Precision raman measurements of some td ions in aqueous solution

293

JoumkofMokcularStnrctuure,79(1982)293-296 Ekvier Scientific PublishingCompany,Amsterdam-

PRECISION

RAMAN MEASUREMENTS

Printed inTheNetherlands

OF SOME Td IONS IN AQUEOUS

SOLUTION

K.J. DEAN and G.R. WILKINSON Department

of Physics, King's

University

College,

of London

(Gt. Britain)

ABSTRACT 2The Raman spectra of SO:', WOZ- and MoO 4 have been studied

in aqueous

solution

and the effect of the alkali metal cations has been carefully + When Li+ or Na were present in solution, recorded as a function of concentration. + the anion vibrational frequency was observed to increase, with Li having the more

with great precision,

The cations K+, Rh+ and Cs+ caused the frequency to decrease, pronounced effect. + + + Precise with Cs causing the largest decrease, followed by Rb and then K . values of the symmetric

stretching

mode frequencies

at infinite

dilution were

The frequency shifts with respect

obtained by extrapolation.

to these values,

were found to depend on the square root of the concentration.

INTRODUCTION paper describes

This

the results obtained from a very high precision

investigation

of the symmetric stretching vibrational mode, of the three tetrahedral anions SO:-, 2wo;- and Moo4 in aqueous solution (ref. I). The sulphate anion has previously been studied association

with

and hydrated with

the

interest The

two

a range

(ref.

group

intensities

been

transition

of metallic

in

and aqueous solution

the study anions

assignment

the determination

of

RESULTS

Very high precision alkali

the anions

In

the case

of ion-pairing

WOf- and Moo4

the vibrational

force

of

(ref. 2-

(ref. 31, in

in anhydrous

research

has

bands

been

and

the solution

5)

concerned

their

studies

9) and bandshape

(ref.

relative the main

(refs.

10,

11).

have also been studied in solids (ref. 12).

of the modes observed

The main interest has

in the spectra

(ref.

13),

and

constants.

AND DISCUSSION measurements

(Li +, Na+, cations 2SO4 , WOZ- and Moo:-

metal

of the

of the Raman-active

using Raman spectroscopy

been the accurate

EXPERIMENTAL

7, 8).

4) and also

(ref.

majority

assignments

(refs.

metal

cations The

6) materials.

theoretical

in solids

has

(ref. 2) and in aqueous solutions

in both solid crystals

K+,

have Rb+

been and

made

Cs+),

of the perturbing on

in aqueous solution,

the vl mode

of vibration

as a function

0022-2860/82/0000-0000/$02.7501982ElsevierScientificPublishing Company

effect

of

the of

of concentration.

The carefully

were performed over a relatively large con-3 (factor of 16 times). This was the range of 1.5 - 0.094 mol dm

centration

recorded experinmnts

dilutions of a solution that was initially made up to a -3 , which was selected after considering the wide of 1.5 mol dm

result of multiple concentration

range of solubilities

of the fifteen (alkali metal) - X04 salts.

the v, band centre relative

to two neon lines (one at a higher

and the other at a lower frequency), be consistently

the very high

By measuring

than v1 -I could of f 0.1 cm

precision

frequency

maintained.

The experimental results (Fig. 1) show quite clearly that the w, mode frequency 2of the SO4 anion is dependent on both the solution concentration and on the particular

cation present

in the solution.

90;!-

I

981.! j-

I-

i-

-

979.! j-

Calculation

Concentration

Fig.

1.

The variation

concentration,

Precise

WOt-

+ 0.1 cm-'

w. = 931.1

f 0.1 cm-'

MOO;- u

0

of the anions in the limit of zero

to (Fig. 2), and are shown below:

981.75

=

for SO:- as a function of the solution

(infinite dilution), have been obtained by extrapolation

diagrams similar

w.

dmm3 1

for the five alkali metal sulphates.

values of the v, mode frequency

concentration

so;-

of the vl frequency

I mol

15

o-75

0 375

0-18-3

0094

5.

I

I

I

I

L

= 896.1

f 0.1 cm-'

from

LI 90i

Na

K

I

I

I

I

07

00

09

10

I

I

Rb

I

c

I

I

I

1c

l-5

16

981-5

979-s

11

l-2 l-3 Radius 18 1

Catlon

Fig. A =

The

2.

sulphate

1.5, B = 0.75,

From

the anion

Li+ having

the more

in solution,

the cation is found

that

resulting reduce

w,

frequency

frequency

in a change

K +,

the results

Rb+

and

presented

polarizability

frequency.

