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EPR detection of the septet ground state of a trinitrene
Volume
2, number
EPR
4
CHEMICAL
DETECTION
OF
THE
SEPTET
E. WASSERMAN*, Bell
Telephone
PHYSICS
GROUND
K. SCHUELLER
Laboratories,
Mm-ray
Received
I II A solution of 1,3,5-triazido-2,4,6-tricyanobenzene [4] III in 2-methyltetrahydrofuran was rapidly cooled to ‘77OK to yield a clear glass. A few minutes irradiation with 3650 A radiation produced a deep blue-black material. The EPR spectrum at 9396 MHz had an absorption at 6792 G, D = 0.9735, E = 0 cm-l [5], as expected for the mononitrene IV obtained by the loss of one molecule of nitrogen from III. The o-cyanophenyl nitrene, obtained by irradiation of the corresponding azide, has D = 0.9716, E = 0 CIX-~. There was no evidence for a quintet dinitrene. That species should have a spectrum similar to IIc, and an absorption near 8400 G [l]. None was found above 7000 G. Presumably, the dinitrene, if formed, is unstable under our conditions. * Also at: Rutgers,
the State University, New Brunswick, New Jersey 08900, USA. ** To our knowledge the initial suggestion for the existence of a quintet system in an m-dicarbene was made by Murray and Trozzolo [2].
Hill,
STATE
OF
A TRINITRENE
and W. A. YAGER N&w Jersey
07974,
USA
12 July 1968
The first report of an organic septet state is made. zero-field interaction in reasonable agreement with a
We have observed the EPR spectrum of the septet state (S = 3) of trinitrene I. To our knowledge this is the first demonstration of an organic system with more than four unpaired electrons. Previously, the EPR spectra of the quintet ground states of the dicarbenes Ha, b and the dinitrene IIc were reported [l]“‘. Ilb has also been studied by Itoh in a single crystal [3].
_Xugust 1968
LETTERS
The trinitrene
has three-fold
simple theoretical treatment.
symmetry
and a
In addition to the mononitrene spectrum. a series of absorptions were observed with the form of the z and x, y lines characteristic of a randomly oriented multiplet with three-fold or higher symmetry [5]. The general appearence of the spectrum resembled the first-order line shape of a septet state with that symmetry; a shape identical to that computed for quadrupoIar resonance by Feld and Lamb [6,7]. For a more detailed analysis, we use the Ha‘niltonian H = DS$, which represents the dipolar’ interaction of unpaired electrons for three-foLd symmetry (E’ = 0). The exact eigenvalues require that the z lines should be spaced at Ho 5 D. f 30’ and f 5D’, where D’ = D/g& and Ho = = 3353 G. The observation of z lines near 2750, 1650, and 420 G, a 1:3:5 shift from Ho. provides the most direct support for the * ssignment of the spectrum to a septet. The z lines at No + D’ and Ho + 30’ are hidden by x, y lines which experience a second-order shift. The high-field limit of the septet absorptions is a z line at Ho + 50’ near 6320 G. From the z and x, y peaks we find DI = = 0.0548 f. 0.008 cm-l. The magnitude of DI may be justified by noting that it is an average of 32.Dij”
$-+,
yij3 G ij where YQ and ZQ refer to the distance and z component of the distance between two unpaired electrons in the ith and jth orbitals [I, S] With six orbitals, each singly occupied, there are I5 pair interactions; D=LCD+ ;S i
The dominant Dij arise from the interaction of an electron in an m-plane “o” orbital with one in a 7~orbital having a large spfn density at the 259
Volume
2. number
4
CHEMICAL
PHYSICS
same univalent nitrogen, Dm. This interaction is just the dipolar energy in the parent mononitrene triplet which we take to be IV. Since the z axis of the triplet is parallel to the bond of the univalent nitrogen, and the .z axis of the septet is perpendicular to the molecular plane by symmetry, we use D v transformed to the septet axes, i.e. This value appears - Dlv f 2 = DQ,i = -0:187 cm-l. for three pairs, one for each univalent nitrogen. The twelve other pairs make much smaller contributions to the sum. These contributions are difficult to evaluate with confidence as a number of them probabiy involve delocalization of the Uunpaired electrons. A crude guess of +0.45 cm-1 is offered as an aggregate vaIue although the uncertainty may well be f 0.3 cm-l. We then find that Dy = & (-3 Y 0.487 + 0.45) = 0.0674 cm-l, in reasonable agreement with the observed value, although given the uncertainties above, a range of 0.047- 0.087 cm-1 is possible. In the development of the EPR absorptions with time the triplet state appears more rapidly than the septet and thus the two are not in thermal equilibrium. With no other multiplets visible, the septet must be the ground state of the system or in thermal equilibrium with a ground state singlet. It is likely that the exchange integrals will be of such magnitude as to lreep the singlet and septet well separated in energy. Therefore, we feel that the septet state is the ground state of the trinitrene just as the quintet is the ground
260
August
LETTERS
1968
state of the dinitrene [1,33. In any case, the temperature depender.ce of the septet spectrum indicates that the species is the ground state or within about 20 small calories of that state.
