Infrared and ultrasonic spectra of sodium thiocyanate and lithium

Infrared and ultrasonic spectra of sodium thiocyanate and lithium thiocyanate in tetrahydrofuran. D. Saar, and S. Petrucci. J. Phys. Chem. , 1986, 90 ...
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J . Phys. Chem. 1986,90, 3326-3330

3326

Infrared and Ultrasonk Spectra of Sodium Thiocyanate and Lithium Thiocyanate in Tetrahydrofuran D. Saart and S. Petrucci* Department of Chemistry, Polytechnic University, Long Island Center, Farmingdale, New York I 1 735 (Received: December 17, 1985)

Infrared spectra of sodium thiocyanate solutions and lithium thiocyanate solutions in tetrahydrofuran (THF) in the wavenumber range 2000-2200 cm-' are reported. The digitized spectral envelope, which is due to the out-of-phase stretch of the SCN anion, is quantitatively described by the sum of three Gaussian-Lorentzian product functions. The three spectral bands are assigned respectively to the N-bonded ion pairs (-2057 cm-' for NaNCS, -2064 cm-' for LiNCS), to the dimers or quadrupoles (-2043 cm-' for (NaNCS)2, 2040 cm-' for (LiNCS),), and tentatively to triple ions (2074 cm-' for sodium thiocyanate, -2086 cm-' for lithium thiocyanate). The maximum absorbances per unit length of cell are expressed as a function of concentrations by cubic polynomial functions. Ultrasonic spectra for sodium thiocyanate solutions in THF in the frequency range 0.5-400 MHz are described by the sum of two Debye relaxation functions. The ultrasonic spectra for sodium thiocyanate in THF are interpreted as due to a two-step process, 2NaNCS NaNCS-NaNCS + (NaNCQ2, depicting the two-step dimerization of the ion pair NaNCS, the intermediate being a solvent-separated species. For lithium thiocyanate solutions in THF, the ultrasonic spectra are described by a single Debye relaxation function and interpreted by the dimerization scheme 2LiNCS LiNCS-LiNCS, the product of the reaction being mostly a solvent-separated species, the "contact* species being probably in lesser amounts than for (NaNCS)2 in accordance with the evidence from the infrared spectra. The limitations of the infrared method in giving reliable formation stoichiometric constants in media of low permittivity are discussed. The advantage of combining the structural information from vibrational spectra to the kinetic results of the ultrasonic spectra for a molecular interpretation of the latter ones is reiterated.

Introduction Recently there has been a renewed interest,' after the classical work of Fuoss and Kraus2 of the thirties, in the association and dimerization of electrolytes in media of low permittivity. The practical relevance of these systems, especially lithium salts in ethereal solutions, is due to their use in the construction of secondary batteries. Knowledge of the state of association of the electrolyte and of the lifetime of the complex species is essential for the optimal choice of solvent and electrolyte. Despite recent advances in application of transport theories of electrolytes3 and theoretical new expressions for the formation constant of dimers ion pairs4 and triplet^,^ our knowledge of the structure of the complex species in solution is scarce or nonexistent. To this end a program by vibrational I R spectra combined to the already existing molecular relaxation dynamic methods (microwave diolectric relaxation and/or ultrasonic relaxation) has been initiated. For the present work the electrolytes sodium thiocyanate and lithium thiocyanate in the solvent THF have been examined. For the IR spectra the intramolecular vibration of the anion, specifically the out-of-phase stretch of the SCN- anion6 (or, in the group frequency jargon, the "CN stretch"), and its changes due to molecular environments have been used as a probe to study the molecular complexation of the electrolytes. Ultrasonic relaxation spectra of the same systems are also reported, using the structural information from the IR spectra, for their interpretation. Experimental Section The IR spectra have been recorded by a Perkin-Elmer 9 8 3 6 spectrometer, using scan times and noise filter modes (4 and 16, respectively) slow enough that no spectral distortion was noticed with respect to slower modes. The cells were calibrated by the fringe method' before each collected spectrum. They were demountable Perkin-Elmer cells with 0.05-mm Teflon spacers. Both CaF2 and NaCl windows were used, ensuring independence of the spectra from the material of the windows, and hence their inertness to possible dissolution into the liquid. The ultrasonic instrumentation, data collection, and method have been described extensively in previous work.s Sodium thiocyanate and lithium thiocyanate (Baker reagents with analysis certificate) were redried Colgate-Palmolive Co., Piscataway, N.J.

