infrared and laser raman spectra of bis(dl-methioninium) sulfate

JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2005; 36: 840–847Published online 22 July 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1358

Infrared and laser Raman spectra ofbis(DL-methioninium) sulfate

S. Ramaswamy,1† R. K. Rajaram2 and V. Ramakrishnan1∗

1 Laser Laboratory, Department of Microprocessor and Computer, School of Physics, Madurai Kamaraj University, Madurai 625 021, India2 Department of Physics, School of Physics, Madurai Kamaraj University, Madurai 625 021, India

Received 29 September 2004; Accepted 25 February 2005

The infrared and laser-excited Raman spectra of bis(DL-methioninium) sulfate crystal were recorded andanalyzed at room temperature. The vibrational assignments of the observed wavenumbers were madebased on group theoretical analysis. The presence of C O group is confirmed. Factor group analysis wascarried out and the number of normal vibrational modes was calculated. The shifting of several stretchingand bending wavenumbers suggests that the extensive intermolecular hydrogen bonding causes bothlinear and angular distortions of several groups. The splitting of the threefold degenerate stretching andbending modes of the sulfate group confirms the distortion of its Td symmetry due to hydrogen bonding.Copyright 2005 John Wiley & Sons, Ltd.

KEYWORDS: bis(DL-methioninium) sulfate; infrared spectrum; hydrogen bonding; factor group analysis

INTRODUCTION

DL-Methionine [DL-2-amino-4-(methylthio)butyric acid orDL-˛-amino-ˇ-methylmercaptobutyrate] is one of the nineessential amino acids needed by humans and is the onlyessential amino acid containing sulfur in its structure.1 Thesulfur-containing amino acids cysteine and methionine aregenerally considered to be non-polar and hydrophobic. Infact, methionine is one of the most hydrophobic amino acidsand is almost always found in the interior of proteins. Apartfrom its role as a protein constituent and as an essentialamino acid, methionine is also important as a donor of activemethyl groups.2 Methionine is a principal supplier of sulfur,which prevents disorders of the hair, skin and nails, helpslower cholesterol levels, reduces liver fat and protects thekidneys. It is found rich in meat, fish, beans, eggs, garlic,lentils, onions, yogurt and seeds.

The findings of Joyner et al.3 suggested that the sulfur-containing amino acids function in sulfide detoxification insymbiotic invertebrates, and that this process depends onammonia assimilation and symbiont metabolic capabilities.Parcell4 studied sulfur-containing amino acids (methionine,

ŁCorrespondence to: V. Ramakrishnan, Laser Laboratory,Department of Microprocessor and Computer, School of Physics,Madurai Kamaraj University, Madurai 625 021, India.E-mail: ramaswamy s [email protected]†Permanent address: Department of Physics, NMSSVN College,Madurai 625 019, India.Contract/grant sponsor: DST, Government of India.Contract/grant sponsor: UGC, Government of India.

cysteine, cystine, homocysteine, homocystine and taurine) inhuman nutrition and their applications in medicine. Organicsulfur complexes, notably the amino acids methionine andcysteine, largely meet the sulfur needs of the body. Zemelet al.5 investigated the role of the sulfur-containing aminoacids in protein-induced hypercalciuria in men. Methioninecontrols the level of beneficial sulfur-containing compoundsin the human body. These sulfur-containing compounds arein turn vital for defending against toxic compounds suchas heavy metals in the liver.6 Methionine helps to reducelevels of histamine, which is an amino acid that controlsdilation of blood vessels and influences brain function.Schizophrenia has also been linked to elevated histaminelevels and methionine has shown promise in the treatmentof the disorder.7

Infrared and Raman spectroscopic studies help in elu-cidating certain structural and bonding features and hencesupplement information obtained from x-ray investigations.There have been several recent vibrational spectroscopicstudies of the methionine complexes of various inor-ganic acids, viz. nitric,8 perchloric9 and orthophosphoricacids10 and the complexes of various amino acids reactedwith sulfuric acid.11 – 13 Many studies have been made ofthe vibrational spectra of methionine, such as those ofbis(L-methioninato)copper(II),1 conformational and infraredspectral studies of L-methionine and its N-deuteratedisotopomer,14 vibrational spectroscopic studies of methio-nine adsorbed on gold and silver surfaces,15,16 normal-modeanalysis of the peptide poly(L-methionine)17 and vibrational

Copyright 2005 John Wiley & Sons, Ltd.

