COMMUNICATIONS TO THE EDITOR
3174 intensity of the y-phase (stable) resonance is directly proportional to the decrease in intensity of the 6-phase resonance. The number of Cr5+spins produced in the present system a t 25" is comparable to the number3 produced at the same chromium composition on alumina by oxidation at 600". Upon the addition of oxygen to the reduced sample at pressures of several Torr a red phosphorescence of the sample was observed which persisted for about 1 min. This effect was not observed when the sample was exposed to oxygen a t -196". For P(02) above about 250 p a very narrow resonance appeared with g = 2.029. At somewhat higher pressures a second sharp line appeared with g = 2.006. These resonances are seen in Figure l b and c and are tentatively ascribed to 02-or 0- on the surface. Further work is in progress including a spectrophotometric and epr study of the phosphorescent state and a study of the P-phase resonance us. temperature and reducing conditions.
accepted rotational inertia h y p ~ t h e s i s . ~ ~The ~ a ,pres~~ ent study has involved measurements of the products from (2) and (3) for Cz to CSalkanes. Standard techniques have been e m p l ~ y e d . ~ &Five ~ ~ per ~ ~ ~cent J 02 scavenger was included in all runs. Any valid experimental demonstration of C-C bond strength effects for (3) necessarily requires the isolation of this mechanistic factor from other possible factors. Intersample comparisons of absolute product yields, for example, can be meaningful for this purpose only if all the experiments are carried out under "constant flux conditions'' (ie., identical tritium atom spectra for all sample^).^ This condition has not been fulfilled in the present study, so that this discussion involves relative intrasample yield comparisons for reactions 3 and 2. Another mechanistic factor that is likely to interfere with the attempted isolation of the Do(R1-Rz) dependence for (3) is the structure f a ~ t o r . ~ I n view of this expectation, the results from the 19 reactions studied were arranged into groups according to the (5) On leave of absence from the University of Puerto Rico. degree of branching at the "attacked" C atom. Results have been obtained for lo,2", 3", and 4" cases,2 D. E. O'REILLY SOLIDSTATESCIENCE DIVISION and the relevant data from the eight 2" cases studied ARGONNE NATIONAL LABORATORY ILLINOIS 60439 ARQONNE, are given in Table I.' DEPARTMENT OF CHEMICAL ENGINEERING F. D. S A N T I A Q O ~ When they are considered as a group, the (R:S) PURDUE UNIVERSITY R. G. SQUIRES ratios in Table I fail to indicate a correlation with WEST LAFAYETTE, IXDL4NA 47907 RECEIVED JUNE 6, 1969
Carbon-Carbon Bond Strength Effects in Recoil Tritium Alkyl Replacement Reactions
Sir: Nuclear recoil tritium atoms react with alkanes principally via hot abstraction (l), substitution (2), and replacement (3).1'2 The sensitivity of (1) to the T*+RH--tHT+
*R
T*+RHT"~RT+H
(1) (2)
