Alkali Metal NMR Chemical Shielding as a Probe of Local Structure: An Experimental and Theoretical Study of Rb + in Halide Lattices | Angel C. de Dios, ² Ann Walling, ² Ian Cameron, ‡,§ Christopher I. Ratcliffe,* ,‡ and John A. Ripmeester ‡ Department of Chemistry, Georgetown UniVersity, Washington, DC 20057, and Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex DriVe, Ottawa, Ontario, Canada K1A 0R6 ReceiVed: NoVember 15, 1999 When Rb + is incorporated into different cubic halide lattices at very low concentrations, the 87 Rb NMR chemical shift becomes more shielded by as much as 282 ppm when the cation is changed from Na + to Cs + . Similarly, in mixed crystals of KCl and RbCl, the average resonance frequency shifts with the degree of incorporation. The 87 Rb chemical shift data for each halide show a near-linear correlation with the average alkali metal-halide interionic distance in the different crystals studied, in good agreement with ab initio calculations which show that indeed the chemical shielding has a strong dependence on the nearest neighbor shell of halide anions and their distance from the Rb + . When the Rb concentration is increased as in KCl/ RbCl solid solutions, the 87 Rb NMR spectra are influenced by a large distribution of K/Rb configurations in the next-nearest-neighbor shell of 12 cations. While the total shift is still largely dependent on the Rb-Cl distance, shells containing both K and Rb atoms give rise to second-order quadrupolar effects and an offset from the isotropic chemical shift. This work is a remarkable demonstration of the sensitivity of the 87 Rb nucleus to its physical environment. Introduction For many years, chemical shifts have been a primary tool for chemists in assigning structural features of new compounds and determining speciation in chemical reactions. For covalent materials, the extensive experience gained from some 40 years of solution NMR spectroscopy was transferred easily to the solid state. On the other hand, for ionic species, this transition is not made so readily as the change in local structure is likely to be far greater on going from solution to the solid state than for covalent compounds. In light of the increasingly important role that the spectra of quadrupolar nuclei are likely to play in the study of materials, especially with the availability of high fields, it becomes of considerable importance to increase our under- standing of the chemical shifts for this vast class of magnetically active nuclides. In a development that has advanced considerably over the last 20 years, the 129 Xe NMR chemical shift of atomic Xe, also known to be very sensitive to local environment, is now understood in terms of local structure. 1,2 Although 129 Xe NMR was used extensively in the study of porous materials (recently reviewed 1 ), the correlation between the chemical shift and the size of the trapping site can now be understood from first principles. 2 Such environmental influences on the chemical shift have also been noted for other situations, e.g., the shifts of guest molecules in clathrate hydrates in cages of different sizes 3,4 ( 13 C in CH 4 , 19 F in CH 3 F, 31 P in PH 3 and 77 Se in H 2 Se), 15 N of NH 4 + in different salts, 5 19 F in BF 4 - salts. 6 The work presented here originally arose out of curiosity about how much the detailed structure of the crystal lattice around an alkali metal ion such as Rb + could influence its chemical shift. For instance, if it were to be claimed that an experimentally observed 87 Rb NMR chemical shift was char- acteristic of rubidium chloride, what exactly would this mean? The alkali metal and ammonium halide salts provide a means of answering this question; all have cubic crystal structures, either of the NaCl or CsCl crystal types, where the cations occupy sites of high symmetry surrounded by a nearest-neighbor shell of six or eight anions, respectively. The cations are located on cubic sites so there is no significant quadrupolar coupling to contend with, except sites affected by defects, and the lines are isotropic. By low-level doping of Rb into the lattices of such salts with the same anion, such that the host lattice parameters are preserved, it becomes possible to vary the average environment of the Rb atom systematically in terms of the nearest-neighbor distance to a particular halide ion. 87 Rb (27.85% abundant, I ) 3 / 2 ) is a relatively heavy atom with many electrons, so one expects a moderately large chemical shift dispersion. The experimental results presented here, obtained some time ago, are intriguing, showing large chemical shifts, with ∼282 ppm chemical shift difference between Rb doped into CsCl and NaBr lattices, and marked correlations with interionic distances. However, these observations have only recently been rationalized on a firm theoretical basis by means of the ab initio calculations of the 87 Rb chemical shielding, also presented here. Since RbCl and KCl are miscible in all proportions to form solid solutions, a range of compositions where the Rb is no longer a trace impurity (5-95% RbCl) were also investigated and the 87 Rb NMR spectra show some very interesting effects. In this context, in a recent report of chemical shifts in mixed K/Rb/X (X ) Br, I) crystals, 7 it was concluded that the shifts * Corresponding author. Phone: (613) 991-1240. Fax: (613) 998-7833. E-mail: cir@ned1.sims.nrc.ca. ² Georgetown University. ‡ National Research Council of Canada. § Current address: Ottawa General Hospital, MRI Unit, Smyth Road, Ottawa. | Published as NRCC No. 43824. 908 J. Phys. Chem. A 2000, 104, 908-914 10.1021/jp994043h CCC: $19.00 © 2000 American Chemical Society Published on Web 01/15/2000