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2-methylenecyclohept-3-en-1,5-diyl (a vinyl-TMM biradical) has a triplet ground state
Tetrahedron Letters,Vol.28,No.Sl,pp Printed in Great Britain
6501-6502,1987
0040-4039/87 $3.00 + .OO Perqamon Journals Ltd.
2-METHYLENECYCLOHEPT-3-EN-1,5-DIYL (A VINYL-TMM BIRADICAL) HAS A TRIPLET GROUND STATE
By Richard J. Bushby*, Christine Jarecki*, Duro Oduwole+ and Keith D. Sales+, *Department of Organic Chemistry, the University, Leeds IS2 9JT. 'Department of Chemistry, Queen Mary College, Mile End Road, London
El
4NS.
A Curie Law study in the temperature range 9-80K shows that 2-methylenecyclohept-3-en-1,5diyl 5 has a triplet ground state. A HMO calculation on a hydrocarbon pibiradicalnormallygives non-bonding MO's.
a degenerate pair of
However, it is not easy to predict whether the ground state should be a
singlet or a triplet.
As well as numerous high level theoretical studies'
predictive rules have been advanced.
three simple
Firstly, Hund's rule2 suggests that the triplet, the
state of highest multiplicity, will always be the ground state.
Secondly, Borden3 has
divided pi-biradicals into disjoint (NBMO'S have no atoms in common) and non-disjoint (NBMO'S with atoms in common) and claimed that whereas the ground state of non-disjoint biradicals will always be the triplet for disjoint biradicals the ground state may become the singlet.
Thirdly, Ovchinnikov4 has claimed that for an alternant pi-biradical the spin
of the ground state will be given by in*-n1/2 where n* and n are the number of starred and unstarred atoms in the alternant pi system.
As, however, experimental results have become
available it has become clear that there are deficiencies in simple and advanced theory alike.
This can be illustrated in the case of the three simplest non-KekulG polyenes,
TMM-1, TME-2, and vinyl-TMM-j5.
On the basis of any simple rule 2-4 TMM should have a triplet ground state and experimental observation of the triplet e.s.r.spectra of the parent 16 and bridged h7 TMM biradicals were followed by Curie Law studiesa confirming that this is indeed the case. However, the best of many theoretical studies suggests that the triplet lies ca. 15.1 kcal below the
6501
6502
lowest single state9 whereas experimental result show the difference to be < 7 kcal for 1" and < 4 kcal for 4".
TME 2 is disjoint and both Ovchinnikov's rule and some more
sophisticated calculations 3,12 suggest that it has a singlet ground state.
A recently
reported Curie Law study has, however, shown that it has a triplet ground state13,14. is therefore of interest to examine the third member of this series vinyl-TMM 3.
It
Recently
we reported the first e.s.r. observation of a vinyl-TMM biradical, the bridged derivative 6.0
t I
positive slope of this line confirms
0.0 0.11
0.00 l/T, K-l-+
that the triplet is indeed the ground state18.
References and Notes 1.
2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14.
15. 16. 17. 18.
W.T. Borden and E.R. Davidson, Ann. Rev. Phys. Chem., 1979, 30, 125; W.T. Borden in "Diradicals",Wiley, New York, 1982. F. Hund, Z. physik, 1925, 22, 345. W.T.Borden and E.R.Davidson, J. Amer. Chem. Sot., 1977, 99, 4587. A.A. Ovchinnikov, Theoret. Chim. Acta, 1978, 2, 297. Non-Kekulh isomers of butadiene and hexatriene. P.Dowd, J. Amer. Chem. Sot., 1966,E, 2587. J.A. Berson, R.J. Bushby, M. Tremelling, and J.M. McBride, J. Amer. Chem. SoC., 1971, 93, 1544. R.J. Baseman, D.W. Pratt, M. Chow, and P. Dowd, J. Amer. Chem. Sot., 1976, 98, 5726; M.S. Platz, J.M. McBride, R.D. Little, J.J. Harrison, A. Shaw, S.E. Potter, and J.A. Berson,s., page 5725. D.M.Hood, H.F. Schaefer and R.M. Pitzer, J. Amer. Chem. Sot., 1978,100, 8009. P.Dowd and M.Chow, 3. Amer. Chem. Sot., 1977, 99, 6438. M.S. Platz and J.A. Berson, J. Amer. Chem. Sot., 1977, 99, 5178. The result is somewhat dependent on the assumed geometry of the system (planar or orthogonal) and the M.O. method used. P. Dowd, W. Chang, and Y.H. Paik, J. Amer. Chem. Sot., 1986,108, 7416 and references therein. It has also been shown recently that the cyclic pi biradical 6, which is predicted to have a singlet ground state, actually has \'/ a triplet ground state, W.R. Roth, R. Langer, M. Bartmann, B. Stevermann, G. Maier, H.P. Reisenauer, R. Sustmann, and W. / \ Miiller, Angew. Chem. (Int. Edn.) 1987, 26, 256. xx . R.J. Bushby and C. Jarecki, Tet. Lett., 1986, 2053. 5:5:2 vol. diethyl ether : isopentane : ethanol. 6 C.P. Poole, 'Electron Spin Resonance: A Comprehensive Treatise on Experimental Techniques',Wiley, New York, 1967, p.784. A positive slope would also apply in the unlikely case of degenerate singlet and triplet states.
