I Phn Chem Solid.\ Vol 59. Nor. 6-7, pp. W5-857, ,998

Pergamon SOO22-3697(98)00042-O 0 1998 Elvwer Science Ltd

Prmted m Great Britam. All rights resewed 0022-3697M $19.00 + 0.00

RAMAN AND INFRARED SPECTRA OF Mn AND Fe HALIDES

TETRAHYDRATED

S. BRUNIa’*, F. CARIATI”, P. FERMO”, G. SPINOLOb and M. MARTINI’ “Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Universita degli Studi di Milano, Via G. Venezian 21, 20133

Milano, Italy hDipartimento di Scienza dei Materiali, Universitl degli Studi di Milano, Via L. Emanueli 15, 20126 Milano. Italy

(Received IO October 1997; accepted I I March 1998)

Abstract-The infrared absorption spectra of the powdered samples and the Raman spectra of single crystals of MnC12.4Hz0, MnBr2.4H20 and FeCl2.4HzO were recorded at liquid nitrogen temperature. Comparing the predicted and observed vibrational bands the spectra have been interpreted paying attention to the site symmetry of the metal atom and assuming only one formula unit in the Bravais cell. 0 1998 Elsevier Science Ltd. All rights reserved

Keywords: Hydrated, hydrogen bond, water vibrations

1. INTRODUCTION

Hydrogen bond is responsible for many of the peculiar

physical and chemical-physical properties of water, but

in spite of a great amount of work [ 11, it still remains quite

elusive and represents a permanent challenge.

Many scientists thought that the study of H20 “fixed”

in a precise spatial position in stoichiometric hydrates

could give considerable help in explaining the informa-

tion obtainable from spectroscopic and crystallographic

studies of water in the solid state, again in the perspective

of understanding the hydrogen bond [I]. The prevision was grounded on the fact that the fre-

quency range in which internal vibrations and hindered

rotations of Hz0 occur is quite well separated from that in

which the lattice vibrations are found. Moreover, it turned

out that lattice dynamics (in principle very complicated

owing to the high number of atoms in the elementary

formula unit) essentially reduces to molecular dynamics

owing to the low interaction among atoms contained in the

elementary unit most often limited to the hydrogen bond

linkages. In distinct cases, such as those in which “lattice”

water is present, Hz0 translations with respect to other

elements of the lattice must also be considered.

Considering now H20 intramolecular oscillations,

vibrational spectroscopy has proven that a direct and

strict relationship exists between stretching frequencies and

hydrogen bond distances [2]; further, stretching and bend-

ing frequencies anticorrelate [3]. It has been observed in

fact, that hydrogen bond decreases the O-H stretching

frequency, depending on its strength, up to - l2%, while it

increases the H-O-H bending frequency up to -6%.

The interpretation of the infrared and Raman spectra

*Corresponding author

we have performed [4-81 on NiX2.6H20 (X = Cl, Br, I),

CoX2.6H2O (X = Cl, Br) and FeC12.4H20, perhaps the

first studies with the ambition of attributing all the peaks

observed in the 4000-400 cm-’ region to corresponding

vibrations, has clarified a number of aspects, but certainly

new experimental results concerning either the structural

or the spectroscopic aspects of the phenomenology can

modify some of the attributions.

The number of bands in the vibrational spectra of

transition metal halide hydrates depends on the molecular

symmetry (static effect) and on the number of molecules

in the Bravais cell (dynamic effect). While the static

effect causes interactions among the vibrations of the

water molecules linked to the same metal atom, the-

dynamic effect causes interactions among the vibrations

of the water molecules of the same Bravais cell, but not

linked to the same metal atom. In order to calculate the

number of vibrations dependent on the static effect, it is

necessary to refer to the symmetry of the site occupied by

the metal atom. On the contrary, in order to calculate the

number of vibrations that depend on the dynamic effect it

is necessary to refer to the point group of the Bravais cell,

i.e. to the factor group. Certainly, the static effect is

stronger than the dynamic one, the latter being directly

connected to the force of the hydrogen bonds. One of the

more evident effects owing to vibration couplings is the

high number of infrared and Raman bands that generally

does not find correspondence with the number of experi-

mental bands. For this reason, in most of the previously

published works, only the vibrational spectra of metal

halide hydrates which have just one formula unit in the

Bravais cell have been completely studied. In a preceding

paper [5] we tried to interpret the vibrational spectra of

FeC12.4Hz0 whose Bravais cell contains two formula

845

846 S. BRUNI ef al.

.

