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Influence of Ionic and Polar Solutes upon G and V of the Amide Rotation in Aqueous Solutions of Dimethylacetamide.

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sis of C,F,N+AsF, in the following way: Since disproportionation occurs in the solid state and at relatively low temperature, only a slight activation energy will be required and
the distribution of the molecules in the solid state will limit
the possible reaction products. The first step is probably
transfer of Fe from the anion to the cation, which is associated with a shift of the lone pair from the N to the As
atoms:
nificant effects on AG* are caused by changes in the polarity
of the solvent['].
The pressure dependence of AG*, defined by the activation volume AV+
CSFSN@AsFa
+ CSF6N0.AsFs
If this is valid, then the charge density values according to
Chambers et al.c4a1would suggest that the Fe transferred to
C5F5N@
is bonded to a carbon atom adjacent to the nitrogen
atom. Redox reaction with the salt still present
CSF~NO.ASFS
+ CsFSN@AsF?+ CSFsN.AsF5+ CSF6N@AsF?
might be the next step.
The formation of C5F7Nfrom C5F6N@
might similarly be
initiated by a slight shift of Fe to AsE, coupled with an
acid/base interaction generating the C5F7N.AsF, adduct.
Procedure
O;AsF, (0.5 mol) is placed in a quartz- or Teflon FEP
reaction vessel connected with a Monel vacuum apparatus.
S02CIF (0.05 mol) and C5F,N (0.5 mol) are then condensed
onto the solid at - 196 "C. On warming to -45 "C, evolution
of O2 ensues. Decomposition of 1 mol of the salt at - 20 "C
affords 0.5 mol of the original pentafluoropyridine and 0.5
mol of heptafluoro-l-aza-1,3-cyclohexadiene,
both as AsF,
adducts (1 :1).
Received. July 14, 1980 [ Z 619 IE]
German version Angew. Chem. 92,956 (1980)
however does not reveal any systematic dependence on the
carbonyl substituents (R = H, CH3, C6HS)or the solvent polarity (CCL, C6D6, (CD3)2CO, (CDs)2SO, CD3CN,
CD30D)12'.For all solvents and substituents studied hitherto
the activation volume is found to AVs zz 10 cm3 mol- ', the
only exception being the dilute aqueous solution
(xDz0>0.98) of the dimethylamides. In these dilute solutions,
the rotation of the dimethylamino group becomes almost
pressure independent (e.g. dimethylacetamide in DzO,
~D,,=0.99, A V * = 0 . 3 cm3 mol-')131.
The preliminary explanation for this unexpected result is
the formation of an open hydration shell around the dimethylamino group, permitting amide rotation to occur without
significant rearrangements of the water molecules in the first
hydration layer. This attempted explanation predicts that the
addition of polar or ionic solutes to the aqueous solutions,
which alters the water structure, should lead to a significant
increase of the AV* observed.
In the first experiments dimethylacetamide (5% w/w) in
aqueous solutions of sodium bromide and urea was studied.
The results are given in Figures 1 and 2. Relatively small
111 T J. Richardson, N. Bartlett, J. Chem. Soc.Chem. Commun. 1974,427.
121 Pure CSFSN.AsFS was produced by tensimetric titration of the components.
Its Raman spectrum and its Debye-Scherrer diagram are identical with those
of the disproportionation product after the more volatile CsF7N.AsF, has
been removed in uacuo.
131 The reaction of O;Sb,F;
with CSFSN gave a product resembling
C,NF: AsF;. In contrast to the corresponding C,Ff-fluoroantimonate(v),
however, the thermal stability is not greater than that of hexafluoroarsenate.
141 a) R. D Chambers, D. T. Clark, T. F. Holmes, W. K . R. Musgraue, I . Ritchie,
J. Chem. SOC.Perkin Trans. I 1974,114 b) I. J. Hotchkiss, R. Stephens, J. C.
Tatlow, J. Fluorine Chem. 10, 541 (1977).
IS] D. A. Long, R. T. Barley, Trans. Faraday SOC.59,599 (1963).
IS] S. L. Bell, R. D. Chambers, W. K. R. Musgraue, J. G. Thorpe, J. Fluorine
Chem. I. 51 (1971).
20 1 ?.l
0
0;:
0.01
,,
10
__ 0.02
mH.Br
Xt4.sr
Fig. 1. AG+ and AV* of the amide rotation in aqueous solutions of dimethylacetamide (DMA), xDMA=O.012 ( - 5 % w/w DMA), as function of the sodium
bromide concentration. (In neat DMA. AV* =7 6 cm' mol- I . )
Influence of Ionic and Polar Solutes upon AG+
and AV% of the Amide Rotation in Aqueous
Solutions of Dimethylacetamidel**]
By Gerhard Volkel, Johann Hauer, and Hans-Dietrich
Liidemann[*'
Dedicated to Professor Ernst Ulrich Franck on the occasion
of his 60th birthday
The free activation energy for the rotation of the dimethylamino group in dimethylamides is altered strongly by variation of the substituent at the carbonyl group. Smaller but sig0.1 0.5
O,!
