Shu Chen KUO, Donald K. Harris, and Ronald Caple University o f Minnesota-Duluth Duluth, Minnesota 55812
I
Mustrating Spectral Simplification by PMR Shift Reagents An undergraduate organic experiment
The following is a description of an undergraduate organic laboratory experiment that we have found to demonstrate, simply yet clearly, spectral simplification through the utilization of pmr shift reagents. This experiment can he approached in a semi-quantitative fashion on the typical 60 MHz nmr instrument and uses the versatile pmr shift reagents tris-(1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionato)europium(III), Eu(fod)a, and the praseodymium analog, Pr(fod)a. The basic theory and application of the lanthanide shift reagents has been presented recently in this Jourol.' That article is required reading for our students doing the experiment described here. The value of these shift reagents i n spectral clarification can clearly he illustrated with the rigid hicyclic ether (I), 1,4-dihydronaphthalene-1.4-oxide. This ether is readily synthesized as indicated2 and is part of our undergraduate project.
(I)
The pmr spectrum for ether (I) is shown in Figure 1. This spectrum is well suited for an approximate lanthanids shift reagent study owing to the simplicity of this symmetrical system and the resulting discernible resonance signals. Furthermore, the rigidity of the ethereal oxygen, the coordinating position for a shift reagent, and the varying distances, Ri, of the protons from the lanthanide make possible a semi-quantitative correlation of the magnitude of induced shift with a function of Ri. The two reagents found to best illustrate spectral simplification were Eu(f0d)a and Pr(fod)a in that they are complementary reagents, Eu(fod)a inducing downfield )~ to upfield shifts. Furthermore, shifts, and P r ( f ~ d leading the reagents can he used as commercially available3 and thev offer no difficultv to the students in handline. -. esoe. ciaily as regards solu6ility problems. If reasonable care is taken in storine the shift reaeents over PzOs the students need not be concerned with the hygro&ipic nature of these reagents. The experiment is considered in two aspects, first in terms of spectral simplification, and secondly, a cnrrelation of rhe induced shift with distance. R,. ~ - is - made ~ ~~~~-~ Figure 2 illustrates the spectral clarification obtained with Eu(fod)a where samples were prepared as described in the experimental procedure. The sizes of the induced shifts are such that the spectrum can essentially he treated by a first-order analysis: the AZBZpattern of the aromatic protons is discernible and JABand JAB, are readily measured. One expects the size of the induced shift to he greatest for the protons closest to the coordination site.' This is dramatically seen by the size of shift for HI and H4 in (I) where they become the most strongly shifted protons in the shifted spectrum (Fig. 21, whereas they ~
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6.0
JD
Wl
70
Figure 1. Pmr spectrum for 0.5 M (I) in CDCIS,60 MHz; shifts listed as 6 values in ppm from internal tms.
I
I
130
ILS
I
I
1
I
80
SD
RD I00
Figure 2. Pmr spectrum of (1) (0.5 M in CDCl3) with 0.2 equiv of Eu(fodla added; shifts listed as 6 values in ppm from internal tms.
50
4.0
15
10
-65
-7.0
, -75
Figure 3. Pmr spectrum of (1) (0.5 M in CDCId with 0.2 equlv of Pr(fodj3added; shifts listed as 6 values in ppm from internal tms.
were the least shifted protons in the uncomplexed spectrum (Fig. 1).These assignments are readily confirmed by the incremental addition of the shift reagent as suggested in the experimental procedure. Figure 3 likewise illustrates one of the pmr shifted spectra using P r ( f ~ d ) The ~ . assignments made with Eu(fod)a 'Peterson, Jr., M. R., and Wahl, Jr., G. H., J. CHEM. EDUC., 49, 790 (1972). Users are referred to this source for a bibliography on shift reagents. 2Fieser, L. F., and Haddadin, M. d., Can. J. Chem., 43, 1599 (1965). The experimental for this conversion is clearly presented in this article. 3Care should be taken to use only the same brand far a given set of soeetra.
