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Calculation of the Charge Distribution in Conjugated Systems by a Quantification of the Resonance Concept.

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The cleavage of 13 from the carrier and its work-up were carried out as dewrihc:(i in ['h].
[I21 S. L. Beaucage, Tdrahedron Left. 25 (1984) 375.
[I31 See [gal. p. 166-1169,
[I41 B. C. Froehler, M. D. Matteucci, Tetrahedron Len. 24 (1983) 3171.
1151 T. M. Cao, S. E. Bingham, M. T. Sung, Tetrahedron Lett. 24 (1983) 1019;
K. Jayaraman, H. McClaugherty, ibid. 23 (1982) 5377.
[I61 H. Seliger, C. Scalfi, F. Eisenbeifl, Tetrahedron Leu. 24 (1983) 4963.
Calculation of the Charge Distribution in
Conjugated Systems by a Quantification of the
Resonance Concept**
By Johann Gasteiger* and Heinz Saller
Dedicated to Professor Rolf Huisgen on the occasion of
his 65th birthday
Fig. I. HPLC separation of the product of the solid-phase synthesis of
DMTrdTTTTA. A =absorption; V=elution volume; P=main product. Column: p-Bondapak C , , (Waters; 7.8 x 240 mm); eluent: 0.1 M triethylammonium acetate buffer (pH 7.0)+20-35% acetonitrile; flow rate: 2 mL/min;
detection: U V (254 nm).
b) Experiment with an automatic synthesizer:
9d (1.2 g) prewashed with 15 mL of CH,CN was treated as described in a)
with tetrazole in CH,CN. The solution of I 1 was poured into the reservoir of
an automatic synthesizer [6]. Four chain extensions [5b] of 12a (4.5 pmol
dA") were carried out (cycle time 23 min). Work-up of the solid phase [Sb],
HPLC of the product on silica gel C t X(Fig. I), and detritylation and desalting of the product fraction (P in Fig. 1) [Sb] gave dTTITA (174 0.D.251)in a
yield of 77%.
The resonance theory has become a fundamental part of
the basic training in organic chemistry."] Although it has a
quantum mechanical basis in the valence bond method, it
has remained only a qualitative concept in the hands of the
organic chemist. Even though individual resonance structures are assigned different weights, there has been no simple method so far to express these weights quantitatively.
We have now developed an empirical procedure that
does just this and enables the charge distribution in TI-systems to b e calculated on the basis of the assignments of
weights to the resonance structures. From these charges,
predictions can then be made as to the physical and chemical properties of compounds. The weight w of each resonance structure consists of a formal (topological) part and
an electronic part. In order to assign the topological weight
w,, the resonance structures are classified according to
whether (a) a reduction in the number of covalent bonds,
(b) a decrease in the number o f aromatic systems, or (c) a
charge separation occurs.
Received: October 8, 1984;
revised: May 15, 1985 [ Z 1031 IE]
German version: Angew. Chem. 97 (1985) 71 I
CAS Registry numbers:
7a, 64325-78-6; 7b, 67219-55-0; 7c, 68892-41-1; 7d, 40615-39-2; Sa, 7863596-8; Sb, 78635-95-7; SC, 78635-97-9; Sd, 74855-51-9; dTTTTA, 80703-91-9;
dT,, 1270-05-9; C12POCH,, 3279-26-3.
R. L. Letsinger, W. B. Lunsford, J. Am. Chem. Soc. 98 (1976) 3655.
M. D. Matteucci, M. H. Caruthers, J . Am. Chem. Soc. 103 (1981) 3185.
S. L. Beaucage, M. H. Caruthers, Tetrahedron Lett. 22 (1981) 1859.
L. A. Carpino, E. M. E. Mansour, J. Knapczyk, J . Org. Chem. 48 (1983)
[51 a) H. G. Gassen, A. Lang (Eds.): Chemical and Enzymatic Synthesis of
Gene Fragments, Verlag Chemie, Weinheim 1982; b) H. Seliger, S.
Klein. C. K. Narang, B. Seemann-Preising, J. Eiband, N. Hauel in [sa],
p. 81 ff.
161 The reactions were carried out in automatic synthesizers from Analysteknik, Vallentuna, and Biosearch, San Rafael, CA. USA. We thank
these companies for carrying out trial syntheses and for the trial use of
an instrument.