IR and laser Raman spectra of bis(DL-methioninium) sulfate 841

spectra and conformational phase transition of crystallineL-methionine.18 In the present investigation, infrared andRaman spectral studies of bis(DL-methioninium) sulfate wereundertaken to elucidate the dynamics of various functionalgroups and the influence of hydrogen bonding on molecularvibrations. Factor group analysis of the compound was alsocarried out.

EXPERIMENTAL

Bis(DL-methioninium) sulfate (2C5H12NO2SCÐSO42�) was

crystallized from an aqueous solution of DL-methionine andsulfuric acid in the stoichiometric ratio 2 : 1 by slow evapora-tion. Colorless, thin, transparent, block-shaped crystals wereobtained after about 2 weeks.

A Bruker IFS 66V Fourier transform (FT) IR spectrometerwas used. An FRA 106 Raman module was used as anaccessory for FT-Raman measurements. The instrument hasa resolution of ¾2–3 cm�1. Multi-tasking OPUS software ona PC/AT 486 computer was used for processes such as signalaveraging, signal enhancement and baseline corrections.

An air-cooled diode-pumped Nd:YAG laser operated at1064 nm and with the laser power output maintained at200 mW was used for Raman spectral measurements. Thespectra were recorded over the range 50–3500 cm�1. Since thegrown crystals were small, the sample was finely powderedand pressed into a small depression on a metal disc andmounted on the sample stage of the sample compartment.For infrared measurements, a globar source was used and thespectra were recorded over the range 400–4000 cm�1. Thesample for this measurement was finely ground and mixedwith KBr. This mixture was then pressed under vacuumat very high pressure to obtain a transparent disc, whichwas then placed in the sample compartment. In the FT-IRmode, the detector was a pyroelectric device incorporatingdeuterium triglycine sulfate (DTGS) in a temperature-resistant alkali metal halide window. For reliability of thedata obtained, Raman spectral measurements were alsomade with the facilities developed in our laboratory.19

RESULTS AND DISCUSSION

Crystal structure analysisThe crystal and molecular structure of bis(DL-methioninium)sulfate was solved by single-crystal x-ray diffractometry.20

The compound crystallizes in the trigonal system withthe space group R3 (C2

3i, No.148), having nine formulaunits per unit cell (Z D 9). The asymmetric part ofthe unit cell contains one methioninium cation and twocrystallographically independent sulfate anions (S2 and S3)sitting on sites of symmetry 3 and 3, viz. (2/3,1/3,1/3) and(0,0,z), respectively. Since the sulfate group cannot haveinversion symmetry, the oxygen atoms of the S2 anion aredisordered over two sites connected by an inversion center.Further, the cation and anions are connected by a system

of hydrogen bonds. In the title crystal, one of the sulfateanions forms a strong O—HÐ Ð ÐO hydrogen bond with themethioninium cation and the structure is further stabilizedby N—HÐ Ð ÐO-type hydrogen bonds. Each sulfate anion issurrounded by six methioninium cations as a cylindrical cageaggregated in the ab plane. The hydrophilic layers at z D 1/3are stacked between hydrophobic layers at z D 1/6.

Vibrational analysisFactor group analysis21 of the title compound gives 420modes of normal vibrations distributed as D 63Ag C63Eg C 77Au C 77Eu, where Ag and Eg species are Ramanactive whereas the Au and Eu species are infrared active.There are no common modes.

The molecular structural formula of bis(DL-methioni-nium) sulfate is given in Fig. 1. The observed infraredand Raman spectra are shown in Figs 2 and 3 and theobserved wavenumbers together with the band assignmentsare presented in Table 1.

Vibrations of the methioninium cationThe methioninium cation consists of a number of func-tional groups such as C O, OH, C—OH, NH3

C, CH3,CH2, O—C O, C—C O, C—N, C—C—N, C—H andC—S—C. The environment of the molecule and extensiveintermolecular hydrogen bonding among the different partsof the molecule may cause several wavenumbers to undergochanges in their position, intensity or degeneracy as com-pared with those of the pure amino acids.

Carbonyl group vibrationsIn a non-ionized carboxylic (COOH) group, the car-bonyl (C O) stretching vibration22 usually appears inthe wavenumber region 1755–1700 cm�1. In the bis(DL-methioninium) sulfate crystal, a band observed at 1732 cm�1

in both IR and Raman spectra corresponds to the carbonyl

Figure 1. Structural formula of bis(DL-methioninium) sulfate(sulfate anions S2 and S3 are sitting on sites of symmetry 3and 3).

