COMMUNICATIONS TO THE EDITOR
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ratios. I n the case of 2-methyl-2-butene the intermediate was written as
so that the relatively large yields of epoxide and aldehyde at 90°K are readily understood. The reaction of 2-methyl-2-pentene provided further evidence3 that where the (0---H) interaction is possible, H migration is inhibited relative to that of the alkyl group. Further, it was shown recently4 that the stereospecificity of 0-atom addition at low temperatures, first noted in the case of trans-&butene at 90°K, increases significantly as the straight-chain, internal olefin increases in size from C,i to Cg. The cis as well as the trans compounds exhibit this effect and can be readily understood when it is recognized that the rate of rotation is inversely related to the moment of inertia, and hence, the mass and size of the rotating group. It is difficult, on the other hand, to reconcile these observations with a biradical intermediate despite the conformational effects proposed by Cvetanovi6. For example, it would be necessary to assume that the migration rate of the most favorably oriented alkyl group in the cis-olefin biradical adduct increase with increasing size and mass. In the previous note CvetanoviB reports the fractional product yields obtained at 298 and 77°K for the reaction of O(3P) and cyclopentene. Besides epoxide formation and H atom migration yielding cyclopentanone, only ring rupture of the c-Cs adduct followed by ring closure to a c-Cd aldehyde is possible. The latter is analogous to alkyl migration in the straightchain adducts but is severely limited by having to surmount a larger energy barrier. Such a process can hardly be expected to compete with the migration of the H atoms despite the loose interaction with oxygen postulated in the Klein-Scheer structure for the intermediate. This is particularly so in the case of cyclopentene where the product c-Cq has more inherent ring strain than does the c-CS adduct. As a matter of fact, the formation at 77°K of even as much as 6% of the product yield as cyclobutylmethanol is a rather surprising result. This suggests an investigation of the reaction of O(3P) with a c-C7 olefin. In this case the product aldehyde would be a with far less ring strain than the c-C7 adduct. Table I gives the fractional product yields obtained for this Table I: Fractional Product Yields for t h e Reaction of O(3P)and Cycloheptane at 77'K EPOXYoycloheptans
Cycloheptanone
Cyolohexylmethanal
0.58
0.28
0.14
reaction at 77°K. As expected, the relative yield of the aldehyde is grea,ter than that of the c-Cd aldehyde in the cyclopentane reaction reported by Cvetanovib. This effect is probably assisted by the (0---H) interaction postulated to occur in the intermediate.
It would therefore appear that the currently available evidence, particularly that obtained at low temperatures, tends to support the Klein-Scheer planar intermediate for the reaction of ground-state 0 atoms with olefins. This structure subsequently undergoes a set of concerted rearrangements4 in which a given group migration and oxygen atom localization occurs simultaneously to produce the final reaction products. Since electron spin is not conserved in the total reaction, the argument that a "triplet biradical" must first be formed before rearranging to singlet products is not compelling. The path by which the intermediate for this process rearranges to form the final products probably involves both energy and steric effects whose details have yet to be entirely elucidated. The spin relaxation process may ultimately prove to be only a minor barrier to the successful completion of these complex intramolecular rearrangements. (3) R. Klein and M . D. Scheer, J. Phys. Chem., 73, 1598 (1969). (4) R. Klein and M. D. Scheer, ibid., 74, 613 (1970).
PHYSICAL CHEMISTRY DIVISION N.4TIONAL BUREAU OF STANDARDS WASHINGTON, D. C. 20234
MILTOND. SCHEER RALPHKLEIN
RECEIVED FEBRUARY 6 , 1970
Nonexcitonic Energy Transfer in Crystalline Charge-Transfer Complexes
Sir: The efficient long-range transfer of excitation energy in molecular crystals has been a well-known phenomenon ever since Bowen2 showed that the emission of crystalline anthracene containing small amounts (1) This work was supported by the National Research Council of Canada. (2) E. J. Bowen, Nature, 142, 1081 (1938); J. Chem. Phys., 13, 306 (1945).
