Polarised IR and Raman spectra of monoglycine dihydrogenphosphate single crystal

Journal of Molecular Structure 708 (2004) 127–144 www.elsevier.com/locate/molstruc

Polarised IR and Raman spectra of monoglycine dihydrogenphosphate single crystal J. Baran*, M. Trzebiatowska, H. Ratajczak Institute of Low Temperature and Structure Research of Polish Academy of Sciences, 50-950 Wrocław 2, P.O. Box 1410, Poland Received 9 February 2004; accepted 2 March 2004 Available online 28 July 2004

Abstract Polarised Raman, IR and FIR spectra of the monoglycine dihydrogenphosphate (glycinium dihydrogenphosphate) single crystal samples are presented and discussed with respect to the crystal structure on the basis of oriented gas model approximation. The IR and FIR spectra were measured by specular reflection method and spectra of an imaginary part of the refractive indices were computed by the KramersKronig transformation. The polarisation properties of the internal vibrations of the glycinium cation, H2PO12 4 anion and hydrogen bonds are predicted and compared to the experimentally determined. q 2004 Elsevier B.V. All rights reserved. Keywords: Monoglycine dihydrogenphosphate; Glycinium dihydrogenphosphate; Polarised reflection IR; Polarised Raman; Glycine

1. Introduction The monoglycine (glycinium) dihydrogenphosphate (GP) crystal contains glycinium cations and dihydrogenphosphate anions [1]. Glycinium cations appear in many crystals exhibiting phase transitions. In the case of TGS crystals, the ferroelectricity is related to the glycinium cation GI which appears in the trans-form (the NH3 group with respect to the CyO bond) [2]. In almost all other crystals, the glycinium cations appear in the cis-form. According to our best knowledge, beside the TGS type crystals, the glycinium cation appears in the trans-form only in the case of the title crystal. In the case of glycinium hydrogenphosphite (GPI) crystal, the inter-phosphite strong hydrogen bonds play the main role in the molecular mechanism of the ferroelectric phase transition [3,4]. Due to this its Tc temperature shifts to higher temperature by ca. 100 K on NHþ 3 and OH groups deuteration. The role of glycinium cation in the ferroelectric phase transition in the GPI [3,4] is not fully understood yet. Beside a very broad interest in the glycine compounds crystals, the vibrational properties of the glycinium cations are not fully understood either. The normal coordinate analysis for the glycinium cation were recently published by Williams et al. [5], * Corresponding author. Tel.: þ 48-71-343-5020; fax: þ48-71-441029. E-mail address: [email protected] (J. Baran). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.03.067

Rosado et al. [6] and Chakraborty et al. [7]. In all these papers, the wave numbers of the bands observed in the spectra of monoglycine hydrochloride (GHCL) were taken into consideration [8]. Williams et al. [5] tried to find the difference between the trans-form and cis-form of glycinium cations involved in a complex with four water molecules. Theoretical results for both these cases appear to be similar. Although, for some of the theoretical modes slightly different wave numbers (not more than ^ 20 cm21) were computed, nevertheless, it was impossible to decide which form appeared in the aqueous solution. In this paper, the polarised Raman spectra (full set) and spectra of imaginary parts of the refractive indices computed from the polarised specular reflection spectra (full set) are presented for the title crystal. The internal vibrations of the glycinium cation, H2PO12 4 and vibrations of hydrogen bonds are discussed taking into account the polarisation properties of the bands, X-ray crystal structure and recently proposed assignments based on the normal co-ordinate analysis.

2. Experimental The single crystals of the GP were obtained by evaporation of the aqueous solution containing glycine and orthophosphoric acid in 1:1 molar ratio. The IR spectra

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Fig. 1. Projection of the structure of the glycinium dihydrogen phosphate crystal onto (010) plane. The optical main directions are denoted as X and Z: The angle between X and the [101] crystallographic direction is equal to 88. The angle between Z and c directions is equal to 238. The orientations of the electric vector for measured polarised reflection spectra are indicated (30, 60, 90 (X), 120 and 1508).

were measured by Bruker IFS-88 spectrometer with resolution of 2 cm21 and very weak apodization. The powder spectra were taken in Nujol and Fluorolube. The specular reflection spectra were measured using Bruker A510/X attachment with the incident angle close to 108. The specular reflection spectra were measured for the (bc) and (ac) planes. The electric vector was perpendicular to the incidence plane and parallel either to the YðbÞ or X and Z axes, respectively. One has to note that the X and Z axes are not parallel to the a and c crystallographic axes. The X and Z axes denote the optical main directions for the visible (yellow) light. Their orientations with respect to the a and c crystallographic directions are shown in Fig. 1. The X axis is very close to the [101] direction (88 towards the a axis) whereas the Z axis forms 238 angle to the c axis. For the (010) sample, the reflectance spectra were measured for the electric vector oriented parallel to Z and X axes and also at 30, 60, 120 and 1508 to the Z axis. The Au wire grid polarisers on the AgBr (4000 – 400 cm21) or polyethylene (500 –80 cm21) substrates were used. The reflection spectra were calculated with respect to the Aluminium coated mirror. The Opus software was used for the Kramers-Kronig transformation (KKT). The reflection spectra were extrapolated to value 0.025 at high wavenumber limit and to 0.15 at low frequency. The Raman spectra were measured with the Jobin-Yvon Ramanor U1000 spectrophotometer equipped with the CCD (4000 – 180 cm21) and photo multiplier (300 – 5 cm21) detectors applying 514 nm line of the Spectra Physics Arþ

laser (power of ca. 100 mW at sample). Acquisition time for the CCD was set for 10 s. The time constant for PHM was set to 0.5 s. The powder Raman spectra were measured by the FT-method (FRA-106 attachment to the Bruker IFS-88 spectrometer). The single crystal spectra were measured by both methods. The back scattering geometry was applied for the FT-Raman spectra measurement and 908 geometry for the classical Raman measurement. The resolution for the Raman experiments was set up to 2 cm21. It was noticed that the measured Raman spectra depended only on the Raman polarizability components, and did not depend on the geometry of the experiments. Therefore, only classical Raman spectra will be discussed in this work. The DSC measurement was performed with the Perkin– Elmer DSC-7 calorimeter equipped with CCA-7 low temperature attachment. No phase transition was noticed in the temperature region between ca. 110 and 450 K.

3. Crystal structure and selection rules The title crystal belongs to the centrosymmetric P21/c space group of the monoclinic system, Z ¼ 4 [1]. The crystal is built of the H2PO21 4 group layers, parallel to the bc plane, alternating with layers of the glycinium cations (Fig. 1). The C –OH group of the glycinium cations (transform) is involved in the hydrogen bonds to the O(4) atoms of the dihydrogenphophate ions with a relatively short ˚ ). The –NHþ O· · ·O distance (2.568(2) A 3 group is involved

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

in three hydrogen bonds; in two (N –H(2n)· · ·O(3) and N – H(1n)· · ·O(5)) with the oxygen atoms of the H2PO21 4 anions and in the third one (N – H(3n)· · ·O(5)) to oxygen atom of the carbonyl group of the neighbouring glycinium cation related by the two-fold screw operation along the YðbÞ axis. The glycinium layer is built of the ribbons being parallel to the ab plane, in which the adjacent glycinium cations are joined through the N –H(3n)· · ·O(5)) hydrogen bonds. There are no strong hydrogen bond interactions between the adjacent ribbons. The glycinium cations in the adjacent ribbons are related by the c-glide plane and the centres of inversion. The H2PO2 4 ions in one layer are related to each other by the two-fold screw operations, c-glide plane and centres of inversion. The anions related by the two-fold screw operations form chains parallel to the YðbÞ axis, in which the adjacent anions are joined by the O(2) – H(2)· · ·O(4i) hydrogen bonds. The c-glide planes transform the anions of one chain into adjacent chains. The adjacent chains are joined within plane by the O(1) –H(1)· · ·O(4) hydrogen bonds. Beside the c-glide planes, the anions of adjacent chains are also related by the centres of inversion. The glycinium cations and the H2PO2 4 anions occupy sites of C1 symmetry. The formal classification of the fundamental modes ðk ¼ 0Þ predicts 168 internal ((27 þ 15)*(Ag þ Bg þ Au þ Bu)), 24 librational (6Ag þ 6Bg þ 6 Au þ 6Bu) and 21 translational (6Ag þ 6Bg þ 5Au þ 4Bu)) modes. As follows from the correlation diagram, each internal mode of the glycinium and H2PO2 4 ions should be split into four components, two of which ðAg ðxx; yy; zz; xzÞ þ Bg ðyz; xyÞÞ are Raman allowed and the other two ðAu ðYÞ þ Bu ðX; ZÞÞ are only IR allowed. The bands observed between ca. 3600 and 350 cm21 arise from the internal modes of the glycinium cations and internal vibrations of the H2PO12 4 anions. The N – H and O –H groups vibrations are in fact the vibrations of the hydrogen bonds. The bands observed below 350 cm21 arise from the deformation vibrations of the glycinium skeleton and from the librational modes and the translational modes of the H 2PO12 anions and 4 glycinium cations. The relative intensity of the bands in the polarised IR spectra may be predicted on the basis of the diffraction data in the oriented gas modes approximation [9,10]. Thus, one should know the orientation of the transition dipole moment (tdm) for each internal mode with respect to the experimental X; Y; Z system. In the case of polarised Raman spectra, the intensity of the bands is determined by (1) the values of the Raman polarizability tensor and (2) by the orientation of the tensor main axes (for each internal mode) with respect to the experimental XYZ system. The components of the Raman polarizability tensor ðaXYZ Þ of the crystal (in the experimental system XYZ) may be expressed by the Raman polarizability tensor of the molecule ðaxyz Þ by the following formula: aXYZ ¼ faxyz fT as described by Turrell [10, page 164], where f is directional-cosine matrice. Unfortunately, the values of the Raman molecular tensor components are