Copyright 2005 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2005; 36: 840–847

842 S. Ramaswamy, R. K. Rajaram and V. Ramakrishnan

Figure 2. Infrared spectrum of bis(DL-methioninium) sulfate.

Figure 3. Raman spectrum of bis(DL-methioninium) sulfate.

stretching mode. This band occurs as expected in bothposition and shape, which leads to a conclusion that ifthe carbonyl group is involved in hydrogen bonding, thebonding must be weak. X-ray studies20 also reveal that thecarbonyl group participated in weak intermolecular hydro-gen bonding (N1—H1CÐ Ð ÐO1 D 2.924 A) with the nitrogenatoms of the amino group.

Hydroxyl group vibrationsThe high-intensity bands at 2977 cm�1 in both the infraredand Raman spectra are attributed to the O—H stretchingmode of the carboxylic group. This mode is shifted tolower wavenumbers by ¾500 cm�1 owing to the presenceof a strong O—HÐ Ð ÐO hydrogen bond (2.57 A) between the

carboxylic group of the methioninium cation and one of thesulfate anions (S3). The very strong Raman peak at 1292 cm�1

is due to the O—H in-plane deformation mode as expected.The out-of-plane deformation mode of the O—H groupis also observed as a very strong peak at 986 cm�1 in theRaman and as a weak peak at 978 cm�1 in IR spectrum. Fromthe x-ray structural data,20 it is evident that the O—HÐ Ð ÐObond is almost linear (bond angle 162°) whereas its bondstrength is greater. Hence the vibrational spectra reflect theresults of the x-ray data so that the O–H stretching mode islowered substantially compared with that of the bendingmode. The band due to C–O(H) stretching vibration isgenerally strong and occurs in the region 1190–1075 cm�1.



Table 1. Experimental wavenumbers (Q�/cm�1) and relativeintensitiesa in the vibrational spectra of bis(DL-methioninium)sulfate

Infrared (Q�/cm�1) Raman (Q�/cm�1) Assignment

2977(s,br) 2977(s) (C) O–H str.; NH3C str.

2957(s) CH3 asym. str.2919(vs) 2919(vvs) CH2 asym. str.

2875(sh) CH3 sym. str.2836(w) CH2 sym. str.

1750–2800 Overtones andcombination bands

1732(vvs) 1732(m,br) C O str.1620(s) NH3

C asym. def.1583(m) 1587(sh)

}1566(sh) 1563(m) NH3

C sym. def.1508(vs,br) C–N asym. str.1464(m) 1470(m)1439(m)

1440(s)

CH3 asym. def.; CH2

sym. def.1426(sh) 1432(vs)

1380(m) CH3 sym. def.1334(sh) 1338(vs) CH2 wag1316(vw) CH2 wag1289(s) 1292(vs) OH in-plane def.1257(s) 1267(w) CH2 twist1237(s) 1243(w) CH2 twist1176(s) 1180(m) C–O (H) str.1159(vw) 1153(m)

}NH3

C rock1141(vw)1107(vvs) SO4

2� asym str.; C–Nsym. str.

1072(sh,br) 1083(m) SO42� asym str.;

C–C–N asym.str.1006(vw) 1017(w) C–N sym. str.978(vw) 986(vvs) SO4

2� sym str.; OHout-of-plane def.

959(w) 950(m)}

CH3 rock934(w) CH3 rock; C–C str.906(sh) C–C str.857(m) 865(w) C–C–N sym.str.837(m) 848(s) C–C str.776(s) 787(s)

}CH2 rock

742(m,br) 752(s)725(m) 727(s) C–S–C asym. str.694(m) 703(s) C–S asym. str.656(vw) 666(vs) SO4

2� asym. def.;O–C O in-plane def.;C–S–C sym. str.

617(vvs) 617(sh) SO42� asym. def.;

O–C O in-plane def.597(sh) SO4

2� asym. def.;O–C O in-plane def.

Table 1. (Continued)

Infrared (Q�/cm�1) Raman (Q�/cm�1) Assignment

540(s,br) 548(s) C–C O in-planedef.; NH3

C torsion458(m) 463(s) SO4

2� sym def.422(vw) C–C–C–C in-plane

def.362(m) C–C–S in-plane def.;

C–C–N def.311(sh) C–C–C–C

out-of-plane def.297(m) C–S–C in-plane def.127(s,br) Lattice modes96(sh) Lattice modes

a Asym, asymmetric; br, broad; def, deformation; m, medium;s, strong; sh, shoulder; str, stretch; sym, symmetric; v, very;w, weak.

