MAGNETIC RESONANCE IN CHEMISTRY, VOL. 30, 338-346 (1992)

Substituent Effects on 'H Chemical Shifts

I-Complete 'H Chemical Shift Assignments of Methyl-Substituted Cyclic Systems

Julie Fisher* and Michael J. Gradwell School of Chemistry, University of Leeds, Leeds LS2 9JT, UK

'H chemical shift assignments are presented for 2-methyladamantane, 2-methylnorbornane (endo and exo) and 2-methylnorbornene (endo and e m ) . Resonance assignment was achieved using a variety of 1D and 2 D homo- and heteronuclear ( 'H-I3C) experiments. The methyl group-induced substituent chemical shift (SCS) is derived and the SCS of protons vicinal to this group is discussed.

KEY WORDS Norbornanes Norbornenes Adamantanes 'H NMR Substituent chemical shifts

INTRODUCTION

The phenomenon of the 'chemical shift' in NMR has been extensively investigated for a range of nuclei. Although it has proved invaluable in the character- ization of a broad range of molecules, i t is yet to be fully understood.

During the 1950s, Dailey and Shoolery' and Allred and Rochow' attempted to correlate 'H chemical shifts with various physical properties, including substituent electronegativity. The 'H chemical shifts of a series of substituted methanes and ethanes were measured and were found to bear a direct linear relationship to sub- stituent electronegativity. However, for systems larger than ethane such empirical rules were not obeyed, and it became clear that the orientation of a proton with respect to the substituent also had to be considered. Zurcher3 addressed this aspect by attempting to deter- mine individual contributions to a substituent chemical shift (SCS). The SCS, Ad, is simply defined as the differ- ence in chemical shift of a proton in a molecule in the presence (6,) and absence (6,) of a substituent X:

A& = 6, - 60

This shift difference can be considered as the sum of four different components : Ad,, , electric field effects due to the difference in the induced dipoles of the C-X bond and the parent C-H bond; Admagn, magnetic field effects, caused by the differences in the magnetic anisotropic susceptibility of the substituent and the parent C-H bond; Advdw, van der Waals effect, due to the difference in the interactions of the proton with neighbouring protons and interactions of the substit- uent X with those protons; and A6,,,,, a difference in intermolecular solvent interactions between the substi-

* Author to whom correspondence should be addressed

tuted and parent molecules. Zurcher could not fully investigate the contributions to the SCS, owing to limi- tations in his experimental data. Relatively low-field spectrometers, and hence poorly resolved spectra and poor geometric data, meant that SCS calculations were under-determined.

More recently, SCSs have been in~es t iga t ed~-~ using model systems with well defined geometries, e.g. nor- bornane, adamantane and cyclohexane. Thus chemical shifts for protons bearing a whole range of orientations with respect to the substituent could be studied. The more recent studies were successful in identifying the most important contributions of a particular substituent to an SCS for all protons, except those vicinal to the substituent. In order to improve these SCS calculations, we need to identify the cause (or causes) of these anom- alous vicinal chemical shifts. We approach this by con- sidering a substituent which has no significant electric field or anisotropic magnetic susceptibility associated with it. These conditions are met by the methyl group, which has therefore been adopted in this work.

The effect of methyl substituents on proton chemical shifts in cyclic hydrocarbons has been investigated for many years. Most notably, Curtis et al.' described anomalous values for the vicinal SCS in some methyl- cyclohexanes. They attempted to explain this effect as arising from interactions of the vicinal protons with gauche, vicinal C-C bonds. Our initial investigations of methyl-substituted cyclohexanes have failed to verify this.

Ideally, a thorough investigation of the methyl SCS requires more chemical shift data, for protons with a range of orientations with respect to the substituent. 'We are in the process of obtaining these data using methyl derivatives of adamantane, norbornane and norborriene (Fig. l), and also other rigid systems, and report here some of our initial findings. The assignment of the 'H NMR spectra of these compounds is presented, fol- lowed by a preliminary discussion of the derived SCS and a brief description of our future aims.

