"Solvent Effects" in 'H NMR Spectroscopy A Simple Undergraduate Experiment J&
A. S. Cavalelro University of Aveiro. 3800 Aveiro. Portugal
NMR spectroscopy has become one of the most powerful analytical techniques in chemistry during the last two decades. I t is almost certain that all chemistry undergraduate students find themselves recording N M R spectra of many compounds involved in their laboratory courses, usually using a low-frequency spectrometer. Frequently the solvents used in NMR studies are deuterated compounds. This f a d causes them to be expensive and they are available throughout the world from only a few sources. I n order to by-pass this situation, and if solvents like CCI4 (or other nondeuterated ones) cannot be used, the spectra of neat liquid samples are recorded. This normallv eives values of chemical shifts that are different from thoseUfound in standard tables, and this is a source of confusion for students. A similar situation applies when concentrated solutions or aromatic solvents are used. The students must be aware of these facts. which are due t o the various terms contributing to so-called "solvent effects".' A series of simple N M R experiments has been devised to familiarize undergraduate students with "solvent effects" o n l H N M R chemicalshifts. If necessary, i t is also possibleto discuss these effects in d e t a i P 3 if the topic has heen only superficially covered in the corresponding lecture course.
solutions and of the neat compounds were recorded using a 60-MHz spectrometer. (The spectrometer had been calibrated, with a reference sample, just before and immediately after running each spectrum. A variation of f0.02 ppm was observed, and this was taken into account for the 6 values of each group of protons.) Thevariationson thechemical shifts of toluene in CCL areshown ic compound manifested by its magnetic anisotropy and molecular shape. Thus, as the toluene is gradually diluted with CCla, the aveiage distance between toluene molecules increases, causing a downfield shift of all protons associated with the aromatic rings.
Several miatur~softolwnr and CCI,, ofp-xyirnr and CCI,,and of tdueneand ClICi~werrmadr.'l'helH NMHspertraof the resulting
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Ronayne, J.; Williams. D. H. In AnnuaiReview of NMR Spectroe copy; Mooney. E. F.. Ed.; Academic: New York, 1969; Vol. 2, p 83. Giinther, H. NMR Spectroscopy; Wiley: New York, 1980; p 89. Williams, D. H.; Fleming. I. Spectroscopic Methods in Organic Chemistry, McGraw-Hill: New York, 1980; p 82.
Variations of 6, in ppm, at 60 MHz. for the aromatic and methyl protons of CaHsCH, with relative proportionrof CCI.. Figure 1.
Volume 64
Number 6
June 1967
549
Figure 3. Typical interaction between aromaticringsgivingrise to upfield shifts of proton resonances due to anisotropic effects.
Figure 2. Variations of SC~H~-SCHC$~. in Ppm, at 60 MHz With relative proponions of CsH,CH, and CHCI..
In Figure 2 is shown the variation of SC~H&HC~~, in ppm, with relative proportions of CHC13 and C6H5CH3. The values plotted here detail a combination of effects which students might be asked to interpret. Chemical shifts of both the CHC1, proton and the CBHSCHB protons are altering in unison. At low concentration of CHC13 (Fig. 2), the aromatic protons experience an upfield shift due to the anisotropic effect (Figs. 1 and 3) and also the CHCb proton is shifted upfield due to the "hydrogen bond" interaction with the aromatic compound (Fig. 4). At high concentration of CHCb the average anisotropic effect upon the CHCb proton is decreased as ia the intermolecular aromatic interaction. The absolute chemical shift of the CHCL proton in 10%C6H5CH3varies from 7.23 to 5.86 ppm in 90% CsHsCH8. On the other hand, the chemical shifts (in ppm) of the CsH&H3 protons in 10%CHC13 are 6.70 (C6Hs)and 1.67 (CHd and in 90% CHC13 are 7.30 (CsHs) and 2.37 (CHd. Thus, one component in the system always affects the chemical shift(s) of the other.
550
Journal of Chemical Education
C1
CL \
cl
!/
C
I &c$ Figure 4. Chloroform/toiuene "complex" showing shielding effect on C H C I ~ proton.
