Infrared and laser Raman studies of l -phenylalanine l -phenylalaninium perchlorate and bis(dl -phenylalaninium) sulphate monohydrate

Spectrochimica Acta Part A 58 (2002) 1923– 1934 www.elsevier.com/locate/saa

Infrared and laser Raman studies of L-phenylalanine L-phenylalaninium perchlorate and bis(DL-phenylalaninium) sulphate monohydrate Beulah J.M. Rajkumar, V. Ramakrishnan * Laser Laboratory, Department of Microprocessor and Computer, School of Physics, Madurai Kamaraj Uni6ersity, Madurai 21, India Received 14 May 2001; received in revised form 4 October 2001; accepted 9 October 2001

Abstract Both crystals under study have two phenylalanine groups in the cationic part of the complex. In the L-phenylalanine L-phenylalaninium perchlorate crystal, two phenylalanine groups share one proton and become monoprotonated. In the bis(DL-phenylalaninium) sulphate monohydrate crystal, on the other hand, both the phenylalanine groups are protonated. This leads to several differences in the infrared and Raman spectra of these two crystals. The presence of both the carbonyl and the ionized carboxylic groups has been identified in the perchlorate crystal, while the sulphate crystal has only the carbonyl group. Extensive hydrogen bonding further leads to the shifting of bands due to several stretching and bending modes. It also reduces the Td symmetry of the anions to C26 symmetry causing the degeneracies of several modes to be removed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectra; Raman spectra; Crystals; L-phenylalanine complexes

1. Introduction Phenylalanine is one of the three naturally occurring aromatic amino acids, the other two being tryptophan and tyrosine. All three are essential amino acids and must be provided in the diet for vertebrates. In addition, phenyalanine and tryptophan are ‘non-functional’ indicating that they do not participate directly in catalytic and enzymatic action. Rather, they play an important role in determining the secondary structure and form* Corresponding author.

ing specificity sites for catalytic action. Velanker et al. [1] have studied the role of phenylalanine in designing specific active site inhibitors against the enzyme of Plasmodium falciparum—the malariacausing parasite. Shumilin and co-workers [2] have studied the crystal structure of the phenylalanine-regulated form of 3-deoxy D-arabinoheptulosonate-7-phosphate synthase (DAHPS) from Escherichia coli crystallized with phosphoenolpyruvate (PEP) and Pb2 + . They have indicated the probable site of inhibitor binding of this enzyme essential for aromatic biosynthesis. There have been several spectroscopic studies on the behavior of many amino acids and pep-

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B.J.M. Rajkumar, V. Ramakrishnan / Spectrochimica Acta Part A 58 (2002) 1923–1934

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tides including phenylalanine and on complexes involving amino acids, organic molecules and metal ions [4– 9]. In this present investigation, the infrared and Raman spectra have been analyzed for two phenylalanine amino acid crystals.

04S) argon ion laser was used as the source of excitation. A suitable notch filter placed before the monochromator filters off the Rayleigh line. The laser power was maintained at 70 mW. The measured spectral lines have a resolution of 2– 3 cm − 1.

2. Experimental 3. Results and discussion Both L-phenylalanine L-phenylalaninium per− and chlorate [C9H11NO2 · C9H12NO+ 2 · ClO4 ] bis(DL-phenylalaninium) sulphate monohydrate 2− [2(C9H12NO+ ] · H2O were crystallized 2 ) · (SO4) by the slow evaporation of an aqueous solution of the amino acid and the corresponding inorganic acid in the ratio of 2:1. Colorless, transparent, needle-shaped crystals were obtained after about 2 weeks. The infrared spectral data were obtained using a BRUKER IFS FTIR spectrometer using the KBr pellet technique. Raman measurements were made employing the technique of Jeyaraj and Ramakrishnan [10]. The 488-nm line of the Spectra Physics (2020-