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not known. Their values ( ¼ 0 or – 0) for a particular internal mode may be predicted assuming some local symmetry (symmetry co-ordinate) for the molecule or its functional group. The symmetry co-ordinates for the glycinium cations can be found in the paper of Williams et al. [5]. For each functional group built of three atoms, we defined the rectangular co-ordinate system ðxyz ¼ wvuÞ defined by the unit vectors parallel to the sum (zðuÞ )the symmetric stretching; nsAB2), difference (yðvÞ )the asymmetric stretching; naAB2) or cross product (xðwÞ )the wagging; vAB2) of the unit vectors parallel to two chemical bonds (A – B) sharing the same atom (A). These data are listed in the Tables for each vibration discussed.

4. Discussion of the results The powder IR and Raman spectra of the GP crystal are shown in Fig. 2. The spectra of the imaginary (kappa ¼ Im(N)) part of the refractive indices are shown in Figs. 3, 4 and 7. The polarised Raman spectra are shown in Figs. 5 and 8. The wavenumbers of the bands, their intensity and proposed assignment are listed in Tables 1 and 2 for IR and Raman, respectively. The bands observed between 3600 and ca. 350 cm21 arise from the internal modes of the glycinium cations, internal modes of the H2PO12 4 anions and vibrations of the O – H· · ·O and N – H· · ·O hydrogen bonds. The bands observed below 350 cm21 arise from the deformation vibrations of the glycinium skeleton, from librational and translational modes of the dihydrogen orthophosphate anions and glycinium cations. 4.1. The internal vibrations of the glycinium cations The internal vibrations of the glycinium cation may be partitioned into those arising from functional groups (NHþ 3, CH2, COOH) and from skeleton (CCN) vibrations [5,8]. Theoretical analysis of the internal vibrations of the glycinium cation was a subject of many papers (Williams et al. [5], Rosado et al. [6] and Chakraborty et al. [7]). However, the proposed assignment is not consistent for many vibrations, particularly those giving rise to the bands in the 1500–1100 cm21 region. The bands observed in the region between ca. 2600 and 3400 cm21 arise from the stretching vibrations of the X–H groups. As all N–H bonds and O–H bonds participate in hydrogen bonds, therefore their stretching bands should be shifted to lower wave numbers, their intensity (increase in IR and decrease in Raman), shape and width should be changed. Usually for such N–H· · ·O and O–H· · ·O hydrogen bonds as those in the title crystal, the stretching vibration bands should be asymmetric from the low wavenumber side and additional structure appears (bands of overtones, whose intensity is increased due to Fermi resonance, and Evans windows).

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Fig. 2. Powder infrared (upper) and FT-Raman (lower) spectra of the GP crystal. Note that only some bands are marked. All bands are listed in Tables 1 and 2.

4.1.1. The vibrations of the CH2 group It is usually expected that the internal vibrations of the CH2 group are best separated from the vibrations of the other part of the glycine molecule. This is particularly expected for the stretching CH2 vibrations. The bands arising from these vibrations are best observed in the Raman spectra. In the powder FT-Raman spectrum, two sharp bands appear at 3023 and 2980 cm21, which formally are assigned to the naCH2 and nsCH2 modes, respectively.

One may note that a separation between these two bands (43 cm21) is smaller than that (61 cm21) in the case of monoglycinium nitrate (MGN) [16]. It is slightly larger than that (31 cm21) observed in the FT-Raman spectrum of monoglycine hydrochloride (3000 and 2969 cm21). The structure of the CH2 group is approximately described by the C2v point group [14, page 329]. The nsCH2 should be polarised along the Z axis in the IR polarised spectra of the GP crystal. The naCH2 is expected

Fig. 3. Spectra of the imaginary part of the refractive indices calculated by the Kramers-Kronig transformation from the polarised IR reflection spectra measured for the (100) sample of the GP crystal. Upper-electric vector parallel to the monoclinic axis YðbÞ: Lower-electric vector parallel to the c crystallographic direction. Note that only some bands are marked. All bands are listed in Table 1.

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Fig. 4. Spectra of the imaginary part of the refractive indices calculated by the Kramers-Kronig transformation from the polarised reflectance IR spectra (4000– 380 cm21) measured for the (010) sample. The orientation of the electric vector with respect to the Z main optical direction for each spectrum is indicated in the figure. Note that only some bands are marked. All bands are listed in Table 1.

in the IR spectrum polarised along the YðbÞ axis and also as a very weak band in the spectrum polarised along the X axis (Table 3). In fact, the nsCH2 band at 2979 ^ 1 cm21 shows the highest intensity in the spectrum polarised kZ: However, the naCH2 mode is observed not only in the spectrum

polarised along the YðbÞ axis (3023 cm21) but also in the spectra polarised between X and Z directions, being the strongest for polarisation 1208 to the Z axis (i.e. 308 to X axis). The stretching vibrations of the CH2 group give well structured and separate bands in the polarised

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Fig. 5. Polarised Raman spectra (3400–180 cm21) of the GP single crystal measured with the CCD detector. Porto notation and expansion coefficient (Ex) of an intensity scale is given for each spectrum in the figure. Note that only some bands are marked. All bands are listed in Table 2.

Raman spectra. The nsCH2 bands (2978 cm21) appear in the YðxxÞZ; ZðyyÞX and YðzzÞX spectra with a very strong intensity; being the strongest in the ZðyyÞX spectrum and the weakest in the YðxxÞZ spectrum. It shows a very weak intensity in the Raman spectra with off-diagonal polarizability components. Assuming that the Raman tensor for

this symmetric mode has a diagonal form, with the ðuuÞ and ðvvÞ polarizability components of similar value, one expects the slightly higher intensity of the nsCH2 band in the spectrum YðzzÞX (0.93) than that in the ZðyyÞX (0.76) spectrum. This disagreement between theory and experiment may be due to involvement of the C –H(1) bond in

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Table 1 Wavenumbers (cm21) and the relative absorbance (ABS*) of the bands observed in the spectra of imaginary part of refractive indices calculated by the KKT procedure from the polarised specular reflectance spectra of the glycine dihydrogenphosphate (GP) single crystal at room temperature Sample ac (Bu)

Sample bc EkYðbÞ (Au)

Ekc (Bu)

EkZ

E at 308

E at 608

EkX

n

ABS

n

ABS

n

ABS

n

ABS

n

n

3162 3123 3066

0.69 1.24 0.65

3163 3122

0.35 0.48

3164 3108 3070

0.41 0.64 0.75

3155

0.33

3023 2973

0.65 0.52

3027 2981

0.64 0.73

3024 2980

0.80 0.89

3026 2980

0.72 0.81

2929

0.61

2930

0.66

2933 2898

0.66 0.63

2840

0.60

2931 2898 2838

0.67 0.63 0.61

2842

0.60

2806 2754 2716 2656

0.55 0.68 0.53 0.65

2751

2524 2495

0.48 0.50

2752

0.57

2750

0.70

2662

0.51

2658

0.57

2550

0.34

2567 2539

0.29 0.36

2447

2317

0.32 2405

0.54

2340

0.46

0.38

2146 2065

0.24 0.24

2146 2057 2001

1951

0.18

1919 1812 1734

0.17 0.18 0.30

1924

1694 1634 1623 1581 1536 1437 1419 1372 1320 1285 1250

0.24 0.24 0.20

2401 2339 2285 2140

0.53 0.42 0.35 0.30

ABS

3074 3046 3034 2981 2961 2926

0.40 0.44 0.43 0.56 0.57 0.69

2841

0.72

2656 2568 2533

ABS

E at 1508

n

ABS

n

ABS

3116

0.47

3158 3117 3074

0.42 0.64 0.74

3250sh** 3160vs 3122vs 3068vs

nNHþ 3 nNHþ 3 nNHþ 3 nNHþ 3

3025 2978

0.70 0.78

3021vs 2976vs

naCH2 nsCH2

0.67 0.61 0.48 0.45 0.57

2927vs

nOH(A) nOH(A) nOH(A)