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0.0 0.0
0.2
0.4
LOO
0.6
CR)
0.8
1.0
Figure 4. Log of lanthanide induced shifts by E u ( f ~ d ) as ~ , determined from Figures 1 ana 2, versus log Ri as given above, where Avi is difference in ppm.
can be nicely confirmed with Pr(fod)a. For example it can be noted t h a t the AzBz has been "inverted" in Figure 3 a s again the A protons, being closer t o the coordination site, are shifted more rapidly. This is beautifully confirmed by the incremental addition of Pr(fod)s where the aromatic hydrogens first coalesce and then separate a s the A protons "pass t h r o u g h the B protons. T h a t in fact the magnitude of the induced shift An, the difference between the values of shifted and unshifted line position for the proton, does correlate with distance a s given in eqn. (1) can be confirmed by the student."his can be done in a semi-quantitative fashion utilizing spectra such a s t h e above.
Acceptable accuracy for the values of Rt are obtainable from Dreiding models. The lanthanide can be assumed to be 3.0 A from the oxygen along the line formed by the intersection of the plane HI-0-HI, and t h e mirror plane bisecting the m ~ l e c u l e (If . ~ Dreiding models are not easily accessible, the following values can be used) 'A more complete form of eqn. (1)would include a dependence upon the angle 8, describing the position of the ith proton relative to the assumed symmetry axis of the lanthanide complex, Aui = K(3 cw2Bi -1)/Ri3. This angular dependence has been shown to be of importance for a number of compounds but a satisfactory correlation is obtained in the present example considering only the internuclear distances, eqn. (1). 5The positioning of the lanthanide complex in the HI-0-H4 plane may seem not to be justified by the symmetry of the molecule. Indeed, calculations in this laboratory indicated an inclination of approximately 10' of the lanthanide complex toward the olefinie center. A slightly better straight line is obtained plotting log Ani versus log R, with this madel.
Figure 5. Log of lanthanide induced shifts by Pr(fodIs, as determined from Figures 1 and 3, versus log Ri as given above, where Avi is difference in ppm.
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RLnVHA= 6.26 A RLnmRB 7.68 Using Eu(fod)a induced shifts from Figures 1 and 2 and the above Ri values, a plot of log Am versus log Rr is shown in Figure 4. A reasonable straight line is obtained with slope approximately -3 which corroborates the R - 3 dependence in eqn. (1).A similar consideration using Figures 1 and 3 yields the R - 3 dependence for the Pr(fod)a induced shifts. Experimental Procedure
The following description of quantities are those corresponding to the results presented in Figures 1, 2, and 3. These are guideline quantities only and there is some flexibility in the amounts used by the students. Figure 1 was obtained with a 0.5 M solution of (I) in deuteriochloroform. This solution is readily made by dissolving 72 mg of (I) (mol wt = 144) in 1 ml of deuteriochloroform and transferring the entire solution to an nmr tube. Two samples can be made in this fashion. The solid shift reagents can be conveniently added directly to these solutions. It is suggested that -207 mg of Eu(fod), (0.2 equiv) be used and likewise -205 mg of P r ( f ~ d (0.2 ) ~ equiv). The shift reagent can be added incrementally in any number of fractions the student wishes. This is especially useful in confirming spectral assignments and the students usually enjoy observing the spectral changes. The total quantity of shift reagent is added for the spectrum used in the Ri correlation plot. To conserve materials the above solutions could be made up in half quantities in 0.5 ml of deuteriochloroform, but the incremental additions are somewhat easier for the students with 1 ml solutions. Normal precautions should he taken in keeping the shift reagent dry by storing over phosphorous pentoxide. The shift reagents used here are commercially available and relatively inexpensive. Possihle sources include Willow Brook Laboratories, P.O. Box 526, Waukesha, Wisconsin, 53186; Eastman Kodak, Rochester, N.Y., 14650; E. M. Laboratories, 500 Executive Boulevard, Elmsdorf, N.Y., 10523.
Volume 51. Number 4, April 1974 / 281
