Polarised FT-IR and Raman spectra of β-d -fructopyranose single crystals

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of Molecular Structure 326 (1994) 109-122

Polarised FT-IR and Raman spectra of P-D-fructopyranose single crystals J. Barana, H. Ratajczaka, E.T.G. Lutzb3*, N. Verhaeghb, H.J. Luingeb, J.N. van der Maasb aInstitute of Low Temperature and Structure Research. Polish Academy of Sciences, Okolna 2,50-950 Wroclaw, Poland bDepartment of Analytical Molecular Spectrometry, Faculty ojchemistry. Utrecht University, P.O. Box 80083, 3.508 TB Utrecht, Netherlands

Received 1I April 1994

Abstract Polarised FT-IR and Raman spectra have been recorded for P-n-fructopyranose single-crystal samples at room temperature. The assignment of absorption bands to the stretching (Lou) and out-of-plane bending (70~) vibrations of the hydrogen-bonded OH groups is proposed on the basis of the ‘oriented gas’ model approximation. For the very weak H-bond the transition dipole moment is located in the direction of the O-H bond whereas it lies in the 0. . .O direction for the stronger H-bond, 0(2)-H. . . O(1). Different couplings between the vOHvibrations are discussed and proposed based on polarised spectra. Assignment is made of the stretching vibrations of the CHz and CH groups. Other vibrations appear to be mixed and very complex. Only a tentative assignment for the C-O stretching vibrations is proposed.

1. Introduction D-Fructose is a well-known member of the ketose family. It is present in nature and known to be the sweetest of the naturally occurring sugars. Shallenberger and Acree’s hypothesis [l] of sweetness suggests that it is induced by the existence of a suitable AH-B system, where A and B are each electronegative atoms in a particular geometric position and H is an acidic proton. According to this model, a molecule exhibits sweet taste by hydrogen bonding with the complementary AI-I-B system on the receptor site. Thus, it has been suggested [2] that the unit giving rise to sweetness is the anomeric hydroxyl group (the AH moiety) and the atom 0( 1) * Corresponding author.

(the B moiety). This proposal is supported by the lower degree of sweetness of ,8-D-arabinopyranose and 2,6-anhydro-D-mannitol; two structural analogues of /3-n-fructopyranose, which lack the hydroxymethyl and 2-hydroxyl group, respectively. So far P-n-fiuctopyranose is the only isomer of n-fructose isolated in crystalline form. Information about the hydrogen bonds can be obtained from vibrational spectroscopy. Raman spectra of aqueous solutions of n-fructose have been already reported [3-51. The relationship between the frequencies of the stretching modes of hydrogen-bonded OH groups and the O-H ... 0 distances has been studied previously for the title compound by Szarek et al. ]6]. Lutz et al. [7] performed variable low-temperature measurements of the powder FT-IR spectra for samples with

~22-2860/94/~07.~ @ 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-2860(94)08346-J

110

J. Baran et al/J. Mol. Struct. 326 (1994) 109-122

different concentrations of deuterium. These experiments prove the presence of vibrational coupling between neighbouring hydroxyl groups with similar vibrational frequencies. In order to verify the presence of vibrational coupling we decided to measure and analyse polarised vibrational spectra of /3-o-fructopyranose single crystals. This paper reports polarised FT-IR and Raman spectra of the title compound measured at room temperature. We tried to find a correlation between the crystal structure determined by neutron [8] and X-ray [9] diffraction methods and the polarised IR and Raman spectra. The properties of the bands in the polarised spectra are important features, which allow us to verify previous assignments.

Single crystals of /3-o-fiuctopyranose were grown from a solution of water and ethanol. The crystals were oriented under a visible light polarising microscope using the X-ray technique. The lattice constants were matched with literature values [9]. Two plates were cut for IR measurement, one parallel to the ab (AT) plane and one parallel to the bc (YZ) plane. The samples were stuck onto KBr windows with paraffin wax and then polished (using ethanol and water) until IR transmission measurements were possible. FT-IR spectra were measured on a Bruker IFS-88 spectrometer equipped with an AgBr wire-grid polariser and DTGS detector. Scanning parameters were: resolution, 2 cm-‘; number of scans, 32; apodization, weak. Accuracy for the peak positions is 1 cm-’ for sharp peaks and 2 cm-’ for broad bands. Raman spectra were measured with a JobinYvon Ramanor UlOOO spectrometer, with 90” geometry, equipped with a photomultiplier on a cubic shaped sample with edges parallel to the orthorhombic axes. The exciting light was the 514.532nm line of a Spectra-Physics Ar+ laser (Model 2016-5) with a power of approximately 1 W.