The crystal L-phenylalanine L-phenylalaninium perchlorate, which has a two-fold symmetric hydrogen bonded dimer, belongs to the space group P212121 and has an orthorhombic geometry. It has four formula units per unit cell [11]. Factor group analysis gives 621 genuine modes of vibration distributed as Y= 156A+155B1 + 155B2 + 155B3. These are recorded in Table 1. The A species is Raman active only, the other three vibrational species being both infrared and Raman active. The crystal bis(DL-phenylalaninium) sulphate monohydrate, on the other hand, belongs to the space group P21/n and has

Table 1 Factor group analysis — L-phenylalanine L-phenylalaninium perchlorate Mode and degree of freedom for each species

Molecular symmetry species

C9H11NO2 · C9H12NO+ 2 Vibrational 564

ClO− 4

Site symmetry species C1

A

Factor group species D2

141A 141B1 141B2 141B3

Td Vibrational 36

A1 E 2F2

Translational 12

F2

Libration 12

F2

A

A

A

9A 9B1 9B2 9B3 3A 3B1 3B2 3B3 3A 3B1 3B2 3B3

int R IR,R IR Crystal space group: P21/n= C2h ; Z= 4, Z B =4. YC +141AIR * =141A +141B1 2 +141B3 . 9H11NO2·C9H12NO2 total acoustic Y int +Y trans +Y rot =15AR+15BIR,R +15BIR,R +15BIR,R . Y vib =156AR+155BIR,R +155BIR,R +155BIR,R . 1 2 3 cryst =Y cryst−Y 1 2 3 ClO− ClO− ClO− 4

4

4

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Table 2 Factor group analysis —bis(DL-phenylalaninium) sulphate monohydrate Mode and degrees of freedom for each species

Molecular symmetry species

2(C9H12NO+ 2 ) Vibrational 576

SO2− 4

Site symmetry species C1

A

Factor group species C2h

144Ag 144Bg 144Au 144Bu

Td Vibrational 36

Translational 12

Libration 12

H2O

A1 E 2F2 F2

A

A

F2 A

9Ag 9Bg 9Au 9Bu 3Ag 3Bg 3Au 3Bu 3Ag 3Bg 3Au 3Bu

C26 Vibrational 12

Translational 12

Libration 12

2A1 B2

A

A1 B1 B2

A

A2 B1 B2

A

3Ag 3Bg 3Au 3Bu 3Ag 3Bg 3Au 3Bu 3Ag 3Bg 3Au 3Bu

Crystal space group: P21/n= C2h ; Z= 4, Z B int =4. Y 2(C 9H12NO+ 2 ) R IR IR int trans rot =144AR g +144Bg +144Au +144Bu . Y SO−+Y SO−+Y SO− 4 4 4 R R IR int trans rot =15Ag +15Bg +15AIR u +15Bu . Y H2O+Y H2O +Y H2O R IR IR vib =9AR +9B +9A +9B . Y g g u u cryst acoustic R IR IR =Y total = 168AR cryst−Y g +168Bg +167Au +166Bu .

a monoclinic geometry [12]. This crystal, too, has four formula units per unit cell and the factor group analysis gives 669 normal modes of vibration distributed as Y = 168Ag +168Bg + 167Au +166Bu (Table 2). Here the Ag and Bg species are Raman active while the Au and Bu

species are infrared active. There are no common modes.

3.1. Vibrations of the phenylalaninium cation Both crystals under investigation (Figs. 1 and

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2) have several common vibrational group frequencies. Their infrared and Raman spectra (Figs. 3– 6), however, reveal that not all of them are identical. The bands observed together with their assignments are tabulated in Table 3. In the L-phenylalanine L-phenylalaninium perchlorate crystal one proton is shared between two phenylalanine groups making the total entity monoprotonated. The anion is the monovalent perchlorate ion. In the case of the bis(DL-phenylalaninium) sulphate monohydrate crystal, two protons attach themselves to two phenylalanine groups. The divalent sulphate ion is the anion in this case. This difference in the formation of the crystals accounts for the differences in the vibrational spectra of both cations. The extensive hydrogen bonding in both crystals causes several bands to undergo changes in their position, intensity or degeneracy from the expected values. The side chains of the samples under study are similar to that of phenylalanine. The assignments proposed for the most of the observed vibrations are made keeping with the observations and the theoretical predictions made for phenylalanine [3,4]. The monosubstituted benzene, in the two crystals under study, may be considered to have a C26 symmetry although the substituent does not strictly lie in the plane of the ring. Under this

Fig. 1. Structural formula of lalaninium perchlorate.

