3078
J. L. Dye, C. W. Andrews, and J. M. Ceraso
Nuclear Magnetic Resonance Studies of Alkali Metal Anions James L. Dye,’ Charles W. Andrews, and Joseph M. Ceraso Depatfment of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received July 28, 1975) Publication costs assisted by the U.S. Energy Research and Development Administration
The NMR chemical shift and line width has been measured for 23Na- in tetrahydrofuran (THF), ethylamine (EA), and methylamine (MA), for 87Rb- in T H F and EA, and for 133Cs- in THF. In all cases, the counterion was the 2,2,2 cryptate complex of the corresponding cation. The chemical shift of Na- is, within experimental error, the same as that calculated for the gaseous anion (based upon the measured value for the gaseous atom) and is independent of solvent. Comparison with the solvent-dependent chemical shift of Na+ provides conclusive evidence that Na- is a “genuine” anion with two electrons in a 3s orbital which shield the 2p electrons from the influence of the solvent. The line width increases from T H F to EA to MA, suggesting either an increasing exchange rate with the cryptated cation or, more probably, the influence of an increasing concentration of solvated electrons. In the case of sodium solutions in all solvents, both Na+C and Na- are detected by their NMR peaks. However, probably because of extreme line broadening, Rb+C and Cs+C are not observed, but only the relatively narrow line of the corresponding anion. The chemical shifts (diamagnetic shift in ppm from the infinitely dilute aqueous ion) are 185 and 197 for Rb- in EA and THF, respectively, and 292 for Cs- in THF, compared with 212 and 344, respectively, for the gaseous Rb and Cs atoms. When 18-crown-6 is used instead of the 2,2,2 cryptand complex, it is still possible to obtain solutions which are about 0.4 M in total metal when methylamine is used as the solvent. However, in this case, both the Na- and the Na+C NMR peaks are exchange broadened, even a t -5OoC, and coalesce as the temperature is raised to about -15 to OOC, depending upon the concentrations. The variation of the rate of exchange of the sodium nucleus between Na+C and Na- with concentration should permit determination of the exchange mechanism. Possible exchange mechanisms and the information obtainable from them are discussed.
Introduction
The evidence for the existence of species of stoichiometry M- in metal-amine and metal-ether solutions is very convincing.l The shift of the optical absorption band with metal, solvent, and t e m p e r a t ~ r e suggests ~?~ strongly that the species is a centrosymmetric anion. However, other models cannot be ruled out on the basis of optical evidence alone. Figure 1 shows three other contenders for the Mstructure. Indeed, one or more of such structures might be responsible for the diamagnetic species in metal-ammonia solutions which, to date, show no specific evidence for the existence of centrosymmetric anions. We believe that the 23Na- NMR spectra in ethylamine (EA) and tetrahydrofuran (THF),4 and the data described in this paper, provide the best evidence so far available that the anion in these solvents is centrosymmetric with two electrons in the outer s orbital. Optical p ~ m p i n g , atomic ~ , ~ beam,’ and NMRs-ll techniques have established the shielding constants of the aqueous cations 23Na+, 87Rb+, and 133Cs+relative to the gaseous atoms with an accuracy of a t least 2%. It is possible to make reliable calculations12 of the shielding constants of Na+(g) and Na-(g) relative to the free atom by using Lamb’s complete expression for atomic diamagnetic shielding13 and analytic Hartree-Fock wave functions.14 The changes in shielding constants are relatively small, amounting to only 7.7 ppm (diamagnetic) for the addition of two 3s electrons to gaseous Na+ to form Na-(g). Corresponding shifts for Rb-(g) and Cs-(g) are also expected to be small. Diamagnetic contributions from solvation are generally only a few parts per m i l l i ~ n , ~ and ~-l~ the major effect of the solvent, both from theoretical expectations The Journal of Physical Chemistry, Voi. 70, No. 26, 1975
and experimental results,ls is a substantial Paramagnetic shift (45 to 75 ppm) upon solvation. The magnitude of this shift correlates very well with the donicity of the sol~ e n t , l that ~ - ~is,~ the ability of the solvent to donate electron density to the cation. It is via the interaction of this solvent electron density with the outer p orbitals of the alkali metal cation which gives the observed paramagnetic shifts. Experimental Section
Solvents and metals were purified as previously de~ c r i b e d The . ~ 2,2,2 cryptand, l (C222), was synthesized by a modificationz4of the method of Dietrich, Lehn, and Sauvage.26The second complexing agent, 18-crown-6, 2 (18-C6), from PCR Inc., Gainesville, Fla., was recrystallized twice from acetonitrile and vacuum dried. The NMR system has been previously described4 except that multinuclear capabilities26have been added.
2,2,2 C r y p f a n d (CZZZ)
18-Crow n-6 (18-C -6)
Chemical Shifts and Line Widths
The key factor which permits us to study the NMR spectra of alkali metal anions is the complexation of the cation
3077
NMR Studies of Alkali Metal Anions
TABLE I: Selected List of Shielding Constants and Line Widths _ _ _ I _ _ . _ _ _ _ _ - ~ _ _ _ _ _ _ _ I
_ i l -
Refc
o(M,,hi vs. Mg),U
Ion
Concn, M
Temp, "C
Solvent
ppm
Auy2,b
HZ
5
-4%
--
-60.5 i 1 5.16 7 43 dil 25 H*0 7 -211.6 i 1.2 dil 25 H,O -344.3 f 5.8 0.02 7 43 25 H2O * dil 16,44 -168 t 2d 25 H2O dil 17 -1 94 (ca1cd)d 25 KO dil -61.2 8.0 11 25 H,Q Sat NaCl -72.2 9.0 0.3 NaI -1 5 MA Na+ 0.25 NaI 25 EA -74.4 17.9 Na+ 0.2 NaPh,B 25 THE' -52.9 23.0 Na+C 0.15 Na+C,Na-1 5 MA -49.8 30.8 Na+C 0.15 Na+C,Na-17 to +1 EA -50.8 120-170 Na+ C 0.15 Na+C,Na-4 TWF -50.4 51 NaGas +2.6 (calcd) 12 Na0.15 Na+C,Na-1 5 MA +1.4 11 Na0.15 Na+C,Na-17 to +1 EA +1.6 6-9 Na0.15 Na+C,Na-4 THF +2.3
