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J. Phys. Chem. 1981, 85,367-374
for CH3D and CH3Br (columns 5 and 6 in Table VI) are basically identical for ranges I1 and 111. Range I represents an extreme case (thermalization of hot D by CH4 is suppressed) yet the CH3D and CH3Br yields are consistent with those values from the other two ranges. Moreover, the quantum yields for Dz(column 4, Table VI) or Hz (column 4, Table VII) are virtually the same for all three ranges. In range I, for example, where thermalization is suppressed, the adjustable barrier width parameters and the initializationtechnique compensate for the suppression of direct thermalization of the hot hydrogens. Thus, the results appear to be more reliable than what might be expected from a relatively simplistic scheme. Further comparisons, which are presented in the following paragraph, support this observation. Tables VI and VI1 list the calculated quantum yields of the major products: DP,CH3D, CH3Br, C2H6formed in the (DBr + CH,) system and Ha, CD3H, CD3Br, CzD6 formed in the (HBr + CD4)system. In our scheme, each adsorbed photon produces a hot hydrogen with an excess energy of about 3 eV. Thus, the quantum yields of the various methyl products (e.g., CH3Br) are a direct measure of the percentage of successful hot hydrogen reactions with methane; however, no provision was made for secondary photochemical reactions. If qi is the quantum yield of the ith product, then the total percentage of successful hot hydrogen reactions with methane is (qCHBD+ qCHsBf + 2qc2HB)X 100% for the (DBr + CH4) system. A similar relationship is obtained for the (HBr CD4)system. For the best-fit set (6-10) in Table VI, we find that the percentage of successful hot D reactions with CH4is between 17 and 18% for the first four experiments and drops to 14% for the last experiment (labeled 10). The mean value over this set of five experiments is 17%. This value for the successful percentage of hot D reactions with CH, is
+
also obtained in the next best-fit set of experiments (11-15). Thus, the predicted percentage of hot D atoms involved in a successful hot reaction with methane is 17%. Similarly,we find with respect to the (HBr CD4)system (see Table VII) that the mean percentage of successful hot H reactions with CD4is 6% from both the best fit and next best fit of experiments (610; 11-15). Martin and Willards quote a value of 17% for 3-eV D atoms undergoing reaction with CH4. Similarly, from their Table 111, column 5, the percentage of successful hot H reactions with CD4is 6.2%. The agreement is excellent between experimentally determined values and our calculations.
+
Conclusion
A semiempirical approach, which permits hot-atom reactions to be melded with nonhot-atom reactions in a kinetic scheme, is outlined and appears to work unusually well for the two cases explored. The simulated results were generally in good agreement with the experimental findings of Martin and Willard. The results pertaining to the percentage of successful hot hydrogen reactions with methane for either the (DBr + CHI) or the (HBr + CD4) system agreed so closely with those of Martin and Willard as to be surprising. The surprise stems from the fact that our values were obtained through detailed calculations involving a large set of hot and nonhot reactions. It would be extremely useful to determine experimentally the quantities of major products formed in these or similar systems and, in an effort to determine whether our method does match reality to the extent that this exercise suggests, to compare them with those predicted by our calculations. Acknowledgment. We thank Dr. Richard Jaffe for his extremely helpful discussions.
ESR Study of ?-Irradiated Substituted Norbornanes in Thiourea Clathrate and Adamantane Matrix. Novel 2-Norbornyl-Type Radicals A. Faucitano,* A. Buttafava, F. Faucitano Martinotti, Istituto di Chmica General8 ed Inorganica dell'Universiti dl Pavia, Italy
and S. Cesca Assoreni, Polymer Research, San Donato Milanese, Italy (Received: October 12, 1979; In Final Form: October 7, 1980)
The y irradiation at 77 K of 2-methyl-2-ethylbicyclo[2.2.l]heptane and of 2-methylene-2-ethylidenebicyclo[2.2.1] heptene in the state of thiourea adducts leads to formation of 2-methyl- and 2-ethylnorbornyl radicals by loss of hydrogen atoms at the substituent sites or by partial hydrogenation of the double bonds. On warming above 77 K after irradiation, the 2-alkylnorbornyls add to double bonds of neighboring molecules yielding new adduct radicals alkylnorbornyl. The reactions can be reversed by UV irradiation at 77 K, thus suggesting that the addition does not proceed beyond the first step. A reaction model based on the geometrical control by the molecular packing within the clathrate channels has been proposed. The irradiation of 2-ethylidenenorbornene in the state of thiourea clathrate or trapped in adamantane matrix yields an allyl-typeradical by loss of a hydrogen atom from the methyl group. The structure and ESR properties of 2-alkylnorbornyls and of the allylnorbornyl have been investigated by MO methods to the INDO and extended Huckel levels of approximation. Introduction The radiation-chemistry studies of rigid polycyclic hydrocarbons is of particular interest from at least two different poink of view: (a) Owing to the variety of molecular geometries available, there is the possibility of obtaining 0022-3654/81/2085-0367$01 .OO/O
information regarding the structure dependency of the radiation-chemical behavior of hydrocarbons; (b) because of the rigidity of the structures which prevents extensive molecular rearrangements following the rupture of C-H bonds, free-radical intermediates having peculiar geome0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 4, 1981
Faucitano et ai.
