Multinuclear Nmr Studies of Alkali Ions in Nonaqueous Solvents

Nuclear magnetic resonance of alkali elements is a very sensitive probe of the immediate chemical environment of an alkali ion in solutions and can be used to study ion-ion, ion-solvent and ionligand interactions. In particular, this technique has been used to determine contact ion pair formation constants, to compare solvating abilities of different solvents and to study the thermodynamics of complexation reactions of alkali ions with macrocyclic polyethers (crowns) and diazapolyoxamacrobicyclic ligands (cryptands) in nonaqueous solvents. INTRODUCTION During the past decade there has been a very rapid expansion of the use of multinuclear NMR for the studies of thermodynamics and kinetics of reactions in solutions. To a very large extent this expansion is the result of progress in NMR instrumentation and, particularly to the introduction of Fourier transform NMR spectroscopy. Among the many nuclei susceptible to NMR measurements probably the most popular group of elements, aside from the proton and carbon-l3, have been the alkalies. All alkali elements have at least one natural isotope with I > 1/2 (Table 1) and, therefore, a quadrupole TABLE 1. Nuclear Properties of Alkali Elements (Natural Isotopes) Nucleus NMR frequency (MHz) at 14.09 kgauss Natural Abundance (%) Spin Sensitivity relative to lH at constant field 6Li 8.827 7.43 1 8.51 x lO" 7Li 23.315 92.57 3/2 0.294 23Na 15.868 100 3/2 9.27 x io2 39K 2.800 93.08 3/2 5.08 x lO 40K 3.480 1.19 x lO2 4 5.21 x lO 41K 1.539 6.91 3/2 8.39 x lO6 85Rb 5.792 72.8 5/2 1.05 x 10-2 87Rb 19.630 27.2 3/2 0.177 133Cs 7.864 100 7/2 4.74 x 10-2 moment; nevertheless with the exception of rubidium, the resonance lines are quite narrow and, in the case of 7Li and l33Cs the natural linewidths are less than 1 Hz. Thus in most cases chemical shifts can be measured quite. precisely. The sensitivities of measurements vary with the nucleus but, in general, they are adequate to detect the resonance signals down to 0.01 M solutions especially when the Fourier transform technique is combined with a high field superconducting solenoid. Figure 1 shows 39K signal for a 0.005 M solution of KPF6 in acetonitrile obtained at a field of 42.3 kG. 102 ALEXANDER I. POPOV Figure 1. Potassium-39 NMR resonance of 0.005 Msolution of KPF6 in acetonitrile. Field strength of 42.3 kG, 2000 scans. Chemical shifts of alkali nuclei are very sensitive to the immediate environment of an alkali ion in solution, thus the alkali metal NMR is a very sensitive technique for the detection of ion—ion, ion-solvent and ion-ligand interactions. The magnitude of the paramagnetic screening constant ap, for an alkali nucleus, is proportional to p LEl where is the expectation value for the outermost p-electron of the element and iE is the average excitation energy (1). Since and tEI both increase with increasing atomic nunter (2), the magnitude and, therefore, the range of c steadily increases in going from Li+ to Cs+. Thus the usual range of the chemical shifts varies from 10 ppm for Li+ to several hundred ppm for Cs+. The popularity of alkali metal NMR is undoubtedly related to the biological importance of lithium, sodium and potassium ions and to attempts to elucidate their behavior in living systems. In addition, the advent of new complexing agents, such as antibiotic ionophores, macrocyclic polyethers (crowns) and diazapolyoxamacrobicyclic ligands (cryptands) which can form stable complexes with alkalies, has opened a new chapter in the coordination chemistry of these elements. CONCENTRATION DEPENDENCE OF CHEMICAL SHIFTS The concentration and counterion dependences of alkali chemical shifts for alkali salts in aqueous solutions were explored in detail by Richards and co-workers (3). As can be seen from Figure 2, the magnitude of the chemical shift strongly depends on the counterion and on the salt concentration. The concentration dependence of the chemical shifts was ascribed primarily to the Debye-Huckel type of cation-anion interactions. Similar measurements in nonaqueous solutions seem to indicate that the cationic chemical shifts result from the formation of contact ion pairs (4), primarily influenced by the nearest neighbors. Thus, for example, in glacial acetic acid solutions, 7Li chemical shifts are only very slightly dependent on the salt concentration (5) although the low dielectric constant of the solvent (6.2 at 25°C) predicts a considerable amount of ionic association and experimental results show that even for strong electrolytes the ion pair dissociation constants are of the order of 10-6 (6). The above NMR results, together with the observation that the frequency of the far infrared band of Li+ vibration in an acetic acid solvent cage is anion independent (7), strongly argue that the ion pairs in glacial acetic acid must be solvent-separated. Concentration-dependent chemical shifts can be used to calculate contact ion pair formation constants. In all such cases the exchange is fast on the NMR time scale and only one, population-average, signal is obtained 6obs = SfXf + 6jpXjp (1) where f and ip are the chemical shifts of the free ion and the ion in the ion pair respectively, and Xf and Xp are the respective populations of the two species. It can be easily shown that Concentration (moles/kg H20) 2 3 4 5 6 7 8 9 0 Figure 2. Potassium-39 chemical shifts in aqueous solutions (3b). where K is the ion pair formation constant, C is the concentration of the salt and y is the mean activity coefficient. The values of K and Sjp can be obtained from a nonlinear leastsquares iteration program (8). INFLUENCE OF SOLVENTS Solvent dependence of alkali chemical shifts is shown in Table 2. Kidd and Bloor argue convincingly that in the case of the 23Na resonance the chemical shifts are determined by the TABLE 2. Chemical Shifts for Alkali Ions in Nonaqueous Solvents; ppm, Negative Shifts are Paramagnetic Vav Reference Solvent L÷(a) Na+c) K' 5+(b,c) Nitromethane 0.4 15.0 21.1 59.8 Acetonitrile 2.8 8.1 0.4 -32.0 Propylene Carbonate 0.6 — 11.5 35.2 Acetone -1.3 9.6 10.5 26.8 FOrmamide — 3.8 — 27.9 Methanol 0.5 3.4 10.1 45.2 Dimethylformamide -0.5 4.8 2.8 0.5 Dimethyl Sulfoxide 1.1 -0.4 -7.8 -68.0 Pyridine -2.5 -1.6 -0.8 -29.4 (a) vs 4.0 M aq. LiClO4, ref. 5; (b) vs aq. soln. at infinite solution; (c) ref. 11; (d) ref. 12; (e) ref. 8. paramagnetic screening constant (9) and this argument certainly must be valid for the 39K and I33Cs resonances. Since the solvent-induced paramagnetic shift is determined by the overlap of the outer p orbital of an alkali ion with the outer orbitals of the solvent molecule, one can expect to find some correlation between the magnitudes of the paramagnetic shifts of the Multinuclear NMR studies of alkali ions 103