vibrational spectroscopic studies of l-argininium dinitrate

JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2003; 34: 50–56Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.938

Vibrational spectroscopic studies of L-argininiumdinitrate

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

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

Received 12 July 2002; Accepted 22 August 2002

The Fourier transform IR and Raman spectra of (C6H16N4O22+·2NO3

−) crystal were studied and theobserved IR and Raman lines were assigned. There is extensive intermolecular hydrogen bonding inthe crystal and this is responsible for the changes in the position and intensity of several bands. Thevibrational spectra show that the carboxyl group forms a strong hydrogen bond whereas the nitrogenatoms of the amino and guanidyl groups of the cation form normal hydrogen bonds with the oxygen atomsof the anions. Copyright 2002 John Wiley & Sons, Ltd.

KEYWORDS: L-argininium dinitrate; Guanidyl group; intermolecular hydrogen bonding; infrared and laser Raman spectra

INTRODUCTION

All proteins are chain-like macromolecules produced by anumber of small units named amino acids. Amino acidsform a very important class of organic compounds thatare of great biological significance. Arginine (˛-amino-υ-guanidinovalerate), a semi-essential amino acid, is one ofthe three diaminomonocarboxylic amino acids, the othertwo being lysine and histidine. It is found in abundancein highly basic proteins of the cell nucleus (histones) andin sperm proteins. Arginine is unique in the sense that itpossesses a guanidine group, owing to which it is even morestrongly basic than lysine, and that it participates in multiplebiological processes including release of several hormones,collagen synthesis during wound healing, antitumor andantibacterial activities and other non-specific immunity.Over the past two decades, studies have shown that arginineplays an important role in many physiological, biologicaland immunological processes beyond its role as a protein-incorporated amino acid. Recent studies have emphasizedon some of the effects of L-arginine on immune cellfunctions, including triggering of L-arginine-nitric oxide andarginase pathways, its biological properties and therapeuticapplications.1

Infrared and Raman spectroscopic studies give struc-tural and bonding features of the crystalline environment

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

and the changes due to biological functioning. Dhanarajet al.2 investigated the vibrational spectroscopic study ofL-arginine phosphate monohydrate (LAP), a new organicnon-linear crystal. Mazumder et al.3 carried out infraredspectroscopy and thermal studies of as-grown L-argininephosphate monohydrate crystals. They analyzed the molec-ular vibration and thermal behavior of LAP crystals. Mukerjiand Kar4 investigated the structural, thermal and infraredspectroscopic studies of non-linear optical crystal L-argininehydrobromide monohydrate. Rahmelow et al.5 obtained use-ful estimates of protein side-chain infrared absorbance fromthe measurements of amino acids and di-, tri- and polypep-tides. They concluded that the —COO� and —NH3

C endgroups of amino acids and peptides differ in their spectro-scopic properties. Petrosyan et al.6 studied the growth andmade an investigation of new non-linear optical crystals ofLAP family by infrared and x-ray diffraction methods. Vibra-tional studies of various crystalline amino acid complexeswith inorganic acids have also been performed.7 – 12

In the present investigation, the vibrational spectra ofa non-linear optical crystal L-argininium dinitrate wereanalyzed. As the compound under study has a large degreeof hydrogen bonding, its spectra are slightly different fromthose of the pure amino acids.13 Factor group analysis of thetitle compound was also carried out.

EXPERIMENTAL

L-Argininium dinitrate crystals were grown from an aqueoussolution of L-arginine and nitric acid by the slow evapora-tion method using a stoichiometric ratio of 1 : 2. Within

Copyright 2002 John Wiley & Sons, Ltd.