3077

0.38

3078

0.55

3020

0.32

3023

0.43

2957 2927

0.63 0.74

0.60

2840

0.54

2963 2930 2896 2841

0.63 0.67 0.56 0.43

2751

0.63

2759

0.53

2763

0.48

2931 2894 2851 2817 2761

0.61

2663

0.47

2661

0.41

2657 2625

0.44 0.33

2656 2615

0.58 0.45

2657vs

0.33 0.41

2544

0.27

2543

0.28

2528

0.35

2524

0.48

2545m

2498 2453

0.37 0.36

2494 2464

0.50 0.48

2393

0.34

2400

0.43

2275 2233

0.27 0.25

0.29 0.27 0.29 0.40

2440 2401

0.52 0.56

2341

0.48

2282

0.40

2142 2056

0.29 0.31

2438 2406 2386 2341 2275 2150 2065

0.43 0.49 0.48 0.44 0.35 0.22

2446 2392

0.37 0.40

2338

0.38

2270

0.33

2145 2072

0.25 0.27

2069

0.34

2275 2237 2136 2052

0.28

1957

0.30

1952

0.48

1955

0.55

0.15

1922

0.26

1914

0.33

1909

0.53

1908 1809

0.60 0.43

1734

0.44

1734

0.25

1734

0.42

1735

0.59

1737 1707

0.62 0.74

1750 1717

0.42 0.37

0.47 0.45 0.23 0.15

1695 1636

0.81 0.80

1690 1636 1623

0.52 0.93 0.39

1693 1636 1533

0.78 0.85 0.55

1699 1636 1531

0.98 0.41 1.44

1636 1620

0.38 0.26

0.30 0.15 0.12 0.16 0.22 0.34 0.64

1534 1451 1416 1377 1320 1277

0.63 0.22 1.15 0.24 0.35 2.18

0.25 1.16 0.25 0.37 2.13

1418

0.78

1376 1321 1280

0.27 0.28 1.59

1.01 0.85

0.24 0.27 1.11 0.19 0.38 1.90 0.54 0.60 1.41 0.92

1451 1416 1375 1321 1277

1148 1139

1534 1450 1416 1376 1321 1277 1251 1213 1147 1140

1219 1147 1139

0.22 1.14 0.92

1215 1150 1140

0.43 0.50 0.49

1530 1448 1419 1376 1322 1287 1246 1202 1150

1.42 0.17 0.21 0.17 0.19 0.37 0.36 1.29 0.85

1107

0.56

1103

0.20

1086 1066

1.64 1.51

1090 1062

0.98 1.59

1055

1.62

1087 1050

1.52 0.92

1.46 1057

1.64

2762vs

2334 s 2276 s

nOH.O (B) O(2)H(2)· · ·O(4)

2142m 2061m

0.19

1980

1081

2839vs

2444 s

0.38 0.43 0.43

0.48

Assignment

E at 1208

2052 1969 1917

1135

Powder

1529

1.80

1419 1376 1282 1250 1206

0.30 0.28 0.62 0.36 1.12

1140

0.37

1088 1060

0.70 1.28

1636 1619 1576 1561 1532 1449 1417

0.80 0.44 0.35 0.34 0.72 0.29 0.69

1320 1280 1249 1206 1148

0.33 1.16 0.48 1.10 1.34

1085

1.96

1912m

nOH.O (C)

1735m

Overtone

nC ¼ O Def. NHþ 3 Def. NHþ 3 Def. NHþ 3 Def. NHþ 3 1529vs Def. NHþ 3 1450vw dCH2 1415 s dCH2 1375w vCH2 1320w tCH2 1278vs nCOH þ dOH(III) 1251 s dOH (I) 1202 s dOH (II) 1146 s rNHþ 3 1138ssh rNHþ 3 rNHþ 3 rNHþ 3 1080vs naPO2 nsPO2 (continued on next page)

1693 s 1634 s 1621ssh 1584w

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Table 1 (continued) Sample ac (Bu)

Sample bc EkYðbÞ (Au)

Ekc (Bu)

EkZ

n

n

ABS

n

1045

2.44

1048

1026

ABS

E at 608

EkX

E at 1208

E at 1508

ABS

n

ABS

n

ABS

n

ABS

n

ABS

n

ABS

1.20

1044 1033

2.13 0.85

1042 1032

2.74 1.37

1044 1032

1.96 1.31

1031

0.83

1032

0.49

985

1.19

991

0.06

991

0.42 982

1.51 977

2.11

920 898 865 787

0.34 0.18 1.04 0.18

Assignment

1044vs 1029ssh

nsPO2 nCN nsPO2(Au)

976vs 957vs 914vs 899vs 863 s

naP(OH)2(Bu) naP(OH)2(Au) nsP(OH)2 rCH2 nCC gOH(I) gOH(III) gOH(II)

0.44 989

1.16 980

958 915 902 865

1.77 1.10 0.86 0.09

777

0.15

549 537

0.18 0.15

512 496

0.63 0.39

401

E at 308

Powder

0.52

337

0.15

254

0.15

190 173 152

0.22 0.23 0.30

919 899 867

0.12 0.29 0.09

921 899 866 791

0.07 0.15 0.53 0.09

0.09 0.20 0.11

920 899 865

0.29 0.19 0.66

0.32

0.22 0.16

920 899 867

920

899 866

865

1.05

768 700 642

0.25 0.08 0.10

770

0.09

768

0.20

768

0.29

0.20

0.09

643

0.08

0.34 0.13 0.17

769

640

767 699 641

644

0.12

642

0.15

539

0.93

538 531

0.80 0.72

539

0.93

0.31

545

0.16

537

0.45

0.29

518

0.37

0.65 0.55 0.10

539

521

541 532 516

0.74 0.88

0.66

525

0.70

0.85

524 511

523

512

506

1.28

506

1.29

402 371

0.45 0.01

450 403 370

0.14 0.29 0.23

403

0.31

367

0.34

303

0.33

246

0.16

246

0.21

191

0.40 160

0.16

402

203 199 161

0.53

0.23 0.26 0.21

142 117

0.00

107 92

0.00 0.04

72

0.04

2.07

138 126 118 110 98 90 78 70 65

0.20 0.09 0.03 0.02 0.02 0.01 0.12 0.05 0.04

401 358

0.57 0.23

404 132

0.42 0.05

0.10

103

0.00

79 71

0.11 0.02

59

0.02

139 123

0.19 0.14

82

0.19

768m 641vw 540 s

522ssh 510vs 498ssh 403vs 370m 339vw 318vw 241 s 190vs 173 s 153 s 139 s 122 s 115? 103vw 90vw 78vw 71vw

dCOH n4PO4 n4PO4 n4PO4 n4PO4 dCOOH dCOOH n2PO4 n2PO4 dCCN dCCN

Tranl or Libr. Tranl or Libr Tranl or Libr Tranl or Libr Lattice vibrations Lattice vibrations

ABS-it is absorbance at maximum of the bands (kmax * n/100), **relative intensity: vs-very strong, s-strong, m-medium, w-weak, vw-very weak, shshoulder, O(1)–H(1)· · ·O(4k), O(2) –H(2)· · ·O(4k), O(6)–H(3f)· · ·O(3).

weak hydrogen bonds with the O(5) and O(1) oxygen atoms. In the literature, there are data on the ratio ðwwÞ=ðuuÞ ¼ r1 and ðvvÞ=ðwwÞ ¼ r2 for the nsCH2 band in the Raman spectra of a-glycine. These ratios have following values r1 ¼ 20:077 and r2 ¼ 1:40 according to Machida et al. [15] or r1 ¼ 20:1276 and r2 ¼ 0:3533 according to our data (unpublished results). These ratios determined for the title crystal are as follows: r1 ¼ 0:526 and r2 ¼ 1:2052: As follows from the comparison of presented data one notices similar values of r2 determined by the Machida et al. [15] (1.40) and that of determined for the title crystal (1.2052).

