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of the North Texas State University Faculty Research Fund. References a n d Notes (1) C. A. Plint, R. M. B. Small, and H.L. Welsh, Can. J. Phys ., 32, 653 (1954). (2) T. Birchall and I.Drummond, J . Chem. SOC. A , 1859 (1970). (3) D. J. Gardiner, R. E. Hester, and W. E. L. Grossman, J. Chem. Phys., 59, 175 (1973). (4) J. H. Roberts, A. T. Lemley, and J. J. Lagowski, Spectrosc. Lett., 5, 271 (1972).
Irish et al. (5) J. W. Lundeen and W. H.Koehler, J. Phys. Chem., 79,2957 (1975). (6) M. Schwartz and C. H. Wang, J. Chem. Phys., 5Q, 5258 (1973). (7) A. T. Lemley, J. H. Roberts, K. R. Plowman, and J. J. Lagowski, J . Phys. Chem., 77, 2185 (1973). (8) D. J. Gardiner, R. E. Hester, and W. E. L. Grossman, J. Raman Spectrosc., 1, 87 (1973). (9) J. H. Roberts and B. de Bettignies, J. Phys. Chem., 78, 2106 (1974). (IO) G. G. Simmestad, G. W. F. Pardoe, and H. A. Gebble, J . Ouant. Spectrosc. Radiat. Transfer, 12, 559 (1972). (11) P. Datta and G. M. Barrow, J. Am. Chem. SOC.,87, 3053 (1965). (12) W. B. Smlth and A. M. Ihrig, J. Phys. Chem., 75, 497 (1971). (13) I. L. Babich, I. I.Kondilenko, P. A. Korotkov, and V. E. Pogorelov, Opt. Specfrosc. (USSR), 29, 537 (1970).
Raman, Infrared, and Ultrasonic Relaxation Studies of Some Sodium and Lithium Salts in Dimethylacetamide' D. E. Irish," S.-Y. Tang, Guelph-Waterloo Centre for Graduate Work in Chemistry, Waterloo Campus, University of waterloo, Waterloo, Ontario, Canada N2L 3 0 1
H. Talts, and S. Petruccl Department of Chemistry, Polytechnic Institute of New York, Brooklyn, New York 11201 (Received April 26, 1979) Publication costs assisted by the Natural Sciences and Engineering Research Council Canada
Raman spectra, some infrared spectra, and ultrasonic relaxation spectra of solutions of LiSCN, NaSCN, LiN03, and NaN03in N,N-dimethylacetamide (DMA) have been obtained and correlated. Vibrational bands of free and ion-paired anions have been identified. Relative integrated intensities have been measured and used to obtain stability constants for the principal equilibria. Ultrasonic relaxation spectra, in the frequency range 3-350 MHz, show a single Debye relaxation. These kinetic data have been interpreted in terms of the Eigen theory, utilizing the conclusions from the Raman study where applicable.
Introduction Raman and infrared spectral studies of electrolyte solutions provide structural information; evidence for contact ion-pair formation, the populations of species in the major equilibria, and the values of contact ion-pair formation constants are frequently 0btainable.~8Ultrasonic relaxation techniques provide information about the kinetics of the formation of the ~pecies.~ Vibrational spectra provide information about species with a lifetime as short as s and ultrasonic methods are currently used for transformations relaxing in a time scale range of lo4 to lo4 s. The motivation for the present study was to assess the possibility of directly correlating and unifying the conclusions from these two lines of experimentation. By making measurements in comparable concentration ranges we believed that we might obtain direct confirmation of the species invoked in the mechanism and that the data might be coupled with advantage. To these authors' knowledge such a direct correlation has never been attempted before. N,N-Dimethylacetamide (DMA) has a dielectric constant of 37.8 a t 25 "C, a dipole moment of 3.81 D, and a wide liquid range;5 the solubility of many electrolytes is high in this solvent. We have chosen for examination of the ion-ion interactions the salts NaSCN, LiSCN, NaN03, and LiN03in DMA. Ultrasonic results are also presented for KSCN and NaN02. In order to establish the molar intensity of the "free" (i.e., solvated) SCN- ion, we have prepared solutions of ammonium thiocyanate containing tetrabutylammonium perchlorate and measured the Ra0022-365417912083-3268501.OOlO
