Demonstration of the degeneracy of the singlet and triplet states in 2,3-dimethylenecyclohexane-1,4-diyl by measurement of its magnetic properties †

Abstract

Magnetic susceptibility and magnetization of 2,3-dimethylenecyclohexane-1,4-diyl (1), a conformationally restricted tetramethyleneethane (TME), have been investigated at 2–20 K and applied fields of 0–5 T. Theoretical curves for a model in which singlet and triplet states are in equilibrium were fitted to the experimental data. In contrast with a previous interpretation of the EPR signal intensities, singlet and triplet states were found to be almost degenerate (2|J/k_B| < 1 K), thus resolving a controversy as to the theoretical and experimental interpretation of the ground state of 1.

References

1. (a) H. C. Longuet-Higgins, J. Chem. Phys., 1950, 18, 265; (b) P. Dowd, Acc. Chem. Res., 1972, 5, 242; (c) L. Salem and C. Rowland, Angew. Chem., Int. Ed. Engl., 1972, 11, 92; (d) J. A. Berson, Acc. Chem. Res., 1978, 11, 446; (e) W. T. Borden, ed. Diradicals, Wiley, New York, 1982; (f) P. M. Lahti, A. S. Ichimura and J. A. Berson, J. Org. Chem., 1989, 54, 958; (g) J. A. Berson, Mol. Cryst. Liq. Cryst., 1989, 176, 1; (h) W. T. Borden and E. R. Davidson, Acc. Chem. Res., 1996, 29, 67; (i) W. T. Borden and E. R. Davidson, J. Am. Chem. Soc., 1977, 99, 4587; (j) A. A. Ovchinnikov, Theor. Chim. Acta, 1978, 47, 297.
2. (a) P. Du, D. A. Hrovat and W. T. Borden, J. Am. Chem. Soc., 1986, 108, 8086; (b) P. Du and W. T. Borden, J. Am. Chem. Soc., 1987, 109, 930; (c) T. P. Radhakrishnan, Chem. Phys. Lett., 1991, 181, 455; (d) B. L. V. Prasad and T. P. Radhakrishnan, J. Phys. Chem., 1992, 96, 9232; (e) P. Nachtigall and K. D. Jordan, J. Am. Chem. Soc., 1992, 14, 4743; (f) J. J. Nash, P. Dowd and K. D. Jordan, 1992, 114, 10 071; (g) J. Pranata, J. Am. Chem. Soc., 1992, 114, 10 537; (h) P. Nachtigall and K. D. Jordan, J. Am. Chem. Soc., 1993, 15, 270.
3. (a) P. Dowd, J. Am. Chem. Soc., 1966, 88, 2587; (b) R. J. Crawford and D. M. Cameron, J. Am. Chem. Soc., 1966, 88, 2589; (c) P. Dowd and K. Sachdev, J. Am. Chem. Soc., 1967, 89, 715; (d) P. Dowd, A. Gold and K. Sachdev, J. Am. Chem. Soc., 1968, 90, 2715; (e) J. A. Berson, R. J. Bushby, M. Tremelling and J. M. McBride, J. Am. Chem. Soc., 1971, 93, 1544; (f) B. K. Carpenter, R. D. Little and J. A. Berson, J. Am. Chem. Soc., 1971, 93, 5723; (g) M. S. Platz, J. M. McBride, R. D. Little, J. J. Harrison, A. Shaw, S. E. Potter and J. A. Berson, J. Am. Chem. Soc., 1976, 98, 5725; (h) R. J. Baseman, D. W. Pratt, M. Chow and P. Dowd, J. Am. Chem. Soc., 1976, 98, 5726.
4. (a) P. Dowd, J. Am. Chem. Soc., 1970, 92, 1066; (b) W. R. Roth and G. Erker, Angew. Chem., Int. Ed. Engl., 1973, 12, 504; (c) T. Beetz and R. M. Kellogg, J. Am. Chem. Soc., 1973, 95, 23; (d) P. Dowd, W. Chang and Y. H. Paik, J. Am. Chem. Soc., 1986, 108, 7416; (e) W. R. Roth, U. Kowalczik, G. Maier, H. P. Reisenauer, R. Sustmann and W. Müller, Angew. Chem., Int. Ed. Engl., 1987, 26, 1285; (f) P. Dowd, W. Chang and Y. H. Paik, J. Am. Chem. Soc., 1987, 109, 5284; (g) W. R. Roth, R. Langer, M. Bartmann, B. Stevermann, G. Maier, H. P. Reisenauer, R. Sustmann and W. Müller, Angew. Chem., Int. Ed. Engl., 1987, 26, 256.
5. (a) T. Matsumoto, N. Koga and H. Iwamura, J. Am. Chem. Soc., 1992, 114, 5448; (b) T. Matsumoto, T. Ishida, N. Koga and H. Iwamura, J. Am. Chem. Soc., 1992, 114, 9952; (c) W. T. Borden, H. Iwamura and J. A. Berson, Acc. Chem. Res., 1994, 27, 109; (d) A. Rajca and S. Rajca, J. Am. Chem. Soc., 1996, 11, 8121.
6. (a) L. C. Bush, R. B. Heath and J. A. Berson, J. Am. Chem. Soc., 1993, 115, 9830; (b) L. C. Bush, R. B. Heath, X. W. Feng, P. A. Wang, L. Maksimovic, A. I. Song, W.-S. Chung, A. B. Berinstain, J. C. Scaiano and J. A. Berson, J. Am. Chem. Soc., 1997, 119, 1406; (c) L. C. Bush, L. Maksimovic, X. W. Feng, H. S. M. Lu and J. A. Berson, J. Am. Chem. Soc., 1997, 119, 1416.
7. H. Iwamura and N. Koga, Acc. Chem. Res., 1993, 26, 346.
8. B. Bleaney and K. D. Bowers, Proc. R. Soc. London, Ser. A, 1952, 214, 451.
9. Another paper in this issue reports the results of a study of the singlet–triplet splitting in the parent TME diradical by photoelectron spectroscopy, E. P. Clifford, P. G. Wenthold, W. C. Lineberger, G. B. Ellison, C. X. Wang, J. J. Grabowski, F. Vila and K. D. Jordan, J. Chem. Soc., Perkin Trans. 2, 1998, 1015.
10. W. R. Roth, M. Biermann, G. Erker and K. Jelich, Chem. Ber., 1980, 113, 586.
11. (a) M. Rule, A. R. Matlin, D. E. Seeger, E. F. Hilinski, D. A. Dougherty and J. A. Berson, Tetrahedron, 1982, 38, 787; (b) B. L. V. Prasad and T. P. Radhakrishnan, J. Mol. Struct. (THEOCHEM), 1996, 361, 175; (c) Z. Havlas and J. Michl, J. Mol. Struct. (THEOCHEM), 1997, 398–399, 281.