However, the r1 values are considerably different (2 0.077 and 0.526, respectively). The band at 1433 cm21 with shoulder at 1423 cm21 in the powder Raman spectrum is due to the scissoring vibration dCH2. Its IR counterparts appear at 1416 and 1450 cm21 (very weak). In the polarised IR spectra, their analogues appear at the same positions being the strongest for polarisation along the Z axis. In the IR spectrum polarised along the YðbÞ axis, two weak bands appear at 1419 and 1437 cm21. Their relative intensity corresponds very well to the predicted ones (108 with respect to Z).

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

135

Table 2 Wave number (cm21) and relative intensity* (Ir) of the bands observed in the polarised and powder Raman spectra of the monoglycine dihydrogenphosphate (GP) single crystal measured at room temperature YðxxÞZ 6161 Dn 3260 3239

ZðyyÞX 9814 Dn

Ir 11 11

3264

3115 3095 3024

18 18

2981 2940

57 21

2762

2666

Dn

Ir 9

17 18 88

2930

15

2855

11

2729

10

9

8

2550 2457

8

2398

7

7

1838 1734

7 14

1693

24

1627

18

3261 3240

YðxzÞX 1579 Dn

Ir 10 11

Ir

3239 3166

30 38

2056

6

106 26

2982

66

2981

31

2982

23

2980

nsCH2

57 53 47

14

2927

17 15 13

2929 2897 2843

2934

2897 2855 2766

2834

nOH(A) nOH(A) nOH(A)

2748

50

2730 2680

13 12

2656

51

2631

11

2275 2146

2549

36

2399

36

9 8

2758

10

2662

9

2653

14

2553

13

33

2054

13

28

1687

13

1633

11

1633

16

1633 1584 1528

5 1501

7

27 46

67 31 10 8

1423 1369 1323

54 51 28

1289

17

1289

6

1289

18

1289

49

7 16 10

67

1139 1100

1033 992

1736 1708

45 32

88 27

1627

29

1533

28

1446

16

1320 1310

59 27

3110

2732 2653 2551 2509 2452

8 7 8

2334

8

2073

8

2010

7

18 19

33

1433 1426 1371 1322

992

14 14

2055

1737

8

33

2750 2730

12

30

1697

992

10

2272

1906

9

5

2843

31

8 8

1732

1069

14

2276

1957 1913

1185 1140 1099

2840

11 11

9 6 5 8

54 27

3023

2981 2941

1433 1426 1369 1322

1033 992

3 4 4 100

19

6

5 10 7

14 17

3023

1733

1184 1139 1101

3161 3119

nNHþ 3 nNHþ 3 nNHþ 3 nNHþ 3 nNHþ 3 naCH2

45 98

7 7 7 7

8 11 10

13

Ir

3081 3024

1433 1423 1369 1321

1187 1142 1101

3263

Dn

Assignment Umax

18 25

7

6 6 6

Ir

FT-R powder

3089 3024

9

2307 2279 2150

Dn

Y(zy)Z 4273

3158 3124 3073 3023

2456 2412

1529

YðxyÞX 2723

21

3022 2998 2981

2652

2053

YðzzÞX 6632

1636

10

1591

8

1447 1435

10 8

1319 1309

8 9

1251

8

2063

1733

Overtone

1697

1531

nCyO nCyO Def.NHþ 3 Def.NHþ 3 Def.NHþ 3 Def.NHþ 3

1432 1424 1370 1320

dCH2 dCH2 dCH2 vCH2 tCH2

1631

nCO þ dOH dOH(I) dOH(II) 1139 rNHþ 3 1100 rNHþ 3 1072 naPO2 nsPO2 1033 nCN 991 nsPO2 (continued on next page) 1289

1136

12

1075

18

1104 1078

14 9

1033 994

18 12

1033 982

8 11

136

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

Table 2 (continued) YðxxÞZ 6161

ZðyyÞX 9814 Dn

YðzzÞX 6632

Dn

Ir

Dn

968 918 912

26 100 81

918 912

36

868

98

649 591

8 6

868 773 648 588

540

11

540

Ir 100

968 918 912

29 29 29

11 5 5 5

868 773 645

100 6 8

868 773 647

34 28 65

8

540 516

9 16

509

8

496 451 417

40 8 10

496

7

418 380

24 10

337

9

132 121

495 449 418 380

7 7 16 8

338

6

253

6

192 176

7 7

517

100

Dn

Y(zy)Z 4273 Ir

919 912

13 12

866 782 650 585 565

38 12 14 15 15

521 512

26 100

8

13 23 33

160 155

20 26

155 140

132

7

Dn

FT-R powder

417 381

921

8

891 867

8 8

566

24

513

12

502

21

57 89 365

44

317

19

190 177

154

85 59

195 171 166

58 37 39

158 137

23

365

12

208 195

13 14

164

52

138

132

Assignment Umax

Ir 967 917

867 775 645 566 540 512

499

10 191

161 155

Ir

16 97 58

55

174

Dn

Ir

YðxyÞX 2723

969 918 913

509

202

YðxzÞX 1579

417 379 364

191 172 162 154 138 132

naP(OH)2 nsP(OH)2 rCH2(?) rCH2 nCC gOH(II) dCOOH

n4PO4 n4PO4 n4PO4 dCOOH dCOOH dCOOH n2PO4 n2PO4 n2PO4 dCCN dCCN dCCN

T or L PO4 T or L PO4 T or L PO4 T or L PO4 Lattice Vibrations

128 119

97

97

86 72 62

84 71 62

120 111

119 111

97 86

97

117 115 103

61

Lattice Vibrations Lattice Vibrations

83 71 62

115 104

83 70 61

61

Lattice Vibrations

Polarised spectra measured with the CCD detector (3600– 180 cm21) and photo multiplier (300– 10 cm21). Powder Raman spectrum measured by the Fourier transformation method. *Ir is given with respect to the strongest band (Ir ¼ 100). The value Umax of the strongest band in each spectrum is listed in the table. This allows to get information on relative intensity of the bands in various Raman spectra.

Assuming that the ðuuÞ Raman polarizability component is larger than the other ones, the Raman band should be the strongest in the YðzzÞX spectrum. In fact the intensity of the 1433 cm21 band (with shoulder at ca. 1423 cm21) is very strong in this spectrum and extremely weak in the other diagonal spectra. The calculated values are r1 ¼ 0:0068 and r2 ¼ 0:378: It is worth to compare these values to those obtained for a-glycine by Machida et al. [15] (r1 ¼ 20:18; r2 ¼ 0:052). We should notice that much more bands are observed in this region than the predicted ones (Ag þ Au þ Bg þ Bu) taking into account the factor group splitting. In this case, either the band at 1433 cm21 or that close to 1423 cm21 in Raman spectra arises from other vibrations

than the dCH2. Very likely, it is the nC – OH mode. It is quite difficult to differentiate them, as both the dCH2 and nC – OH exhibit similar polarisation properties. The band observed in the region close to 1370 cm21 is due to the wagging CH2 vibration. It is observed only in the Ag type Raman spectra. In the IR spectra, it is observed in the spectrum polarised along YðbÞ axis (1372 cm21) and in the spectra polarised in the (010) plane, where it is the strongest in the spectrum polarised along the X axis, what corresponds well to the predicted orientation (1028). The bands at ca. 1322 cm21 should arise from the twisting vibration of the CH2. It appears in all Ag Raman spectra at 1322 cm21 and also in the Bg spectra at 1320 cm21, being

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144 Table 3 Directional cosines of the bonds and transition dipole moments for some simple internal vibrations of the glycinium cation in the GP crystal Bond or mode

˚) Lengths (A

COOH CyO C–OH vCOO dCyO dCOH nsCOOH naCOOH CCN CC CN vCCN ns CCN na CCN dCC dCN CH2 C(1)–H(1) C(1)–H(2) vCH2 ns and dCH2 na and rCH2 dC(1)–H(1) dC(1)–H(2)

125.1(2)8 1.206(3) 1.301(3)

113.3(2)8 1.500(3) 1.478(3)

0.93(2) 0.95(2)

X

Y

Z

Qa

0.8891 20.3434 0.4090 20.2050 0.8455 0.5918 0.6944

20.3949 0.1386 0.9123 0.1078 20.3852 20.2778 20.3008

0.2310 20.9291 20.0150 0.9728 20.3699 20.7567 0.6535

75 21 93 178 134 142 47

0.6310 20.8788 0.3637 20.2250 0.9039 0.6850 0.3093

20.2837 0.3257 0.9302 0.0381 20.3649 20.2326 20.1689

20.7221 20.3493 20.0476 20.9737 20.2232 0.6903 20.9359

139 68 173 13 166 45 162

20.1936 20.3921 0.9182 0.1761 0.3546 0.3456 0.0547

20.8094 0.7346 20.3486 20.0663 0.9348 0.4728 0.5821

0.5546 0.5536 20.1882 0.9821 20.007 0.8105 20.8113

35 161 102 10 90 23 176

The X; YðbÞ and Z denote the experimental coordinate system determined by the main optical directions of the crystal for the visible light. a Q—the angle between the Z axis and the dipole moment projection onto the (010) plane.

