Narayanaswamy, C K

Rao, K Mallikarjuna

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Infrared spectrum of anhydrous citric acid in the solid state—IB y  K . M a l l ik a b ju n a  B ao  a k d  C. K . N a b a y a n a s w a m yDepartme7it of Physics, Indian Institute of Technotoigy, Madras-^Q[Received 12 August revised 14 October and 12 December 1969)The infrared Bpoctra o f anhydrous citiic acid in mull and in X B r matrix have boon recorded in the range 700-4000c with a Perkiii-Elmer Infrared Speoirophotomeler, model 221 equipped with sodium chloride optics. The observed frequencies arc assigned tentatively to their respective characteristic groups. The frequencies duo to the hydrogen bonds in the unit oell, are assigned with reference to the data known from Hadzi (1966). The results are reported m this communication.I n tb o d u o tio nThere are only a few investigations that are to be found in the literature on tlui infrared and Raman spectra of citric acid either in solution or in the solid state. Passerini (1935) and Duval (1956) have recorded the infrared spectrum in the solid state without presenting any analysis. There is no investigation reported on the infrared spectrum of citric acid in solution. Edsall (1937) has investigated the Raman spectrum in solution and has reported the Raman frequencies without any analysis except indicating the C 0  and CHg frequencies. Nisi (1931), Thatte et al (1936) and Canal et al (1938) have investigated the Raman spectrum of citric acid monohydrate in the solid state. Nisi has confined himself to the region of water of crystallization only and reported the same. Thatte et al liave photographed the Raman spectrum in the molten state and discussed the 0  =  0— 0  group frequencies only. Canal et al have photographed the spectrum using powder technique and reported the data without any analysis. The authors have attempted to analyse the vibrational spectrum of this molecule in the solid state. Thus they have recorded the infrared spectrum of anhydrous citric acid in mull ai^d in KBr matrix and have assigned the frequencies to their respective characteristic groups.E x f e b im e n t a lCitric acid monohydrato of AR grade from S. Merck & Co. was procured and purified by repeated recrystallization. A hot saturated solution of the subs­tance in distilled water at about 70°C was cooled slowly to room temperature over several days. It is known that citric acid becomes anhydrous above the tempera­ture 36.7°0 and so wo have obtained the crystals of anhydrous citric acid by theIndian J. Phya. 44 , 34- 38 , (19 '?0 )34Infrared spectrum of anhydrous citric acid 35a b o v e  process of cooling. This anhydrous citric acid was used to record the spectra in mull as well as with a KBr pellet. The spectra are reproduced in figures 1 and 2,E esultbThe obseiwed frequencies with assignments are given in table 1. Also a pictorial diagram of anhydrous citric acid molecule is given in figure 3a. The Raman frequencies of citric acid in the polycrystalline state from Landolt and Bernstein tables (1951) are also given for comparison.D iscussionAnhydrous citric acid w’ith the chemical formula CgHgO, is a tribasic acid with an OH group attached to the middle carbon atom. There is no data avai­lable on the molecular symmetry of this molecule in the free state. The crystal structure has been fully worked out by Nordman et at (1960).To facilitate understanding of the assignment of observed frequencies to their functional groups, a few salient details on the structure of the anhydrous (!itnc! acid molecule are given here. Tlie aliphatic chain Cj—C5 is approxi­mately planar, but is slightly bent away from the carboxyl OsCflOg. All the carboxyls are coplanar with their a-carbons. The carboxyl group O1C1O2 IS at an angle of 3.3°, while the carboxyl O3C6O4 is approximately at right angles to that plane. There is approximate parallelism between the carboxyl OsCgOg to the plane CaCgO; containing the a-hydroxyl. In the unit cell the carboxyl group O1C1O2 is coimected by two equivalent hydrogen bonds to an equivalent group across a ceuitro of symmetry, i.o., the bonding is of the dimer typo.The free state symmetry of the molecule is only and the site symmetry is also G-^  although wo could consider C?® to bo the approximate site symmetry from the view point o f the structural details already given above. Since in the present woi'k we have aimed at only the characteristic group frequency assign­ments and not the assignment of symmetry species of the modes of vibration, the site symmetry has not been discussed further. The observed frequencies are dis­cussed below in terms of their functional groups.CHj frequenciesThe antisymmetric stretching frequencies of this group appear in the region 2920-2948 cm'^ as a broad region of absorption and the symmetric stretching frequencies appear in the region 2885-2895 cm~  ^ in the mull spectrum. These frequencies are absent in the KBr pellet spectrum and this may bo so because the pressure applied might be too high and some solid state reaction might take place between KBr and the substance. The region 1100-1400 cm~  ^ is a little