Thomas S. Spencer and c. Michael O'Donnell Colorado Stote University Fort Collins, 80521
ClNDP Experiment Using a Permanent Magnet NMR
The undergaduate chemistni student often considers nmr only as i n analytical tool to he used in the identification of organic compounds. The followina- ex~eriment is . designed to show thestudent that other phenomena such as nuclear dynamic polarization can be studied through nmr. Specifically, we will discuss the use of chemically induced nuclear dynamic polarization (CINDP) which is an enhanced proton resonance signal resulting from an Overhauser effect due to coupling of the nuclear spin with an electron spin. A second aspect of the experiment is that i t can be carried out on R permanent magnet nmr, wh~chmakes [he exper). ment accessible to chem~srn.students at most schools. T h ~ s provides a means for extending the utility of such an instrument into the undergraduate P-Chem lab. Of course, the experiment can be carried out on higher resolution nmr spectrometers such as the Varian A-60; however, the experimental procedure will deal with the permanent-mapet type while suggestions at the end of the experimental section will enabie one to use the higher resolution instruments. Theory
In this experiment we are concerned with hydrogen proton nmr which is discussed in many textbooks ( I ) . We shall begin with the a and P spin states formed when electron and proton moments are placed in a magnetic field (Fig. 1).At room temperature a thermal equilibrium exists between the a and B levels with a Boltzmann distribution between the number of a states, N,, and the number of p spin states, Ns
5 in lo5 nuclear states, N,/NB = 1.000047; hence, when energy is absorbed by the system, the number of 0 states rapidly becomes equal to the number of a states and net a P absorption is not observed because the number of induced absorptions ( a P ) is equal to the number of ina).However, three mechanisms are duced emissions ((3 present whereby the system can relax from the B to the a state, such that radiationless @ a transitions occur, thereby allowing a 0 absorption to be experimentally observable (1). Spin-lattice relaxation is simply a B - a conversion whereby the excited state energy is given up to other degrees of freedom in the surrounding lattice or solvent molecules (2). Spin-spin or transverse relaxation results from coupling of the excited nuclear spin with other nuclear spins, where a mutual exchange of spins occurs. Thus, the excited spin state relaxes to the lower spin state. These relaxation mechanisms depopulate the excited spin state 0 absorption to he observed in the rapidly and allow a typical nmr experiment (3). The Overhauser effect, which a transitions, causing stronger nmr adsorpenhances (3 tions, is discussed below. The Overhauser effect occurs when nuclear and electron spins couple, allowing a relaxation mechanism in which both nuclear and electron spins change simultaneously, i.e., U&N peaw. The resulting relaxation processes are labeled X and Y in Figure 2. The enhancement of the nmr signal can be expressed by the factor (I)
-
--
-
-
-
-
where g~ is the nuclear g factor, BN is the nuclear Bohr magneton, H i s the applied magnetic field, k is the Boltzmann constant, and T is the temperature of the system. Atroom temperature the difference in population between the a and B states for H = 7000 gauss is on the order of
ZERO
FIELD
Fi
ELECTRON
Figure 1. Effect of an applied field on nuclear spin states. H is the applied magnetic field. W is the probability of spontaneous transitions which produce a thermal equilibrium with the Boltzmann factors for n and 0 levels given at the right. P is the probability of transitions (a 0 and 4 - a ) induced by an oscillating field with energy, hu = ~ N ~ N H .
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152
/ Journalof Chemical Education
NUCLEAR
Figure 2. Nuclear Overhauser effect. 0, and a, are the electron spin states with spontaneous transition probabilities. We. 4,P.v DenN and adiw - a&v are the nuclear spin states coupled with the 8, and n, electron spins, respectively. An enhanced nmr signal is produced by cnanges in N , , / N n resulting from the effect ot pracesses X and Y o n the thermal equilibrium produced by W,,S.
-
where 1 indicates the nmr absorption signal brought about by spin-lattice and transverse relaxation, s is the degree of saturation of the electron spin resonance, 5 is a relative relaxation factor determined primarily by X and Y, and ( g p l g ~ p is ~ )the electron-proton moment ratio. The above enhancement effect is 10-100 times the original nmr absorption signal (2, 3). The [ factor can he either positive or negative depending on the relative magnitudes of X and Y, resulting in greatly enhanced absorption in the former case or Ru +uuemission in the latter. I n the present experiment. 2-iodopropane and n-burvllithium react t a ~form propvl and butyl free radicals ( 4 . 5 1 PrI
+ BuLi
+
Pr.
+ Bu. + LiI
(3)
The electron spin states of Pr. and Bu. free radicals are populated so that coupling between proton and electron spins is possible. One can assume that approximately equal numbers of a, and p. states are produced followed by thermal relaxation to a Boltzmann distribution, N,/NB = 1.03, a net p, - a , transition shown as the process We in Figure 2. If, however, either of the processes X or Y occurs more rapidly than We, (1) a net enhanced relaxation process &BN CtNLYp occurs such that enhanced nmr absorption can he observed experimentally, X > Y >> We,or (2) a net nuclear excitation process occurs, B ~ N OL&N,Y > X >> We and an emission p~ LYNis ohserved in the nmr experiment. The nuclear spin states are said to he polarized a t this point so that if X o r Y >> We Pr. 'Pr. *Bu. (4) and Bu.