Wavenumber (cm-l)

Fig. I. LNT infrared spectra of: (a) MnC12.4H20; and (b) MnBr2.4H20 in the region of water stretching modes (solid lines) and second derivative curves (dotted lines).

units. In this case, we put forth a hypothesis according to

which, if the intermolecular hydrogen bonds are weak, it

is possible to neglect the dynamic effect and study the

vibrational spectra exclusively on the basis of the site

symmetry of the metal atom (that is the molecular

symmetry of the hydrate compound).

The aim of this work is to interpret Raman and infrared

spectra of MnC12.4H20 and MnBrz.4H20, that present

isomorphous crystals of low symmetry [9] having four

formula units in the Bravais cell, and to reconsider the

spectra of FeC12.4H 20.

2. CRYSTAL STRUCTURES AND VIBRATIONAL SYMMETRIES

The crystal structure of MnC12.4Hz0 has been deter-

mined by X-rays [IO] and neutrons [ 1 I] diffraction. The

space group is P2 ,ln (factor group C’j,,) with four formula

units per Bravais cell (monoclinic cell). The structure

consists of discrete cis octahedral groups with each

manganese atom co-ordinated to two chloride ions and

to four oxygen atoms. The manganese ions occupy a C,

site and the water molecules are all unequivalent.

Four of the eight hydrogen atoms form normal (e.g. not

bifurcated) hydrogen bonds to chloride ions of another

molecule. Two hydrogen atoms form hydrogen bonds to

oxygen atoms (of another molecule) and each of the last

two hydrogen atoms forms bifurcated hydrogen bonds

with two chloride ions. On the whole, the hydrogen bond

interactions are somewhat weak as judged by the dis-

tances H. . Cl (ranging from 2.201 to 2.374 A for normal

hydrogen bond and from 2.495 to 2.948 A for bifurcated

hydrogen bond) and H. . .O (1.967 and 2.014 A, respec-

tively). As stated above, MnBr2.4H20 is isostructural

with MnC12.4H20.

Tetrahydrated Mn and Fe halides 847

1700 1600 1500

Wavenumber (cm-l)

Fig. 2. LNT infrared spectra ok (a) MnC12-4Hz0; and (b) MnBrr4H20 in the region of water bending modes (solid lines) and second derivative curves (dotted lines).

The crystal structure of FeC12.4H20 was determined

by X-rays [ 121 and neutrons diffraction [ 131. The space

group is P2 t/c (factor group C&) with two formula units

per Bravais cell (monoclinic cell). The iron ions occupy a

C; site. The molecular structure consists of a tram

octahedron in which the four co-ordinated water mo-

lecules lie at the comers of the equatorial plane forming

two different diagonals. Consequently, there are two

different couples of water molecules in the octahedron.

The octahedra are linked by hydrogen bonds among

hydrogen atoms and chlorine ions. The H. . Cl distances

have values ranging from 2.2 I4 to 2.289 A.

The 60 atoms of the Bravais cell of MnX2e4H20 (X =

CI,Br), according to the standard factor group analysis

[14], give rise to 177 optical modes that can be divided

into 90 Raman active modes and 87 IR active modes

(45A, + 45B, + 44A, + 438.). They can also be

grouped into: (a) water internal modes (stretching

modes, 8A, + 8B, + 8A, + 8B,, and bending modes,

4A, + 4B, + 4A, + 4BJ; (b) water libration modes

(12A, + l2B, + l2A, + l2BJ; (c)octahedron internal

vibrations (l5A, + l5B, + l5A, + 15BJ; (d)

octahedron libration modes (3A, + 3B, + 3A, + 3BJ;

and (e) octahedral translations (3A, + 3B, + 2A, +

2BJ. The 30 atoms of the Bravais cell of FeC12-4H20 give

rise to 87 optical modes (21A, + 2lB, + 23A, + 22BJ

that can be divided into: (a) water internal modes

(stretching modes, 4A, + 4B, + 4A, + 4B, and bending

modes, 2A, + 2B, + 2A, + 2BJ; (b) water libration

modes (6A, + 6B, + 6A, + 6BJ; (c)octahedron internal

vibrations (6A, + 6B, + 9A, + 9BJ; (d) octahedron

libration modes (3A, + 3B,); and (e) octahedron transla-

tions (2A, + B,).

