ANALYSIS BY RAMAN SPECTROSCOPY OF THE STRUCTURE AND THE DYNAMICS OF SOME POLYATOMIC ANIONS IN AQUEOUS SOLUTIONS

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ANALYSIS BY RAMAN SPECTROSCOPY OF THE STRUCTURE AND THE DYNAMICS OF SOME POLYATOMIC ANIONS IN AQUEOUS SOLUTIONS

M. Perrot, F. Guillaume

To cite this version:

M. Perrot, F. Guillaume. ANALYSIS BY RAMAN SPECTROSCOPY OF THE STRUCTURE AND THE DYNAMICS OF SOME POLYATOMIC ANIONS IN AQUEOUS SOLUTIONS. Journal de Physique Colloques, 1984, 45 (C7), pp.C7-161-C7-167. �10.1051/jphyscol:1984717�. �jpa-00224282�

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment a u n09, Tome 45, septembre 1984 page C7-161

ANALYSIS BY RAMAN SPECTROSCOPY OF THE STRUCTURE AND THE DYNAMICS OF SOME POLYATOMIC ANIONS IN AQUEOUS SOLUTIONS

M. Perrot and F. Guillaume

Laboratoire de Spectroscopie Infrarouge, L.A. 124, Universite' de Bordeaux I , 33405 Talenee, France

Resume -Apartir des profils vibrationnels de diffusion Raman I

VV et IVH dlanions en solutions aqueuses, enregistrgs B diverses temperatures et con- centrations, il est possible d'obtenir d'importantes informations sur la structure et le comportement temporel de l'environnement B courte distance de ces ions en solution. Cette methode qui consiste B analyser les fonctions de correlation Raman isotrope et anisotrope permet de suivre de facon ind6- pendante la dynamique orientationnelle et la dynamique vibrationnelle de quel- ques anions en solution aqueuse liquide et vitreuse.

Abstract - From the vibrational IVV and IVH Raman profiles of several anions in aqueous solution at various temperatures and concentrations, one may obtain important information on the structure of the time-varying local order existing around these aquated ions. This technique of analysing Raman isotropic and anisotropic correlation functions allows for the independent studies of the orientational and the vibrational dynamics of some anions in liquid and glassy aqueous sol.utions.

INTRODUCTION

Infrared and Raman vibrational spectroscopy have been used for a long time to obtain information on the structure of liquid water and ionic aqueous solutions. Neverthe- less, the corresponding spectra are broad, complex, difficult to analyse without decompositions through structural models and not very sensitive to temperature and pressure effects. Moreover, important salt concentrations are needed in order to notice appreciable changes between the spectra of pure water and of ionic solutions (I - 3 ) .

Improvement in spectroscopic techniques and in numerical evaluation of the spectral data, as well as the introduction of new theoretical approaches like Fluctuation- Dissipation theorem and correlation functions formalism, make it now possible to study the dynamics of polyatomic anions in water by analysing their own vibrational profiles (4-8Y.

This method involves the numerical Fourier transformation of the spectral frequency data of a convenient vibrational fundamental of the solvated anion into the time domain which is usually called "Fluctuation Spectroscopy1'. Thereby, direct information on the mechanism of anion-water interaction and on the temporary structure of ionic solutions may be obtained.

Every spectral profile I(w) is related via Fourier transform to the correlation function of a macroscopic quantity A(t) :

1 (w) = F.T. <A(O)

.

Nevertheless, by a careful choice of the experimental conditions, it is possible to

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984717

C7-162 JOURNAL DE PHYSIQUE

o b t a i n some s p e c t r a l p r o f i l e s f o r which t h e a s s o c i a t e d c o r r e l a t i o n f u n c t i o n r e f l e c t s t h e dynamics of only one o r two simple molecular v a r i a b l e s . Thus i n t h e c a s e of aqueous i o n i c s o l u t i o n s , one may c o n s i d e r t o s t u d y t h e s p e c t r a l p r o f i l e of a p a r t i - c u l a r v i b r a t i o n of t h e i o n f o r v a r i o u s temperatures, p r e s s u r e s and s a l t c o n c e n t r a t i o n s , so t h a t t h e a n a l y s i s of t h e corresponding time c o r r e l a t i o n f u n c t i o n s would give

important i n f o r m a t i o n on t h e molecular dynamics of t h e s e s o l u t i o n s .

