NOTES
Nov., 1961
2103
stable than the boat, though comparable to the crown form. For the calculation we chose param130°, eters relatigely favorable for stability: 6 a = 1.33 A., b = 1.46 A., and c = 1.50 A. The energy in this configuration was found to be El,chlr
=
8a!
+ 6.5 8’1
39
Conclusions The boat configuration is favored energetically B. C R O W N ( D 4 d ) over the crown and chair configurations by about 3.5 Po139 in these calculations, thus agreeing with Bastiansen’s experimental results favoring the boat configuration. It is interesting to note that although the calculation is intended primarily to give relative results for several possible configurations, the predicted stabilization energy of the boat configuration is comparable to the experimental resonance energy. The energy of four C . CHAIR ( C 2 h ) isolated double bonds would be 8k(1.33)$139 or 9.5P0139, so our calculations predict a stabilization energy of 0.3/P01391, or about 11 kcal./mole, using -37 kcal./mole as a reasonable estimate of Po1 39.697 The experimental resonance energy is 4 kcal. /mole. Fig. 1.-Ground state pi electron energies calculated for It may be possible to estimate the relative stathree configurations of cgclooctatetraene. bility of various configurations of other non-planar Using Rastiansep’s molecular parameters, e = T electron systems in this way. Reliable quantita126.5’’ a = 1.33 A., b = 1.46 A., and Mulliken’s tive calculations, or comparisons of configurations differing widely in C-C-C bond angles would redata for k ( p ) , one obtains quire a more careful treatment which included E,,bost -= 8a: 9.8fia1.39 change in hybridization and an explicit evaluation Crown Configuration (Da).-All eight bonds are of the energy of the u system. equivalent in this configuration (Fig. lB), with a (6) G. Glockler, J Chem P h y s , 21, 1242 (1953); R. S. Muiliken s n d R G Parr,t b d , 19, 1271 (1951) relative twist, 4, given by
+
cos I$ =
‘COS
1
6
+ 2 COS 7r/4 - 1 1+ e
I
COS
Solution of the secular equation gives a ground state energy of = 801
E,mown
For p
=
1.42A., 6
+ 8.668p4
125O, thisgives
=
EI,crown
= 8a
+ 3.3P01.a9
Since ,BO1 39 is negative, this configuration is unstable with reference to the boat configuration by 5.5 f1°1.391. Assuming even the most favorable bond length, that appropriate to benzene, c = 1.39 A., and a relatively flattened molecule, e = 130°, gives E X ~ r o m n=
801
+ 6.W1
39
which is still 3.6 lPo1.39 less stable than the boat form. Chair Configuration (Czh).-The chair configuration has three kinds of bonds (Fig. 1). The four type b bonds are twisted by an angle $ J b given by COS 4 b
=
cotze
just as in the boat configuration. The two type a bonds are untwisted, but the remaining two type c bonds are twisted by an angle cpc given by COS
,
I
= 12Cot2
e-
11
The twist in the c bond is considerable, so that neighboring p orbitals are eclipsed completely in the vicinity of 6 = 125’. A numerical solution of the secular equation involving three different resonance integrals showed that the chair configuration is considerably less
(7) H. D. Springall, T. R. White and R. C. Cass, Trans. Faraday Soe., PO, 815 (1954).
