COMMUKICATIONS TO THE EDITOR
2714
Vol. 66
TABLE I ELECTRON AFFINITIES -Calculated Compound
Exptl. (e.v.)
w =
1.4
OJ
=
3.80
TIuckel results were available. I t is suggested that future calculations of electron affinities of alternant aromatic hydrocarbons using the w-technique em(e.v.)-o = ploy the value w = 3.77 until additional experi3.73 SCF“ mental work dictates an alternative value. This -1.42 -1 4 0 f 0 . 2 procedure should give very good approximations -0.08 - 0 . 2 O f . 2 to the electron affinities where such may be re0.58 0 . 6 1 f . 2 quired, e.g., in charge transfer studies. We should .95 ... point out that the very good agreement between the 1.19 ... calculated and experimental values for these com0.17 .25 f . 2 pounds provides a further confirmation of the validity of the recent experimental values determined .37 ... by Wentworth and Becker.6 We wish to thank .47 ... Dr. W. E. Wentworth for helpful discussion.
Benzene . . . 4.32 -1.59 Naphthalene . . . 5.33 -0.25 Anthracene 0.42 5.84 0.42 Naphthacene . . . 6.14 .79 Pentacene . . . 6 . 3 3 1.04 Phenanthrene .20 5.44 0.01 3,4-Benzophenanthrene .33 5.56 .21 Chrysene .33 5.66 .31 Benzanthracene .46 5.81 .46 .61 ... Pyrene .39 5.80 .42 .57 . 5 5 f .2 Triphenylene .14 5.32 -0.03 .12 ... Pentaphene . . , 5.87 0.58 .73 ... Picene . . . 5.73 .41 .59 ... Perylene . . . 6.04 .73 .88 ... l12-3,4-Dibenzanthracene . . . 5.74 .43 .58 ... l12-5,6-Dibenzanthracene . . . 5.79 .50 .65 ... l12-7,8-Dibenz. . . 5.75 .46 .62 ... anthracene 3,4-5,6-Dibenxophenanthrene . . . 5.66 .3i .52 ... 1,12-Benzoperylene . . . 5.87 .57 .73 ... Coronene ... 5.66 .39 .54 Biphenylene . . . 5.74 .26 .42 a Error estimated by J. R. Hoyland in a private communication to R. S. B.
... ...
son in his very successful calculation of ionization potentials using an approximation similar to that of eq. 1. We also have included the SCF calculated values of Hoyland and Goodman.6 As noted by Ehrenson, the electron ailhities calculated using w = 1.4 are significantly larger than the theoretical or experimental values. However, if w = 3.80 is used, then a better correlation between the w-technique values and either the experimental or theoretical values is noted. Ehrenson used w = 3.8 on the basis of the value of w which would give graphite an electron affinity of 4.39 e.v. (work function of the crystal) 2.53 e.v. (correction from crystal to vapor).8 In fact, our agreement with experiment or SCF theory is excellent except for phenanthrene, 3,4-benzophenanthrene, and triphenylene. For these compounds, it appears that w = 3.73 gives much greater agreement with experiment. Ehrenson has noted that the w-technique introduces a correction for a-electron repulsion and u-electron rearrangement which is omitted in the Huckel MO method. Hoyland and Goodman6 also have emphasized the importance of these factors as well as reminimization of the negatively charged state. The necessity of using w = 3.73 for the three compounds mentioned above may be caused by varying a and a-deformation within the series of aromatic compounds which are reflected in a varying value of w . Table I also includes calculated values of electron affinities of several alternant sromat,ic hydrocarbons for which the necessary
-
( 8 ) L). It. Kearns and M. C i v i u , J. Chtm. P k y s . , 34,2026 (1981).
( 9 ) National Science Foundation Codperatwe Predoctoral Fellow 1962/1963. On leave from Texaoo Research Laboratories, Bellaire, Texas.
