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X-Ray Structure Analysis of the -Cyclodextrin-Krypton Inclusion Complex A Noble Gas in an Organic Matrix.

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statefh1.The shifts of -5.9ppm (C', C") and f 4 . 6 p p m (C',
C3) for (2) also indicate polarizatjon effects in the rr-system.
Benzofulvene (3)" tl, however, shows better agreement for
C3, C 4 and C'. The diene system of (3) and the heptafulvene
( I ) can thus be regarded as essentially normal conjugated
olefins.
According to the 'H-NMR data[71and the 13C-NMR data
of the five-membered ring, sesquifulvalene ( 4 ) whose dipolar
resonance structure contains an aromatic five-membered and
seven-membered ring does not show a more extensive charge
separation than is found in ( 5 ) . The value for C ' * differs
from the calculated value by only 7.4 ppm, which may indicate that the excess of positive charge found at thecorresponding position in the pentafulvenes is delocalized over several
C atoms in the seven-membered ring of ( 4 ) . In fact the sum
of the shifts of C', C2 and C3 in (4)is greater than in ( I ) .
From the I3C-NMR data it can thus also be concluded that
pentafulvene ( 2 ) , heptafulvene ( I ) and sesquifulvalene ( 4 )
should be regarded as largely olefinic structures. The small
extent of charge separation in these compounds is also shown
by the relatively small dipole moments['21. Formation of an
aromatic seven-membered ring in ( I ) and ( 4 ) requires not
only charge separation but also overcoming of steric strain,
and it seems that the corresponding energies are not sufficiently
compensated by the gain in delocalization energy.
+
Received. May 24, 1974 [Z 56 IE]
Revised' June 24, 1974
German version: Angew Chem. 87.595 (1974)
CAS Registry numbers:
( 1 ) : 539-79-1
( 4 ) : 1961-84-8
(2) : 497-20-1
( 5 ) : 4401-17-6.
( 3 ) 2471-84-3
Cell constants
[A]
a
b
C
[ l ] Fulvenes, Part l3.-Part
28, I I9 (1974).
123
12: M Nnrenschri~ariderand A. Frq,, Chimia
"C-NMR Spectroscopy, Part 6.--Part
horn, Helv. Chim. Acta 56, 2680 (1973).
5 : G . Mullrr and W ron Pkilips-
[3] Cf.,e.g.,K.Hufnrr,Angew.Chem. 75,1041 (1963): Angew. Chem. internat.
Edit. 3, 165 (1964).
[4] M . Niwrtischirander and W K . Sdirnh, Chimia 26, 194 (1972).
[5] H . E . Zimmrrmun and L. R. Sousa. J. Amer. Chem. SOC.94. 834 (1972).
[6] R. Hollensfeiii, W yon Philipshorn, R Vogdi, and M . Neiren.schrwndrr,
Helv. Chim. Acta 56, 847 (1973).
[7] Analysis of the AA'BB' system of the five-membered-ring protons of
sesquifulvalene ( 4 ) (270MH2, ChDh) gives J ? x=5.2Hz: J * . u = 2 2 H 2 ;
J,, =2.2Hz: J,,9 = 1.5 Hz.
[a]
D. J BmreI/i. T G. Andrew, Jr.. and P. 0.Crews, J. Amer. Chem.
SOC.91. 5286 (1969).
[9] G. 5. Sacifsky and K. Namikawo, J. Phys. Chem. 68, 1956 (1964).
[lo] D. E. Dormun, M . Juiifrlaf, and J. D. Roberts, J. Org Chem. 36, 2757
(1971).
[I I ] M . Neucnschwaiider, H . P Fuhmi, H . Lrhmonn, and R I'ogdi, Chimia
28, lI5(i974).
[I21 Pentafulvene (2): 0.42 D [ P . A . Buron, R. D. Brown, F . R. Btrrilen,
P. 1. Domaille and J . E. Kent, J. Mol. Spectroscop. 43. 401 (197211: for
the thermally very unstable heptafulvene ( I ) : ca 1.1 D: sesquifulvalene ( 4 1 .
2.1 D.
552
X-Ray Structure Analysis of the a-Cyclodextrin-Krypton Inclusion Complex: A Noble Gas in an Organic
Matrix[**]
By Wolfram Saenger and Mathias N o l t e m q w [ ' ~
In aqueous solution, a-cyclodextrin (cyclohexaamylose, r-CD),
the smallest of the cyclic oligosaccharides obtained by degradation of starch by Bacillus macerans, forms inclusion complexes
with substances (guest molecules) whose nature extends from
hydrophobic to hydrophilic, e. g. paraffins, aromatic compounds, fatty acids, amines, halogens, and salts". *I. Hydrophobic interactions, hydrogen bridge bonds, van der Waals and
dipole-dipole forces have already been discussed as bonding
forces that might stabilize these inclusion compounds''], but
not all of these need be considered for all types of inclusion
complexes.
X-ray structure analyses 3f the adducts of cr-CD with water"],
iodinef4' and l - p r o p a n ~ l have
[ ~ ~ clarified the nature of the
conformational changes that can be observed spectroscopically
during the formation of the inclusioni7! These show that
the a-CD molecule in the crystalline water adductr3I or in
"aqueous solution" takes up a strained conformation which
is relaxed on formation of an adduct with any guest molecule
other than water.
