Trapping of Atomic Hydrogen in Si8O12-Cages
J. Phys. Chem. A, Vol. 101, No. 44, 1997 8365
(4) Hall, J. L.; Schumacher, R. T. Phys. ReV. 1962, 127, 1892.
(5) For RdH, see: Agaskar, P. A. Inorg. Chem. 1991, 30, 2707. For
R ) Me, Et, Pr, see: Olsson, K. Ark. Kemi 1958, 13, 367. For RdCyx,
see: Barry, A. J.; Daudt, W. H.; Domicone, J. J.; Gilkey, J. J. Am. Chem.
Soc. 1964, 86, 1120. For RdVi, see: Voronkov, M. G.; Martynova, T. N.;
Mirskov, R. G.; Bely, V. I. Zh. Obshch. Khim. 1979, 49, 1522.
TABLE 4: Experimental Conditions, Applied for the
a
Irradiation of Cyclohexane Solutions
signals
due to Htr
•
no.
experimental conditions
1
2
300 mg of EtT
300 mg of EtT
He-purged
8
, 3 mL of c-C
, 3 mL of c-C
6
H
12, air
,
not detectable
not detectable
H
6 12
(6) El-Abbady, A. M.; Anderson, L. C. J. Am. Chem. Soc. 1958, 80,
8
1
737.
(7) (a) Hoebbel, D.; Wieker, W. Z. Anorg. Allg. Chem. 1971, 384, 43.
b) Agaskar, P. A. Inorg. Chem. 1990, 29, 1603 (starting from Si(OMe)4,
this procedure avoids separation of crystalline D4R-silicate).
(8) (a) B u¨ rgi, H.-B.; Calzaferri, G. HelV. Chim. Acta 1990, 73, 698.
b) Calzaferri, G. Nachr. Chem. Tech. Lab. 1992, 40, 1106.
3
300 mg of EtT
He-purged, 25 mg of I
8
, 3 mL of c-C
H
6 12
,
intense signal
(
2
4
5
170 mg of H:PrT
170 mg of H:PrT
He-purged
8
, 2 mL of c-C
8
6
H
12, air no longer detectable
, 2 mL of c-C
H
6 12
,
no longer detectable
(
6
170 mg of H:PrT
He-purged, 25 mg of I
8
, 2 mL of c-C
H
6 12
,
intense signal
(9) (a) Hassler, K. Monatsh. Chemie 1984, 115, 713. (b) Uhlig, W. In
Organosilicon Chemistry. From Molecules to Materials; Auner, N., Weis,
J., Eds.; VCH: New York, 1994; p 21. (c) Agaskar, P. A. Synth. React.
Inorg. Met.-Org. Chem. 1990, 20, 483.
2
a
γ-dose: 120 kGy, Room Temperature.
(10) Feher, F. J.; Budzichowski, T. A. Organometallics 1991, 10, 812.
variety of other systems: Whereas hydrogen atoms trapped in
(11) Greenwood, N. N.; Earnshaw, A. Chemie der Elemente, 1st ed.;
VCH: New York, 1988; p 1035.
(12) Estimated as arithmetical mean from the corresponding atomic
values for proper hybridization. For source of data, see: Huheey, J. E.
Anorganische Chemie; Walter de Gruyter: Berlin, 1988; p 155 ff.
glassy or amorphous matrices show a more complex time-
temperature behavior,20 the uniform traps in the present case
give rise to simple first-order kinetics. At this time nothing
can be said about the fate of the hydrogen atoms thermally
removed from the cages.
(13) Podbereskaya, N. V.; Magarill, S. A.; Baidina, I.A.; Borisov, S.
V.; Gorsh, L. E.; Kanev, A. N.; Martynova, T. N. Zh. Strukt. Khim. 1982,
2
1
6
3, 120. Bieniok, A. M.; B u¨ rgi, H.-B. J. Phys. Chem. 1994, 98, 10735.
14) For example, see: Zeldes, H.; Livingston, R. Phys. ReV. 1954, 96,
702. Hase, H.; Miyatake, Y. Chem. Phys. Lett. 1995, 245, 95.
15) Trammell, G. T.; Zeldes, H.; Livingston, R. Phys. ReV. 1958, 110,
30. Bales, B. L.; Lesin, E. S. J. Chem. Phys. 1976, 65, 1299. Poole, C. P.;
The value of the activation energy determined for this process
and some more general aspects make the following hypothesis
reasonable: In order to be suitable for getting trapped, the kinetic
energy of hydrogen atoms has to fit an “energetic window”.
This means, at one hand, that they have to possess enough
energy to overcome the energetic barrier to enter the cage and
on the other hand their energy should not considerably exceed
the activation energy mentioned above.
(
(
Farach, H. A. J. Magn. Reson. 1976, 5, 305
(16) Foner, S. N.; Cochran, E. L.; Bowers, V. A.; Jen, C. K. J. Chem.
Phys. 1960, 32, 963.
(17) Morton, J. R.; Preston, K. F.; Strach, S. J.; Adrian, F. J.; Jette, A.
N. J. Chem. Phys. 1979, 70, 2889.
From the preceding facts the following conclusions may be
drawn: 1. There are both positive and negative contributions
to the finally detected concentration of trapped hydrogen atoms
(
(
(
18) Shevtsov, V.; Masaki, N. Chem. Phys. Lett. 1995, 244, 188.
19) Adrian, F. J. Phys. ReV. 1960, 32, 972.
