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ESR spectra of mixed singlet-quintet states in diacetylene crystals
ESR SPECTRA
15 March 1981
CHEMICAL PHYSICS LETTERS
Volume 78, number 3
OF MIXED
SINGLET-QUINTET
STATES
IN DIACETYLENE
CRYSTALS
R.A. HUBER, M. SCHWOERER PII_I a falrsches inshht t, Oniversctat Ban ercth, D-S%‘0 Bayrecc th, West Ger:nan_v H. BENK
Instrtut fiir Theoretuche
P&wh-. Ted I, Utliverslt~t Stuttgart,
D-7000 Stuttgart.
West GermanJ
and H. SIXL PI~ysikalrsches Itrstitrrt, Ted 3. Unil emtat
Stuttgart,
D- 7000 Stuttgart.
Received 14 January 1981, m final form 2 februdrl
West Germaa,lqr
1981
ESR spectra of four different blcxbenes in a dlacetylene crystal are fitted to the theory of mixed singlet-quintet states. hl~ng is due to electrostatic .md mdgnetlc dipole coupling between a paw of triplet carbetzes. The tit is perfect. It determines for each blcarbene the energ) difference between the ground state which IS a cmglet m zero order and the excited states which are quintets m zero order.
1. Introduction l
The recent interest in diacetylene crystals is due to their reactiirity in the solid state. As was shown by Wegner and co-workers [ I] highly perfect polymer diacetylene single crystals are obtained from the cxresponding monomer crystals under the Action of pressure, heat, UV- or y-irradiation. In recent experiments concerning
the mechanisms
of the solid state polym-
enzation reaction a series of different intermedtate states has been stabilized at low temperatures and analyzed by ESR and optical spectroscopy [Z--5] _ The different dimer, trimer. tetramer, etc. intermediates of this unusual photochemical reaction are claw-
fied mto S = 0, S = 1 triplet and S = 2 quintet states- A blcarbene structure of a planar tetramer, consisting of two mesomeric forms, is shown schematically in scheme I_ In this configuration two triplet states are coupled electrostatically to a pair state which is characterized by the Intrinsic fine structure parameters D, and E, and the orientation cpof the fine structure tensor of the individual triplet states and by their distance R 12_ 416
0 009-26
R-c_!
’
z
P P @ -c* ,&c-c* C;
’
c-
I4
d
b I
I
7: : ‘7 7 -‘c-c,\ ‘c_C%C c_CZC -c+&Z=-c-c* C- . d d R’
’ R-C%-c .
2 b
Scheme 1. Bicarbene configuration. The bicarbene tetramer molecule consists of four diacetylene units with two identical mesomeric trxplet carbene chain ends. D, and ET are the triplet fme structure parameters of the S = 1 carbene species and 6 r and $2 are their wavefunctions. 9 gives the orientation of the triplet fme structure z aGs with respect to the crystal C3aGs in a pkmar arrangement of the two mesomeric tetramer structures_
In this paper the theory of mixed singlet-quintet pair states [6] is applied to the fiie structure analysis of the bicarbene ESR spectra, obtained during lowtemperature photopolymerization of perdeuterated TS-diacetylene crystals (bis(p-toluene sulphonate) ester of 2,4-hexadiyne-I ,6diol). Our previous interpretations [3,4,7] used pure quintet states; the present
14/81 /OOOO-0000/$02_50
0 North-Holland
Publishing Company
Volume 78, number
CHEMICAL
3
PHYSICS
theory accounts for the singlet-quintet mixing which becomes important if AeSQ is not large as compared to the intrinsic fine structure splitting parameters 0, and ET_ AeSQ is the energy difference between the ground state which is a singIet in zero order and the excited states which are quintets in zero order.
