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Geometry and n-Ionization Energies of Alkyl-Substituted Triketones.

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field shift of the resonances of the olefinic protons (multiplet
at ~=1.2-2.0) and a concomitant upfield shift of the
bridge-proton AB system (7=9.87 and 11.05 with J =
- 10.7 Hz). The UV spectrum of ( 3 ) recorded in ethanolic
perchloric acid, like that of protonated ( 2 ) , exhibits
excellent agreement with the spectrum of the ion (Z)['].
As an acid (3) reacts with alkali metal hydroxides to give
salts (12) that afford dark red aqueous solutions. A pK,
value of 8.25 is measured for (3) by potentiometric
titration in water. ( 3 ) is thus not quite as strong an acid as
cr-tropolone (7.00)191but is more acidic than benzo[d]tropolone (10.2)[ '1.
ed as 21.8 kcal/mol, the staggered arrangement of methyl
groups appearing only marginally more favorable ( N0.4
kcal/mol) than the eclipsed one. Kerr effect measurements"] and electronic spectraf3] indicate deviations
from planarity in biacetyl and triketones, respectively ;
according to these results the torsional angles in dimethyltriketone should be wl,w2 Z O O , 90". Further evidence
for a possible non-planar arrangement of the three carbonyl
groups in ( 1 ) is provided by CNDO CI calculations of
excitation energies; MIND0/2 (1.95 D) and CND0/2 dipole moments (1.67 D)[41are in good agreement with the
experimental value of 1.93 D.
The acidity of (3) permits methylation of the compound
with diazomethane in the absence of a catalyst, the ethers
(13) (m.p. 122°C) and (14) (m.p. 143°C) being formed in
the ratio 6 :1(total yield 64%).Since (14) can be synthesized
independently by reaction of (9) with sodium methoxide,
the structural assignments of both ethers are proved beyond
doubt. Diazomethane methylation of (3) probably proceeds via an intimate ion pair"'] formed from the methyl
diazonium ion and (12), thus precluding any assessment
of the equilibrium mixture of (3) and (ZU) from the ratio
of (13) and (14).
Assuming the validity of the MIND0/2 model in which
the acetyl groups protrude in opposite directions from the
C-CO-C
plane we found a preference for a helix conformation in tri- and tetraketonesf51(Fig. 1).
Received: November 29,1971 [Z 552b IE]
German version: Angew. Chem. 84,208 (1972)
[l] Presented in part by E . Vogel at the XXIII IUPAC Congress,
Boston, Mass., July 1971.
121 W Grimme, H . Hoffunn, and E. %get, Angew. Chem. 77, 348
(1965); Angew. Chem. infernat. Edit. 4, 354 (1965); E. Vogel, R. Feidmann, and H . Duwel, Tetrahedron Lett. 1970, 1941.
[3] W Grimme, J . Reisdora W Jiinemann, and E. Vogel,J. Amer. Chern.
SOC.92,6335 (1970).
[4] E. Vogel and H . D. Roth, Angew. Chem. 76, 145 (1964); Angew.
Chem. internat. Edit. 3,228 (1964).
[ S ] The terms syn and unti refer to the position of the bridge protons
relative to the cycloheptatriene structural unit.
[6] J . Reisdorff; Dissertation, Universitat Koln 1970.
[7] The pK, values were determined spectrophorometrically in aqueous
sulfuric acid utilizing Hammett H, values: cf. H . H o f m n n , Dissertation, Universitat Koln 1967.
[8] W Grimme, E. Heilbronner, G. Hohlneicher, E. Vogel, and J.-P.
Weber, Helv. Chim. Acta 51, 225 (1968).
[9] J . W Cook, A . R . Gibb, R. A. Raphael, and A . R . Someruilfe, J. Chem.
SOC.1951,503.
[lo] B. E. Bryonr and K C . Fernelius, J. Amer. Chem. Soc.76, 3783
(1954).
[ll]
J . S . Pyrek and 0. Achmatowiczjr., Tetrahedron Lett. 1970,2651.
Geometry and n-Ionization Energies
of Alkyl-Substituted Triketones[**I
Fig. 1. Helix model of dialkyl-substituted triketones [6].
By Jiirgen Kroner and Walter Strack"]
The dimethyl derivative (pentanetrione) (1) was selected
as a model compound for the study of the hitherto unknown
molecular geometry and the photoelectron (PE) spectra of
open-chain triketones.
