COMMUNICATIONS
[2] For a review, see T. Bach, Liebigs Ann. 1997, 1627 ± 1634.
[3] a) T. Bach, Tetrahedron Lett. 1994, 35, 1855 ± 1858; b) T. Bach, Liebigs
Ann. 1995, 1045 ± 1053; c) T. Bach, K. Jödicke, K. Kather, R. Fröhlich,
J. Am. Chem. Soc. 1997, 119, 2437 ± 2445.
[4] a) R. E. Schwartz, J. Liesch, O. Hensens, L. Zitano, S. Honeycutt, G.
Garrity, R. A. Fromtling, J. Onishi, R. Monaghan, J. Antibiot. 1988, 41,
1774 ± 1779; b) J. H. Johnson, D. W. Phillipson, A. D. Kahle, J.
Antibiot. 1989, 42, 1184 ± 1185.
[5] For previous syntheses of preussin, see A. Kanazawa, S. Gillet, P.
Delair, A. E. Greene, J. Org. Chem. 1998, 63, 4660 ± 4663, and
references therein.
[6] For examples, see a) C. Rivas, R. A. Bolivar, J. Heterocyclic Chem.
1976, 13, 1037 ± 1040; b) D. R. Morton, R. A. Morge, J. Org. Chem.
1978, 43, 2093 ± 2101; c) S. R. Thopate, M. G. Kulkarni, V. G. Puranik,
Angew. Chem. 1998, 110, 1144 ± 1147; Angew. Chem. Int. Ed. 1998, 37,
1110 ± 1112.
[7] For examples, see a) J. D. White, D. N. Gupta, J. Am. Chem. Soc. 1968,
90, 6171 ± 6177; b) J. J. Partridge, N. K. Chadha, M. R. Uskokovic, J.
Am. Chem. Soc. 1973, 95, 532 ± 540; c) S. W. Baldwin, M. T. Crimmins,
J. Am. Chem. Soc. 1980, 102, 1198 ± 1201; d) T. Hansson, B. Wickberg,
J. Org. Chem. 1992, 57, 5370 ± 5376; e) A. B. Smith III, G. A.
Sulikowski, M. M. Sulikowski, K. Fujimoto, J. Am. Chem. Soc. 1992,
114, 2567 ± 2576.
[8] M. J. S. Carpes, P. C. M. L. Miranda, C. R. D. Correia, Tetrahedron
Lett. 1997, 38, 1869 ± 1872.
[9] A. L. J. Beckwith, C. L. L. Chai, J. Chem. Soc Chem. Commun. 1990,
1087 ± 1088.
Ethenedithione (S C C S):
Does It Obey Hundꢀs Rule?**
Ngai Ling Ma and Ming Wah Wong*
One of the fundamental rules governing the electronic
structure of molecules is Hundꢁs rule of maximum multi-
plicity. In order to minimize Coulombic repulsion, two
electrons in a pair of degenerate orbitals prefer to be
unpaired. Thus, for a molecule like O2, the stability of the
three lowest electronic states is in the order Sg > 1Dg > 1Sg .
For the linear cumulated carbon oxides and carbon sulfides,
XCnX (X O and S; n is even), such species have two
electrons in the degenerate p HOMO, and hence are expected
to have a triplet ground state. However, for the smallest
member of the cumulated carbon sulfides, ethenedithione
3
(S C C S), the nature of its ground state remains contro-
versial. C2S2 was first predicted by Schaefer et al. to be an
experimentally accessible species in the gas phase that should
have a triplet ground state.[1] The existence of this transient
molecule has been demonstrated by mass spectrometry[2±4]
and matrix isolation IR[3, 5] and UV[3, 5, 6] spectroscopy. Based
on the unusual thermodynamic stability and extreme inter-
molecular activity, Wentrup et al. suggested that C2S2 is a
triplet species.[3] On the other hand, Maier et al. observed a
very weak signal in the ESR spectrum of C2S2, which
indicated that it could possess a singlet ground state. In
addition, they performed complete active space (CAS) SCF
and CISD calculations and found that the ground state of C2S2
is 1Dg, with a singlet ± triplet (S ± T) gap estimated to be 3 and
[10] a) S. C. Freilich, K. S. Peters, J. Am. Chem. Soc. 1981, 103, 6255 ± 6257;
b) S. C. Freilich, K. S. Peters, J. Am. Chem. Soc. 1985, 107, 3819 ± 3822.