A NEW ME:THOD FOR STUDYING PORE SIZES BY THE USE OF DYE LUMINESCENCE BY JEROME L. ROSENBERG A N D DOVALD J. SHOMBERT Contnbutaon X o . 108P f r o m the Department 0.1’ Chemzstry, Unzverszty of Pzttsburgh, Pzttsburgh 1 3 , Pennsylaanza Recezaed January 10, I Q f i l
I n connection with some recent work on the mechanism of the reaction of oxygen with photoexcited adsorbed dyesY2 we found that the penetration of oxygen through the pores of the adsorbent was rate-limiting in some cases. I t occurred to us that photochemical observations under these conditions might be used to study the pore characteristics of the adsorbent. This note summarizes the experimental basis and outlines the possibilities of application for such a porosimeter. Procedure.-The method of study has been described in detail previously.2 Briefly, a suitable phosphoreecent dye was adsorbed on the porous substance. The principal criteria in the selection of the dye wrre high adsorbability (1) Presented before the 138th National Meeting of the American Chemical Society, New York, September 16, 1960. This work was supported by the National Science Foundation under Grant NSFG6271. The material presented here is abstracted from a dissertation presented t o the University of Pittsburgh by Donald J. Shombert in partial fulfillment of the requirements for the Ph.D. degree in January, 1959. (2) J. L. Rosenberg and D. J. Shombert, J . Am. Chsm. Soc,, 89,
YOTES
2104
t
\
Vol. 65
between the gel granules and flow within the pores of a single granule. Adsorption can be excluded as the rate-limiting process on the groiunds (a) that the amount of oxygen adsorbed on the gel at these temperatures is insufficient t>o account, for the quenching o b ~ e r v e dand , ~ (h) that t,he kinetic evidence, from both steady stat.e and transient experiments, indicates that oxygen quenching is a diffusion-limited process,2 about one in ten collisions being effective in quenching. Flow between the granules is quite fast, and limitations due to this region can be eliminated by using samples with a small number of granules. We found that if the oxygen was added after the flash, the time of rise of chemiluminescence intensit'y depended on the time for passage of gas between the granules and the rise time could be shortened to less than 0.1 second by reducing the number of granules in the sample. The time for decay after the maximum, however, was independent of the number of granules in the sample. Tbercfore the decay time in oxygen is related t,o t,he time of flow of oxygen through the pores of a single granule. (b) Experiments of this kind were performed under identical conditions for two samples that differed only in the average pore size of the gel. In both cases the gel contained 2.5 micromoles of acriflavine per gram, the temperature was -looo, the chemi-luminescence was observed through a 500 mp interference filter, and the oxygen pressure after admission was 0.04 mm. The two gels had the same granule size and equal amounts were used for the two experiments. One gel, a standard Davison preparation, had fairly small internal pores, 30 to 50 A. The other gel, made by sintering of pure silica beads, is knoJvn to have fa,irly large internal pores, about 300 A., and to be free of small pores.4 The final decay of luminescence was much faster in the large pore gel, presumably because oxygen can reach the dye sites more rapidly (Fig. 1). Similar differences in the slopes of decay of the /%phosphorescence were observed for samples that differed in internal pore size. Discussion We propose that these differences in luminescence decay rate might be used to determine the internal pore sizes of adsorbent substances. The method is applicable to any substance which can adsorb a suitable phosphorescent dye, so long as the adsorbent transmits the dye's excitation and luminescent bands. Complications due to dye-gel interaction5have ruled out a complete kinetic analysis of the system. Kevertheless, a single experimental parameter, such as the rat,e constant for the apparently exponent,inl decay portion of t,Ee chemi-luminescence curves in Pig. 1, should be relatable to an
-t 0.8 1.2 1.4 Time, sec. Fig. 1.-Effect of pore size on rate of chemi-luminescence. Concentration, 2.5 pmole acriflavine/g. silica; observation wave length, 500 mp: temperature, - 100"; oxygen pressure at infinite time, 0 04 mm. Arrows denote admission of oxygen. Large- and small-pore gels as described in text. 0.4
0.6
(minimum of 10-8 mole of dye per gram of adsorbent) and a phosphorescence, of convenient mean life (about one second), quenched readily by ouygen. Typical systems are acriflavinesilica gel and fluorescein-alumina. The adsorbate sample was placed on a vacuum line, where it was baked out. Its phosphorescence, excited with a flash lamp, was intercepted by a photomultiplier tube, and the intensity-time curve wae displayed on an oscilloscope. While the phosphorescence still was decaying oxygen was admitted to the sample through a solenoid-operated mercury valve. By a proper choice of optical filters between the sample and the photomultiplier, it was possible to observe either the direct triplet to ground delayed emission of @-phosphorescenceor the chemi-luminescence accompanying the oxygen-dye reaction (Fig. 1 in ref. 2 ) . The former process could be ohserved readily at any tewperature from -160" to room temperatiire; the latter was most conveniently studied a t - 100".