0022-3654/86/2090-3326$01.50/0

at -70 and 110 OC, respectively, in vacuo (- 1 mm), in volumetric flasks until constancy of weight. Solutions were prepared by volume addition of tetrahydrofuran (THF) (Baker ACS) dried over molecular sieves (SA). The molecular sieves had been dried for many hours at 140 OC and c d e d in vacuo. Infrared spectra of the T H F product as received showed a faint band at -3300 cm-' which disappeared after a few days' exposure to molecular sieves. This indicated absence of water in this product within the resolution of the 983G spectrometer set with an ordinate scale of 0.25 in absorbance ( A = O.O02S/minimum division).

-

Results and Calculations Some of the infrared spectra expressed in absorbance vs. wavenumber (cm-I) are depicted in Figures 1 and 2. The spectra for sodium thiocyanate show rather dramatically the relative change of the two visible bands with total concentration of electrolyte. For lithium thiocyanate the satellite band is the one at lower wavenumbers at variance with the case of sodium thiocyanate, indicating qualitatively a different molecular distribution of the species present for the two electrolytes. For the quantitative interpretation of the spectra they were digitized with a resolution of 2.5 cm-I (1.25 cm-I near the peaks and shoulders). It is known that Lorentzian functions are particularly applicable to gases and Gaussian functions are particularly applicable9 to solids in terms of the band shape of vibrational (1) Farber, H.; Irish, D. E.; Petrucci, S. J. Phys. Chem. 1983,87, 3515. Harada, Y.; Salomon, M.; Petrucci, S. J . Phys. Chem. 1985, 88, 2005 and literature quoted therein. (2) Fuoss, R. M. J . Am. Chem. SOC.1934,56, 1127, 1031; 1936,58,982. Fuoss, R. M.; Kraus, C. A. J . Am. Chem. SOC.1935,57, 1; 1933.55.3614, (3) Delsignore, M.; Farber, H.; Petrucci, S. J. Phys. Chem. 1985,89,4968. (4) Maaser, H.; Delsignore, M.; Newstein, M.; Petrucci, S. J. Phys. Chem. 1984,88, 5100. (5) Delsignore, M.; Farber, H.; Petrucci, S. J. Phys. Chem. 1986, 90, 66. (6) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 2nd ed.; Academic: New York, 1975; Chapter 4. (7) Reference 6, Chapter 2. (8) Petrucci, S. J. Phys. Chem. 1967, 71, 1174. Darbari, G. S.; Richelson, M. R.; Petrucci, S. J. Chem. Phys. 1970, 53, 859. Bonsen, A.; Knoche, W.; Berger, W.; Giese, K.; Petrucci, S. Ber. Bunsenges. Phys. Chem. 1978, 82* 678. Chen, C.; Wallace, W.; Eyring, E.; Petrucci, S. J . Phys. Chem. 1984, 88. 2541. (9) Dogonadze, R. R.; Itskovitch, E. M.; Kuznetsov, A. M.; Vorotyntsev, M. A. J. Phys. Chem. 1975, 79, 2827.

0 1986 American Chemical Society

The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3327

Spectra of NaNCS and LiNCS

TABLE I: Calculated Infrared Parameters from Fitting the Mgithed Infrared Spectra to the Gaussian-brentzian Product Function'

~~

-

~

c,

I, mm 0.057 0.057 0.050 0.051 0.059

M

0.198 0.148 0.103 0.050

0.025

"2043

Ao1043

0.79 0.60 0.40 0.17 0.080

(A")I/2

2043.5 2043 2042.5 2042 2042

13 13 13 13 13

A02057

32057

0.47 0.42 0.25

2057 2056 2057 2056.5 2057

0.14

0.087

(Ay)I/2

13

13 13 13 13

A02074

%074

0.10 0.06 0.02 0.005

2074 2074 2074 2074

(Ay)I/Z

13 13 13 13

LiSCN I, rnm 0.056 0.056 0.057 0.061 0.056

c, M

0.268 0.210 0.100 0.066 0.042 'uj

0.20 0.14 0.05 0.03 0.016

%wl

(A")I/Z

A02064

"2064

(A")I/Z

A020*s

32086

(WI/Z

2038 2040 2040 2040 2040

14 14 14 13 13

1.70 1.26 0.67 0.58 0.29

2065 2065 2064 2063 2064

14 14 14 13 13

0.12 0.12 0.04 0.015 0.018

2088 2086 2086 2086 2086

14 14 14 13 13

and

= ( A ~ ~ ) ~ / ~ /for 1 . 4NaSCN 6 and LiSCN in THF.