848 S. BRUNT et al.

3550 3450 5350 3250

Wavenumber (cm-')

Fig. 3. LNT polarised Raman spectra of MnC12.4Hz0 in the region of water stretching modes (solid lines) and portions of the second derivative curves (dotted lines).

3. EXPERIMENTAL

Crystals of MnC12.4H20 and MnBrp.4Hz0 were

prepared at 32°C from a saturated aqueous solution of

the corresponding salts and FeC12.4Hz0 was grown at

room temperature from a solution of the salt in the

presence of small pieces of pure iron,

The crystals were grown in plastic boxes whose

temperature was maintained constant. The rate of eva-

poration was adequately controlled by covering the

crystallisation containers.

The Raman spectra were obtained with a Jasco TRS-

300 spectrophotometer equipped with a diode array

detector. The measurements were carried out at liquid

nitrogen temperature (77 K) using the line at 488 mn of

an Ar ion laser. Both 90” scattering and back scattering

geometries were adopted.

The crystals belong to the monoclinic system and

consequently present a binary symmetry axis correspond-

ing to the crystallographic axis b, while a and c axes are

not mutually perpendicular and lie in a plane perpendicular

to b. The orientation of the crystals was rather immediate

owing to the fact that the macroscopic morphology

evidenced the b axis [ 151.

Taking into account that for the monoclinic system

only light propagating perpendicular to the b axis retains

its polarisation [ 161, we had to select the propagation

directions in the UC plane.

The Raman spectra were recorded with respect to three

orthogonal axes of which they axis is coincident with the

b axis while x and z axes were utilised as propagation

directions of the light.

To the same co-ordinate system Loudon [ 171 referred

the Raman tensor for a monoclinic crystal.

The infrared spectra were performed using a Digilab

FTS 40 spectrophotometer at 77 K. The measurements

Tetrahydrated Mn and Fe halides 849

I

3500 3400 3300

Wavenumber (cm-‘)

Fig. 4. LNT polarised Raman spectra of MnBr2.4Hz0 in the region of water stretching modes.

were performed on pellets of KBr and KCI, depending

on the examined halide, in the spectral region

4000-400 cm-‘, and of polyethylene in the spectral

region 400-100 cm-‘. To obtain a better assign-

ment the spectra of deuteriated samples were also

recorded.

4. INFRARED AND RAMAN SPECTRA OF MnX2-4H20 (X = Cl, Br)

4.1. Water internal modes region (3600-1500 cm-‘)

In the unit cell there are 16 water molecules and so we

should expect 48 internal modes: 32 in the stretching

vibrations region and 16 in the bending vibrations region

(see above). If we look at the stretching bands shown in

the infrared spectra of Fig. I and in the Raman spectra of

Figs 3 and 4 and at the bending bands shown in the

infrared spectra of Fig. 2, we observe only eight bands

owing to stretching modes and four bands due to bending.

One proposed explanation of this behaviour is that each

of the four crystallographically distinct and asymmetric

waters of the formula unit has only one bending and two

stretching frequencies of mixed infrared and Raman

character. According to this hypothesis, no coupting

exists among the water molecules.

The interpretation of the spectra should be made

considering only one formula unit in the Bravais cell

and taking account of the site symmetry C, of the

manganese ion. In fact, in this case, we neglect both

dynamic and static vibrational coupling.

Indeed, it is worth noting that in a site of C, symmetry,

owing to lack of symmetry elements, even the coupling

among water molecules linked to the same metal atom is

forbidden. On these grounds, for MnXz.4H20 (X =

CI,Br) one can calculate 42 bands, all infrared and

Tab

le

I. V

ibra

tiona

l fr

eque

ncie

s (c

m -

‘)