A MIND0/2 minimization['] as a function of the torsional
angles o,and w, about the CC axes reveals the most stable
form to be a non-planar conformation (2) (cf. Fig. 1)with
an equal and opposite deviation of the two acetyl groups
from the C-CO-C
plane (w, ,a2
= k90"); the difference
in energy between the conformations ( I ) and (2) is calculat-
['I
Dr. J. Kroner and Dip].-Chem. W. Strack
Institut fur Anorganische Chemie der Universitat
8 Miinchen 2, Meiserstrasse 1 (Germany)
This work was supported by the Deutsche Forschungsgemeinschaft.
[**I
220
According to Shimanouchi and M i ~ u s ~ i r nthe
a ~ spatial
~~
arrangement of the carbon centers can be described in
terms of the angle of rotation (T) about the helical axis, the
distance (p) from the axis, and the chain 1engtheni;g ( d )
per CC segment (Fig. 1).Values of 7 = 107 p = 0.62 A, and
d = 1.08 8, are obtained for dimethyltriketone with wl,
02=
i.90". Dimethyltriketone thus proves to have a
significantly larger helix radius than, e. g., polyethylene
= 0.45 8,) and polytetrafluoroethylene (w = 18 ',
(w=Oo,
p=0.45 ) f 7 , 8 1 . An increase in the torsional angle along
the series dimethyl-, diisopropyl-, di-terf-butyltriketone,
owing to competitive steric interactions of the alkyl groups
with each other and with the central carbonyl oxygen,
should lead to an increase in the helix radius (p) with a
concomitant compression of the helix cylinder (d).
O,
R
Angew. Chem. internat. Edit. 1 Vol. 11 ( 1 9 7 2 ) / N o . 3
The n-ionization energies['] of alkyl-substituted triketones,
as determined by photoelectron spectroscopy, can also be
discussed on the basis of the calculated geometry of dimethyltriketone.
The considerable energy difference between orbitals of
"nonbonding" (n)-electron pairs in a-diketones"'~ and
a,P-triketones"'I has its origin in their through-bond interaction1131by way of the o skeleton. The first bands in the
PE spectrum of the simplest open-chain triketone (1) are
found at 9.52, 10.97, and 12.30 eV; they can be assigned to
ionization from "nonbonding" orbitals (n,, n,, n3).
Combination of the orbitals n , ( -), n 2 (+), and n3(-)[I4],
which are threefold degenerate under conditions of slight
through-space interaction["], with lower lying asymmetric
and symmetric CCC CY orbitals having the correct symmetry properties yields the highest occupied energy levels
which, for C,, symmetry, correspond to the irreducible
representations b,, a,, and b, (Fig. 2). These n-splittings
will be discussed with the aid of the simpler planar model.
Assuming the validity of Koopmans theorem"'] the vertical
ionization energies of dimethyltriketone, as measured by
PE spectroscopy, are compared with the orbital energies
calculated for 0 , , ~ = 0 "and k90" by the MIND0/2 and
modified CND0/2 methods (Table).
As in diketones['0-121, the energies of the n-orbitals and
their splitting in triketones proves to be only slightly
dependent upon the torsional angle, thus offering some
justification for a discussion on the basis of the planar
model.
In contrast to dimethyltriketone, An is found by experiment
to be 2.78 eV (nl = 9.04, n2= 10.48, n, = 11.82 eV) for diisopropyltriketone, and 2.59 eV (nl =9.00, n z = 10.28,
n3= 11.59 eV) for di-tert-butyltriketone. A decrease in the
splitting would correspond to an increase in the torsional
angle and an enlargement of the helix radius along the
series R = CH, zz CH(CH,), <C(CH,), if the differing electronic effects of the alkyl groups on n, and n3 are neglected.
The observed splittings are also in good agreement with
theoretical predictions (2.72 eV)['O1 for a hypothetical
cyclopropanetrione. Increasing n-splitting is observed
within the series of di-, tri-, and tetraketones['!
Received: June 23,1971 ; revised: September 13,1971 [Z 553 IE]
German version: Angew. Chem. 84,210 (1972)
'1
,
I
:
:+
b,
A
Fig. 2. Qualitative representation of the n-orbital interactions in planar
triketones.