the strongest in the XðxyÞZ spectrum, what well corresponds with the prediction. In the polarised IR spectra, it (1320 cm21) is the strongest for the polarisation along the Z axis (Bu spectra) and also in the spectrum polarised along the YðbÞ axis (Au spectrum). Its intensity is too strong in this spectrum, as its tdm should be almost perpendicular to YðbÞ axis. The rocking rCH2 vibration is expected in the IR spectrum polarised along the YðbÞ axis. In fact, two bands at 902 and 915 cm21 are observed therein, one of which may be assigned to this mode. We assign the band observed at 902 cm21 ðEkYðbÞÞ to this mode. Its Bu counterpart (polarised ’ YðbÞ) appears as depolarised band at 899 cm21 with quite low intensity. This corresponds with the expected polarisation for this mode. Such properties correspond with that observed in the polarised IR and Raman spectra of monoglycine nitrate [15]. In the case of the MGN crystal, the rCH 2 vibration appears at 916 ^ 1 cm21. In fact, in the Raman spectra of GP there is also a very strong band at 918 with a shoulder at 912 cm21. Weak Raman bands are usually observed for rCH2 mode. The strong Raman band at 918 cm21 arises from the symmetric stretching vibration of the H2PO12 4 anions. Thus, the shoulder at 912 cm21 is also due to the rocking rCH2 (Ag component).

137

4.1.2. The COOH group vibrations The wave number of the nCyO (1692 ^ 1 cm21) is lower than that observed in the case of MGN (1726 cm21) [11] and in the case of GHCl (1716 cm21) [8] although the ˚ , respectively) CyO distances (1.206(3), 1.193 and 1.204 A are similar in these three crystals. The carbonyl group is not involved in any hydrogen bond in the case of GHCl crystal [21]. ˚ ) [22] In the case of MGN (N –H(2)· · ·O(1 h); 2.890 A ˚ and GP (NH(3n)· · ·O(5); R(N· · ·O) ¼ 2.887(3) A) crystals, the carbonyl oxygen atoms are proton acceptors in similar hydrogen bonds. Therefore, it is clear that the low wave number (1692 ^ 1 cm21) of the n CyO in the title crystal is due to the trans-type structure of the glycinium cation. The tdm of this mode is almost parallel to the CyO direction (Table 3 and Figs. 3 and 4). The r1 ðxx=zzÞ and r2 ðyy=zzÞ ratios for the n CyO in the polarised Raman spectra of the title crystal have the values equal to 0.3652 and 0.5336, respectively. The x is perpendicular to the plane of the COOH, the z is parallel to the CyO bond and the y is perpendicular to the x and z (i.e. perpendicular to the CyO bond). The stretching vibration of the C –OH gives rise to the IR band at ca. 1278 cm21. Its polarisation (at 308 to Z) almost exactly corresponds with the expected one. The Raman counterpart appears at 1289 cm21. It is observed only in the Ag -type ðxx; yy; zz; xzÞ spectra. The difference between IR and Raman wavenumbers is due to the factor group splitting. There is also difference between the Au (1285 cm21, kYðbÞÞ and Bu (ca. 1277 cm21) modes in the IR spectra. Moreover, the wave numbers of the Bu bands (’ YðbÞÞ are changing between 1282 and 1277 cm 21. The nCO H mode is strongly mixed with the in-plane bending mode of the ˚ ). This O(6) – H(3)· · ·O(3f) hydrogen bond (2.569(2) A hydrogen bond is much stronger than its analogue in the ˚ )). ThereMGN crystal (O2) – H(6) – O(5), Roo ¼ 2.648 A fore, the n C – OH appears at lower frequency in the case of MGN (1224 cm21) than that in the title crystal. One of the deformation vibration of the COOH appears at ca. 644 ^ 2 cm21. It is polarised perpendicular to the YðbÞ axis and shows the highest intensity for polarisation kX: It seems to be best described by the mode defined as in plane (COOH) bending mode of the C – OH (dCOH). The position of this band is slightly higher than that (639 cm21) observed in the GHCl crystal, however, considerably lower than that (664 cm21) in the spectra of the MGN crystal [8,16]. In the Raman spectra of the GP crystal, there are bands observed in between 645 and 649 cm21. It is surprising that even in the Ag-type Raman spectra, this mode exhibits different position. In the spectra of MGN (575, 507, 496 cm21) and of GHCl (563, 499 cm21) crystals [8,11,16], there are other bands arising from the deformation vibrations of the COOH group. Their assignments is not uniform [5 –8]. The bands at ca. 570 cm21 are assigned either to the wagging [5,6,8,11] or to much more complex (daCO þ dNCC) mode [7]. In the case of MGN [16], this mode was identified as dCOOH.

138

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

A weak band at 566 cm21 is observed only in the Raman powder spectrum of the GP crystal. No band close to 566 cm21 is observed in the polarised IR spectra, either. In the polarised Raman spectra (Bg ; YðxyÞ; YðzyÞZ), there is a weak band at ca. 568 cm21, and extremely weak bands at 590 (Ag ; ZðyyÞXÞ; 593 (Ag, YðxxÞZ) and at 587 cm21 ðBg ; YðxyÞXÞ: It is difficult to propose any assignment, as in this region the deformation vibration modes of the H2PO4 anion also appear. A shoulder at ca. 496 cm21 observed in the IR and Raman powder spectra also arises from the deformation vibration of the COOH group. The polarisation properties of the last band in the IR (medium intensity band polarised kY) and Raman (weak bands at 498 cm21 in the YðxxÞX; ZðyyÞX and YðzzÞX) spectra suggest that they arise from the Au and Ag type modes, respectively. Very likely its Bu-type mode appears at 508 cm21 in the IR spectra polarised perpendicular to YðbÞ (between 120 and 1508), whereas its Bg mode appears at 504 cm21 in the YðzyÞZ) Raman spectrum. Such polarisation allows to assign it either to the dCOH or to the sCOOH. Similar bands are observed in the spectra of MGN crystal (496 and 508 cm21 in polarised IR and at 499 and 508 cm21 in polarised Raman spectra). 4.1.3. The skeleton CCN vibrations The stretching vibrations of the CCN skeleton of the glycinium cation may be considered either as two independent stretching modes nCC and nCN or as nsCCN or naCCN [5 – 8].

As follows from Table 3, there is essential difference between the tdm orientations for these vibrations. All these modes should give relatively weak bands in the spectra polarised along the YðbÞ axis. The band at ca. 865 cm21 should be assigned to the nCC mode (1398) as it is the strongest in the IR spectrum polarised at ca. 1208 with respect to the Z axis. The nsCCN mode should be polarised at 138 to the Z: The polarised Raman spectra (very strong band at 868 cm21 in the spectra YðxxÞZ and YðzzÞX and very weak one in others) are also in agreement with the simple nCC model. The tmd of the nCN mode should be oriented at 688 to the Z axis. In fact this mode gives rise to the band which is best seen in the spectra polarised between 60 and 908. The strongest intensity of the band at 1033 cm21 in the YðxxÞZ Raman spectrum also well corresponds with the expected polarisation. One should note that a strong intensity of the Raman band at 1033 cm21 indicates a simple nature (nCN) of this mode. 4.1.4. The NHþ 3 group vibrations Bands at 3122, 3068 and a shoulder at ca. 3160 cm21 in the powder IR spectrum arise from the stretching vibrations of the NHþ 3 group. Simultaneous analyse of the polarisation properties for the NHþ 3 stretching bands in the polarised IR and Raman spectra shows that their polarisation properties are not determined by the hydrogen bonds of the NHþ 3 group (see Fig. 1 and Table 4). Assuming that the tdm of

Table 4 The geometry parameters and directional cosines for the N –H· · ·O type hydrogen bonds in the GP crystal Bond or mode

˚) Distance (A (Angle (deg))

N –H(3n)· · ·O(5) NH(3n) N· · ·O(5) H(3n)· · ·O(5) gNH dNH ( ’ .· · ·O) dNH ( ’ N· · ·O) N –(H2n)· · ·O(4) NH(2n) N· · ·O(4) H(2n)· · ·O(4) gNH dNH ( ’ H· · ·O) dNH ( ’ N· · ·O) dNH ( ’ NH) N –H(1n)· · ·O(3) NH(1n) N· · ·O(3) H(1)· · ·O(3) gNH dNH ( ’ H· · ·O) dNH ( ’ N· · ·O) dNH ( ’ NH)