man intensity of the v(C-N) stretching mode. Experimental Section N,N-Dimethylacetamide (Aldrich Chemicals, 99+ %) was redistilled twice in vacuo in all-glass apparatus with no grease in the ground joints. The column was a 3 f t Vigreux line. LiN03 (Fisher Certified), NaSCN (Baker Analyzed), KSCN (Baker Analyzed), NaN03 (Fisher Certified), NH4SCN (Fisher Certified), NaN02 (Fisher Certified), and NH4N03(Baker Analyzed) were dried to constant weight in an oven at 120 "C and atmospheric pressure. LiSCN.xHzO (Alfa Inorganics) was dried in vacuo at room temperature to the approximate composition LiSCN.H,O; it was then converted into the anhydrous form with constant weight by slowly raising the temperature over a several-day period to 110 "C. Shortening the process led to decomposition of the salt and the appearance of a yellow-green color. NaC104-H20and LiC104 (G. F. Smith Reagent) were dried in vacuo a t 60-70 "C. Anhydrous tetrabutylammonium perchlorate and NaC104 (G. F. Smith Reagent) were employed without further purification. Solutions were prepared by weight, filtered through Millipore FHL PO 13OO,0.5-pm filters directly into Pyrex glass capillaries, and sealed for Raman study. Spectra were excited with the 514.5-nm line of a Spectra-Physics argon ion laser operating at 0.8 W and were recorded on a Jarrell-Ash Raman system.6 Solutions contained both the electrolyte of interest and a second salt to provide an in@ 1979 American Chemical Society
The Journal of Physical Chemistty, Vol. 83, No. 25, 1979 3269
Sodium and Lithium Salts in Dimethylacetamide
ternal intensity standard; quantitative intensity data were obtained by running the sequence standard-samplestandard. The spectral slit width was 2 or 3 cm-l and scanning conditions were typically 1000 or 3000 cm-l, 10 s, and 0.1 cm-l s-l. Infrared spectra of the same solutions used in the ultrasonic study were recorded on a PerkinElmer 521 instrument using demountable cells with CaF2 windows. The sample was run against a solvent blank. Even though the cell depth was matched by using the fringe method, the absorbance data were not considered sufficiently reliable for quantitative analysis. The frequency data complemented the Raman spectra, however. Digitized band profiles were resolved into component bands by employing the computer program CURVERe7 The equipment and procedure for the pulsed ultrasonic measurements were as described in previous papems The attenuation coefficients cy (neper cm-’) were calculated from the collected attenuations (dB) vs. distances as slopes by fitting the data with a linear regression computer program. The calculated correlation coefficients, r2, were greater than 0.995. Crystal displacements in the liquid were always below R2/2X (with R the radius of the emitting crystal and X the wavelength of sound) so as to remain within the Fresnel zone. A Kai calibrated attenuator which could be read down to 0.1 dB was used for frequencies below 10 MHz. The attenuation coefficients, a, and corresponding frequencies, f , were fitted to the Debye single relaxation function4 a / f = IA/[1
+ (f/fR)21j + B
2100
2020
2060
p/ C M-’ Figure 1. CN stretching region of Raman spectra of LiSCN/UMA solutions: (A) 0.1285 M; (6)0.8121 M; (C) 1.830 M.
(1)
by multiple linear regression to the form z = a. alx a,y
+
+
To this end eq 1 was rearranged as 1 a / f L = ( A + B ) - ;cy + --f2B fR
fR2
where z = a/?, x = CY, and y = f“; then a. = A + B, al = - l / f R 2 and a2 = B/fR2. For the case of the Li+ salts and NaNOz the fit was rather poor because most of the attenuation data had to be collected at frequencies above f R , and because of the enhanced statistical weight of the high-frequencydata due to the f“ term. A similar undesirable weighting of the high-frequency data results by following the Mikhailov p r o ~ e d u r e . ~I t was found convenient in these cases to follow the iteration procedure previously describedlO (choosing a tentative B value and calculating f R and A by simple linear regression).
Results and Discussion I. Vibrational Spectra. The linear thiocyanate anion generates a three-line vibrational spectrum nominally as follows: vl(C-N) 2059 cm-’, v2 (degenerate bend) 480 cm-l, v3(C-Q)735 cm-l. The formation of either N- or S-bonded complexes gives rise to spectral changes that have been well d~cumented.ll-~~ For N-bonded coordination v1 increases, v3 increases, and v2 is largely unchanged; for S-bonded coordination v1 increases marginally more, and v2 and v3 decrease. For all of the cases studied in this work SCNis believed to be N bonded to the cation as in the studies of Paoli et al.13 Typical Raman spectra are illustrated in Figure 1. Clearly, as the Concentration of salt increases the 2059-cm-l band of the free SCN- gives way to a 2075-cm-l band, assigned to the Li+NCS- ion pair. Similarly two bands are observed in the infrared spectrum (Figure 2); when the experimental contour, converted from
-l_.L,----.-. .Lo-
2160
2120
2080
-
2040
‘J
2000
v (cm-l)
Figure 2. CN stretching region of the infrared spectrum of a 0.435 M LiSCN/DMA solution.