15 March 1981
LETTERS
and the pure singlet (S) carbene pair states [6]. The Zeeman term HZ is dependent on the external magnetic field B. and is given by - (.$
+$) ,
with the isotropicg
factorg
fiz =gru8 8,
(4) = 2.0023
and Bohr mag-
netOn pg. Fmally, the first term Hspin resulting from the magnetic dipolar interaction within and between
the tnplet carbene chain ends is given by [6]
2. Spin hamiltonian The system of two coupled triplet carbenes shown in scheme 1 is described by the hamiltonian A_ 1 _ N= If, +HO -r-Hz _ (1)
The first contribution & arises from the magnetic dipolar interaction between each of the four electrons of the two triplet carbene chain ends. The second contribution k. is due to the electrostatic interactions within and between the individual triplet carbenes. The third contribution kz accounts for the Zeeman splitting in the presence of an external magnetic field
Ij,pin =O,(~,i
- SSl)
- is,“) + E& + qgz ^ ^ +xs**s2x +xAS:,s,, + xcY(&z~2Y
+ s,,
&)
- g;,,) -X(1
+A)&&a
-
(5)
0, and E, are the fine structure parameters of the identical triplet carbene chain ends. x,y and z are the principal axes of the corresponding fine structure tensors_ The intercarbene magnetic dipolar interaction is represented by the parameter
SOIn order to obtain a pure spin hamiltonian which explicitly is only dependent on the spin variables, the hamiitonian (1) has to be averaged over the pure orbital wavefunction I,!Jof the bicarbene structure [6],
x =g’j&lo/4ir)T)R-3
~>=crLI~lJ/>=(Ei,)+(~o)+~~.
A = 1 - 3 s&p
(2)
+ E,(~~~ - ~:y)
_
(6)
The geometrrcal factors A and Q!are only dependent on the orientation y of the fine structure tensor z axis with respect to the crystal b axis, ~
CY= g sm 2~.
(7)
In the case of tden_tic_altriplet carbenes the effective spm hamiltonian IY$n = (I?! obtained in this way is reduced to the form- [6]
In contrast to the spin hamiltonian (3) for mixed singlet-quintet states the simple spin hamiltonian [3,4] for pure quintet states
fi$r,
I-ippi, =DQ(iz
= I&,
+ ; Ae,&,
+ $)*
+ fj,
(3)
_
.!?, and & are the triplet spin vector operators of the individual triplet carbene chain ends. This spin hamiltonian accounts for (1) the magnetic dipolar interaction, (2) the electrostatic interactions, and (3) the Zeeman interactions. The electrostatic energy term resulting from the orbital contrjbu$ons +#_J is diagonal in the total spin operator S = S, + S, in contrast to= fjspin, which in addition has nokvanishing off-diagonal matrix elements_ Therefore the five pure quintet spin pair states and also the singlet spin pair state are mixed due to the intrinsic magnetic dipolar spin-spin interaction. The principal contribution to the spin hamiltonian (3) is the second term where AeSQ = is the electronic energy gap between the pure quintet (Q) eQ
-
- ii*)
+EQ(iz
- $1,
03)
with i = 4, + i2 and S = 2 is only an approximation of the general situation which fails in the limit 1AESQ [ 4 6 the opposite limit 1Ae8Q 1S 6 the spin hamiltonians (3) and (8) become equivalent [3,6] _ The energies of the mixed singlet-quintet spin pair states and the corresponding microwave transitions are obtained by diagonalization of the energy matrix using I?:& of eq. (3) IDQ
[_
rn
10~
[
3. ESR spectroscopy 3.1. Experimental
es
ESR experiments were performed with a variable 417
CHEMICAL PHYSICS LETTERS
Volume 78, number 3
temperature cryostat and with a helium bath cryostat using a Bruker ESR spectrometer operating at 9.308(l) GHz. The perdeuterated TS-dtacetylene crystals were prepared by Krijhnke [8]. They were immersed in hqurd helium during the ESR experiments. The quintet states were generated by UV irradiation of the crystal using a xenon h&-pressure arc (300 W) and a 3 mm Schott UC1 1 filter and a water filter in addition.