(Fig. 1) has been determined by X-ray [9] and neutron diffraction [8] techniques. Different notations for the orthorombic axis are used by the authors of these studies. For uniformity we will use those of Takagi and Jeffrey [8] (a = 9.191(2), b = 10.046(2) and c = 8.095(2)). The crystal belongs to the P2i2i2i space group. The primitive unit cell contains four molecules, Formal analysis of the fundamental modes (k = 0) is given in Table 1. P-DFructopyranose molecules occupy sites with Ci symmetry in the crystal. Therefore, all external and internal modes (66) are allowed in IR and Raman. As follows from the correlation diagram (Table l), each vibration should be split into four components (A + B1 + B2 + Bs) due to the correlation field (Davydov) effect. All are allowed in Raman whereas only the B modes are IR-active. In view of the weak correlation field effect, the frequencies of the bands arising from the same internal vibration but observed at different polarisation directions may be similar. The ratio of their intensities (dichroic ratio) could be determined on the basis of the ‘oriented gas’ model approximation [14]. In this approach the ratio of intensities of the bands in the polarised IR spectra are determined by the square of the directional cosines of the transition dipole moment (TDM) of the vibration of interest. Although the intensity of you bands in Raman spectroscopy is commonly very weak, owing to the polarisability, for crystalline sugars, it can be very informative [15]. From theory it is known that there is a slightly more complex relation between the intensities of the bands in polarised Raman spectra. They are also determined by the directional cosines, albeit in higher power. Since the molecules occupy Ci positions, group theory cannot be used to determine the TDM (axes of the Raman polarisability tensor) orientation of the internal vibrations of the molecule. However, for the well localised (pure) vibrations one may assume a certain direction for the TDM. This will be discussed in the following sections of this paper.

3. Crystal structure and selection rules

4. Results and discussion

2. Experimental

The structure of crystalline /3-n-fructopyranose

FT-IR spectra are shown in Figs. 2 and 3 in the

J. Baran et al.lJ. Mol. Struct. 326 (1994) 109-122

111

(a)

W

structure of orthorombic P-D-fntctopyranoside crystals (P212121)

Fig. 1. Molecular structure of D-fructopyranose (a); crystal structure of ,L-D-fructopyranose (b); hydrogen-bonded chain in the crystal

Bl (c). regions 3700-2000 cm-l and 2000-400 cn--’ respectively. The positions of the band maxima are listed in Table 2. Polarised Raman spectra are shown in Figs. 4 and 5 for the regions 3700-2OOOcrC and

2000-lOcm-’ respectively. The positions of the Raman bands are listed in Table 3. An exploded view of the Ok-stretc~ng region is presented in Figs. 6 (IR) and 7 (Raman). Fig. 8

112

J. Baran et al./J. Mol. Strut.

Table 1 Correlation diagram and classification of the fundamental (k = 0) modes for a ~-o-fru~topyranose crystal (space group P2,2,2, = 0;; z = 4) Site group

Factor group

Cl

4

Modes8

326 (1994) 109-122

\ POWdH deutemted ~-D-fr~clapyranase

Selection rules

T,

T

R Ni IR

3

3

66

2

3

66

B2

0 1 1

B3

1

2 3

3 3

66 Y(= b) 66 X(= a)

Raman Powder

A

A Rt

Z(= c)

XX,YY,ZZ xy

xz yz

a Acoustic, T,; translational, T, librational, R, and internal, Ni. Single crystal sample XY

E II Y

deuterated p-D-fructopyranose

wavenumbers

icrn-l]

Fig. 3. Complementary IR spectra in the region 2000-4#cm-’ of the compounds listed in Fig. 2.

Single crystal sample

XY

shows the IR spectra of the powder sample measured in the region 900-550cm-’ at 300 and 110 K. The intensities of the OH bands in Fig. 7 are scaled, taking into account the laser power. 4.1. Stretching vibrations of the OH groups

Single crystal sample

3oGo wavenumbers

2600

YZ

2200

[cm-‘1

Fig. 2. IR spectra in the region 4000-2000cm-’ of crystalline fi-D-fructopyranose measured as a powder (deuterated and nondeuterated) and single crystals XY and YZ measured at different polarisation directions.

As follows from the diffraction data [8], all OH groups of ~-D-f~ctopyrano~ are involved in hydrogen bonding (see Fig. 1 and Tables 4 and 5 where the structural parameters are listed}. Each of the hydroxyl groups forms an intermolecular hydrogen bond. Except for 04-H, all hydroxyl groups are linked into an infinite chain and appear to be both proton donor and proton acceptor. Some protons, H(1) and H(3), are involved in a bifurcated hydrogen bond. In order to find some correlation between the polarised IR and/or Raman spectra and the crystal structure, we

J. Baran et al.lJ. Mol. Struct. 326 (1994) 109-122 Table 2 Wavenumbers (cm-‘) of the bands in the spectra of powder and single-crystal samples of fl-o-fructopyranose

Tabie 2 (~ntinued)

Powder

Normal

Normal

3524

Crystal Partly de&d.