Vibrational spectra of L-argininium dinitrate 51

Table 1. Factor group analysis of L-argininium dinitrate

(Crystal space group: P21 D C2; Z D 2; ZB D 2)

Mode anddegrees of

freedom foreach species

Molecularsymmetry

species

Sitesymmetry

speciesC1

Factorgroupspecies

C2

C6H16N4O22C

VibrationalA

84A168 84B

intC6H16N4O22C D 84AIR,R C 84BIR,R

2(NO3�) D3h

Vibrational A01 12A

(intramolecular) A002 A

24 2E0 12B

Translational A002 A

6A12 E0 6B

Libration A02 A

6A12 E00 6B

int2�NO3�� C trans

2�NO3�� C rot2�NO3�� D 24AIR,R C 24BIR,R

vibcryst D total

cryst � acoustic D 107AIR,R C 106BIR,R

1 week, colorless, transparent, needle-shaped crystals of L-argininium dinitrate were obtained.

A Bruker IFS Fourier transform (FT) IR spectrometer wasused for the IR spectral measurements with a ‘globular’source and the spectra were recorded over the range4000–400 cm�1. The sample for this measurement was finelyground and mixed with KBr. This intimate mixture wasthen pressed under vacuum at very high pressure to obtaina transparent disk, which was then placed in the samplecompartment.

The Raman spectral measurements were carried out withthe facility developed in our laboratory.14 A Spectra-Physicsmodel 2020-04S argon ion laser operating at 488 nm was usedas the source of excitation. A suitable notch filter was placedbefore the monochromator to suppress the Rayleigh line.The laser power was maintained at 70 mW. The measuredspectral lines had a resolution of 2–3 cm�1 and user-friendlysoftware obviates the need for a strip-chart recorder. Forreliability of the data, Raman spectral measurements werealso made with a DILOR Z24 laser Raman spectrometer witha krypton ion laser operating at 647.1 nm and the laser powerbeing maintained at 200 mW.

RESULTS AND DISCUSSION

In L-argininium dinitrate crystals, the two nitrate groups areplanar whereas the argininium cation is in a slightly foldedconformation. The available potential donors are involved

in extensive intermolecular hydrogen bonding in the crystal.The crystal belongs to the monoclinic space group P21. In theunit cell, the title molecules are related to each other througha twofold screw axis.15

Factor group analysis16 of the system gives 213 genuinenormal modes of vibrations distributed as

D 107A C 106B

excluding the three non-genuine normal modes (acousticmodes). Here the A and B species are both infrared andRaman active. The results of the factor group analysis arepresented in Table 1.

The configuration of one argininium cation and twonitrate anions is shown in Fig. 1. The observed infrared andRaman spectra are shown in Figs 2 and 3 and the observedwavenumbers together with the proposed assignments aregiven in Table 2.

Vibrations of the argininium cationThe argininium cation, C6H16N4O2

2C , is made up of severalfunctional groups. The characteristic wavenumbers of thesegroups are expected to undergo changes in their intensityand position according to their environment. Figure 2 showsthe FTIR spectrum of L-argininium dinitrate recorded inthe range 4000–400 cm�1. The high-wavenumber region ofthe spectrum consists of bands due to NH3

C , NH2, CH2

and (C)O–H stretching vibrations. The lower wavenumber

Copyright 2002 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2003; 34: 50–56

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

Figure 1. Structural formula of L-argininium dinitrate.

region contains bands due to deformation vibrations of thevarious groups. The bands due to lattice vibration usuallyappear below 300 cm�1.

Our group had already investigated the crystal structureof L-argininium dinitrate by the single-crystal x-ray diffrac-tion method.15 From the structural details, it had been foundthat in the crystalline state, the argininium molecule is dipro-tonated, one at the carboxyl group (COOH) and the other atthe guanidyl group [(H2N)2

C CNH]. The wavenumbers of thevarious functional groups are modified owing to the exten-sive intermolecular hydrogen bonding extended throughoutthe crystal. The vibrational wavenumbers for L-arginine, astrong amino acid, as reported by Krishnan et al.,13 exhibitmarked wavenumber differences. These are attributed to thepresence of hydrogen bonding.