180
90
0 180
12
Fme stmctru e analysis
Fig_ 1 shows the angular dependence of the resonance fields of four different bicarbene species 1-4. The crystal is rotated with the external magnetic field B, m the stacking plane of the monomer TS molecules which is identrcal with the plane of the resulting polymer backbone [9] _ The points are taken from the ESR spectra. The curves are the calculated “Am e 1” ESR transitions using the spin hamiltonian (3). Owing to the two different orientations A and B of the monomer molecules in the unit cell [9] each anisotropy 1s obtamed twice with extrema at the angles 5~ from the b axis. In the computer analysis of the fme structure anisotropies of fig. 1 the specific parameters D,, E,, R ,?, Peso of the spin hamiltonian (3) and the orientation cpof the fine structure tensor are automatically varied untrl the square sum of the deviations from the experimental points becomes minimal_ The essential results of the computer fits are listed in table 1. From the temperature dependence of the relative ESR signal intensities the sign of DQ x $0, has been determined to be positive, using the procedure, descrrbed by Hornig and Hyde [lo] which was applied to a srx-level system of one singlet and five quintet states.
0 @ 180 0, z 90
DO/
I
Fig. 1. Angular dependence of the resonance fields BO of 4 different bicarbene structures l-4 in perdeuterated TSdiacetylene crystals. The crystal is rotated with the external magnetic field B. in the plane of the polymer backbone_ The b axis is the direction of the polymer chain. y and z are the principal axes of the tine structure tensor. The calculated curves are computer fitted to the experimental points. 418
15 March 1981
3.3. Thermal activation At very low temperatures the absolute intensities I of the ESR signals approach zero. All ESR signals are thermally activated. The points in fig. 2 show the temperature dependence of the ESR intensities I of the bicarbene states 2-4. The specific temperature dependence was used to discriminate the specific angular dependencies of the bicarbene species 1-4 in fig. 1.
15 March 1981
CHEMICAL PHYSICS LETTERS
Volume 18, number 3
Table I Fine structure parameters (DT, E,) and average distances RI2 of the indrvidual triplet carbenes within the bicarbene structures l-4. ACZSQ and AE$Q are the energy separations of the zero-order singlet and qumtet pan states deduced from the ESR fme strUCtUre analysis and from the temperature dependence of the ESR signal intensity, respectively. The calculated parameter Y is a measure of the singlet-qumtet mixing (Y = 0 ; pure quintet) [6 J Bicarbene
DJitc
E#c
(cm-‘)
(cm-‘)
1
O-296(6)
2 3 4
0.293(l) O-292(1) 0.290(l)
-0.0080(6) -0.007 l(1) -0.0072(2) -O-0069(1)
~.______
The curves in fig. 2 are calculated
R12
10~ l/T f.5 +
eXp(L&&/kT)]
,
using the approxi-
(9)
is Boltzmann’s constant_ In all cases the energy separation AE& is assumed to be large compared to the Zeeman and fine structure splitting and small compared to the singlet-tiz@t energy separation Ae ST = eT - es, where T indicates the triplet spin pair state as discussed by Benk and Srxl [6] _ The specific activation energies of the different dicarbene species obtained in this way are listed in table 1.
k
4. Discussion The computer fit of fip 1 rs perfect. The activation energies Aeso deduced from the fine structure anal-
ysis are in excellent
-
>,I2 Z=12 >12 212
mate relation
agreement with the values AE$Q T/K
Fig. 2. Temperature dependence of the bicarbene ESR signal intensities_ The calculated curves have been fitted to the experimental points with the activation energies AegQ (table 1).
GQ
Y
(cm-t)
(cm-r)
O-53(2) l-55(5) 4.5 (1) 15 (2)
2.2(8) 4.7(4) 14 (1)
O-53(5) O-27(4) 0.1 l(3) 0.04(2)
AesQ
(A)
deduced independently from the temperature dependence. The distances R,, can be estimated to be larger than 12 a Owing to the decrease of all electric interaction integrals with increasing distance R 12, a corresponding decrease of the activation ener,gy AE,Q is expected. Therefore, the chain length of the brcarbenes increases in the sequence 4,3,2,1. From new experimental data we presume that there exist at least two shorter bicarbenes. If we assume that the shortest existing bicarbene 1s located at a trimer molecule [ 111, the quintet states, described in the present paper, are due to bicarbenes located at oligomers with more than 4 units. A measure of the singletqumtet mixing is given by the parameter Y (0 =GY< 1)
WI, Y= (1+
lAesQ1/20,)-:
,
(10)
listed in table 1. Due to the small activation energies the Y parameters range half way between the limiting cases of pure quintet spin pair states (Y = 0) and the totally mixed singlet-quintet spin pair states (Y = l). Therefore the bicarbene species 1,2 and 3 are mixed singlet-quintet pair states with W < 2. Only species 4 may be described by S = 2 to a good approximation_ In contrast to the asymmetric triplet carbenes discussed by Bubeck et al. [2] the fine structure parameters D, and E, of the individual triplet carbenes forming the bicarbene structure seem almost independent on the chain length. They are comparable with the ‘%rirner” AC carbenes. In all cases the orientation of the fine structure tensor of the mixed singlet-quintet pair states is very close to the orientation of the triplet fine structure tensor of the individual carbene centers. 419
Volume
78, number
CHEMICAL
3
Acknowledgement
PHYSICS
LETTERS [3]
R. Huber and M. Schwoerer,
15 March 1981 Chem. Phys. Letters
72
schungsgemeinschaft.
