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Estimation of Spiro-Interaction (Spiroconjugation) in Tetravinylsilane.

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Theoretically through-conjugation can arise by participa-
sI
tion of an-type i bonding MO (Si-C hyperconj~gation[~’)
on the silicon atom in
or the d, AOs (p,d,
x conjugation. If (I) has C,, symmetry then, provided that
through-conjugation is a real phenomenon, three x MOs
of different energy are to be expected: the xl(bl) HOMO
having a node in each Si-C bond, the next lower xz(a2)
MO having one node at Si, and finally the x3(bl)MO without a node. x l corresponds to the antibonding x1 MO and
n3 to the bonding x 2 MO in trimethylvinylsilane (2)[’];
the x2(a2)MO in (1) is new and has no analog in (2)[51.
L
6
1
Received: February 18, 1972 [Z 637a IE]
German version: Angew. Chem. 84,550 (1972)
[I]Theory and application of photoelectron spectroscopy, Part 8. We
are indebted to the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie for their support of this work, and to the
Deutsches Rechenzentrum, Darmstadt, for carrying out the calculations.-Part 7 : Ref. [9].
[2] C. J . Attridge, Organornetal. Chem. Rev. A 5,323 (1970).
[3] C. Eaborn and S . H . Parker, J. Chem. SOC.1954, 939; C. Eaborn,
ibid. 1956, 4858; J. Organometal. Chem. 20, 49 (1969); W Hanstein,
H . J . Berwin, and 7:G . Daylor, J. Amer. Chem. SOC.92,829 (1970); C. G.
Pitt, J. Organometal. Chem. 23, C 35 (1970); J . M . Jerkunica and T G .
Paylor, J. Amer. Chem. SOC.93,6278 (1971); U . Weidner and A. Schweig,
Angew. Chem. 84, 167 (1972); Angew. Chem. internat. Edit. 11, 146
I
10
12
1L
1P lev1
xz(az)is expected to lie a little lower than x,(b,) since the
above conjugation mechanisms of the saturated silicon
atom should only exert a slight effect. Such arguments
would predict that the first peak in the PE spectrum of ( I )
consist of two bands@) and @ in contrast to that of (2)[’].
The spectrum of ( 1 ) is reproduced in Figure 1. The intensity ratio of the first two peaks is 2:316](the corresponding
intensity ratio in the spectrum of (2) is 1:316]).
The expanded spectrum (Fig. 1b) clearly shows the splitting
of the first peak into two bands @ and @ separated by
0.2 eV. This finding demonstrates that a saturated Si atom
between two x systems cannot be regarded as performing
an isolating function.
8
l o
rn
9
10
-
11
IP [eVl-
8
16
18
20
12
Fig. 1. Photoelectron spectrum (a)of tetravinylsilane (1). The spectrum
(b) shows the first two peaks on an expanded scale.
-t
lo-
>
al
-
Y
a
11 -
(1972).
[4] E . A. !l Ebsworth in A . G . MacDiarmid: Organometallic Compounds of the Group IV Elements, Vol. 1. Dekker, New York 1968.
151 U . Weidner and A . Schweig, Angew. Chem. 84, 167 (1972); Angew.
Chem. internat. Edit. 11, 146 (1972); U. Weidner and A . Schweig, J. Organometal. Chem., in press.
[6] The intensity value of 3 for the second peak in the PE spectra of ( I )
and (2) is attributable to ionization of two Si-C o MOs in addition to
the bonding nMOs (nn for (1) and x 2 for (2)).
[7] The He1 (584 A) photoelectron spectra were recorded with a PS 16
spectrometer produced by Perkin-Elmer Ltd. Beaconsfield (England).
[8] 0. Klemperer: Electron Optics. University Press, Cambridge 1953,
p. 414.
[9] U . Weidner and A . Schweig, J. Organometal. Chem., in press.