RAMAN SCATTERING ANALYSIS

V i b r a t i o n a l Raman Spectroscopy i s p a r t i c u l a r l y w e l l adapted t o t h i s kind of s t u d y because t h e Raman spectrum of l i q u i d water i s n o t v e r y i n t e n s e ( 1 , 3 ) and i s a l s o w e l l s e p a r a t e d i n frequency from t h e v i b r a t i o n a l p r o f i l e s of s e v e r a l polyatomic a n i o n s . I n t h e v i b r a t i o n a l Raman s c a t t e r i n g p r o c e s s , t h e molecular q u a n t i t y involved i s t h e d e r i v a t i v e of t h e molecular p o l a r i z a b i l i t y t e n s o r 8 w i t h r e s p e c t t o a normal c o o r d i n a t e Q. For a t o t a l l y symmetric v i b r a t i o n of a l i n e a r o r a symmetric-top a n i o n , s e v e r a l a u t h o r s have r e c e n t l y shown t h a t two d i f f e r e n t Raman p r o f i l e s may be obtained according t o two d i f f e r e n t s c a t t e r i n g geometries u s u a l l y c a l l e d IVV and IVH (12,131.

I n t h i s c a s e , t h e ropert ties of t h e second rank t e n s o r 8 g i v e r i s e t o two s p e c t r a : an i s o t r o p i c spectrum Iiso(u) = I v v ( u ) - 4/3 IVH (w) = F.T. G . (t)

I S 0 / 2 /

an a n i s o t r o p i c spectrum Ianiso(w) = 15 IVH (w) = F.T. G

a n i s o ( t ) / 3 / r e l a t e d t o t h e two c o r r e l a t i o n f u n c t i o n s :

a ^B 2 ^-+ +

and G anise ( t ) = (-) < Q ( o >

.

.

a ^Q

In t h e s e e q u a t i o n s , Q i s t h e normal c o o r d i n a t e of t h e v i b r a t i o n , P2 i s t h e second Legendre polynomial of t h e u n i t v e c t o r ?i l y i n g on t h e main symmetry a x i s of t h e i o n ,

i s t h e t r a c e and t h e a n i s o t r o p y of t h e molecular p o l a r i z a b i l i t y .

From t h e s e r e l a t i a n s h i p s , i t appears t h a t t h e i s o t r o p i c p r o f i l e i s only r e l a t e d t o t h e v i b r a t i o n a l dynamics, w h i l e t h e a n i s o t r o p i c p r o f i l e r e f l e c t s b o t h t h e v i b r a t i o n a l and t h e o r i e n t a t i o n a l dynamics of t h e anion surrounded by t h e water molecules.

EXPERIMENTAL

The experimental r e s u l t s d i s c u s s e d h e r e concern t h e t o t a l l y symmetric v i b r a t i o n a l Raman p r o f i l e (V ) of s e v e r a l a n i o n s a s n i t r a t e , c a r b o n a t e , cyanide, t h i o c y a n a t e and s u l f a t e i n aqueous s o l u t i o n s f o r temperatures between -200 and 1 + 80 C e l s i u s and a t v a r i o u s c o n c e n t t a t i o n s .

The e x c i t a t i o n i s o b t a i n e d w i t h an Argon l a s e r and t h e s p e c t r a a r e recorded on a Coderg T 800 t r i p l e monochromator connected t o a PDP 1 1 computer which allows f o r d a t a a c q u i s i t i o n , d a t a accumulation and numerical t r e a t m e n t s a s s p e c t r a l moments d e t e r m i n a t i o n , c o r r e l a t i o n f u n c t i o n d e r i v a t i o n s and v a r i o u s adjustments between t h e o r e t i c a l and experimental p r o f i l e s .

RESULTS AND DISCUSSION

As we have a l r e a d y shown t h a t t h e IVV and IVH Raman p r o f i l e s a l l o w f o r independently

study t h e v i b r a t i o n a l and t h e o r i e n t a t i o n a l dynamics of some aquated a n i o n s , we s h a l l r e p o r t on our main r e s u l t s i n t o t h e s e two domains.

1 ) O r i e n t a t i o n a l Dynamics

The e a r l i e s t a n a l y s i s s t a r t e d w i t h t h e o r i e n t a t i o n a l dynamics because s e v e r a l dynamical models were a l r e a d y proposed (14-16) t o cover many mechanisms from f r e e r o t a t i o n t o a n g u l a r jumps o r r o t a t i o n a l d i f f u s i o n .

