NMR Analysis of 1- and 2-Adamantanol An Advanced Undergraduate Laboratory Project Charles D. Schaeffer, Jr. Elizabethtown College, Elizabethtown, PA 17022 Claude H. Yoder Franklin & Marshall College, Lancaster, PA 17604 The great value of lanthanide shift reagents in the simplification of NMR spectra has been thoroughly reviewed (1-14). There are, however, relatively few experiments designed to demonstmte t& technique. We report here a project used.in our iunior-year laboratories that involves the preparation of two aln)hols, t hr charnctcrizntion of t hese compounds, and the use of a shift rement for structure determination and peak assignment. Adamantane is a particularly interesting system for NMR analysis. Although the structural formula does not reveal its high symmetry to the casual observer, the molecule has tetrahedral svmmetrv. and annlication of these svmmetrv onerations &eels that it has &ly two types of carbon atoms&, conseouentlv. onlv two tvues of hvdro~ens. The rieiditv of the . struciure aisb has severii consequences. The vicinal-proton coupling usually observed in other molecules is very weak in adamantane derivatives (15) because of the Karplus dihedral angle-coupling constant relationship (16).Secondly, the internal rotational motions that sometimes affect shifts and coupline are in this case absent. 1" order to make use of a shift reagent. n suitahle functionill group must he present on the adamantane. The OH group is
CHCI.
1
Figwe 1.90-MHz 'H NMR spectrum 01 ladamanlano1 (a) and ?-adamanlano (b) in CDCh w8mout shin reagsnl(60 mg of alcohol m 2 mL of CDCI, in eacn
case),
frequently employed because of its strong complexation to the lanthanide atom. I t is particularly appropriate in this case because of the relatively easy preparation of the alcohols. Actually, there are two possible adamantanols: the derivative with the OH on the bridgehead (I) and the 2-derivative (11). l-Adamantanol is prepared by bromination of adamantane followed bv nucleor~hilicsubstitution with hvdroxide ion. 2-~damsnianolis secured by oxidation of adamantane with sulfuric acid followed by reduction with sodium horohydride. l-Adamantanol has the simpler proton and carhon-13 spectra because of higher symmetry; the region of the ring protons and carbons will both display four distinct areas. On the other hand, the lower symmetry of the 2-derivative leads to seven 13Cresonances and nine areas for the ring protons. The 'H (17-22) and I3C (23-29) NMR assienments for both alcohols have i~eenthe subjects of many studies. One N M K text t3u) reorodurcs the nrtmln NMR of 2-ndnmantanol but inadver&ntiy labels the pioton attached to carbon 2 as the OH proton. The 90-MHz proton NMR spectra of 1-and 2-adamantanol (Fig. l a and h) consist of closely spaced lines and are difficult to interpret. Nevertheless, solutions of these alcohols in CDCl3 with varying proportions of Eu(fod)3or its totally deuterated analog permit the assignment of all protons. Figures 2 and 3 show the 90-MHz proton spectra that result from a mixture of Eu(fod)s and the alcohols (for 2-adamantanol an equimolar ratio is required; for l-adamantanol, a smaller amount of shift reagent suffices). The McConnell-Robertson equation (31)
shows the realtionship between the lanthanide-induced chemical shift of the ith nucleus, Avi, where ri is the distance of the i t h nucleus to the lanthanide ion, and 0;is the angle hetween the principle magnetic axis of complex (usually assumed to he colinear with the lanthanide-alcohol bond axis) and a line drawn from the nucleus i to the lanthanide ion. The angle term gives positive values except when 0i is greater than 55" but less than 125" (9).At these angles, induced shifts will he in the opposite direction to the normal shift, e.g., upfield for Eu(fod)a. In most cases, however, the angle term is negliVolume 62
Number 6
June 1985
537
gihle, and a simplified version of the McConnell-Robertson equation may he used Avi = Klri3
(2)
Accordine to this eauation. the nrincivle factor determinine NMR resonance is the cube of the the shift of a distance senaratine the metal ion from the nucleus that is responsihle'for thatresonance. That is, the closer the nucleus is to the metal.. the ereater will he the observed shift. Tentative peak assignments can he made by consideration of annroximate internuclear distances. For examnle, the high& frequency peak a t 18.5 ppm in the spectr"m of 1adamantanol in Figure 2 has experienced the greatest shift (17 ppm relative to its position in the unshifted spectrum) and can therefore he attributed to the proton closest to europium; that is, the OH proton. The peak at 4.7 ppm has shifted 3 ppm and can thus he assigned to the six equivalent protons attached to the 2,8,9 carhon atoms (see tahle). Likewise, the peak a t 3.0 ppm is due to the 3,5,7 protons, and the second order AB douhlets at 2.3-2.7 ppm arise from the geminal coupling of the cis and trans protons attached to carbons 4,6,10. The potentially complicating vicinal couplings are generally not resolved (vide supra) in the adamantane framework. In order to make the assignments more precise (furexample. to determine which doul)let belonea to the c i . 