The C–O(H) stretching mode is observed as a strong band at1176 cm�1 in the IR spectrum and the corresponding Ramanband is observed as a medium-intensity band at 1180 cm�1.

NH3C group vibrations

The NH3C group, which forms a part of the crystal, has

C3v symmetry in the free state with a pyramidal structure.Its normal modes of vibrations are �1�A1�, �2�A1�, �3�E� and�4�E�. Among these vibrations, the symmetric stretching andbending modes (�1 and �2) are non-degenerate whereas theasymmetric stretching and bending modes (�3 and �4) aredoubly degenerate.23 All these modes are both IR and Ramanactive. The symmetry of the amino group may be lowered,thereby causing the shift in the vibrational wavenumbersas the NH3

C group is attached to the rest of the moleculethrough hydrogen bonding.

For the NH3C group, the asymmetric and symmetric

stretching modes are expected in the region 3150–3000 cm�1,whereas the asymmetric and symmetric deformation modesof this group appear in the regions 1660–1610 and1550–1480 cm�1, respectively.24,25 The strong and broad bandat 2977 cm�1 in the IR spectrum is assigned to the stretchingvibrations of the NH3

C group. As all the three N—H bondsof the amino group take part in intermolecular hydrogenbonding, the wavenumber of the stretching vibration of theNH3

C group is lowered.The doubly degenerate asymmetric deformation vibra-

tion of this group is observed as a doublet, a strong peak at1620 cm�1 in the IR spectrum along with a medium-intensitypeak at 1583 cm�1. The appearance of the strong and broadband at 2977 cm�1 in the IR spectrum and the splitting ofthe degenerate mode (�4) suggest that the symmetry of theNH3

C group is lowered from C3v to C1 in the crystal. Howeverthe symmetric deformation mode is observed at 1563 cm�1

(medium) in Raman. The bands around 1155 cm�1 in both



spectra are tentatively assigned to the NH3C rocking mode as

expected.26 The NH3C torsion mode is observed at ¾540 cm�1

in the IR spectrum and at 548 cm�1 in the Raman spectrum.As this band is overlapped with the in-plane deformation ofC—C O group, it appears to be highly intense and broadin the IR spectrum.

O—C O and C—C O group vibrationsThe ˛-branched aliphatic monocarboxylic acids generallyexhibit three strong bands due to the in-plane vibrationof the O—C O group, at ¾655, 635 and 620 cm�1, whichare not usually well resolved in the region 665–610 cm�1.27

In the title crystal, the in-plane deformation bands of theO—C O group are observed at 656, 617 and 597 cm�1 inthe IR spectrum. The band at 617 cm�1 appears to be strongowing to the overlapping of asymmetric deformation modeof sulfate anions present in the crystal.

In addition, a strong band is found at 555–520 cm�1,which is attributed to the in-plane vibration of the C—C Ogroup.27 The strong and broad band at 540 cm�1 in theIR spectrum and the corresponding strong Raman peak at548 cm�1 are due to the in-plane deformation mode of theC—C O group. The results of these modes of vibrationscorrelate well with those observed for other crystallinecomplexes, viz. L-methionine L-methioninium perchloratemonohydrate,9 bis(L-ornithinium) chloride nitrate sulfate11

and L-argininium dinitrate.28

Methyl and methylene group vibrationsThe characteristic wavenumbers of CH3 and CH2 groups mayshift in their position depending on the electronegativity ofthe group attached to them.23 In the methioninium cation,both CH3 and CH2 groups have sulfur, as the adjacentatom which has a higher electronegativity than carbon.This makes the stretching and bending modes of vibrationsmuch stronger and shifts them to slightly lower and higherwavenumbers, respectively, compared with those of thecorresponding straight-chain alkane.29,30

The asymmetric and symmetric stretching modes of themethyl group are expected to occur at 2970 and 2880 cm�1,respectively. In the title crystal, the high-intensity Ramanband at 2957 cm�1 corresponds to the CH3 asymmetricstretching mode. The Raman line at 2875 cm�1 (shoulder)confirms the symmetric stretching mode of CH3. The CH2

asymmetric and symmetric stretching vibrational modes areexpected to occur at 2935 (strong) and 2865 cm�1 (weak),respectively.29 The very strong band at 2919 cm�1 in both theIR and Raman spectra is easily assigned to the asymmetricstretching vibration of the CH2 group. The weak band at2836 cm�1 in the Raman spectrum is due to the symmetricstretching vibration of the CH2 group.