(1980) 10. [4] C. Bubeck, W_ Hersel, W. Neumann, H. Slxl and J_ Waldmann, Chem. Phys. 51 (1980) 1. [5] H. Sixl, W_ Hersel and H.C. Wolf, Chem. Phys. Letters _ 53 (1978) 39; W. Hers& H. S~vl and G. Wegner, Chem. Phys. Letters
References
73 (1980) 288. [6 ] H. Benk and H. Saul, Mol. Phys. (1981),
This
work
is supported
111 G. Wegner, hlakromol. PI
420
by the Deutsche
For-
Chem. 145 (1971) 85, K. Takeda and G. Wegner, hfakromol. Chem. 160 (1972) 349. C. Bubeck, H. SLXI and H.C. Wolf, Chem. Phys. 32 (1978) 231. C. Bubeck, HI. Stxl and W. Neumann. Chem. Phys. 48 (1980) 269; W. Neumann and H. SIXI, Chem. Phys. 50 (1980) 273.
to be published.
[7]
M. Schwoerer, R.A. Huber and W. Hartl, Chem. Phys. 55 (1981) 97. [S] C. Krbhnke, Dissertation, Freiburg (1979). [V] H. Elchele. h-i. Schwoerer, R. Huber and D. Bloor, Chem.
Phys. Letters 42 (1976) 342; V. Enkelmann, Acta Cryst. B33 (1977) 2842. [IO] A.W Homig and J.S. Hyde, Mol. Phys. 6 (1963) 33; R. Huber, M. Schwoerer, C. Bubeck and H. Sill, Chem. Phys. Letters 53 (1978) 35. [I 1 ] W. Neumann, private communication.
15 March 1981
CHEMICAL PHYSICS LETTERS
Volume 78, number 3
OF MIXED
SINGLET-QUINTET
STATES
IN DIACETYLENE
CRYSTALS
R.A. HUBER, M. SCHWOERER PII_I a falrsches inshht t, Oniversctat Ban ercth, D-S%‘0 Bayrecc th, West Ger:nan_v H. BENK
Instrtut fiir Theoretuche
P&wh-. Ted I, Utliverslt~t Stuttgart,
D-7000 Stuttgart.
West GermanJ
and H. SIXL PI~ysikalrsches Itrstitrrt, Ted 3. Unil emtat
Stuttgart,
D- 7000 Stuttgart.
Received 14 January 1981, m final form 2 februdrl
West Germaa,lqr
1981
ESR spectra of four different blcxbenes in a dlacetylene crystal are fitted to the theory of mixed singlet-quintet states. hl~ng is due to electrostatic .md mdgnetlc dipole coupling between a paw of triplet carbetzes. The tit is perfect. It determines for each blcarbene the energ) difference between the ground state which IS a cmglet m zero order and the excited states which are quintets m zero order.
1. Introduction l
The recent interest in diacetylene crystals is due to their reactiirity in the solid state. As was shown by Wegner and co-workers [ I] highly perfect polymer diacetylene single crystals are obtained from the cxresponding monomer crystals under the Action of pressure, heat, UV- or y-irradiation. In recent experiments concerning
the mechanisms
of the solid state polym-
enzation reaction a series of different intermedtate states has been stabilized at low temperatures and analyzed by ESR and optical spectroscopy [Z--5] _ The different dimer, trimer. tetramer, etc. intermediates of this unusual photochemical reaction are claw-
fied mto S = 0, S = 1 triplet and S = 2 quintet states- A blcarbene structure of a planar tetramer, consisting of two mesomeric forms, is shown schematically in scheme I_ In this configuration two triplet states are coupled electrostatically to a pair state which is characterized by the Intrinsic fine structure parameters D, and E, and the orientation cpof the fine structure tensor of the individual triplet states and by their distance R 12_ 416
0 009-26
R-c_!