Estimation of Spiro-Interaction(Spiroconjugation)
in Tetravinylsilane[‘]
By Ulrich Weidner and Armin Schweig[*]
The term “spiroconjugation”[21 denotes the interaction
between two mutually perpendicular x systems separated
by a tetrahedral atom. Such a system is present in tetravinylsilane
By comparison of the photoelectron
(PE) spectra of ( 1 ) and dimethyldivinylsilane (2) 141 we
[*I
Dip].-Chem. U. Weidner and Prof. Dr. A. Schweig
Physikalrsch-chemisches Institut der Universitat
355 Marburg, Bregenstrasse 12 (Germany)
Angew. Chem. internal. Edif. 1 Yo!. I 1 (1972) N o . 6
rn
Si [CH=CH21&
111
IH$I,Si ICH=CH,I,
121
Fig. 2. Comparison of the MO and energy-level diagrams of tetravinylsilane ( I ) (DZdsymmetry) and dimethyldivinylsiiane (2) (Czvsymmetry). The spiroantibonding and spirobonding interaction in the MOs
xz(az)and nn(b,) of ( I ) are illustrated with the aid of the 2p. AOs on
the four C atoms adjacent to the Si atom drawn looking down the S,
axis.
have now been able to detect a spiro-interaction in (1) and
to estimate its magnitude.
The PE spectrum[51of ( I ) is reproduced in Figure 1a, and
the first two peaks of this spectrum are shown on an expanded scale in Fig. 1b. The ratio of the correctedL6]intensities of the first two peaks is 4 :3 ; the corresponding ratio
for (2) is found to be 2:3[41.These intensity ratios are
readily understandable in the light of Figure 2 which demonstrates how the energy-level and MO diagrams of ( I )
can be derived from those of (2jt4]. The Si-C antibonding
and bonding MOs xl(bl) and x3(bl) in (2) correspond to
the degenerate MOs xl(e) and n4(e) in ( I ) . Instead of the
antisymmetric and symmetric Si-C combination o1(b,)
and 02(al)in (2) the single Si-C MO ol(b2)is retained in
( I ) . The x2(a2)MO unequivocally established in (2) splits
into the spiroantibonding and spirobonding MOs xz(a2)
537
and a3(bl) in ( I ) . Particular interest attaches to the magnitude of the splitting. We estimate a value of 0.2-0.3 eV for
the spiro-splitting in ( I ) from the structure and width of
the first peak in Figure 1b. To the best of our knowledge,
this is the first spiro-splitting to be measured by PE spectroscopy and, in particular, the first detection of spiroconjugation both in a non-spiro compound and in an organosilicon
compound.
The preceding communication in this series"] showed that
for unsaturated benzenoid and nonbenzenoid hydrocarbons the n-orbital energies &,(a)= - I v , calculated on the
basis of Koopmans' theorem from the ionization potentials
can be parametrized by a simple H M O model when
first-order bond localization['] is accounted for by a
perturbation method13]. The relevant regression function
is
Received: February 21,1972 [Z 637b IE]
German version: Angew. Chem. 84, 551 (1972)
/:.j=-(m
+ PSI) + b x ( ~ g . , , pP,)(po
- P
~ J
(1)
P"
[l] Theory and application of photoelectron spectroscopy, Part 9. We
are grateful to the Deutsche Forschungsgemeinschaft and the Fonds
der Chemischen Industrie for their support of this work, and to the
Deutsches Rechenzentrum for carrying out the calculations.- Part 8:
ref. [4].
where
a) cr+pxJ=~YMo
is the orbital energy of the n orbital QJ
in the usual Hiickel approximation ;
b) pp, is the bond order between the bonding centers p,v
of the neutral hydrocarbon M ;
[2] H . E. Simmons and 7: Fukunaga, J. Amer. Chem. SOC.89, 5208
(1967); R. Hofmann, A. Imamura, and G . D. Zeiss, ibid. 89, 5215 (1967);
R. Bosche, A. S . Dreiding, and E . Heilbronner, ibid. 92, 123 (1970);
A . Tajiri and 7:Nokajima, Tetrahedron 27, 6089 (1971); M . J . Gotdstein
and R. Hofmann, J. Amer. Chem. SOC.93,6194 (1971).
