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Hydrogen Bridges with Nitrogen as Donor and Acceptor.

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could be shown by NMR spectroscopy that acidic rearrangement of the diols ( I ) gave a new intermediate X,
which on further treatment with acid was converted into
(2) and finally into ( 3 ) .When the experimental conditions
were chosen suitably, the compounds X could also be isolated in crystalline form [for R=p-CH,C6H4,
R'=p-CH,0C6H4, Z=C(CH,), in CH,OH at -50°C
with 1 equiv. of CF,COOH in 70% yield; m.p. 115'Cl.
NMR spectra (coupling of the P,y protons: 0.7-1 Hz),
IR spectra (C=C vibration at ca. 1635, ether bands at
1080 cm- I), and UV spectra (h,,, =310-330 nm,
~=3300-8000) confirm the oxepin structure (4) of X.
Further structural proof is provided by the I3C Fourier
transform NMR spectra. Photolysis of the oxepins ( 4 )
leads to 2-oxabicyclo[3.2.0]hepta-3,6-dienes. The oxepins
must have been formed by 1,Zmigration of the hydroxyl
group of (A) to give (B) and deprotonation of (B) to the
benzene epoxide (C).
Lecture at Tiibingen on February 12,1971 [VB 291 IE]
German version : Angew. Chem. 83,449 (1971)
atom, causes such great steric hindrance that one of the
other conformers is thermodynamically more favorable.
Experimental findings support this view. Systematic increase in size of the substituents R' in ( 2 a ) (H, CH,, C2H5)
first increases the bathochromic shift of the NH bands,
this being explained by the compression of the bridge;
with iso-C,H, and tert-C,H9, however, there is no longer
any indication of hydrogen bridges, so that for (211) with
substituents of this size a conversion into another conformation is assumed. Increasingly strongly electron-attracting substituents R2 (CH,, C,H,, p-C6H4Cl,p-C6H4N02)
permit closer approach of the donor and acceptor owing
to increase in the N-H bond moment ;this can be recognized by increasing bathochromic shifts of the NH bands. In
the sulfur compounds ( 2 b ) , however, even C6H5 as R2
leads to such an increase in the acidity of the N-H bond
that the proton transfer (1)+(2) no longer occurs, and
exchange reactions take place instead.
Lecture at Hamburg (Germany) on February 16,1971 [VB 292 IE]
German version: Angew. Chem. 83,449 (1971)
[l] G.Schwenker and R . Koib, Tetrahedron, ZS, 5437,5549 (1869).
Hydrogen Bridges with Nitrogen as Donor and
By Gerhard Schwenker"'
The products obtained on reaction of amidines with
heterocumulenes (isocyanates or isothiocyanates) differ
according to the type of substitution of the amidine, the
reactivity of the heterocumulene, and the experimental
conditions"]. Amidines with hydrogen on N-I give dipolar
molecules ( I ) which afford carbamoyl- ( 2 a ) or (thiocarbamoy1)-amidines (2 b ) by proton transfer.
In (2a), but not in (2b), there are strong intramolecular
hydrogen bridges recognizable by the bathochromic shift
of the NH bands to around or below 3000cm-'. The
cause is as follows: In an unperturbed system of five sp2hybridized atoms the donor and the acceptor approach
one another to within about 2.4 in the conformers able
to form intramolecular hydrogen bridges. This requires
an extremely strong, symmetrical bridge, which however is
unlikely. Thus one must assume appreciable widening of
the relevant angle and as a consequence a closer approach
of the peripheral substituents. The range of existence of
these conformers is thus limited by the steric demands of
the peripheral substituents and size of the angle broadening
required, the latter decreasing with increasing polarity of
the N-H bond and thus with increasing bond enthalpy.
In (2 b ) the sulfur atom, which is larger than the oxygen
Improved Determinations of Intermolecular
By Bernhard Schramrn[*]
A knowledge of intermolecular potentials is an essential
basis for statistical calculations of macroscopic properties
of matter. Information about potential energy curves can
be gained, e.g. from the temperature dependence of the
virial coefficients or transport phenomena of gases and
from measurements of the scattering cross-section with
the aid of molecular beams.
Let us assume that the potential energy between two
particles depends only on their mutual separation. It must
be made up of a long-range attractive component (responsible for condensation of gases) and a repulsive component
that is effective at shorter ranges (responsible for separation in crystal lattices). Superposition of the two components gives a potential energy minimum having coordinates
rminand E. The best known expression for potential energy
that contains only these two free parameters is the LennardJones potential :
On suitable choice of rminand E, it can be used for calculation of the second virial coefficient and transport parameters over wide ranges of temperature. However, more
accurate measurements uncover deviations that cannot be
accounted for by experimental error. Moreover, two
different sets of potential parameters are required for
calculations of virial coefficients and transport factors
These drawbacks can be lessened by permitting free
choice of a third parameter such as the power of the
repulsive term. This leads to the n - 6 potentials by way of
the equation :
cp(r)= E/n - 6[6(rmin/~)"
- n(rmin/r)61
[*] Prof. Dr. G. Schwenker
Pharmazeutisch-Chemisches Institut der Universitat
75 Karlsruhe, Kaiserstr. 12 (Germany)
[*I b r . B. Schramm
Physikalisch-chemisches Institut der Universitat
69 Heidelberg 1, Im Neuenheirner Feld 1 (Germany)
Angew. Chem. internat. Edit. / Vol. 10 (1971) 1 No. 6
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hydrogen, bridge, acceptor, nitrogen, donor
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