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Ionization Potentials of Substituted Benzenes.

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ing x-bond in ( 3 ) , as for exo-cyclopropanonorbornene,
whereas for interaction with the other x-bond in (3) p,,=O,
as for endo-cyclopropanonorbornene["'.
Assignment of the spectra of ( 4 ) , (51, and (6) is simplified
by a characteristic set of bands that corresponds to the two
e, orbitals and the a, orbital of cyclobutane["]. As a result
of substitution by two sp3-centers, the center of the e,
orbitals is shifted by 1.7 eV from 11.35 in cyclobutane['"
to 9.65eV in ( 4 ) . For the cyclopropane ring of ( I ) this
effect is distinctly smaller, being 1.25eV[']. The correlation diagram for ( 4 ) , (5), and (6) in Fig. 3 is very similar
[S] J . A . Pople, D. L. Beueridge, and P. A. Dobosh, J. Chem. Phys. 47,
2026 (1967).
[9] The measurements were made with a PS 16 spectrometer from
Perkin-Elmer Ltd. (Beaconsfield, England). The accuracy is 0.03 eV.
[lo] The value &,(a')= -9.05 eV from (7) was used for the n-bonds in
(2) and (51, and the mean of the n-orbital energies of ( 8 ) [&Ja,)
+~,(b,)]/2= -9.16eV for the n-bonds of (3).
[ I l l P. Bischof, E . Haselbach, and E. Heilbronner, Angew. Chem. 82,
952 (1970); Angew. Chem. internat. Edit. 9, 953 (1970).
Ionization Potentials of Substituted Benzenes"]
By Martin Klessinger"'
Within the HMO approximation simple perturbation
calculations quite often lead to surprisingly good agreement between .theory and experiment (cf. Dewar's PMO
method[']). Another impressive example is provided by
the removal of the degeneracy of the highest occupied
benzene x-orbital (el,) under the inductive influence of
substituents, which is directly measurable by photoelectron
spectroscopy (PES).
Assuming that in the HMO approximation the effect of a
substituent X on the x-electron system is given by a change
6a,(X) in the Coulomb integral a , of the substituted C
atom p, one obtains for the change of the orbital energy
Ei[21
la,]-
a'
Fig. 3. Correlation diagram of compounds ( 4 ) -(6).
to that in Fig. 2, but the interactions are smaller because
of the reduced p-character of the cyclobutane orbitals;
accordingly only the p-contribution of the cyclobutane
orbitals given by eqn. (I) was considered in the HMO
calculation of the interaction. Also, as for (2) and (3)
it was taken that Spv= -0.3eV or p,,=O and it was
assumed that the cyclobutane orbitals are stabilized by
0.15 eV per double bond with respect to (4)['01,which leads
to good agreement with the experimental values.
The marked decrease in energy of the a"(e,) and the a'(a,)
orbitals of the cyclobutane part on going from ( 5 ) to (6)
is surprising. This may be due to the fact that the o-contribution is greater than for cyclopropane, thus shifting these
orbitals to lower energies on introduction of double bonds,
similarly as for ordinary o-orbitals.
A
S
Fig. 1. n-Orbitals of benzene.
In Fig. 1 the occupied x-MOs mi= ?criqp of benzene are
shown diagrammatically ; they belong to the irreducible
representation A,, and El, of the point group D,, and
have the orbital energies E(aZu)=ct+ 2p and .z(e,,)a+ p.
According to Koopmans' theoremc3]the negative orbital
energies are set equal to the vertical ionization potentipls
or the band maxima in the PE spectrum respectively :
Received: February 2, 1972 [Z 623a IE]
German version: Angew. Chem. 84,543 (1972)
[I] Photoelectron Spectra of Organic Compounds, Part 1.
[2] P. Bischof, E . Heilbronner, H.Prinzbach, and H . D. Martin, Helv.
Chim. Acta 54, 1072 (1971).
[3] The hydrocarbons ( 1 ) - ( 6 ) were prepared by anodic bisdecarboxylation of the appropriate maleic acid adducts [4] and were purified
by preparative gas chromatography.
[4] H. Westberg and H.Dauben, Tetrahedron Lett. 1968, 1523
[5] A . D. Walsk, Nature 159, 167, 172 (1947); Trans. Faraday SOC.45,
179 (1949).
[6] Since there is also a combination of C H orbitals with the same
symmetry behavior, this M O contains a small contribution from H-l s
AOs.
171 L. Salem and I . S . Wight, J. Amer. Chem. SOC.91, 5947 (1969).
Angew. Chem. internat. Edit. 1 Vol. 11 (1972) 1 No. 6
In Fig. 2 the experimental PE spectra of some methyl- and
fluoro-benzenes are compared with the negative orbital
energies - E~ calculated according to eqn. (1) with the coefficients cpishown in Fig. 1 and 6ct,(CH3)= + 1.0 eV and
6a,(F)= - 1.2 eV.