The CH3 asymmetric deformation mode is observed as atriplet around 1440 cm�1 in both the IR and Raman spectra.As this mode is overlapped with the symmetric deformationmode of the methylene group, the intensity of the methyl

group vibrations appears to be strong. The medium-intensityband at 1380 cm�1 in the IR spectrum reveals the presence ofthe CH3 symmetric deformation mode. The rocking modes ofthe methyl group are observed weakly at 959 and 934 cm�1 inthe IR spectrum. The corresponding Raman peak at 950 cm�1

(medium) is attributed to the same mode of vibration.In the —CH2 —CH2 — group, the deformation of one

CH2 group hardly exerts any force on the carbon of the nextCH2 group because of the C—C bond. Hence the couplingis very weak between adjacent methylene groups. Sincethe CH2 deformation oscillators are weakly coupled, thein-phase and out-of-phase vibrations have nearly the samewavenumbers.23 The wagging, twisting and rocking modesof the CH2 group have also been identified and assigned.

Side-chain C—S—C group vibrationsThe x-ray structural data for the title compound20 revealthat the sulfur atom (Sυ) of the methioninium cation isdisordered over two positions. Thus the methioniniumcation in the asymmetric part of the unit cell has differentconformations about C˛ —Cˇ —C� —Sυ skeleton. The side-chain conformation angle for the major disorder componentof the methioninium cation is trans–trans–gauche II, whereasit is trans–gauche I–gauche I for the minor component.

The methionine side group, —CH2 —CH2 —S—CH3,which is similar in structure to 2-thiabutane, can assumedifferent molecular forms in solution. In the Raman spectrumof 2-thiabutane in the liquid state, Hayashi et al.31 observedand reported three intense bands at 654, 680 and 727 cm�1.They assigned these three bands to the C–S stretchingwavenumbers, of which the 727 cm�1 bands were attributedto the trans form, the 680 cm�1 bands to the gauche form, andthe 654 cm�1 bands to both forms.

Lord and Yu32 predicted the contribution of the methio-nine to the spectrum of native lysozyme. From inspectionof the spectra of 2-thiabutane and aqueous methionine, theyconcluded that the strong vibrational modes at 724, 700 and655 cm�1 are due to the C—S stretching modes of the trans,gauche and both forms, respectively. They also concludedthat the spectral lines undergo a change in intensity undercertain conditions. For the crystal under investigation also,the three strong bands are observed in the Raman spectrumat 727, 703 and 666 cm�1. These results are in good agree-ment with the results obtained for 2-thiabutane for C—Sstretching vibrations and also reflect the x-ray structuralinvestigations.20 The equal intensities of these lines indicateequal population ratio of the molecular forms.

CH (chain) group vibrationsThe hydrocarbon CH stretch occurs near 2900 cm�1 andis usually lost among other aliphatic absorptions. TheCH deformation mode absorbs weakly in hydrocarbonsat 1350–1315 cm�1 in the IR spectrum.29 In the presentinvestigation, the CH stretching mode observed around2920 cm�1 is superimposed by the very strong asymmetric



stretching vibrations of methylene group. However, theCH deformation vibration mode has not been identifieddistinctly as this mode lies in the region of CH2 deformationvibrations.

C—N and C—C—N group vibrationsThe absorption bands arising from C—N and C—Cstretching vibrations are observed in the wavenumberregion 850–1150 cm�1.29 The medium-intensity IR bandat 857 cm�1 and the corresponding weak Raman band at865 cm�1 are readily assigned to the C—C—N symmetricstretching vibration. The broad shoulder band at 1072 cm�1

in the IR spectrum is assigned to the C—C—N asymmetricstretching vibrations. As this vibration is overlapped withthe asymmetric stretching vibrations of the sulfate anion, itappears as a broad band in the IR spectrum.

