The AgH is +0.13 ppm for syn-CH3 and -0.28 ppm for anti-CH3 as the former lies in front of the carbonyl reference plane and the latter, behind the reference plane. The A$His -0.28 ppm for the 7-methyl group in accordance with the carbonyl plane rule. Carbonyl Plane Rule for Hexafluorobenzene If hexafluorobenzene is used instead of benzene as the aromatic solvent, the carbonyl plane rule is exactly reversed (5). "The protons lying in front of the reference plane are shielded -ve) and those behind the plane are deshielded (As@== +ve)."
(Ar=
collision complex model is intuitively very appealing to the organic chemist but is empirical. We advocate the use of solvent-induced shifts for separation and assignment of proton signals whenever possible, because the method is simnle. ex~edient.and inex~ensive.and the i d u t r can hr recuwred fnjm the solvents easily. The ad\,antazci and nses uf r\SIS cannot hr werern~~h;~sized. 'ru uume fr& literature (4) "solvent shifts can be used to differeitiate between axial and eauatorial orotons or methvl . .arouDs . adiacent tocarhun?l and ;o l u w e prurunj hchiud or in ti-mt niche krtu eruuu. The knowledae derived in t his manner is uhviousl~ of great utility in the aetermination of structure, stereochemistry, and conformation." Acknowledgment The author thanks Drs. I. S. Bhardwaj and S. Sivaram for facilities.
The ASIS are not only useful in the assignment and separation of signals ( 6 )but also are used often for determining a preferred conformation. Many examples have been reported (1, 7). The table lists some representative data on ASIS for a few monoterpenes. A similar carbonyl plane rule has been proposed for the aromatic solvent pyridine (pyridine-db). The only change is in the position of perpendicular reference plane which now passes through the two C, carbons (8,9).
Literature Cited (1) Willisms, D. H.. Pure Appi
Chrm., 40,25 (19741. 12) For revievn on solvent induced shifts, see Lsuzlo, P..Progr. NMR Speciiosc., 3, 231 11967);Ronalme, J., and Williams, D. H.. Ann. Rev. NMR Sixrlmsc., 2.83 (1969); ~ h a c c aN. , S., and wi1liams.D. H.,"ApplicationsofNMRSpedroseopy in Organic Chemistry:Hoiden-Dsy,San Francisco, 1964. (3) Connolly, J. D., and McCrindle, R.,Chem. Ind. ILandon), 379 (1965). (4) Wiiliams, D. H.,sndBhsecs. N. S.,Telrahedron,21,2021 11965). (5) Tori, K., Horik, I., Sigemoto, H., and Umemoto, K.. Tetrahedron Lett., 2199 i\.",",. lP7Xi (6) Ledall, T., Tetrahedron Lolf., 1683 11968). 17) See Horibo, I.. Sigemoto, H., andTori, K., TrfrohodronLatt.,2844 (1976) for anap. p1ieation to the conformational ane1ysia of sesquitemenes. For a cavestaee, Baldwin, J., J. Org. Chem,30,2423 11965);for cedranoneconfonnation by ASIS seekdriguez. M., and Bertran, J. F., Org Mogn. Reson. 13.263 (19801. (8) Williams, D. H.. Tetrahedron Lett., 2305 (1966). I91 For a leading reference on pyridine induced shifts see, Demarco, P. Y.,Farkas, E., Doddrell, D., Mylari, a. L., and Wenkert. E., J. Amer Chrm. Sac, 80, 5480
The ASIS for other classes of rornpounds have a h been studied and plane rules wggrsrrd i i m ls~,tuncs,lactnm