L-phenylalanine

L-pheny-

Fig. 2. Structural formula of bis(DL-phenylalaninium) sulphate monohydrate.

approximation, the 30 possible fundamental vibrations are distributed as Y= 11A1 + 3A2 + 6B1 + 10B2. All the species are both infrared and Raman active except the A2 species, which is only Raman active. In mono substituted benzenes, all the CH stretching vibrations lie in the 3100–3000 cm − 1 region and multiple bands are expected in the infrared spectra of which at least three are prominent. One strong band is usually noticed in the Raman spectra [13]. For the crystals under study, the strong band 3060 cm − 1 in the Raman spectra of both crystals is assigned to this mode. In the infrared spectra, however broad bands are observed centered  3070 cm − 1 but these are not as intense as expected. This is because the bands due to the stretching modes of the CH2 and the NH+ 3 groups also lie in the same region. Of the ring stretching modes the most prominent is the w8 mode, which involves the quadrant

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stretching of the C···C bonds in the ring. The two quadrant stretching components are resolved into the w8a and w8b modes in mono-substituted benzene compounds irrespective of the nature of the substituent [14,15]. These have been identified for both crystals and lie in the same region as that expected [C, c, C%, c%] for phenylalanine [3,4]. The semicircle ring-stretching mode mixes with the CH bending mode to give rise to two bands w19a and w19b. The former is noticed as a strong band in the infrared and a weak one in the Raman spectra as expected  1490 cm − 1 [3,4]. The sextant ring stretching mode, w14, also shows a band in the expected region for monosubstituted benzene compounds, confirming that this is a specific mode of vibration [14]. The w1 ring-stretching mode, which is the ring-breathing mode, is known to be substituent sensitive [15]. It shifts to higher bands for heavier substituents. Here it is observed for the bis(DL-phenylalaninium) sulphate monohydrate crystal 1050 cm − 1 [I%, i%]. For L-phenylalanine L-phenylalaninium perchlorate, each

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benzene ring has a larger substituent. The w1 mode, therefore, occurs at a higher wavenumber and is masked by the strong phenyl ring-carbonstretching mode— a characteristic of alkyl substituent [G, g]. Among the in-plane ring bending modes, noticeable in monosubstituted benzene crystals are the trigonal ring breathing mode, w12, and the two degenerate modes w6a and w6b due to bending by quadrants. The w12 bending mode is specific in form and wavenumber irrespective of the substituent. It occurs as a strong Raman band  1000 cm − 1 [j, j%] and is weak [J%] or not noticeable in the infrared spectrum as expected [3,4]. The w6a is a non-specific mode while the w6b bending mode is expected around 618 cm − 1. In both crystals, the degenerate bending modes of the corresponding anions overlap these modes. The out-of-plane sextant ring deformation mode (w4) is expected to be a very intense band in the infrared spectra and weak in the Raman spectra. These modes have been observed for both

Fig. 3. Infrared spectrum of L-phenylalanine L-phenylalaninium perchlorate.

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Fig. 4. Raman spectrum of L-phenylalanine L-phenylalaninium perchlorate.

crystals 700 cm − 1 in the infrared spectra and are absent in the Raman spectra. The in plane CH deformation mode of monosubstituted benzene, w3, has been assigned ( 1250 cm − 1) and this lies in the same region as the CH2 twisting mode. The modes w9a and w15 have their vibrational bands very close to each other  1170 cm − 1 and in these crystals they are masked by the degeneracy lifted asymmetric stretching modes (F2) of the anion. The w18a mode has also been assigned for both crystals. The w18b mode has been observed only for the sulphate crystal as a shoulder at 1066 cm − 1. The weak bands observed in the Raman spectra 950 cm − 1 are assigned to the CH out of plane bending mode (w5). The w11 mode appears as a strong infrared band  750 cm − 1 as this is associated with a large change in dipole moment when the five adjacent hydrogen atoms move in phase, out of the plane of the benzene ring.