Primary amines with secondary ˛-carbon atoms (CH—NH2) absorb weakly at 1043–1037 cm�1 and even morestrongly at 1140–1080 cm�1.29 The very strong band at1107 cm�1 and a very weak band at 1006 cm�1 in the IRspectrum are assigned to the C—N symmetric stretchingvibrations. The high-intensity broad band at 1508 cm�1 inthe IR spectrum is attributed to the asymmetric stretchingmode of the C—N bond12. This band appears broad owingto the overlapping of overtone of the CH2 rocking modeof vibration, the fundamental of which is a strong bandat 752 cm�1. However, in some crystalline complexes, thismode is observed as a split band as it is in Fermi resonancewith the CH2 rocking mode.8 – 10 The C—C—N deformationmode is also observed in Raman spectrum, which is assignedat 362 cm�1 (medium).

Skeletal vibrationsThe skeletal stretching vibrations of the amino acids areall coupled together. Of the skeletal vibrations, the C—Cstretching absorptions lie in the region 1260–700 cm�1 andthe deformation bands occur below 600 cm�1. The bands at934 cm�1 (weak), 906 cm�1 (weak shoulder) and 837 cm�1

(medium) in the IR spectrum are assigned to the C—Csymmetric stretching vibrations. The presence of C—S—Cgroup is confirmed by assigning the strong band at 727 cm�1

and a medium-intensity band at 297 cm�1 in the Ramanspectrum to C—S—C asymmetric stretching and in-planedeformation vibrations, respectively. The medium-intensityband at 362 cm�1 in the Raman spectrum is due to theC—C—S in-plane deformation mode.30

The methioninium cation has a C—C—C—C skeleton,which is considered to be made up of two coupledoscillators.29 The in-phase and out-of-phase vibrations ofthe C—C—C—C skeleton are observed at 422 and 311 cm�1,respectively, in the Raman spectrum. The lower wavenumbervibrations (below 250 cm�1) are assigned to the lattice modes.

Other vibrationsThe IR spectra of the compound obtained in KBr, Nujol andFluorolube generally show a strong broad absorption band

around 3440 cm�1.33 For the bis(L-methioninium) sulfatecrystal also, a strong and broad band at 3427 cm�1 isobserved in the IR spectrum which correlates well withbis(L-ornithinium) chloride nitrate sulfate.11

Hydrogen bonding: spectroscopic featuresThe stability of the structure of amino acid crystals is usuallydetermined by the set of hydrogen bonds present in the unitcell. The hydrogen-bonding network in solid-state aminoacid crystals makes the amino acids particularly interestingfor spectroscopic investigations. Vibrational spectroscopicmethods of detecting and studying hydrogen bonds are wellknown.34

Although both IR and Raman spectroscopic methods giveinformation regarding hydrogen bonds, the IR spectra in thehydrogen bond stretching region will be highly complicatedowing to the presence of overtone and combination bands.However, in the Raman spectra, the bands due to hydrogenbonding vibrations can be easily identified as they appearwith poor intensity.35

According to structural data for the bis(DL-methioninium)sulfate complex,20 a very strong hydrogen bond existsbetween the oxygen atoms of the carboxylic group andsulfate group with the O—O bond distance being 2.570 A(the O—O bond distance varies from 2.7 to 2.9 A for normalhydrogen bonds). Owing to the increase in the strength of theO—HÐ Ð ÐO hydrogen bond, the force constants of donor andacceptor groups are very much influenced and the O—Hstretching wavenumber is lowered by about 500 cm�1 andalso becomes very strong and broad in both the IR and Ramanspectra. The highly intense bands at 2977 cm�1 in both spectracorrespond to the stretching mode of the hydroxyl group.Further, the x-ray data20 reveal that the O—HÐ Ð ÐO bond(bond angle 162°) is almost linear and hence the bendingwavenumbers are not much different from the expectedrange, indicating that the linear distortion in this system isgreater than the angular distortion. A strong peak observedat 1732 cm�1 in both spectra confirms the presence of ahydrogen-bonded carbonyl group in the monomeric form.

The amino N atom of the methioninium cation formsN—HÐ Ð ÐO hydrogen bonds with the O atoms of the sulfateanion. In addition, this N atom also forms an intermolecu-lar hydrogen bond with a symmetry related O atom of thecarboxylic group.20 In the compound under study, a classIII hydrogen-bonding pattern36 is observed, involving onetwo-centered and two three-centered hydrogen bonds.