’
z
P P @ -c* ,&c-c* C;
’
c-
I4
d
b I
I
7: : ‘7 7 -‘c-c,\ ‘c_C%C c_CZC -c+&Z=-c-c* C- . d d R’
’ R-C%-c .
2 b
Scheme 1. Bicarbene configuration. The bicarbene tetramer molecule consists of four diacetylene units with two identical mesomeric trxplet carbene chain ends. D, and ET are the triplet fme structure parameters of the S = 1 carbene species and 6 r and $2 are their wavefunctions. 9 gives the orientation of the triplet fme structure z aGs with respect to the crystal C3aGs in a pkmar arrangement of the two mesomeric tetramer structures_
In this paper the theory of mixed singlet-quintet pair states [6] is applied to the fiie structure analysis of the bicarbene ESR spectra, obtained during lowtemperature photopolymerization of perdeuterated TS-diacetylene crystals (bis(p-toluene sulphonate) ester of 2,4-hexadiyne-I ,6diol). Our previous interpretations [3,4,7] used pure quintet states; the present
14/81 /OOOO-0000/$02_50
0 North-Holland
Publishing Company
Volume 78, number
CHEMICAL
3
PHYSICS
theory accounts for the singlet-quintet mixing which becomes important if AeSQ is not large as compared to the intrinsic fine structure splitting parameters 0, and ET_ AeSQ is the energy difference between the ground state which is a singIet in zero order and the excited states which are quintets in zero order.
15 March 1981
LETTERS
and the pure singlet (S) carbene pair states [6]. The Zeeman term HZ is dependent on the external magnetic field B. and is given by - (.$
+$) ,
with the isotropicg
factorg
fiz =gru8 8,
(4) = 2.0023
and Bohr mag-
netOn pg. Fmally, the first term Hspin resulting from the magnetic dipolar interaction within and between
the tnplet carbene chain ends is given by [6]
2. Spin hamiltonian The system of two coupled triplet carbenes shown in scheme 1 is described by the hamiltonian A_ 1 _ N= If, +HO -r-Hz _ (1)
The first contribution & arises from the magnetic dipolar interaction between each of the four electrons of the two triplet carbene chain ends. The second contribution k. is due to the electrostatic interactions within and between the individual triplet carbenes. The third contribution kz accounts for the Zeeman splitting in the presence of an external magnetic field
Ij,pin =O,(~,i
- SSl)
- is,“) + E& + qgz ^ ^ +xs**s2x +xAS:,s,, + xcY(&z~2Y
+ s,,
&)
- g;,,) -X(1
+A)&&a
-
(5)
0, and E, are the fine structure parameters of the identical triplet carbene chain ends. x,y and z are the principal axes of the corresponding fine structure tensors_ The intercarbene magnetic dipolar interaction is represented by the parameter
SOIn order to obtain a pure spin hamiltonian which explicitly is only dependent on the spin variables, the hamiitonian (1) has to be averaged over the pure orbital wavefunction I,!Jof the bicarbene structure [6],
x =g’j&lo/4ir)T)R-3
~>=crLI~lJ/>=(Ei,)+(~o)+~~.
A = 1 - 3 s&p
(2)
+ E,(~~~ - ~:y)
_
(6)
The geometrrcal factors A and Q!are only dependent on the orientation y of the fine structure tensor z axis with respect to the crystal b axis, ~
CY= g sm 2~.