0749-1 58 1/92/040338-09 $05.00 0 1992 by John Wiley & Sons, Ltd.

Received 12 October 1991 Accepted 23 November I991

COMPLETE 'H CHEMICAL SHIFT ASSIGNMENTS OF METHYL-SUBSTITUTED CYCLIC SYSTEMS 339

"5, "3,

2-rnethyladamantane (1) 2-rnethylnorbornane (2 = endo; 3 = exo)

H3"

2-rnethylnorbornene (4 = endo; 5 = em)

Figure 1. Systems investigated.

RESULTS

2-Methyladamantane

Analyses'.* of the 'H NMR spectra of some 2- substituted adamantanes have been reported. The 'H NMR spectrum of 2-methyladamantane itself was pre- viously measured at 60 MHz by Fort and Schleyer.' At 60 MHz the spectrum is poorly resolved. At 400 MHz, the 'H NMR spectrum of 2-methyladamantane (Fig. 2) is relatively sharp and consists of a 3H doublet, due to the methyl group, and a series of broad, but resolved, resonances for the ring protons. These resonances can be readily assigned by the use of a phase-sensitive COSY'o and 'H/13C correlation spectra."

The methyl doublet [J(CH3, A) = 7.18 Hz] at 1.030 pprn (given in Ref. 9 as 1.04 ppm) is coupled to the multiplet at 1.838 ppm; this is therefore assigned to the geminal proton, A. Protons B and I are assigned using the 'H/13C correlation spectrum, the 13C spectrum having been assigned previ~us ly '~ . '~ . Carbon C-/? has a correlation with the 'H multiplet at 1.580 ppm, which is thus proton B. Similarly, a correlation is observed between C-E and the proton at 1.710 ppm. Hence the I protons resonate at 1.710 ppm.

The C-ysyn (CC, DD') protons are resolved at 1.924 and 1.476 ppm in the 'H/13C correlation spectrum. The proton at 1.924 ppm is coupled to I. Of C and D, only

C is in the correct orientation for 4J W-coupling to I ; hence, C is assigned to 1.924 ppm. By elimination, therefore, we can assign D to 1.476 ppm.

The C-6,,, carbon is correlated (lH/13C correlation spectrum) to a 'H multiplet at 1.768 ppm. The assign- ment of this resonance position to proton H is sup- ported by observation of couplings to C and D. G can be assigned to 1.860 ppm from the 'H/13C correlation spectrum.

Finally, the remaining, y-anti protons, E and F, res- onate at 1.739 and 1.821 ppm; their couplings to B are observed. The proton at 1.739 ppm is coupled to C, hence this must be proton E. By elimination, therefore F absorbs at 1.821 ppm.

2-Methy lnorbornane

The relative configuration of 2-substituted norbornanes and norbornenes (endo or exo) is readily determined. In the case of an endo compound, a large vicinal coupling is observed between the H-4 proton and the H-2x proton. In contrast, for the exo isomer, this coupling (to H-2n) is less than 1 Hz. However, a relatively large W- coupling is observed from the bridge proton (H-7a for norbornanes or H-7s in norbornenes, as shown in Fig. 1) to H-2 in the exo but not in the endo isomer. Following these guidelines, it has been possible to

340 J. FISHER AND M. J. GRADWELL

I

I I I , , , , I , . , , , , , I , , I I I I . . , I I <---r--v-I-r-v 1.95 1.w 1.85 1.80 1.75 1 . 7 0 1 - 8 5 1.80 1.55 1.50 1.4s

PPH

Figure 2. 400 MHz ' H NMR spectrum of Zmethyladamantane (ca. 10% in CDCI,)

specify unambiguously the configuration of each com- ponent of the exo- and endo-2-methylnorbornane mixture.