EllX B3

EIIY B2

EIIY

EIIZ

B2

%

3520

3521

3524

3524

3521 3465

3420

3420

3420

3420

Powder

875 865

3210

3360 3273 3204

3397 3356 3273 3204

3013

3013

3014

3358

3360

3200 3014 2605 2523 2496 2394

1430 1398

1394 1355 1350 1340

1335 1297 1275 1265 1251 1232

1312 1297 1286 1271 1265 1249

1427

1428

1429

1399

1398

1398

1431 1411 1398

1337

1335

1344 1335

1344 1339

1296

1296

1300

1265 1250 1232

1264 1251 1233

1264 1252

1176

1175

1175

1176

1151

1150

1149

1149

1134 1093

1135 1094

1077

1079

1135 1094 1085 1081

1056 1030

1054 1032

1050 1031

978

978

979

981

925

925

926

923

1275 1266

1207 1188 1176 1168 1149 1141 1135 1094 1080 1043 1052 1032 979 960 924

1101 1076 1060 1047 992 979 960 940 927 904

1080 1060

Crystal Partly deutd. 875

EM’ B,

EIIY

EllY

EIIZ

B2

B2

Bl

875 867

875 867

875 867

824

827

875

855 826 819

3411 3403 3360 3270 3230 3195 3013 2990

113

784 687 628 599 574 544 526 469 444 425 408 387 324 311 303 290 280 267 244 235

821 816 804 771 625 585

786

784 682 628 600 580

783 685 629 600

521

540 520

784 691 628 595 570 540 525

469

465

464

469

425 404

425 408

426 411

426 406

628 598 572 540

526 492

428 404 390 321 307

818

293 258 252 239

assume that the stretching vibration of each OH group is independent of those of the remaining OH groups of the same fructopyranose molecule. This seems to be logical since each OH group participates in a hydrogen bond of different strength. Keeping this in mind, we assume that the TDM of vOH is determined by either the O-H, H +- .O or the 0. * 10 direction (see Tables 4 and 5) and that in those directions the largest component of the Raman pola~sibility tensor appears. Except for the sharp band at about 352Ocn-‘, previously assigned to the stretching vibration of the weakest hydrogen bond O(4)H - - . O(2’) f6,7], the observed bands are broad both in XR (Fig. 2, see also Fig. 6) and in Raman spectra (Fig. 4, see also Fig. 7). The intensities of the bands at about 352Ocm-’ are similar in the IR spectra polarised parallel to the

114

J. Baran et al.lJ. Mol. Strut. 326 (1994) 109-122

81

z (X2) Y

52

z (YZ) x

-_:I s_k _i--

A

xwz

A

Z(WY

A

lfi 3em

1

-’

3200

zeoo

2400

2

wa~enumber~ [cm-‘]

d 2wo

1600

1200 wavenumbers

Fig. 4. Raman spectra of single-crystal samples in the region 4000-2OOOcn-’ measured at different polarisation directions. X, Y and 2 axes. This seems to be in accordance with the assumption that a TDM of the Vou vibration of this hydrogen bond is predominantly oriented along the 0(4)-H bond (Table 4). In the Raman spectra the strongest A-type band appears at the (XX)geometry; compared to the by) and (zz) spectra it is at least eight and nine times stronger (scaling remark!), Note that the position of the IR band polarised along the Y axis (3524cm-i, B2) is at a slightly higher frequency than those in the spectra polarised along the X (Bs) and Z (Bi) axis (3521 cm-i). A similar frequency shift is observed in the Raman spectra, where the band (3526cm-i) in the xz spectrum (B2) appears at higher frequency than those (3521 cm-‘) in the A ((XX), (JJ~)and (zz)) and Bi CJCC) spectra. The band at about 352Ocn-’ is very weak in the Bsbz) spectrum. The observed frequency difference is due to a Davydov type interaction between the

800

400

0

[cm-‘]

Fig. 5. Complementary Raman spectra in the region 2000400 cm-’ of the compounds listed in Fig. 4.

stretching vibrations of the symmetry-equivalent 0(4)-H . . .0(2’) bonds. This interaction is strongest for the O-H groups related by the two-fold screw axis parallel to Y(b). Assuming that for stronger H-bonds the TDM is oriented along the O... 0 rather than along the O-H axis (see Table 4) the stretching vibration of the strongest hydrogen bond at 3204cm-’ in the IR spectra, polarised along the Z and Y axes, should be assigned to the 0(2)-H .. .O( 1’) bond. In the Raman spectra, this band is weak (M 3200 cm-‘). According to the directional cosine values this vibration should not be observed in the (XX)Raman spectrum whereas it should be present in the cvu>and (zz). As it happens it is barely present in the (zz) spectrum, the major contribution being in the by) spectrum. This assignment corresponds with previous literature proposals [6, 71.

