Debra A. Heldman and Hans-Georg Gilde1 Marietta Coiieae Marietta. OH 45750
II
Use of a Lanthanide NMR Shift Reagent in the Analvsis of the Trans-Retitin1 spectrum
.
Since their introduction by Hinckley ( I ) in 1969,lanthanide
NMR shift reagents have proved to be extremely valuable by producing firscorder speitra of many organic compounds. A discussion of the theory and application of shift reagents can be found in one of the many journal articles or bwks published on the subject (2-6).In spite of the great utility of shift reagents, only a few student experiments making use of them have been reported (7-9). The undereraduate laboratbw experiment described herein demonstrates the value of theshik reagent E u ( f o d ) in ~ the NMR spectral analysis of lrans-retinol (Fig. I). This study
This linear relationship was expected because the complexation between shift reagent and suhstrate is an equilibrium and because a spectrum represents a cnncentration-weighted average of the chemical shifts of the uncomplexed suhstrate and the lanthanide-substrate complex (11).Thus, the linearity of the plot confirmed that the peak assignments had been made consistently. In addition, the initial chemical shift of a given proton could be determined bv extra~olationto zero Eoncentration of shift reagent. Although the relationship between chemical shift and
' First Annual Student Affiliate Meeting for the Central Region of the American Chemical Society, March 00-31,1979.
Figure 1. Structure of trans-retinol. The numbering scheme is used to identlhl protons In subsequent illustrations. considers the following aspects of shift reagent use: (1)simplification of the spectrum, (2) the relationship between lanthanide-induced shift and concentration of shift reagent, and (3) the relationship between induced shift and lanthanideproton distance. Results and~lscusslon Simplification of the spectrum was the first problem. The poor resolution of the NMR spectrum of 0.30 M frans-retinol in CDCll allowed positive nssignment of only those peaks representing the methyl groups (Fig. 2). These assignments were made bv cornoarison with ouhliahed data IlDl.'l'he addition of ~uifod)a:however, pioduced sufficient ;esolution for identification of all vinvlic hvdroeens. - . the methvlene hvdrogens adjacent to the hydroxi1 group, as well as ail meth;l group hydrogens. Figure 3 indicates the spectrum of transretinol with the addition of Eu(fod)a. The second problem considered was the relationship hetween the lanthanide-induced shift and the concentration of the shift reagent. Plotting of chemical shift ( v ) versus concentration of Eu(fod)a revealed a linear relationship (Fig. 4).
Figure 3. NMR spectrum of 0.30 M trans-retinol with the addition of 0.5 mole equivalent Eu(todh; 60 MHz; TMS used as internal standard.
- ,
.I0
c . .~I r d w ,
F m e 2. NMR spechum of 0.30 Mhanp~etinolin CDCb: 60 MH2; TMS used as internal standard.
390 1 Journal of Chemical Education
.I0
(1,
Figure 4. Resonance positions of 0.30 M tmnwetlnol versus concentrationof Eultodk. Analogous plots were obtained forthesix vlnylic hydrogens(H - 1 H - 61.
concentration of Eu(fod)., was linear at nearly all concentrations of the shift reagent. deviations were observed when high concentrations of ~ b ( f o d(i.e., ) ~ greater than 1mole equivalent) were used. These deviations can be attributed to shift reagent dimerization (11) and/or the finite magnitude of the formation constant of the retinol-Eu(fod)z complex (3). Finally to validate the proton assignments in trans-retinol a simplified form of the McConnell-Robertson Equation (12) was applied. This equation indicates that the pseudocontact contribution to the lanthanide-induced shift is as follows:
where Aui is the lanthanide-indueed chemical shift of the ith proton, K is a proportionality constant, and R; is the average distance between the lanthanide ion and the ith proton in the lanthanide-substrate complex.2 The following logarithmic form of the equation provides a simple method for plotting data (13): log Aui = -3 log Ri + log K
A plot of log Au versus log R should give a straight line of slope -3. Our results closely approximate the logarithmic McConnell-Robertson Equation (the table and Fig. 5), thereby demonstrating predominant pseudocontact interaction between Eu(fod)a and trans-retinol. Experlrnental Procedure
.21
I
I5
.I*
I 0
I., mli, Five 5. Lqlof lanmanideirducedshin -ur lg of lanhnid%protandisfance for 0.30 Mtrans+etinol in the presence of 0.5 mole equivalent Eu(tCdh. AnalD ~ O U Sgraphs can be plmed at each concernration of Eu(kd1r.