DEPARTMENT OF CHEMISTRY 1 7OF HOUSTON ~ HOUSTON, TEXAS
~
DONALD R. SCOTT’ RALPH ~ S. BECKER ~
RECEIVED OCTOBER 10. 1962
ISOPROPYLBENZENE CONVERSION ON PRE-IRRADIATED SILICA-ALUMINA
Sir: I n y-irradiation of isopropylbenzene adsorbed on microporous silica-alumina (400 m.”g.), G(benzene) was observed to be greater than one at electron fractions of isopropylbenzene, F , ranging from 0.0107 down to 0.00106 and to fall to G(benzene) = 0.40 at F = 0.00046.1 These results were interpreted as indicative of energy transfer from the solid to the adsorbed isopropylbenzene. Further, the persistence of such a high yield as G(benzene) = 1.13 to such a low electron fraction as F = 0.00106, a t which presumably 99.9% of the radiation energy is initially absorbed in the solid and which corresponds to about 0.6% surface coverage, suggested that transfer of energy is rapid relative to the decay time of the responsible transfer entity produced by radiation in the solid. Additional evidence now has been obtained in support of these interpretations. Using the same chemicals and procedures as previously reported, the following experiments were performed in which about 30 g. of silicaalumina (400 m.”g.) was irradiated to a dose of 5.3 X 1021e.v. at 36’ in the absence of isopropylbenzene, and then about 0.2 g. of isopropylbenzene was introduced to the pre-irradiated solid and products were recovered. 1, Irradiation imparted a very dark, essentially black, color to the solid. KOgas was obtained from the sulid. With liquid nitrogen on the reaction cell which had been attached to a vacuum line, isopropylbenzene was distilled onto the dark solid after it had stood for 2 hr. at room temperature following irradiation. The cell with contents then was brought to room temperature, and the uppermost beads of solid were observed to be partially decolorized. The beads appeared white on the periphery first with dark centers, and decolorization proceeded inward to the bead centers. At the same (1:
R. R. IIentz, J . Phys. Chem., 66, 1625 (1062).
~
Dee., 1962
COMJIUSTCATIOSS TO THE EDITOR
time decolorization proceeded down the column of beads in the reaction cell and appeared to be essentially complete in 3-4 hr. The decolorization process apparently is governed by isopropylbenzene diffusion. (Kohn2has observed similar phenomena in decolorization of irradiated silica, silicaalumina, and alumina by hydrogen, ethylene, water vapor, and ammonia.) Cell and contents stood overnight and products were removed. Only total gas and benzene could be measured with reasonable accuracy: F = 0.0070; G(tota1 gas) = 0.011; G(benzene) = 0.42. 2. The cell with irradiated solid was kept in liquid nitrogen from the termination of irradiation to introduction of isopropylbenzene : F = 0.0067; G(tota1 gas) = 0.012; G(benzene) = 0.42. After product recovery the cell with decolorized solid was opened to the vacuum line and pumped overnight a t 10-5-10-6 mm. Another 0.2-g. portion of pure isopropylbenzene was introduced to give F = 0.0068 and allowed to stand overnight on the solid. There was no detectable product formation or isopropylbenxene decomposition. 3. Isopropylbenzene was allowed to distil onto irradiated solid which was at room temperature (this process required only 10 min.). Decolorization proceeded much more uniformly over the whole solid in this case, although the rate still was slightly more rapid in the uppermost beads than in the lowest. Complete decolorization required about 3 hr.: F = 0.0070; G(tota1 gas) = 0.009; G(benzene) = 0.43. 4. Irradiated solid was maintained a t 100’ for four hours. The solid remained quite dark, but some bleaching did occur. Isopropylbenzene then was introduced as in the third experiment, and decolorization proceeded in the same fashion : F = 0.0068; G(tota1 gas) = 0.001; G(benzene) = 0.33. Thus, a 10-fold decrease in gas yield and a 21% decrease in benzene yield occurred as a result of the 4 hr. a t 100’. Since complete deactivation of the solid was observed in experiment 2, the deactivation could not have been due to the product recovery procedure (4 hr. at 100’)1 but must have been due to the prior contact with isopropylbenzene. 5. Irradiated solid was stored in the sealed reaction cell for 1 year a t room temperature prior to introduction of isopropylbenzene. Darkness of the solid after the year’s storage had definitely decreased to a shade judged roughly equivalent to a dose one-fourth of that originally received: F = 0.0072; G(tota1 gas) = 0.0043; G(benzene) = 0.41. Under conditions comparable with these experiments, irradiation of isopropylbenzene adsorbed on the solid gave G(benzene) = 1.05 and G(tota1 gas) = 0.219. The gas was 95% hydrogen.’ These experiments demonstrate the longevity a t room temperature of certain benzene-producing ‘‘excited states of the solid” formed by irradiation. (The general expression “excited states of the solid” is used in the absence of definitive evidence for the mechanism of energy storage in the solid. It is considered probable that the long-lived excited (2) H. W. Kohn, Nature, 184, 630 (1959).