2-endo-Methylnorbornane (major component). The 400 MHz 'H NMR spectrum of 2-methylnorbornane (mixture of endo and exo isomers) is shown in Fig. 3. The 13C NMR spectra of both isomers have been assigned pre- v iou~ ly . '~ With the aid of a 'H/13C correlation spec- trum (Fig. 4), all of the methine resonances can be identified. C- 1 is correlated to the bridgehead pattern at 1.982 ppm, which is thus due to H-1. A correlation is observed between C-2 and the multiplet at 1.895 ppm, which is assigned to H-2x. A large coupling was observed between this proton and H-1, verifying that this is indeed the endo isomer. Finally, C-4 is correlated to the bridgehead resonance at 2.1 14 ppm, assigned to H-4. The 3H methyl doublet is resolved at 0.931 ppm

The other protons are assigned using the 'H/13C correlation spectrum and a DQF COSY spectrum" on the mixture of isomers (Fig. 5). Correlations are observed between C-3 and two protons at 1.741 ppm and 0.532 ppm. The resonance at 1.741 ppm is strongly coupled to H-4, and is thus assigned to H-3x. Therefore, by elimination, H-3n is the proton absorbing at 0.532 ppm. C-5 is correlated to multiplets at 1.468 ppm and 1.084 ppm. The H-3x and H-4 protons are coupled to the proton attached to C-5 at 1.468 ppm. This is assign- ed to H - ~ x , which shows a W-coupling and a vicinal coupling, respectively, to those two protons. In the same way as H-3n was assigned, H-5n is assigned to 1.084

[J(CH,, H - 2 ~ ) = 6.93 Hz].

ppm. In the case of C-6, we see correlations to protons at 1.273 and 1.549 ppm. Couplings are observed between the C-6 pattern at 1.273 ppm and H-2x and H-1. This is thus H - ~ x , which has a vicinal coupling to H-1 and a W-coupling to H-2x. The other proton absorbing at 1.549 ppm is thus H-6n. By virtue of a W-coupling to H-3n, H-7a is assigned to the resonance at 1.253 ppm. The final proton, H - ~ s , is observed at 1.331 ppm. We can verify this assignment by a W- coupling observed between this proton and H-6n.

2-ero-Methylnorbornane (minor component). As for the endo isomer, all of the methine resonances are assigned unambiguously from the 'H/* 3C correlation spectrum' ' (Fig. 4). C-1 is correlated to the multiplet at 1.820 ppm, which is thus due to H-1. C-4 correlates with the bridgehead pattern at 2.161 ppm, hence this is assigned to H-4. Similarly, C-2 is correlated to the resonance at 1.494 ppm. This is therefore H-2n; this assignment can be verified by the coupling observed from this reson- ance to the methyl group at 0.861 ppm [J(CH,,H-?.n)

In contrast to the endo isomer, assignment of the other shifts using the COSY spectra is more difficult, partly owing to overlap with resonances for the evsdo isomer, and some of the assignments required verifica- tion using ID decoupling difference spectra.' '

C-3 is correlated to protons absorbing at 0.930 and 1.425 pprn. A large coupling is observed between H-4 and the proton absorbing at 0.930 ppm. This is assigned to H-3x. The assignment was confirmed by recording a 1 D decoupiing difference spectrum; saturating the res-

= 6.82 Hz].

COMPLETE 'H CHEMlCAL SHIFT ASSIGNMENTS OF METHYL-SUBSTITUTED CYCLIC SYSTEMS

-436 -cti,.

H3x'

34 I

Figure 3. 400 MHz ' H NMR spectrum of 2-methylnorbornane (ca. 10% in CDCI,; mixture of endo and ex0 isomers). resonances belonging to the minor, exo isomer).

