (12) C-CN sym. stretch (9.) CH2 rocking (5) C-CN antisym. streteh

3 16 R.S. KRISHNAN AND K. BALASUBRAMANIAN SI. No G2BaClv H (I0) 1309 (5) 1333 (20) 1407 (12) 1429 (20) 1455 (15) GzHCI 1122 ] TABLE I (Contd.) BaCIv2H20 a-glycin Asiglm.nt3 114'3 NHa + rocking 1323 CH, wag3ing 1330,, COO- sym. stretch 1441 CHe. sym. bending 1459,, (12) 1498 (2) 1564 (3) 1613 (3) 1(23 (4) 1659 (4) ( , NH, sym. deformation 1506 " 1563 NH3 de3. deformation COO- antisym. stretch 1640 " 1668 C=O ionised carboxyl (2d) 2700 (Id) 2745 (2d) 2785 (3d) 2865 (5d) 2900 (10d) 2920 ( OO 26" I N-H.".0 { ; O-H'"CI or " = N-H;..CI 275e o o N-Iq...O [ =O-H--.CI or 23o a'n-h..: :".:::% --I N-H" (25) 3000(25) 3033 (2d) 3054 (2d) 3141(2d) 3440(6d) 3487 (6d) C-H stretch 3008 C-H stretch 3145 N-H. "CI N-H-" "CI NHa + stretch O-H sym. stretch O-H antisym, stretch

4 Raman Spectrum of Diglycine Barium Chloride Monohydrate 17 The spectrum of diglycine barium chloride exkibits 43 Raman lines. Of these, 9 Raman lines in the region cm. -x come under the classifieation of external oscillations and the rest falling in the region cm. -a are due to the internal oscillations of the atoms constituting the structure. The Raman lines observed in the spectrum are very sharp excepting the lines in the region and cm. - There are two diffuse and broad Raman lines at 3440 and 3487 cm. -1 due to OH oscillations. In the region cm. -1 there are three bands which are of low intensity (3033, 3054 and 3141 cm.-). As in the case of glycine and other addition compounds of glycine, in this case also there are a series of Raman bands i, the region cm. - (2596, 2700, 2745, 2785, 2865, 2900 and 2920 cm.-x). Some of these bands are also observed in the Raman spectrum of diglycine hydrochloride (Balasubramanian, 1961) with almost the same frequency shifts. There are two very intense lines at 2970 and 3000 cm. -1 due to C-H oscillations. In the region cm. - there are 9 sharp Rarnan lines. The Raman line at 1429 cm. -1 is the most intense, and those at 1407, 1455 and 1480 cm. -1 occur with medium intensity, and others ate of very low intensity. There are 7 Raman lines in the region cm. - which are of medium intensity, excepting the line at 1333 cm. - which is very intense. There is a mercury line A 2625 "2 A in this region which is very weak in intensity compared to the other mereury lines. The observed high intensity in this very region is therefore due to the presence of the Raman line at 1333 cm. -1 The Raman lines at 304, 325, 509 and 580 cm. -I are of very low intensity. The lines at 304 and 325 cm. - are broad. Of the nine Raman lines in the region cm. -1, the line at 121 cm. -a is the most intense. The lines at 182 and 207 cm. -1 are broad and diffuse. The lines in the region cm. -1 consist' of two distinct groups. The iines at 46, 59 and 70 cm. - form one group and those at 90 and 100 cm. -1 forro another group. The lines in these groups are of medium intensity. 4. DIscussIoN The assignments for the various vibrational frequencies observed in diglycine barium chloride monohydrate have been made from a comparison of its spectrum with that of glycine artd some of its addiª compounds that have been investigated. lnternal Oscillations.PThe most striking feature in the spectrum of GBaC12.H20 is the absence of the Raman lines , 560 and 717 cm. -1 which ate characteristic of crystauine BaCI.2HO. The line at 560 era. -1

