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n-Ionization Potentials of Alkyl Bromides.

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Confirmation of independence from chain length could be provided by the completed analysis of tricarbonylbicyclo[4.4.1]tindeca-l,3,5-trienechromiumcurrently under investigation.
A comparison of the bond lengths (A) found in the systems
bound to the metal atom is given for (2)--(5) :
(21
(3)
(4)
(5)
The Figure, which is a superposition of the projections of the
two structures (2) and ( 3 ) onto their respective mirror planes,
shows that, relative to the iron complex in which the metal
atom makes only four contacts with theligand, the chromium
atom is moved sideways thus participating more with the
ligand as a whole. These relative positions are in accord with
the ideas of 4- and 6 z-electron involvement, respectively, by
the M(CO)3 groups.
cation we wish to demonstrate that the photoelectron (PE)
spectroscopic determination of the ionization potential I(n)
of those processes RBr + RBr+ + e, in which the electron e
comes from the atomic orbital 4 pz+ or 4 p x , is a convenient
and sensitive tool for the investigation of inductive and
(hyper)conjugative effects. a-Bonded bromine atoms represent a n ideal probe for the measurement of such effects
for the following reasons:
1. The n-ionization potential of their 4 p z electrons lies between 9.5 and l l eV, i.e. in a range between the normal
ionization potentials of the C = C TT electrons ( z 9 e V ) and
the C-C a- and C-H o-electrons (Z > 10.5 eV).
2. Owing to the nonbonding character of the 4 p z atomic
orbitals of the bromine atom, the n-ionization bands observed in the PE spectrum of RBr exhibit sharp ( r 0 transitions (cf. Fig. 1) which are clearly distinguishable from the
other broad bands. They are easy to locate and can be
measured with high precision (cf. ref. 121).
3. With few exceptions (see below), the PE spectrum of RBr
shows two sharp bands separated by 0.30 0.01 eV and
which correspond to n-ionization (cf. Fig. 1). This makes
their identification even easier and increases the accuracy of
the measurement of the ionization potential.
4. Alkyl bromides are more readily accessible, more stable,
and more suitable for PE spectroscopy than the corresponding
iodides.
i
Ilnl,
CH3CH2Br
Figure: Superposition of the tricarbonyl complexes of iron(0) and chromium(0) with tricyclo[4.3.1.01,6]deca-2,4-die~e,( 2 ) and ( 3 ) , respectively.
The molecules are made to coincide in the projection along the direction
C-1-C-6. The larger circles refer to complex ( 2 ) .
Received: September 5, 1969
[ Z 154b IE]
German version: Angew. Chem. 82, 293 (1970)
Publication delayed at authors’ request
[ * ] R. L. Beddoes, P. F. Lindley, and 0.S. Mills
Department of Chemistry
University of Manchester
Manchester 13 (England)
[1] E . Vogel, W . Wiedemnnn, H . Kiefer, and W. F. Harrison,
Tetrahedron Letters 1963, 673.
[2] W.-E. Bleck, W. Grimme, H . Giinther, and E. Vogel, Angew. Chem. 82, 292 (1970); Angew. Chem. internat. Edit. 9, 303
(1970). We are grateful to these authors for the supply of
crystalline samples.
[3] B. F. Hnllnm and P . L . Pnuson, J. chem. SOC.(London)
1958, 642.
141 0 . S. Mills and C. Robinson, Acta crystallogr. 16, 758
(1963).
[51 P . E . Bnikie and 0. S . Mills, J. chem. SOC.(London) A
1968, 2104.
[ 6 ] P . E. Bnikie and 0 . S. Mills, J. chern. SOC.(London) A
1969, 328.
[71 C . J . Fritchie jr., Acta Crystallogr. 20, 27 (1966).
10
11
By Joseph A . Hnshmnll and Edgar Heilbronner[*l
The lone pairs (n) of the a-bonded bromine atom of a n alkyl
bromide RBr occupy the atomic orbitals 4 p x + and 4p;s-,
and, to a first approximation, 4 pa. In the present communiAngew. Chem. internat. Edit.
/ Vol. 9
(1970)
/ No. 4
--
11,
15
16
17
Figure I . Photoelectron spectrum of ethyl bromide (recorded with a
modified spectrometer 171 manufactured by Perkin-Elmer Ltd. (Beaconsfield, England). The n band is recorded at reduced sensitivity. The decrease in intensity with increasing energy is in part due to the instrument.
The method is illustrated by the following Table of ionization
potentials I(n)l, I(n)z of a series of alkyl bromides RBr, their
I(n)z)/2, and their differences
mean values I(n> = (Z(n)l
A(n) = I(n)1- I(n)z.
+
Table: n-Ionization potentials of alkyl bromides RBr (in eV). Mean
error ca. f0.015 eV. I(n)l and I(n)z correspond to the 0-0 transition.