transmittance to absorbance, was fitted with two Lorentzian-Gaussian product functions, two lines coincident with those observed in the Raman spectrum were found. Likewise both infrared and Raman spectra of NaSCN/ DMA solutions show two bands but these are more closely spaced (Figure 3); they occur at 2060 and 2068 cm-l. For 0.18 M KSCN only a single symmetrical 2060-em1 band was observed. The ultrasonic absorption data indicated no relaxation for 0.15 M KSCN but did indicate definite relaxation processes for both NaSCN and LiSCN. Both the 480- and 735-cnrl bands are obscured by intense lines ofthe solvent. In order to confirm the existence of the ion pairs and to deduce the formation constant for the process, the Raman intensities of the NaSCN/DMA solutions were quantitatively analyzed and used to calculate the average ligand numbers li = CB/CT, where CBis the concentration of bound SCN- and CT is the total concentration of Na+. The formation constants were extracted from the n,CFdata by the GAUSS-z method.“J5 First it was necessary to establish the molar intensity, JF,of the “free” SCN- ion. Solutions of NH4SCN containing a small amount of Ru4NC104generate a single v(C-N) band and it was assumed, therefore, that the stoichiometric salt concentration was equal to the “free” SCN- concentration, CF.Relative /1931, were integrated intensity ratios, IF = 1206~[Ci041 plotted against CF(Figure 4,supplementary material) and linearity was observed. The internal intensity standard (610;) compensates for differences in the refractive index.
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The Journal of Physical Chemistry, Vol. 83, No. 25, 1979 I
I
I
”.
UOSCk
2313
N O
2363
t /c I C
0.2
I
1
0.4
0.6
0.8
CF
Flgure 5. Observed average ligand number, A , vs. the concentration of “free” SCN- for NaSCN/DMA solutions: (0) with tetrabuiybmmonium perchlorate as standard; (8)with sodium perchlorate as standard. The solid line has been calculated for
81.
mation curve was fitted by the equation Pl[SCN-] + 2&[SCN-I2 1 + Pl[SCN-] + &[SCN-I2
Flgure 3. cuRvm-analyzed Raman contours of NaSCN/DMA solutions: (A) 0.1014 M; (B) 1.109 M; (C) 1.543 M.
0
Irish et
K = 1.49 M-’.
The slope of the curve, defined by a least-squares analysis, was 1.71. The concentrations of “free” SCN- in the NaSCN solutions were then calculated from the I F / J F ratios, and CB was obtained from CT - CF. Thus the formation curve (Figure 5) was generated. Some samples containing more than the stoichiometric amount of Na+ were prepared by dissolving NaC104 in the NaSCN/DMA solutions. The data from these fall on the same curve. Clearly, from the nature of the curve, only mononuclear complexes are f ~ r m e d .A~single K1 = P1 equal to 1.49 f 0.01 M-l (25 “C) adequately fits the data; the curve computed from ri = PICF/(l + &CF) is shown in Figure 5. The data are presented in Table I. A critical discussion of the reliability of the stability constants evaluated from Raman intensities and the potential discrepancy between these p’s and values obtained from a model which includes outer sphere ion pairs is presented in the Conclusions section of this paper. Similar data for LiSCN in DMA (Table I) exhibit-more scatter when plotted as It vs. Cp, despite high-quality spectra (Figure 1) and repeated experiments. The for-
with PI = 0.75 f 0.17 M-l and P2 = 1.80 f 0.07 M-2at 25 “C. The fundamental modes of vibration of the nitrate ion consist of vl, A{ (1050 cm-’), v2, A/ (830 cm-’), v3, E’ (1380 cm-l), and v4, E’ (720 cm-l). Of these, vl, v3, and v4 are Raman active and v2, v3, and v4 are infrared active. The positions are sensitive to the medium; for example, in DMA v1 occurs at 1040 cm-l. The 720-cm-l region of the Raman spectrum is masked by an intense band of DMA; similarly the 1300-1500-~m~~ region is obscured by several bands of this solvent. This is unfortunate because the observation of a doublet in the 720-cm-l region is a clear indication of the existence of both free and ion-paired nitrate ions and equilibrium constants have frequently been inferred from the intensities of this ~ a i r ; ~changes B characteristic of ion pairing also occur in the 1300-1500cm-l region and these are also masked by lines of the solvent. In some cases the symmetric stretch vl, Al’ is markedly perturbed by ion pairing. For example, for the crystalline hydrates of Mg(N03),,outer sphere nitrate gives rise to a 1060-~m-~ line and inner sphere nitrate gives rise to 1050-cm-l (tetrahydrate, monodentate orientation) and 107O-cm-’ (dihydrate, bidentate orientation) lines.le These lines, altered in position and broadened, are also apparent in the spectra of ionic liquids (water-salt ratios