Results The rate of either type of luminescence decay following admission of oxygen was found to depend on the rate of penetration of oxygen through the pores of the adsorbent. This fact was proved by two types of observation on adsorbates of acriflavine on silica gel. (a) Although the phosphorescence a t - 100' persisted for about a second if oxygen a t a pressure of mm. was admitted to the sample within 0.1 second after a flash, the luminescence was completely quenched in less than 10 milliseconds if the same pressure of oxygen was equilibrated with the sample before the flash. The rate-limiting factor in the former case must be the flow of oxygen to the site of dye adsorption. Greater discrepancies existed at higher pressures. Two regions of hindered gas flow can be distinguished in these gels, flow
(3) Oxygen adsorption was not measured under these experimental conditions, h u t estimates were made by extrapolating data from higher pressures as determined for similar samples by W ,D. Urr Chem., 36, 1831 (1932). These extrapolations indicate t h a t the mol? ratio of adsorhed ovvgrn t o dye was IPCS than 0.01. (4) We are indehted t o Dr. W. Keith Hall. of Xlellon Inatitutr. for furnishing this silica sample, described as X-88 a e r o g ~ l1'1 Fig. 5 of W. K. Hall, S. RlacTvf=r and H. P. TVPber. I n d . Eng. Chem.. 63, 421 (1060). Curve C in this same referenre describes a material similar in porosity t o the Davison gel used by u s . (.5) .J. L. Rosenherg and D. J. Shombert, J. Am. Chem. Soc., 33, 3252 (1960).
n.
xov., 1961
SOTES
2105
average pore size. The relationship could be determined by calibration against a primary poresize measurement. The range of 10 to 1000 A. in pore diameter would be involved. If the method were to be used, adsorbent granules should be of the same over-all dimensions to avoid differences in the rate of flow between granules. At least one granule dimension should be greater than 1 millimeter so that the inter-granular flow does not become ratelimiting.
noticed. The epitaxially grown nickel films mostly developed parallel orientations. They were occasionally mixed with a small amount of crystals which were aximuthally rotated by 4 j 0 , ie., they thus had (110) axis parallel to (110) axis of NaC1. { 111] twinned structures of the oxide were observed also. In some cases the patterns due to unknown oxide were predominant. Using 220 rings of either X i 0 or Xi, the dhkl values were evaluated and rings indexed. Table I shows thme values which agree well with a hexagonal structure having a. = 4.61 A., co = 5.61 A. and colao= 1.22. AN OXIDE OF TERVALENT NICKJ3L It is, however, interesting to note that cobaltic oxide (Cos03) has a similar hexagonal structure BY P. S.AGGARWAL AND A. GOSWAMI (UO = 4.64 A., co = 5.75 A., eo/@ = 1.24) with its d National Chemical Laboratorv, Poona-8, Indza values and intensities of rings4 very close to those of Recaved Aprzl 1 1 , 1961 the oxide of nickel mentioned above (Table I). From While iron and cobalt exist in di- and tervalent the consideration of similarities in the properties of states in the oxides of compositions MO, M20; cobalt and nickel compounds and their isomorand M304, the existence of anhydrous Ni203 has phous nature and also of the fact that similar been doubted by many workers. Attempts to pre- chemical compounds have similar structures, it pare this by heating the hydroxide, basic carbonate may be concluded that the observed hexagonal or nit,rate of nickel in air or oxygen resulted in the structure is very likely due to the formation of formation of NiO only.' Cairns and