(Received
in UK 28 October
1987)
6501-6502,1987
0040-4039/87 $3.00 + .OO Perqamon Journals Ltd.
2-METHYLENECYCLOHEPT-3-EN-1,5-DIYL (A VINYL-TMM BIRADICAL) HAS A TRIPLET GROUND STATE
By Richard J. Bushby*, Christine Jarecki*, Duro Oduwole+ and Keith D. Sales+, *Department of Organic Chemistry, the University, Leeds IS2 9JT. 'Department of Chemistry, Queen Mary College, Mile End Road, London
El
4NS.
A Curie Law study in the temperature range 9-80K shows that 2-methylenecyclohept-3-en-1,5diyl 5 has a triplet ground state. A HMO calculation on a hydrocarbon pibiradicalnormallygives non-bonding MO's.
a degenerate pair of
However, it is not easy to predict whether the ground state should be a
singlet or a triplet.
As well as numerous high level theoretical studies'
predictive rules have been advanced.
three simple
Firstly, Hund's rule2 suggests that the triplet, the
state of highest multiplicity, will always be the ground state.
Secondly, Borden3 has
divided pi-biradicals into disjoint (NBMO'S have no atoms in common) and non-disjoint (NBMO'S with atoms in common) and claimed that whereas the ground state of non-disjoint biradicals will always be the triplet for disjoint biradicals the ground state may become the singlet.
Thirdly, Ovchinnikov4 has claimed that for an alternant pi-biradical the spin
of the ground state will be given by in*-n1/2 where n* and n are the number of starred and unstarred atoms in the alternant pi system.
As, however, experimental results have become
available it has become clear that there are deficiencies in simple and advanced theory alike.
This can be illustrated in the case of the three simplest non-KekulG polyenes,
TMM-1, TME-2, and vinyl-TMM-j5.
On the basis of any simple rule 2-4 TMM should have a triplet ground state and experimental observation of the triplet e.s.r.spectra of the parent 16 and bridged h7 TMM biradicals were followed by Curie Law studiesa confirming that this is indeed the case. However, the best of many theoretical studies suggests that the triplet lies ca. 15.1 kcal below the
6501
6502
lowest single state9 whereas experimental result show the difference to be < 7 kcal for 1" and < 4 kcal for 4".
TME 2 is disjoint and both Ovchinnikov's rule and some more
sophisticated calculations 3,12 suggest that it has a singlet ground state.
A recently
reported Curie Law study has, however, shown that it has a triplet ground state13,14. is therefore of interest to examine the third member of this series vinyl-TMM 3.
It
Recently
we reported the first e.s.r. observation of a vinyl-TMM biradical, the bridged derivative 6.0
t I
positive slope of this line confirms
0.0 0.11
0.00 l/T, K-l-+
that the triplet is indeed the ground state18.
References and Notes 1.
2. 3. 4. 5. 6. 7. 8.
9. 10. 11. 12. 13. 14.
15. 16. 17. 18.
W.T. Borden and E.R. Davidson, Ann. Rev. Phys. Chem., 1979, 30, 125; W.T. Borden in "Diradicals",Wiley, New York, 1982. F. Hund, Z. physik, 1925, 22, 345. W.T.Borden and E.R.Davidson, J. Amer. Chem. Sot., 1977, 99, 4587. A.A. Ovchinnikov, Theoret. Chim. Acta, 1978, 2, 297. Non-Kekulh isomers of butadiene and hexatriene. P.Dowd, J. Amer. Chem. Sot., 1966,E, 2587. J.A. Berson, R.J. Bushby, M. Tremelling, and J.M. McBride, J. Amer. Chem. SoC., 1971, 93, 1544. R.J. Baseman, D.W. Pratt, M. Chow, and P. Dowd, J. Amer. Chem. Sot., 1976, 98, 5726; M.S. Platz, J.M. McBride, R.D. Little, J.J. Harrison, A. Shaw, S.E. Potter, and J.A. Berson,s., page 5725. D.M.Hood, H.F. Schaefer and R.M. Pitzer, J. Amer. Chem. Sot., 1978,100, 8009. P.Dowd and M.Chow, 3. Amer. Chem. Sot., 1977, 99, 6438. M.S. Platz and J.A. Berson, J. Amer. Chem. Sot., 1977, 99, 5178. The result is somewhat dependent on the assumed geometry of the system (planar or orthogonal) and the M.O. method used. P. Dowd, W. Chang, and Y.H. Paik, J. Amer. Chem. Sot., 1986,108, 7416 and references therein. It has also been shown recently that the cyclic pi biradical 6, which is predicted to have a singlet ground state, actually has \'/ a triplet ground state, W.R. Roth, R. Langer, M. Bartmann, B. Stevermann, G. Maier, H.P. Reisenauer, R. Sustmann, and W. / \ Miiller, Angew. Chem. (Int. Edn.) 1987, 26, 256. xx . R.J. Bushby and C. Jarecki, Tet. Lett., 1986, 2053. 5:5:2 vol. diethyl ether : isopentane : ethanol. 6 C.P. Poole, 'Electron Spin Resonance: A Comprehensive Treatise on Experimental Techniques',Wiley, New York, 1967, p.784. A positive slope would also apply in the unlikely case of degenerate singlet and triplet states.
(Received
in UK 28 October
1987)