strengths of attacked R-H bonds has been established,2p3and the present study has involved a search for C-C bond strength effects for (3). Studies of (3) have included (i) a l k a n e ~ , ~ a -(ii) ~ f halocarbon~,4b,~o (iii) alkyl silanes,4d and (iv) alkyl cyclopropane^.^^ The sum of yields from (3) in alkanes varies between 10 and 15% of the sum of yields from (1) and (2). Experimental investigations of the intramolecular competition between T-for-Rl and T-for-Rz reactions have demonstrated that the lighter products are invariably obtained in highel. yields [i.e., if ml < m2, (YR~T/YR*T) > 1.Olj2 and these observations have led to the widely The Journal of Physical Chemistry
(1) (a) A. P. Wolf, Aduan. Phys. Org. Chem., 2, 201 (1964); (b) R. Wolfgang, Progr. Reaction Kinetics, 3, 97 (1965); (c) R. Wolfgang, Ann. Rev. Phys. Chem., 16, 15 (1965). (2) The symbols used in the text include the following: (*) denotes excess translational energy; ml and mz denote the masses of alkyl radicals R1 and Rz; YR,Tand Y,, denote the absolute yields from reactions 3 and 2; l o , 2O, 3O, and 4" denote primary, secondary, tertiary, and quaternary carbon atoms; Do(R1-Rz) denotes the ordinary 298OK R1-Rz bond dissociation energy; and (R:S) and (R:S') are defined below (note 7). (3) (a) J. W.Root, TV. Breclrenridge, and F. S. Rowland, J . Chem. Phys., 43, 3694 (1965); (b) E. Tachikawa and F. S. Rowland, J . Amer. Chem. SOC.,90, 4767 (1968); (0) E. Tachikawa and F. S. Rowland, ibid., 91, 559 (1969); (d) T . Tominaga and F. S. Rowland, J . Phys. Chem., 72, 1399 (1968); (e) T. Tominaga, A. Hosaka, and F. S. Rowland, ibid., 73, 465 (1969); (f) C. C. Chou, T . Smail, and F. 8. Rowland, J . Amer. Chem. SOC.,91, 3104 (1969). (4) (a) D. Urch and R. Wolfgang, ibid., 83, 2982 (1961); (b) R. Odum and R. Wolfgang, ibid., 83, 4668 (1961); (c) R. A. Odum and R. Wolfgang, ibid., 85, 1050 (1963); (d) J. Witkin and R. Wolfgang, J . Phys. Chem., 72, 2631 (1968); (e) Y. N. Tang and F. S. Rowland, ibid., 69,4297 (1965); (f) J. W. Root, Ph.D. Dissertation, University of Kansas, 1964. (5) (a) S. W.Benson, J. Chem. Educ., 42, 502 (1965); (b) J. A. Kerr, Chem. Reu., 66, 465 (1966); (0) D. M. Golden and 9. W. Benson, ibid., 69, 125 (1969). (6) (a) J. W. Root and F. S.Rowland, J. Amer. Chem. Soc., 84, 3027 (1962); (b) J. W. Root and F. 9. Rowland, ibid., 85, 1021 (1963). (7) The per-bond relative substitution yield values used in these calculations are as follows (ref 4f): C&T, 0.64 f 0.03; CsH7T, 0.71 f 0.03; i-C4HoT, 0.76 f 0.03; n-CaHoT, 0.74 f 0.03; neo, f 0.03. CaHiiT, 0.79 0.03; &CsHiiT, 0.77 f 0.03; W C ~ H I I T0.79 The (R:S) and (R:S') ratios are defined as
*
in which n j R ! and njH denote the respective numbers of RI groups and H atoms in substance j and rjj denotes the per-bond substituttion yield factors cited above.
COMMUNICATIONS TO THE EDITOR
3175
Table I : Alkyl Replacement Reactions a t 2' Carbon in Alkanes Unoor yield ratio Reaction
Cor yield ratio
Do(Ri-Rz),
10 (R:S)
10 (R:S')
eV
2.0rt0.1 1.5 1.3 1.5 2.1 1.8 1.8 2.4
1.1 f O . l 1.1
3.69 f 0.04 3.69 3.69 3.69 3 . 5 6 =!= 0 . 0 4 3.56 3.56 3.47 f0.04
1.2 1.2 1.6 1.7 1.7 2.2
-??
3.0 -
'L 2 . 0 1.0-
-0.0
B 3.40
iLJ
3.80
3.60 D,(R,-R,)
(eV)
Figure 1. Variation of (R: S') with Do(RrR2). Experimental points: 0 , reactions at 1" carbon; A, reactions a t 2' carbon; +, reacticns at 3" carbon; and B, reactions at 4' carbon; ( ) indicates multiple values plotted a t one point. Experimental uncertainties in (R: S') are covered by plot points unless otherwise indicated; uncertainties in Do(R~-R*) are always f 0 . 0 4 eT'.