The oxygen atoms of the sulfate and carboxylic groupsare hydrogen bonded to the amino nitrogen atom with nearlythe same distances (average distance 2.926 A) from them andhence in similar environments. Therefore, the wavenumbersare not expected to differ drastically from the free ion sym-metry. The sulfate anions adopt a tetrahedral configurationwith their average O—S—O angle being 109.46° and theaverage O—S bond distance 1.469 A. These values for sul-fate anion are not much different from those for the free



ion. This is reflected in the IR and Raman spectra wherethere is not much deviation in the stretching and bendingvibrational wavenumbers of both NH3

C and SO42� groups,

which indicates that the effect of hydrogen bonds is normal.In the present investigation, the tetrahedral (Td) symmetry ofthe sulfate group is distorted slightly and not affected muchby hydrogen bonding. All the expected wavenumbers of thesulfate group were also observed and assigned.

Vibrations of the sulfate ionIn the free state, the sulfate ion has tetrahedral (Td) symmetrywith its vibrational modes distributed as D A1 C E C 2F2.The A1 and E species are Raman active only whereas the F2

species are both IR and Raman active. A1 is a one-dimensionalspecies, E is doubly degenerate and the F2 species havethreefold degeneracy. These modes are expected to occur at981, 451, 1104 and 613 cm�1, respectively.37,38

As the symmetric stretching mode �1 of the sulfate groupis expected to be very strong in the Raman spectrum, the verystrong band at 986 cm�1 is easily assigned to this mode. Inthe IR spectrum, this mode is observed as a very weak bandat 978 cm�1, which is forbidden under free ion symmetry.The intensity of the breathing mode �1 in the IR spectra ofsuch compounds is a measure of the extent of distortion ofthe ions at the lower symmetric lattice site compared withthat of the free ion.39 The activation of this inactive IR modeis due to the site symmetry effect (from Td to C3/C3i). Themedium-intensity band at 458 cm�1 in the IR spectrum andthe corresponding strong band at 463 cm�1 in the Ramanspectrum are assigned to �2 modes.

The triply degenerate asymmetric stretching vibration(�3) is expected in the region around 1050–1100 cm�1 forthe sulfate group.24 Lower site symmetry of molecularunits in the lattice may lift the degeneracy of degeneratenormal modes and also make allowance for (in the case offree molecules) silent modes in the IR or Raman spectra.Thus, in the case of tetrahedral SO4

2� ions, loweringthe Td symmetry results in splitting of the threefolddegenerate S—O stretching and SO4 bending modes �3

and �4 into three components.39 The broader shoulderedpeak at 1072 cm�1 and the highly intense band at 1107 cm�1

in the IR spectrum and a medium-intensity peak at1083 cm�1 in the Raman spectrum are assigned to thismode of vibration. The components of the triply degenerateasymmetric deformation vibrations (�4) are observed at617 cm�1 with two peaks on either side (656 and 597 cm�1) inthe IR spectrum and at 666 and 617 cm�1 in the Ramanspectrum, which correlate well with bis(L-ornithinium)chloride nitrate sulfate.11

CONCLUSION

The IR and laser Raman spectral data have been assignedand interpreted for various functional groups of the bis(DL-methioninum) sulfate crystal. The methioninium cation is

formed by protonation and the sulfate group forms the anion.The shifting of several stretching and bending wavenumberssuggests that the hydrogen bonding present in the crystalcauses both linear and angular distortions of severalfunctional groups. The presence of doublets and broadbands in the spectra indicate the different conformationsof the methioninium cation. The splitting of the threefoldstretching and bending modes of the SO4

2� group confirmsthe distortion in its Td symmetry due to hydrogen bonding.

AcknowledgmentsThe authors are grateful to DST, Government of India, New Delhi, forestablishing the laser laboratory and also to the UGC, Government ofIndia, New Delhi, for having recognized our group’s (V.R.) activitiesas a thrust area of research in DRS-Phase II and COSIST programmesin the School of Physics and also for having provided assistance toour laboratory. One of the authors (S.R.) thanks the managementof NMSSVN College, Madurai, India, for their encouragement. Oneof the authors (V.R.) is grateful to DST, Government of India, NewDelhi, for financial assistance in the form of a research project tosupport this work.

REFERENCES

1. Wagner CC, Baran EJ. Acta Farm. Bonaerense 2002; 21: 287.2. Jain JL. Fundamentals of Biochemistry. S. Chand: New Delhi, 1990.3. Joyner JL, Peyer SM, Lee RW. Biol. Bull. 2003; 205: 331.4. Parcell SW. Altern. Med. Rev. 2002; 7: 22.5. Zemel MB, Schuette SA, Hegsted M, Linkswiler HM. J. Nutr.