of M

nC12

.4H

20

Ram

an

Infr

ared

5

Ass

ignm

ent

YY

xx

3497

34

99

- -

3425

34

27

3395

33

94

- -

3363

33

43

3294

-

- 32

90

1649

16

49

- -

1 -

1594

15

94

- -

750

sh

753

734

734

712

706

642

- 63

2 -

623

627

577

580

- -

512

516

- -

467

- 36

1 36

9 29

3 29

0 -

- -

- 20

4 20

0 -

160

160

132

129

- -

- -

122

-

107

99

- -

81

78

zz -

3492

34

27

- 33

91

3363

32

94

3290

16

49

- -

1594

76

5 - - 71

5 sh

- 63

2 - 570

523 -

487

sh

- 36

9 sh

35

2 30

0

282

sh

210 - 160

139

- - - 115

- 84

XY

3498

-

3426

33

95

-

3359

-

3290

16

49

- -

1594

- - 730

710

642 - 619

576 - 507

482

sh

- 367

300

- sh

283 - 198

161

132

- - - 104

- 85

xz

3497

-

3426

33

95

-

3356

-

3287

16

49

- -

1594

- - 72

6 70

0 - - 622

577

523

507

485 - 360

346

- sh

282 - 199

169

- - - 123

109

- 87

YZ

3493

-

3422

33

94

-

3354

33

00

3291

16

49

- -

1594

76

0 sh

-

727

sh

710 - - 616

570 - 508 - - 360

302

- sh

282 - 196

162

138

- - 118

107

- 87

3507

-

3438

33

93

-

3364

-

3290

16

53

1635

16

12

1586

76

0 75

2 74

0 sh

69

2 - - 629

567

530

sh

- -

433

sh

366

2%

335

264

221

200

170

150

sh

139

131

sh

- 106 95

84

O-H

st

retc

hing

H-O

-H

stre

tchi

ng

WL

Mn-

0 st

retc

hing

OID

WL

=

wat

er

libra

tion

mod

e.

OID

=

octa

hedr

on

inte

rnal

de

form

atio

n m

ode.

sh

=

shou

lder

.

Tab

le

2. V

ibra

tiona

l fr

eque

ncie

s (c

m-‘

) o

f M

nBr?

.4H

?O

Ram

an

YY

xx

Z

Z

XY

X

Z

YZ

Infr

ared

A

ssig

nmen

t

3515

- - 34

44

3401

33

67

3320

- 16

35

- - I5

82

744

sh

734

702

684 - 624 - 580

- 515

495

350

- - 285

- 168

201

- 139

129

II0

93

76

68

3514

- 34

62

sh

3443

34

00

3364

-

3315

16

35

- - 15

82

745 -

706

sh

687

630 - - 578

546

523

- 508

353

- 30

0 sh

29

1 - 16

7 20

2

- 152

- 129

110

92

68

3514

-

3464

sh

34

47

3401

33

60

-

3315

16

35

- - 15

82 - 740 - 690 - 625 - - 545

- - 493 - 338

- 287

- I69

I99 - I52 - 127

II0 97

- -

351s

-

3464

sh

34

43

3397

33

60

-

3315

16

35

- - 15

82

- 736

703

sh

680

628 - - 580

548

- 516

497

350

- 300

286

168

201

I78

I52 - 129

II0

92

- 68

3513

-

3460

sh

3443

33

97

3368

3318

-

1635

- -

1582

- 73

4 70

6 sh

683

628 - - - 547 - 516

497

350

300

286

- 167

202

178

152

- 127

I09 91

67

3514

3460

sh

3443

33

97

3368

3318

-

1635

- -

I 582

- 737

706

683 - 626

611 - 547 - 516

495

352

- 298

285

- 168

202

I78

152

- 127

I09 91

- 67

3524

sh

34

72

sh

-

3433

33

94

-

3329

-

1643

16

28

1608

15

81

- 722 - 672 - - - 587

- - - 469

344

320

300

279

I89

I60 - - - I39

123

II0

- 85

-

O-H

st

retc

hing

H-O

-H

ben

din

g

WL

Mn

-0

stre

tch

ing

Mn-

Br

stre

tchi

ng

OID

WL

= w

ater

hb

ratlo

n m

od

e. 0

10

= o

ctah

edro

n In

tern

al d

efo

rmat

ion

mo

de.

sh

=

sho

uld

er.

852 S. BRUNI et al.

I I

750 650 550 450

Wavenumbsr (cm -1)

Fig. 5. LNT polarised Raman spectra of MnClr.4H~O in the region of water libration modes.

Raman active, distributed according to this scheme: 8

stretching vibrations, 4 bending vibrations, 12 water

libration bands, IS octahedral internal vibrations (which

can be grouped into 4 u(M-0) stretching modes, 2 u(M-

X) stretching modes and 9 deformation modes) and 3

octahedron libration bands. Consequently, the spectra

reported in Figs l-4 can be thoroughly interpreted.

We wish to point out the fact that Raman spectra were

recorded with different polarisations of incident and

scattered radiation, in order to obtain bands of intensities

corresponding to the six distinct elements of the scatter-

ing tensor (see experimental). If the vibrational analysis

of the crystal would be based on the factor group

symmetry we should have recognised bands correspond-

ing to eight stretching vibrations of As symmetry and

eight of B, symmetry. On the contrary, just eight bands

were observed with only minor intensity changes in all

the examined polarisations, as shown in Table I and

Table 2 and in Figs 3 and 4.

