Mutual interactions in solutions of polar substances as observed in the Raman effect—III

Speetrochimica Acta,1963,Vol.10,pp.339 to 344. Pergamon PressLtd. Printed inNorthern Ireland

Mutual interactions in soIutions of pokr substances as observed in the Raman effect--III. ~et~~ol soIutio~ of Li, Na, I&, Ca and Ba perchlorates S. MINC and S. KUROWSKI Department of Physical Chemistry, Warsaw University, Warsaw Laboratory of Electrochemistry, Institute of Physical Chemistry, Polish Academy of Sciences, Warsaw (Received 26 &Far& 1962) Abstract-The influence of Li ClO,, Na ClO,, Mg(ClO,),, Ca(ClO,), and Ba(ClO.,), on the Raman band parameters of the C-O stretching mode (1033 cm-l) and the CH, group stretching vibration (2835 cm-l) of methyl alcohol were investigated. The changes in the integral molar intensity, position of the maximum, and half-width of both lines are given aa a function of the concentration. The mechanism of ~larization of the methanol molecules in the solvation sheath of cations is discussed.

INVESTIGATIONS on the influence of perchlorates

of alkali metals and alkaline-earth metals on the Raman band parameters of the solvent molecules were made to elucidate the mechanism of polarization of the bonds of these molecules on the solvation sheath of the solute cations. The choice of the electrolytes is connected with the work of K~CKI [l, 21. He observed rather unexpected changes in the intensity of the C-O stretching band under the influence of Li, Ca, Zn, chlorides, Since these changes were produced by cations, chloride anions as well as by ion pairs and complex ions (as in the case of ZnCl,) it seemed reasonable for us to investigate this problem in simpler systems. The perchlorate salts were chosen because of the small charge density of perchlorate anions and their relatively weak tendenoy to form complex ions, p~ticularly with cations of the alkali and alkaline-earth metals. The results on the investigation of similar systems by GOUSEAU [3] can only be partially employed, since he did not determine the changes in the integral molar intensity of the Raman band of methanol and of the ClO,- ion. EXPERIMENTAL

The spectrum was recorded by a photoelectric method with the aid of a Hilger The apparatus, the conditions of recording, and the method of determining the Raman line parameters have been described in Part I [I]. The perchlorates were prepared according to the method of WILLARD and SMITH (41. They were dissolved in distilled methyl alcohol to a concentration close to saturation. When the basic solution was not clear, it was trickled through a Schott G-4 funnel in a moisture-proof arrangement. The concentration ofperchlorates in the basic solutions was determined by a weight method. The solutions were obtained by diluting the basic solution. In order to determine the concentration of methanol in the diluted solutions the density was determined at 25%. E-612 spectrograph.

[1] Z. I@:cKI, ~~ec~roe~j~. Acta 18,1155 (1962). [2] Z. K~CKI, Spectrochim. Acta 18, 1165 (1962). [3] J. GOUBEATJ, 2. phyaik. Chem. Leipzig R38, 45 (1937). [4] H. H. WILLARD and G. F. SMITH, J. Am. Chem. Sot. 44, 2255

339

(1922).

S. MINCand S. KUROWSKI

340

The water content (determined by means of the Fischer agent) did not as a rule exceed 0.2 per cent by weight. Only in the case of the Mg(ClO,), did it attain 2 per cent. RESULTS The changes in the integral band intensity, position of the maximum and halfwidth of the C-O stretching mode (1033 cm-r) and the CH, group stretching vibration (2835 cm-l) are shown in Table 1 and in Figs. 1 and 2.

140 H .E ,B 120

2835cm-1

H

80

0

I

I I.0 Concentration,

I,,

I 2.0

Fig. 2 I 30

mole/l

Figs. 1 and 2. I&qpl molar intensity of 1033 cm-l and 2835 cm-l lines, vs. eon~~ntration. p’ LiCIO,, A N&IO,, i3 Mg(ClO,),, (3 Ca(ClO& o Ba(CIO,),

1033 cm-l band I,, the integral molar intensity, increases monotonically with an increase in the electrolyte concentration, the greater the density of the cation electric charge the stronger the increase: while the frequency of the maximum of this line decreases and the half-width increases. The contour of the 1033 0n1-~ band becomes diffuse with an increase in the concentration; the greater the density of the cation charge the more diffuse the band becomes, while the asymmetry of the contour of this band increases in comparison with the band contour in the pure solvent. However, the difference between the frequency at the maximum yg and the mean frequency 13does not exceed 3 cm-l: Y,, - i; < 3 cm-l, where