[7] c:P D. Tu, R. Wu in L. Grossmann, K. Moldave (Eds.): Merhods in Enzymoloql;. Yo/. 65, Academic Press, New York 1980, p. 620ff.; R. Frank,
H. Blocker in [Sa]. p. 225ff.
[Sl a) N. K. Mathur, C. K. Narang, R. E. Williams: Polymers as Aids in Oryanrc Chemistry. Academic Press, New York 1980; b) for examples see:
[Sa], p. 174- 197.
191 S. P. A d a m . K. S . Kavka, E. J. Wykes, S. B. Holder, G. R. Galluppi, J.
Am. Chum. Soc. 105 (1983) 661; L. J. McBride, M. H . Caruthers, Terrahedron Leu. 24 (1983) 245.
[lo] T. Dorper. E:L. Winnacker, Nucleic Acids Res. 11 (1983) 2575.
[ I I 1 A. D. Barone. J.-Y. Tang, M. H. Caruthers, Nucleic Acids Res. I 2 (1984)
405 I
Anyew. Chem Inr. Ed. Engl. 24 (1985) No. 8
Each of these formal situations is assigned a factor,
whose value was determined as described below. The total
topological weight w, of a resonance structure is given by
Equation (d).
The electronic weight w, must express how well, for example, the atom A in Equation (c) can donate its lone pair
of electrons and how stable a negative charge on atom C
is. In order to determine this, we used the electronegativity
According to Mulliken's definition,I2' the electronegativity is determined by the ionization potential I P and the
electron affinity EA [Eq. (e)].
Priv.-Doz. Dr. J. Gasteiger, Dipl.-Chem. H. Saller
Organisch-chemisches lnstitut der Technischen Universitat Miinchen
Lichtenbergstrasse 4, D-8046 Garching (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft;
computer time was provided by the Leibniz Rechenzentrum Miinchen.
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I n order to further refine these ideas, the concept of orbital electronegativity was introduced.['.41 According to
this concept, the electronegativity is no longer a purely
atomic property, but is also dependent on the type of orbital. An s p orbital, for example, has a higher electronegativity than an sp3 orbital. Furthermore, the orbital electronegativity is dependent on the occupation number of the orbital ;[4.51 an unoccupied orbital attracts electrons more
strongly than a singly or doubly occupied orbital. We selected a second degree polynomial [Eq. (f)] to express the
dependence of the orbital electronegativity on the occupation of an orbital (or, equivalently, on the charge q).
placed per cycle. New electronegativity values [Eq. (f)]
are calculated in the next cycle using the resulting
Eight cycles have proved to be a good compromise between computing accuracy and computing time; the computing times are then always extremely short.
The six factorsf were fixed using properties that directly
depend o n the charge distribution. This is true, for example, of I3C-NMR shifts as long as one class of compounds
is used (constant mean excitation energy"']), or of ESCA
x,.=o, + b,,.q + c,:qZ
shifts, which can be easily interpreted using a potential
Three sets of physical data were used to optimize
The coefficients a,, b,, and c, could be c a l c ~ l a t e d [ ~ ~ ~rnodel.["l
the factors by the Simplex procedure:"'] A) the I3C-NMR
from published data on valence-state ionization potentials
shifts of the para C atom in twelve monosubstituted benand electron a f f i n i t i e ~ . ~ ~ . ~ '
On the basis of the partial equalization of orbital eleczenes; B) the I3C-NMR shifts of twelve C atoms in nine
substituted pyridines; C) the C 1s ESCA shifts of eleven C
tronegativities, we had already developed a procedure for
atoms in seven fluorinated olefins.
the calculation of charge distribution in o-bonded moleThis gave the following factors:
= 0.65 ; fb = 0.30;
cules,'61which has proved suitable for the reproduction of
f,=0.133; f e = 1.OO;f,,=O.5O;f,=O.33. The charge values
a variety of physical and chemical
calculated from them reproduced these three sets of data
The two resonance structures in each of the equations
well (r= correlation coefficient: s =standard deviation): A)
(a) to (c) differ by electron transfers between the n orbitals
r=0.986, s=1.04 ppm (see Fig. 1); B) r=0.991, s=1.36
of two atoms. The availability of the electrons in these orppm; C) r = 0.987, s = 0.34 eV.
bitals is characterized by the corresponding n-orbital electronegativity
the coefficients a,, b,, and c, in Equation
(f) that are necessary for its determination have been published already.['] The value of the charge q in Equation (f)
134is obtained from Equation (g), in which the different effect
of o and TI charges must be taken into consideration in de132termining the factor f,,.