115

J. Baran et al./J. Mol. Struct. 326 (1994) 109-122 Table 3 Wavenumbers (cm-‘) of the Raman bands of a fl-n-fructopyranose single crystal Z(xx)Y A

X(z) Y Zcvy)X Z(xz) Y A A R2

3521

3521

3521

z(,Vz)xY(Jx)z

X(yy)Z

B3

BI

A

3521 3514 3425

3521

3526 3425 3407

3400 3394 3350 3015 2992 2962

3400

3397

3193 3015 2990 2960

3190 3015 2991 2962

3016 2991

3402 3397

3350 3190 3015 2991 2962

3360 3016 2992 2962

3190 3016 2991 2962

2940 2934 2925 2902

2941

2941

2925 2902 2892

2925 2904

Table 3 tcontinu~)

Z(xx)Y X(zz)Y Z(yy)X

Z(xz)Y

Z~z)XYdyx)Z X(yy)Z

A

A

A

B2

B3

BI

A

1081 1066

1080

1081 1065 1048

1081 1063

1081 1061

1082 1062 1049

1081 1064 1049

1030 981 927 876 867 826 783

1036 979 926 874 868 820 793

982 926 873 870 820 785

978 927 874

677

699

628

628

630

630

595

594 575

601

595

526 466

526 471

527 471 463 432

527

977 927 874

1048 1035 978 924 874

819

819

819

778

778

779

2924 2903 2890 2196 2731 2690 2609

2390 2150 1960 1470 1453 1435 1413

2940 2934 2924 2902

2780 2670 2593 2508

2941 2924 2903 2898 2856 2780 2733 2660 2604 2520 2484 2390

2933 2922 2902

2512

2782 2727 2650

2734

625 594

521

526

462

462

462

427

427

420

421

530

2732 2660 2526

2523

779

423 420

429

401

1470 1458 1435

1425 1413

1412

1413 1399

1371

1386 1372

1472 1455 143.5

1473 1433

1473 1457 1436

322 297

419

420 394

1370 1346

1403

1399

1371

188 1373

1343

153 138

1334

1333 1324

1220 1177

1243 1221 1178

1145

1145

1145

249 181 152 133 117 108

1325 89

1261 1252 1242 1222 1176

324

1417

1340

1303 1278 1260

428

401

279 249

1349

1324

427

345 1470 1455 143s

1470 1456

463

416

2160

1389

1276 1261 1253

630 625 59s

626 594

2856 2730

819

658

2957 2941

978 926 874

1278 1263

1176 1153 1139 1088

1267 1252 1219 1176

1136 1088

1276 1270 1251 1240 1226 1178

1252 1245 1221 1178

1144

1145

f090

1088

71 58 48

91 78 71 48

272 249 236 183 161

289 272 258 234

287

292 279 254

249 236

249 176

179 160

156 133

131

131

109 93 83

109

103

110

73 58 45 39

73

133

166 138

85

85

59

74 59

44

For the assignment of the three remaining hydrogen bonds, 0(1)-H.V.0(3’)-H.~~0(5’)H. * - 0(2’) (see Fig. 1, Table S), to the appropriate stretching vibrations three band maxima remain

J. Baran ef al.lJ. Mol. Struct. 326 (1994) 109-122

116

A

3500 wavenumbers

1

I

3300

3100

[cm-l]

3 3600

3480

3360

wavenumbers

Fig. 6. Expanded OH-stretching region of the IR spectra (see Fig. 2).

namely 3420, 3397 and 3360 cm-‘. The IR spectral data in the region 3425-3250 cm-’ are obscured by the presence of a very broad low-intensity absorption at 3273 cm-’ . This band appears to be polarised along the Z and Y axis but is absent in the Raman spectra. With reference to previous low-temperature and deuteration experiments [7] and the absence of this band in the OD spectral region, no logical assignment can be presented for this band. It might originate from some local inclusion of ethanol/water impurities in the crystal. Lutz et al. [7] suggested vibrational coupling between the stretching vibrations of 0(3)-H and 0(5)-H on the basis of isotopic (deuteration) changes but they do not exclude coupling between 0(1)-H and 0(3’)-H. In fact the O-H bond lengths for 0(3)-H and 0(5)-H are virtually identical, (0.964(3) and 0.963(3) A respectively). However, the corresponding 0 . . .O distances,

3240

3120

[cm-‘]

Fig. 7. Expanded OH-stretching region of the Raman spectra (see Fig. 4).