The existence of a hydrogen bond between the carboxylgroup of the argininium cation and nitrate anions showsthe protonation of carboxylate ion in the amino acid. Thefour nitrate ions in the unit cell are involved in hydrogenbonding for stabilizing the crystal structure. The intense bandat 2960 cm�1 in the Raman spectrum indicates the presenceof the carboxyl OH group and it is assigned to the OHstretching vibration. The very weak band at 1357 cm�1 anda strong band at 980 cm�1 in the infrared spectrum are dueto the OH in-plane and out-of-plane deformation vibrations,respectively. The medium intensity band at 1180 cm�1 inthe Raman spectrum is attributed to the C–O(H) stretchingvibration, which is absent in the infrared spectrum. Thedownshifting of the hydroxyl group stretching wavenumberis due to the presence of strong O–HÐ Ð Ð O hydrogen bonding.

The —[NH3]C group exists with a pyramidal symmetryin the free state. Under this, it has C3v symmetry andits normal modes of vibrations are �1�A1�, �2�A2�, �3�E)and �4�E). These four vibrations are both infrared and

Tra

nsm

ittan

ce /

%

20

60

100

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber / cm-1

Figure 2. Infrared spectrum of L-argininium dinitrate.


Vibrational spectra of L-argininium dinitrate 53In

tens

ity /

Arb

. Uni

ts

3250 2750 2250 1750 1250 750 250

Wavenumber / cm-1

Figure 3. Raman spectrum of L-argininium dinitrate.

Raman active.17 In the Raman spectrum, the medium-intensity bands around 1688 and 1594 cm�1 are attributedto the asymmetric and symmetric deformation modes ofthe —[NH3]C moiety, respectively. The asymmetric andsymmetric stretching modes of the —[NH3]C group arealso identified. The shoulder peak with medium intensity at3359 cm�1 in the Raman spectrum is due to N–H stretchingand NH2 (guanidyl) asymmetric stretching vibrations. Themedium-intensity broad band in the infrared spectrum at3214 cm�1 is due to the symmetric stretching mode of NH2.The deformation, twisting and wagging modes of NH2 groupare also identified and assigned. In the title compound, eachof the N—H bonds of the —[NH3]C group and the guanidylgroup are hydrogen bonded (normal hydrogen bonds)with the oxygen atoms of surrounding NO3

� groups andtwo of them are chelated three-centered hydrogen bonds.15

These hydrogen bonds are responsible for the lowering ofthe stretching wavenumbers and the shift of deformationmodes to higher wavenumbers. The bands around 1144and 1113 cm�1 in the infrared spectrum are assigned to therocking mode of the —[NH3]C moiety.

When a hydroxyl group is present in a molecule with acarboxyl group, or when an acid is dissolved in a solventcontaining a hydroxyl group, there is an opportunity for theformation of hydrogen bonds. When this happens, symmetryis destroyed and the carbonyl vibrations appear in both theinfrared and Raman spectra. When the carbonyl group ishydrogen bonded but not dimerized, a band appears around1720 cm�1 in both the infrared and Raman spectra. Whenthe carboxyl group is dimerized, a band is observed inthe 1680–1640 cm�1 region in the Raman spectrum and atslightly higher wavenumbers in the infrared spectrum. Whenthe carbonyl group is not hydrogen bonded as mentionedabove, a band appears at around 1735 cm�1 in both theinfrared and Raman spectra.18 L-Argininium dinitrate hasa characteristic >C O group which gives rise to a verystrong and broad band at ¾1729 cm�1 in the infrared

spectrum and the corresponding Raman line has a mediumintensity at 1734 cm�1. The fact that this is well within theexpected wavenumber range of the carbonyl stretching modeindicates that the >C O is unlikely to be involved in thehydrogen bonding.

˛-Branched aliphatic monocarboxylic acids usuallyexhibit three strong bands at ¾655, ¾635 and ¾620 cm�1 thatare not usually well resolved in the region 665–610 cm�1

owing to the in-plane vibration of the O—C O group. Inaddition, a strong band is found at 555–520 cm�1, whichis attributed to the in-plane vibration of the C—C Ogroup.19 The in-plane deformation modes of the O—C Oand C—C O groups are also identified and assigned.

