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Ionization Barrier of Trityl Chloride in Sulfur Dioxide.

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off the solvent, is distilled at 10 torr. Spectroscopically and
analytically pure ( 2 a ) is obtained in 70 % yield.
Received: January 12, 1977 [Z 657 IE]
German version: Angew. Chem. 89,265 (1977)
CAS Registry numbers:
(1 a ) , 156-05-8;(1 b ) , 25209-46-5;(1 c ) . 61752-37-2;( I d ) , 61752-38-3;(1 c ) ,
6828-41-7; ( I f ), 61752-39-4;( I g ) , 61752-40-7;( 2 a ) , 100-42-5; ( 2 b ) ,
1073-67-2;(2c), 622-97-9;(2ri), 16939-57-4;( 2 e ) , 300-57-2; 2 - ( 2f),
61752-41-8;E 4 2 f ), 61752-42-9;( Z q ) , 61752-43-0;P(C6H&, 603-35-0;
dietbyl azodicarboxylate, 1972-28-7;1-acetoxybutadiene, 151 5-76-0;methacrylic acid, 79-41-4
Since the calculation of k - requires knowledge of the equillibrium constant of dissociation, only the temperature dependence of k'L1 = I/T~,," (the reciprocal lifetime of all ionic
forms)was considered in the compilation of kinetic data (Table
1) for recombination.
I(.
ki
Rexo
ki
R@
+
PCO"
0.78
1.22
1.70
Xo
free ions
have so far been studied by two different methods: (i) determination of the solvolytic behavior of RXIZ1and (ii) examination of the kinetics of recombination of Re and Xe by flow
methodsr3a]or relaxation techniquesr3b1.All these methods
use time-dependent changes in concentrations in order to
determine kinetic parameters. We now report a study of
the ionization kinetics of triphenylmethyl chloride in SOz
under equilibrium conditions by NMR spectroscopy.
When a solution of trityl chloride in SO2 is cooled down,
the center of gravity of the 'H-NMR signals migrates downfield, indicating a shift of the equilibrium from covalent to
ionic species. In addition, the signals exhibit broadening below
-40°C and ultimately split into a spectrum of ionic and
covalent trityl chloride. In order to allow a quantitative evaluation of the 'H-NMR spectra we used trityl chloride deuterated
in the ortho and para positionsr4]which at low temperatures
only gives rise to singlets at 6=7.37 (covalent trityl chloride)
and 7.93ppm (all ionic forms). Line shape analysis of the
coalescence of these two signals (Fig. 1)results in temperaturedependent values of the average lifetimes T~~~ (all ions) and
T,,, (covalent form). The rate constant kl can easily be calculated from T~~~ according to
activity of free chloride ions
[*I Prof. Dr. H. Kessler, DipLChem. M. Feigel
Institut fur Organische Chemie der Universitat, Laboratorium Niederrad
Theodor-Stern-Kai 7, D-6004 Frankfurt am Main 70 (Germany)
256
0.00107
k-2
ion pair
ma:
=!h
0.43
By Martin Feigel and Horst Kessler[*]
Rate constants for ionization and dissociation of a compound RX via ion pairs into free ions according to the following
simplified Winstein scheme
k- i
0'
in SO,
Ionization Barrier of Trityl Chloride in Sulfur Dioxide"]
RX
H'
0'
H'
G . W! Moersch, A. R. Burkert, J. Org. Chem. 36, 1149 (1971);A. P.
Krapcho, E. G . E. Jahngen Jr., ibid. 39, 1650 (1974).
S.Hara, H. Taguchi, H . Yamamoto, H . Nozaki, Tetrahedron Lett. 1975,
1545; A. Riittimann, A . Wick, A. Eschenmoser, Helv. Chim. Acta 58,
1450 (3975).
W! Adam, J . Baeza, J.-C. Liu, J. Am. Chem. Soc. 94,2000 (1972).
7: Mukaiyama, Angew. Chem. 88, 1 1 1 (1976);Angew. Chem. Int. Ed.
Engl. 15, 94 (1976).