2.930(2) and 2.805(2) A, differ considerably. The 0.. .O distances for the hydrogen-bond pairs 0(1)-H and 0(3/)-H are less different (2.930(2) and 2.858(2) A, respectively), whereas the correspondingO O-H bond lengths (0.964(3) and 0.972(3) A, respectively), differ only slightly. Without invoking any inter-hydrogen bond coupling and the orientation of the TDM along the O-H, H+..O or 0.. .O axis, there is no agreement between the polarised spectra and the crystal structure as shown in Table 5. Assuming the orientation of the TDM along the 0 . . .O direction again no agreement is found with the polarised spectral observations (Table 5). In order to explain this discrepancy, one may consider coupling between vibrations of adjacent hydrogen bonds (OH groups) in the chain. The simplest approach considers in-phase (v,) and

J. Baran et al./J. Mol. Struct. 326 (1994) 109-122

-

T=llOK

.---

T=fJOOK

117

3 s 8 f

s

I

I

!

I

I

1

I

700

800

1

wavenumbers

600

5

[cm-l 1.

Fig. 8. IR powder spectra in the region 900-550cm-’

out-of-phase (vaS) coupling between the vibrations of the neighbouring OH groups, 0(3)-H and 0(5)-H, as proposed by Lutz et al. [7] (Model I) and coupling between 0(1)-H and 0(3)-H (Model II). The directional cosines of the TDM for these vibrations and proposed assignments are

measured at 300 and 110 K.

listed in Table 6. It appears that neither Model I nor Model II corresponds fully with the observed polarised spectral intensities. A tentative assignment including disagreements is presented in Table 6. It is difficult (or impossible), however, to find definitive proof that this ‘local’ type of vibrational

Table 4 The square of the directional cosines for the transition dipole moments (TDM) of the vOH stretching vibration of the strongest (0(2)-H. 0( 1’)) and weakest (0(4)-H.. O(2’)) hydrogen bond assuming that the TDM is determined by one of the following directions: O-H, H ‘0 or 0.. ‘0 H-Bond

Bond

Distance (A)

co? (TDM)

Assignment of frequency (cm-‘)

a(x)

b(Y)

c(Z)

IR

Raman

0(2)-H. 0( 1’) Angle 154.6”

O-H H...O o...o

0.979 1.750 2.668

0.222 0.003 0.043

0.322 0.493 0.446

0.456 0.505 0.510

3204 (Y, Z)

3 190 (YJJ,zz, YZ)

0(4)-H. . . O(2’) Angle 159.8”

O-H

0.948

0.503

0.292

0.205

3521 (X,Z) 3524 (Y)

3521 (xx,yy,zz) 3526 (xz) 3514 (Jz)

H...O o...o

2.065 2.972

0.805 0.722

0.158 0.200

0.037 0.078

118

J. Baran et al./J. Mol. Struct. 326 (1994) 109-122

Table 5 The square of the directional cosines for the vOHstretching vibration of three hydrogen bonds for which coupling is possible H-Bond

Bond

Distance (A)

cos2 (TDM)

Assignment of frequency (cn-‘)

a(x)

b(Y)

c(Z)

IRa

Raman

O(l)-H...0(3’) Angle 152”

O-H H...O o...o H...06,

0.972 1.695 2.859

0.791 0.399 0.542 0.014

0.024 0.300 0.184 0.628

0.185 0.301 0.273 0.359

3420 ( Y, A’*)?

3425 (xz, zy)?

0(3)-H.. .0(5’) Angle 169.3”

O-H

0.964

0.091

0.241

0.668

3397 (Z)

3400 (zz, xx, yx) 3397 CVY)

H...O o...o H...06,

1.977 2.930

0.139 0.123 0.067

0.106 0.145 0.902

0.755 0.732 0.031

O-H H...O o...o

0.963 1.869 2.805

0.137

0.290 0.382 0.357

0.573 0.611 0.609

3360 (X, Y*, Z)

3360 (xx, xy, yz)

0(5)-H.. .0(2’) Angle 163.2”

a An asterisk denotes disagreement with prediction.

coupling really occurs. The effect of Davydov (crystal) coupling on peak positions in the polarised IR and Raman spectra can be (easily) recognised when comparing band maxima in different polarisation . .0(2’) with its peak posidirections, viz. 0(4)-H. tion at 3524-3520cm-‘. 4.2. Bending vibrations of the hydroxyl groups The in-plane bending vibrations of the OH groups, bon, are strongly mixed with other internal vibrations of ,0-n-fructopyranose [lo] and most

of the bands in the region 1500-800 cm-’ are sensitive to deuteration (see Fig. 3). Therefore, it is difficult to give a correct assignment of these bands to the appropriate So, modes. The out-of-plane bending vibrations, Ton, can be easily recognised because of their larger bandwidth. The 7or.r bands appear to be more pure which follows from the observed isotopic shift ratio (1.34-l .40). Similar bands have been observed in the spectra of reference compounds like adamantanol-1 (722 and 655cm-t) [ll], 2-phenyladamantanol-2 (600 cm-‘) [ 121 and 2-