2715
states responsible for benzene formation are trapped electrons and/or concomitant positive holes. *) Further, these experiments suggest that benzene formation involves energy transfer from “excited states of the solid” which are destroyed thereby. If one molecule of benzene is produced per “excited state,” then G(benzene) = 0.42 gives 7 X 10’’ excitations per gram of solid. The approximately 20-fold lower G(tota1 gas) in these experiments is consistent with the previously proposed mechanism for hydrogen formation’ in that hydrogen atoms formed on irradiation of the solid woiild be recaptured at surface sites in the absence of adsorbed isopropylbenzene. The results of experiment 5 suggest that although gas formation on contact of isopropylbenzene with the pre-irradiated solid may be associated with visible “excited states,” benzene formation is not but probably involves “excited states” which absorb in the ultraviolet. Doseeffect and other studies are in progress to determine the nature and concentration of defects responsible for the energy storage and to explain the 40% lower G(benzene) as compared with that in simultaneous irradiation of solid and adsorbed isopropylbenzene. To the author’s knowledge these experiments constitute the first conclusive demonstration of a chemical conversion due to transfer of energy stored in a solid (without destruction of the ~ o l i d ~by ,~) exposure to high-energy ionizing radiation. (3) T. Westermark and B. Grapengiesser, Arkiv Kemi, 17, 139 (1961). (4) T. Westermark, N. Biesert, and B. Grapengiesser, ibzd., 17, 151 (1961).
SOCONY MOBILOILCOMPANY, INC. ROBERTR. HENTZ RESEARCH DEPARTMENT PRINCETON, NEWJERSEY RECEIVED SEPTEMBER 31, 1962
ELECTRONIC PARAMAGNETIC RESONAKCE OF MANGANOUS ION IN CllLCIUM PYROPHOSPHATE AND CALCIUM FLUORIDE Sir: Kasai‘ recently published electronic paramagnetic resonance (e.p.r.) spectra of divalent manganese in calcium fluorophosphate (fluorapatite) and calcium compounds used in the synthesis of the fluorophosphate. We believe that the e.p.r. spectra shown in the publication are incorrect for divalent manganese in calcium pyrophosphate and in calcium fluoride. We have prepared polycrystalline alpha calcium pyrophosphate and beta calcium pyrophosphate by pyrolysis of dibasic calcium orthophosphate, CaHP04, a t 1200 and 1050°, respectively.2 The chemical identity and crystalline form of the pyrophosphates were confirmed by the weight loss during preparation, X-ray diffraction analysis, and measurement of the refractive indices. Manganese was introduced in the atomic proportion, Mn/Ca = 10-3, by slurrying the CaHP04 in (1) P. H. Kasai, J . P h y s . Chem., 66, 674 (1962). (2) A. 0. McIntosh and W. L. Jablonski, Anal. Chem., 38, 1424 (1956).