Besides the benzene ring, other functional groups are also present in the cation of both crystals under study. The amino group has a C36 symmetry and its normal modes of vibration are A1 (w1), A2 (w2) and E (w3 and w4). All of these are both infrared and Raman active with the asymmetric stretching and bending modes being doubly degenerate. In the two cations, all the three hydrogens of the amino group participate in hydrogen bonding formation leading to the NH···O type of bonds with the oxygens of the anion. In addition, in the L-phenylalanine L-phenylalaninium perchlorate crystal, hydrogen bonds with the carbonyl group are also formed while in the case of the bis(DL-phenylalaninium) sulphate monohydrate, the amino groups form additional hydrogen bonds with the water molecule. These hydrogen bonds result in the weakening of the NH bonds causing a lowering of the stretching and a raising of the deformation modes. The

B.J.M. Rajkumar, V. Ramakrishnan / Spectrochimica Acta Part A 58 (2002) 1923–1934

asymmetric stretching mode is lowered by about 200 cm − 1 in both crystals. The strong NH+ 3 symmetric deformation mode lies in the same region as the strong ring stretching modes w8a and w8b and is not distinguished as a separate band. The asymmetric deformation modes, however, are observed at 1679 and 1693 cm − 1 as a weak shoulder and are about 30 and 50 cm − 1 higher than that expected for the free ion [16] for the perchlorate and sulphate crystals, respectively. This difference is attributed to the fact that in the former, some of the hydrogen bonds are bifurcated leading to a further weakening of the NH bond strengths. The large difference in wavenumber shifts between the bands due to the stretching and deformation modes indicates that the linear distortion is much greater than the angular distortion in both cases. This suggests that the interaction of the group with the environment is less related to symmetry and more through the formation of hydrogen bonds. The weaker NH+ 3 torsion and rocking modes are not

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observed as they also lie in the same region as the anion deformation modes. A study of the X-ray analyses of both crystals [11,12] reveals that for the CH2 group the CH bond length varies from 0.93 to 1.01 A, for the sulphate crystal while it is 0.97 A, for the perchlorate crystal. This is the reason why the CH2 stretching modes appear as distinct bands in the perchlorate crystal [a] whereas they lie within a broad band for the sulphate crystal [a%]. The wagging mode of the CH2 groups has been assigned as well. The bands due to CH stretching modes in both cases is superimposed by that of the much stronger aromatic CH stretching modes. The CH deformation mode, in bis(DL-phenylalaninium) sulphate monohydrate appears as a doublet. This mode lies in the same region as the overtone of the strong w4 mode at 692 cm − 1 of the benzene ring. Fermi resonance thus takes place giving rise to two strong bands at 1405 and 1358 cm − 1 [F%]. The CCCC skeleton leads to three different CC stretching

Fig. 5. Infrared spectrum of bis(DL-phenylalaninium) sulphate monohydrate.

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Fig. 6. Raman spectrum of bis(DL-phenylalaninium) sulphate monohydrate.

modes and these have been observed for both crystals. The in phase and out of phase vibrations of the CCCC skeleton have also been identified. In the bis(DL-phenylalaninium) sulphate monohydrate crystal, a band is observed  1730 cm − 1 [B%, b%] which is due to the CO stretching mode. This band occurs as expected both in shape and in position and it leads to the conclusion that if the carbonyl group is involved in hydrogen bonding, it must be a weak one. The X-ray data also reveals that only one of the two CO groups participate in a very weak hydrogen bond with the water molecule (O···HO = 3.301 A, ). The case of the L-phenylalanine L-phenylalaninium perchlorate crystal, however, is quite different. Here the band due to CO stretching mode has changed both in form and wavenumber. It is weak and broad in the Raman spectrum and is downshifted by almost 20 cm − 1 [B, b]. This indicates that the carbonyl group in this crystal is involved in strong hydrogen bonding. However, the presence of a medium intensity band at 1423 cm − 1 in the infrared spectrum suggests the exis-

