Communications to the Editor
2048 (11) H. W.Ousterhoudt, J. fhys. Chem., 7 8 , 408 (1974). (12) A. Despic and G. J. Hills, Trans. Faraday SOC.,53, 1262 (1957). (13) A. Yasuda, C. E. Lamze, and L. D. Ikenberry, Makromoi. Chem., 118, 19 (1968). (14) M. H. Cohen and D. Turnbull, J. Chem. fhys., 31, 1164 (1959). (15) M. Rinaudo, B. Loiseleur, M. Milas, and P. Varoqui, C. R. Acad. Sci., 272, 1003 (1971). (16) B. A. Soldano and G. E. Boyd, J. Am. Chem. SOC., 75, 6107 (1953). (17) S. Lifson and J. L. Jackson, J. Chem. fhys.. 36,2410 (1962).
(18) J. H. B. George, R. A. Horne, and C. R. Schlaikjer. J. Electrochem. SOC., 117, 892 (1970). (19) G.E. Boyd, J. fhys. Chem., 78, 735 (1974). (20) M. D. Kalinina and N. I. Nikolaev, Russ. J. fhys. Chem., 45, 1290 (1971). (21) G. L. McVay and D. E. Day, J. Am. Ceram. SOC.,53, 508 (1970); J-P. Lacharme, C.R. Acad. Sci., 270, 1350 (1970). (22) Yasuda assumed this fraction to be the weight fraction of water, while we have taken it as equal to (1 - Vp),the volume fraction of water.
COMMUNICATIONS TO THE EDITOR
Multiplicity of the Reacting State in the Photoadditson of Carbon Tetrachloride to Anthracene Publication costs assisted by the National lnstitutes of Health
Sir: The photochemical addition of carbon tetrachloride to anthracene has been known for some time.1 Although it has been assumed to be a reaction occurring from the excited singlet state of anthracene,l-3 Hardwick concluded from flash spectroscopic studies of triplet quenching6 that the reaction is very likely to take place entirely from the anthracene triplet state.? Because of our interest in quenching of excited triplet states8 and in heavy atom elucidation of photochemical and photophysical processes?JO we have investigated the photoaddition of carbon tetrachloride to anthracene in benzene solutions containing varying concentrations of bromobenzene as a heavy atom additive. Assuming that the only effect of bromobenzene is to convert anthracene excited singlets to triplets,ll the following kinetic scheme may be written for the case in which the photoaddition takes place from the anthracene singlet state.12 hv
A -+- A*(1)
(3) CC14
A
+ A*(l)+products k4
(4)
k5
+ A*(l)-+dimer
(5)
(7) The Journal of Physical Chemistry, Voi. 80, No. 18, 1976
If this formulation is correct, the Stern-Volmer slope for bromobenzene quenching of anthracene fluorescence (from plots of F’IF VS. [bromobenzene], where F’ and F are the fluorescence intensities (both in the presence of CC14) in the absence and presence of bromobenzene, respectively) and the Stern-Volmer slope for bromobenzene quenching of anthracene disappearance (from plots of vs. [bromobenzene], where @d’ and @d are the disappearance quantum yields in the absence and presence of bromobenzene, respectively) should be the same. Using solutions of 2.4 X M anthracene and 0.13 M CC14 in benzene, with bromobenzene concentrations of 0,1.2,1.52, 1.9, and 2.3 M and following the photochemical reaction by monitoring the decrease of anthracene absorbance at 378 nm, we observed a Stern-Volmer slope of 0.68 M-l (standard deviation, 0.02) for fluorescence quenching and a SternVolmer slope of 0.72 M-I (standard deviation, 0.06) for reaction quenching. Since these slopes are the same within experimental error, we conclude that the photochemical reaction of anthracene with carbon tetrachloride takes place entirely from the singlet state of anthracene. At our experimental concentrations we observed a 480-fis lifetime for the anthracene triplet in the absence of carbon tetrachloride and a k , for CC14 quenching of anthracene triplets of 1.5 X l o 4 M-l s-l, in good agreement with the k , value determined by Hardwick.6 A t the CC14 concentration used in this study, half of the anthracene triplets were quenched by carbon tetrachloride. Since photochemical adduct formation does not appear to follow, we conclude that the net result of triplet quenching is enhanced radiationless decay of the triplet excited state. Charge transfer interactions have been proposed in the cc14 quenching of naphthalene triplets by Schulte-Fr0h1inde.l~ In the present case, an electron transfer complex can be calculated from the equation of Weller14 and from anthracene and CC14 redox datal5 to lie 1.97 f 0.1 eV above the ground state. Since this is slightly higher than the 1.82-eV energy of the anthracene triplet, electron transfer would require thermal activation. In that case homolysis of the C-C1 bond, which is thought to lead to adduct formation,l,2 may be less likely than other processes which lead to ground states.
2047
Communications to the Editor
This study provides another example of the elucidation of photochemical reactions through the use of heavy atom effects.16 The technique should find use in still other investigations.
TABLE I : Diamagnetic Susceptibility of Cyclic PKetoacetals
RCOCH,CH/'THa '0-CH,
Acknowledgment. This work was supported by a National Institutes of Health Research Grant (GM 15238).