R
I
I(n)l
10.53
10.30
10.18
10.13
10.12
n-Ionization Potentials of Alkyl Bromides111
13
E leVi
12
9.95
10.10
10.04
I1
~
10.85
10.61
10.49
10.44
10.41
10.24
10.41
10.34
2
I
I -a(n)
10.69
10.455
10.335
10.285
10.265
10.095
10.255
10.19
0.32
0.31
0.31
0.31
0.29
0.29
0.31
0.30
Remarks:
1. In the case of the radical cations RBr+ with R = methyl or
rert-butyl, in which the bromine atom is situated o n a threefold axis, Z(n)l and I(n), correspond to the states *nS/z and
305
which show a -0.31 eV splitting due to spin-orbit
coupling [31. This splitting is still observed even when RBr n o
longer possesses a threefold axis of symmetry. Apparently,
the local cylindrical symmetry at the bromine atom is disturbed to a negligible extent by a n alkyl group of low symmetry. This is shown by the fact that the splitting 4(n)
remains constant within the limits of experimental error.
'n112,
2. Figure 2 shows the dependence of the mean ionization
potential E)o n R. On the average, I(n)is shifted by-0.2 eV
per a-methyl substituent and by -0.09 eV per P-methyl substituent.
121 D . C. Frost, C. A . McDowell, and D . .4. Vroom, J. chem.
Physics 46, 4255 (1967); W . C . Price, ibid. 4 , 539, 547 (1936).
131 G. Herzberg: Molecular Spectra and Molecular Structure.
Van Nostrand, New York 1950, Vol. I, p. 214.
[41 K . Wutnnube. T. Nakayania, and J. Mottle, J. quantitative
Spectroscopy radiative Transfer 2, 369 (1962); M . I . Al-Joboury
and D . W. Turner, J . chem. S O C . (London) 1964,4434; M. J . S .
Dewar and S. D . Worley, J. chem. Physics 50, 654 (1969).
[ 5 ] A . D . Wulsh, Trans. Faraday SOC.45, 179 (1949).
161 G. A . Oluh and C. U.Pittnow, Advances physic. org. Chem.
4 , 333 (1966).
[7] D. W . Turner, Proc. Roy. SOC.(London) A 307, 15 (1968).
ESR Spectra of Radical Anions of Some
6a-ThiathiophtheneDerivatives111
=c
@ Br
By Fabian Gerson, Rolf Gleiter, Josef Heinzer, and
Hans Behringer [*I
I
-
>
2?
E
t
1oc -
102 -
tv"
$r
I
I
I
&
I
The results of the ESRstudies reported in this communication
present an opportunity for testing the MO models proposed [2,31 for 6a-thiathiophthene. The experimental data
allow conclusions to be drawn regarding the symmetry of the
system in general and the form of the lowest vacant orbital
in particular.
Tables 1 and 2 list the coupling constants of the magnetic
nuclei for the radical anions of the 6a-thiathiophthene
derivatives ( I ) to ( 4 ) [41. The experimental values were
refined with the aid of a computer programlsl; thus the relative experimental error should not exceed 1 Wherever the
assignment could not be deduced directly from the ESR
spectrum it was made on the basis of analogy with the corresponding data for known radical anions. (The number of
x.
3. With reference to the first band in the PE spectrum of
RH [41, the corresponding band of the series of e bands in the
PE spectrum of the alkyl bromide RBr is shifted by 0.4 eV
towards higher energies if we confine our attention to the
adiabatic ionization potentials.
4. While the results for cycloalkyl bromides (e.g., cyclo-
hexyl bromide, r(n)= 10.02 eV; A(n) = -0.29 eV) are in line
with the predicted values, the group R = cyclopropyl represents a special case. The PE spectrum of cyclopropyl
bromide contains only one clear n-ionization band with a
sharp 6 0 transition at 10.41 eV, which can be ascribed to
the 4 pr lone pair.
Coupling constants (in gauss) for [ I ) @ and (210.
Table 1 .
methyl or ethyl
trime ylene
radical
anion
0;:
425
(I)@
I
1-25
2.17
(two)
I
I
6.35
I
I (six)
-
1
-
I
~~
4.10
GO.15
(four)
~
2.05
_(six)
(four)
_
4.08
-
3.54
h*hp
The 4px orbital, which is oriented parallel to the C7-C3
bond of the cyclopropyl group, conjugates with one of the
Walsh orbitals (51 of this group and thus gives rise to a finestructured, broad band at 9.53 (adiabatic) and 9.66 eV
(vertical) (cf. ref. 161).
1.19
phdY3jPh
s-s-s
s-s-s
Ph
=
Phenyl
f 41
Coupling constants (in gauss) for [3)@and (4IQ.
Table 2.
[*I Dr. J.
A. Hashmall and Prof. Dr. E. Heilbronner
Physikalisch-chemisches Institut der Universitat
CH-4056 Basel, Klingelbergstrasse 80 (Switzerland)
[l] Applications of Photoelectron Spectroscopy, Part 11. This
work is part of Project No. 2.120.69SR of the Schweizerischer
Nationalfonds. - Part 10: E . Heilbronner and K . A . Muszkat,
J. Amer. chem. SOC.,in press.
H
(3)
Received: January 29, 1970
[Z 163 IE]
German version: Angew. Chem. 82, 320 (1970)
(3p
1
C25
1
1.58
(two)
_
(two)
(two)
H
0.31
(two)
2.98
(two)
GO.15
(six)
(two)
-40
-
i
-
Angew. Chem. infernat. Edit.
i 1 i
1.37
0.45
1.58
(four)
(four)
(two)
Vol. 9 (1970) 1 No. 4
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