20) Plonka, A. Magn. Reson. ReV. 1990, 15, 83. Blumen, A.; Klafter,
(i.e., it is a net effect). 2. Both the presence of radical
J.; Zumofen, G. Phys. ReV. B 1983, 27, 3429. Verdi, L.; Miotello, A. J.
Phys. IV 1992, 2, C2-231.
scavengers and a lower irradiation temperature favor processes,
which are responsible for filling the cages.
The way radical scavengers favor the filling is not yet fully
understood. Possibly, the following points might be of impor-
tance:
(
21) Millar, D. M. Thesis, Dissertation Abstracts, 1988/89, B, 49; p
08-B, order no. DA8803141.
22) To avoid the voluminous IUPAC names (e.g., 1,3,5,7,9,11,13,15-
1
(
3,9 5,15 7,13
octapropylpentacyclo-[9.5.1.1 .1 .1 ]octasiloxane) of these compounds
the following designations were used, derived partly from those used in
silicone chemistry: H, Me, Et, Pr, Cyx, Vi, and Ph stand for hydrogen,
methyl, ethyl, propyl, cyclohexyl, ethenyl, and phenyl, respectively. T8
stands for the cube-shaped Si8O12 unit. Q8 denotes [Si8O12]O8/2, and M
means (CH ) SiO1/2. So the example mentioned above simply writes as PrT8.
1. Selective scavenging of radicals which are capable of
recombining with hydrogen atoms; the selectivity could be
brought about by energy dependent reaction cross-sections. 2.
selective scavenging of reactive (hot) particles capable of
emptying “filled” cages.
3
3
(23) This was accomplished by thoroughly heating the upper half of
the sealed ampule/ESR sample tube by means of a microburner while
keeping the lower half of the tube and thereby the sample itself immersed
in liquid nitrogen. After that, the tube was totally immersed in the liquid
nitrogen, turned upside down and subsequently transferred to the spec-
trometer. The radicals in the sample itself are not affected by this procedure.
Acknowledgment. Prof. M. Meisel of the Humboldt Uni-
versity is acknowledged for giving access to his labs, where
the preparative work was done. We wish to thank D. Gassen
and Dr. E. Janata of the Hahn-Meitner-Institut Berlin GmbH
for their helpfulness and technical assistance in performing the
γ-irradiations. Acknowledgments are due to Dr. V. Seefeld,
ACA Berlin, Germany, for providing tetramethylammonium-
D4R-silicate. M.P. wants to thank Dr. D. Pfeifer, BAM Berlin,
for encouraging discussions.
•
•
(24) Preliminary experiments using H :Q8M8 as well as H :PrT8 have
shown the adsorption equilibrium with oxygen to be reached within a few
seconds. The (completely reversible) response to oxygen depends on the
substituents attached to the silicon atoms of the Si8O12-cage. While the
•
signals due to H tr in MeT8 and HT8 do not respond to changes in atmosphere
(
Ar f O2), they do increase considerably in all other compounds covered
•
so far. For the case of H :Q8M8 see ref 2.
(25) The successful encapsulation in this case is indicated by the
additional splitting of the H-hyperfine lines in the irradiated solid due to
superhyperfine interaction with 31P: a31P ≈ 2.33 mT.
References and Notes
(26) All substances changed color upon addition of solid iodine.
Therefore, due to adsorption, the actual concentration of I2 in/at the solid
is likely to be much higher than the concentration of the gaseous scavengers.
While the RT -type compounds turned more or less pink/red, Q M turned
(
1) For example, see: Bornhauser, P.; Calzaferri, G. J. Phys. Chem.
1
996, 100, 2035. Earley, C. W. J. Phys. Chem. 1994, 98, 8693. Feher, F.
8
8
8
J.; Budzichowski, T. A. Polyhedron 1995, 14, 3239. Sellinger, A.; Laine,
R. M.; Chu, V.; Viney, C. J. Polym. Sci. A 1994, 32, 3069. Mehl, G. H.;
Goodby, J. W. Angew. Chem. 1996, 108, 2791. Murugavel, M.; Voigt, A.;
Walawalkar, M. G.; Roesky, H. W. Chem. ReV. (Washington, D.C.) 1996,
yellowish brown. There is indication for iodine penetrating the bulk of the
substance in case R * H, D, Me.
(27) On/2 indicates n adjacent bridging oxygen atoms.
(
28) Accomplished by nonlinear-least-squares fitting of mixed Gauss-
9
6, 2205.
Lorentz profiles and integration of the latter using the program package
(
2) Sasamori, R.; Okaue, Y.; Isobe, T.; Matsuda, Y. Science 1994, 265,
isotrop.
1
691.
(
29) Latest results obtained by γ-irradiation of EtT8/C6D12/I2 provide
(
3) For example, see: Duval, E.; Serughetti, J.; Louat, R. Solid-state
1
•
convincing proof for the incorporation of solvent derived as well as
substituent-derived hydrogen (deuterium) atoms.
Comm. 1970, 8, 1155 ( H in lithium fluoride) Kazumata, Y. J. Phys. Soc.
Jpn. 1973, 35, 1442 ( H in neutron-irradiated lithium fluoride) Ogoh, K.;
Takaki, S.; Yamanaka, C.; Ikeya, M.; Ito, E. J. Phys. Soc. Jpn. 1996, 65,
3
•
(30) This statement does not exclude the involvement of intramolecular
processes, although they are unlikely.
1
•
8
44 (most recent example, H in coesite and stishovite at room temperature).