ACKNOWLEDGEMENT The authors are most grateful to Dr. K. Wallen, fels for supplying a precursor for the synthesis of the triazide and to Drs. S. H. Glarum, R. W. Murray and A. M. Trozzolo for interesting discussions_
REFERENCES [I]
[Z] [3] [4] [5] [S]
E. Wasserman, R. 1%‘.Murray, W.A. Yager, A.M. Trozzolo and G. Smolinsky, J. Am. Chem. Sot. 89 (1967) 5076. R. W. Murray and A.M. Trozzolo. J. Org. Chem. 2G (1961) 3109. K. Itoh. Chem. Phys. Letters I (1967) 235. K. Wallenfels, F. Witzler and K. Friedrich, Tetrahedron 23 (1968) 1845. E. Wasserman. L.C. Snyder and W.A. Yager, J. Chem. Phys. 41 (1964) 1763. B.T. Feld and W. E. Lamb Jr., Phys. Rev. 67 (1945)
[7] E’Burns. [a] J. Hiychi,
J. Appl. Phys. 32 (1961) 2048. J. Chem. Phj%. 39 (1963) 1847.
2, number
EPR
4
CHEMICAL
DETECTION
OF
THE
SEPTET
E. WASSERMAN*, Bell
Telephone
PHYSICS
GROUND
K. SCHUELLER
Laboratories,
Mm-ray
Received
I II A solution of 1,3,5-triazido-2,4,6-tricyanobenzene [4] III in 2-methyltetrahydrofuran was rapidly cooled to ‘77OK to yield a clear glass. A few minutes irradiation with 3650 A radiation produced a deep blue-black material. The EPR spectrum at 9396 MHz had an absorption at 6792 G, D = 0.9735, E = 0 cm-l [5], as expected for the mononitrene IV obtained by the loss of one molecule of nitrogen from III. The o-cyanophenyl nitrene, obtained by irradiation of the corresponding azide, has D = 0.9716, E = 0 CIX-~. There was no evidence for a quintet dinitrene. That species should have a spectrum similar to IIc, and an absorption near 8400 G [l]. None was found above 7000 G. Presumably, the dinitrene, if formed, is unstable under our conditions. * Also at: Rutgers,
the State University, New Brunswick, New Jersey 08900, USA. ** To our knowledge the initial suggestion for the existence of a quintet system in an m-dicarbene was made by Murray and Trozzolo [2].
Hill,
STATE
OF
A TRINITRENE
and W. A. YAGER N&w Jersey
07974,
USA
12 July 1968
The first report of an organic septet state is made. zero-field interaction in reasonable agreement with a
We have observed the EPR spectrum of the septet state (S = 3) of trinitrene I. To our knowledge this is the first demonstration of an organic system with more than four unpaired electrons. Previously, the EPR spectra of the quintet ground states of the dicarbenes Ha, b and the dinitrene IIc were reported [l]“‘. Ilb has also been studied by Itoh in a single crystal [3].