(&j)1/2

are expressed in cm-I. 1.51

THF

of NaSCN 0.148 M in 0.

'

On5:

2 1'00 ' ' 2075 ' 20'50

'

'

Intririi ide'ctrii of NaSCN 0.0503M in '

:

t

\

0.4

'

'

'

'

'

'

'

20'26 '

'

'

'

'

'

'

2000 :'

THF

0.01 2150

0.4.

2125

2100

2075

2050

2025

G(c 1m ) -

t

Figure 2. Representative infrared spectrum for the CN stretch of lithium thiocyanate in THF.

< 0.3 0.2-

f

8 e c t r u m lor N m M 0.1ODM h THF Mpllh.d hhued

BoHd h. 18 the 8Un

Of

IhreW

0.1.

V

(cm-1)Fz-ZOS7 em-1

-

0.0sj

V O'

'

'

2100 '

'

Ckm-1)

(cm-1)-

20'75

'

'

2050

'

'

20'25

'

'

iobo

Figure 1. Representative infrared spectra for the CN stretch of sodium

thiocyanate in THF at various concentrations.

spectra. For the present case the empirical Gaussian-Lorentzian product function already used beforelo has been applied l+-

Figure 3. Digitized infrared spectrum and calculated spectrum sum of three Gaussian-Lorentzian product functions for sodium thiocyanate in THF. Cell length I = 0.057 mm. with A: the absorbance at the center-band wavenumber ej, ulthe variance uj = (Au)1/2/1.46, with ( A P ) ~the / ~bandwidth at (1/2)4" for each band. For the spectra of the present work it has proven to be necessary to use three Gaussian-Lorentzian bands for a quantitative interpretation of the absorbance envelope; hence A = CAI

(10) Irish, D. E.;Tang, S.Y.;Talts, H.; Petrucci, S.J. Phys. Chem. 1979,

83, 3268.

J

( j = 1, 2,

3)

(11)

The parameters used to interpret the spectra are collected in Table

Saar and Petrucci

3328 The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 t

Dbm-1)

Figure 4. Digitized infrared spectrum and calculated spectrum sum of three Gaussian-Lorentzian product functions for lithium thiocyanate in THF. Cell length I = 0.056 mm.

WHr)

-

Figure 6. Excess sound absorption per Wavelength p vs. frquencyjfor a representative ultrasonic spectrum of lithium thiocyanate in THF at 25 OC.

TABLE II: Ultraaodc Relaxation Parameters and Sound Velocities for NISCN and USCN in THF at t = 25 O C iOi78, iO-k,cm c,

M

105pI fi, MHz 105pII fiI, MHz

cm-' s2

S-1

NaSCN 0.20 315 0.158 260 0.11 210 0.056 95

90 80 70 65

40 35 15 10

12 10 7

6

124 124 126 130

1.289 1.304 1.284 1.291

112 122 126 124

1.282 1.279 1.283 1.283

LiSCN

I(Wd

-

Figure 5. Excess sound absorption per wavelength p vs. frequencyffor a representative ultrasonic spectrum of sodium thiocyanate in THF at 25 O C .

I for all the concentrations of sodium thiocyanate and of lithium thiocyanate investigated in THF. Figures 3 and 4 report representative plots of the digitized spectra for sodium thiocyanate and lithium thiocyanate in THF with the indicated band components (dashed lines) and their sum (solid lines) describing the absorbance envelope. Representative plots of the ultrasonic spectra for sodium thiocyanate and lithium thiocyanate in THF are reported in Figures 5 and 6. For the case of sodium thiocyanate two Debye processes are necessary to interpret the spectra expressed as excess sound absorptions per wavelength g = (a- B f )u/f vs. the frequencyf. In the above, CY is the sound attenuation coefficient, B is the background sound absorption ratio a / f at f >>fifiIwith fi andfiI the two relaxation frequencies, B = ( a / f ) p h J Iand ,, u is the sound velocity. Therefore

where gIand pIIare the maximum sound excess absorptions for the two Debye processes centered a t fi and fir, respectively. Table I1 collects all the calculated relaxation parameters and the sound velocities u for the concentrations investigated for sodium thiocyanate and lithium thiocyanate in THF a t 25 OC.