164(3) 0.91(4) 2.877(3) 1.97(4)

a

170(3) 0.93(4) 2.859(3) 1.94(4)

178(3) 0.94(4) 2.900(3) 1.96(4)

Y

Z

Qa

0.1322 0.3073 0.3839 0.3979 0.8333 0.8645

0.8744 0.8738 0.8620 20.4780 20.1687 20.0903

20.4667 20.3772 20.3310 20.7830 0.5265 0.4945

164 141 131 153 58 60

20.6201 20.6845

0.0705 0.1367

0.7814 0.7160

142 134

20.4987

20.8045

20.3230

57

0.5319 0.6058

20.5782 20.5899

0.6187 0.5338

49

20.3766 20.3974

20.5982 20.5871

20.7076 20.7056

28 29

0.2775

0.6558

20.7021

158

0.8747 0.8839

20.4747 20.4607

20.0977 20.0809

96 95

X

The X; YðbÞ and Z denote the experimental coordinate system determined by the main optical directions of the crystal for the visible light. Q—the angle between the Z axis and the dipole moment projection onto the (010) plane.

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

the stretching N –H· · ·O bonds are parallel either to N – H, N· · ·O or to the H· · ·O directions, one may notice that these directions are very similar for all three hydrogen bonds due to the fact that their NHO angles are higher than 1608. As these three hydrogen bonds are very similar, therefore their stretching (and deformation) vibrations will be coupled and could be described as symmetric and asymmetric stretching vibrations. However, we cannot predict the orientation of the tdms (or orientation of the main axis of the Raman polarizability tensor) for such modes. The factor group splitting should also be taken into account in the analysis. Note that the wave numbers of the IR and Raman bands are not identical. It is also worth to mention that the wave numbers of the Ag type Raman bands arising from the same vibration show considerable differences. It should be also mentioned that it is possible to propose some assignment of the bands separately for IR and for Raman taking into account only hydrogen bonds. However, such assignments are not consistent. Four bands at 1634, 1621(sh), 1582(sh) and 1529 cm21 observed in the powder IR and two bands at 1632 and 1532 cm21 in the powder Raman spectrum arise from the deformation vibrations of the NHþ 3 group. Usually, the low frequency band is considered as symmetric deformation ssNHþ 3 , whereas the high frequency bands as asymmetric deformation vibrations, saNHþ 3 . In that case, it is obvious that the former one should be stronger in IR than the latter one. It is clearly seen that this is not obeyed in the spectra of the title crystal. The tdm of ssNH3 can be almost parallel to the CN bond. This assumption is fulfilled quite well. The polarisation properties of the ssNH3 band and band of the nCN (1032 cm21) are similar. Note that in the IR spectra of the (010) sample the polarisations properties of the ssNH3 (1529, kX) are perpendicular to those of the saNH3 (1636, 1620 cm21; kZ). It is rather difficult to discuss the rocking vibrations of the NH þ 3 group as in the expected region (1140 – 1120 cm21) the stretching vibrations of the H2PO12 4 anions may appear as well. 4.2. The OH· · ·O hydrogen bonds vibrations The strong and broad absorption, with complicated additional structure observed in the region ca. 2900 –1800 cm21 arises from the stretching vibrations of the O – H· · ·O hydrogen bonds. The N –H· · ·O hydrogen bonds give rise to the strong absorption observed above 3000 cm21. However, the nNH absorption of the NH· · ·O hydrogen bond is characterised also by some additional bands (overtones) observed below 3000 cm21. These bands are superimposed with the broad bands arising from the stretching OH vibrations of the O –H· · ·O hydrogen bonds present in the crystal. As the O· · ·O distances of the hydrogen bonds in the title crystal are in the range ˚ , they should give an absorption with the 2.568 –2.599 A characteristic ABC structure, where the C band (at ca.

139

1800 ^ 100 cm21) should have the lowest intensity [12]. The A band is expected in the region between 3000 and 2600 cm21, whereas the B band should appear at ca. 2300 ^ 100 cm21. The intensity of the A and B bands may be comparable. The polarised spectra allow us to get some information about stretching vibrations of each O – H· · ·O HBs. This follows from the fact that three O – H· · ·O HBs present in the title crystal are almost perpendicular to each other. The geometry parameters and their directional cosines are listed in Table 5. Thus, the weakest O(2) – ˚ ) is directed along the Y H(2)· · ·O(4k) (Roo ¼ 2.596(3) A axis, whereas the other two almost identical hydrogen bonds are nearly perpendicular to the YðbÞ axis. As follows from Table 5 and Fig. 1 the O(6)· · ·O(3f) direction makes an angle 1368 to the Z axis, whereas the O(1)· · ·O(4) direction makes an angle of 458 to the Z axis in the projection onto (010) plane. Thus, in the spectrum polarised along YðbÞ axis, the broad absorption with centre at ca. 2900 (A) and the broad band at ca. 2300 (B) arise from the stretching vibration of the weakest hydrogen bond. The A (ca. 2850 cm21) and B (ca. 2400 cm21) bands, the best observed in the IR spectrum polarised at 608 to Z axis (or parallel to the c axis for the sample (100)) arise from the nOH of the second interphosphate hydrogen bonds (O(1) –H(1)· · ·O(4k)). The third hydrogen bond O(6) – H(3)· · ·O(3f); although very similar to (O(1)– H(1)· · ·O(4k)) exhibits other type of absorption. Thus, the C band at ca. 1900 cm21 is present (see the spectrum polarised at 1508 to Z) for nOH of this hydrogen bond. Very likely there is same interaction between this band and the nCyO; clearly seen in the spectrum polarised at 1208. The bands at 768 cm21 (IR) and at 775 cm21 (R) in the powder spectra are due to the out-of-plane bending modes gOH of the O –H· · ·O hydrogen bonds. However, in the polarised IR spectra more bands appear in this region, namely at: 767 ðkXÞ; 777 ðkYÞ and 793 cm21 ðkZÞ: The band at 793 cm21 ðkZÞ arises from the gOH of the O(1) – ˚ ) hydrogen bond. The H(1)· · ·O(4k) (Roo ¼ 2.568(3) A ˚ ) hydrogen bond O(2) – H(2)· · ·O(4k) (Roo ¼ 2.596(3) A gives gOH band at frequency of ca. 768 cm21. Its intensity is the strongest in the spectra polarised in between 30 and 90 ðkXÞ direction. The band at 777 cm21 polarised along the YðbÞ axis arises from the gOH of the (O(6) – ˚ hydrogen bond. Such a H(3)· · ·O(3); Roo ¼ 2.569(3) A low wave number of the last mode is surprising. In the case of the MGN crystal the gOH of the O – H· · ·O hydrogen ˚ , OHO angle 1478) appears at bond (Roo ¼ 2.644 A 895 cm21. In the powder IR spectrum of the title crystal, there is a band at 899 cm21. However, its behaviour on cooling down (Fig. 6) does not support such assignment. On the other hand, the low temperature behaviour of the band at 768 cm21 uniquely supports proposed assignment. Thus, already at 250 K, one may notice two shoulders at 782 and 795 cm21 on the high frequency side of the 770 cm21 band. Three well shape bands at 803, 789

140

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

Table 5 The geometry parameters and directional cosines for the O –H· · ·O hydrogen bonds in the GP crystal Bond or mode

˚ ) (Angles (deg)) Distance (A

O(6) –H(3)· · ·O(3f) O(6) –H(30 H(3)· · ·O(3f) O(6)· · ·O(3f) gOH dOH ( ’ H· · ·O) dOH ( ’ O· · ·O) dOH ( ’ OH) O(2) –H(2)· · ·O(4k) O(2) –H(2) H(2)· · ·O(4k) O(2)· · ·O(4k) gOH dOH ( ’ H· · ·O) dOH ( ’ O· · ·O) dOH ( ’ OH) O(1b)H(1)· · ·O(4k) O(1b)–H(1) H(1)· · ·O(4k) O(1)· · ·O(4k) gOH dOH ( ’ H· · ·O) dOH ( ’ O· · ·O) dOH ( ’ OH)

176(4) 1.01(4) 1.56(4) 2.569(2)

174(5) 0.73(5) 1.87(5) 2.596(3)

175(4) 0.87(3) 1.70(4) 2.568(2)

Y

Z

Qa

0.6887 0.6512 0.6665 0.3092 0.6931 0.6785 0.6422

20.2341 20.2673 20.2542 20.7612 0.5909 0.5967 20.2420

20.6863 20.7102 20.7009 0.5701 0.4130 0.4287 0.7273

135 137 136 28 59

20.0007 20.0811 0.0588 20.6097 0.7885 0.7904 0.7926

20.9899 0.9937 20.9938 20.1122 0.0164 20.0126 20.0872

0.1420 20.0777 0.0931 20.7848 20.6148 20.6124 20.6034

0 46 32 38 129 128 127

20.5574 0.7206 20.6982 0.6931

0.5256 0.1339 0.1220 20.1315

20.6428 0.6801 20.7056 20.7088

41 47 45 136

20.1793 20.4572

20.9338 20.8405

0.0072 20.2911

92 58

X

Assignment

777 1278

41

768 1202

793 1250

The X; YðbÞ and Z denote the experimental coordinate system determined by the main optical directions of the crystal for the visible light. Q—the angle between the Z axis and the dipole moment projection onto the (010) plane.

a

and 774 cm21 are observed in the spectrum measured at 12 K. In the Raman spectra, only the bands at 773 cm21 (Ag type symmetry) and at 782 cm 2 1 (Bg, Y(xy)X, very weak) appear.