NMR Spectra
All NMR spectra were recorded on a Varian T-60 nuclear magnetic resonance spectrometer. NMR solutions were prepared by an "incremental dilution technique" (14). In a small test tube, a given amount of Eu(fod)swas dissolved in 1ml of a stmk solution of 0.30M trans-retiwl in CDC1, A 0 . 5 d aliquot of the resulting solution was removed end placed in an NMR tube: tetramethvlsilane (TMS) was added as an internal reference. The 0.5 ml of so6tion remainin; in the test tube was diluted to I rnl with more of !he refind i t m k sdution. The pr(~edurewas repeated !mil dl the desired conrentratinns of Etdfcd)? had been ohtaincd. ~~~~~
~~
~~~
Distance Measurements A Dreiding model of trons-retinol was used todetermine the distance (R) between each proton and the Eu3+ of the shift reagent. The location of Eu3+was assumed to he 1.5A from the hvdroxvl oxveen atom (3)..The R values recorded were the averages the distances measured on the two most favored conformations of trons-retinoL3
of
Lanthanlde-Induced shln (Av) and Lanthanlde-Proton Dlstance ( R ) lor 0.30 M trans-retlnol In the Presence of 0.5 Mole Equivalent E"lf0d.l
protan
ANHZ)~
-
130 26 8 8 498 314 87 82 34 16 16
CHs I CHa - 2 CHa - 3 (CHsh 4 C& OH H-I H-2 H-3 H-4 H-5 H-6
-
LOSAV) 2.11 1.41 0.903 0.903 2.70 2.50 1.94 1.91 1.53 1.20 1.20
R~AI
logR
4.4 8.8 13.5 11.7 3.0 3.7 5.7
0.643 0.944 1.13 1.07 0.477 0.568 0.756 0.799 0.892 100 1.03
6.3 7.8 10.0 10.8
AVvalues ware determined hom Flgues 2 and 3. a RvaIws were determined tm Dreiding mod%lr.
2 A more detailed description of the MeConneU-Robertsonequation
is K(3cos2oi - 1) R? where K is a constant and Oi is the angle between the major magnetic axis and the line drawn between the lanthanide ion and the proton of interest. Au; =
Lllerature Cited (11 Hinckley, C. C..J Amor Chem Soc., 91,5160 (1969). (2) Kime, K. A. and Siavers, R.E.. Aldriehimica Aeta. 10.54 (19771 (31 von Ammon, R. and Fiseher, R.D.,Angrm. Chem. intarnot. Edit., 11,675(1972). (4) Peterson, Jr., M. R. end WahLJr..G.H., J. CHEM. EOUC., 49,790 (19721. 151 Csmobell.J. R.. Aldrichimice Act0 4.55 119711. ,.=w
.".n
,...,.
=,".
(7) McGoran, E. C., Cutter, B., and Morse. K.. J. CHEM. EDUC.. 56.122 (19791. (8) Kuo, S. C., Hanim, D. K.. and Caple, R.,J. CHEM. EDUC., 51.280 (19741. (91 FIaer, L. F.and W i l l i a m n , K. L.,"OrgmieExperimcnts:4thEd.. D. C. Hesthand
Although different lanthanide to oxygen distances have been reported (6.13).a distance of 2.3 A according to a referee is a preferred compromise. For our purposes we elected to use the lower value of those reported.
Co..Lerington.MA. 1979.p.68. (10) Moueseron-Csnet. M. and Meni. J.-C.,Bull. Soc Chim. Fr. 3285 (1966). (11) Armitage. I. M.. Ha1l.L. D., M ~ ~ h a l 1 . A G..and . WwwbeIo~,L.G . & " N u ~ I ~ M a p . e t i c Resonance Shift &agents: (Editor:Siewrs. R. E.1, Academic Press, Nev York, NY. 1973. Chanter I
Volume 57, Number 5, May 1980 1 391