Do(Rl-R2) values. This simple (R: S) comparison, however, is based upon reaction 2 as an internal standard, a procedure which can be valid only provided intermolecular variations are negligible for all the substituassociated with (3) probably increases in the order tion reactions involved. Tests for the possible im1" < 2" < 3" < 4". portance of variations in ( 2 ) can be based upon intraThese results neither depend upon the rotational sample comparisons of several competing replacement inertia hypothesis nor are necessarily inconsistent with reactions for ab single Rl-R2 species. Of course the it. Whereas the rotational inertia model provides a reactions to be compared must belong to the same mechanistic basis for prediction of relative rates for structure class. Suitable data for this purpose are competing alkyl replacement reactions, the present listed in Table I for three 2" reactions for n-CjH12 and findings indicate that absolute alkyl replacement reactwo 2" reactions for n-CdHlo. The reactions, (R:S) tion cross sections vary with C-C bond strength.6 values, and Dl3(R1-R2)values for the n-CsH12 cases Further studies in progress include constant flux experiinclude: (i) 'I'-for-CzHs, 1.8 f 0.1, 3.56 eV; (ii) ments with alkanes, amines, and other group IVa alkyl T-for-n-C3H,, 1.8 & 0.1, 3.56 eV; and (iii) T-for-CHS, derivatives. 1.3 f 0.1,3.69 eV. The corresponding data for n-CdHlo include: (iv) T'-for-C2Hs,2.1 f 0.1, 3.56 eV; and (v) Acknowledgment. The author wishes to acknowledge T-for-CH3, 1.5 0.1, 3.69 eV. For both these molestimulating discussions with Professors B. S. Rabinocules a 0.13 eV increase in Do(RI-Rz) is accompanied vitch, University of Washington, and D. L. Bunker, by a 300/, decrease in (R :S). These observations sugUniversity of California, Irvine; cooperation of the gest that additional corrections should be applied to University of California, Berkeley, nuclear reactor staff ; the data. and support of the U. S. Atomic Energy Commission.s Relative substitution reaction efficiencies for alkanes (8) This work has been supported by the U. S. Atomic Energy Comhave been measured through mixture t e c h n i q u e ~ , ~ f ?mission ~ under Contract No. AT-(11-1)-34, Agreement 158. and the (R:S') ratios in Table I include corrections for CROCKER NUCLEAR LABORATORY AND JOHK W. ROOT this factor.' The corrected (R:S') ratios for all four DEPARTMENT OF CHEMISTRY classes of alkyl replacement are plotted vs. Do(R1-R2) UNIVERSITY O F C.4LIFORNIA values in Figure 1. The results suggest separate corDAVIS,CALIFORNIA95616 relations for the 1" and 2" cases, which are approxiRECEIVED JUNE 12, 1969 mately linear over the range of available data. A comparison of relative (R:S') values a t 3.47 eV gives 1.00, 0.49, 0.29, and0.13 (*lo%) forthe l o ,2", 3", and 4" cases, respectively, suggesting roughly 50% reducCorrection to "Fluorescent Yields of tions in (R:S') associated with each alkyl substituent 1,2,3,4-TetrahydronaphthaleneExcited bonded to the "attacked" C atom. An indication of relative bond-strength sensitivities for l o , 2", and 3" i n the Region 2850-3100 A" reactions is provided by the line slopes from Figure 1. The normalized slope values are 1.0, 0.6, and 0.3 Sir: In a recent communication,' we reported the ( A 15%), respectively, indicating that (i) the 3" reacresults of a liquid-phase study of tetralin fluorescence, tions are least sensitive to Do(R1-R2), (ii) energy range as well as results obtained in the presence of biacetyl. differences are probably important for the four classes (1) M. Grossman, G. P. Semeluk, and I. Unger, J. Phys. Chem., 73, of reactions, and (iii) the average reaction energy 1149 (1969). Volume 78, Number 0 September 1960