The almost quantitative agreement between HMO predictions and experiment, which is not limited to mono- and
[*] Prof. Dr. M. Klessinger
Organisch-Chemisches Institut der Universitat
44 Miinster, Orleans-Ring 23 (Germany)
525
where S E , is calculated from eqn. (1)with Scc,(CH,) = 0.85 eV
and 6ctp(F)=- 1.10 eV and the o inductive effect is given by
AI,(CH,)=0.05 eV and AI,(F)= -0.10eV.
Y
,
.
t
i
8
I
,
Further investigations of the influence of substituents on
the PE spectra of aromatic and conjugated systems are in
progress ; the results reported here give hope that they will
lead to more detailed information about the ordering of
orbitals in benzene and similar systems.
CH,
,I
II I,
I
The fact that for 1,(3) the same correlation exists in the
HMO data as for 1,(1)and 1,(2) can be regarded as support
for the assignment, proposed by Turner et al.[*],of these
bands to an ionization from the lowest occupied z-MO
(a,,)[']. If eqn. (1)is valid for substituted polyenes in a way
analogous to that for benzene derivatives, the two n-ionization potentials in fluorinated butadienes should be at
higher energies than in unsubstituted butadienes. That,
however, is not the case if, according to Brundle and Robin[''] 1,(2)= 11.47 eV is assigned to o-ionization for
butadiene, so that the bands 1,(1)=9.06 eV and 1,(3)
= 12.23 eV must correspond to a-ionizations, whereas in
1,1,4,4-tetrafluorobutadienethese x-ionizations are unambiguously correlated with 1,(1)=9.38 eV and 1,(2)
= 12.04 eV.
I
I
1
10
Received : February 2, 1972 [Z 623 b IE]
German version: Angew. Chem. 84, 544 (1972)
12
1L
16
18
20 8
10
12
16
li
18 20
Fig. 2. PE spectra of methyl- and fluoro-benzenes, and correlation diagrams of the n- M O s derived from eqn. (1).
di-substituted benzene derivatives, can be further slightly
improved by dividing the inductive effect of the substituents
into the x-contribution given by eqn. (1)and a o-contribu-
[I] Photoelectron Spectra of Organic Compounds, Part 2.-Part 1 :
Angew. Chem. 84, 544 (1972); Angew. Chem. internat. Edit. 11, 524
(1972).
[Z] M . J . S. Dewar: The Molecular Orbital Theory of Organic Chemistry. McGraw-Hill, New York 1969.
[3] 7: Koopmans, Physica I , 104 (1934).
[4] That corresponds approximately to the field effects as and xE in
the FCT theory of M . Godfrey, J. Chem. SOC.B 1971, 1534.
[ 5 ] The measurements were made with a PS 16 spectrometer from
Perkin-Elmer Ltd. (Beaconsfield, England). Within the experimental
error of i0.03eV the values agree with those quoted in the literature
for mono- and p-di-substituted benzenes [6, 71.
Table 1. Vertical ionization potentials of substituted benzenes (calculated values in parentheses) [5].
Substitution
1"(1)
1"(21
9.2,
9.3
9.0
9.0,
9.1,
8.6
8.9,
8.6,
8.6
( - ) el,
(9.20) a,(A)
(9.01) a,@)
(9.01) b,(S)
(9.15) b,(A)
(8.68) b,(S)
(8.89) a"(S)
(8.68) e"(S)
(8.63) a,(A)
11.6 ( - ) a z u
11.4 (11.41) b,
11.2, (11.22) b,
11.2 (11.22) b ,
11.2, (11.22) b,,
10.9, (11.03) b,
11.0 (11.03) a"
11.0 (11.03) a;
10.9 (10.83) b,,
(9.72) b,(S)
(10.00) b,(A)
(10.00) a,(A)
(10.19) b,,(S)
12.1,
12.4,
12.4,
12.6
X=F
1-
(CIJ
1.21,3194-
(C2J
(C2,)
(Dzh)
9.3,
9.6
9.6
9.4
(9.35)a,(A)
(9.63) a,(S)
(9.63) b,(S)
(9.45) b,,(A)
9.7,
9.9,
10.0
10.2
(11.88) b,
(12.17) b ,
(12.17) b,
(12.17) b,,
[a] S and A denote the symmetry of those benzene M O s from which the M O of the stated symmetry derives
tion A&@) which is proportional to the number n of sub~tituents[~?
This is made
in
in which the
rc-ionization potentials[51 are compared with the values
(3)
526
[6] A. D. Baker, D. P. May, and D. W ?itrner, J. Chem. SOC.B 1968,22.
[7] I . D. Clark and D. C. Frost, J. Amer. Chem. SOC.89,244 (1967),give
adiabatic ionization potentials for some fluorobenzenes.
[8] A. D. Baker, C. R. Brundle, and D. W Turner, Int. J. Mass. Spectrom.
Ion. Phvs. I . 327 (1968).
~,
191 For a literature review on assignments of the benzene spectrum
see S. D. Worley, Chem. Rev. 71, 295 (1971).
[lo] C. R . Brundle and M . B. Robin, J. Amer. Chem. SOC.92, 5550
(1970).
I
,
Angew. Chem. internat. Edit. 1 Vol. 11 (1972)
1 No. 6
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