Analysis of Xylene Mixtures Using Binuclear Lanthanide(lll)-Silver(1) NMR Shift Reagents Thomas J. Wenzel and Mark D. Russett Bates College, Lewiston. ME 04240 Xylene isomers are difficult compounds to distinguish and auantifv in mixtures. Gas chromatoera~hic " . analvsis reauires the use of specially designed columns ( I , 2). A common method for auantifvine - xvlene isomers. and one that is emplayed freq"ently as an undergraduate laboratory experiv . report ment (2. 3). is the use of infrared s ~ e c t r o s c o ~This will describe an alternative to infrared spect'r© fo; the quantification of xylene mixtures. The method employs NMR spectroscopy and involves the use of binuclear lanthanide(II1)-silver(1) NMR shift reagents. Applications of lanthanide shift reagents are well documented (4.5). Lanthanide tris beta-diketonates are suitable for use with oxveen.- and nitroeeu-containineu com~ounds. Several undergraduate experiments utilizing lanthanide tris beta-diketonates have been described (6-9).Binuclear lanthanide (111)-silver(1) complexes are effective for shifting the NMR spectra of soft Lewis bases such as aromatics. olefins, halogenated compounds, and phosphines (10-13): The binuclear reagents are formed in solution from alanthanide tris beta-diketonate and a silver beta-diketonate. The lanthanide ion causes the shifts in the NMR spectrum of the substrate (S), and the silver ion provides a bridge between the lanthanide and the soft Lewis base. Equations 1 and 2 are representative of the equilibria involved with the binuclear reagents.
- -
-
+ Ag(@-dik)= Ag[Ln(p-dik)J Ln(@-dik)3 Ag[Ln(p-dik),] + S = (S)Ag[Ln(@-dik),]
(1) (2)
Sterirand indurtiveet'ferts influenre the bondin~ofsil\,er lo aromatic sut~stmtes.Silver preferentially bonds to aromatic substrates a t positions removed from steric encumbrances. The arrows in structures 1-3,
show the preferred hinding sites for the xylene isomers. The hindine constants for o-. m-. and D-xvlene with silver nitrate in metKanol-water mixtures are i.43,1.35, and 1.14, respectively (14). These values reflect the relative distance of the preferred hinding site in each isomer from the steric hindrance of the methyl groups. Competitive bonding is observed for a mixture OF xyienes when the concentration of shift reagent is less than the concentration of xylenes. In this case thelargest shifts would be expected for the resonances of o-xylene and the smallest for those of p-xylene. The shifts in the NMR spectrum of a substrate in the presence of a binuclear reagent are expected to he dipolar in oriein (15.16). 3). relates the . . . The dipolar shift eauation (ea . sh& to r, the distance between thk lanthanihe ion and the nucleus of interest. and 8. and angles between the principal rnngnetir axis and thr line dmwn frnm the lanthanide ion to the nuclcw of interest. Usine- this e~uarionand the preferred hinding sites, AU/U= K(3 cos2R - 1)/r3
(3)
one would expect the methyl resonance of p-xylene to exhibi t the lareest shift and that of o-xvlene the smallest. Such a " situation would occur in the absence of competitive effects, that is, when the concentration of shift reagent if appreciably greater than the concentration of xylenes. The experiment is best performed when the concentration of shift reagent is less than that of xylenes. Reliable resolution of the methyl resonances of the three isomers is obtained, and the cbst of shift reagent is kept to a minimum. The resolution of the methyl resonances is sufficient to permit quantification of mixtures of xylenes at 60 MHz. Experimental The reagents necessary for the experiment are Pr(fod)s (fad = 6,6,7,7,8,8,8-heptafl~010-2,2,-dimethy1-3,5-ndin), Ag(fod), and carbon tetrachloride with tetramethylsilane(TMS)added a s a zero reference. The Pr(fod)z(17) and Ag(fod) (11)can be synthesized by literature methods or purchased from commerical sources. Xylene mixtures can be handled in two ways. One is to prepare a solution of xylenes in CCll so that the total concentration is 0.3 M. The second is to use xylene mixtures with no solvent. The procedure
Volume 64
Number II November 1987
979
1.2 1 .o 0.8 0.6
0.4
0 = ortho 0 = meta A
-
para
0.2
Figure 2. Plot of the ianthanide-induced shins (US) for me methyl resonances of O, m, and pxyiene versus me shin-reagent-ta-substrate ratio ([Ln]/[S]). The LIS values were recorded for a xylene mixture with a total xylene concentration of 0.1 M.