Discussion ( a ) Vibrational Spectra. The thiocyanate anion is konwn to be a potentially ambidentate ligand. It is a linear ion which may ligate through the nitrogen or sulfur atom. The three frequencies of the thiocyanate ion are the out-of-phase stretch fil, with characteristic frequency at 2050 cm-', the in-phase stretch fi3 with

0.10 370 0.066 140 0.042 150 0.024 85

60 50 45 35

characteristic frequency at 735 cm-I, and the bending (double degenerate) mode fi2 with frequency fi2 = 480 cm-'. These group frequencies (used here as a jargon in place of the more correct term wavenumber) change characteristically depending on the coordination mode. Specifically, N-bonded complexes cause an increase of f i l , v2 does not change, and fi3 increases, whereas S-bonded complexes show a larger fiI increase but a decrease of both fi2 and fi3, both being more diagnostic than pI in determining the bonding end of the anion. However, solvent bands often has established obscure the c2 and fi3 regions. Previous that with "hard" poorly polarizable ions as Na+ and Li+ the ion pairs contact complexes are N-bonded with a respective increase of -8 and 15 cm-I of the center-band frequency with respect to the spectroscopically free (ion and outer-sphere ion-paired) SCN- anion. On the basis of the above, one would attribute the band at ~ 2 0 5 7cm-l for NaNCS and ~ 2 0 6 4cm-I for LiNCS in T H F (Table I) to contact ion pairs. N o free ions or outer-sphere ion pairs are detectable. Indeed both these species, if present, would cause the appearance of an additional band at -2050 cm-I, which is absent for the present systems. In past work in this 1aboratoryl3 and previously by Chabanel et a1.l2 the band at -2040 cm-I has been attributed to contact dimer ion pairs or quadrupoles (NaNCS)2 and (LiNCS)2. One has to keep in mind however that the bands at 2057 and 2064 cm-I, respectively, could also be accounted for by solventseparated dimers (NaNCS-.NaNCS)2 and (LiNCS-.LiNCS)2 which would be spectroscopically indistinguishable from the monomer species NaNCS and LiNCS.

-

(1 1) Gans, P. Vibrating Molecules; Chapman and Hall: London, 1971 : Chapter 9. (12) Paoli, D.; Lucon, M.;Chabanel, M. Spectrochim. Acta, Part A 1978, 34, 1087 and previous literature quoted therein. (13) Saar, D.; Brauner, J.; Farber, H.; Petrucci, S.J . Phys. Chem. 1978, 82, 545.

The Journal of Physical Chemistry, Vol. 90, No. 15, 1986 3329

Spectra of NaNCS and LiNCS

( / = lenQh1 0 1 Infrared cell1

&03d' :

o

o

Figure 7. Maximum absorbances vs. concentration of the GaussianLorentzian bands for sodium thiocyanate in THF.

Therefore, for instance for sodium thiocyanate solutions any calculation of formation constants based on relations as AvzOn = tPkpand Aom = tqlcqis destined to fail if outer-sphere dimers are present. In the above cp and tq are the extinction coefficients or molar absorptivities of the ion pair and quadrupole, respectively, cpand cq the corresponding concentrations, and I is the length of the cell. In other words, even if assumptions as 2tp = cq are correct, the calculated Kq = cq/c: is going to be void of significance if outer-sphere dimers are present. This is because, in this case, AoW = tqcqContaft and A02057 = t ( c cqoUtersphcre). The calculated Kq = cqmnhct/(cp c~tcrsphc~c)Pc)2Pwou1d then become strongly concentration-dependent. The presence of solvent-separated dimers is consistent with the results of the ultrasonic analysis of the data given below. The small bands at 2074 and 2086 cm-' respectively for sodium thiocyanate and lithium thiocyanate have a less definite interpretation than the ones for the other two bands which has also been confirmed by normal-coordinate analysis.', We tentatively attribute these bands at 2074 and 2086 cm-'to triple ions existing at lower concentration than the other species. The latter is suggested by the smallness of the AO's. Because of the above considerations, we think that it is safe not to overextend vibrational spectrometry beyond its present capability in trying to calculate formation constants at low permittivities where, because of electrostatic forces, a larger population of outer-sphere dimers may exist. The same warning referred to ion pairs, in solvents of intermediate permittivity, was given by some of us in a past work.I0 The method may be approximately correct for ion pairs in aqueous solutions because of its large permittivity, although even in this case for metal(I1) sulfates, a large concentration of outer species ion pair appears to exist.I4 Accordingly, we have only calculated the concentration dependence of the individual maximum absorbances per unit length of cells I by fitting the data to cubic polynomials as reported below. For sodium thiocyanate in T H F A02043/1 = -0.31 6 0 3 . 1 ~-I-2187.5~'- 8 6 4 9 ~ ~r'; = 0.998