tries around the free valence centers can often be obtained. Pursuing an investigation started in this field,1-3in this paper we refer to the results of an ESR study of some irradiated alkyl- and alkenyl-substituted norbornanes and norbornenes, namely, 2-methyl-, 2-ethyl-, 2-methylene-, and 2-ethylidenenorbornane and 2-ethylidenenorbornene (H4MNB, H4ENB, H,MNB, H2ENB, and ENB, respectively). These hydrocarbons have been irradiated in the 4
H,MNB
H4ENB
H,MNB
H,ENB
ENB
state of inclusion compounds in thiourea (H4MNB,HIENB, H2ENB, H,MNB) and in adamantane (ENB). The calthrate technique, beside affording the usual advantage (better resolved ESR spectra, stablization of transient intermediates4),adds further points of interest which are related to energy-transfer phenomena between the matrix and the host compounds and to the possibility of geometrical control of bimolecular reactions within the channel structure.
Experimental Section H4MNB,H4ENB, H2ENB, and HzMNBwere obtained by controlled hydrogenation of commercially available 2-methylene- and 2-ethylidenenorbornene, over Pd/C catalyst. All of the compounds were submitted to repeated fractional distillation prior to use, and their purity was checked by GLC on 4-m Carbowax 20M columns. The thiourea adducts were precipitated as needlelike crystals by adding -20% of the pure hydrocarbons to saturated 2-propanol solutions of thiourea. Adamantane doped with ENB (mixture of geometrical isomers5)was prepared according to the procedure described in the literature! The irradiations and ESR measurements were performed following the methods described in previous Calculations The simulation of first-order isotropic ESR spectra and molecular orbital calculations to the INDO' and extended Hucke18 levels of approximation were performed on a Honeywell 6030 computer by using Fortran IV QCPE programs. In the extended Huckel theory (EHT) approach, the k constant in the Wolsberg-Helmholtz equation for the approximation of off-diagonal resonance integrals Hw was taken equal to 1.75; the diagonal elements were essentially those of Skinner and Pritchard (HIS= -13.6, CZs= -21.4, CPp= -11.4 eV).9 The isotropic proton hfs was calculated from the coefficients of the HISatomic orbital in the MO of the unpaired electron, by the equation aH= 878C1,2.lo Within the INDO framework, the equation (1)A. Faucitano, F.Faucitano Martinotti, and S. Cesca, J. Chem. Soc.,
Perkin Trans. 2, 1694 (1974).
(2) A. Faucitano,A. Buttafava, F. Faucitano Martinotti,and S. Cesca, J . Chem. SOC., Perkin Trans. 2, 1017 (1976). (3) A. Faucitano,A. Buttafava, F. Faucitano Martinotti, S. Cesca, and R.Fantechi, J. Phys. Chem., 81, 354 (1977). (4)A. Faucitano, A. Perotti, G. Allara, and Faucitano Martinotti, J. Phys.Chem., 76, 801 (1972). (5) C. Tosi, F. Ciampelli, and N. Cuneli, J. Appl. Polym.Sci., 16,801 (1972). (6)D.E.Wood and R. V. Lloyd, J. Chem. Phys.,53, 3932 (1970). (7)A. Pople and 0. L. Beveridge, "Approximate Molecular Orbital Calculation", McGraw-Hill, New York, 1970. (8) R. Hoffmann and W. N. Lipscomb, J. Chem. Phys., 2179 (1962). (9)H. 0.Pritchard and H. A. Skinner, Chem. Reu., 55, 745 (1955).
