RELATIVE SIGNSOF H-H COUPLING CONSTANTS FOR ABX SYSTEMS
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Relative Signs of H-H Coupling Constants for ABX Systems in Four- and Five-Membered Saturated Ring Compounds
by Richard H.Cox and Stanford L. Smith Department of Chemiatry, University of Kentucky, Lexington, Kentucky
40606
(Received December 6, 1966)
Double irradiation (spin-tickling) experiments on the ABX portions of the nmr spectra from 1,4-diphenyl-2-azetidinone, styrene carbonate, and the vinyl chloride Diels-Alder adduct of hexachlorocyclopentadiene are described. The geminal and vicinal H-H coupling constants are shown to have opposite signs in all three molecules. These results are in accord with recent theoretical predictions.
0 Exact computer analyses1-' of carefully calibrated nmr spectra for systems of three or more spins have P shown that geminal and vicinal H-H couplings may have either the same or opposite signs. In recent years these conclusions have been verified by double~ ~ across resonance experiment^.*-'^ Thus, 2 J couplings 'A spa hybridized carbons in saturated systems are usually found to be n e g a t i ~ e , ~ - * ~whereas ~ - ' ~ ~ for ~ ~ ~sp2 ~~ B ~ ~be either positive or hybridized systems 2 J may I negative depending on the nature of substituents attached to the system.14 Similarily, the sign of 2 J ~ ~ in three-membered heterocyclic depends on the heteroatom (e.g., ' J H Hin styrene oxide is +5.66 HZ while 2 J in~styrene ~ sulfide is -1.38 Hz).l6 This problem is of continuing significance, both from the practical aspect of spectral analysis and from the theoretical viewpoint of elucidating the quantum mechanical origin and mechanism(s) of spin-spin coupling. Recently, while investigating the solvent dependence of H-H couplings in small ring compounds, the relative sign of the geminal H-H coupling constant in these systems became of critical importance.16 In (1) R. R. Fraser, R. V. Lemieux, and J. D. Stevens, J. Am. Chem. order to obtain further information concerning the sign SOC.,83, 3901 (1961). (2) F. Kaplan and J. D. Roberta, ibid., 83, 4474 (1961). ~ ring systems, spin-tickling exof 2 J in~ saturated (3) K. L. 8ervis and J. D. Roberts, J . Phus. Chem., 67, 2886 (1963). periments were carried out on the ABX system in four(4) R. R. Fraser, Can. J. C h . ,40, 1483 (1962). and five-membered saturated ring compounds. We (5) C. A. Reilly and 3. D. Swalen, J. Chem. Phys., 32, 1378 (1960). report here the relative signs of the H-H coupling con(6) C.A. Reilly and J. D. Swalen, ibid., 35, 1522 (1961). stants in 1,4diphenyl-2-azetidinone(I), styrene car(7) H. Finegold, Proc. Chem. Soc., 213 (1962). bonate (11), and the vinyl chloride Diels-Alder adduct (8) R. Freeman, K. A. McLaughlan, J. I. Musher, and K. G. R. of hexachlorocyclopentadiene (111). Pachler, Mol. Phye., 5 , 321 (1962).
II
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Volume 71, Number 6 May 1967
RICHARD H. Cox AND STANFORD L. SMITH
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Experimental Section Procedures described in the literature were used to prepare 1,4-diphenyl-2-azetidinone (I)l6 and styrene carbonate (11),17 The vinyl chloride Diels-Alder adduct of hexachlorocyclopentadiene (111) was a gift from Dr. P. E. Hoch. Samples were prepared as 10 mole yo solutions in d-chloroform containing about 4% TMS as an internal standard and lock signal. Spectra were obtained with a Varian HA-60-IL spectrometer operating in the frequency-sweep mode. A Hewlett-Packard 201CR frequency oscillator monitored by a Hewlett-Packard 521CR frequency counter was employed to provide the weak perturbing radiofrequency field applied to a particular transition.