The >CH group is also next to a carbonyl group.The >CH deformation absorbs weakly in hydrocarbonsat 1350–1315 cm�1 in the infrared spectrum. This vibra-tion is distinctive in the Raman spectrum.18 As expected,in this environment, the >CH bending mode occurs asa weak band at 1443 cm�1 in the infrared spectrumand a strong band at 1435 cm�1 in the Raman spec-trum. The >CH stretching vibrations, which occur around2928 cm�1,18,20 are overlapped by the stronger CH2 stretch-ing vibrations.

The wavenumber of the CH2 vibrational mode dependson its immediate environment. In this investigation, in theRaman spectrum, a very intense peak occurs at 2928 cm�1

and a weak peak at 2902 cm�1. These are due to the asym-metric and symmetric stretching modes of the methylenegroups, respectively. The shoulder peak at 1464 cm�1 inthe infrared spectrum is due to the methylene deforma-tions. The in-phase CH2 rock vibrations cause a promi-nent band with a splitting at 729 and 719 cm�1 in theRaman spectrum. The intensity of this split band is mainlydue to the presence of more CH2 groups in the crys-talline state.18 The CH2 twist and wag vibrations are alsoassigned. Thus, the appearance of multiple bands confirmsthat the argininium molecule has a long chain of methy-lene groups.

The asymmetric motion of the branched carbon atomagainst its neighbors causes several vibrations. Motionalong the C—N bond can be described as asymmetricC—C—N, whereas motion at right-angles to the C—Nbond can be described as asymmetric C—C—C. Primaryamines with secondary ˛-carbon atoms (CH–NH2� absorbweakly at 1043–1037 cm�1 and even more strongly at1140–1080 cm�1.18 The highly intense band at 1080 cm�1

in the Raman spectrum and a medium-intensity band near1009 cm�1 in the infrared spectrum are attributed to theasymmetric vibrations mentioned above.

The skeletal stretching vibrations of the amino acidsare all coupled together. Of the skeletal vibrations, the C–Cstretching absorptions occur in the region 1260–700 cm�1 andare normally weak. The C–C deformation bands occur below600 cm�1 and these also are weak.19 L-Argininium dinitratehas a C—C—C—C skeleton, which can be considered as



Table 2. Observed wavenumbers (�) for the vibrational spectra ofL-argininium dinitratea

Infrared�/cm�1

Raman�/cm�1 Assignment

3359(sh) NH2 asym. str.; NH str.3214(m,br) 3125(m) NH2 sym str.3058(sh) NH3

C asym. & sym. str.3009(w,br)

}2950(sh) 2960(vs) (C) O–H str.

2928(vs) CH2 asym. str.2902(w) CH2 sym. str.

2650–1900 Overtones and combination bands1729(vvs,br) 1734(m) C O str.

1688(m) NH3C asym. def.

1631(vs) 1641(s) C N str.1616(sh) 1625(s) NH2 def.1593(sh) 1594(m) NH3

C sym. def.1513(vs) CNH group (NH bend, CN str.)1464(sh) 1461(s) CH2 in-plane def.1443(w) 1435(s) CH def.1413(s,br)

}CH2 wag

1385(vs) 1383(w) CH2 wag1357(vw) 1367(m) OH in-plane def.1327(vs) 1328(s) NO3

� asym. str.1305(sh)

}NO3

� asym. str.1223(s) 1273(w,br) CH2 twist1204(s)

}CH2 twist

1180(m) C–O(H) str.1144(s) 1164(m) NH3

C rock1113(vs)

}1125(m)

}1080(vs) C–N str.

1040(vs) 1046(vvs) NO3� sym. str.