Figure 1. ESR spectrum of 2-methylnorbornyl radical: (A) recorded at 240 K following y irradiation and UV bleaching at 77 K; (B) computer simulation: a(1H) = 39.5 0,a(1H) = 22.3 G, a(2H) = 7.1 G, a(1H) = 22.3 G, line width = 5.2 G.
aH = 5 3 9 . 8 6 was ~ ~used; ~ ~ ~PSHSHis the 1s orbital unpaired spin density at the hydrogen nucleus, obtained from the elements of the (Y and 0spin density matrices. In all MO calculations on compounds where the undistorted bicyclo[2.2.l]heptyl ring is present, the following geometrical parameters, derived from the work by Hideyuki Konishi et al.I1 have been employed: LHCH
110" 94" diedro 115"
c;-c; C-H
LC,C,C, 43'1' Lc1c2c3
104" 104" 106"
1.53 A 1.56 A 1.55 A 108 A
Results ESR Spectra and Nature of the Radicals. Following the irradiation at 77 K, the low-temperature (-90 K) ESR spectra of thiourea clathrates are characterized by the presence of an asymmetrical doublet with a center displaced toward low field. This signal is superimposed to a larger splitting component which it will be shown is due to alkylnorbornyl radicals. The doublet is commonly encountered in irradiated thiourea clathrates independently of the nature of the guest compounds;12as a consequence it must be attributed to a species from the radiolysis of thiourea. The large g factor (>2.01) strongly suggests that a significant portion of odd electron density is localized on a sulfur atom; this, is sufficient to rule out anion radicals of the type (H2N),CS-. A more reasonable hypothesis might be based on the radical cation (H2N)2C+=S or on neutral thiyl radicals (H2N)2(R)CSformed by addition of radicals precursors R. to C=S bonds. When activated C-H or double bonds are available in the guest compound (as in alcohols and vinyl monomers), the sulfur radicals are capable of undergoing postirradiation hydrogen ab(10)G. R.Underwood and R. S. Givens, J.Am. Chem. SOC.,90,3713 (1968). (11)Hideyuki Konishi, Hiroshi Kato, and Teijiro Yonezawa, Bull. Chem. SOC.Jpn., 43, 1676 (1970). (12)A. Faucitano, A. Buttafava, and F. Faucitano Martinotti, J Chem. SOC.,Perkin Trans. 2, 1014 (1976).
ESR Study of y-Irradiated Substituted Norbornanes
The Journal of Physical Chemlstty, Vol. 85, No. 4, 198 1 360
TABLE I : INDO and Extended Huckel Calculations of Proton hfs for 2-Methylnorbornyl Radical. Effect of Nonplanarity of Radical Center
INDO
- 30"
ext Huckel
-10" 0" t10" t20" 4.1 3.0 1.9 1 7.2 6.3 5.3 37.9 35.0 31.0 39.2 37.7 39.0 3ex0 32.4 32.8 32.0 27.8 30.7 iendo 24.3 -1.0 -1.1 -1.0 -0.3 -0.7 -0.1 0.0 0.0 -0.1 5em 0.0 0.1 0.1 0.1 0.1 endo 0.1 6O , 6.3 4.8 4.0 3.1 2.0 1.o -1.3 -1.5 -1.6 -1.6 -1.5 -1.3 Sendo -1.0 -1.1 -1.1 -1.2 7sin -0.9 7,ti 0.2 1.0 1.9 2.8 3.7 (CH,), 23.3 25.6 24.8 24.6 23.2 8 is defined as the angle of the C,-CH, bond with the C,C,C, plane. 4a
-20"
+30" 0.9 26.1 30.2 -0.6 0.0 0.1 0.2 -1.1 -1.2 4.4
21.9
A
0" 2.3 28.8 23.7 0.0 1.2 0.0 3.1 0.2 0.0 1.2 16.8
\
-20"
expt
3.7 29.8 21.6 0.0 2.0 0.0 3.7 0.4 0.0 0.8 16.1
7.1 39.5 22.3