The in-plane bending dOH mode of the O(1) – H(1)· · ·O(4k) hydrogen bonds gives rise to the band at 1250 cm 21 polarised along YðbÞ axis according to the expectation. The dOH of the second inter phosphate

Fig. 6. Powder IR spectra of the GP in the region between 1350 and 470 cm21 measured at 250 K (upper) and at 12 K (lower).

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

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Table 6 Correlation diagram and proposed assignment of the bands arising from the internal vibrations of the dihydrogenphosphate anion in the GP crystal Group of Site C1

Modes Interchange Ci

Factor C2h

naPO2

nsPO2

na(POH)2

ns(POH)2

1069vw

992vs

968vw

918vs

1075vw

982vw(?)

982vw(?)

918vw

1081(kY)

1024(kY)

958(kY)

915(kY)

1085( ’ Y)

1044( ’ Y)

977 ( ’ Y) 985(? ’ YÞ

920( ’ Y

O(2) –H(2)· · ·O(4k) hydrogen bond appears at 1202 cm21 in the powder IR spectrum, and it is polarised perpendicular to the YðbÞ axis with the highest intensity between 90 ðkXÞ and 1508, thus also very well corresponding to the expected polarisation. Position of this band is dependent on polarisation and changes between 1213 cm21 ðkZÞ and 1202 cm21 (at 1208). It is characteristic that the dOH of the O(2) –H(2)· · ·O(4k) HB is not observed in the Raman spectra at all, whereas for the former (O(1) –H(1)· · ·O(4k)) HB extremely weak band at 1250 cm21 is observed in the YðzyÞZ spectrum. The dOH mode of the O(6) – H(3)· · ·O(3f) HB gives rise to the IR band at 1278 cm21 and it is strongly mixed with the nCOH. For this reason, this band is also observed in the Raman spectra. Similar properties of this mode are observed in the GHCl spectra, where it appears at 1224 cm21 in IR and at 1219 cm21 in Raman spectra. 4.3. Internal vibrations of the dihydrogenphosphate anions group were The stretching vibrations of the H2PO12 4 discussed by Cruiskshank and Robinson [17]. The P –O and P –O(H) stretching modes for H2PO12 4 in solution give rise to the bands at 1072 (nsPO2), 1150 cm 21 (( naPO2), 878 cm21 ((nsP(OH)2) and at 947 cm21 (ns P(OH) 2). These frequencies are considerably different from those (n1 ¼ 980(A1) and n3 ¼ 1082(F2) according to Herzberg [13],] or n1 ¼ 938(A1) and n3 ¼ 1017 (F2) cm21 according to Muller et al. [18]) observed for the isolated PO32 4 ions of the Td symmetry. The deformation modes for the H2PO12 4 anions may appear in the same regions as those observed for 21 the PO32 [13] 4 anions i.e. between 567 [18] and 515 cm 21 for the n4 (F2) and between 420 [18] and 363 cm [13] for the n2 (E). In the region of stretching vibrations of the P – O and P –OH bonds, the wave numbers of the bands in the powder Raman spectrum (917, 967, 991, 1033, 1075sh and 1100 cm21) are different from those (899, 914, 957, 976, 1023, 1044 and 1080 cm21) observed in the powder IR spectrum of the title crystal. The bands at ca. 1100 (rNHþ 3 ), 1033 (nCN) and 899 cm21 (rCH2) arise from the glycinium

cation vibrations. It is clearly seen, that the bands observed in the Raman spectrum are not observed in the IR and viceversa. This observation strongly suggests either a strong Au – Ag splitting or presence of internal vibrations active only in Raman (symmetric type modes) or only in IR (asymmetric type modes) spectra. In the polarised IR spectra additional bands (920, 991, 985, 981, 977, 1050, 1066 cm21 (all polarised ’ Y), and at 1026 cm21 (kY) are observed beside those already observed in the powder IR spectrum. The strongest Raman bands at 918 cm21 and at 992 cm21 arise from the symmetric stretching vibrations nsP(OH)2 and nsPO2, respectively, corresponding to the Ag type modes of the unit cell group. A very weak band at 991 cm21 ð’ YÞ appears also in the polarised IR spectra. This clearly follows from the correlation diagram shown in Table 6. As the lengths of the POH bonds (1.559(2) and ˚ ) are almost identical therefore, one may expect 1.556(2) A a very strong (effective) in-phase and out-of-phase couplings between the stretching vibrations of these bonds. In that case, one may predict the orientation of the tdm for these vibrations (Table 7). The polarisation properties of the nsP(OH)2 bands in the IR spectra do not correspond to those expected for the simple nsP(OH)2 mode Table 7 Directional cosines of the bonds and of the transition dipole moments for some simple internal vibrations for the H2PO12 4 ions Bond or mode

˚) Lengthsa (A

X

Y

Z

Qa

P –O(4) P –O(3) v PO2 ns and sPO2 na and rPO2 P –O(1) P –O(2) vPO2 ns and sPO2 na and rPO2

1.510(2) 1.497(2)

20.0698 20.7079 20.5921 20.7101 0.3811 2.0448 0.9229 0.03320 0.7179 20.6117

0.4878 20.06949 0.6817 20.1889 0.7067 0.6861 20.3787 0.6956 0.2513 0.6729

20.8702 0.1271 0.4298 20.6784 20.5959 0.7261 0.0676 20.6370 0.6488 0.4160

5 100 126 46 123 176 86 118 48 126

1.559(2) 1.556(2)

The X; YðbÞ and Z denote the experimental coordinate system determined by the main optical directions of the crystal for the visible light. a Q—the angle between the Z axis and the dipole moment projection onto the (010) plane.

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(Table 7). Theory predicts the highest intensity of this mode in the IR spectrum of the (010) sample polarised at ca. 488 to Z axis, whereas the strongest band appears in the spectrum polarised along the Y axis. In the region of the asymmetric stretching vibration of the POH bonds two bands appear in the IR (at 957 and 976 cm21) and Raman (967 and at 991 cm21) powder spectra. The IR band at 957 cm21 is polarised along the YðbÞ axis, whereas the band at 977 cm21 appears in the spectrum polarised at 1508 to Z axis. It should be mentioned that the position of the band polarised perpendicular to Y axis is either dependent on polarisation and changes between 985 and 977 cm21 or few bands (977, 982 and 985 cm21) appear in the spectra of the (010) sample. The deference between the wavenumbers of the bands in the spectrum polarised along YðbÞ axis (Au, 957 cm21) and those in the spectra of the (010) sample (Bu) may be explained by the factor group splitting. The polarisation dependence of the bands in the spectra of the (010) sample may be due either to the Fano effect [19] or really few bands appear therein. Two bands at 968 and 992 cm21 are observed in the YðxxÞZ; YðzzÞX and YðxzÞY polarised Raman spectra (Ag). The 992 cm21 band is also observed in the ZðyyÞX Raman spectrum. Additionally a weak band at 982 cm21 appears in the YðzyÞZ Raman spectrum (Bg). Thus, the polarised Raman spectra suggest a presence of few bands in this region of the IR spectra. The IR bands at 977 cm21 (’ YðbÞ; 1508) and at 958 cm21

(kYðbÞ) may arise from the naP(OH)2 modes. Their polarisation properties correspond to the expected ones. Their Raman counterparts appear at 967 (Ag) and at 982 cm21 (Bg). The very strong bands observed at 1044 cm21 and at 1081 cm21 in the powder IR spectrum arise from the symmetric and asymmetric stretching vibrations of the PO2, respectively. Thus, they appear at lower frequency than those proposed by Cruiskshank et al.[17]. Only a very weak shoulder at 1075 cm21 is observed in this region of the powder Raman spectrum. In this region of the polarised Raman spectra, there are bands at 1075 cm21 ðXðxyÞXÞ and at ca. 1100 cm21. The later band (1100 cm21) may, however, arise from the rocking mode of the NHþ 3 . The strong Raman band at 992 cm21 (YðxxÞZ; ZðyyÞX; YðzzÞX; YðxzÞX) arises from the symmetric stretching mode nsPO2. Such a big splitting between the IR and Raman spectra can be explained on the basis of the correlation diagram C1 ! Ci ! C2h, in which the coupling between the vibrations of the neighbouring anions related by the inversion symmetry operation in the layer is the most important. This approach well explains both the Raman and IR polarised spectra in the region of the nsPO2 and also in the region of the naPO2, for which the Raman bands are very weak. However, a very weak IR band observed at 991 cm21 in the spectra polarised ’ YðbÞ (E at 90(kX) 2 180 (kZ)) appears unexpectedly.