tence of the ionized carboxylic group as well. This band is attributed to the symmetric stretching mode of this group [E]. The medium intensity shoulder at 1513 cm − 1 [D] is assigned to the asymmetric stretching mode of the CO− 2 group. From the spectroscopic data, it is thus clear that L-phenylalanine L-phenylalaninium perchlorate has the characteristics of the carbonyl and the ionized carboxylic groups. This may be attributed to the fact that the carbonyl groups of phenylalanine groups share a proton, which appears to give rise to a symmetric OH···O hydrogen bond. Hence there is a possibility of tunneling effect of the proton involved in hydrogen bonding between the two phenylalanine groups. X-ray structural investigations also reveal that there is a very strong hydrogen bond between the two phenylalanine groups through the shared proton and the OH···O distance being 2.44 A, . The two CO groups of the perchlorate crystal have bond lengths of 1.218 and 1.227 A, , respectively, while the corresponding distances in the sulphate crystal is 1.198 and 1.182 A, . The CO(H) bond on the other hand measures 1.283 and 1.267 A, , respec-

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Table 3 Observed vibrational bands (w¯ ) for L-phenylalanine L-phenylalaninium perchlorate and bis(DL-phenylalaninium) sulphate monohydrate L-phenylalanine L-phenylalaninium

perchlorate

bis(DL-phenylalaninium) sulphate monohydrate

Assignment

Infrared w¯ /cm−1 Raman w¯ /cm−1 Infrared w¯ /cm−1 Raman w¯ /cm−1

3084 s,b 3016 m,sh 2932 w,sh

3134 s 3062 s

3450 3062 2997 2912

w w,sh s,b m,sh

3046 sh 2984 m [a] 2941 m [a]

2581 w 2000 −1800 1712 s,b [B] 1679 w,sh

1703 w,b [b]

1614 1595 1513 1496 1453 1443 1423

s [C] s [C] m [D] s m m m [E]

1605 s [c] 1578 m [c]

1354 1321 1281 1243 1209

m w m w,sh s [G]

1348 m 1328 m,b

1143 sh [H] 1117 vs [H] 1081 vs [H]

1028 sh

933 sh [K] 871 857 810 796 764 746

w,sh [L] m [L] w,sh m w s

700 s

1500 w 1453 m

1200 s [g]

3125 m 3065 s 3031sh 2984 m,b [a’]

2700 2663 2581 2350 −1900 1735 1693 1621 1594 1564

w w s

vs [B’] w,sh w,sh s [C’] s [C’]

1480 vs 1460 m,sh

1405 1358 1324 1301 1264 1215 1180 1110

s [F’] s [F’] m m w,sh s [G’] s [H’] s,b [H’]

1066 1043 1021 982

w,sh w,sh [I’] s w,sh [J’]

1743 m [b’] 1609 sh 1589 s [c’] 1565 sh [c’] 1500 w 1453 w, b

1375 w,b

1250 m 1179 s [h’] 1125 w[h’]

1125 w,b [h] 1094vw [h]

1034 1002 953 933

m vs [j] w,sh s [k]