[11] For a review, see H. Buschmann, H.-D. Scharf, N. Hoffmann, P. Esser,
Angew. Chem. 1991, 103, 480 ± 518; Angew. Chem. Int. Ed. Engl. 1991,
30, 477 ± 515.
[12] S. Saijo, M. Wada, J. Himizu, A. Ishida, Chem. Pharm. Bull. 1980, 28,
1449 ± 1458.
[13] E. Hardegger, H. Ott, Helv. Chim. Acta 1955, 38, 312 ± 320.
[14] J. Ackermann, M. Matthes, C. Tamm, Helv. Chim. Acta 1990, 73, 122 ±
132.
1
9 kJmol , respectively. Hence, C2S2 was suggested by Maier
et al. to be one of the first examples for the ªviolation of
Hundꢁs rule in an equilibrium structureº.[5]
[15] P. G. Gassman, S. J. Burns, J. Org. Chem. 1988, 53, 5574 ± 5578.
[16] T. Bach, Angew. Chem. 1996, 108, 976 ± 977; Angew. Chem. Int. Ed.
Engl. 1996, 35, 884 ± 886.
[17] The force field calculations (MM3*) were conducted with Macro-
model4.5 (G. Chang, W. C. Guida, W. C. Still, J. Am. Chem. Soc. 1989,
111, 4379 ± 4386). The coupling constants were calculated by an
extended Karplus function. The ratio of the various conformers was
deduced from a Boltzmann distribution at 373 K. As a simplification a
butyl side chain at C5 was assumed instead of a nonyl group. We thank
Dipl.-Chem. Thomas Trieselmann (research group of Prof. Hoffmann)
and Dr. Ruth Gschwind cordially for their help in the course of the
structure elucidation.
Given that the reported S ± T gap is rather small using
moderate levels of theory, whether C2S2 is exists as a singlet or
triplet remains uncertain. Herein we report ab initio calcu-
lations that are at a significantly higher level of theory than
those reported previously in order to establish definitively the
nature of the ground electronic state for ethenedithione.
First, we examined the energy difference of the three lowest
3
states of C2S2, namely Sg , 1Dg, and 1Sg , using various single-
[18] D. P. Curran, N. A. Porter, B. Giese, Stereochemistry of Radical
Reactions, VCH, Weinheim, 1995, pp. 120 ± 121.
3
1
determinant methods (Table 1). The Sg and Dg states were
[19] T. Bach, Liebigs Ann. 1995, 855 ± 866.
obtained by using an unrestricted Hartree ± Fock (UHF)
starting point. At the HF/6-31G* level, the 1Dg and 1Sg states
1
lie 31 and 98 kJmol , respectively, above the 3Sg state.
Inclusion of electron correlation at the MP2 level significantly
1
lowers the energy gap by 15 and 61 kJmol , respectively. It is
important to note that the hS2i values of the UHF wave-
3
1
functions of the Sg (2.10) and Dg (1.07) states are signifi-
cantly different from that of the corresponding pure spin
states. In particular, the 1Dg state is severely spin contaminated
[*] Prof. Dr. M. W. Wong, Dr. N. L. Ma
Department of Chemistry,
National University of Singapore
Kent Ridge, Singapore 119260 (Singapore)
Fax: (65)7791691
[**] M.W.W. would like to thank the National University of Singapore for
financial support (grant no. 970620).
3402
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1433-7851/98/3724-3402 $ 17.50+.50/0
Angew. Chem. Int. Ed. 1998, 37, No. 24