1009(m) C–N str.980(s) 953(s) OH out-of-plane def.922(s) 906(m) C–C skeletal str.873(s,br) 859(w) NO3

� sym. def.; C–C skeletal str.815(vs) 820(s) NH2 wag and twist

789(w) C–C skeletal str.748(w) 729(s) CH2 rock731(m)

}719(s)

}CH2 rock

707(vw) 696(w) NO3� asym. def.

640(vs) 648(w)617(w)

}O–C O in-plane def.

594(w)527(vs) 530(s) C–C O in-plane def

438(m) In-phase def. of skeletal carbon chain398(m) Out-of-phase def. of skeletal carbon chain297(m)234(m)195(m)

Lattice vibrations157(s)125(w)

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


Vibrational spectra of L-argininium dinitrate 55

coupled oscillators.17 The three bands around 906, 859 and789 cm�1 in the Raman spectrum are attributed to the C–Cskeletal stretching vibrations, just as Dhamelincourt andRamirez20 identified three C–C bands (around 970, 918 and806 cm�1� in the infrared and Raman spectra of L-glutamicacid. Here, the band at 873 cm�1 in the infrared spectrumis overlapped by the stronger and broader NO3

� symmetricdeformation modes. The deformation modes of the skeletalcarbon chain are also assigned.

In addition to the peptide bonds, which are the basis of thesecondary structure prediction, some functional groups inthe side-chains of the different amino acid residues contributeto the spectral region 1750–1480 cm�1.5

Both arginine and lysine have basic amino side-chain endgroups. The presence of a guanidyl group is identified by theappearance of strong absorbance around 1625 cm�1 in theinfrared spectrum.5,18 The very strong peak at 1631 cm�1 isdue to the C N stretching vibrations of the guanidyl group.Here, the guanidyl group is not only protonated but alsoinvolved in the formation of normal hydrogen bonds. Theside-chain guanidinium group of arginine causes the intenseabsorption at 1513 cm�1. In the guanidinium group, thereare three C—N bonds. If they are of different bond lengths,there can be multiple bands due to the C–N stretchingvibrations. However, in the present case, only one strongband is observed. This indicates that all the C—N bonds arenearly equal in length. This is in good agreement with thex-ray structural results.15

Hydrogen bondingAn important feature of amino acids in the crystalline state isthe extensive network of hydrogen bonding that connectsthe molecules together. These hydrogen bonds give thestructures cohesion in three dimensions. In the zwitterionicstructure, the —[NH3]C group is a good hydrogen bonddonor and the carboxyl group is an excellent acceptor, andstrong hydrogen bonds are formed between these groups.

L-Argininium dinitrate has nine different hydrogenbonds, which can be grouped into three categories. Thereare four nitrate ions in the unit cell and they help in stabi-lizing the crystal structure through extensive intermolecularhydrogen bonding.

The first type of hydrogen bond is the O—HÐ Ð ÐO bondinvolving the carboxylic group and the oxygen of the nitrategroups. This bond is a very strong bond with the averageO—O distance being 2.653 A (for normal hydrogen bonds,the bond distance lies in the range 2.7–2.9 A). This is evidentfrom the infrared and Raman spectra of the compoundwhere the O–H stretching wavenumbers were loweredby about 500 cm�1 and also became broad. The bendingwavenumbers, however, are not much different from theexpected range (1380–1280 cm�1�, indicating that the lineardistortion is much greater than the angular distortion.11

The second category of hydrogen bonds is the N—HÐ Ð ÐObond. All the amino and guanidyl group nitrogen atoms

of the argininium cation are linked with nitrate groupsthrough these types of hydrogen bonds. All the N—HÐ Ð ÐObond lengths lie in the range 2.823–3.086 A and may beconsidered as normal hydrogen bonds. There are six suchhydrogen bonds in this crystal.

The third category is the chelated three-centered hydro-gen bonds between the nitrogen atoms of the amino andguanidyl groups with the nitrate anions. The bond lengths ofthese hydrogen bonds are in the range 2.855–3.212 A, againshowing that they are normal hydrogen bonds.