The infrared spectra (see Fig. I) in the water stretching

region show five bands instead of the eight bands

calculated. In our opinion, this behaviour is due to low

resolution or to the coincidence of some bands.

4.2. Water libration modes region (800-400 cm-‘)

The region of the water libration modes in the hydrate

crystals has already been clarified [8]. Taking into

account, in analogy with the case of the stretching

vibrations, the differences among the water molecules

and considering the presence of only one molecule in the

Bravais cell, we calculated 12 water libration modes

distributed in four groups each containing three bands

(a rocking, a wagging and a twisting vibration).

The Raman spectra in the region of the water libration

modes are reported in the Figs 5 and 6 and the IR spectra

are reported in Fig. 7. The experimental values of the

frequencies are reported in Table I. Examining these data

it is possible, as in the case of the stretching vibrations, to

Tetrahydrated Mn and Fe halides 853

r

750 850 550 450

Wavenumber (cm-f)

Fig. 6. LNT polarised Raman spectra of MnBrr.4HrO in the region of water libration modes.

establish that the vibrational frequencies of the spectra of MnC12.4H20 and of MnBrzs4H20 can be assigned con- sidering the site symmetry of the metal atom and only one molecule in the Bravais cell.

4.3. Octahedron internal vibrations region (400-

80 cm-‘)

In this region, once more on the grounds of the site symmetry and considering the presence of only one molecule in the Bravais cell, we calculated 15 bands for the octahedron internal vibrations. In the IR and Raman spectra of MnX2.4H20 (X = Cl, Br) reported in the Figs S- IO we observed I2 non coincident bands (Fig. 8 IR; Figs 9 and 10 Raman). The values of the frequencies are reported in Tables I and 2. The vibration bands of this region can be completely interpreted taking into account the site symmetry and one formula unit in the Bravais cell.

5. INFRARED AND RAMAN SPECTRA OF FeC12-4H20

The crystal of FeC12.4H20 has two molecular units in the Bravais cell. Nevertheless, from the vibrational spectra

we collected [5] a lower number of bands than that expected on the basis of the factor group analysis was observed. According to the procedure followed for the hydrate compound of manganese, we treated the case of FeC12.4Hz0 on the basis of the site symmetry C, of the iron atom and considering one molecule in the Bravais

cell. In this case, we must observe that the four water molecules of the formula unit are divided into two groups of two molecules each. The two water molecules of each group are related by the symmetry operations of the C, point group.

Consequently, between the two water molecules there is a vibrational coupling owing to the static effect which produces two IR (A,) and two Raman (A,) stretching vibrations. In total, for the four water molecules of the

854 S. BRUNI er al.

750 650 550 450

Wavenumber (cm-’ )

Fig. 7. LNT infrared spectra of: (a) MnCl,.4H20; (b) MnC12.4D20; (c) MnBr+lHgO in the region of water libration modes.

formula unit, we expected eight stretching vibrations

(4As + 4A,). Analogously, we calculated four water bending vibrations (2A, + 2A,).

In order to characterise all the Raman active bands, we recorded the polarised spectra of the crystal and we obtained four stretching and two bending bands in the region of water internal vibrations. In the IR spectra we observed as well four stretching and two bending water vibrations. As these results were in accordance with our predictions, we considered the opportunity to analyse the whole vibrational spectra considering the site symmetry of the iron atom and only one molecule in the Bravais cell. Consequently, we calculated 12 water librations (6A, + 6A,), 15 octahedron internal vibrations (which can be grouped into four (2A, + 2AJ u(Fe-0), two (AK +A,) u(Fe-Cl)), nine (3As + 6AJ deformation modes) and 3 octahedron librations (3A,). The values of the IR and Raman bands frequencies are reported in Table 3.