+

+

+ 5 1 6 . 6 ~+ 483c2 - 4942c3; r2 = 0.998 A02074/1 = 0.0131 - 5 6 . 3 4 ~+ 1195~'- 2 3 3 4 ~ ~ r2 ; = 0.999 For lithium thiocyanate solutions in T H F A02040/1 = -0.0006 51.980~+ 379.80~'- 2 8 5 . 9 5 ~ ~ ;rz = 0.99999 A02064/l= -0.252 1610.7~+ 5395.9~' + 1 3 . 4 9 0 ~ ~ 9 ; = 0.9987 A02086/l = 0.1170 - 17.480~' + 1254.4~'- 3 3 2 0 . 8 ~ ~ 9 ; = 0.9925

+

+

(14) Eigen, M.; De Maeyer, L. In Itwestigation of Rates and Mechanisms of Reaction. Weissberger, A,, Ed.;Wiley: New York, 1962; Vol. VIII, Part 11.

C(mol/dn+)--r

Figure 8. Maximum absorbances vs. concentration of the GaussianLorentzian bands for lithium thiocyanate in THF.

The above expressions have been calculated by nonlinear regression giving 50% statistical weight to the origin. It is noteworthy that the bands at 2043 and 2065 cm-' for sodium thiocyanate and lithium thiocyanate solutions may also be expressed as simple linear functions of c with an almost comparable determination coefficient 9 (dashed lines in Figures 7 and 8). For sodium thiocyanate in T H F

A02043/1 = -0.34

+ 714.2~; 9 = 0.997

For lithium thiocyanate in T H F

Aozo65/l = 3.08 + 111.1~; r2 = 0.9948 From the analysis of the IR spectra it would appear therefore that contact dimers are the dominant species for sodium thiocyanate in T H F with an almost comparable amount of "spectrmcopically" free monomers (free ion pairs and outer-sphere dimers). On the contrary for lithium thiocyanate in THF, the latter two species appear as the dominant species. This is in line with the relative solvation tendencies of Li+ vs. Na+ ions. (b) Ultrasonic Spectra. The ultrasonic spectra and the data collected in Table I1 for sodium thiocyanate in T H F can now be interpreted on the basis of the structural information gathered through the IR spectra. Simple concentration distribution calculations of the various species present in T H F with reasonable formation constant parameters as Kp= lo* M-I, KT = lo2 M-I, and Kq = 10 M-' (for the ion pair, triple, and quadruple formation constants) reveal that in the concentration range 0.05-0.2 M the ion pair and quadrupole concentrations are of the same order of magnitude but that cp,cq>> cT, the triple ion concentration. It is therefore unlikely that the source of any of the two Debye relaxation processes be due to equilibria involving triple ions. Accordingly, we have interpreted the ultrasonic relaxation spectra for sodium thiocyanate in T H F according to the two-step dimerization equilibrium

+

A02057/l = 0.1996

A%0071' 0.3

0.2

0.1

l

2AB

k k-i

AB-AB

k-1

(AB),

where AB = NaNCS, AB-AB = NaNCS-NaNCS, and (AB), = (NaNCS),. The above leads'$ to two relaxation times and Til, where 71,JI-I

S=

TI-'

= &[S

+ k-'+ k2 + k-, = 4kl(AB)(k2 + k-2) + k-'k-,

+ TIL'

P = T