J. Phys. Chem. 1995, 99, 3793-3797
3793
Hydration of Lithium Ion in Aqueous Solution+ Wolfram Rudolph,* Murray H. Brooker, and Cory C. Pye Chemistry Department, Memorial University of Newfoundland, St. John's, Newfoundland, Canada AIB 3x7 Received: September 21, 1994; In Final Form: December 7, 1994@
Raman spectra of aqueous lithium salt solutions (LiC1, LiBr, and LiC104) were measured from saturation concentrations to more dilute solutions at room temperature. In the saturated LiCl and LiBr solutions a polarized band at 380 and 335 cm-', respectively, was detected. This band was assigned to a symmetric vibrational mode from an inner sphere complex LiCVBr(OH2),+ where n is probably three. In more dilute aqueous LiCl and LiBr solutions the high frequency component disappeared, and a new polarized band at 255 cm-' (FWHH60 cm-') was observed and assigned to the Li-0 symmetric stretching motion of Li(OH2)4+. This is the first definitive report of the symmetric stretching motion of an univalent cation. Ab initio geometry optimizations and frequency calculations at the 3-21G and 6-31G*level were performed on the tetraaquolithium species and support our assignment.
1. Introduction Although the hydration of Li+ in aqueous solution was studied extensively by a broad variety of structural'-6 and physicochemical techniques,'-" the hydration number of the first sphere is still not established. Hydration numbers between 3.3 and 9.1 have been reported and depend on the method of study. Nevertheless the knowledge of the hydration structure of electrolytes is important for the understanding of transport properties in solutions and in membranes. Improved information is required on the average coordination number, the structural geometry, and the residence lifetime of the water molecules in the first coordination sphere. Previous attempt^'^-'^ to identify the Li-0 modes of the Li(OH2),+ hydrate by Raman spectroscopy are inconsistent. The measurements were complicated by overlap from the low frequency bands of water and the intense quasi-elastic wing of the central Rayleigh line. It has been shown r e ~ e n t l y ' ~that J~ for most accurate relative intensity measurements it is essential to normalize the low-frequency Raman data for the BoseEinstein temperature factor, B, and a frequency factor in order to present the data in a spectral form that is directly related to the relative molar scattering factor, SQ(V) = (aa/aQi)*, where Qi is the mass-weighted normal coordinate. The relation between Qi and qi, the dimensionless normal coordinate is the following: Qi = (2n~Pi/h)-~'Lqi. It should be pointed out that also another normalization procedure is used in the literature, namely R,(P). This form of the spectrum has not been normalized to include the frequency factor and will be proportional to the molar scattering activity, S,(V) (see ref 16 and references therein). The reduced or RQ(P)spectrum was constructed after eq 1:
RQ(5)= Z(5)Q0 -
(1)
Equation 1 is valid under the condition that the normal coordinates are taken to be harmonic and the polarizability expansion is terminated after the fiist-order term (i.e., the double harmonic approximation). The RQ(B)spectrum has been related
* Corresponding author, permanent address: TU Dresden, Universitaetsklinikum "Carl Gustav Carus", D-01307 Dresden, Fetscherstrasse 74, Germany. t Parts of the material was presented at the Ninth International Conference on Molten Salts, San Francisco, May 22-25, 1994. 'Abstract published in Advance ACS Abstracts, February 15, 1995.
to the vibrational density of states spectrum and may also be considered as the energy absorbed in a scattering process (see ref 16 and references therein). Raman difference methods employed in earlier studies12-14 cannot be considered reliable because it has been shown17 that dissolved ions alter the low-frequency Raman spectrum of water. Difference spectra obtained by subtracting the spectrum of pure water from the spectrum of an aqueous solution must be viewed with caution. Raman spectra (in RQ format) were measured for lithium salts (LiX: X = C1-, Br-, C104-) at room temperature with different mol ratios LiX:H20 in order to characterize the cation influence on the low frequency Raman area of water. Studies of the perchlorate solutions have the advantage that the intensity due to water in the low frequency region is reduced. It was felt necessary to corroborate the experimental results with theoretical calculations. For this purpose, Hartree-Fock calculations with three different basis sets were employed. The geometry of the tetraaquolithium ion was optimized as explained in detail in the Experimental Section.