Mutual Table

interactions

in solutions

of polar

substances

as observed

in the Raman

effect-III

1. The dependence of molar integral intensity, I,, position of maximum-rs, width, S, of 1033 cm-l and 2835 cm-l bands on electrolyte concentration

I,,

Molar integral intensity

Position of maximum, vo LiCIO,

0.000 0.053 0.263 0.493 1.18 1.96 3.86

100 102 105 103 115 117 145

1033 1031 1031 1030 1030 1029 1026 NaClO,

0.000 0.294 0.588 0.882 1.176 1.764 2.35 2.94

100 100 100 101 101 108 121 122

1033 1033 1033 1032 1031 1029 1029 Mg(ClO,),

0.000 0,063 0.289 0.594 0.71 1.04 1.08

100 102 105 140 166 174 -

1033 1033 1033 1033 1032 1034 1031 Ca( ClO,),

0.000 0.048 0.095 0.496 0.97 1.49 2.05

100 98 101 106 118 129 163

1033 1033 1032 1031 1031 1029 1026 Ba(ClO,),

0.000 0.149 0.280 0.96 1.38 2.31

100 102 109 106 113 140

1033 1033 1032 1031 1031 1024

and

half-

2835 cm-l

1033 cm-l

Cone. (moles/l.)

341

I,, Molar integral intensity

Half-width,

b

in methanol

solution

18.6 18.7 19.5 19.5 24.0 26.7 29.4 in methanol 18.6 18.6 17.2 19.8 20.1 21.4 21.0 23.8 in methanol 18.6 19.9 23.4 28.8 30.6 35.4 36.5 in methanol 18.6 19.2 20.6 23.7 25.0 27.0 27.0 in methanol 18.6 18.6 21.8 23.6 23.0 25.2

100 103 105 105 104 97 96

Position of maximum, VO

Half-width,

2835 2835 2837 2837 2839 2843 2847

10.3 11.0 11.4 11.0 11.8 11.4 12.2

2835 2835 2837 2837 2838 2839 2839 2839

10.3 11.1 11.4 12.6 12.8 12.6 12.4 12.2

2835 2835 2837 2837 2841 2839 -

10.3 10.5 12.5 13.3 14.3 13.3

2835 2835 2835 2836 2838 2842

10.3 10.7 10.7 12.0 12.1 12.1 -

2835 2836 2835 2840 2842 2847

10.3 10.7 11.4 12.5 12.9 12.9

solution 100 103 100 103 101 95 98 86 solution 100 99 109 104 121 135

solution 100 99 100 94 95 99 solution 100 96 101 101 103 93

6

342

S. MINCand S. KUROWSKI

2835 cm-l band I,, integral molar intensity of this band, decreases slightly with an increase of the electrolyte concentration in methanol solutions for Li ClO,, Na ClO,, Ca(ClO,), and Ba(ClO,),, while in the case of Mg(ClO,), it increases. The frequency of the maximum v,, and the half-width 6 increases with an increase in the electrolyte concentration in every case. The contours become only slightly diffuse. DISCUSSION

We shall interpret the results under the following assumptions: (i) The solvation of the perchlorate ions is negligible. (ii) The perchlorate ions show little tendency to form complex ions with alkali metals and alkaline-earth metal cations. (iii) Ion pairs formed in large concentrations are solvated only from the positiveion side. The two first assumptions are well founded experimentally [2, 51. The third assumption is a rather obvious consequence of the first two. On the basis of assumptions (i), (ii) and (iii) we can ascribe all observed changes in the Raman line parameters of the methanol molecule to the action of the intense The cation electric field of the cations on solvent molecules in their direct vicinity. electric field cause the negative poles of the molecular dipoles to orient themselves in the direction of the cation field. The molecular dipole is situated on the bisector of the angle formed by the axes of the lone pairs. They are then subject to the strongest polarization. The electrons from the C-O and O-H bonds move to the orbital of the lone pairs, as the result of which these bonds are weakened and, consequently, the frequency of the stretching vibration also decreases. Under the influence of the cation electric field the hybridization of the lone-pair orbitals also changes. It can be shown [S, 71 that the Coulomb interaction of the cation with the electron cloud of the lone pair will increase with the participation of the 2s electrons in these orbitals until the digonal hybridization is attained. The hybridization of the lonepair orbitals is close to tetrahedral, (which would signify an angle 105” 56’ [S] between the axes of the OH and CO bonds). Such a change must accompany the opposite displacement of the 2p electrons from the lone-pair orbitals to the y(OH) and y(C0) bond orbitals. The increase in the participation of 2p electrons in the y(CO) and y(OH) orbitals causes a weakening of these bonds and consequently the decrease of the frequency of their stretching vibrations. The increase in the participation of 2p electrons in the w(CO) bond may arise partly also from the displacement of 2p electrons from the y(CH) orbitals. The opposite displacement of 2s electrons from y(C0) orbitals to the lone pair and y(CH) orbitals will stabilize energetically the [5] I. 0’ M. BOCKRISand B. E. CONWAY,Modern Aspec‘tsof Electrochemistry.