The normal N—HÐ Ð ÐO hydrogen bonds and the chelatedthree-centered hydrogen bonds influence the various vibra-tional modes of the —[NH3]C group and the guanidyl group.The bond lengths of all the N—HÐ Ð ÐO bonds, includingthe chelated three-centered hydrogen bonds in the crys-tal, vary from 2.823 to 3.212 A. These normal hydrogenbonds are responsible for the lowering of the stretchingwavenumbers and the shift of deformation modes to higherwavenumbers than the expected range. For hydrogen bond-ing, rocking modes are most sensitive than the asymmetricstretching modes.

From the infrared and Raman spectra, the influence ofhydrogen bonding on the nitrate group was also analyzed.The nitrate anions exist in a planar orientation with theO—N—O angle varying from 118.6 to 120.8°. The averageN—O bond lengths are 1.243 and 1.240 A for the two nitrategroups in the molecule. This is not much different from thatof the free ion, ¾1.24 A. There is no appreciable variationin the vibrations of NO3

� ion, indicating that the effect ofhydrogen bonds is normal.

Vibrations of the nitrate ionL-Argininium dinitrate has two nitrate groups. The nitrateanions in this crystal exist in the planar form. The four nitrateanions in the unit cell help in stabilizing the crystal structurethrough the hydrogen bonding network. When the nitrateion is free, it has D3h symmetry and its normal modes ofvibrations belong to the species A0

1(�1�, A002(�2� and E0(�3 and

�4�. The �1 mode is Raman active, the �2 mode is infraredactive and the �3 and �4 modes are both Raman and infraredactive. In its free ion state, �1 (symmetric stretch) occursat 1049 cm�1, �2 (symmetric bend) at 830 cm�1 and �3 and�4 (asymmetric stretch and asymmetric bend) at 1355 and690 cm�1, respectively.21

The �1 mode of vibration is observed at 1040 cm�1 as avery strong band in the infrared spectrum and at 1046 cm�1

as a very strong band in the Raman spectrum. Here theinfrared inactive mode is excited owing to the local crystallineenvironment. The wavenumber is also downshifted by about10 cm�1. The intense infrared peak at 873 cm�1 is assigned tothe �2 mode of vibration. Here the forbidden Raman line isalso excited owing to the site symmetry effect and is observedwith a weak intensity at 859 cm�1.

The asymmetric stretching wavenumber �3, at about1327 cm�1, is both infrared and Raman active. In the Raman



spectrum, this �3 mode of vibration has lost its degeneracyand occurs as two peaks instead of one. The site symmetryeffect may be responsible for the removal of degeneracy.

The asymmetric bending mode is also both infrared andRaman active. It occurs at 707 cm�1 in the infrared spectrumand 696 cm�1 in the Raman spectrum, which are againslightly higher than the free ion value. As expected, thestretching modes occur at slightly lower wavenumbers andthe bending modes occur at slightly higher wavenumbersthan that expected for the free ion state owing to theextensive intermolecular hydrogen bonding and the localsite symmetry effect.

CONCLUSION

The infrared and Raman spectral bands of L-argininiumdinitrate have been assigned. The argininium cation isdiprotonated. The nitrate groups form the anions, andthe anions stabilize the structure through intermolecularhydrogen bonding in the crystal. The downshifting of thewavenumbers of several stretching modes and the corre-sponding increase in those of the bending modes confirm theoccurrence of extensive intermolecular hydrogen bonding.

AcknowledgementsThe authors are grateful to DST, Government of India, New Delhi,for establishing the laser laboratory. The UGC, Government ofIndia, New Delhi, is acknowledged for recognizing our groupactivities as the thrust area of research in DRS-Phase II andCOSIST programmes in the School of Physics and for providingassistance to our laboratory. One of the authors (S.R.) is indebtedto the UGC, Government of India, New Delhi, for the award ofthe teacher fellowship under the Faculty Improvement Programand the authorities of NMSSVN College, Madurai, India, for theirencouragement

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vibrational spectroscopic studies of l-argininium dinitrate

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