The orbital sequence results from the differing number of
antibonding and bonding n/o interactions. According to
model calculations no significant admixture of antibonding CCC o* contributions is to be expected in the range
of highest occupied energy levels considered in the present
case. The MOs corresponding to the first three ionizations
do show a preference for n-components but they also undergo strong delocalization over the o skeleton.
Table. Vertical ionization energies I E , (ev), calculated n-orbital energies (eV) and n-splitting of dimethyltriketone.
Orbital type
Symmetr) [a]
n1
n2
n3
b,
a1
bl
IE,
MINDO/2 [b]
CND0/2 [b,c]
MINDO/Z [d]
CNDOj2 [c, d]
9.52
-9.25
-9.22
-9.58
-9.29
10.97
-11.27
-11.45
-11.00
- 11.03
12.30
-12.14
-12.30
-12.24
- 12.66
I4n
-na)l
The Equivalent Orbital Method of Interpreting
Photoelectron (PE) Spectra :Neopentane"]
By Werner Schmidt and Bernard T Wilkins"]
2.78
2.89
3.08
2.66
3.37
[a] Based on (planar) C,,-geometry; [b] w,, w,=O" [16]; [c] energy
zero at 1 eV; [d] w ~ w. 2 = +90" [16]
Angew. Chem. internat. Edit. 1 Vol. 11 (1972) 1 No. 3
[l] M . J. S . Dewar and E. Haselbach, J. Amer. Chem. SOC.92, 590
(1970); N. Bodor, M . J. S . Dewar, A . Harget, and E . Haselbach, ibid. 92,
N . Bodor,
3854(1970); N.Bodorand M . J.S.Dewar,ibid.92,4270(1970);
M . J . S . Dewar, and S. D. Worley, ibid. 92, 19 (1970).
[ 2 ] J. E. LuValle and V. Schomaker, J. Amer. Chem. SOC.61,3520(1939);
P. H . Cureton, C . G. LeFPare, and R . J. W LeFkcre, J. Chem. SOC 1961,
4447.
[3] M . Cultin and C. L. Wood, J. Amer. Chem. SOC.62, 3152 (1940);
L . Horner and F. Maurer, Chem. Ber. 101, 1783 (1968).
[4] J . Del Bene and H . H. Jafe, J. Chem. Phys. 48, 1807,4050 (1968);
49, 1221 (1968); 50, 563, 1126 (1969). The Ohno two center y-relationship was used instead of the Pariser-Parr relationship (cf. K . Ohno,
Theor. Chim. Acta 2,219 (1964)).
[S] J. Kroner and W Struck, to be published.
[6] The protons of the methyl groups were neglected in the Figure.
171 T Shimanouchi and S.-I. Mirushima, J. Chem. Phys. 23, 707 (1955).
[8] K . Morokuma, J. Chem. Phys. 54,962 (1971).
[9] The photoelectron spectra were recorded with a Perkin-Elmer
PS 16 spectrometer (He I lamp, electrostatic sector fieid 127").
[lo] J. R. Swenson and R. Hoffmann, Helv. Chim. Acta 53,2331 (1970).
1111 D. 0. Cowan, R. Gleiter, J. A . Hashmall, E. Heilbronner, and
I/. Hornung, Angew. Chem. 83, 405 (1971); Angew. Chem. internat.
Edit. 10,401 (1971).
[12] J. Kroner, W Strack, and D. Proch, to be published.
[13] R . Hoffmann, A . lmamura, and W J. Hehre, J. Amer. Chem. SOC.
90,1499 (1968).
[14] Signs refer to the C,, symmetry operation of rotation about the
central CO bond.
1151 ?: Koopmans, Physica I , 104 (1933).
[16] Distances: CO=1.20, CC=1.47, C-CH,=1.54,
CH=I.O9A;
angles: CCO=123, OC-CH3=122.5,CCH=109.5"(cf.
12, 31).
While the analysis of the electronic structure of molecules
containing IT bonds or lone pairs is already quite far
advanced, the study of saturated organic compounds by
[*] Dr. W. Schmidt and B. T. Wilkins, B. A.
School of Molecular Sciences, The University of Sussex
Brighton BN 1 9 QJ (England)
221
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geometry, alkyl, triketone, substituted, energies, ionization
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