9 - and which belongs to the trans-4,6,10 protons< incremental amounts of shift reagent may he added and spectra recorded after each addition. The shifts are then plotted versus the shift reagentadamantan01 molar ratio as shown in Figure 4 for 1adamantanol. The greater the incremental rate of shift, as reflected in a steeper slope of the line, the smaller the through-space distance (r;) between the nucleus and europium. In Figure 4, the line with the smallest slope was constructed from the shifts of the low frequency douhlet. This doublet must he assigned, therefore, to the proton farthest from europium. The determination of distances to the europium atom can he made with some certaintv in the adamantanols hv measurements from molecular models made to scale. he distances in the tahle were obtained hv measuring the distance of closest approach of a freely rotating europ&m atom attached to oxygen, assuming an Eu-0 distance of 2.51 &sum of covalent radii), a 115' angle at oxygen (17),a C-0 distance of 1.43 A, and C-C and C-H distances of 1.54 A and 1.09 3, respectively. I t is clear from the tahle that the trans-4,6,10 protons in l-adamantanol are farthest removed from europium and are therefore responsible for the low frequency douhlet. Notice that the 3,5,7 protons arecloser in number of bonds to the cis-4,6,10 protons, although they are actually farther in direct through-space distance from these cis protons. The 'H NMR spectrum of 2-adamantanol can he analyzed in a similar fashion, except that the two sets of protons re.. sponsihle for the peak a t 4.5 ppm in Figure 3 cannot he re-
25.0
15.0
5.0
8
Figwe 3 . 9 M H z 'H NMI specbum of 60 mg of 2-adamanlano1in 2 mL of W l s with 380 mgof Eu(fcd)~.A = 211 proton). B = cis-8.10(2 protons), C = 1.312 protons). D = trans-8.10 (2 protons). E = 7 (1 proton), F = cis-4.9 (2 protons). G.H = trans-4.9 (2 protonsl overlappingwilh 6 (2 protons), I = 5 (1 proton). OH oroton not shown.
solved a t 90 MHz, as might he anticipated on the basis of their similar throueh-snace distances (5.0 and 4.9 A for nrotons trans-4,9, a n l 6 , respectively, from the tahle). Because the carhon atoms are roughly the same distances as their attached protons from europium, they will experience similar lanthanide-induced shifts. As may he seen in Figures 5 and 6, the greater range of 13C chemical shifts makes the effect much less dramatic. Nevertheless, the closest carhon atoms are shifted to the greatest extent. The four lines in the proton-decoupled 13C spectrum of l-adamantanol (Fig. 5a) are readily assigned by a combination of extent of lanthanide shift (Fig. 6) and off-resonance multiplicities (Fig. 5h). Carbon C(1) is an off-resonance sinelet. C(2.8.9) . . . . and C(4.6.10) are off-;esonance triplets, and ~73,5,7)is an off-resonance doublet. The two sets of carbons giving rise to off-resonance triplets can he distinguished by their relative lanthanide shifts using spectral data derived from Figure 6. Experimental
Syntheses The preparation of l-bromoadamantane, l-adamantanol,and 2adamantanone are adapted from the procedures described by Monson (32). Preparation ofl-Bromaadornontane.Note: This reaction must be carried out in a hood using great care (gloves and goggles)
MOLE RATIO F ~ u 2e 9cw& 'H NMR *bum of60 mg of 1-adamantanal m 2 m- of W I s w ~ m80 mg 01 E ~ l l c d ,A~ = Oh proton. B = 2.8.9 (6 protons). C = 3.5.7 I3 protons). D = ar4.6.10 (3 protonsl. E = tmns4.6.10 (3 potonsl
538
Journal o f Chemical Education
Fogwe 4. 'H NMR chemical shihs la carom-bonded protons ol t-adamaman01 (60 mg of alcohol in 2 m. of COC131 versbs fncreasing Eulfodhlaaamantanol mo ar ratios. A = 2.8.9 (6 protons). B = 3.5.7 (3 pratonsl. C = cir4.6.10 (3 protons), D = mns-46.10 (3 protons).
reflux. The conversion is about 80% effective and the product has a melting point of 258-262T in a capillary tube.
NMR The NMR spectrum of 60.0 mg (0.394 mmol) of the adamantanol in 2.0 mL of CDC13 is first obtained without shift reagent. A trace amount of TMS may be added as an internal reference, but this can cause complications later in the experiment. The small amount of residual CHC5 present in the deuterated solvent is normally sufficient to serve as a standard that can be used to estimate TMS-based chemical shifts. Using this solution, 40.0 mg (0.0386 mmol) of Eu(fodh shift reagent (Aldrich Chemical Company) is then added to obtain a solution whose shift reagent/adamantanol molar ratio may be computed. The spectrum should be obtained a t sweep widths of both 10 and 20 ppm. If it is desirable to eliminate proton iesonances (0.42 oom) caused bv the shift reaeent. totallv deuterated E u ( f ~ ..d ) ~ - d . ~mav hr wbst~rutetlI M o r t c m T h l ( ~ k d In?. , Alfa I'nxlurb,, a l t h o u g h t h e r u s t is over rhrrc runes na m u