The determination of the electronic weight we is based
on the classical idea that those resonance structures are
more important ("have lower energy") in which the negative charge is localized on the more strongly electronegative atom. This can be incorporated into the calculation by
the use of the difference of the TC electronegativities of the
two atoms i and j, which have differing formal charges,
e.g., A and B in (a) or A and C in (c). In addition, the second term of Equation (h) takes into consideration the electrostatic potential of the neighboring atoms Ni of the atom
i and N, of the atom j. This potential is assumed to be proportional to the sum of the charges qk, ql on these neighboring atoms.
The total weight of each resonance structure is calculated from the topological weight and from the electronic
weight according to Equation (i).
By adding the changes in charge of the individual resonance structures and taking into consideration the scaling
factor,f,, which is the same for all resonance structures, the
charge distribution is obtained. The two terms in Equation
(h), however, are themselves dependent on the charge distribution [cf. Eq. (f)]. In order to take this into consideration, the charge transfer was distributed over several cycles, n,, so that only a certain part of the charge, qc, is dis688
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Fig. 1. "C-NMR shifts (6 values) of the puru carbon atom in monosubstituted benzenes versus the total charge, Q,c,,,,l,of this carbon atom.
The charge values are also helpful for the calculation of
other physical data. Thus, for example, a good correlation
was found between the charge of the para C atom o f monosubstituted benzene derivatives and the chemical shift of
the protons bonded to it. Furthermore, for a large data set
of molecules (48 points) comprising conjugated olefins,
acetylenes, and carbonyl compounds, a linear relationship
between charges and C 1s ESCA shifts was obtained. In
addition, good agreement between experimental and calculated dipole moments was observed (see Table l). This
emphasizes the broad general significance of the charge
The accuracy of the charge values has been demonstrated here on the basis of physical properties. Resonance
theory is most important, however, for the interpretation of
chemical reactivity, in particular, of aromatic substitution.
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Angew. Chem. Int. Ed. Engl. 24 (1985)No. 8
Table I. Experimental and calculated dipole moments (in Debye).
3.2 I
6.9 I
The weighting of the resonance structures now enables
quantitative statements to be made. They are used in the
program system EROS['3' for the prediction of reactions
and the planning of syntheses. Use of the procedure presented here, along with methods to determine the inductive
effect['41and the p~larizability,"~]
allows the reactivity of
complex organic molecules to be estimated automatically.
The course of organic reactions can thereby be predicted.
Received: February 28, 1985;
revised: May 21, 1985 [Z 1196 IE]
German version: Angew. Chem. 97 (1985) 699
[I] See, e.g., R. Huisgen in Gattermann-Wieland: Die Praxis des organischen
Chemrkers. d e Gruyter, Berlin 1959, 39th ed., p. 377-396.
[2] R. S. Mulliken, J . Chem. Phys. 2 (1934) 782.
131 J. Hinre, H. H. Jaffe, J. Am. Chem. SOC.84 (1962) 540; J . Phys. Chem. 67
(1963) 1501.
[4] J. Hinze, M. A. Whitehead, H. H. Jaffk, J. Am. Chem. SOC.85 (1963)
[5] R. P. Icrkowski, J. L. Margrave, J . Am. Chem. SOC.83 (1961) 3547.
[6] J. Gasteiger, M. Marsili, Tetrahedron 36 (1980) 3219.
171 M. Marsili, J. Gasteiger, Croat. Chem. Actu 53 (1980) 601.
181 J. Gasteiger, M. Marsili, Org. Magn. Reson. 15 (1981) 353.
[9] M. D. Guillen, J. Gasteiger, Tetrahedron 39 (1983) 1331; J. Gasteiger, M.