Taking into account the spectra of the deuterated compound we observed, with respect to our previous

work, that the Raman band at 213 cm-’ assigned to a u(Fe-0) stretching vibration must be better assigned to an octahedral internal deformation and that the two IR bands at 228 cm-’ and at 197 cm-’ previously assigned,

respectively, to a u(Fe-Cl) stretching and to an octahedron deformation, must be assignment.

swapped in the

6. CONCLUSION

After a close examination of the IR and Raman spectra of MnX2e4H20 (X = CI,Br) and of FeC12.4H20 we can conclude that these spectra have to be interpreted con- sidering the site symmetry of the metal atom and only one molecule unit in the Bravais cell. This interpretative scheme, besides allowing a reliable assignment of the

Tab

le 3

. V

ibra

tiona

l fr

eque

ncie

s (c

m -‘

) of

FeC

lz.4

H20

Ram

an

Pre

sent

wor

k

Infr

ared

A

ssig

nmen

t r

Ref

. (5

1 Y

Y

xz

XY

R

ef.

[8]

Ref

. [7

] T

his

wor

k

‘%I

3431

34

30

3430

34

30

- -

- O

-H

stre

tchi

ng

A,

3415

34

12

3412

-

- -

- 33

90

3387

33

81

3375

-

- -

3374

33

71

- -

- -

- - -

- -

- -

3462

A

” -

- -

- -

- 34

04

- -

- -

- -

3383

-

- -

- -

- 33

67

1660

16

60

1660

16

60

- -

- H

-O-H

st

retc

hing

A

, -

- -

- 16

30

1630

16

30

- -

- -

- -

1660

A

” -

- -

- -

- 16

10

632

639

639

sh

639

- -

- W

L

A,

613

620

620

620

sh

- -

- 55

7 55

8 55

8 -

- -

550

sh

550

- -

- - 50

0 50

0 -

- -

- - 48

2 48

0 48

4 sh

48

4 -

- -

- -

- -

- -

665

- -

- -

- -

630

- -

- -

- 58

8 59

8 -

- -

- -

- 52

8 -

- -

- -

484

482

213

370

- -

- -

- 31

0 31

7 31

7 31

7 -

- -

- -

- 39

8 38

6 38

2 -

- -

- 34

0 34

8 34

4 -

- -

- I8

5 19

9 19

2 -

- -

228

- -

197

I95

- 21

3 -

- -

- 15

2 15

9 16

0 16

4 -

- -

- -

I I5

1 I

4 11

5 -

197

- -

- 22

8 -

- -

- 17

2 16

1 -

- -

- -

142

142

- -

- -

122

- -

- 11

6 -

- -

- -

103

- - 85

86

83

-

OL

- A

s

WL

=

wat

er l

ibra

tion

mod

e. O

ID

= o

ctah

edro

n in

tern

al d

efor

mat

ion

mod

e. O

L =

oct

ahed

ron

libra

tion

mod

e. s

h =

sh

ould

er.

Fe-

O

stre

tchi

ng

A,

A.

Fe-

Cl

stre

chin

g A

,

A”

OID

A

Z

A”

856 S. BRUNI er al.

J IO 300 200 100

Wavenumbw (cm-’ )

Fig. 8. LNT infrared spectra of: (a) MnClz.4H20,(b) MnC12.40,0; (c) MnBr2.4HzO in the region of octahedron internal modes.

300 100

Wavonumbw (cm-1 )

Fig. 9. LNT polarised Raman spectra of MnC12.4H,O in the region of octahedron internal vibrations.

Tetrahydrated Mn and Fe halides 857

400 300 200

Wovenumber (cm -9 )

100

Fig. IO. LNT polarised Raman spectra of MnBrZ.4Hz0 in the region of octahedron internal vibrations.

vibrational spectra, finds its chemical basis in the

weakness of the intermolecular hydrogen bonds, already

evidenced by X-ray and neutron diffraction studies of the

examined compounds.

I.

2.

3 .

4.

5.

6.

REFERENCES

Falk, M. and Knop, 0.. Water in Stoichiomerric Hydrates in Water: (I Comprehensive Treatise, Vol. 2, ed. E. Franks, Plenum Press, New York, 1972, pp. 55- 173. Mikenda. W., J. M&c. Strucf., 1986, 147, I. and references therein. Falk, M., Specrrochim. A&o, 1984, MA, 43. and references therein. Agullo-Rueda. F., Calleja, J. M., Martini, M., Spinolo. G. and Cariati, F., J. Raman Specrrosc., 1987, 18, 485. Cariati, F., Masserano, F.. Martini, M. and Spinolo. G., J. Raman Sperrosc.. 1989,20, 773.

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I 1. El Saffer, Z. M. and Brown, G. M., Acfrr Ccw. B. I97 I, 27. 66.

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15. Groth, P.. Chemische KristnNogruphie, I Teil. Verlag Wilhelm von Engelman, Leipzig, 1906.

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