Table 6 The square of the directional cosines of the TDM of the vibrations obtained from in-phase (v,) and out-of-phase (v,) coupling between the stretching of 0(3)-H with 0(5)-H (Model I) and 0(3)-H with 0(1)-H (Model II), respectively Assignment of frequency (cm-‘)

cos2 (TDM) 4x) Model I

0(3)-H

va

0.002

Z(l)-H

0.791 0.298

Raman

0.997 0.002 0.185

3397 (Z) 3360 (X, 3420 (X, Y) T*,Z)

= 3400 (zz, yy, xx) 3360 (xz, 3425 (xx) zy)

3420 (A’, rC) 3397 (Z) ?? 3360 (X, Y,Z)

3425 (xz, zy) = 3400 (zz, yy, xx) 3350 (xx)

+ ~2 and Y)

0.001 0700 0.024 and 0(1)-H

IR”

c(Z)

b(Y) and 0(5)-H

Model II

0(3)-H

va

0.459

0.037

+ v2 and v, 0.504

&-H

0.137 0.379

0.290 0.457

0.573 0.164

’ An asterisk denotes disagreement with prediction.

J. Baran et al.lJ. Mol. Strut. 326 (1994) 109-122

119

Table 7 The square of the directional cosines for the bending vibrations of the hydrogen bonds co? (TDM)

Bonds and vibrations

0(1)-H. .0(3’) Out-of-plane In-plane O(2)-H...O(l’) Out-of-plane In-plane 0(3)-H.. O(5’) Out-of-plane In-plane 0(4)-H.. .0(2’) Out-of-plane In-plane 0(5)-H.. .0(2’) Out-of-plane In-plane

Assignment of frequency (cm’)

x(a)

y(b)

Z(c)

IR

Raman

Aa Bb A B

0.205 0.103 0.004 0.106

0.026 0.211 0.950 0.765

0.769 0.687 0.046 0.128

540 (Z)

530 cvr)

A B A B

0.547 0.027 0.231 0.751

0.026 0.484 0.652 0.194

0.426 0.489 0.118 0.055

680 (Z, y)

677 (xz)

A B A B

0.067 0.746 0.841 0.162

0.612 0.049 0.147 0.710

0.321 0.205 0.012 0.127

A B A B

0.409 0.047 0.087 0.449

0.051 0.606 0.658 0.102

0.540 0.347 0.255 0.449

A B A B

0.322 0.496 0.175 0.001

0.003 0.266 0.705 0.442

0.675 0.238 0.121 0.557

<400

< 400

572 (X y,z)

575 fjJz)

a The TDM of the out-of-plane mode is perpendicular to the plane defined by the C-O-H atoms; the TDM of the in-plane bending mode lies in that plane and is perpendicular to the O-H bond. b The TDM of the out-of-plane mode is perpendicular to the plane defined by the O-H ‘0 atoms; the TDM of the in-plane mode lies in that plane and is perpendicular to the O-H bond.

ethynyladamantanol-2 (655 cm-‘) [ 131,with similar hydrogen bonds to the title compound, and 0. . .O distances of 2.749,2.813 and 2.839 A, respectively. The approximate orientation of the TDM for the You modes has been predicted on the basis of two different models: model A with the TDM of the out-of-plane mode perpendicular to the plane defined by the C-O-H atoms and model B with the TDM of the out-of-plane mode perpendicular to the plane defined by the 0-H. . .O atoms (see Table 7). Predictions from model B seem to fit better than those based on model A. However it is recognised that, in agreement with the stretching vibrations, the TDM is dependent on the strength of the hydrogen bond. The bands at about 690 (l/Z) and 685 cm-’ (I/Y) are assigned to You of the shortest (strongest) 0(2)-H.. . O(1) hydrogen bond. Upon cooling to 110 K in the powder spectrum this peak shows

a blue shift to about 710cm-’ and as expected it also becomes sharper (Fig. 8). In the region between 600-500cm-’ a complex broad band is observed with at least two maxima at 571 and 53Ocm-‘. These bands are polarised parallel to X (571 cm-‘) and Z (53Ocm’) and not to Y. For the thicker XY sample, only the lowfrequency band is barely present. Assignment of one of these bands to You of 0(3)-H. . .0(5’) is obvious but the involvement of 0(1)-H . . . 0( 3’) and/or 0(5)-H. . .0(2’) is questionable. Upon cooling, these bands shift to approximately 588 and 566cm-’ (110K) in the powder spectrum (Fig. 8). The You band of the ‘weak’ 0(4)-H.. .0(2’) hydrogen bond is expected at the lowest frequency, beyond the spectral region studied (< 400 cm-‘). An interesting phenomenon can be noticed in the