_Xugust 1968
LETTERS
The trinitrene
has three-fold
simple theoretical treatment.
symmetry
and a
In addition to the mononitrene spectrum. a series of absorptions were observed with the form of the z and x, y lines characteristic of a randomly oriented multiplet with three-fold or higher symmetry [5]. The general appearence of the spectrum resembled the first-order line shape of a septet state with that symmetry; a shape identical to that computed for quadrupoIar resonance by Feld and Lamb [6,7]. For a more detailed analysis, we use the Ha‘niltonian H = DS$, which represents the dipolar’ interaction of unpaired electrons for three-foLd symmetry (E’ = 0). The exact eigenvalues require that the z lines should be spaced at Ho 5 D. f 30’ and f 5D’, where D’ = D/g& and Ho = = 3353 G. The observation of z lines near 2750, 1650, and 420 G, a 1:3:5 shift from Ho. provides the most direct support for the * ssignment of the spectrum to a septet. The z lines at No + D’ and Ho + 30’ are hidden by x, y lines which experience a second-order shift. The high-field limit of the septet absorptions is a z line at Ho + 50’ near 6320 G. From the z and x, y peaks we find DI = = 0.0548 f. 0.008 cm-l. The magnitude of DI may be justified by noting that it is an average of 32.Dij”
$-+,
yij3 G ij where YQ and ZQ refer to the distance and z component of the distance between two unpaired electrons in the ith and jth orbitals [I, S] With six orbitals, each singly occupied, there are I5 pair interactions; D=LCD+ ;S i
The dominant Dij arise from the interaction of an electron in an m-plane “o” orbital with one in a 7~orbital having a large spfn density at the 259
Volume
2. number
4
CHEMICAL
PHYSICS
same univalent nitrogen, Dm. This interaction is just the dipolar energy in the parent mononitrene triplet which we take to be IV. Since the z axis of the triplet is parallel to the bond of the univalent nitrogen, and the .z axis of the septet is perpendicular to the molecular plane by symmetry, we use D v transformed to the septet axes, i.e. This value appears - Dlv f 2 = DQ,i = -0:187 cm-l. for three pairs, one for each univalent nitrogen. The twelve other pairs make much smaller contributions to the sum. These contributions are difficult to evaluate with confidence as a number of them probabiy involve delocalization of the Uunpaired electrons. A crude guess of +0.45 cm-1 is offered as an aggregate vaIue although the uncertainty may well be f 0.3 cm-l. We then find that Dy = & (-3 Y 0.487 + 0.45) = 0.0674 cm-l, in reasonable agreement with the observed value, although given the uncertainties above, a range of 0.047- 0.087 cm-1 is possible. In the development of the EPR absorptions with time the triplet state appears more rapidly than the septet and thus the two are not in thermal equilibrium. With no other multiplets visible, the septet must be the ground state of the system or in thermal equilibrium with a ground state singlet. It is likely that the exchange integrals will be of such magnitude as to lreep the singlet and septet well separated in energy. Therefore, we feel that the septet state is the ground state of the trinitrene just as the quintet is the ground
260
August
LETTERS
1968
state of the dinitrene [1,33. In any case, the temperature depender.ce of the septet spectrum indicates that the species is the ground state or within about 20 small calories of that state.
ACKNOWLEDGEMENT The authors are most grateful to Dr. K. Wallen, fels for supplying a precursor for the synthesis of the triazide and to Drs. S. H. Glarum, R. W. Murray and A. M. Trozzolo for interesting discussions_
REFERENCES [I]
[Z] [3] [4] [5] [S]
E. Wasserman, R. 1%‘.Murray, W.A. Yager, A.M. Trozzolo and G. Smolinsky, J. Am. Chem. Sot. 89 (1967) 5076. R. W. Murray and A.M. Trozzolo. J. Org. Chem. 2G (1961) 3109. K. Itoh. Chem. Phys. Letters I (1967) 235. K. Wallenfels, F. Witzler and K. Friedrich, Tetrahedron 23 (1968) 1845. E. Wasserman. L.C. Snyder and W.A. Yager, J. Chem. Phys. 41 (1964) 1763. B.T. Feld and W. E. Lamb Jr., Phys. Rev. 67 (1945)
[7] E’Burns. [a] J. Hiychi,
J. Appl. Phys. 32 (1961) 2048. J. Chem. Phj%. 39 (1963) 1847.