A Qualitative Approach to the
Donald E. McGreer and M. M. Mocek University of British Columbia Varcouver, British Columbia, Canada
Study of Complex NMR Spectra
Roberts (1) recently has described the fundamentals of NMR spectroscopy in a manner that is useful for teaching the subject to undergraduates. When presented in this way we find, however, that it leaves a student lacking in the appreciation for more complex spectra, which are by far the more common forms encountered in practice. We have found that a useful extension to Roberts description of NMR is the application of the experimental approach to changing a simple spectrnm into a more complex spectrum. This procedure makes it unnecessary to consider a t this stage a mathematical description of complex spectra and gives a student a feeling for such spectra which, it is hoped, will spur him to further study of the subject. Nuclear magnetic resonance spectra are simple, i.e., can be described by first-order theory, when the chemical shift (A in cps) between two signals is much greater than the spin-coupling constant ( J in cps). This is the case in ethyl ether (see Fig. 1) where A is 135 cps and
J is 7.0 cps and for which we have a clearly defined triplet and quartet as predicted by the first-order methods described by Roberts (1). A spectrum is complex when the values of A and J are nearly equal. For such a situation first-order theory does not predict the spectrum and in general there are more lines than predicted by the first-order theory. Such spectra are disrussed thoroughly in texts and reviews (2-4) and the line structure of most spectra can be calculated if sufficient variables are known. Simple spectra can be made complex in two ways. First, since A varies directly with the applied field Ho used to obtain resonance and since J is field independent, a decrease in Ho will reduce A and can cause the ratio of J / A to approach 1. For example a spectrum which has a value of J / A of 0.2 a t 60Mc will closely resemble the first-order spectrnm. At 16Mc, however, where J / A is 0.75 many second-order effects will have appeared. This can be illustrated by the spectrum of ethyl alcohol for which Arnold (5) reports seeond-order splitting for the CHa of ethyl alcohol at 30Mc. This splitting is less complex a t 60Mc. Most laboratories are not equipped for determining spectra
,
L
7 Figure 1.
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8 7
Ethyl ether.
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2.0
I
I
I
3.0
4.0
5.0
7 Figure 2.
Bendydrol in acetone
. 6.0
a t a number of field strengths and thus this method is little used. Second, A can often be changed by changes in solvent. Some signals are particularly sensitive to concentration and to the nature of the solvent. This is the case with the OH signal of an alcohol which can often be moved from the 4.r region to the 9r region by solvent changes. If the OH is coupled to a proton which is not affected by solvent and which lies in the 4-9r region, then A can be varied to give a large variation in the value of J / A . I n this way many simple spectra can be made complex. This approach was fint used in the study of the coupling of the OH signal of benzyl alcohol with its CH2 component by Corio, Rutledge, and Zimmerman (6). The advantage of studying complex spectra by transforming simple spectra into complex spectra is that J is not affected by the change and thus J can he found from the simple spectra and used in the study of the complex spectra (the J for an alcohol does depend on its environment due to the orientation of the OH function but such changes on dilution are assumed small). A second advantage is that several variations of the one type of coupling are available from the one sample. I n order to describe more fully the use of solvent shifts in the study of complex spectra it is necessary first to discuss in general terms the factors influencing the spectra of alcohols. I n carbon tetrachloride or rhloroform the OH of alcohols is usually not split, although such splitting is expected due to the coupling between the OH and adjacent CH. Thus of some 2.5 spectra of alcohols (7% solutions in deuterochloroform) displayed in the Varian Catalog (7) only three show this roupling. On the other hand, as the undiluted liquid or in solvents like acetone or dimethyl sulfoxide it is quite common to observe the CH to OH coupling. The absence of splitting of the OH peak has been attributed to an exchange process (8) in which the proton of one molecule transfers to a second a t a rate that is sufficientlyfast that the lifetime of a proton on any molecule is much less than the lifetime of lo-" lo-= sec required for NMR to observe a single state. The result is that the proton of the OH sees a number