E. Brunn, R. Huisgen, Angew. Chem. 81, 534 (1969); Angew. Chem.
Int. Ed. Engl. 8, 513 (1969); 0.Mitsunobu, M . Eguchr, Bull. Chem.
SOC.Jpn. 44, 3427 (1971);A . K . Bose, B. Lal, W. A. Hoffman, M . S.
Manhas, Tetrahedron Lett. 1973, 1619.
N . Rabjohn, Org. Synth. Coll. Vol. 3, 375 (1955);J . C . Kauer, ibid.
4 , 41 1 (1963).
3.26
6[ppml8.0
1.5
7.0
Fig. 1. Lineshapeanalysis (right)of the FT-'H-NMR spectra (left) of nonadeuteriotrityl chloride in SO2 (0.1 M). The individual mean lifetimes of the ionic
and covalent species are given by T~~~ = r/pcov and rcor= T / P ~ ~ "p.; = population
of the species S; K,on=p,on/peov.
Table 1. Activation parameters for trityl chloride in SO2 (0.1 M).
[kcal/mol]
AH
* [kcal/mol]
AS* [cal/mol.K]
Ionization ( k , )
Recombination ( k - l ) [a]
10.12f0.05
5.34k0.5
-22.9 k 3
10.06+0.05
10.6 k0.5
+ 2.6 +3
[a] Recombination pseudo first order, see text.
Whereas enthalpy contributions are mainly responsible for
the recombination barrier, the ionization barrier is largely
determined by the strong negative activation entropy (Table
1). The higher order of the solvent molecules around the
ions than around covalent RX lowers the entropy. It follows
from our data that in the transition state the solvation is
comparable with that of ion pairsrs1.
Regarding the results obtained in less polar solvents the
value of AG?, in SO2 appears high. For example the barrier
was estimated to be lower than 4 kcal/mol from SNlreactions
of trityl chloride in benzener7! Furthermore, in dichloroethane
the recombination of trityl cations with chloride anions is
diffusion controlledr3b1.Since the population of ions and covalent compound are similar in SOz (measureable populations
p s are an essential condition for kinetic NMR measurements
Angew. Chem. Int. Ed. Engl. 16 (1977) No. 4
at equilibrium; cf. Fig. I), according to the Hammond postulate
the transition state in our case is more similar to the covalent
compound than on collapse of the high-energy system of
dissociated ions in dichloroethandB1.Dynamic NMR measurements therefore permit the study of barriers of ionization
equilibria, and should thus provide a deeper insight into
the transition state of ion recombinations in solution.
stat
Received: January 18, 1977 [ Z 658 IE]
German version: Angew. Chem. 89,266 (1977)
CAS Registry numbers:
Trityl chloride, 76-83-5
~
[l] NMR SpectroscopicStudies on Kinetics and Thermodynamics of Reversible Dissociation Reactions, Part 3. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.-Part 2: M . Feigel, H . Kessler, Tetrahedron 32, 1575 (1976).
[2] D. J . Raber, J . M . Harris, P . u. R. SchkJJerin M . Szwarc: Ions and
Ion-pairs in Organic Reactions, Vol. 11. Wiley, New York 1974, pp.
247-374.
[3] a) C. D. Rifchie, Acc. Chem. Res. 5, 348 (1972); b) L. M . Dorfmann,
R. J . Sujdak, B. Bockrath, ibid. 9, 352(1976).
[4] J . I . Brauman, M! C . Archie, J. Am. Chem. SOC.92, 5981 (1970).
[5] The entropy of trityl chloride ion pairs in SO2 is not known exactly.
WhileK, hasbeendeterminedas0.014atO0C[6a], a value of K , =0.0025
can be calculated from the results of temperature-dependent conductivity
measurements [6b]. In [6b] AS? was determined as -34.5 e.u. Our
own concentration-dependent NMR measurements give AS?= - 37.5
e.u.forthedeuteratedcompound,butavalueofK, =0.0044on extrapolation to 0°C.
[6] a) N . N . Lichtin, Prog. Phys. Org. Chem. 1, 75 (1963); b) E. Clougherry,
Ph. D. Boston University 1966, Diss. Abstr. B 27, 1438.