I n t h e c a s e of t h e n i t r a t e a n i o n , t h e experimental r e s u l t s (Fig.1) on t h e r e o r i e n t a - t i o n of t h e C3 a x i s of symmetry show t h a t f o r very low temperature, i n t h e v i t r e o u s s t a t e , t h e I and IVH p r o f i l e s a r e almost e x a c t l y i d e n t i c a l proving t h a t t h e orientationaYVdynamics i s t o t a l l y f r o z e n out. On t h e o t h e r hand, a s t h e temperature i s r a i s e d , t h e IVB p r o f i l e becomes l a r g e r than i t s corresponding I e x h i b i t i n g t h a t t h e o r i e n t a t ~ o n a l dynamics of t h e a n i o n i n t o i t s water favoured by i n c r e a s i n g t h e temperature ( 6 , 8 ) .

vitreous $tote

- ^{5 0 0 0 + 5 0 cm-'}

F i g u r e 1 - IVV and IVH ( V, 1050 cm-I ) Raman p r o f i l e s of t h e NO- 3

anion i n g l a s s y (- 200 C) and l i q u i d (+ 80 C) aqueous s o l u t i o n .

From t h e r a t i o G

a n i s o ( t ) / Giso ( t ) , an o r i e n t a t i o n a l c o r r e l a t i o n time

and an a c t i v a t i o n energy of t h i s dynamical process may be o b t a i n e d .

I n t h e c a s e of t h e n i t r a t e a n i o n , t h e o r i e n t a t i o n a l c h a r a c t e r i s t i c time ( t a b l e 1) v a r i e s between 2.4 ps. and 0.7 ps. The a s s o c i a t e d a c t i v a t i o n energy i s found around 2 . 5 Kcal. p e r mole, a v a l u e q u i t e c l o s e t o t h e energy needed t o break a weak O . . . . H hydrogen bond.

Table 1 - O r i e n t a t i o n a l c o r r e l a t i o n time (PS) f o r NO3 a t v a r i o u s temperatures. -

C7-164 J O U R N A L DE PHYSIQUE

Similar results have been obtained for other anions of different shape and size (table 2) permitting to conclude that the orientational dynamics of the main symmetry axis is going faster for the smallest anions as the correlation time is 10 ps for the large SCN-, 4.8 ps for CO;-, 1.3 ps for NO; and only 0.8 ps for the small CN- anion.

Table 2 - Orientational correlation time (ps) for several anions at room temperature.

/

cN-

I

Lastly, for the cyanide anion, one may also obtain the corresponding vibrational infrared profile related to the dynamics of the derivative of the molecular dipole m n t . In this case, the simultaneous knowledge of the reorientation of the first rank tensor (dipole moment) and the second rank tensor (polarizability tensor) allows to obtain the mechanism of the orientational dynamics (5). The best fit model for cyanide is instantaneous large angles jumps reorientations between equivalent water-anion interaction sites.

From this set of results, we may conclude that the small anions like CN-, NO;, CO;- and SCN- have a fast orientational dynamics into their aqueous solvation sphere and that this dynamics is favoured by increasing the temperature and may be totally stopped at low temperature in the vitreous state.

2) Vibrational Dynamics

Even though isotropic Raman profiles seem simpler to analyse as they are only governed by

one

However, within some experimental constraints one of these origins may be prevalent and governs almost alone the isotropic Raman profile of the anion in the aqueous solution. It is now generally accepted that the dephasing process is predominent for the vibrational relaxation of the anions (6,8,19). This corresponds physically to consider that all the anions in solution do not have exactly the same water environment giving rise to a vibrational frequency distribution around a mean fre- quency value. Moreover, these environments may change in time so that the vibrational frequency fluctuates.

This results in various halfwidths and shapes of the isotropic spectra according to the different anions (Fig.2).

Figure 2 - Isotropic Raman spectra of NO; and SCN- anions in aqueous solution at room temperature.

In order to account for this phenomenon, a dynamical model has been proposed by Rothschild (11,17) following the work of Kubo (20). This model uses a Gaussian

distribution of the environment and a Markovian process for the frequency fluctuations.

This model has the advantage to give a correlation function :

which is easily tractable and uses physical paramaters (M2 is the second spectral moment defining the Gaussian distribution and -cw is the correlation time of the

fluctuations). Moreover, it also reaches the two good limits of fast and slow modulations (11,17).

Applying this theory to our experimental results one may deduce that : i) For one molar aqueous solutions of ~ 0 4 , ~ 0 -- 3 ~and SCN- while the ~ 0 ~ -rw

are almost identica4 (around 0.3 ps), the associated second moments M2 vary between 40 cm-2 and 520 c m (table 3). This essentially show that the environment distribu- tion of the water molecules depends on the size and the shape of the solvated anion.

Table 3 - Fluctuation characteristic times and vibrational second moments for one molar solutions at room temperature.