7
6
5
4
3
2
1
Oppm
Figure 1.Proton NMR spectra of a mixture of xylenes (0.3M) in CCi4 at 60 MHz with (a) no shin reagent. (b) 0.1 M Pr(f~d)~-Ag(fod),and (c) 0.4 M Pr(fodk
for using the shift reagent is to weigh into a small, stoppered test tube or vial the appropriate amount of Pr(fodh (0.0513 g) and Ag(fod) (0.0202 g) so that addition of 0.5 mL of solvent will result in a solution approximately 0.1 M in shift reagent. To this is added 0.5 mL of the unknown xylene solution (0.3 M xylenes in CClr) or 0.5 mL of CC14and 6pL of the xylene mixture. The vial is covered with aluminum foil to exclude light and shaken vigorously for 1-2 min. (The silver beta-diketonates exhibit a slight sensitivity toward light that is accentuated when in solution. The foil covering is used only as a precautionary measure. It has been found that the shifts in the spectrum of a substrate in the presence of a hinuclear reagent remain constant for at least four days if the solution is covered to exclude light (11)).The mixture is then centrifuged and the supernatant decanted into an NMR tube for analysis. The supernatant can be conveniently removed with a disposable pipet. The NMR tube is covered with aluminum foil prior to recording the spectrum. Results and Dlscusslon A tvnical result is shown in Figure 1.The unshifted sDectrum o f a mixture with a total xylene concentration of 0.3 M is shown in Firurc la. The methvl resonanre of the o-isomcr is resolved from the m- and p-methyl resonances hut is too close a t 60 MHz to obtain reliable integrations. The spectrum recorded in the presence of the shift reagent (0.1 M) is shown in Figure lb. The shift reagent used in the example is Pr(f~d)~-Ad(fod), which shifts the resonances in an upfield direction. Complete resolution of the three methyl resonances is obtained and the relative amounts of the xylene isomers can be determined. Typical results obtained by the students were within f 1% of the true values. Absolute concentrations of the isomers could be obtained through the addition of a suitable internal standard. One likely candidate for an internal would be chloroform. The chloroform
resonance would be the furthest downfield in the shifted spectrum. The result when too high a concentration of shift reagent is added is illustrated in Figure lc. In this spectrum the methyl resonance of the meta isomer is shifted to the point atwhichit overlaps with the resonance of the orthoisomer. A plot of the lanthanide-induced shifts for the methyl resonances of the xylenes versus shift-reagent-to-substrate ratio ([Ln]/[S]) is shown in Figure 2. The methyl resonance for the ortho isomer is shifted the furthest of the three a t low concentrations of shift reagent. A reversal of the relative magnitude of shifts for the meta and ortho isomers is observed a t [Ln]/[S] values greater than two. At the highest [Ln]/[S] value, the shift of the methvl resonance of the para isomer is equalto that of the ortho isomer. The relative shifts at the highest ILnl/lSl value are consistent with re dictions based oithe riipoia; ;hift equation. More complex mixtures including ethylbenzene andlor toluene can also he analyzed using the binculear reagents. A literature report has demonstrated the use of the downfield reagent Yb(fopd)s-Ag(fod) to resolve the methyl resonances of toluene; o-, m-, and p-xylene; 1,2,3-, 1,2,4-, and 1,3,5trimethylbenzene; and 1,2,4,5-tetramethylbenzeneis unleaded gasoline (13).
a .
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Journal of Chemical Education
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15. McConnell, H. M.;Robertson, R. E. J. Chem Phys 1958,29,1361. Ifi. Bleaney. B. J. M o g n Reaon. 1972.8.91, 11. Sprinwr,C. S., Jr.; Meek.D. W.:Sievexs,R.E.1norp.C k m . 1961.6.1105.
