Infrared active vibrational modes of lithium hydroxide monohydrate

INORG. NUCL. CHEM. LETTERS Vol.16, pp.159-163. © Pergamon Press Ltd. 1980. Printed in Great Britain.

0020-1650/80/0301-0159502.00/0

INFRARED ACTIVE VIBRATIONAL MODES OF LITHIUM HYDROXIDE MONOHYDRATE

Yo sh iyuki Hase Instituto de Qu~mica, Universidade Estadual de Campinas, Caixa Postal 1170, 13100 - C a m p i n a s ,

(Received 7 November

SP, Brasil

1979)

Recently Germick and Harmon reported the infrared active fundamental frequencies of solid -i state lithi~n hydr~cide monohydrate in the 4000 - 200 cm region told the observed spectral data were discussed taking the H/D isotope effect on the fundamental vibrations into account -i (i). Two intense bands at 1005 and 860 om were assigned to the rotational lattice modes of water molecules of crystallization in comparison with the CH 2 bending modes of solid state -I CH2CI 2. The rotational lattice modes of the hydroxide ions were found at 680 and 635 cm and -i these bands were shifted to lower frequency region upon deuteration end found at 490 cm Three bands at 460, 412 and 336 cm -I did not show remarkable frequency shifts upon deuteration end were assigned to the Li + translational lattice modes.

In the present paper, the infrared

spectral data are studied for four isotopically substituted lithium hydroxide monohydrates, 6LiOH.H20, 7Li~H.H2 O, 6LiOD .D20 and 7LiOD .D20 , and new vibrational assignments of the lattice modes are discussed by considering the H/D and 6Li/7Li isotope effects.

EXPER ]MENTAL

6LiOH.H20 was prepared by the direct reaction of 6Li metal the aqueous solution was evaporated using a vacu~n system.

(>95.0% enriched) with H20 and

Lithium hydroxide monohydrate

ccmmercially obtained was recrystallized from aqueous solution and used as 7LiOH.H20 to measure the infrared spectrum.

The deuterated campounds, 6LiOD.D20 and 7LiOD.D20, were prepared by

The repeated recrystallizations of 6LiOH.H20 end 7LiOH.H20 from heavy water (>99.5% enriched). -i infrared spectra were recorded in the frequency region from 4000 to 200 em , on a Perkin-Elmer IR 180 spectrophotometer, for Nujol mulls between two cesium iodide or polyethylene plates. The spectral resolution was typically 1.0

-

2.0 om-I

,

but a resolution of 3.0

--

4.0 cm -I WaS

also used for the intense and broad bm~ds.

RESULTS AND DISCUSSION

Since the crystal s t r u c ~ r e

of lithium hydroxide monohydrate belongs to the factor group

C2h3 = C2/m with Z = 4, each Bravais unit cell contains two formula units of LiOH.H20 ( 2 - 6 ) Figure I shows the crystal structure determined by the neutron diffraction technique ( 6 ) .

In

this study, the x- and y-axes are chosen to be parallel to the crystallographic a- and b-axes

159

160

Vibrational Modes of Lithium Hydroxide Monohydrate

and the z-axis is perpendicular to the plane defined by the x- and y-axes.

The results of

the factor group analysis for LiCK.X20 , where X is H or D, are summarized in Table i with the descriptions of the vibrational modes and the structure of the reduced representation

""

',,,

," k,'l

", ,- %

.,"

of the thirty-three optically active normal vibrations

\\

_,-" h~" ,~.,

"i

',,, ,,

is found to be Fvib = 9a + 9b + g g + 9b . Consequently, there are fifteen

6a U

w'l

U

infrared active fundamental vibrations, in which four vibrations are the internal modes and eleven vibrations the lattice modes, for

C

each isotopically substituted compound.

\

In

the present study, however, fourteen infrared

Y

X

Z

bends were observed for each compound. The -I observed frequencies, in cm , are tabulated

(O)

Li

(e)

o

in Table 2, with their relative intensities.

H

The frequency ratios which have information

(.)

about the 6Li/7Li and H/D isotope effects on FIG. i

the observed frequencies were calculated and

Crystal structure of lithium hydroxide monehydrate, LiOH.H^O, determined by • . Z neutron dlffractlon study. Structural data were transferred from Reference 6.

the results are also listed in Table 2 with the tentative band assigoments. The fundamental vibrations of the four internal modes are expected in the frequency region above ii00 cm -I and the fundamental

bands will show the characteristic frequer~y shifts upon deuterium substitution.

