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Optical Activity of Oriented Molecules.

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[6] R . P. Barinsk!'. Zh. Strukt. Khim. I . 200(1960).
[7] H . Binder and Ch. Elschunbroich. Angew. Chem. 85, 665 (1973): Angew.
Chem. internat. Edit. 12, 659 (1973).
[S] F. 7: Delbeke. E. G. Claeys, G. P. Van der Kelen, and Z. Erckhaut.
J. Organometal. Chem. 25. 213 (1970).
[9] Ch. Elschmbroich, J. Organometal. Chem. 22, 677 (1970).
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Optical Activity of Oriented Molecules[*'1
-1
-2
By Hans-Georg Kuball and Ties Karstens"'
A C
Measurement of the optical rotatory dispersion (ORD) and
the circular dichroism (CD) of isotropic solutions has proved
to be valuable in structural elucidation. Since it follows from
the theory"' of optical activity that O R D and C D depend
upon the direction of radiation of the light in the molecule,
investigations on oriented molecules should yield additional
structural information. So far, this effect has only been detected
in flowing media or in electrical fields with oriented molecules
of high polymers[21.
Small molecules cannot be oriented suficiently by these
methods'3, 4! However, a very high degree of orientation can
be achieved by their inclusion in liquid crystals-especially
in nematic phases-and
orientation of the nematic phase
in electrical or magnetic fields. Incorporated optically active
molecules convert nematic phases into cholesteric phases['],
and the optical activity of the solution is then determined
by the liquid crystal and no longer by the included molec ~ l e '4~j (induced
.
optical activity).
However, this optical activity induced in solution can be
compensated by a further optically active component. For
this purpose a solvent mixture of two compounds is used
which on their own give cholesteric phases of opposite helicity.
At a certain temperature (T,,,,,),this mixture, which normally
forms a cholesteric phase, behaves as a nematic phasec6](compensated nematic phase). T,,, is a function of the mixing
ratio (in present case cholesteryl chloride and cholesteryl laurate in weight ratio 1.8: 1 ; Tn,,~36"C, depending upon additive). If the optically active substance is dissolved then the
nematic phase is transformed into a cholesteric phase owing
to the induced optical activity. A change in temperature to
TAemcompensates this effect, i. e. the nematic phase is restored
(ITne,,,-TAe,,,1~ 3 ~ 4 ° CAfter
) . orientation of the molecules of
this nematic phase in an electrical field (20-30 kvjcm) the
optical activity can be measured with a light ray that propagates along the optical axis['?
Investigations were performed on the steroids 4-cholesten-3one ( I ) and 3P-acetoxy-5-cholesten-7-one
(2) in the nematic
phase at TAemand at a temperature (T=8O0C) at which the
mixture is isotropic (Figs. 1 and 2). From the degree of polarization P#O (Figs. l a , 2a) it follows that the molecules in the
liquid crystal d o not display an isotropic distribution. Since
P is larger for ( I ) than for (2), either the orientation must
be greater for ( I ) than for (2) or, as is more likely, the
direction of the transition moment describes a smaller angle
with respect to the orientation axis in the case of ( I ) than
in the case of (2). The orientation axis is the longest molecular
axis, since in this case the dissolved molecules are included
in the liquid crystals in similar manner to the molecules of
the liquid crystals themselves.
['I
Prof. Dr. H.-G. Kuball and Dr. T. Karstens
Fachbereich Chemie der Universitat
675 Kaiserslautern, Postfach 3049 (Germany)
[**I Optical Activity of Oriented Molecules, Part 1. This work was supported
by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie.
176
E
___ --AEfield
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AE
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30
0.5
15
-0.5
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-1.5
I
,
,
25
30
I
35.1O3[cm.
