Proton NMR Spectra Deceptively Simple and Deceptively Complex Examples J. E. Gurst University of West Florida, Pensacola, FL 32514 C. M. Dellinger Louisiana State University, Baton Rouge, LA 70803 JohnJacobus Tulane University, New Orleans, LA 70118 T h e concents of nuclear snin-snin . . couoline . .. in NMR spectroscopy areBddressed in most recent undergraduate organic chemistrv texthooks The utili7ation of spin-spin . . couplina . . constants roderivestructural information, particularly from is )commonplace. . Difficulvicinal Drown couolines ( ~ .J..H .. H ties in the interpretation of NMR spectra can arise when experimental results do not coincide with expected spectral parameters. We describe herein relatively simple NMR experiments that demonstrate unexpected results of the deceptively simple and deceptively complex types. ~
A6m > lO,%Jfi
~~
~
In order to observe spin-spin coupling between two nuclei a t least three conditions must he met 1) both nuclei must possess spin; 2) the nuclei must be proximate, the number of intervening bonds is generally four or less; and, 3) the nuclei must be chemically shifted relative to each other. The first two conditions are amply met in various molecules nossessine ethane-like fraements. The third condition is satisfied, in most instances, if the suhstitution patterns of the two rarhons are different. i.e.. if the molecule lacks in any conformation either a planeor axis of symmetry. The spinspin coupling pattern observed for such an ethane-like fragment (in the absence of other spin-spin interactions) can be of three types: 1) AX, consisting of two sets of doublets, widely separated in
chemical shift (A6),each of the four lines of equal intensity, the two pairs of lines each separated by the coupling cons t a n t 3 J ~Such ~ . spectra are generally observed when
2) AB, consisting of two sets of doublets with the outside lines
less intense than the inside lines, the chemical shift difference being in the range 0 < A6m. < 10 $JAB,with the separation between the paired outside and inside lines equaling the coupling constant ' J m 31 .42. ctrnsisting of a smgie line. The t w o nuclei may be either the end result rs identirhemicallv or necidcntlv .euui\,alent: . eal. The coupling constant is not directly observable in either case. A single line is observed for equivalent nuclei (AbM = 0 , I 3 J d 2 0)or for accidently (magnetically)equivalent nuclei (06.4~= 0, IJ d f O).' Although numerous examples of the spectral types mentioned agave exist, examples of single molecul&that can exhibit varvine alone- the continuum from AX to Az " -sDectra . are relatively rare. Thus, in order for the student t o see the various coupling patterns, a variety of different molecules must he prepared, and purified, and their individual spectra recorded. We report herein two simply prepared molecules, one which exhibits (at 60 MHz) spectra along the AX to A2 continuum as a function of solvent and another which exhihits spectra of an unexpected type in particular solvents. In the former case a t the A? limit the spectrum is deceptively s i m ~ l e :in the latter case the soectra are deceptively complex. 1; both cases the deception arises because chemical shifts between chemically distinct nuclei hecome small in relation to a coupling constant. ( 2 5 3 s ) and (25,3R) Ethyl-2,3-dlbromo-3phmylpropanoates
-
' A problem of nomenclature arises here; the letters A. B, . . .
p~
generally refer to relative chemical shifts. It two chemically nonequivalent nuclei are accidently magneticaily equivalent, their designations would generally be identical.
Volume 62
Number 10 October 1985
871
..
Flgule 2 90 Mrlz spectrum 01 melhinc and methyl regoan of lla (R = R' = Ch,l in CDCI, Ester melnyl grmp ominea hom spectrum.
Fiaure 1. 9 0 MHz s ~ e c t r aof methine rsoion of l l a I R = Figure 3.90 MHz specbum of methine and methyl regionof lla (R = R' = CHS) in benzene-ds. Ester methyl group omined hom spectrum.
Table 1. NMR Parameters ot lla (R = C8H&R' = Et) Volume Fraction CDCll OMSOds
1.00 0.75 0.67 0.50 0.33 0.25 0.00
0.00 0.25 0.33 0.50
0.67 0.75
1.00
Chemical Shift (ppm) A B
5.30 5.33 5.37 5.40 5.44 5.60 5.65
lA6/3&). 60 MHz
90 MHz
2.6 1.9
3.9
4.78 4.95 5.03. 5.18
2.6
1.1 1.5
5.24
5.40 5.55
1.0 Ob
0.7
A b In HI:%
= 12.0 Hz for all specIra. " 8 < 0.1 ppm (see led).