-
-
-
--
where the * indicates a nuclear polarized species. Rapid formation of products by these polarized nuclei leads to enhanced absorption or emission signals in the nmr spectrum. Exchange
"Pr + RI { *Bu. + RI
Termination 'Pr.
+ 'Bu.
-
-+
+
'PrI "BuI
+ R.
+ R.
emissions from *BuI, *PrI, or polarized products, if the polarized radicals react rapidly enough. The original nmr signals of the unpolarized species (Fig. 3) are distinguished by the septet from the methine proton in 2-iodopropane at 4.1 ppm and the triplet from the terminal protons in l-iodohutane a t 3.2 ppm. When the reaction takes place, the sequence of nmr spectra in Figure 4 is observed. Note the enhanced absorptions and emissions in the iodopropane septet and the appearance of the iodobutane absorptions and emissions at 4.2 ppm. The effect of polarization in the products is not readily observed because of interference from solvent methyl and methylene protons in the 1-2 ppm range ( 6 ) .The series of peaks ohsewed from 5-7 ppm can be attributed to vinyl protons in the products. Thus, one observes enhanced nmr signals in free radical reactions due to dynamic nuclear polarization arising from the Overhauser effect. 'Experimental
The fallowing procedures were found to give goad results on the Varian EM-300;suggestions for other instruments will be made at the end of this section. First, the permanent magnet nmr should he tuned by the lab instructor prior to the lab for best results. Attention should be paid to field homogeneity and temperature variations during the experiment which might create distracting results for the undergraduate unfamiliar with the operation of the instrument. When one prepares the samples, a 500 syringe is helpful, and n-hutyl-lithiumshould be handled under the hood or preferably in a dry box for it will burn on contact with moisture. Portions of n-BuLi in hexane and 2-iodopropane should be used to give approximately the same stoichiometric ratio as 500 4 of 2.4 M n-BuLi and 200 PI of 2-iodopropane. The n-BuLi solution is added to clean dry nmr tubes for as many runs as are desired (sometimes the excitement of the first run causes the results to he less than spectacular). Then a solution with 200 4 of 2-iodopropane and 500 4 of n-hexane is made up in an nmr tube. Finally, a sample of l-iodobutane is made up.
(5)
* Products
Hence, one expects to see enhanced nmr absorptions or
I
6
Figure 3. NMR spectra of reactants. The refererrce is TMS. 2-iodo. propane, l-iodobutane, and n-butyl-lithium were run in n-hexane solvent.
I
I
4
I
S ppm
I
I
2
Figure 4. NMR spectra of reaction mixture at 0.0. 0.5, and 10.0 mi". The sample is 500 pi of 2.4 M n-butyl lithium with 200 & ~ofi 2-iodopropane injected by syringe. For 0.0 mi" the sample is 200 ,ti of 2-iodopropane in 500 111 of n-hexane. The scans, 10 ppm in 2 min. were run at regular intervals from the beginning of the reaction. The intensity scale is reduced by a factor of 10 at approximately 2.4 ppm.
Volume 50, Number 2, February 7973 / 153
The 2-iodopropane sample is run, after the field has been zeraed with a T M S sample, and the spectrometer is adjusted according to the procedure in the manual associated with the particular instrument so that the s a m ~ l esienal .. is ootimized. After the sienal has been recorded. 200 u1 of 2-rudopn~panel i added tu one of the tuhea runtarnmg n-BuLi and the tuhe ralready in the spinner) is quickly placed in the cavity without a cap. Scans are then made a t 2-min intervals until the reaction is complete, 1C-15 min. A few experimental conditions should be noted. The reaction as reported was run a t a room temperature of 23-24'C. If room temoerature is above that level. the reaction will ~ r o c e e dmuch more rapidly. Alio, the temperature in the rmm should he nmstant during the expermem in order tu prevent drift of the maanetlr iield. Finally, the n-hutyl-lithhlm solution should he handled rarefully, including the disposal of excess amounts in the syringe in a solvent such as isopropanol.
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154 /Journal of Chemical Education
If the experiment is run on a larger instrument such as s Varian A-60 NMR Spectrometer, much higher resolution can be expected However, one must he sure that the cavity temperature is less than 24°C or the reaction will proceed too rapidly to be followed by the nmr spectrometer. When the variable temperature control is used for the A-60 spectrometer, care must he taken when the cap is placed on the sample cavity after the sample is introduced, since any movement of the probe will cause the magnet to go out of adjustment and the field will have t o be readjusted. Literature Cited
(m.
111 Carrington, A.. and McLschlan. A. D.. "Infrodudion to Magnetic Resonance." Harppr and Row. New York. 1967. I21 Lawlor. R. G.. J Amsr Chem. Soc.. 89.SSL1lL9671. (31 Cocivers. M., J. A m m Chem. Sac. 90.3261 (19681. (41 Lepley. A. R.. and Landau. R. L.. J. Arne?. Chem Soc.. 91.148 (19691. ( 5 ) Lepley, A . R . , J Arne,. Chem. Soc.. 91.749(1969l. (61 Ward. H.R., Lawler, R. G., J. Amrr. Chem Soc.. R9.551Rl19671.