The?Journal of Physical Chemistru, VoL 7 4 , N o . 18, 1970
COMMUNICATIONS TO THE EDITOR
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of tetracene consists exclusively of tetracene fluorescence. According to the exciton theory13a packet of excitation can hop from one lattice position to another, visiting many thousands of lattice sites during its lifetime. I n a mixed crystal containing a “guest” molecule whose emitting state is below that of the “host” crystal lattice, the excitons produced initially by excitation of the host are trapped at the guest sites, resulting in selective emission from guest molecules. Exciton transfer is generally considered to be the mechanism of sensitized fluorescence in molecular crystals and similarly ordered aggregates. We have found an apparent exception to this behavior in mixed crystals of a more complicated type, wherein two kinds of charge-transfer complexes are incorporated into a single crystal lattice. I n a hydrocarbon-trinitrobenzene (TNB) mixed-donor4 complex, some of the hydrocarbon lattice positions are occupied by guest hydrocarbon molecules of another species. Such a system, exemplified by phenanthrene-TNB containing small amounts of anthracene-TNB, is formally equivalent to a mixed crystal of the two chargetransfer complexes and furnishes a useful means of studying excitation transfer between two different charge-transfer states, in this case, from phenanthreneTKB to anthracene-TNB. The fluorescence of pure phenanthrene-TNB is most efficiently excited at 430 nm, and extends over a broad region between 15,000 and 21,000 cm-l a t room temperature. If 1% of the phenanthrene molecules in the lattice are replaced by anthracene, most of the emitted light still consists of host charge-transfer fluorescence. Only at guest-complex concentrations of 10 mol % and above is the crystal fluorescence exclusively that of anthracene-TNB. Host and guest C T fluorescence intensities are about equal at 4 mol % anthracene-TNB concentration; this torresponds to a mean separation of approximately 170 A between neighboring guest complexes along the linear donor-acceptor stacks of the crystal. This distance is of the order of the distances usually associated with dipole-dipole resonance transfer16suggesting that this process may account for most of the sensitized fluorescence that is observed here. This behavior is in striking contrast to that of other crystalline systems such as tetracene-in-anthracene, where the similar transition between host and guest emission occurs over a guest concentration range of to mol %.‘j These results show that exciton migration is not an efficient energy transfer process in crystalline charge-transfer complexes. This conclusion lends support to the view7 that excitation of a crystalline charge-transfer complex results in the formation of “localized excitons” whose migration is limited by the molecular reorientation that must accompany the excited center as it travels through the crystal. The Journal of Physical Chemistry, Vol. 7 4 , No. IS, 1970
(3) For a brief and literate review, see the article by M. W. Windsor in D. Fox, &I. W. Labes, and A. Weissberger, “Physics and Chemistry of the Organic Solid State,” Vol. 11, Interscience, New York, N. Y., 1965. (4) S. K. Lower, Mol. Cryst., Liq. Cryst., 5, 363 (1969). (5) Th. Forster, “Floreszenz Organischer Verbindungen,” Gottingen, Vandenhoeck, and Ruprecht, 1951, Chapter 13. (6) F. R. Lipsett and A . J. Dekker, Can. J . Phys., 30, 165 (1952); J. Ferguson, Aust. J . Chem., 9, 160 (1956). (7) R. M. Hochstrasser, 9 . K. Lower, and C. Reid, J . Chem. Phys.,
41, 1073 (1964).
DEPARTMENT OF CHEMISTRY SIMOKFRASER UNIVERSITY BURN.4BY 2, B. c., CANADA
STEPHEN K. LOWER
RECEIVEDFEBRUARY 4, 1970
Effect of pH on the Ultrasonic Absorption of Aqueous Solutions of Proteins
Sir: Recently, Kessler and Dunnl reported results of ultrasonic investigation on aqueous solutions of bovine serum albumin (BSA). They attributed the observed changes of absorption at pH below 4.3 and above 10, respectively, to conformal changes and expansion of the protein molecule. This note is to report new data, taken on BSA, P-lactoglobulin (PL), and lysozyme at pH ranging from 1 to 13.3, which suggest that proton transfer reactions contribute significantly to the absorption of dilute protein solutions at pH lo. Figure 1 shows that in the acid pH region the frequency-free absorption a / N 2 (a is the absorption coefficient and N the ultrasonic frequency) goes through a maximum for a value ~ H ofMthe pH. Several facts seem to indicate that the absorption maxima cannot be explained by conformal change or dissociation of the protein in subunits. (1) Such reactions are characterized by pH’s which depend upon the nature of the protein2 while Figure 1 shows that within the accuracy of the experiments PHM is independent of the nature of the protein. (2) Unlike BSA and pL, lysozyme does not exhibit any conformal change2 or dissociationa for protein concentration c , below 0.01 g/cm3, as used in this work and for 2 < pH < 4.5, that is, precisely the pH region in which curve 3 relative to lysozyme shows a maximum similar to the ones observed for ,8L and BSA. (3) The absorption of lysozyme solutions is practically constant in the pH range 4.5-8.5 in which dimerization3 and conformal change4 are known to (1) L. Kessler and F. Dunn, J . Phys. Chem., 73, 4256 (1969). (2) C. Tanford, Aduan. Protein Chem., 17, 69 (1962). (3) A . Sophianopoulos and K. Van Holde, J . B i d . Chem., 239, 2516 (1964). (4) J. Owen, E. Eyring, and D. Cole, J . Phys. Chem., 73, 3918 (1969).