The structure analysis of the krypton adduct reported below
was designed to explain how an apolar guest behaves that
can use only van der Waals forces apd in its dimensions
(4A) resembles the water molecule (3.8 A). The a-CD-krypton
adduct first prepared by Cramer and Heng1ein161was crystallized at 3 and at 14 atm of krypton pressure (Table 1). 3900
reflexion intensities were measured for both crystals with
a diffractometer, and the structures were solved and refined
to a crystallographic reliability index of 8.6% in both cases.
14
p2 12 $ 2 ,
14.337 (7)
37.402 ( I 3)
9.446 (4)
1.51
C.X~HM,OJO-(0.28
HzO.0.76 Kr).5 H 2 0
14.299 (7)
37489 (15)
9.407 (4)
Table 2. Occupation of the voids in the a-CD molecule in the krypton
adducts studied and in the water adduct. The occupation factor m amounts
t o 1.0 for full occupation of a n atomic position. Positions that have van
der Waals separations and thus can be occupied simultaneously are marked *.
W =oxygen atom of the included water.
~-~
__
Atom
Occupation factor m
4 atm
I5 arm
K r adduct
*0.?8
0.13
0 14
*0.21
-
Water
adduct
0.28
0.07
0.46
0.13
0.08
*1.0
'1.0
W I
w2
The crystal structures of the krypton adducts formed at 3
and at 14atm differ hardly at all in the positions of the
['I
Priv.-Doz Dr. W. Saenger and DiplLChem. M. Noltemeyer
Max-Planck-Institut fur experimentelle Medizin, Abt. Chemie
34 Gottingen, Hermann-Rein-Strasse 3 (Germany)
[**I
This work was partly supported by the Deutsche Forschungsgemeinschaft.
Angew. Chem internat. Edir.
/ Vol. 13
( 1 9 7 4 ) 1 No 8
Fig. 1.Views of the a-CD-krypton adduct formed a) at 3 and b)at 14 atm; c) structure of the a-CD.ZH,O adduct. Hydrogen bonds are denoted by broken
lines, the positions of the oxygen atoms of included water molecules are denoted by W and those of the krypton atoms by KR. The numbering used in the text
for the six glucose residues is shown in Fig. 1 c. O = Oxygen, o=carbon.
a-CD and five molecules of water of crystallization, but they
d o differ in the krypton contents and in the positions that
the noble gas atoms take u p (Table 2). As Figures l a and
1 b show, the a-CD molecule exists in an almost hexagonal
structure, similar to that observed in the isomorphous adducts
with iodine and 1-propanol; the six C(6)-0(6) bonds point
radially “outwards” from the center of the molecule. The
krypton positions are only statistically occupied (Table 2).
The statistical distribution of the krypton over several positions is due to its failure to fill the void in the cyclodextrin
with a diameter of 4-5 A.
For the x - C D . 2 H 2 0adduct (Fig. 1 c) the situation is substantially different: The two included water molecules are fixed
in definite, fully occupied positions by hydrogen bridge bonds
formed to two O(6)-H hydroxy groups that are twisted
“inwards”. Here, unlike the situation for the krypton adduct,
the a-CD molecule fits the relatively small van der Waals
radius of the water molecule (3.8A) because it contracts its
hole by twisting the glucose residues 1 and 5. This conformat i ~ n ‘ ~which
],
the “empty” x-CD molecule assumes also in
aqueous solution before forming an inclusion compound with
a molecule other than water, has an increased energy content.
When a krypton atom is included, a conformational change
Angew. Chrm. internat. Edit.
1 Vol. 13 ( 1 9 7 4 ) 1No. 8
of the a-CD molecule occurs together with a decrease in
itsenergy content, whereby the adduct is stabilized. In addition
hydrophobic interactions may stabilize the x-CD-krypton
adduct, whereas van der Waals forces can hardly be of importance because of the mutually unfavorable space-filling.
Complex formation as a result of energy decrease due to
conformation effects may play a general role in biochemistry,
e. g. in enzyme-substrate interactions[*!
Received: May 9, 1974 [Z 55 IE]
Revised: June 24.1974
German version. Angew. Chem. 86. 594 (1974)
CAS Registry numbers:
a-Cyclodextrin-krypton adduct: 52165-27-2
F. Cramrr and H . Hurrler, Naturwissenschaften 24. 625 ( I 967).
[2] D. W Griffirhsand M. L Bmder. Advan. Catal. 23, 209 (1973).
[I]
[3] P . C. Manor and W Surngvr. J. Amer. Chem SOC. YO, 3630 (1974).
[4] R. K . McMullun, W Saengrr, J . Fuvos, and D. Mootz, Carbohyd. Res.
31, 21 I (1973).
[ 5 ] W Saenger, R. K . McMullun, J . Fuyos, and D. Moor-, Acra Crystallogr.
press.
( 1974).in
[6] F . Cramer and I;. M. H m g l c w i . Chem. Ber. YO. 2561 (1957).
[7] D A . Rrrs, J. Chem. Sac. B IY70, 877.
[ 8 ] D. E . Koshland, Jr., Sci. Amer. 229, N o 10, p. 52 (1973).
553
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matrix, structure, complex, organiz, inclusion, analysis, gas, krypton, cyclodextrin, ray, noble
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