1981; 111: 545.6. Donaldson WE, Leming TK. J. Nutr. 1984; 114: 2155.7. Braverman ER. The Healing Nutrients Within. Keats Publishing:

New Canaan, CT, 1997.8. Pandiarajan S, Umadevi M, Briget Mary M, Rajaram RK,

Ramakrishnan V. J. Raman Spectrosc. 2004; 35: 907.9. Briget Mary M, Umadevi M, Pandiarajan S, Ramakrishnan V.

Spectrochim. Acta, Part A 2004; 60: 2643.10. Rajkumar BJM, Ramakrishnan V. Spectrochim. Acta, Part A 2001;

57: 247.11. Ramaswamy S, Rajaram RK, Ramakrishnan V. J. Raman

Spectrosc. 2005; 36: 12.12. Jinnah MMA, Umadevi M, Ravikumar B, Ramakrishnan V.

Spectrochim. Acta, Part A 2004; 60: 2977.13. Rajkumar BJM, Ramakrishnan V. Spectrochim. Acta, Part A 2002;

58: 1923.14. Cao X, Fischer G. J. Phys. Chem. A 2002; 106: 41.15. Cooper E, Krebs F, Smith McD, Raval R. J. Electron Spectrosc.

Relat. Phenom. 1993; 64/65: 469.16. Lee H, Kim MS, Suh SW. J. Raman Spectrosc. 1991; 22: 91.17. Tandon P, Gupta VD, Prasad O, Rastogi S, Gupta VP. J. Polym.

Sci. 1999; 35: 2281.18. Grunenberg A, Bougeard D. J. Mol. Struct. 1987; 160: 27.19. Jayaraj SE, Ramakrishnan V. Spectrochim. Acta, Part A 1995; 51:

979.20. Ramaswamy S, Sridhar B, Ramakrishnan V, Rajaram RK. Acta

Crystallogr., Sect. E 2004; 60: 01 691.21. Fateley WG, Dollish FR, McDevitt NT, Bentley FF. Infrared and

Raman Selection Rules for Molecular and Lattice Vibrations. Wiley:New York, 1972.

22. Liu J, Wu Y, Hu S, Lan G, Zheng J. J. Raman Spectrosc. 1998; 29:185.

23. Nakamoto K. Infrared and Raman Spectra of Inorganic andCoordination Compounds. Wiley: New York, 1986.



24. Bellamy LJ. The Infra-red Spectra of Complex Molecules. Wiley:New York, 1975.

25. Hubert Joe I, Philip D, Aruldhas G, Botto IL. J. Raman Spectrosc.1991; 22: 423.

26. Tsuboi M, Takenishi T, Nakamura A. Spectrochim. Acta 1962; 10:271.

27. Socrates G. Infrared Characteristic Group Frequencies. Wiley: NewYork, 1980.

28. Ramaswamy S, Rajaram RK, Ramakrishnan V. J. RamanSpectrosc. 2003; 34: 50.

29. Colthup NB, Daly LH, Wiberley SE. Introduction to Infrared andRaman Spectroscopy. Academic Press: New York, 1990.

30. Dollish FR, Fateley WG, Bentley FF. Characteristic RamanFrequencies of Organic Compounds. Wiley: New York,1973.

31. Hayashi M, Shimanouchi T, Mizushima S. J. Chem. Phys. 1957;26: 608.

32. Lord RC, Yu N. J. Mol. Biol. 1970; 50: 509.33. Philip D, Aruldhas G. Acta Chim. Hung. 1990; 127: 717.34. Hamilton WC, Ibers JA. Hydrogen Bonding in Solids, WA

Benjamin Inc.: New York, 1968.35. Krishnan RS, Sankaranarayanan VN, Krishnan K. J. Indian Inst.

Sci. 1973; 55: 66.36. Jeffrey GA, Saenger W. Hydrogen Bonding in Biological Structures.

Springer: Berlin, 1991.37. Ross SD. Inorganic Infrared and Raman Spectra. McGraw-Hill: New

York, 1972.38. Massaferro A, Kremer E, Wagner CC, Baran EJ. J. Raman

Spectrosc. 1999; 30: 225.39. Lutz HD, Haeuseler H. J. Mol. Struct. 1999; 511: 69.


infrared and laser raman spectra of bis(dl-methioninium) sulfate

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