J. Phys. Chem. 1880, 84, 1129-1 133
x ( x ) is related to the potential energy of interaction between K and e-. The potential energy should be strongly screened in IIME, so we take x ( x ) = 0. The symbol D is the sum of the diffusion coefficients of K and e- in DME. Most likely, D D;. With eq 4, we may evaluate eq 2 readily as
-
8,
1 -j x-2 O(x - ro) dx = IZ 4irD
-= k, 4irD
L l e x - 2
ro
dx =
___ 1
4irDr0 (5)
The significance of eq 5 is the following: From a knowledge of D, ro, and k,, we may compute Q, and hence P8, Assuming each photon hv absorbed produces one pair (Ke-), then P, is the ultimate quantum yield for photodetachment. Assuming D x D;, 47rDr0 depends only upon the solvent amd is independent of the solute. Hence, if photodetachment of Na- can be carried out in DME, one should expect QJkz = Q2Na/kN,. Similar ideas have been advanced recently by Tachiya and Sano.2
1129
(1) S. A. Rice and J. K. Baird, J. Chem. Phys., 69,1989 (1978). ( 2 ) M. Tachiya and H. Sano, J. Chem. Phys. submitted for publication.
SEDDON. Sodium is insoluble in DME so the relevance of this analysis is obscure. DYE. Although I think it likely that the spin-paired species in ammonia is e-.M+.e-, your data do not rule out M+-e?- and the bare e?- would always form the ion pair by Coulomb attraction. Your data verify nicely the stoichiometry M- but not the nature of this species.
SCHINDEWOLF. Indeed, that is all thermodynamics can do. It is believing rather than knowing that the Coulomb repulsion in e&, even in contact with a cation, would blow up the diamagnetic species. I dislike the two electrons in one cavity also on the grounds that the optical spectra and the conductivity are only little affected in the spin pairing concentration range.
Spin-Lattiice Relaxation in the Alkaline Earth Hexaammlnes M. J. Mobley, W. S. Glaunslnger, and R. F. Marzke" Departments of Chemistry and Physics, Arizona State University, Tempe, Arizona 85281 (Received July 16, 1079) Publication costs assisted by Arizona State University
The proton spin-lattice relaxation times Tlof Ca("3)6, SI("&, and Ba("3)6 have been measured at 8 MHz in the range 30-200 K by using standard pulsed NMR techniques. Dispersed samples were prepared from ingots of the compounds and were mixed with alumina powder to ensure electrical isolation of the metal grains. The samples exhibited T1behavior differing significantly from that of ammonia. Three different mechanisms for nuclear relaxation appear to operate in this temperature range: (1)rotational diffusion of ammonia at high temperatures (above -100 K), (2) activated rotation of ammonia about the pseudo C3 axis in the intermediate temperature range (-50-100 K), and (3) rotational tunneling which results in temperature independent Tlvalues in the low temperature range (below -50 K). Activation barriers to ammonia diffusion and rotation were determined. Nonexponential relaxation was observed in the vicinity of the TIminima. These samples exhibited asymmetricalline shapes expected of metal powders with an average particle size somewhat larger than their skin depths. Knight shifts in the narrow line region were determined to be less than 10 ppm, in agreement with recent measurements by cw techniques. The liqe widths also agreed, to within experimental error, with previously published cw data.
Introduction Recent neutron diffraction and NMR studies have indicated interesting and unusual structures and molecular motions for the alkaline earth hexaammines, Ca("3)6, ST("^)^, and Ba(NH3)e.1-3Powder neutron diffraction studies on deuterated forms of these compounds have shown that they crystallize in a body-centered-cubic structure with the metal atom octahedrally coordinated of the structure of by six ammonia l i g a n d ~ .Refinement ~ Ca(ND& has indicated a distorted ammonia geometry, the ammonia ligands being nearly planar with one short N-D bond (0.94 A) and two extremely long N-D bonds (1.39 A). The pseudotrigonal axis of each ammonia molecule is not coincidmt with the Ca-N bond, but makes an angle of roughly 13" with it.2 Similar distorted geometries were proposed for the protonated compounds to account for the narrow (