Butterworth, London (1945). [6] C. A. COULSON,l’alelzceClsrendonPress, Oxford (1953). [7] S. KUROWSKI.Dissertation (1961). Institute of Physical Chemistry, Polish Academy of Sciences. [S] C. H. TOU~NESand A. L. SCRAWLOW,Aficrowuve Spectroscopy. McGraw-Hill, New York (1955); D. J. SWALEN,J. Chem. Phys. 23, 1739 (1955); E. IVASH and D. DENIAON,J. Chem. Pkya. 21, 1804 (1953).

Mutual interactions in solutions

of polo substances a.aobserved in the Reman effect-111

343

solvation complexes and at the same time cause a strengthening of the v(CH) orbitals. This conclusion is in good agreement with the observed increase in frequency of the CH, group stretching vibration (Table 1). Hence we may infer that the increase in the p-character of y(C0) two-electron orbital is responsible for the increase of the scattering cross-section in the Raman spectrum of the CO bond stretching vibration. The spatial distribution of the electron cloud in the perturbed y(C0) bond in methanol molecules of the solvation sheath of cations is concentrated on the C-O bond axis to a greater degree than in the case of the unperturbed molecules. At the same time, its density decreases as a result of inductive displacement in the direction of the lone-pair orbitals. Hence the changes of the C:-0 bond polarizability tensor during the stretching vibrations will be greater in t’he perturbed molecules than in the unperturbed ones. Thus the t’ensor of the derivative of the C-O bond polarizability with respect to the R,. int,ernal co-ordinate particularly its main component parallel to the bond axis (i3~l~/iYRco)oincreases. In Part IV the results of an approximate calculation of the cross-section for scattering in the Raman spectrum are given for the stretching vibrations of the C-O methanol bond. The calculations show that the hybridization of the oxygen and carbon orbital as a result of the increase in the participation of Zip electrons has the greatest influence on the increase in the intensity of this line (1033 cm-l). The displacement of the electrons and the changes in hybridization of the molecular orbital produced in the solvent molecules by the anions have a direction opposite to that caused by the cations ; therefore the displacement and intensity of the 1033 cm-l stretching vibration of the CO bond should be opposite to the changes produced by the cations. The size of the changes in the 1033 cm-l parameters in perchlorate solutions of methanol depends, according to our results, on the cation electric-charge density. In the case of chloride solutions of the same cations in methanol, the effect of the action of the chloride anion is, according to KFCKI [2], greater than the effect of the perchlorate anion ; the results obtained for the LiCl solution in methanol would prove this, but the situation is additionally complicated by the association in ion pairs or even the formation of complexes. VV’e can interpret the changes in the integral molar intensity of the stretching vibrations of the CH, group if we consider the influence of the polarization and changes in the hybridization of the v(CH) orbitals on the value of the tensor (aa,,/a&.u),,. Such calculations were not made because of the unclear situation connected with the Fermi resonance in this group, e.g. according to REITZ and SABATHY [9] the 2835 cm-l line can be one of the components of the resonance doublet of the stretching vibration of the CH, group and the overtone of the deformation vibration of this group with a frequency of 1432 cm-l. If, however, the 2835 cm-l line was not a component of the resonance doublet but closely corresponded to the stretching vibration of the CH, group, then the observed slight decrease in the integral molar intensity and the increase in the vibration frequency can be explained by the displacement of the 2s electrons of the [9]A. W. REITZ and R. SABATHY, 2. physik. Chem.

Leipzig

B41 151 (1935).

344

S. MINC and S. KUROWSKI

carbon atom from the y(C0) orbital to the orbitals of the methyl group and the opposite displacement of the 2p electrons. The increase in the role of the (2s) function in the bond orbitals causes an increase strengthening of the bond. Since the increase in the participation of the (2s) function in the y(C0) orbital causes a decrease in the value of (&,,,/i3Rco),,, one can assume on the basis of this analogy that there also occurs a corresponding decrease in the tensor (&too/aR,,), for the C-H bond of the methyl group. authors wish to thank Professor W. KOEOSand Docent Dr. Z. numerous stimulating discussions.

Acknowledgements-The

K~CKI

for