Fig. 7. Spectra of the imaginary part of the refractive indices calculated by the Kramers-Kronig transformation from the polarised reflection FIR spectra (450– 80 cm21) measured for the (100) and (010) samples. Orientation of the electric vector is indicated in the figure.

J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

The bands of n4PO4 deformation vibrations of the H2PO12 4 groups in the NaH2PO4·2H2O crystal appear at 518, 528, 521, 535 and 550 cm21 [20]. Very similar frequencies are observed in the powder Raman spectra (566, 540, 512 cm21) and IR spectra (540, 522sh, and 510 cm21) of the title crystal. It is interesting that for the Ag type Raman spectra, additional splitting is observed (511 cm21 in YðxxÞZ and ZðyyÞX and at 518 cm21 in YðzzÞX and YðxzÞX). The n2PO32 bands appear at 376, 386, 396, 414 and 4 426 cm21 in the Raman spectra of NaH2PO4·2H2O [20]. Three bands at 365, 379 and 418 cm21 are observed in the powder Raman spectrum and only two bands at 370 and 403 cm21 in the powder IR spectrum of the title crystal. The 419 and 381 cm21 band are of Ag type symmetry, whereas the band at 366 cm21 is of the Bg type symmetry. In the polarised IR spectra the band at 403 cm21 is depolarised, as it is observed in all measured spectra with almost the same intensity. The second band at 371 cm21 exhibits very weak

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intensity and is observed only in the spectrum polarised along the Z axis. 4.4. The region below 300 cm21—the lattice vibrations The FIR (Fig. 7) and Raman spectra (Fig. 8) of the title crystal and MGN [16] are not similar in the region below 300 cm21. It is obvious as in this region the translational and librational modes of anions appear, which are different in both these crystals. The wavenumbers of the bands in the region below ca. 160 cm21 are quite similar to those observed in the Raman spectra of the NaH2PO4·2H2O [20]. Therefore, we assign these bands to the translational and librational modes of the H2PO12 4 anions. The bands at ca. 250, 173 cm21 arise from the skeleton deformation vibrations of the glycinium cations, as their analogues are also observed in the FIR spectra of the MGN crystal [16]. Comparing the FIR spectra of the title crystal to those of

Fig. 8. Polarised Raman spectra (250–10 cm21) of the GP single crystal measured with the photo multiplier detector. Porto notation and expansion coefficient (Ex) of an intensity scale is given for each spectrum in the figure.

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J. Baran et al. / Journal of Molecular Structure 708 (2004) 127–144

MGN and GHCl, one can notice the lack of the band at ca. 310 cm21, which may arise from the CCNþ deformation vibration. This may be related to different structure of the glycinium cations in the title crystal (trans-form) and in the MGN and GHCl (cis-form) crystals.

Acknowledgements This work was financially supported by the KBN (project No. 7 T09a 014 20). The CCD detector was founded by the Polish Sciences Foundation (Fundacja na Rzecz Nauki Polskiej-Subin 96).

5. Conclusions The knowledge of the crystal structure allows to predict approximate polarisation properties of the bands in polarised IR and Raman spectra on the basis of the oriented gas model approximation for some internal modes of the glycinium cation and for stretching vibrations of the H2PO12 anions. The stretching vibrations of the NHþ 4 3 group are not determined by its hydrogen bonds. This is due to the similar strength of the formed N – H· · ·O hydrogen bonds. The internal vibrations of the CH2 group are separated from the other parts of the molecule. The observed disagreement between the theoretical and experimental intensity of the nsCH2 mode in the polarised Raman spectra may be caused by participation of the C – H(1) bond in two weak hydrogen bonds with the O(5) and O(1) oxygen atoms. The trans-form of the glycinium cation in the title crystal may be characterized by lower wavenumber of the nCyO mode (1694 ^ 2 cm 21) than that (ca. 1720 ^ 10 cm21) observed in the other crystals containing cis-form of glycinium cation. However, this shift may be also caused by the involvement of the carbonyl oxygen atom in the hydrogen bond with the N – H(3) group. Moreover, the polarised IR spectra indicate a strong interaction between the nCyO mode and the C band of the stretching vibration of the C –O(6) –H(3)· · ·O(3) hydrogen bond. The stretching vibrations of the C –C and C –N bonds are not coupled to each other. As the H2PO12 4 anions form the layer in the crystal structure, therefore, a strong dynamic interaction is observed between their stretching vibrations. The strongest interaction appears between the anions related by the inversion centres. The polarised properties of the O –H· · ·O hydrogen bonds vibrations bands are well described assuming that the tdm of the nOH is parallel to the O – H or O· · ·O direction, of the dOH is perpendicular to the O –H or O· · ·O directions and of the gOH is perpendicular to the plane defined by the O – H· · ·O atoms.

References [1] M.T. Averbuch-Pouchot, A. Durif, J.C. Guitel, Acta Crystallogr. C44 (1988) 99– 102. [2] S. Hohino, T. Okaya, R. Pepinsky, Phys. Rev. 104 (1956) 819. [3] M.T. Averbuch-Pouchot, Acta Crystallogr. C49 (1993) 815. [4] J. Baran, K. Łukaszewicz, A. Pietraszko, M. S´ledz´, J. Mol. Struct. 61 (2002) 155. [5] R.W. Williams, V.F. Kalasinsky, A.H. Lowrey, J. Mol. Struct. (Theochem) 281 (1993) 157–171. [6] M.T. Rosado, M. Leonor, T.S. Duarte, R. Fausto, Vib. Spectrosc. 16 (1998) 35– 54. [7] D. Chakraborty, S. Manogaran, J. Phys. Chem. A 101 (1997) 6964–6969. [8] R.K. Khanna, M. Horak, E.R. Lippincott, Spectrochim. Acta 22 (1966) 1759–1771. [9] H. Poulet, J.-P. Mathieu, Spectres de vibration et symetrie des cristaux, Gordon and Breach, Paris-Londres and New York, 1970. [10] G. Turrell, Infrared and Raman Spectra of Crystals, Academic Press, London and New York, 1972. [11] R.K. Khanna, M. Horak, E.R. Lippincott, Spectrochim. Acta 22 (1966) 1801–1811. [12] A. Novak, Structure and Bonding 18 (1974) 177. [13] G. Herzberg, Infarared and Raman Spectra of Polyatomic Molecules, D. VanNostrand Company, Inc., New York, 1945. [14] R.S. Krishnan, P.S. Narayanan, Crystallography and Crystal Perfection; Proceeding of a Symposium held in Madras, 14–18 January, Academic Press, London, New York. (1963) 329. [15] K. Machida, M. Mori, A. Kagayama, J. Raman Spectrosc. 9 (1980) 139. [16] J. Baran, unpublished results. [17] D.W.J. Cruiskshank, E.A. Robinson, Spectrochim. Acta 22 (1966) 555 –563. [18] A. Muller, B. Krebs, J. Mol. Spectrosc. 24 (1967) 180– 197. [19] M.P. Marzocchi, L. Angeloni, S. Sbrana, Chem. Phys. 12 (1976) 349 –354. and papers quoted therein. [20] N. Toupry-Krauzman, H. Poulet, M. Postollec, J. Raman Spectrosc. 8 (1979) 115. [21] A.R. Al.-Karaghouli, F.E. Cole, M.S. Lehmann, C.F. Miskeli, J.J. Verbist, T.F. Koetzle, J. Chem. Phys. 63 (1975) 1360. [22] P. Narayanan, S. Venkataraman, J. Cryst. Mol. Struct. 5 (1975) 15.