COMMUNICATIONS TO THE EDITOR
1334
terial. Powder diffraction patterns of both materials are identical. We therefore conclude that barium ion exchange of Linde 4-A does not cause the crystals to become amorphous. It is the very calcination required to activate the zeolite for sorption which causes the Ba-A crystals to collapse and be amorphous to X-rays. This structure collapse is reflected in the lack of water sorption capacity shown in Table I1 for Ba-A. The loss in stability of the dehydrated Linde A structure in the barium form is probably due to the large internuclear distances which must be expected between the barium ion and the oxygen atoms of the lattice and concomitant large charge separation. Previous reports of the decomposition of Linde A caused by barium exchange1v2 must have been based on X-ray studies of calcined samples. Although calcination does destroy the crystal lattice, any ion-exchange studies involving Linde 4-A and BaClz a t moderate temperatures in aqueous solution will be valid because the system is reversible.
CH2-CH2
I
CH2
\/ 0
I
CH2 -% CH2---CH2*
\ /
+ CH20
(5)
CH2
A Hanovia ,3500 mercury full arc was used as a light source. Since tetrahydrofuran absorbs3 strongly in the region 2100-1800 A, the effective wavelength was in the 2100-1900-A region. Products were analyzed gas chromatographically on a 25-ft silver nitrateethylene glycol (25% by weight on Chromosorb) column. Unless otherwise stabilized by collision, the excited cyclopropane formed in reaction 5 will isomerize to propene. The result of a series of runs a t different pressure and a t 120" are shown in Figure 1. Included in the same figure are the results obtained by Frey and Kistiakowsky in the photolysis of the keteneethylene system,2 where the fractional yield of cyclopropane was plotted against the initial pressure of the reactant . RESEARCH DEPARTMENT HOWARD S. SHERRY From the thermochemical data,4 reaction 5 is 28 SOCONY MOBILOIL COMPANY, INC. kcal/mole endothermic. Since energy absorbed in PAULSBORO, NEWJERSEY the system is of the order of 143 kcal/mole, an excess RECEIVED JANUARY 20, 1966 energy of -115 kcal/mole is available to be carried over to the products as vibrational energy. The data given in Figure 1 undoubtedly demonstrate this. Two notable features are obvious from the data. First, the highest pressure required to stabilize all of the On the Reaction of Vibrationally cyclopropane is much less than that required in the Excited Cyclopropane' ketene-ethylene system. Second, the fractional yield at high pressure (0.72 t o be compared with 0.60 obSir: The photolysis of ketene in the presence of an tained by Frey and Kistiakowsky) is less than unity. excess of ethylene is known to produce vibrationally This might imply the formation of additional propene excited cyclopropane.2 I n such a system the vibraby secondary reaction. If it is assumed that the degradation of vibrational hv CH,=C=O CH2: CO (1) energy of excited cyclopropane occurs on every collision, then the rate constant for the isomerization step (3) CH,: C2H, (2) is given by
+
-
-
+ A
tional energy-rich cyclopropane undergoes isomerization to propene or collisionally deenergizes to normal cyclopropane depending upon the total pressure of the system. Thus far very little is known about the forma-
tion of vibrationally excited cyclopropane by direct molecular split in a photochemical reaction. We wish to report that one of the primary processes in the photolYsis Of tetrahydrofuran bads to the formation of excited cyclopropane. The Journal of Physical Chemistry
where Pt = yield of propene a t pressure i, Pa = yield of propene a t high pressure, A, = yield of cyclopropane a t pressure i, and /3 = collisional rate constant. At 120" the average value for /ciao = 6.01 X lo7 sec-'. This value is to be compared wit'h 1.11 X 1O'O sec-' (1) Supported, in part, by the U. S.Atomic Energy Commission. (2) H . M. Frey and G. B. Kistiakowsky, J . Am. Chem. Soc., 79, 6373 (1957); H. M. Frey, Proc. Roy. SOC. (London), A251, 575 (1959). (3) L. W, Pickett, et al., J. Am. Chem. SOC.,73,4865 (1951). (4) J. D. Cox, Tetrahedron, 18, 1337 (1962).
COMMUNICATIONS TO THE EDITOR
IO-
C H2+ CH2=C H2-A*--
1335
as that produced by the chemical reaction of methylene with ethylene.
C H3 CH = CH2
08 -
-
06 -
Acknowledgment. The author is thankful to Dr. K. 0. Kutschke for his interest in this work.
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PRESENT STUDY
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B. C. ROQUITTE
DIVISIONOF PURECHEMISTRY NATIONAL RESEARCH COUNCIL OTTAWA,CANADA
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RADIATION RESEAXCH LABORATORIES MELLON INSTITUTE PITTSBURGH, PENNSYLVANIA
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RECEIVED JANUARY 31, 1966
I
Cation Exclusion from Gels1 In his comment on an earlier note of Tien,2a Maatman2b concludes that there “. . . seems to be no need to postulate. . . ion exclusion from small pores in alkali metal cation-silica gel systems,” and he cites as evidence the invasion of even the smallest pores of silica gels by the large hydrated A13+ ion. However, the argument may be spurious, for the mechanism of ion exclusion probably does not entail simply the mismatch of ion and pore sizes. The structure of liquid water is different near a gas-water, liquid-mater, or solid-water interface than in the bulk water. The water region near the interface will thus incorporate or exclude ion species relative to the bulk solution in accord with the effect of these species on water structure. For example, the surface electrical conductivity of aqueous electrolytic solutions is less than the bulk conductivity due to enhanced water structure and/or ion e x c l u ~ i o n . ~If~there ~ are charge sites on the surfaces of the pores, the situation will be further complicated by ion exclusion arising from the electric double layer.5 I n a system of great surface area, such as a gel, where the amount of water near a surface is comparable to the amount distant from a surface, the effects of interface-produced perturbations of the concentration of ionic constituents should become apSir:
obtained by Frey and Kistiakowsky in their system. I n this system the average lifetime of the vibrationally excited cyclopropane is of the order of sec. This lifetime is about three orders of magnitude longer than that obtained by Frey and Kistiakowsky in their system. However, our value is similar to the average lifetime of a reacting molecule in the thermal isomerization of cyclopropane5a t 445”. An estimate of the energy content of the vibrationally excited cyclopropane may be obtained from the Rice, Ramsperger, and KasseP equation for unimolecular reaction where the rat.e constant is given by kis0 =
[E, -E,Eaot]’-’A
Substituting kiso = 6.01 X lo7 sec-I, S = 13, Eaot= 65 kcal, and A = sec-l into the above equation, the energy of the excited cyclopropane molecule was found to be E, = 86 kcal. This value is much lower than a recently estimated7 value of 103 kcal/mole for methylene the photochemical system ethylene (from ketene). The lower energy content of cyclopropane in the present system is indicative of the fact that energy stabilization by collision occurs a t a much lower pressure (Figure 1) than that required in the methylene :znd ethylene system, The longer lifetime is also another indication of lower energy content’. In summary it therefore, that photolysis of tetrahydrofuran yields energy-rich propane but in an energy state which is not as “hot”
+
(1) This work was supported in part by the Officeof Naval Research.
~ ~ n f ~ b ~ : , ~ 9 ~ ~ ; ~ ( ~ ~ ~ ~ ; w’ Chem.3699350 (1965); (b) R *
Maat-
(3) S. Bordi, Ann. Chim. (Rome), 48, 327 (1958). (4) S. Bordi and F. Vannel, ibid., 54,710 (1964).
(5) L. Dresner, J . P ~ U SChem., . 69,2230 (1965).
Volume 70, Number 4
April 1966