(7)
In the case of tden_tic_altriplet carbenes the effective spm hamiltonian IY$n = (I?! obtained in this way is reduced to the form- [6]
In contrast to the spin hamiltonian (3) for mixed singlet-quintet states the simple spin hamiltonian [3,4] for pure quintet states
fi$r,
I-ippi, =DQ(iz
= I&,
+ ; Ae,&,
+ $)*
+ fj,
(3)
_
.!?, and & are the triplet spin vector operators of the individual triplet carbene chain ends. This spin hamiltonian accounts for (1) the magnetic dipolar interaction, (2) the electrostatic interactions, and (3) the Zeeman interactions. The electrostatic energy term resulting from the orbital contrjbu$ons +#_J is diagonal in the total spin operator S = S, + S, in contrast to
-
- ii*)
+EQ(iz
- $1,
03)
with i = 4, + i2 and S = 2 is only an approximation of the general situation which fails in the limit 1AESQ [ 4 6 the opposite limit 1Ae8Q 1S 6 the spin hamiltonians (3) and (8) become equivalent [3,6] _ The energies of the mixed singlet-quintet spin pair states and the corresponding microwave transitions are obtained by diagonalization of the energy matrix using I?:& of eq. (3) IDQ
[_
rn
10~
[
3. ESR spectroscopy 3.1. Experimental
es
ESR experiments were performed with a variable 417
CHEMICAL PHYSICS LETTERS
Volume 78, number 3
temperature cryostat and with a helium bath cryostat using a Bruker ESR spectrometer operating at 9.308(l) GHz. The perdeuterated TS-dtacetylene crystals were prepared by Krijhnke [8]. They were immersed in hqurd helium during the ESR experiments. The quintet states were generated by UV irradiation of the crystal using a xenon h&-pressure arc (300 W) and a 3 mm Schott UC1 1 filter and a water filter in addition.
180
90
0 180
12
Fme stmctru e analysis
Fig_ 1 shows the angular dependence of the resonance fields of four different bicarbene species 1-4. The crystal is rotated with the external magnetic field B, m the stacking plane of the monomer TS molecules which is identrcal with the plane of the resulting polymer backbone [9] _ The points are taken from the ESR spectra. The curves are the calculated “Am e 1” ESR transitions using the spin hamiltonian (3). Owing to the two different orientations A and B of the monomer molecules in the unit cell [9] each anisotropy 1s obtamed twice with extrema at the angles 5~ from the b axis. In the computer analysis of the fme structure anisotropies of fig. 1 the specific parameters D,, E,, R ,?, Peso of the spin hamiltonian (3) and the orientation cpof the fine structure tensor are automatically varied untrl the square sum of the deviations from the experimental points becomes minimal_ The essential results of the computer fits are listed in table 1. From the temperature dependence of the relative ESR signal intensities the sign of DQ x $0, has been determined to be positive, using the procedure, descrrbed by Hornig and Hyde [lo] which was applied to a srx-level system of one singlet and five quintet states.
0 @ 180 0, z 90
DO/
I
Fig. 1. Angular dependence of the resonance fields BO of 4 different bicarbene structures l-4 in perdeuterated TSdiacetylene crystals. The crystal is rotated with the external magnetic field B. in the plane of the polymer backbone_ The b axis is the direction of the polymer chain. y and z are the principal axes of the tine structure tensor. The calculated curves are computer fitted to the experimental points. 418
15 March 1981
3.3. Thermal activation At very low temperatures the absolute intensities I of the ESR signals approach zero. All ESR signals are thermally activated. The points in fig. 2 show the temperature dependence of the ESR intensities I of the bicarbene states 2-4. The specific temperature dependence was used to discriminate the specific angular dependencies of the bicarbene species 1-4 in fig. 1.
15 March 1981
CHEMICAL PHYSICS LETTERS
Volume 18, number 3
Table I Fine structure parameters (DT, E,) and average distances RI2 of the indrvidual triplet carbenes within the bicarbene structures l-4. ACZSQ and AE$Q are the energy separations of the zero-order singlet and qumtet pan states deduced from the ESR fme strUCtUre analysis and from the temperature dependence of the ESR signal intensity, respectively. The calculated parameter Y is a measure of the singlet-qumtet mixing (Y = 0 ; pure quintet) [6 J Bicarbene
DJitc
E#c
(cm-‘)
(cm-‘)
1
O-296(6)
2 3 4
0.293(l) O-292(1) 0.290(l)
-0.0080(6) -0.007 l(1) -0.0072(2) -O-0069(1)
~.______
The curves in fig. 2 are calculated
R12
10~ l/T f.5 +
eXp(L&&/kT)]
,
using the approxi-
(9)
is Boltzmann’s constant_ In all cases the energy separation AE& is assumed to be large compared to the Zeeman and fine structure splitting and small compared to the singlet-tiz@t energy separation Ae ST = eT - es, where T indicates the triplet spin pair state as discussed by Benk and Srxl [6] _ The specific activation energies of the different dicarbene species obtained in this way are listed in table 1.
