A Simplified Version of a Classic Proton Exchange NMR Experiment Asimple upgrade of a classic almhol exchange experiment (1)that demonstrates changes in chemical shift, spin-spin coupling, and integration has heen used as an introductory experiment for neophyte N M R users. Preparing and maintaining alcohols that are dry enough to show spin-spin mupling of their hydroxylic protons is not always trivial. Transferring these solutions to NMR tubes sometimes introduces moisture sufficient ta catalyze rapid proton exchange and effectively demuple any hydroxylic protons. Clearly, it would be highly desirable to find systems that are less sensitive to atmospheric moisture, making them easier to handle and keep dry and that can be used in deuterated solvents. We have found that solutions of 1-phenylethanol, 1, 2phenylethanol, 2, or 3-phenylpropanol, 3, (approximately 'H-NMR Data: Chemical Shift (ppm), O.OSp/mL in CDCla containing 1% (vlv)TMS) when dried for ~ ~ l t i ~ and i i ~coupling i t ~ constants (HZ) only a few hours over oven-dried 4-.& molecular sieves show of the Hydroxylic Proton of 1 3 mupling of their hydroxylic protans to their a-CH's. These solutions, which are easy to handle and require little or no Compound Anhydrous Acidic special precautions to exclude atmospheric moisture, have JAB(H~) S(OH) been stared over molecular sieves in septum-capped amber S(0H) bottles for over 3 months with no deterioration in thequality 1 2.60 (d) 3.8 5.48 (s) of the spectra produced. 2 2.26 (t) 6.1 4.53 (s) As shown in the table, the hydroxylic protons of 1 4 , when 3 2.15 (t) 5.4 4.73 (S) dry, couple to their a-CH's with coupling constants ranging from 3.8 to 5.4 Hz and multiplicities predicted by the "n + 1" rule. The remaining resonances in these compounds are quite easy to assign, even by a novice N M R user. Adding 2-3 drops of trifluoroaeetie acid directly ta the NMR tube and mixing thomughly by inverting the tube causes rapid exchange and effectively decouples the hydroxylic protons from the a-CH's. The hydroxylic proton resonances collapse to a sinelet. which are shifted downfield. and the multiolicities of the a-CH's sienals are made simoler a s well. (See the table.', Increasing the amount uf acid k e a r e s the rhahve amount of single: with a further downfield shin. We feel that thew changes in chemical shift, couplingparterns, and relative integral values a r e a lurid demonstratinnof rapid proton exchanar and a splend~dfirst XMR experiment.
Literature Cited 1. Clams, G.;Cmmwel1.N. H.J. Chpm Edur
1971,48,20%203.
'~uthor10 whom correspondence should be addressed.
Marw Pagnotta,' James Carter, and Carrie Armsby Barnard College, Columbia University New York, NY 10027-6598
162
Journal of Chemical Education