895 w [l] 844 w 808 m 767 s 760 w

1059 1016 1002 953

sh,w [i’] m,sh s [j’] w,sh

924 m 874 s 829 s

938 m 891 w,b 831 m

779 763 720 692

765 w

s s, sh s s

720 m

OH sym str (H2O) NH+ 3 asym str 8 CH str; CH str. (chain) NH+ 3 sym str CH2 asym str CH2 sym str Overtones and Combination bands O–H str Overtones and Combination bands C =O str NH+ 3 asym def H2O sym def 8 quad. Ring str. (w8a) 8 quad. Ring str. (w8b) CO-2 asym str 8 s.c. ring str.+ CH def. (w19a) 8 s.c. ring str.+ CH def. (w19b); CH2 sym. Def CO-2 sym str Fermi resonance C-H def (chain) 8 sextant ring str. (w14) CH2 wag; OH i.p. def. 8 i.p. CH def. (w3); CH2 twist; CH o.p. def (chain) 8 ring–carbon str. SO2 asym.str SO2 asym.str ; SO2 sym.str; CN sym. str. ClO2 asym.str ClO2 asym.str ClO2 sym.str; CN sym. str. 8 i.p. CH def (w18b) 8 ring breathing (w1) 8 i.p.CH def (w18a) 8 i.p. ring def (w12) 8 o.p. CH def. (w5) ClO2 sym. str. SO2 sym. str. C-C str C-C str CO-2 sci. C-C str 8 o.p. CH def. (w11) H2O rock 8 o.p ring def. (w4)

B.J.M. Rajkumar, V. Ramakrishnan / Spectrochimica Acta Part A 58 (2002) 1923–1934

1932 Table 3 (Continued) L-phenylalanine L-phenylalaninium

perchlorate

bis(DL-phenylalaninium) sulphate monohydrate

Assignment

Infrared w¯ /cm−1 Raman w¯ /cm−1 Infrared w¯ /cm−1 Raman w¯ /cm−1 644 m [M’] 617 s [M’] 577 s [M’] 626 s [M] 608 s [N] 600 s [N]

627 s [m] 594 w,sh [n] 563 w [n]

484 m,b

487 s 466 sh

672 w [m’] 612 m [m’]

497 s

472 m [N’] 443 m [O] 432 m [O]

438 sh [o] 344 w 251 w

484 m [n’] 421 w [n’]

359 250 176 119

w w s s

SO2 sym. def,. SO2 rock ;8 i.p. ring def (w6) SO2 rock ClO2 sym. def. ClO2 rock ;8 i.p. ring def (w6) ClO2 rock H2O wag ClO2 sym. def. ClO2 tor. SO2 sym. def. SO2 tor. CO-2 rock C-C-C-C i.ph. vib. C-C-C-C o.ph. vib. Lattice modes Lattice modes

asym., Asymmetric; b, broad; def., deformation; quad., quadrant; i.p., in plane; m, medium; o.p., out of plane; i.ph., in phase; o.ph., out of phase; 8, phenyl ring; vib., vibration; s, strong; s.c., semicircle; sci., scissoring; sh, shoulder; str., stretch; sym., symmetric; v, very; w, weak. The symbols within square brackets correspond to the labeling of the bands in Figs. 3–6.

tively, in the perchlorate crystal, the corresponding distances being 1.309 and 1.319 A, in the sulphate crystal. These bond lengths confirm that in the perchlorate crystal, the CO of one phenylalanine group loses its double bond character while at the same time the C(OH) bond of the other phenylalanine group gets a partial double bond character. Hence, the CO− 2 group cannot be considered to be a pure ionized carboxylic group as it does not have a symmetrical bond of the expected bond length 1.248 A, . It is this asymmetrical nature that causes the bending modes (scissoring and rocking) of the ionized carboxylic group to occur as doublets [L, l, O, o]. Thus, the spectra reveal features of both the carbonyl and the ionized carboxylic group.

3.2. Vibrations of the perchlorate and sulphate anions In the free state both the perchlorate and the sulphate ions have a Td symmetry with its vibrational modes distributed as Y =A1 +E +2F2.

The A1 and E species are Raman active only while the F2 species are both infrared and Raman active. A1 is a one-dimensional species; E is doubly degenerate while the F2 species has a threefold degeneracy [17]. These modes are expected to occur at 928, 459, 1119 and 625 cm − 1, respectively, for the perchlorate ion, while for the sulphate ion they are expected at 983, 450, 1105 and 611 cm − 1, respectively [16]. Both the perchlorate and the sulphate ions are known to have coordination with other ligands through hydrogen bonds [16,17]. The bonding can be unidentate, bidentate or bridging. In the case of unidentate bonding, the symmetry reduces to C36 while for the other two types of bonding it is C26 [16,18]. In the present case, the anions act as a bridge between the two phenylalanine groups of the respective molecular units through hydrogen bonds in which all four oxygen atoms are participating. Hence, it is possible to consider that the symmetry is reduced to C26. Under these circumstances, the vibrational species are redistributed as Y = 4A1 + A2 + 2B1 + 2B2 with all the species being

