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Y. M. Cahen, P. R. Handy, E. T. Roach, and A . I . Popov
Spectroscopic Studies of Ionic Solvation. XVI. Lithium-7 and Chlorine-35 Nuclear Magnetic Resonance Studies in Various Solvents Yves M. Cahen, Paul R. Handy, Eric T. Roach, and Alexander 1. Popov* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Received January IO, 1974; Revised Manuscript Received September 9, 1974) Publication costs assisted by Michigan State University
Chemical shifts of lithium-7 nucleus were measured in 11nonaqueous solvents against 4.0 M aqueous lithium perchlorate solution. Lithium perchlorate, chloride, bromide, iodide, triiodide, and tetraphenylborate were used. The shifts ranged from +2.80 ppm for acetonitrile, down to -2.54 ppm for pyridine. In dimethyl sulfoxide and dimethylformamide no evidence for contact ion pairing was observed. Formation of contact ion pairs was particularly evident in tetrahydrofuran, nitromethane, and tetramethylguanidine. The large broadening of the 35Cl resonance of the perchlorate ion in these solvents is in agreement with the above explanation. In contrast to sodium-23 nmr, no correlation was found between limiting chemical shifts in different solvents and the Gutmann donor numbers of these solvents.
Introduction of HC104 a t -946 f 6 ppm from concentrated aqueous HC1 solution with a line width of 0.15 f 0.005 G, Le., 62.5 Hz. Previous studies in this laboratory1 and e l s e ~ h e r e ~ - ~ The same observations were made by Saitog for the chemihave shown that sodium-23 nmr offers a very sensitive cal shift. In the latter case the line width was less than 0.1 probe of the environment of sodium ions in various solG, i.e., -42 Hz. vents and solvent mixtures. It was of interest to us to exThe relatively narrow line width of the 35Cl resonance for tend such studies to salts of other alkali metal ions in order perchlorate solutions is probably due to the highly symto determine the influence of the cation on the ionic equimetrical electronic density around the nucleus and one can libria and ionic species present in various nonaqueous solexpect its broadening when the perchlorate ion is involved vents. in interactions, such as contact ion pair formation, which The exchange of ions between different environments is will destroy such symmetry. usually rapid with respect to the nmr time scale, resulting In this paper we report the influence of the solvent, of in only one resonance signal at an average frequency deterthe concentration, and of the counterions on the 'Li chemimined by the magnetic shielding and lifetime of the nuclecal shifts. We also report 35Cl nmr line width of lithium us in each of the sites. Alteration of parameters such as perchlorate solutions in methanol, acetone, tetrahydrofurconcentration, counterions, and solvent produces changes an, tetramethylguanidine, and acetonitrile. in the relative proportion and type of environment which may be reflected by alteration of the nmr spectra on chemiExperimental Section cal shift and/or line shape, and/or line width of the obA . Reagents. Lithium perchlorate and lithium chloride served resonance. (Fisher) were dried at 190' for several days. Water content The properties of sLi nucleus are quite favorable for nmr was found to be, respectively, 0.2 and 0.6 wt %. Lithium iostudies. The resonance lines of Li+ ion in solutions are exdide (K & K Laboratories) was purified by recrystallization ceptionally narrow and chemical shift can be measured from acetone and dried under vacuum over P205 2 days a t with considerable accuracy. room temperature, 1 day a t 56.3', and 2 days at 82'. The Nuclear magnetic resonance studies on simple lithium purified salt contained 0.1 wt % of water. Lithium bromide salts in different solvents have been rather sparse. Craig (Reagent grade, Matheson Coleman and Bell) was dried at and Richards5 report on 7Li spin-lattice relaxation rates 190' for 3 days and contained 0.3 wt % of water. Lithium for lithium chloride solutions in water, methanol, formic tetraphenylborate was prepared from NaBPh4 (J. T. acid, and dimethylformamide and observe, inter alia, that Baker) by methathesis reaction,1° then dried over PzOb no significant variation in the chemical shifts was observed under vacuum for 12 hr at room temperature followed by with a change in salt concentration or in the solvent. Sub24 hr at 82'. The purified salt contained 0.1 w t % of water. sequent studies by Maciel, e t ~ l . and , ~ by Akitt and After drying, all the lithium salts were stored in a drybox Downs,7 however, showed that the frequency of the 'Li resonance is quite sensitive to the environment. The chemical under dry nitrogen atmosphere. Solutions of lithium triiodide were prepared by the addishifts in different solvents range from 2.90 ppm upfield tion of equimolar amounts of iodine to a lithium iodide so(from water) in acetonitrile, to 2.26 ppm downfield in pyrilution in a given solvent. Reagent grade iodine (Baker) was dine. The above studies also indicated that in certain solvents, the chemical shift was influenced by the anion of the used as received. B. Solvents. Nitromethane (spectroscopic grade, Allithium salt. Very few results are available on chlorine nmr in nondrich) was fractionally distilled and dried over freshly actiaqueous solvents and particularly on lithium perchlorate vated 5A Linde molecular sieves for 24 hr. Water content was found to be