bit complicated, in the case of carboxylic acids having a number of COOH groups and because of waggings of CHg group some o f the skeletal frequencies and carboxyl group frequencies appear in this region. So in this region assignmentsI^’ahle 1 Infrarod froquencios of anhydroup citric acid from mull and KBr pellet spectra, with tentative assignments, 700-4000 region.36 K . M allikarjm ia  R a o  and  C. K . N a ra y a n a sw a m yKaman frocjH. from Lantloll & BoniHtein (1051)Infrared freqs. present authorsFrtjq cm  ^^ Intensity Freq. cm“ ^H701(»71( 1)(I)( 1)1442 ( 1)1730 (0)2984 (b)0877750281 937 } 96010441 1073 1 1207 1280 1307 13281 1342 1 1407 1 1413 J1425 1430 1400 1089  ^1095 \ 1704 J 17161730 1 1741 V 1749 J 2325 1 2340 J 26301 2650 1 27341 2770 J 284512860 J2885292012948]32033277334333073486IntensityAssignment ^ Angle deformationO O CHg rookingC-C bending and OH out o f pla.no bonding.CHa rocking C-0 strntohes o f C-OHC-C stretching.C-0 stretching of carboxyl CHa wagging > OCO bonding of COOHm 1 .5r ]CHa scissoringm.m.br C-0 stretching m C-OH deformationni CHo scissoringwI C 0 stretchingOH stretching lowered dueto hydrogen bonding.Innkavvw 6r| -fc frequencies lowered duo vvw.br1.0 hydrogen bondings.brCHa stretching CHa stretching0 -H  stretchings (fundamentals and overtones and combinations)aro made in (lomimrison Avilh the aswgnmouts for adipio acid (hio to Anantanarayanan (1964). The other frequoncios of this group appear at 1400, 1413, 1407 cm-^ assigned to OHg scissoring inodes and at 1307 cm“ ’ a broad band assigned to OHg wagging modes. Tlie rocking vibrations of this group appear at 960 and 775 cm“ .^COOH frequenciesThe frequencies at 1689, 1694 and 1704 cm -’ are attributed to C 0  stretch­ings of this group These frequencies are in accordance with the fact that there is dimerization in the unit coll. From the literature it is knoAvn that the dimer type molecule will give C 0  stretching frequencies in the range 1690-1720 cm -’ The other froipioncies of this group, namely C—O stretchings appear at 1280, 1425 cm -’ , the COO angle bendings of COOH group appear at 1328, 1342 cm -i and the angle deformation frequencies appear at 687 cm-’ . The assignments arc given in table 1.C— OH frequencies—Q fitrotchings ol this group appear at 1073, 1044 cm"’  and the C OH in plan<‘ bending appears at 1430 cm-’ . Those arc given the assignments in com­parison with other compounds containing C—OH group.C— C frequenciesTt is known from the anhydrous citric acid molecular structure given in figiij'e 3a that the molecule consists of a zig-zag chain of carbon atoms and so one Slioulfl expect the GC group to give rise to stretchings and bondings. Tire fre- qnency at 1207 ent-> is attributed to the stretching vibration of this group and tlio hand at 928 eni-> is attributed to the bendings of tliis group.Hydrogen hand frequenciesThe range of those frequencies is identified using the details available from Hadzi (1965), According to Hadzi these bands form a characteristic trio near 2800, 2500 and 1900 cm -’ , if tho carboxyl groups act as proton donors but they are somewhal. lower (about 2700, 2200, 1600 cm -’ ) for hyebogen phosphate and similar groups. Thus the frequencies in the range 2776 to 2325 cm-’  and the frequencies at 1749 to 1730 cm -’ assigned to the OH stretches are lowered due to hydrogen bonding. Ex'cept for identifying the range of these frequencies, any other discussion is difficult in tho absence o f studios on single c-rystals.A ckno w led g em en tThe authors wish to thank the Director, Regional Research Laboratory, Hyderabad, for having permitted them to record the spectra reported in this paper. One of tho authors (K. Mallikarjuna Rao) wishes to express his gratitude to the Director, Indian Institute of Technology, Madras, for the grant of a Research Fellowship. .Infrared spectrum of anhydrous citric acid 37R e f e r e n c e sAnanianarayanaii V. lOO-l SpcctrocMm. Acta. 20, 197.(?anal S & Piorre Peyrot 1938 Compt, Rend 206, 1179.Duval a  1955 Anal. Chim Acta. 13, 427.Edaall J. T. 1937 J . Chem. Phys. 5, 508.ITadzi D ., 1965, Pure and Appl, Chem., 11, 435.Landolt. & BomsI.om 1951 Springer Verlag rf- Co. (Berlin), Baful 1 , Teil 2, 534.Nifu H . 1931 Jap. J . Phyx. 7, INordmaii E., W eldon A, S. & Patterson A . L , 1960 Acfa. Cn/sf. 18, 418.Passerini L. 1936 Oazz. Chim Jtal. 65, 502Thatte V. N & Aakhedl<ar W . 1930 Z  Phynil', 100, 450.38 K .  M a W ik a rjiin a  R a o  a n d  C . K .  N a r a y a n a s w a m yK . IVCaUikarjuna. R ao &  O. K . Narayanasw am y1eg.. I.-s ■§s &O S3I iS^  ES5| 51C. Mallikarjuna Rao & O. 1C. NarayaixaswamyK . Mallikarjuna R ao & C. K . K arayanaswam yFigure 3a, Anhydroua citric acid molecule w ith  H  atom s removed.F igure 3b. H ydrogen bondm g and m olecular paokUig in anhydrous citric acid  un it cell. U n it oelhdim ensions : a  =  12 .82A , & =  5 .62A ,c =  11 .64A , 111.2^* space group Z  4 m olecules.

Infrared Spectrum of Anhydrous Citric Acid in the Solid State-I

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Infrared Spectrum of Anhydrous Citric Acid in the Solid State-I

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