of environments which are averaged in the measurement to give a single peak rather than a split peak. The exchange rate is catalyzed by both acids and bases and thus in the presence of such catalysts all alcohols show no splitting for the OH. The use of acetone and dimethyl sulfoxide as solvents to permit the observation of spin-spin splitting of the OH peak ran be demonstrated for methanol and 2,2,2 trifluoroethanol. Both of these alcohols do not show splitting in the pure state even with rigorous purification. In the solvents acetone (9, 10) and dimethyl sulfoxide (10, I f ) , however, splitting of the OH is observed. I t has been proposed that hydrogen honding of the alcohol to the solvent slows the rate of proton exchange to permit observation of the splitting (9). The second feature of the spectra of alcohols which is important to our discussion is hydrogen bonding. Alcohols undiluted or in solution hydrogen bond to form dimers, trimers, and polymers. Each form is expected to give its own NMR signal, but because of the rapid change of the state of agglomeration in the
time required for NMR measurement, the OH protons are found in a number of states of agglomeration and the chemical shift position of the OH is a weighted average for these species (12). Dilution in a solvent favors formation of the species of lower states of agglomeration and the chemical shift will approach that of the monomer. This corresponds in a shift from low field to high field. For ethanol dilution with carbon tetrachloride shifts the OH signal from 5 to 9.5 .r (13). Dilution shifts of this magnitude do not always occur. For example benzhydrol in acetone shows little dilution shift. This is probably because under these conditions benzhydrol already exists as a monomer hydrogenbonded to solvent. A shift of the OH signal can still be
Figure 3. A-E-addition of prtionl of ethanol to o rdution of ben2hydrol in acetone for which J = 4 cps. J/A for A = 0.071.8 = 0.28, C ; .0.54 and D = 0.95.
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produred, however, by addition of other solvents with different hydrogen bonding properties. Thus the OH of benzhydrol in aretone is shifted to lower field by the addition of either ethanol or dimethyl sulfoxide. On the basis of the above discussion it can be seen that the OH signal ran be shifted quite simply and thus values of J / A for many types of spectra can be obtained t o provide a n experimental means for converting simple spertra into more romplex spectra. I n the following sertion we will describe the application of some of these principles. The simplest first-order spectrum showing splitting is that in whirh a single proton is coupled with a second proton such that .J/A is small. This is an AX system (14) and leads to a pair of doublets and is illustrated by benzhydrol in aretone where J is 4 cps and J / A is 0.071 (see I'ig. 2). Addition of ethanol moves the OH doublet down field toward the C H doublet as shown in Figure 3 and rhanges the AX system into an AB system. The OH triplet of the ethanol is not shown hut appears separately a t higher field. It is immediately apparent that as .J/A increases toward one the outer signals of the pair of doublets weaken compared to the inner signals. Finally the transitions responsible for the outer signals become forbidden and the inner signals coalesce. Theoretical treatment (15) of the AB system tells us that J and A are related to the sparings of the signals according to the equation (' = l/2(As
/
Benryl alcohol.
+ .J~)I/Z
where 2C - J is equal to the spacing between the two inner signals. Sinre J is known from the first-order spertnun A can readily be calculated. A more complex system is illustrated by benzyl alcohol (see Fig. 4) in whirh the OH and adjacent CH? make up a simple AX2 system. This leads to a doublet for the CH2and a triplet for the OH for which J is 5.6 cps and J / A is 0.10. Addition of acetone causes a highfield dilution shift of the OH signal changing the A X 2 system into an ABz system. (See Fig. 5.) These spectra clearly show how coniplex spectra ran have more lines than experted for the corresponding firstorder system. For the AB2 system there are theoretically eight lines and these are numbered in Figure 5. Lines 5 and 6 are not resolved in this instance. Theory predicts these eight lines plus a n additional line of lorn transition probability and values of A and J can be calculated from them. The separation between line 3 and the midpoint between lines 5 and 7 gives the value of A. Roberts (16) clearly illustrates the relation between J / A and the line positions by graphical means. I n our case J is known from the first,order spectrum, but if it were not knou-n it could be obtained from the graph. 360
Figure 4.