[3] According to definition (IUPAC: Nomenclature of Organic Chemistry, Sect. A. Butterworths, London 1966) spiro compounds consist of
two rings with one common atom (spiro atom). Hence (1) is not a spiro
compound. However, ( 1 ) possesses two mutually perpendicular n
systems separated by a tetrahedral atom, thereby fulfilling the requirements (cf. ref. 121) for spiro conjugation.
[4] U . Weidner and A. Schweig, Angew. Chem. 84,550 (1972); Angew.
Chem. internat. Edit. 11,536 (1972).
[5] The He1 (584 A) photoelectron spectra were recorded with a PS 16
spectrometer produced by Perkin-Elmer Ltd., Beaconsfield (England).
[6] 0. KIernperer: Electron Optics. University Press, Cambridge 1953,
p. 414.
c) p ~ , , = p , , - ~ , ~ is
c , the
~ corresponding bond order in
the radical cation M+(JI;'), i.e. M f in the configuration
in which JtJ is only singly occupied;
d) p o = 213 is the standard bond order in benzene ;
e) b is a factor dependent upon the force constants of the
(J bonds and upon the derivative dJ3ldR of the resonance integral J3with respect to the length R of the a bond.
a and
Summation is performed over all bonds.
An impressive test of the quality of the ionization potentials
I:,, calculated from equation (1) is provided by a photo-
QlQcJc)c&)
0 0 0
The Photoelectron Spectra of Benzenoid
Hydrocarbons C, ,HI ['I
TE
By Franz Brogli and Edgar Heilbronner"'
BP
BA
While there has been an abundance of calculations of
n-orbital energies, particularly those of fused benzenoid
hydrocarbons, since the fundamental publications by
Erich Hiickel in the 1930's, experimental verification of the
theoretical values obtained for large systems (more than
Table 1. Vertical ionization potentials of the five isomeric benzenoid hydrocarbons C,,H, 2 . Iv,,(n)= measured
vertical ionization potentialsinev; /:,,=calculated according to(1)with -ci=5.782eV, -p=3.199 eV, b=5.106
eV. The designation of the orbitals $, refers to the following symmetry groups: TE, D 2 h ;BA, C,; BP, C 2 > CH,
;
C 2 h ;TR, D,, (cf. ref. LSJ). Values in parentheses refer to shoulders and are therefore accurate to one decimal place
only.
7.01 6.88
a,
b,, 8.41 8.34
b,, (8.6) 8.59
a,
9.56 9.53
b,, (9.7) 9.78
b,, 10.25 10.31
14 C atoms) has been lacking. The low vapor pressure of
these compounds precludes measurement of their highresolution photoelectron spectra with spectrometers of
conventional construction. However, recent advances
have now rendered even
involatile
accessible to study by photoelectron spectroscopy.
[*] DipLChem. F. Brogli and Prof. Dr. E. Heilbronner
Phvsikalisch-Chemisches Institut der Universitat
CH-4056 Basel, Klingelbergstrasse 80 (Switzerland)
538
a"
a"
a"
a"
a"
a"
7.42 7.40
8.03 8.14
8.82 8.78
9.34 9.29
9.90 9.87
10.40 10.38
b,
7.62
a,
8.00
a2 8.96
b , (9.13)
b,
9.95
a, 10.26
1.71
8.00
8.93
9.05
10.04
10.25
a,
a,
b,
b,
a,
b,
7.61 7.60
8.10 8.19
8.68 8.64
9.44 9.41
9.73 9.69
10.52 10.58
e"
7.86
a;
8.63
err 9.66
a; 10.05
7.98
8.68
9.67
9.93
electron spectroscopic investigation of the five isomeric
benzenoid hydrocarbons of molecular formula C 18H12 :
tetracene (TE), 1,2-benzanthracene (BA), 3,4-benzophenanthrene (BP), chrysene (CH), and triphenylene (TR).
The PE spectra were recorded with a PS 16 spectrometer
(Perkin-Elmer, Beaconsfield, England) whose specifications correspond to those given by Turner[41.Table 1 lists
the vertical n-ionization potentials I,,,, their assignments
to the corresponding a orbitals (cf. Ref. [ 5 ] ) and the values
Angew. Chem. internat. Edit.
Vol. 11 (1972)
1 No. 6
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