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Nuclear Resonance Studies of Molecular Movements and Phase Changes in Solid Organic Substances.

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The hydrogen is eliminated as a proton in condensations with
stannyl compounds R3Snse-X8@, where advantageously
X = -N(CzH5)2.
The radical decomposition of diacyl peroxides is extremely
powerfully induced by stannyl radicals. As shown by 1 8 0
labeling, they attack the 0-0 bridge, the more readily
the more the bridge is deprived of electrons by the influence
of substituents. Radical decomposition of numerous azo compounds, R-N-N-X-Ror
R-X-N=N-X-R,
is also powerfully induced, e. 4. :
Here again electron deficiency on
is important, In every
case investigated the mechanism is synchronous (SR2).
Now experimental difficulties have been overcome, the organic chemistry of lead is developing rapidly [21. On the basis
of the observation that negative substituents, X, on lead can
be readily replaced 121, sensitive organolead hydrides are n o
longer prepared in the pure state but are formed in situ by the
following equilibria:
R3Pb-CH2-CH=CH2+
(R3Pb)zOf 2 R3'SnH
R3'SnH 2 R,'Sn-CH2-CH=CH2
R3PbH
+
e (R3'Sn)20+
2 R3PbH
They are valuable for specific hydrogenations and are useful
for additions to C = C , C=C, SCN-, and OCN-groups (with
conjugated systems 1,4-addition is possible), and also as catalysts for polyadditions.
N,N-Dialkylplumbylamines (e.g., R3Pb-N(CzH&) can also
undergo numerous 1,2- or 1,4-additions, e.g., t o isocyanates,
nitriles, isocyanides, aldehydes, or carbodiimides. In this way
- 0 - P b - or new -N-Pb--- groups are formed and further
syntheses become readily accessible.
Lecture at Hamburg (Germany) on June 30 1967 [VB 97 IEI
German version: Angew. Chem. 80, 48 (1968)
[*I Prof. W. P. Neumann
Institut fur Organische Chemie der Universitiit
63 Giessen, Ludwigstr. 21 (Germany)
[l] For a recent review see W. P. Neumann: Die Organische
Chemie des Zinns. Enke Verlag, Stuttgart 1967.
[2] W . P. Neumann and K . Kiihlein, Lecture at the 2nd International Symposium on Organometallic Chemistry, Madison,
Wisconsin, U.S.A., Sept. 3, 1965, partly reported in Angew.
Chem. 77, 808 (1965); Angew. Chem. internat. Edit. 4, 784
(1965); Tetrahedron Letters 1966, 341 5, 3419, 3422.
Nuclear Resonance Studies of Molecular
Movements and Phase Changes in Solid Organic
Substances
By W . Miiller- Warmuth [*I
Toluene is an example of a system with several solid phases.
Its properties can be continuously varied, by careful supercooling, over a wide range from the mobile t o the highly
viscous glassy phase. A crystalline phase also exists. Transition of the phases into one another can be followed by the
temperature-dependence of the spin-lattice relaxation and
by the second moments of the nuclear resonance curves.
Hindered rotation of the methyl group was studied for a
number of solid e t h e r s W Many ethers are crystalline, some
are glassy, and some exist in several phases side by side, depending on the thermal treatment. The marked narrowing of
the 1H-NMR lines that is observed o n phase transition from
the glassy t o the crystalline state well below the melting point
Angew. Chem. internat. Edit. 1 Vol. 7 (1968) 1 No. 1
can be interpreted as isotropic movements of whole molecules.
Although rotation of the methyl groups normally leads t o
line narrowing and maxima in the relaxation rates only at
elevated temperatures (usually above 77 OK), the second
moment for all three xylenes was lower by a factor of 2 than
that expected for a rigid lattice, even at very low temperatures
(2 "K) 121. Relaxation studies gave a similar surprising
result[21. With a working frequency of about 14 MHz,
maxima in the spin-lattice relaxation occurred a t 77 "K
(0-xylene), 51 "K (p-xylene), and 16 "K (m-xylene). The
values for the relaxation decreased only very slowly with
decreasing temperatures. This behavior can be explained as
due to reorientation processes of the methyl group by a
tunnel effect. This mechanism is effective even at very low
temperatures for the torsion ground state. In the relaxation,
restrictions t o energy transfer between various degrees of
freedom in the organic solid can be noticed. A cosine law
with two terms for the contributions with six-fold and threefold symmetry
as potential function, The potential
barriers lie between o,2 and 2,3 kcal,mole.
27* 1967
[VB 102 IE]
Lecture at Kie', Germany, O n
German version: Angew. Chem. 80, 85 11968)
-~
['I Prof. W. Miiller-Warmuth
Magnetic Resonance Laboratory EURATOM CCR
Ispra (Italy)
[l] K. Grude, J . Haupt, and 19: Miiller- Warmuth, Z. Naturforsch.
Zla, 1232 (1966).
[2] J. H a u p f and W . MiiNer- Wormuth, unpublished.
Investigation of Proton-Transfer Processes by
Electrochemical Relaxation Techniques
By H . W . Niirnberg 1*I
The dissociation and recombination of carboxylic acids in
aqueous solution is an important example of proton-transfer
processes that are of significance in many chemical and biochemical reactions. The rate constants of these very fast
reactions (-ry2 M 10-6-10-8 sec) can, for example, be determined by suitable voltammetric techniques when the equilibrium
HA
fd.\
kr
H @ + AO
is displaced by means of the coupled electrode process of
hydrogen evolution at mercury
H a + e<>
+
' I 2 H2
and the velocity is measured via the current passing the electrode or a component of this current.
The rate constants kd and kr can be determined from the
limiting current values that are obtained from the resulting
current-voltage curve (polarogram) provided that the current
is measured so fast (tl M 30-1 psec) that it is not merely the
rate of diffusion of the reaction components that determines
the value of the current. The method of High Level Faradaic
Rectification 111 enabled satisfactory measurements to be
made for the first time in the p.ec range and thus made
possible the determination of kinetic data for 17 structurally
different carboxylic acids [21. The results are corrected for the
influence of double layer effects at the electroder31 and are
therefore the true values in 1 M lithium halide solution.
On the basis of the results obtained, the following conclusions
may be drawn: In many cases the carboxylic acids behave
normally, i.e. recombination is diffusion controlled in each
case with the same k, value, and k d is proportional to the
equilibrium constant Kc. The order of the values ofkd and Kc
is in general agreement with the extent of inductive and
mesomeric effects arising from the structure of the respective
acid.
79
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