2. Experimental Section LiCl and LiBr (Fisher certified) were used without further purification to prepare saturated stock solutions at 22 "C. Distilled water was treated with activated charcoal to remove fluorescent impurities and filtered through a fine sintered-glass frit. All diluted solutions were prepared by weight. The density of the solutions were checked with a 5 mL pycnometer. LiC104 was prepared by reacting LiZCO3 with HC104. The LiC104 was recrystallized twice, and from these crystals a stock solution was prepared. Raman spectra were obtained with a Coderg PHO spectrophotometer after sample excitation with a Control Laser Model 552 argon ion laser. The 90' scattering geometry was employed with the incident light analyzed with Polaroid film to give the ~I[X(ZZ)Y]and IL[X(ZX)Y] scattering geometries. A quarterwave plate before the entrance slit served to compensate for the grating polarization preference. Raman wavenumber accuracy was f 2 cm-I and was checked with plasma lines of the laser. The laser power at the sample was about 800 mW. The slit width of the double monochromator were set at 4 cm-l. The PMT was cooled to -30 "C, and the signals were collected via photon counting. The scan rate was 50 crn-'/min, and the
0022-3654/95/2099-3793$09.00/0 0 1995 American Chemical Society
Rudolph et al.
3794 J. Phys. Chem., Vol. 99, No. 11, 1995 total counts were collected for the time intervals that corresponded to a 0.5 cm-' spectral interval. Six to eight spectra at each orientation were collected, and the digital output was stored on the Memorial University VAX 4500 computer. Spectra were signal averaged and smoothed twice with a Savitsky-Golay three-point smoothing function. A baseline program was applied which corrected the measured intensity, Z(P), for the fourth power scattering factor and normalized the spectra. These normalized Z(V) spectra (which should be independent of excitation intensity) must be further normalized to account for the temperature factor B = [ l - exp(-hVc/kr)] and the frequency factor to give the RQ spectra as shown in eq 1. The reduced spectra are most important in the low-frequency region where the effect of temperature is most pronounced. The normalized isotropic spectra in RQ format was constructed by the subtraction
- 4/3RQ(y)l
RQ(v)iso= RQ(v)f[
.-It A /1 4M)
G
'F _I 2
1001
1 1030 O t
fly) = 1
i=l
+ 4 (P - V,d2/bL
(3)
where Zi are the band heights at the peak positions Vi, bGi, and are the full widths at half height (FWHH) of the Gaussian and Lorentzian fractions of the bands and n the number of component bands. The FW" bi of the two Lorentzian-Gaussian product functions were determined by an iterative procedure solving the nonlinear equation
300
400
500
600
900 1000
700 800
wavenumber Icrn.7 Figure 1. Isotropic Raman spectra in R-format for LiCl solutions at three different mol ratios LiC1:HZO at room temperature between 40 cm-l and 900 cm-l.
(2)
Zi exp[-4 ln(2)(P - Q2/bii1
200
100
0
700
Digitalized isotropic spectra in RQ format (in the following text shortly R format) were fitted to a sum of LorentzianGaussian product functions:20 n
200
.-
1
n
mol ratio LiBr : H20 400-
1 : 2.72
.-
$
300:
1 : 4.46
2
200
.-
1
1 : 6.90
1 : 10.52
hi
O=
exp(- 1n(2)b;/bii)
1
- + bflbii
- 112
(4)
The Lorentzian-Gaussian shape ratio can be defined as R = bG/(bG hl).Pure Lorentzian band shape character is obtained for R = 1 (in the case b a >> hl)and a pure Gaussian band shape for R = 0 (in the case bGi