Whereas,

in a decrease largest

increase

in the decrease

in the

perturbation

effect,

anion

but

anion

from of

its

Cs+

of

the vibrational

act

results

seen

vibrational

in the water vibrational

of smaller

in

snd

(Fig.

the water

1) and

The

of

commensurate

Na*

I>.

It

the anion perturbed, and Na+ the

both

anion, From

vibrational

polarizability results + cation Li which causes

cation

with

that

the effective

anion

is responsible The

Cs+

polarizability.

a reduction

The

be Lif

surrounding

water

water

frequency.

voJ with

(ref.

will

cations

the dissolved

the effective

present

2) shows

surrounding

molecules

I) that

than

(Fig.

the anion

the effective

in

value

in with

then K+.

molecules

then

present

than vo,

or C.s+ were

interaction

frequency.

frequency.

concentrations:

value

Rh+

K+,

to a lower

in

polarizability

magnitude,

when

by I&+

value,

(Fig.

for

-I+ or Na were

to a higher

the water

in an increase

an increase anion

of

to increase

it can be

Li

a polarizability

normal

polarizability

that when

shifted

presented

size

dm-?

Whereas,

was

through

of anion

shifted

followed

if the polarizability

or increased

mol

was

ef feet.

of the data

the

the effective

whereas,

D = 0.188

pronounced

analysis

as a function

1) it can be seen

w, mode

the anion

perturbs

is reduced

the

(Fig.

the most-pronounced

A detailed

water

C = 0.375,

the results

solution,

having

w, frequency

for induces

the smaller

the largest a positive effect

that

296 it has

on the polarizability

an increase Rb+,

with

increases

produced

rational by

Rb+, The

in the water the smallest

frequency. with

dilution The

also

in

been

these The

three

observed

of the

that

and

change

increase

three

the

frequency

to K+.

being caused

studied, in

shifts,

root

and being

in a decrease

being

or decrease

Rb+

due

decrease

anions

perturbations then

being

result

on the square

of the observed by WOi-

K+,

largest

decrease

increase

depend

cations

the

frequency

frequency

effective

The

cations

largest

frequency

frequency,

followed

polarizability,

polarizability

smallest

to the

magnitude

anion,

IO I1 12 13

the

change

ly related It has

by

of water.

caused

due

all produce + to Cs , then

The

polarizability

the

anion

by

Cs+,

has

been

found

the polarizability with

to be

vib-

followed

by K+.

respect

of the solution

is found

in

Cs+

the

to

to be inverseof the water.

the infinite

concentration largest

for

(ref.

I).

the SOi-

MOO:-.

K.J. Dean, Ph.D. thesis, London, 1981. M.A. Lopez-Bote and S. Montero, in Proc. 6th Int. Conf. Raman Spec., Vol. 2, 1978, p.310. and Yu. A. Pervak, J. Appl. Spec., Vol. 2 I, 1974, NO. 2, Ya. Kushnirenko pp.1050-1054. S. Montero, R. Schmolz and S. Haussuhl, J. Raman Spec., Vol. 2, 1974, pp. 101-113. B.J. Berenblut, P. Dawson and G.R. Wilkinson, Spectrochimica Acta, Vol. 29A, 1973, pp.29-36. P. Dawson and G.R. Wilkinson, Spectrochimica Acta, Vol. 27A. B.J. Berenblut, 1971, pp.1849-1863. M.A. Lopez-Bote and S. Montero, J. Raman Spec., Vol. 9, 1980, pp.386-392. H. Schulze, N. Weinstock, A. Muller and G. Vandrish, Spectrochimica Acta, Vol. 29A, 1973, pp.1705-1709. W.A. Adams and A.R. Davis, J. Phys. Chem., Vol. 78, 1974, R.M. Chatterjee, pp.246-250. Kazuyoshi Fujita and Masao Kimura, J. Raman Spec., Vol. 11, 1981, pp.108-111. S. Ikawa, M. Yamada and M. Kimura, J. Raman Spec., Vol. 6, 1977, pp.89-91. R.H. Busey and 0-L. Keller, Jr., J. Chem. Phys., Vol. 41, 1964, pp.215-225. F. Gonzalez-Vilchez and W.P. Griffith, J. Chem. Sot. Dalton Trans., Vol. 13, 1972, pp.1416-1421.