A
12 II IO
a
e 7
6 5
4 3
21
Results and Discussion Freeman and Andersong have described the effect of a weak perturbing radiofrequency field when applied to individual lines in an nmr spectrum. When the perturbing radiofrequency field is set at the frequency of a particular resonance line and the magnitude of this field is about equal t o the line width in Hz, transitions which share a common energy level with the irradiated line (and only those transitions) will be split into doublets. From results obtained by irradiating, in turn, several lines in the spectrum, it is then possible to trace out the energy level diagram of a nuclear spin system and, if the spectrum is close to first order, to determine the relative signs of the coupling constants. Figure 1A shows the ABX portion of the nmr spectrum of 1,4-diphenyl-2-azetidinone (I). The results of an experiment in which line 12 was irradiated are shown in Figure 1B. Changes in the spectrum show that line 1 and 2 of the A region have the same X spin state as do lines 5 and 6 of the B region. The results show J A B(gem) to be different in sign from JAx (trans) and J B X (cis).'* A similar experiment in which line 9 of the X region was irradiated (Figure IC) is consistent with the geminal coupling constant being opposite in sign to the two vicinal coupling constants. The nmr spectra of I1 and 111 are similar to the one shown in Figure 1A for I. Experiments, similar to the above, carried out on I1 and I11 give identical results. The chemical shifts and coupling constants obtained from exact computer analysislg of the nmr spectra of I, 11, and I11 are given in Table I. Making the usual assumption that vicinal H-H couplings are positive,z0*21 the absolute sign of the geminal H-H coupling constants is given as negative. The sign of the geminal H-H coupling constant in 111 has been suggested earlier to be negativezz and is verified here. Since it is highly unlikely that changing p substituents in the hexachlorobicycloheptene system The Journal of Phyeical Chemistry
Figure 1. The ABX portion of the 60-MHz spectra of 1,4-diphenyl-Z-azetidinone(I): A, normal spectrum; B, line 12 irradiated; C,line 9 irradiated.
would cause z J t o~change ~ by a factor of 2, it follows that the sign of 'JnH in the remainder of these adducts studied previouslyz3 is also negative. For I and 11, computer analyses with both a positive and a negative (9) R. Freeman and W. A. Anderson, J . Chem. Phys., 37, 2053 (1963). (10) R . Freeman and N. 9. Bhacca, ibid., 38, 1088 (1963). (11) D. D. Elleman and S. L. Manatt, J . Mol. Spectry., 9, 477 (1962). (12) K. A. McLaughlan and D. H. Whiffen, PTOC.Chem. SOC.(London), 144 (1962). (13) S. L. Manatt, D. D. Elleman, and S. J. Brois, J . Am. Chem. Soc., 87, 2220 (1965). (14) C. N. Banwell and N. Sheppard, Discussions Faraday SOC.,34, 115 (1962), and references contained therein. (15) Both couplings are solvent dependent. S. L. Smith and R. H. Cox, J . Chem. Phys., 45,2848 (1966). (16) H. Gilman and M. Speeter, J . A m . Chem. SOC.,65, 2255 (1943). (17) L. R. Morris and D. J. Hubbard, J. Org. Chem., 27, 1451 (1962). (18) A mnemonic method for establishing the relative signs of
coupling constants from double-resonance data has recently been given. E. F. Friedman and H. S. Gutowsky, J . Chem. Phys., 45, 3158 (1966). (19) J. D. Swalen and C. A. Reilly, ibid., 37, 21 (1962). (20) M. Karplus, J . Am. Chem. SOC.,84, 2458 (1962). (21) P. C. Lauterbur and It. J. Kurland, ibid., 84, 3405 (1962). (22) A. A. Bothner-By, private communication reported in ref 23. (23) K. L. Williamson, J . A m . Chem. SOC.,85, 516 (1963).
RELATIVE SIGNSOF H-H COUPLING CONSTANTS FOR ABX SYSTEMS
Table I: Calculated Chemical Shifts and Coupling Constants a t 60 MHe' Cornpound
JAB (gem)
I I1 111'
-15.16 -8.64 -13.32
JAX
(:?one)
JBX (cie)
2.73 5.66 7.78 8.07 3.16 8.20
V A ~
174.17 258.22 130.23
' Calculated probable errors are