--

anion

/

ii) For a given anion as the temperature is raised, M increases and 2 T w

decreases (table 4). This is characteristic of a broadening of the distribution and an increase of the velocity of the fluctuations. Increasing the temperature gives rise to a faster modulation conducing to the homogeneous limit.

Table 4 - Fluctuation characteristic time and vibrational second moment for one molar NO3 solution at various temperatures.

I

/

1

C7-166 JOURNAL DE PHYSIQUE

i i i ) For t h e v i t r e o u s s t a t e , .rU becomes g r e a t and f o r t h e s e low t e m p e r a t u r e s , t h e system reach almost t h e Gaussian s t a t i c l i m i t . I n t h e s e c o n d i t i o n s , one may not completly n e g l e c t t h e v i b r a t i o n a l energy r e l a x a t i o n p r o c e s s which t a k e s p l a c e on t h e 10 picoseconds time s c a l e and may s l i g h t l y i n f l u e n c e t h e s p e c t r a l p r o f i l e

i i i i ) A t l a s t f o r t h e one molar l i t h i u m n i t r a t e s o l u t i o n , t h e p r o g r e s s i v e a d d i t i o n of l i t h i u m c h l o r i d e o r t h e e l e v a t i o n of temperature o r a l s o t h e d e c r e a s e of p r e s s u r e give r i s e t o t h e a p p a r i t i o n of a s l i g h t dissymetry on t h e i s o t r o p i c Raman p r o f i l e of t h e v, v i b r a t i o n of t h e n i t r a t e a n i o n ( f i g u r e 3 ) .

P R E S S U R E E F F E C T ,1052.5

without LiCl with 8 M LiCl

1000

9 0 bars

7

Figure 3 - L i + c a t i o n e f f e c t and p r e s s u r e e f f e c t on t h e i s o t r o p i c spectrum of t h e v , NO- v i b r a t i o n .

3

This e f f e c t g i v e s evidence f o r t h e formation of anion-cation i n t e r a c t i o n s c h a r a c t e r i z e d by t h e e q u i l i b r i u m :

between s o l v e n t s e p a r a t e d ions p a i r s ( o r e x t e r n a l sphere s o l v a t i o n ) and c o n t a c t i o n s p a i r s ( o r i n t e r n a l sphere s o l v a t i o n ) . From our experimental r e s u l t s and t h e i r a n a l y s i s u s i n g non-Gaussian d i s t r i b u t i o n f u n c t i o n of t h e v i b r a t i o n a l frequency ( 2 1 ) , we a r e a b l e t o conclude t h a t t h i s e q u i l i b r i u m p o s s e s s e s a n e g a t i v e AH and a p o s i t i v e AV.

These r e s u l t s a g r e e w i t h t h e w e l l known s t r o n g i n t e r a c t i o n p r o p e r t i e s e x i s t i n g between water and t h e l i t h i u m c a t i o n (22-24).

CONCLUSION

I n t h i s paper we have t r i e d t o p o i n t o u t t h a t Raman v i b r a t i o n a l spectroscopy i s a technique w e l l adapted t o t h e s t u d y of t h e s t r u c t u r e and t h e dynamics of simple anions i n aqueous s o l u t i o n s . From t h e time F o u r i e r a n a l y s i s of t h e v i b r a t i o n a l p r o f i l e s , one may o b t a i n s e p a r a t l y t h e c h a r a c t e r i s t i c times of t h e v i b r a t i o n a l and of t h e o r i e n t a t i o n a l dynamics of t h e aquated anions and a l s o u s e f u l information on t h e water-anion and cation-anion i n t e r a c t i o n s and t h i s f o r v a r i o u s temperatures and p r e s s u r e s .

N e v e r t h e l e s s , s e v e r a l l i m i t a t i o n s do e x i s t f o r t h i s type of s t u d y a s f o r example t h e o b l i g a t i o n t o use a t l e a s t s a l t c o n c e n t r a t i o n s around one mole p e r l i t e r and a l s o t h e requirement t o a n a l y s e p r o f i l e s w e l l i s o l a t e d over a l a r g e frequency range.

Several o t h e r techniques a s NMR r e l a x a t i o n t i m e s , i n f r a r e d a b s o r p t i o n , d e p o l a r i z e d Rayleigh s c a t t e r i n g and q u a s i - e l a s t i c i n c o h e r e n t n e u t r o n s c a t t e r i n g may a l s o provide v a l u a b l e complementary i n f o r m a t i o n f o r such anion-water i n t e r a c t i o n s t u d i e s .

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