Since the

shortest O-H- " 0 distances for H20 and OH- sites were found to be about 2.68 and 3.19 ~ ( 6 ), the effective hydrogen bond system m a y be considered only for the former sites and the H20 and

TABLE 1 Factor Group Analysis of Lithium Hydroxide Monohydrate 3 C2h

n

a

g g

a

T'

N

Activity ~(~-

b

R'

(LiOX.X20)

~X20

6X20

OK-

X20

Li +

CK-

X20

9

i

1

i

I

I

I

2

i

Ram~m

9

0

I

0

I

2

2

I

2

Raman

6

0

1

i

i

i

( 1

I

i )*

Infrared

9

i

I

0

i

2

( 2

2

2 )**

Infrared

U

b U

N, number of vibrational degrees of freedom for Bravais unit cell; n, number of internal modes; R' , number of rotational lattice modes; T', number of trmlslational lattice modes. 9, valence bond stretching mode; 6, valence angle bending mode. * One acoustic mode belongs here. ** Two acoustic modes belong here.

Vibrational

OH- stretching modes are expected

Modes of Lithium Hydroxide

the O-H'''O hydrogen bonds

( 7 - 9 ).

OH- (bu) , asymmetric H20 (b u)

By considering

bm~ds,

of

the observed frequencies and band widths, -i can be assigned respectively to the

compounds

(au) are found

to the corresponding -I

at 1580 and 1168 cm

two weak bands are also observed, for each isotopically

hydroxide monohydrate, 7LiOH.H20

and O-H stretching frequencies

m~d syrr~etric H20 (a u) s t r e t c h i n g modes and t h r e e bands a t 2632,

The H20 and D20 bending modes

fundamental

distances

intense bands at 3575, 3100 and 2800 cm

2320 and 2160 cm -I of the deuteriL~n substituted modes.

161

in the frequency regions -2900 and -3600 cm -I, respectively,

from the relations between the 0 .... 0 interatomic

three infrared

Monohydrate

in the frequency region above Ii00 cm -I.

are found at 2540 and 2355 cm -I and attributed

O-D stretching In addition to the

substituted

lithium

These bands for 6LiOH.H20

and

to the combination modes between the

infrared active H20 bending and Raman active OH

or H20 rotational

lattice modes or between the

Rmnan

or H20 rotational

lattice modes.

active H20 bending

and infrared

active OH

for 6LiOD.D20 and 7Li(I).D20 are found at 1860 ~ d In the frequency region below ii00 cm rotational

lattice modes are expected.

for four isotopically Previously, Gennick H20 rotational

substituted

, only the bands relating

The observed

infrared

two infrared

are reproduced

and -i ,

in Figure 2.

intense bands at 1005 and 860 cm -I to the

in comparison with the band assignments

of solid state dichloromethane

to the translational

spectra, from Ii00 to 250 cm

lithium hydroxide monohydrates

and Harmon assigned

lattice modes

-1

The bands

1740 cm -1

of the CH 2 bending modes -i to the

and also two infrared intense bands at 680 and 635 om

TABLE 2 Observed

Infrared Frequencies,

in cm

-i

, of Lithium Hydroxide Monohydrate

Frequency 6LiOH.H20

7LiOH.H20

(LiOX.X2)

Ratio

6LiOD.D20

7Li(~D.D20 6H/7H

6D/7D

6H/6D

7H/7D

2632 s

1.000

1.000

1.358

1.358

~OX

(6H)

(7H)

(6D)

(7D)

3575 s

3575 s

2632 s

3100 s,br

3100 s,br

2320 s,br

2320 s,br

1.000

1.000

1.336

1.336

~as~nX20

2800 s,br

2800 s,br

2160 s,br

2160 s,br

1.000

1.000

1.296

1.296

symX20

2540 w

2540 w

1860 w

1860 w

1.000

1.000

1.366

1.366

2355 w

2355 w

1740 w

1740 w

1.000

1.000

1.353

1.353

1580 s

1580 s

1168 s

1168 s

1.000

1.000

1.353

1.353

~2 o R'OX

see text

994 s

994

s

730 s

730 s

1.000

1.000

1.362

1.362

854 s

854 s

632 s

632 s

1.000

1.000

1.351

1.351

684 s

680 s

510 s, sh

507 s,sh

1.006

1.006

1.341

1.341

634 s

634 s

468 s,sh

467 s,sh

1.000

1.000

1.358

1.358 1.019

527 s,sh

494 s,sh

520 s,sh

485 s, sh

1.067

1.072

1.013

486 s

456

480 vs

450 vs

1.066

1.067

1.013

1.013

440 m

413 m

419 m

398 m

1.065

1.053

1.050

1.038

332 m

332 m

314 s

314 s

1.000

1.000

1.057

1.057

s

s, strong; m, medium;

w, weak; v, very; br, broad;

sh, shoulder.

# Accidental

R'X20

T 'Li +

T'(IK ,X20 degeneracy.

#

]62

Vibrational Modes of Lithium Hydroxide Monohydrate

(A) i

llO0

!

900

(c.)

J

I

J

s

i

300

|

1100

900

900

i

i

700

I

I

500

i

300

Frequency (an -1)

.