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J. Baran et al/J. Mol. Strut. 326 (1994) 109-122

Table 8 The square of the directional cosines of the TDM for the C-H vibrations Bondsa

Vibrations

cos’ (TDM)

Assignment of frequency (cm-‘)

x(a)

Y(b)

Z(c)

IR

Raman

C(l)-Hz 1.092(3) 1.092(3)

“a “S w

0.890 0.090 0.020

0.085 0.303 0.612

0.026 0.607 0.348

2990 (_I’,Y) 2932 p

2992 (xx) 2991 (JY, zz, xz, yz) 2934 (zz)

C(6)-Hz 1.095(3) 1.093(3)

vu “9 w

0.445 0.549 0.005

0.550 0.438 0.013

0.005 0.013 0.982

3013 (X,Y,z) 2920 p

3015 (xx,xz,yy,zz,yz) 2924 (xx, YY,xz, YZ)

C(3)-H 1.097(2)

VCH

0.033

0.947

0.020

2939 p

2941 (yy, xx, zz, yx)

C(4)-H 1.097(2)

“CH

0.055

0.913

0.032

2960 p

2962 (JJ~,xx, zz, yz)

C(5)-H 1.101(3)

“CH

0.050

0.095

0.855

2901 p

2902 (zz, xx, yy, xz, yz)

a Lengths in angstroms.

region 780-850cm-’ of the powder IR spectrum (Fig. 8). Two bands at 784 and 819cm-’ (with a shoulder at approximately 826cm-i) appear at room temperature. Upon cooling to 110K new bands at about 805, 798 and 826cm-’ (shoulder) appear. In view of their (relatively) high frequency these bands cannot be assigned to You vibrations but the temperature sensitivity shows that somehow they must be related to a vibration sensitive to (intermolecular) interaction. 4.3. Stretching vibrations of the CH2 and CHgroups The /3-D-fructopyranose molecule contains two CH2 and three CH units. The corresponding stretching vibrations are supposed to be pure and easy to assign. Szarek et al. [6] proposed an assignment inconsistent with the one based on polarisation measurements and presented in Table 8. Taking into account polarisations of IR and Raman bands one has to assign the band at about 3014cm-i to the asymmetric stretching vibration (Vc&) of the C(6)H2 group and that at 2990cm-’ to the same vibration of the C(1)H2 group, consistent with conclusions drawn from the spectra of P-Dfructopyranose and L-sorbose published by Szarek et al. [6]. In these two molecules the only structural difference is the substitution of the OH group at

the C(5) atom, equatorial and axial. Compared with the hydroxymethyl C(1)H2-OH group, a somewhat larger steric influence of this OH group on the adjacent C(6)H2 group is to be expected. This explains the difference in the band maxima of the r+H vibrations of the C(6)H group (3014 and 3006cm-i for fructose and sorbose, respectively) and the similarity between the VoH vibrations of the hydroxymethyl groups (2990 and 2988 cm-‘). In the polarised IR spectra information about absorptions from the stretching vibrations of the CH groups is blocked by the paraffin (Nujol) bands. The Raman spectra are well polarised in this region. The very strong bands at 2962, 2941 and 2902 cm-’ can be assigned to the stretching vibrations of C(4)-H and/or C(3)-H and C(5)-H, respectively. Notice that it is impossible to discriminate between C(4)H and C(3)H as their polarisation properties are almost identical (see Table 8). The bands at 2934 and 2924 cm-’ are assigned to the symmetric stretching vibration of the C(1)H2 and C(6)H2 groups, respectively. 4.4. The C-O stretching vibrations According

to

Wells and

Atalla

[lo]

these

J. Baran et al/J.

121

Mol. Struct. 326 (1994) 109-122

Table 9 The square of the directional cosines of the TDM for the C-O vibrations Bond

w)-w) C(2)-O(2) C(3)-O(3) C(4)-O(4) C(5)-O(5) C(2)-O(6) C(6)-O(6)

Bond length (A)

1.414 1.407 1.420 1.418 1.426 1.413 1.430

Assignment of frequency (cm-‘)

co? (TDM) Y(a)

Y(b)

Z(c)

IR

Raman

0.059 0.070 0.932 0.258 0.069 0.933 0.060

0.938 0.927 0.038 0.354 0.918 0.015 0.063

0.003 0.004 0.029 0.389 0.013 0.052 0.877

1135 (Y,x) 1149 (Y,X,z) 1094 (X, Y) 1080 (X, Y,Z) 1050 (X,Y) ? ?