. 6 U . S O

((') indicates

onance position of H-4. H-3n gives the multiplet at 1.425 ppm. There are correlations from C-5 to reson- ances at 1.105 and 1.442 ppm. The lower field of these, at 1.442, is also coupled to H-4, and i s therefore H-5x. The proton absorbing at 1.105 ppm is thus H-5n. H-1 is coupled to H-2n, the H-7s/a protons and a multiplet at 1.462 ppm. Therefore, the resonance at 1.462 ppm must be due to H-6x. We can therefore assign, by consulting the 'H/13C correlation spectrum, PI-6n to 1.140 ppm. Finally, H-2n is coupled to the bridging proton at 1.035 ppm; this is therefore H-7a, which is 45 W-coupled to H-2n. By elimination we can assign H-7s to 1.333 ppm.

2-Meth ylnorbornene

2-endo-Methylnorbornene (major component). The H NMR spectrum of 2-endo-methylnorbornene (Fig. 6) was assigned using a phase-sensitive DQF COSY spectrum" (Fig. 7), recorded on a mixture of endo and exo isomers. As for the norbornane derivative, the endo isomer is the major isomer. The methyl doublet ( J = 6.89 Hz), resolved at 0.779 ppm, is coupled to the multiplet at 2.097 ppm; this is assigned to H-2x. H-2x is coupled to the bridgehead resonance at 2.656 ppm, which is thus due to H-1. As for 2-methylnorbornane, this coupling confirms the configuration at the 2- position. By elimination, H-4 can therefore be assigned

to the other bridgehead pattern at 2.743 ppm. Both H-2x and H-4 are coupled to the resonance at 1.867 ppm; this is assigned to H-3x. H-3x and H-2x are coupled to the multiplet at 0.415 ppm, which must be due to IJ-3n. A W-coupling is observed from H-3n to the bridging proton at 1.389 ppm, which can only be H-7s. The other bridging proton at 1.252 ppm is H-7a. The olefinic proton absorbing at 5.944 ppm is coupled to H-1 and is thus assigned to H-6. We also observe a coupling from H-5, absorbing at 6.126 ppm, to H-4.

2-cxo-Methyhorbornene (minor component). The assign- ments described here were made with the use of differ- ence decoupling experiments, in addition to the phase-sensitive DQF COSY1o (Fig. 7).

H-2n can be readily assigned. This is the multiplet at 1.455 pprn which is coupled to the methyl doublet at 1.06 ppm. The two bridgehead protons absorb at 2.393 and 2.780 ppm. Both bridgehead patterns are coupled with niultiplets at 1.291 and 1.369 ppm. These are there- fore the bridge protons. The lower field bridgehead proton at 2.780 ppm is also coupled to the rnultiplet (resembling a doublet of triplets) at 1.028 ppm. We can therefore assign this resonance to H - ~ x , and the lower field bridgehead proton absorbing at 2.780 ppm to H-4. Hence H-1 absorbs at 2.393 ppm. H-3x is coupled to a multiplet at 1.291 ppm, which is thus due to H-3n. H-2n

342 J. FISHER AND M. J. GRADWELL

Figure 4. 400 MHz 2D 1H/13C correlation spectrum of 2-methylnorbornane (mixture of endo and ex0 isomers). ((') indicates resonances of the minor, e m isomer).

DISCUSSION

Table 1 presents chemical shift data for compounds 1-5. The derived SCS for the methyl substituent (calculated using the shifts for the parent compounds reported in Ref. 4) are presented in Table 2.

is coupled to the resonances at 1.291 ppm, but not to the bridge pattern at 1.369 ppm. Hence H - ~ s , which W- couples to H-2n, is assigned to 1.291 ppm. H-7a, which does not W-couple with H-2n, therefore absorbs at 1.369 ppm. H-1 is coupled to the olefinic proton at 6.100 ppm, which is assigned to H-6. Hence H-5, which couples with H-4 absorbs at 5.998 ppm.