[7] E . D. Huyhes, C. K . Inyold, S. F. Mok, S. Parai, I! Pocker, J. Chem.
SOC.1975, 1265.
kcal mol was determined by
[8] A recombination barrier of AG:,,=8.1
an indirect method for tri-p-tolylmethyl chloride in methylene chloride
[9a]; no change in line shape has been reported on direct measurement
of the ionization of this compound in SO,/CD2Cl2 [9b].
[9] a ) H. H. Freedman, A. E. Young, !I R . Sandel, J. Am. Cbem. SOC. 86,
4722 (1964); b) M . Nojima, M . Ishiyama, N . Tokura, Kogyo Kagaku
Zasshi 73. 2306 (1970).
Comparison of Calculated and Experimental Electron
Difference Densities of T e t r a c y a n o e t h y l e n e [ * * ]
By Hans-Lothar Hase, Karl- Wilheim Schulte, and Armin
Schweigr]
It is assumed[” that electron difference densities[*] in the
region of chem’ical bonds can be determined with an accuracy[31of 0.05 e/A3.We now report quantum chemical ab initio
results which cast doubt upon this expectationL4]for the standard example tetracyanoethylene (TCNE)[’].
D
.
,_
I
Fig. 1. Calculated (“4-31 G” basis) static ( s t a t ) and dynamic ( d y n ) and
experimental ( e x p ) electron difference densities in the molecular plane ( A )
and perpendicular to the molecular plane through the C=C ( B ) , C-C
(C), and C s N bonds (D) of tetracyanoethylene (TCNE). The contour
negative
lines mark differences of 0.1 e/A3 in all cases; positive lines (-),
experimental).
lines (----), zero line (.... calculated and
shows sections through the resulting static (star) and dynamic
( d y n ) densities and also through the experimental (exp)
densities [in the molecular plane ( A ) , perpendicular thereto
through the C=C (B), C-C (C), and C=N bonds ( D ) ] .
As shown by the figure, overall agreement between the calculated and the experimental difference densities is surprisingly
good. However, some considerable discrepancies (0.5 e/A3 for
W N and 0.4e/A3 for C-C) are found for the difference
densities in the bonds (Table 1).
The deviations of the calculated values to be expected as
a result of using the incomplete “4-31 G” basis have been
estimated for the model compound cyanogen (NCCN). For
Table 1. Peak heights in the difference densities of tetracyanoethylene (TCNE).
Peak heights in the difference densities
r431
Difference density type
4-31
G
experimental
For this purpose the static and dynamic electron difference
densities were calculated in “4-31 G quality[61.Figure 1
C=-N
C=C
C-C
lone pair
static
dynamic
0.7
0.4
0.5
0.3
0.6
0.2
1.1
0.3
dynamic
0.9
0.4
0.6
0.4
Table 2. Peak heights in the difference densities of cyanogen (NCCN).
Peak heights in the difference densities
[*] Prof. Dr. A. Schweig, Dr. H.-L. Hase, Dipl.-Chem. K.-W. Schulte
Fachbereich Physikalische Chemie der Universitat
Auf den Lahnbergen, D-3550 Marburg (Germany)
p’] Part 3 of “Comparison of Observed and Calculated Electron Densities”,
DFG-Sonderforschungsbereich 127 (“Crystal Structure and Chemical Bonding”). Presented in part at the “Sagamore V” conference, Killjava (Finland),
August 1 6 2 0 , 1976.-Part 2: H . Imgartinger, H . L. Hase, K.-W Schulte,
A. Schweig, Angew. Chem. 89, 194 (1977); Angew. Chem. Int. Ed. Engl.
16, 187 (1977).
Angew. Chem. l n t . Ed. Engl. 16 ( 1 9 7 7 ) No. 4
~
~
3
1
-
Difference density type
C=N
C-C
lone pair
DZ+P
(near HF)
static
dynamic
1.0
0.65
0.6
0.4
1.1
0.35
4--31
static
dynamic
0.8
0.47
0.4
0.24
0.40
1.1
257
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