Ott2 prepared the oxide of tervalent nickel (Xi203),as in the from solutions a compound of the composition case of cobaltic oxide (Co207). It may, however, yi~O3.2H20, which decomposed to NiTOs H20 be pointed out here that, in the absence of accurate and finally to KiO, as revealed by X-ray studies. intensity data of Co108 the comparison cannot he No line characteristic of S203was a t all detected. carried out too far. Rooksbya showed, by X-rays, that different oxides ( 4 ) A S.T.N. Card No. 2-0770. of nickel, Hack or otherwise, obtained from various sources consisted only of N O . During electron diffraction studies of evaporated films of nickel, on PRIMARY STEPS I N THE PHOTOLYSIS OF hot rocksalt substrates in uucuo, me observed many METHYL CARBONATE' rings in diffraction patterns, which could not, be BY M. H. J. WIJNEN explained by the presence of Ni and 5 0 alone, but Radzatzon Research Laboratotzes, Mellon Instztutr. Pittsburgh. P a . required the esistence of an oxide having a hesaRecezved M a v 1, 1961 gonal structure. Nickel was evaporated from a nickel filament Investigations of the photolysis of methyl ace(spec. pure, supplied by MIS.Johnson and Matthey tate2.3have shown that the main primary process & Co., London) a t a pressure of about 10-' to produces methoxy radicals according to mm. (obtained by a rotary oil pump) on the cleavCHZCOOCHT + h v --+ CHSCO + CHBO age face of rocksalt crystals kept a t about 400'. After removal of the films from the substrate in the A similar step in the photolysis of methyl carbonate usual way. these were examined in the E D . cam- would lead to CH3OCO and CH30 radicals and possibly to 2CH30 and CO if the CH30C0 radiera by transmission methods. cals would decompose into carbon monoxide and TABLE I" methoxy radicals. This investigation has been a0 = 4 . 6 1 m = 4.64 undertaken to investigate the feasibility of using ci, = 5 . 6 1 co = 5.75 Ni20z co/m = 1 . 2 2 CorOa' co/m = 1 . 2 4 the photolysis of methyl carbonate as a source for Intensity d Intensity hkl d (X-rays) methoxy radicals.
-
3.23 2.80 2.30 2.02 1.77 1.62
vf
..
..
S
m3 S
p 3
101 002 110 200 112 202 21 0 004
3.21 2.87 2.33
.. 1.78 1.63 1.57 1.39
1.40 f 1.11 f ... ... a v, very; f , faint; s, strong; and m, medium.
90 100
100 I
.
.
100 90 -50 90
...
Pat terns thus obtained, consisting of rings and spots, mostly were due to nickel, though sometimes extra rings of X i 0 and the unknown oxide were (1) R. W. Cairns and E. O t t . J . A m . Chem. Soc., 65, 527 (1933). (2) R. W. Cairna and E. Ott, ibid., 66, 534 (1933). (3) H. P. Rooksby, Nature, 162, 304 (1943).
Experimental Since it has been observed4 that methyl carbonate decomposes thermally on quartz, the photolysis was studied at 6 and a t SO" only. The usual photochemical technique hab been applied. A Hanovia Type 73A10 (S-500)medium pressure arc was used to obtain the data a t SO". Constant temperature a t 80" was maintained by an aluminum block furnace. Temperature control a t 6' was obtained by placing the cell in a mater-bath and transmitting the light through a 5 mm. layer of water into the cell. A Hanovia medium pressure arc (Type 16A13) was used as the light source for the experiments at 6". Analyeis of the reaction products (1) ThiR investigation was supported, in part, by the U. S. iltoniic Energy Commission. (2) (a) W. L. Roth and G. K. Rollefson, J . A m Chem. Soc., 64, 490 (1942); (b) P. .4usloos. Can. J . C h e m , 36, 383 (1958). (3) (a) M. H. J. Wijnen, J . Chem. Phys., 27, 710 (1957); (b) 28, 271 (1958): (c) 28, 939 (1958). 4 ' (4) M. H. J. Wijnen, abad., 34, 1465'(1961).