XM
References and Notes (1) E. JSBowen and K. K. Rohatgi, Discuss Faraday SOC., 14, 146 (1953), and references therein. (2) M.I. Ivanoff, BUN.SOC.Chim. Bee., 71, 759 (1962). (3) Oster4has presented results which suggest that diphenylanthracene gives radicals from CC14 quenching of the singlet excited state only, but that both excited singlet and triplet states of benzene react with CC14 to produce radicals. Results for anthracene are not explicitly given. See also the discussion of Birks.' We thank a referee for pointing out these references. (4) G. K. Oster: Acta Phys. Polon., 26, 435 (1964). (5) J. B. Birks, "Photophysics of Aromatic Molecules", Wiley-lnterscience, New York, N.Y., 1970, pp 439-441. S. Kusuhara and R. Hardwick, J. Chem. Phys., 41,2386 (1964). S. Kusuhara and R. Hardwick, J. Chem. Phys., 41, 3943 (1964). , J. K. Roy, F. A. Carroll, and D. G. Whitten, J. Am. Chem. Soc., 96, 6349 (1974). , A. R. Gutierrez and D. G. Whitten, J. Am. Chem. Soc., 96, 7128 (1974). (a) F.A. Carroll and F. H. Quina, J. Am. Chem. SOC., 98, 1 (1976); (b) F. H. Quina, 2. Hamlet, and F. A. Carroll, submitted for publication. For evidence supparting this assumption, see T. Medinger and F. Wilkinson, Trans. Faraday Soc., 61, 620 (1965). The dimerization of anthracene is also included as a singlet process: cf. E.J. Bowen, Adv. Photochem., 1, 23 (1963). S. Ander, H. Blume, G. Heinrich, and D. Schulte-Frohlinde, Chem. Commun., 745 (1968). H. Knibbe, D. Rehm, and A. Weller, Ber. Bunsenges. Phys. Chem., 72,267 (1968). C. K. Mann and K. K. Barnes, "Electrochemical Reactions in Non-Aqueous Systems", Marcel Dekker, New York, N.Y., 1970. For related applications, see F. Wilkinson and J. T. Dubois, J. Chem. Phys., 48, 2651 (1968); R. B. Cundall, D. A. Robinson, and A. J. R. Voss, J. Photochem., 2, 239 (1973-1974); W. I. Ferree, Jr., 6. F. Plummer, and W. W Schlornan, Jr., J. Am. Chem. Soc., 96, 7741 (1974); R. H. Fieming, F. H. Quina, and G. S. Hammond, ibid., 96, 7738 (1974); D. 0. Cowan and J. C. Koziar, ibid., 97,249 (1975); A. Gupta, R. J. Kelley, E. M. Evleth, and G.S. Hammond, J. Chem. Phys., 63, 5496 (1975). Department of Chemistry, Davidson College, Davidson, N.C. 28036.
Felix A. Carroll*17 David G. Whitten"
Department of Chemistry
University of North Carolina Chapel Hill, North Carolina 275 14 Received February 23, 1976
Effect of Ring Closure on the Diamagnetic Susceptibility Contributions of Oxygen Atoms
Sir: The present study of the diamagnetic susceptibility of cyclic P-ketoacetals was undertaken to reveal how the contribution of an oxygen atom to the total diamagnetic susceptibility is affected when a second oxygen atom is attached to the same carbon atom. Cyclic alkyl P-ketoacetals were synthesized by the standard methods3 and their diamagnetic susceptibilities (all the susR-COCH2-CH
loci' \
R
CH, CZH, n-C,H, n-C,H,, a References
Expt
From Baudet's method8
70.45 82.01 93.38 116.19
80.96 92.35 103.66 126.36
Calcd From Haberditzl's methodb 69.80 81.15 92.50
115.20
xo 3.45 3.55 3.56 3.60
4 and 5. b Reference 6.
force was on the order of f0.05 mg and reproducibility of the results was quite satisfactory. The Gouy tube was hung in such a manner that its one end remained in the field so that the susceptibility may be calculated by the well established method. XM values of these compounds have also been calculated by Baudet's wave mechanical m e t h ~ dand ~ , the ~ data are recorded in Table I. XM is given by the relation
where BE, ISE, and NBE represent bonding electrons, inner shell electrons, and nonbonding electrons, respectively. The near constancy of the deviation for the homologus series suggests that the factor which is responsible for the deviation may be common and contributing almost to the sanie extent in these compounds. The reason for the poor agreement between the experimental and calculated XM values is that the contributions of the electrons in different states do affect diamagnetism and have not been fully accounted for in theoretical calculation. HaberditzF has modified Baudet's method and has taken into account the contribution of the electrons in different states. X M values calculated by Haberditzl's method show a good agreement between the calculated and experimental XM values (Table I). The diamagnetic contribution of the oxygen atoms present in the form
-c:
0-C-
O-C-
I
has been calculated taking the most reliable values as X C H ~= 11.36, X C H ~= 13.36, XCH = 9.36, and xco = 6.75 in ketones2 and following the additivity rule, xo(av) in this class of compounds is 3.5 against xo = 5.37 in alcohols and ether; xo = 5.4 for g1ycols;l and xo = 4.7 for alkyl noncyclic 0-keto acetals, R-CO-CHpCH(OR')2. These values indicate that ring closure is primarily responsible for the low value of xo in cyclic acetals, rather than the fact that a second oxygen is attached to the same carbon atom.
OCH2
ceptibility values are in cgs units) were measured with a sensitive Gouy balance calibrated with a number of standard substances. The accuracy of the determination of the Gouy
Acknowledgment. The author expresses his sincere thanks to Professors R. C. Mehrotra, K. C. Joshi, and J. N. Gaur for providing facilities in this department. The Journal of Physical Chemistry. Vol. 80, No. 18, 1976