k
4. Discussion The computer fit of fip 1 rs perfect. The activation energies Aeso deduced from the fine structure anal-
ysis are in excellent
-
>,I2 Z=12 >12 212
mate relation
agreement with the values AE$Q T/K
Fig. 2. Temperature dependence of the bicarbene ESR signal intensities_ The calculated curves have been fitted to the experimental points with the activation energies AegQ (table 1).
GQ
Y
(cm-t)
(cm-r)
O-53(2) l-55(5) 4.5 (1) 15 (2)
2.2(8) 4.7(4) 14 (1)
O-53(5) O-27(4) 0.1 l(3) 0.04(2)
AesQ
(A)
deduced independently from the temperature dependence. The distances R,, can be estimated to be larger than 12 a Owing to the decrease of all electric interaction integrals with increasing distance R 12, a corresponding decrease of the activation ener,gy AE,Q is expected. Therefore, the chain length of the brcarbenes increases in the sequence 4,3,2,1. From new experimental data we presume that there exist at least two shorter bicarbenes. If we assume that the shortest existing bicarbene 1s located at a trimer molecule [ 111, the quintet states, described in the present paper, are due to bicarbenes located at oligomers with more than 4 units. A measure of the singletqumtet mixing is given by the parameter Y (0 =GY< 1)
WI, Y= (1+
lAesQ1/20,)-:
,
(10)
listed in table 1. Due to the small activation energies the Y parameters range half way between the limiting cases of pure quintet spin pair states (Y = 0) and the totally mixed singlet-quintet spin pair states (Y = l). Therefore the bicarbene species 1,2 and 3 are mixed singlet-quintet pair states with W < 2. Only species 4 may be described by S = 2 to a good approximation_ In contrast to the asymmetric triplet carbenes discussed by Bubeck et al. [2] the fine structure parameters D, and E, of the individual triplet carbenes forming the bicarbene structure seem almost independent on the chain length. They are comparable with the ‘%rirner” AC carbenes. In all cases the orientation of the fine structure tensor of the mixed singlet-quintet pair states is very close to the orientation of the triplet fine structure tensor of the individual carbene centers. 419
Volume
78, number
CHEMICAL
3
Acknowledgement
PHYSICS
LETTERS [3]
R. Huber and M. Schwoerer,
15 March 1981 Chem. Phys. Letters
72
schungsgemeinschaft.
(1980) 10. [4] C. Bubeck, W_ Hersel, W. Neumann, H. Slxl and J_ Waldmann, Chem. Phys. 51 (1980) 1. [5] H. Sixl, W_ Hersel and H.C. Wolf, Chem. Phys. Letters _ 53 (1978) 39; W. Hers& H. S~vl and G. Wegner, Chem. Phys. Letters
References
73 (1980) 288. [6 ] H. Benk and H. Saul, Mol. Phys. (1981),
This
work
is supported
111 G. Wegner, hlakromol. PI
420
by the Deutsche
For-
Chem. 145 (1971) 85, K. Takeda and G. Wegner, hfakromol. Chem. 160 (1972) 349. C. Bubeck, H. SLXI and H.C. Wolf, Chem. Phys. 32 (1978) 231. C. Bubeck, HI. Stxl and W. Neumann. Chem. Phys. 48 (1980) 269; W. Neumann and H. SIXI, Chem. Phys. 50 (1980) 273.
to be published.
[7]
M. Schwoerer, R.A. Huber and W. Hartl, Chem. Phys. 55 (1981) 97. [S] C. Krbhnke, Dissertation, Freiburg (1979). [V] H. Elchele. h-i. Schwoerer, R. Huber and D. Bloor, Chem.
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