lournul of Chemical Education
Figure 5. A-H-Addition of portions of acetone to benzyl olcohol for which J = 5.6 cpr. Per cent by wt. of benryl alcohol and J/A for A = 78, 0.14.B = 67.0.21.C = 55.0.37. D = 51.0.60,
E=45.l,F=43,1.3,G=42.1.9mdH=40
From these two examples it can he seen that many spectra can he created by use of the wide variety of alcohols available. For example methanol gives an AB3 system ( l l ) ,isohntyl alcohol gives the AB2 part of an AB2X system and 2,2,2-trifluoroethanol gives the AB2 part of an AB,X, system. The application of solvent shifts in studying spectra is not restricted to alcohols, as has been shown by Schaefer and Schneider in their excellent study on vinyl bromide (17). They found that different solvents could change the relative positions of the geminal hydrogens from -4 to f 9 cps. The solvents used were benzene, n-hexane and acetone. We have found that a great deal of information can be ohtained from the spectra of alcohols in general when fullest use is made of the solvent effects. Citronello1 shows a single peak for the OH when undiluted or in carbon tetrachloride. I n dimethyl sulfoxide the OH is a triplet a t 6.81 r clearly indicating this to be a primary alcohol (see Fig. 6). Changes in the spectrum hetween the undiluted sample and the sample in dimethyl sulfoxide permits easy assignment of the peak a t 6.58 T to the CH1 coupled to the OH. The triplet a t 4.96 r is due to the vinyl CH.
the OH confirms the secondary nature of this alcohol. Assignments to all hydrogens in a series of carhohydrates has been made using these and related methods (18).
The spectra reported herein were ohtained on a Varian Associates A-60 instrument. We wish to thank Mrs. E. Brion for determining most of the spectra.
Figure 7.
Diocetone glucose in acetone with water added
Literature Cited (1) ROBERTS, J . I)., J. CHEM.EUUC.,38, 581 (1961). (2) ROBERTS,J. D., "An Introduction to the Analysis of Spin-
Spin Splitting in High-Resolution Nuclear Magnetic Resonance Spectra," W. A. Benjamin, Inc., New York, 1961.
Figure 6.
Citronellol in dimethyl rulfoiide.
eitronellol
dixcetone glucose
Diacetone glucose in acetone has its OH peak hidden among other CH peaks. On the addition of water the OH douhlet becomes clearly identifiable as it shifts to lower field (see Fig. 7). The doublet structure for
(3) POPLE,J. A,, SCHNEIDER, W. G., AND BERNSTEIN, H. J., "High-resolution Nuclear Magnetic Resonance," McGraw-Hill Book Company, Inc., New York, 1959. (4) CORIO,P. L., Chem. Revs., 60, 363 (1960). J . T., Phys. Rev., 102, 136 (1956). (5) ARNOLD, R. L., AND ZIMMERMAN, J. R., (6) CORIO,P. L., RUTLEDGE, J . Molenrlar Spectroscopy, 3 , 592 (195'3). N. S., JOHNSON, L. F., A N D SCHOOLERY, J . N., (7) BHACCA, "Varian High-Resolution NMR Spectra Catalog," Varian Associates, Palo Alto, Calif., 1962. (8) POPLE,J. A,, ET AL., op. eit., p. 417. (9) CORIO,P. L., RUTLEDGE, R. L., A N D ZIMMERMAN, J. R., J . Am. Chem. Soe., 80,3163 (1958). (10) Unpublished results by Mocek, M. M. (11) KIVELSON, D., AND KIVELSON,M. G., J. M o l e ~ u l aSpec~ trcscopy, 2, 518 (1958). (12) POPLE,J. A,, ET AL.,op. eit., chap. 15. (13) COHEN,A. D., AND REID,C., J . Chem. Phys., 25,790 (1956). (14) JACKMAN, L. M., "Applications of Nuclear Magnebir Resonance Spectroscopy in Organic Chemistry,.," Pergaman Press, New Yark, 1959, p. 89. (15) POPLE.J. A,, ET AL.,op. cit., p. 121. J. D., "Analysis of Spin-Spin Splitting." p. 70. (16) ROBERTS, J., A N D SCHEIEIDER, W. G., Can. J . Chem.,'38, (17) SCHAEFER, 2066 (1960). R. J., HALL,L. I)., HOUGHL., (18) ABRAHAM, LAN, I