0
I*]Prof. Dr. H:D.
Liidemann, Dr. G. Volkel, Dip1.-Phys. J. Hauer
lnsritut fur Biophysik und Physikalische Biochemie der Universitat
Universilatsstrasse 31, D-8400 Regensburg (Germany)
[**I
This work was supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie.
Angew. Chem. In:. Ed. Engl. 19 (1980)No. I 1
',
2
1
1,
I
'
3
(
I
' ,
,4
m(NH2)2C0
-
0.01 0.02 0.03 0.04 0.05 0.06 0.07 x
(
~
Fig. 2. A c t - and AV* of the amide rotation in aqueous solutions of dimethylacetamide (DMA), xDMA= 0.012 ( x 5% w/w DMA), as function of the urea concentration. (In neat DMA. A V + =7.6 cm' mol- '.)
0 Verlag Chemie, GmbH, 6940 Weinheim, 1980
OS70-0833/8O/l111-0945 $02.50/0
945
~
~
concentrations of sodium bromide do increase AV* significantly. 5% w/w of dimethylacetamide dissolved in a 0.14 mola1 solution of NaBr yields an activation volume of
AT/* = 5.6 cm3 mol-I, which is fairly close to the results
found in the common solvents.,It must be concluded that the
average distribution OF the water molecules in the immediate
vicinity of the dimethylamino group is drastically altered,
and that the unique features of the binary aqueous solutions
are destroyed almost completely by addition of small
amounts of ionic solutes. The changes in AV* caused by
comparable amounts of urea are smaller than the effects observed with NaBr. This might be explained qualitatively by
the different influence of the urea molecules, which are incorporated into the hydrogen bond network of water and
thus cause only local disturbance of the average water structure, while the electric charges of the sodium and bromide
ions will cause longer-range changes in the water structure.
Our results show that the hydrophobic solvation of the
apolar moieties of the dissolved molecules is significantly influenced by relatively small additions of ions, and indicate
that the determination of AV* for the rotation of the dimethylamino group is a sensitive probe for structural changes of
the hydrate structure.
The steepest increase of A V* is found in sodium bromide
solutions with an ion/DMA ratio smaller than 1:6. It is thus
unlikely that the effects observed can be explained by a preferential solvation of the ions by the DMA molecules. This
assumption is further corroborated by the variation of X D M ~
at constant x N ~ B ~At
. x D M A = O . 0 1 2 and ~ ~ ~ ~ , = 0 . a0value
003
of A V* = 3.8 cm3 mol - is found. A V* remains essentially
unchanged on lowering the DMA concentration to
XDMA = 0.0024.
AG* (cf. Fig. 1 and 2) is not influenced by the addition of
NaBr or urea. This shows that the determination of A V* by
high pressure studies can yield information about solvent
structure which could not be obtained from the ambient
pressure activation parameters AG*, A H * , and AS*.
Experimental
A V* was derived from the pressure dependence of the ‘HNMR signals of the N-CH3 groups between 0.1 and 150
MPa. The rate of rotation was determined by computer simulation of the exchange broadened spectra in the region of
the coalescence temperature ( Tc: 370-380 K). Observation
frequency: 100.1 MHz. Details of the high pressure apparatus and the simulation procedure have been published previ0usly[~.~1.
Received: December 4. 1979 [Z 620 IEj
German version: Angew. Chem. 92,955 (1980)
Publication delayed at authors’ request
[ I ] L. M . Jackman in L. M. Jackman,
i? A . Cotton: Dynamic Nuclear Magnetic
Resonance Spectroscopy. Academic Press, New York 1975.
121 R. Rauchschwalbe. G. Volkel, E. Long, H.-D. Ludemann, J . Chem. Res. ( S )
448, (M) 5325 (1978).
[3] G. Volkel, E. Long. H:D. Liidemann, Ber. Bunsenges. Phys. Chem. 83. 122
(1979).
BOOK R E V I E W S
Physical Chemistry of High Temperature Technology. By E.
T. Turkdogan. Academic Press, New York 1980. xv, 447
pp., bound, $. 49.50.
The author of this book has been active in applied research in the steel industry For over 30 years and was also involved in some basic work through numerous investigations.
Here he gives a comprehensive account of the fundamental
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generally of little practical value. The present book confines
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of solid and liquid chalcogenides, carbonates, and silicates,
after which, in the second part, it shows examples of their application (mainly in the field of ferrous metallurgy). To this
extent, the phrase “high temperature technology” as used in
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The thermodynamics of metals, alloys, oxides, and silicates take up much of the first part, and in addition transport
phenomena in particular are treated very exhaustively. In the
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kinetics of individual processes (e.g. calcination, the blastfurnace process, steel production). 900 references indicate
original papers and compilations of data (not only Amencan) giving further information. The book constitutes a welcome supplementation of Darken and Gurry’s monograph; it
is an introduction into an industrially particularly important
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principles.
Lothar Riekert [NB 520 IE]
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