B.J.M. Rajkumar, V. Ramakrishnan / Spectrochimica Acta Part A 58 (2002) 1923–1934

both infrared and Raman active except the A2 species. In L-phenylalanine L-phenylalaninium perchlorate, the bridging is further complicated because the perchlorate ion is disordered and this increases the number of hydrogen bonds for these oxygen atoms. A careful study of the hydrogenbonding scheme indicates that although the NH···O hydrogen bonds involved in bridging vary from 2.906 to 3.278 A, , only two oxygen atoms form hydrogen bonds of equal length (2.954 and 2.956 A, ) [11]. Further, the OClO angles are also on an average, closer to the normal 108° for only two out of a possible 12. The degeneracies of the different modes are, therefore, lifted and the stretching and bending modes of the ClO− 2 groups are identified [H,h, K,k, M,m, N,n]. Although these modes appear distinctly different when they are wide apart, they tend to merge into each other when they occur close together. Hence, the disordered nature of the perchlorate ion makes it difficult to identify the two ClO2 groups as distinct. In the bis(DL-phenylalaninium) sulphate monohydrate crystal, again all the four oxygen atoms of the sulphate group participate in the hydrogen bond formation. The NH···O hydrogen bonds involve all the three hydrogens of the amino group of one phenylalanine group and two hydrogens of the amino group of the other phenylalanine group. Structural data show that two of the SO bonds are stronger than the other two (bond lengths 1.452 and 1.482 A, , respectively) [12]. Correspondingly, the stronger SO bonds form weaker NH···O hydrogen bonds confirming the reduction of symmetry of the ion from Td to C26 with two distinct SO2 groups being involved. As compared with the perchlorate ion, the sulphate tetrahedron is much less distorted. The ion is not disordered and of the six possible OSO angles four are close to the expected 108° —the other two being 113 and 111°, respectively. The asymmetric and symmetric stretching modes of the SO2 groups, therefore, appear as a strong broad band rather than as two distinct bands in the infrared spectrum [H%h%]. The infrared inactive mode (torsional vibration) continues to be inactive in both the crystals.

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3.3. Vibrations of water The broad band centered on 3000 cm − 1 in the infrared spectrum of bis(DL-phenylalaninium) sulphate monohydrate coupled with the appearance of a weak band at 3450 cm − 1 is indicative of the presence of water molecule in the crystal. The latter is assigned to the symmetric stretching mode of water. The weak shoulder  1615 cm − 1 is assigned to the bending mode. The fact that these are not much shifted from the expected values indicates that the hydrogen bond is weak and the distortion of the water molecule is negligible. This is also confirmed by the X-ray results, which indicate that the OH···O hydrogen bond lengths are 2.905 and 3.301 A, and both of these are weak bonds. The molecule continues to be bent with the OH···O angle being 105°. The rocking and wagging modes have also been assigned. The twisting mode is masked by the deformation mode of the SO2 group [M%, m%].

4. Conclusions The various infrared and Raman modes have been identified. It has been confirmed that the formation of the cation in both crystals is different. Each phenylalanine group in the bis(DLphenylalaninium) sulphate monohydrate accepts one proton making the cation diprotonated. In the perchlorate, on the other hand, both phenylalanine groups share one proton making the cation monoprotonated. The presence of the CO group in the sulphate crystal is confirmed but in the perchlorate crystal, it shows the characteristics of both the carbonyl and the ionized carboxyl group. The asymmetric nature of the CO− 2 group leads to the appearance of a couple of doublets.

Acknowledgements The authors are thankful to Professor R.K. Rajaram, School of Physics, Madurai Kamaraj University, for many useful discussions. They also thank DST and CSIR, Government of India, for financial assistance. One of the authors (B.J.M.R)

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thanks the UGC, New Delhi, for a teacher fellowship under the faculty Improvement programme, and the Principal of Lady Doak College, Madurai for encouragement.

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