Fig. 1. 4-Cholesten-3-one ( I ), dissolved in cholesteryl chloride/cholesteryl
/ ( E I IEII and
laurate (cf. text).-a) Degree of polarization P = ( E I I - E ~ )+EL);
E L are the molar decadic absorption coefficients of the sample for light
polarized parallel and perpendicular, respectively, to the optical axis. grlrld
=AE~~.,,JE~~.,~
and g,,=Asi,Jq,,
are the dissymmetry factors.-b) Circular
dichroism
and molar decadic absorption coefficient & M d = '/A (&[I+ 2 ~ 1 ) .
measured at Tnem-c) Circular dichroism A E , , ~and A& and molar decadic
absorption coefficient
and E, measured in the mixture in the isotropic
state (T=80"C)and in n-heptane ('F=2O"C).
In all cases, the AE and E spectra (Figs. 1 c, 2c) are practically
identical in the isotropic mixture and in n-heptane (T= 20°C),
i.e. the mixture displays no anomalous solvent effects. Moreover, the absorption spectrum
does not deviate significantly from the spectrum in the isotropic medium. In contrast,
the C D is completely different when recorded for the solution
in the field. Whereas the C D signal is merely enhanced for
( 2 ) and its frequency dependence remains the same (Fig.
2b), the sign and the frequency dependence change for ( I )
(Fig. 1 b). This effect can be seen clearly from the dissymmetry
factors (Figs. 1 a, 2a).
The optical activity of ( I ) and (2) is largely determined
Since this group has
by the chromophore C=C--C=O.
approximately mirror symmetry in ( I ) and (Z)[*], the circular
dichroism of isotropic solutions will differ in sign. On the
basis of these assumptions, the difference between Asfield in
(I) and (2) should arise from the differing arrangement of
the chromophore in the molecule. When measurements are
performed in the oriented state, the direction of the light
ray runs parallel to the longest molecular axis. However,
the chromophore is arranged differently relative to the longest
molecular axis in ( I ) and (2), and is therefore traversed
differently by the light ray.
Angen. Chrm. intrrnar. Edit. J Val. 14 ( 1 9 7 5 ) i N o . 3
The resultsdescribed here indicate that measurement of optical
activity on oriented systems yields not only information about
absolute configurations but also provides further structural
P
[7] Owing to theorientation with an electrical field. the phase has a symmetry
axis in the direction of the field (optical axis of system). Measurement
perpendicular to the optical axis is impossible at present because the linear
dichroism is very large. O n filling the cells care should be taken that no
additional linear dichroism arises at the cell windows.
[ 8 ] C . Djrrusci. R . Rinikrr. and B. Rinikur. J . Amer. Chem. S O C . 78. 6377
(1956);H . Z i f e r and C . H . Robinson, Tetrahedron 14, 5803 119681.
01
Isolation of Tetrazene, N&[ll
4
0.5
.10-2
2
1
-1
-2
I
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/ i
I
I
II
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15
By Nils Wiberg, Heiner Bayer, and Heinz Bachhubrr[']
The only hydrogen compounds of nitrogen that have previously been unambiguously characterized are ammonia
(NH,), hydrazine (N2H4), diimine (N2H2)"' and hydrogen
azide (N3H). We have now succeeded in preparing a further
nitrogen hydride, namely tetrazene (N4H4). which, unlike the
previously known compoundscontaininga one-, two- or threemembered nitrogen chain, is based on a skeleton of four
nitrogen atoms.
For synthesis of this compound, tetrakis(trimethylsily1)tetrazenel4]which is readily accessible by acid-catalyzed dimerization of bi~(trimethylsilyl)diimine[~~
is treated with trifluoroacetic acid in methylene dichloride at -78°C. The tetrazene,
being only sparingly soluble in methylene dichloride, is thus
precipitated as a colorless solid; it can be purified by sublimation.
A C
(Me3Si)2N-N=N-N(SibIe,)2
10
+ 1 v3<'-rool~
- P,t-roosi,~HzN-N=N-NH2
E
30
05
15
Fig. 2. 3 P-Acetoxy-5-cholesten-7-one
(Z), dissolved in cholesteryl chloridejcholesteryl laurate (cf. text and legend to Fig. 1).
information, e. g. regarding the arrangement of the chromophore relative to the molecular axis. Theoretical evaluation
and the consequences of applying the sector rules, eft., will
be reported later.