(D) s

llO0

t

700 500 Frequency (cm-I )

•

700

I

I

500

I

&

300

i

11O0

i

900

Frequency (cm -1 )

a

i

i

700

i

500

I

|

300

Frequency (cm-1 ) FIG. 2

Infrared spectra of (A) 6LiOH.H20, (B) 6LiOD.D20,

(C) 7LiOH.H20, and (D) 7LiOD.D20.

OH- rotational lattice modes for reason that these bands showed the characteristic band shifts upon deuteration but were not assigned as the H20 rotational ones ( 1 ).

However, the Gennick

and Harmon's assignments of the rotational modes seem .unreasonable because the Li .... 0 bonding character is essentially different from the C-CI valence bond.

According to Ferraro and Walker

(i0) , the rotational mode of the bridging hydroxide ion between two metals was observed at 955 -1 -1 cm and shifted to 710 cm upon deuteration. Furthermore, an accidental degeneracy may be expected for two C~- rotational lattice modes (au and b u) since the OK- site is not effectively hydrogen bonded through the hydrogen or deuterium atom ( i ) and the center of gravity of OX is -I located almost on the oxygen atom. Therefore, the b ~ d s at 994 and 730 cm can be assigned to the accidentally degenerated OH

and (X) rotational modes, respectively.

Three remaining H/D

sensitive bands at 854, 684 and 634 om-i of 6LiOH.H20, 854, 680 and 634 om-I of 7LiOH.H20, 632, i 6 I 7 • 510 and 468 ore- of LiC~.D20 and 632, 507 and 467 ore- of LiOD.D20 are undoubtedly asslgned to the X20 rotational lattice modes as wagging (bu, rotation about the x-axis), twisting (au, rotation about the y-axis) and rocking (bu, rotation about the z-axis) modes.

As easily found

in Table 2, these X20 rotational modes are not vibrationally mixed with the Li +, C~- and X20 translational lattice modes, but the detailed band assigr=nents are not discussed in this study.

Vibrational Modes of Lithium Hydroxide Monohydrate

Since three acoustic modes of LiCE.X20 are classified into a

and b

U

163

syn~etry species, the

U

Li +, C~- and X20 site translations are mechanically mixed, as a matter of course, to form the infrared active fundamental modes of the translational lattice vibrations.

The bands at 527,

486 and 440 cm -I of 6L iOH. H20 are 6Li/ 7Li-sensitive and shifted to 494, 456 and 413 cm -I upon 7Li-substitution.

With due consideration for the relative intensities, the corresponding bands

for 6LiOD.D20 are found at 520, 480 and 419 cm -I and those for 7LiOD.D20 at 485, 450 and 398 -i on From the reason that the 6Li/7Li frequency ratios for these bands are about I .065 and close upon #m(TLi)/m(6Li ) = 1.080, where re(y) indicates the atomic mass of Y, these infrared bands can be assigned approxlmately as the Ll

translatlonal lattlce modes along the x- (bu) ,

y- (au) and z- (bu) axes, though the H/D isotopic frequency shifts are also observed slightly. Therefore, three translational lattice modes not mentioned above can be described approximately as the mixed modes between the C~ pure OK

and X20 site translations.

The H/D ratios expected for the

and X20 translational motions are given by •m(OD-)/m(OH- ) = 1.029 and #m(D20)/m(H20 ) =

1.054, respectively, ~ e r e m(z ) indicates the molecular weight of Z.

In practice, however, one

infrared band is observed in this study for each isotopically substituted compound.

The bands

at 332 cm -I for 6LiOH.H20 and 7LiOH.H20 and at 314 om-I for 6LiOD.D20 and 7Li(1).D20 are easily assigned to one of the translational lattice modes and two remained translational modes will be -I observed in the frequency region below 200 cm

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I.

I. GENNICK and K. M. ~ O N ,

2.

R. PEPINSKY, Z. Krist. 102, 119 (1940).

Inorg. Chem. 14, 2214 (1975).

3.

H. RABAUD, C. R. Hebd. Seances Acad. Sci. 241, 1959 (1955).

4.

H. RABAUD and R. GAY, Bull. Soc. Fr. Mineral. Cristallogr. 80, 166 (1957).

5.

N. W. ALCOCK, Acta Crystallogr. B27, 1682 (1971).

6.

P. A. AGRON, W. R. BUSING and H. A. LEVY, Chemistry Division Annual Progress Report, ORNL 4791 UC4, Oak Ridge National Laboratory, Oak Ridge (1972).

7.

E. R. LIPPINCOTT and R. SCHROEDER, J. Chem. Phys. 23, 1099 (1955).

8.

K. NAKAMOTO, M. MARGOSHES and R. E. RUNDLE, J. b~n. Chem. Soc. 77, 6480 (1955).

9.

G. C. PIMENTEL and C. H. SEDERHOIM, J. Chem. Phys. 24, 639 (1956).

i0.

J. R. FERRARO and W. R. WALKER, Inorg. Chem. 4, 1382 (1965).