Table 1. Assigned 'H chemical shifts for the methyl derivatives'

1 2 3 4 5

Proton S(ppm) Proton S(pprn) Ref. 16 Proton S(ppm) Proton S(ppm) Ref. 16 Proton S(ppm)

A 1.838 H- I 1.982 H-1 1.820 H-I 2.656 2.6 H - l 2.393 6 1.580 H-2x 1.895 H-2n 1.494 H-2x 2.097 2.02 H-2n 1.455

D 1.476 H-3n 0.532 0.55 H-3n 1.425 H-3n 0.415 0.41 H-3n 1.291 E 1.739 H-4 2.114 H-4 2.161 H-4 2.743 2.7 H-4 2.780

G 1.860 H-5n 1.084 H-5n 1.105 H-6 5.944 5.91 H-6 6.100

I 1.710 H-6n 1.549 H-6n 1.140 H-7a 1.252 1.25 H-7a 1.369 CH, 1.030 H-7s 1.331 H-7s 1.333 CH, 0.779 0.78 CH, 1.069

C 1.924 H-3x 1.741 1.79 H-3x 0.930 H-3x 1.867 1.84 H-3x 1.028

F 1.821 H-5x 1.468 H-5x 1.442 H-5 6.126 6.11 H-5 5.998

H 1.768 H-6x 1.273 H-6x 1.462 H-7s 1.389 1.33 H-7s 1.291

H-7a 1.253 H-7a 1.035 CH, 0.931 0.85 CH, 0.861

"Ambient temperature, ca 10% in CDCI,. Shifts quoted relative to TMS as internal standard (6 = 0.0 ppm); quoted shifts have an accuracy of i0.003 ppm.

COMPLETE 'H CHEMICAL SHIFT ASSIGNMENTS OF METHYL-SUBSTITUTED CYCLIC SYSTEMS 343

Table 2. Derived SCS for the methyl derivatives (A6 = 6,, - 6")'

1 2 3 4 5 Proton M(pprn) Proton M(ppm) Proton Ad(ppm) Proton M(pprn) Proton M ( p p m )

A 0.085 H-1 -0.210 H-1 -0.372 H-1 -0.185 H - 1 -0.448 €3 -0.294 H-2x 0.424 H-2n 0.332 H-2x 0.494 H-2n 0.504

D -0.277 H-3n -0.630 H-3n 0.263 H-3n -0.534 H-3n 0.340

F 0.068 H-5x 0.003 H-5x -0.029 H-5 0.141 H-5 0.01 3 G -0.014 H-5n -0.078 H-5n -0.057 H-6 -0.041 H-6 0.115

I -0.043 H-6n 0.387 H-6n -0.022 H-7a 0.179 H-7a 0.296

C 0.171 H-3x 0.270 H-3x -0.541 H-3x 0.264 H-3x -0.575

E -0.014 H-4 -0.078 H-4 -0.031 H-4 -0.098 H-4 -0.061

H -0.106 H-6x -0.198 H-6x 0.009 H-7s 0.076 H-7s -0.022

H-7s 0.150 H-7s 0.152 H-7a 0.072 H-7a -0.146

a Data quoted relative to shifts presented in Ref. 4.

.60

. 80

1.00

1.20

1.40

1.60

1.80

2. OD

2.20 PPn

2.00 1.80 1.60 1 .40 1.20 1.00 . 80 .6D PPU

Figure 5. 400 MHz phase sensitive DQF COSY spectrum of 2-methylnorbornane (mixture of endo and ex0 isomers).