The following facts indicate that the nitrogen hydride isolated
has the structure ofa trans-Ztetrazene: 1 . Quantitative analysis
of the thermolysis products (see below) yields a nitrogenbydrogen ratio of 1 :1, i. e. the nitrogen hydride has the molecular
formula (NH),.-2. The mass spectrum obtained for the isolated compound at an ionization energy of 70eV in the gas
phase at 5 x lo-' torr shows the molecular ion N4H; as an
intense line at m/e=60. If the ionization energy is lowered
from 70 to IOeV, then, as expected, the mass groups of ionic
fragments N,H; with one, two or three nitrogen atoms disappear almost completely and there remains only the signal
of the molecular ion. Thus x in the molecular formula (NH)x
that follows from the analytical investigations must be replaced
by the number 4.-3. By germylation with N,N-diethyl(trimethylgermy1)amine the nitrogen hydride N4H4 can be converted by the reaction (E = Ge)
Received: September 16, 1974 [Z 140 IE]
German version: Angew. Chem. 87, 2M) (1975)
CAS Registry numbers:
( I ) , 601-51-0: (Z), 809-51-8
[I] 1. Tinoco, J r . and W G . Hammerle, J. Phys. Chem. 60, 1619 (1956):
N . Go,J. Chem. Phys. 43, 1275 (1965);J. Phys. SOC. Jap. 23, 88, 1094 (1967);
Y N . Chiu, J . Chem. Phys. 52, 1042 (1970);A. D. Buckinykam and M . B.
Soc. A 1971, 1988.
[2] I . Tinoco, Jr., J. Amer. Chem. SOC. 81, 1540 (1959); R. Mandel and
G. Holiwarrh, J. Chem. Phys. 57, 3469 (1972);Biopolymers 12, 655 (1973);
and references cited therein.
[3] J . K u n z and A. McLean, Nature 136,795 (1935);J . Kunz and A . Babkock,
Phil. Mag. 23, 616 (1936);J . K u n z and R . G. La E m . Nature 140, 194
( 1937).
[4] A . D. Buckinykam, G. P . Caesar, and M. 8. Dunn, Chem. Phys. Lett.
3, 540 ( 1969).
[5] E . Friedel, C. R. Acad. Sci. Paris 1923,475;H . Stegemrier, K . J . Mainusch,
and E . Striyner, Chem. Phys. Lett. 8, 425 (1971).
161 J . M . Pochan and P . F. Erhardr, Phys. Rev. Lett. 27, 790 (1971): E.
Frirdrl, Ann. Phys. Paris 18, 273 (1922); H . Baesslrr and M . M . Labrs,
J. Chem. Phys. 51, 1846 (1969):52, 631 (1970);E. Sackmann, Chem. Phys.
Lett. 3,253 (1969);J. Amer. Chem. SOC. YO, 3569 (1968).
Dunn, J. Chem.
Anyew. Chem. intrrnat. Edit. J Vol. 14 ( 1 9 7 5 1
No. 3
into the derivative, tetrakis(trimethylgermyl)-2-tetrazene
which could be synthesized by an independent routeL5'.N4H4
also reacts analogously with N,N-diethyl(trimethylstanny1)amine (E = Sn). Thence we conclude that N4H4has the structure
2-tetrazene.-4. The synthetic route leading to tetrazene consists of protolysis of tetrakis(trimethylsilyl)-2-tetrazenewhich
according to X-ray structure analysis[6fhas the trans-configuration; and, since a change of configuration during protolysis
is unlikely, the 2-tetrazene prepared in this way probably
also has the trans-configuration['!
Thermolysis of tetrazene
XzN + H2N-NHZ
HZN-N-N-NH,
N=N=N
['I
Prof. Dr. N. Wiberg, Dipl.-Chem. H. Bayer, and Dr. H . Bachhuber
tnstitut fur Anorganische Chemie der UniversitPt
8 Miinchen 2, Meiserstr. I (Germany)
171
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