5 18 R.S. KRISHNAN AND K. BALASUBRAMANIAN is very intense in the spectrum of barium chloride (Galy, 1952). This might support the suggestion of Pfeiffer (1927) that in the crystalline state, the moleoular groups in each unit cell have the structural formula BakOOC--C/Ha--NH a... CI. llao. OOC--CHt--N Ha C1 In any case it is necessary to have the complete structure analysed with the help of X-rays or neutrons before one can establish this formula. The frequency shifts of the bending, stretching, twisting and wagging oscillations of the CH2 group in. the spectrum of glycine appear to be unaffected by the addition of HCI or BaCI2. The frequency shifts of the oscillations of the NH3 - groups and COO- groups are lower in the case of the barium chloride addition compound as compared with HC1 addition compound and pure glycine. The lowering effect of the frequency in the case of oscillations of the CO0- group is more pronounced than in lhe case of the oscillations of the NH3 + group. This follows from the fact that the Ba atom is attached to the earboxyl group as per the structural formula given above. Although the C-CN antisymmetric stretching oscillation has nearly the same frequency shift in the spectra of GzBaClz. HzO, G2HCI and glycine, the frequency shifts of the C-CN symmetric stretching oscillation and of the C-CN bending oscillation ate higher in the spectrum of G2BaCI2.H20 than the corresponding frequency shifts in the speetra of G2HCI and glycine. The presence of Raman lines (3141, 1564, 1498, and l124cm. -x) attributable to NHa + groups and the line at 1659 cm. - attributable to COOgroups indicated that the glycines exist in the zwitterion form inside the lattice. As in the case of glycine, the C-H stretching oscillations give rise to two Raman lines and C-CN symmetric stretching to one Raman line at 904 cm. -1 in the spectrum of G2BaCI 2.H20. In the case of all the other addition compounds of glycine such as G3HSO, GsHSeO4, etc., one observes three Raman lines due to C-H strctch oscillations and one Raman tine at 870 cm. -1 for C-CN stretching oscillation. It can therefore be eoncluded that the glycine units in diglycine barium chloride monohydrate are only in one forro, namely, zwitterion forro as in crystalline -glycine. In the other addition compoundsthere are two types of glycines present in the structure. The diffuse lines appearing in the region 2500 cm. - to 3100cm. - are due to hydrogen bonded oscillations of the type N-H...O, O-H.--C1 and

6 Raman Spectrum of Diglycine Barium Chloride Monohydrate 19 N-H- 9.CI. The lines arising from hydrogen bonded O-H- 9 9 C1 and N-H-.-C1 oscillations are present in the barium chloride and hydrogen chloride addition compounds and not in a-glycine (see columns 2, 3 and 4 in Table I), The presence of the water of crystallisation in diglycine barium crystal is confirmed by the appearance of two fairly sharp lines at 3440 and 3487 cm. -1 in its Raman spectrum. The X-ray studies (Vaidya and Balasubramanian, 1963) have clearly indicated the presence of only one molecule of water of crystallisation per molecule of G2BaCI. The splitting of the O-H mode in the Raman spectrum arises from the fact that each unit cell contains more than one molecule. The frequency shifts of these O-H vibrations are much lower than those for free O-H stretching vibrations indicating the influence of hydrogen bonding. It follows therefore that the hydroxyl group is bonded to the chlorine atom through hydrogen bond inside the crystal. In the case of barium chloride dihydrate one observes 4 Raman lines due to O-H vibrations. External Oscillations.--G2BaC12.HzO belongs to the space group Vh 1G and there ate four molecules in the unit cell. From group theoretical caiculations made taking Ba as one unit and (COO.CH2.NHaC1) as another unit, ir is found that the lattice Raman spectrum of diglycine barium chloride monohydrate should exhibit 24 lines (12 translatory and 12 rotatory). The recorded spectrum exhibits 9 Raman lines of which the two intense but diffuse lines at 182 and 205 cm. -a should be attributed to the low frequency vibrations of the hydrogen bond, i.e., NH.. "O or OH" "CI. These lines ate also found in the spectra of many hydrcgen bonded crystals. The lattice line at 121 cm. -1 stands out prominently because of its intensity and sharpness. 5. SUMMARY The Raman spectrum of diglycine barium chloride monohydrate in the single cryslz,l forro has been recorded using excitation. 43 Raman lines (9 lattice and 34 internal) have been recorded. Satisfactory assignments have been given for most of the observed Raman lines. It is concluded from a comparison of the Raman spectrum of this compound with those of glycine and of other addition compounds of glycine, that the glycine unit exists in the zwitterion form in the structure of diglycine barium chloride monohydrate.

7 2O R. S. KRISHNAN AND K. BALASUBRAMANIAN 6. REFERENCES 1. Balasubramanian, K. 2. Bernal, J. D. 3. Galy, A. 4. Krishnan, R. S. and Balasubramanian, K. 5. Pfeiffer, P. 6. Vaidya, S. N. and Balasubramanian, K... Pror Ind. Ac Sel., 19 DA, Z. Krist., 1931, 78, Comp. Rend., 1952, 235, Proc. Ind. Acad. $cl, 1958, 48 A, Organische Molekulver bindungen, F. Enke, Stuttgart, lnd. Jour. Pure and AppL Phys. 1963, 1, 433.

8 >- x.,. '),., III "" I I ll..,. I I ).)." I., I \I _,,..-. I I II ",.... 1"1 : : :i I 0., I' t11 VI ::. II (. I -I I 0 I..., 1 II 1I1 1 1 r g" 1!n, $ li : I :;, ; '! I ", I :; j, I I I I ' " - ']1 '"... N I I 0... b) ii I!I i.! '," \ I \, " II " c,lli,i I (a) FIG. I. (a) The Raman spectrum of diglycine barium chloride monohydrate taken with medium spectrograph and A excitation. (b) Its microphotometer record.?tl l:l ;:JC/). ;, l:l ;:J... l:l ;:J I:: ;:I l:l... ;:J j;) ;:s r>... j;) p.. :.?>- - l:"'<.?<...