344 J. FISHER AND M. J. GRADWELL

r - m - 7 PPH

6 . 0 5 6. 00 5.95 H3x

8.10

7 -I----- -.-"?-"

2. ED 2 .40 2.20 2 . 0 0 1 .80 1 .60 1 .40 1 . 2 0 1.00 .a0 . t o

Figure 6. 400 MHz 'H NMR spectrum of 2-methylnorbornene (ca. 10% in CDCI,; mixture of endo and exo isomers). (( ') indicates

PPH

protons belonging to the minor, ex0 isomer)

As discussed above, the vicinal SCS is anomalous, but has been found to be consistent. Consider the vicinal methylene protons of the bicyclic (norbornane/ norbornene) systems; protons with an approximately eclipsed configuration with respect to the methyl group (W-3n in 2, W-3x in 3, H-3n in 4 and €3-3x in 5) experi- ence a shielding effect of -0.5 to -0.6 ppm, depending on the precise geometry. Those protons bearing a dihe- dral angle of 120" to the substituent (H-3x in 2, H-3n in 3, I-I-3x in 4 and €3-311 in 5) are deshieided by ca. 0.25- 0.35 ppm. In methylcyclohexane the axial and equato- rial protons vicinal to the methyl group each bear a dihedral angle of ca 60", and yet their chemical shifts are affected by the methyl group to different extents. The axial proton is shielded by 0.29 ppm, in line with the observations made above, whilst the equatorial proton is only very slightly shielded (-0.02 ppm)I7.

The vicinal, methine protons (B in 1 and H-1 in all of the other molecules), have torsional angles in the range 40-75" with respect to the methyl substituent. The shieldings observed are in the range -0.185 to -0.448 ppm, showing an angular dependence similar to that of the vicinal methylene protons.

There is clearly a correlation between dihedral angle and methyl-induced chemical shift; we are now in the process of performing a more detailed analysis of the data presented here in order to characterize this relationship fully. Clearly, in order to produce a quanti-

tative assessment of our results, it is necessary to derive accurate atomic coordinates. We are approaching this primarily by the use of geometry optimizations, at a variety of levels, depending on the individual system under consideration (from molecular mechanics to the ab initio). It is worth noting that problems have been identified, particularly with norbornane-type systems,' in attempting to compute a geometry. These problems are primarily due to the inability to quantify the bond angle strain in such molecules. However, it is possible to monitor geometric distortion which may be present in these bicyclic systems, but which is not accommodated in geometry optimizations, using scalar coupling data. In particular, it has been shownt9 that 3J(13C, 'H) couplings have a strong, Karplus-type" geometric dependence. This is particularly appropriate in this work, as it is possible to measure the couplings of the methyl carbon to the vicinal protons. in this way we may be able to account for the anisotropic shielding found for the vicinal protons in methylcyclohexane.

It is clear that present theoretical approaches do not account for the experimentally derived, vicinal SCS in Table 2. The most dramatic illustration of this is the relatively large shielding effect observed for those protons which are eclipsed with the methyl group; present theory3 would predict that these protons experi- ence a small deshielding effect due to van der Waals interactions with the substituent. Our future work will

COMPLETE 'H CHEMICAL SHIFT ASSIGNMENTS OF METHYL-SUBSTITUTED CYCLIC SYSTEMS 345

i--

' 8 ca 1 !

I

I ' Q

+ ..- a .

. ..

-. ,.'a

i I .8

- .B

I _

- 1.0

- 1.2

- 1.1

,- 1.6

- 1.8

- 2.0

- 2.2

- 2.1

I - I ' J ~ l ' l ' l ' l ~ l ' l ' l ~ l ~ l ~ l ~ 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 .8 . 6 . 4

PPH

Figure 7. 400 MHz phase sensitive DQF COSY spectrum of 2-rnethylnorbornene (mixture of endo and ex0 isomers). expansion to aliphatic reg ion.

therefore be focused on finding how systematic this vicinal shielding effect is, and how it can be accounted for.

ethyl acetate, with a catalytic amount of 10% palladium on carbon (Aldrich) added, gave 2-methyladamantane.