1139 (xz,yz) 1145 cvy, zz, xx) 1087 (_w,xwz) 1081 tjy,xx, zz,xz,yz) 1048 tj~y,zz) ? ?

vibrations are expected in the region 1200900 cm-’ and can be strongly mixed with other vibrations. Szarek et al. [6] assigned only the band at 1175crY’ to the C-O stretching and bending modes of the hydroxymethyl. As follows from our data this band is sensitive to deuteration and probably arises from in-plane bending vibrations of the hydrogen bonds. Three strong bands at 1101, 1076 and 1047cn-’ are present in the IR spectrum of deuterated /3-o-fructopyranose. Their counterparts are found at about 1094, 1080 and 1052 cm-’ in the spectra of the non-deuterated compound. According to the polarisation properties of these bands, listed in Table 9, an assignment to the C-O stretching vibrations of the C(3)0, C(4)O and C(5)O groups, respectively, is most likely. The bands at 1149 and 1135cm-’ might be assigned to the C-O stretching vibrations of the C(l)0 and C(2)O groups; however, this seems to be in conflict with earlier conclusions based on primary and secondary aliphatic alcohols [16]. The C-O stretching vibration of primary alcohols has been assigned to absorptions at lower wavenumber than for secondary alcohols, 1080-lOlOcm-’ and 1120- 1090 cm-’ respectively. It is difficult to propose a reliable assignment for the vibrations of the C-O-C vibrations as they are strongly mixed with the C-C stretching vibrations.

isation directions, based on calculations on the ‘oriented gas’ model, is established for the orientation of the TDM of the weakest proton donor O(4)H. . *O(2) (3520 cm-‘) along the O-H bond and of the TDM of the strongest H-bond 0(2)H...O(l) (3204cm-‘) in the direction of the 0 . . .O axis. However, as all vibrations of the OH groups are not pure, the transition dipole moment of the particular stretching vibrations will not be oriented exactly along the O-H or 0 .. - 0 directions of the hydrogen bonds to which the band is formally assigned. No foolproof assignment could be made for 0(1)-H, 0(3)-H and 0(5)-H. In addition to vibrational coupling Davydov splitting is recognised, e.g. a minor shift of peak positions at different polarisation directions. The bandwidth of the OH-stretching vibrations is strongly dependent on the strength of the H-bond, stronger H-bonds show broader bands. The out-of-plane Ton bending modes of the strongest hydrogen bonds are assigned taking into account the relation with the OH-stretching region, e.g. the strongest H-bond gives rise to the highest Ton frequency [13]. The stretching vibrations of the CH;! and CH groups appear to be ‘pure’. Their polarisation properties in Raman spectra (and for a limited region of the IR spectra) correspond very well with prediction based on the assumed ‘oriented gas’ model.

5. Conclusions

References

The best agreement between experimentally observed band intensities and predicted polar-

[l] R.S. Shallenberger 480.

and T.E. Acree, Nature, 216 (1967)

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J. Baran et al./J. Mol. Struct. 326 (1994) 109-122

[2] R.S. Shallenberger, Pure Appi. Chem., 50 (1978) 1409. [3] M. Mathlouthi and D.V. Luu, Carbohydr. Res., 78 (1980) 225. [4] M. Mathlouthi, C. Luu, A.-M. Meffroy-Biget and D.V. Luu, Carbohydr. Res., 81 (1980) 213. [5] T.W. Barrett, Spectrochim. Acta, Part A, 37 (1981) 233. [6] W.A. Szarek, S.-L. Korppi-Tommola, H.F. Shurvell, V.H. Smith, Jr. and O.R. Martin, Can. J. Chem., 62 (1984) 1512. [7] E.T.G. Lutz, Y.S.J. Veldhuizen, J.A. Kanters, J.H. van der Maas, J. Baran and H. Ratajczak, J. Mol. Struct., 270 (1992) 381. [S] S. Takagi and G.A. Jeffrey, Acta Crystallogr. Sect. B, 33 (1977) 3510. [9] J.A. Kanters, G. Roelofsen, B.P. Alblas and I. Meinders, Acta Crystallogr., Sect. B, 33 (1977) 665.

[lo] H.A. Wells, Jr. and R. Atalla, 3. Mol.. Struct., 224 (1990) 383. [l l] J. Baran, M. Wierzejewska-Hnat, E.T.G. Lutz and J.H. van der Maas, J. Mol. Struct., 244 (1991) 87. [12] J. Baran, M. Wierzejewska-Hnat, E.T.G. Lutz and J.H. van der Maas, J. Mol. Struct., 237 (1990) 355. [13] E. Steinwender, E.T.G. Lutz, J.H. van der Maas and J.A. Kanters, Vib. Spectrosc., 4 (1993) 217. [14] G. Turrel, Infrared and Raman Spectra of Crystals, Academic Press, New York, 1972. [15] J. Giermanska and M. Szostak, J. Raman Spectrosc., 22 (1991) 107. 1161 R.G.J. Miller and H. Willis, Irscot Infrared Structural Correlation Tables, Heyden, London, 1969.