The addition of.bromine to E-ethyl cinnamate (I, R = CsHs; R' = Et) leads to the enantiomeric ethyl-2,s-dibromo3-phenylpropanoates (ethyl erythro-2,3-dibromodihydrocinnamate) (IIaand IIb, R = CGHS;R' = Et), i.e., the reaction proceeds in a stereospecific anti manner. The NMR uarameters for the 2- and 3-protons are presented in ~ a b i 1. e In CDC13 solution the 2- and 3-protons exhibit the expected AB pattern; the chemical shift djfferenceis 0.52 ppm & I ~ ~ J H H= 12.0 Hz. As the volume fraction of DMSO-d6 relative to CDC13 is increased, the chemical shifts of both resonances shift downfield and the chemical shift difference (A6) progressively decreases (Fig. 1). Concomitant changes occur in the appearance of the spectra; as A6 decreases the outer lines of the AB pattern decrease in intensity and the inner lines increase in intensity. In pure DMSO-ds a t 60 MHz the two 872
Journal of Chemical Education
resonances (of the rhemically distinct nuclei) are accidently magneticnllg equivalent. A single resonance is observed for the two nuclei. The cbanre in solvent from CDCl, to DMSOdg reduces the AB spectyum to Az, thereby suppressing the coupling constant information available in the AB spectrum. Spectra in which coupled chemically (or symmetry) distinct nuclei are accidentlv maeneticallv eauivalent are eenerallv described as being "dece&ively s&pie." For the sake of illustration, spectra of the methine proton region of IIa at 90 MHz are shown in Figure 1.At 90 MHz in DMSO-d~,the methine protons are not m i t e accidentlv e q u i v a ~ eand ~ t the four-fine AB pattern is bbservable with A~A= B 8.8 Hz (4.1ppm). At 60 MHz, A6 6 Hz, and since 'JHH = 12.0 HZthe separation of the unobserved outer lines must be a t least 24 Hz. The separation of the strong inner lines thus must be less than 1.5 Hz. Thev are observed as a single line. I t should be noted that the coudine . " constant 3J9.1 - " is invariant in the solvents employed. This invariance indicates that the conformation of the molecules does not chanee appreciably with solvent. Further, the magnitude of the c i pling constant (12.0 Hz) indicates that the bromines adopt the anti conformation.
-
(2R, 3 s ) and (2S,3R)-Melhyl-2,3-dlbromobutanoate
Addition of bromine to methyl-E-but-2-enoate (methyl crotonate) (I, R = R' = CHa) leads to the enantiomeric (2R, 3s)- and (ZS, 3R)-methyl-2,3-dibromobutanoates (IIa, R = R' = CH3). Assuming the preferred conformer (of the pair of
Table 2.
NMR Parameters of 11 (R = R' = CHd in CDCl,
Chemical Shins (ppm)
Coupling Constants (Hz)
This value lo aooumed
n n
X4
Figure 4. 90 MHz spectrum of lla (R = R' = CHI) in DMSWs. MeMlne reglon shown. Upper nace is four fold amplification of H-3 absorption.
proton appears as a doublet exhibiting 3 J ~ of 3 -10.5 Hz. This simplification results from the 4 . 5 ppm downfield shift of the C-2 proton resonance while the C-3 resonance remains unchaneed. The compntelspectrum for the methine region of IIa (R = R' = CH3) is shown in Figure 5. The vicinal coupling constant 352.3 can be directly measured from the separation of the low-field doublet (Fig. 4) while the coupling constant 3Jw (H-3 to methyl) is ohserved six times in the high-field methylene region. The NMR parameters of IIa (R = R' = C H 3 in CDC13 were determined by selective spin-spin deconpling experiments a t 200 MHz. These parameters (Table 2) were employed to calculate spectra identical to experimental spectra determined a t 60, 90, and 200 MHz. Indeed the large coupling constant predicted is calculated, but it is unobservable directly. The spectra (in CDC13 and in henzene-d6) are termed "deceptively complex." The vicinal (HZ-H3) coupling constant of IIa (R = R' = CH3) is large in the three solvents employed. The complexity observed in CDC13 and henzene-ds is not a function of the conformation of the molecule but rather is afunction of near accidental coincidence of the H-2 and H-3 resonances.
Concepts
Figure 5. Computed "stick" spectrum of methine region of lla (R = R' = CHI) in DMSOd6.
enantiomers) to be that in which the bromines are anti, the expected vicinal C-2H-C-3H coupling constant (3J2-3) should be -10-12 Hz. I t is initially surprising to observe a spectrum a t 90 MHz in either CDC13 or benzene-ds consisting of two exceedingly complex regions of absorption (Figs. 2 and 3), in addition to the anticipated ester methyl singlet. The unanticipated multiplets are centered a t -1.9 and 4.5 ppm with relative integrated intensities of 32. The deceptive complexity of the spectrum is markedly reduced when DMSO-ds is employed as solvent (Fig. 4). The methyl resonance appears as a clean douhlet, the methine proton a t C-3 appears as a douhlet of quartets, and the C-2
The determination of the spertral propertirs of the two molecules mentioned abovr can he illustrnti\e of a number of important concepts: the stereospecificity and stereochemistry of the addition reaction, the utilization of vicinal coupling constants to determine conformational preferences, and the utilization of solvent effects to help decipher snectra of the dece~tivelvsimple or deceptively complex Gpe. If two spectiometkrs operating a t different field streneths are available the advantage of higher field strength to remove near accidental degeneracy canbe demonstrated. Experimental Ethyleinnamate (I,R = CeHs;R' = Et) andmethylcrotonate (I,R = R' = CH3)were both obtained from Aldrieh Chemical Company and were used as received. To solutions of 0.01 moles of the esters (1.76 and 1.00 g, respectively) in 20 ml of CCll were added solutions of 0.011 moles of Br2(1.76 g) in 20 ml of CClr.The reaction mixtures were stirred at room temperature for 1.5 h. The solvent and excess Brzwere removed by vacuum distillation. Both products are originally isolated as oils; the dibromoeinnamate product crystallizes on standing. Further purification of the products is not required. All spectra were recorded on 0.1 M solutions in the specified solvents with TMS as the internal standard.
Volume 62
Number 10 October 1985
873