2-Methylnorbornane was prepared by Super Hydride (Aldrich) reduction of the mesylate (methanesulphonate) prepared from norbornane-2-methanol (Aldrich), using the method of Holder and M a t t ~ x - 0 . ~ ~ The olefin, 2- methylnorbornene, was synthesized from norbornene-2- methanol via an analogous route. Compounds

NMR spectra A Grignard reaction2' was used to prepare 2- methyladamantan-2-01 from a commercial sample of adamantan-2-one (Aldrich). 2-Methyleneadamantane All spectra were recorded on a Bruker AM400 spec- was then prepared from 2-methyladamantan-2-01 using trometer at ambient temperature. All spectra were refer- the method of Alford et a1.22 Finally, hydrogenation of enced to Tetramethylsilane (TMS) as internal standard 2-methyleneadamantane, at 200 psi in dry, distilled (6 = 0.0 ppm).

EXPERIMENTAL

346 J. FISHER AND M J. GRADWELL

The 1D 'H NMR spectrum of 2-methyladamantane was recorded with sweep width 846.02 Hz in 8K real data points, giving a total acquisition time of 4.84 s, over 64 transients. A Gaussian window function was applied (G.B. = 0.30; L.B. = -1.2) and the data were zero-filled to 16K points prior to Fourier transform- ation, giving a final digital resolution of 0.21 Hz per point.

The 1D 'H NMR spectrum of 2-methylnorbornane was recorded with a sweep width of 915.75 Hz (45" flip angle) in 8K real data points. The total acquisition time was 4.47 s. The number of transients recorded was 64. The data were resolution enhanced using a Gaussian window function (G.B. = 0.3; L.B. = - 1). The spec- trum was zero-filled to 16K data points, giving a final digital resolution of 0.24 Hz per point.

The 1D 'H NMR spectrum of 2-methylnorbornene was recorded with sweep width 2688.17 Hz in 32K data points. The acquisition time was 6.095 s, over 64 tran- sients. The data were resolution enhanced using a Gaussian window function (G.B. = 0.3; L.B. = -0.5). The data were zero-filled once to give a final digital resolution of 0.082 Hz per point.

All COSY experiments were recorded in the phase- sensitive mode. A total of 512 t , increments were recorded, with final memory sizes of 2K in F , and zero- filled once in F,, 64 scans recorded for each increment, with two dummy scans. An unshifted sine-bell window function was applied in both dimensions. For 2- methyladamantane the sweep width was 846.02 Hz in F , (423.01 Hz in F l ) , giving a total acquisition time of 1.21 s. A relaxation delay of 2.2 s was used. The final

digital resolution was 0.83 Hz per point in F , . For 2- methylnorbornane the sweep width was 915.72 Hz in F , (457.88 Hz in Fl). A relaxation delay of 2.5 s was used. The final digital resolution was 0.89 Hz per point in F , . For 2-methylnorbornene the sweep width was 2688.17 Hz in F , (1344 Hz in Fl). The acquisition time was 9.38 s. A relaxation delay of 2.8 s was used and the final digital resolution was 2.63 Hz per point in F , .

All 'H/I3C correlation experiments were recorded at 100.61 MHz for 13C. The number o f t , increments was 128. Final memory sizes were 2K data points in F , and 3.56 points in F , . For 2-methyladamantane, sweep widths were 8620.69 Hz in F , and 423.01 Hz in F, . The acquisition time was 0.12 s, with relaxation delay 1 s, over 256 transients per increment. In addition, two initial dummy transients were used per increment. The final digital resolutions were 8.42 Hz per point in F , and 6.61 Hz per point in F , . Sweep widths for 2- methylnorbornane were 8064.52 Hz in F, and 457.88 Hz in F , . This gave an acquisition time of 0.13 s, with relaxation delay 1 s. The number of transients per t , i 'crement were 416, with two dummy transients. The final digital resolutions were 7.88 Hz per point in F , and 7.15 Hz per point in F,.

Proton chemical shifts were obtained via spectral simulation (2-endo-methylnorbornene), or measured using rows and columns of relevant 2D matrices.

Acknowledgement

We thank the SERC for a Quota award to M. J. G.

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