D. Guillen, J . Chem. Res. IS) 1983, 304; (Mj 1983, 2611.
[lo] M. Karplus, J. A. Pople, J. Chem. Phys. 38 (1963) 2803.
1111 C. Nordling, Angew. Chem. 84 (1972) 144; Angew. Chem. Inr. Ed. Engl.
11 (1972) 83.
[I21 J. A. Nelder, R. Mead, Comput. J. 7(1965) 308.
(131 J. Gasteiger, C. Jochum, Top. Curr. Chem. 74 (1978) 93; J. Gasteiger,
Chim. lnd. (Milano) 64 (1982) 714.
[I41 M. G. Hutchings, J. Gasteiger, Tetrahedron Lett. 24 (1983) 2541.
(151 J. Gasteiger, M. G. Hutchings, J . Chern. Soc. Perkin Trans. 2 1984.
a Paramagnetic Molybdenum(1v) Compound
with a Metal-Metal Bond
By Ulrich Miiller, * Paul Klinyelhoyer, CIaus Friebel, and
Juryen Pebler
Many compounds are known in which two transitionmetal elements are joined via sulfur or halogen atoms and
which further contain metal-metal bonds.['] The metal-metal bond is recognized, apart from its length, by the existence of diamagnetism or only a very weak paramagnetism. Paramagnetic species occur when an odd number of
[*I Prof. Dr. U. Miiller, DipLChem. P. Klingelhofer, Doz. Dr. C. Friebel,
Dor. Dr. J. Pebler
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (FRG)
Anyew. Chem. Int. Ed. Engl. 24 (1985) No. 8
electrons are present-e.g., in 1-3,12-41
in which the metal
atoms have two different oxidation states.
[(NC)4Mo(p-SZ)(p-SOz)Mo(CN)4]5" 1'"
[CI W(p-SPh)2(p-C I) WCI 4'
3 141
We have now obtained the title compound, a paramagnetic compound with an even number of electrons, in
which a Mo-Mo single bond is present and in which, at
the same time, each molybdenum atom possesses an unpaired electron.
When a suspension of molybdenum pentachloride in
CHzClz is added to an equimolar solution of NEt,SH in
CHzClz frozen at 77 K, and the mixture is allowed to thaw,
the following reaction occurs with formation of HCI.
(M0C15)~ 2 SH"
After ca. 15 h of stirring, an insoluble, greyish green
product, which has not yet been identified, is filtered off;
after addition of CCI, to the filtrate, the tetraethylammonium salt of 4 (27%) crystallizes out at 278 K.
A sulfido complex containing pentavalent molybdenum
is not present, in contrast to what the composition
NEt4MoSC14 might lead one to suppose. As observed for
other thiomolybdates,['' the sulfido ligand is oxidized to
the disulfide and the molybdenum is reduced to Mo'". The
presence of a disulfide in 4 follows from the IR spectrum
(v(SS)=609 cm- ') and by the X-ray structure determination (Fig. I).'"] The disulfide group links the two molybdenum atoms, which, in addition, are bridged by two chlorine atoms. If the disulfide group is viewed as only a single
ligand, then 4 consists of two face-sharing octahedra: the
bond angles Mo-CI-Mo (68.0') and Mo-S-Mo (69.2')
are only slightly smaller than the ideal value (70.5") for
regular face-sharing octahedra.
Fig. I.Structure of 4 in the crystal o~(NE~~)~[CI,MO(S,)CI,MOCI,]
with ellipsoids of thermal vibration (50% probability at 295 K). Distances [pm]; standard deviations: Mo-Mo 0.2, Mo-CI 0.3, Mo-S 0.4, S-S 0.7 pm. The ion is
situated o n a crystallographic mirror plane that intersects the atoms S(I),
S(2), CI(1), and Cl(2).
The Mo-Mo distance (276.3 pm) supports the existence
of an Mo-Mo bond. Even though two d electrons are
available per molybdenum atom, it can only be a single
bond since, according to the magnetic susceptibilities and
the electron spin resonance spectrum, each Mo atom still
has available a n unpaired electron.
Between 4 and 160 K the magnetic susceptibility approximately obeys the Curie-Weiss law. The fitting of the
measured data gives a magnetic moment of 1.40 pB per
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distributions, calculations, conjugate, quantification, system, concept, resonance, charge
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