Free radicals in an Adamantane matrix. XI. Electron paramagnetic resonance study of conformations in the $beta$-halo-tert-butyl radicals
The $beta$-halo-tert-butyl radicals were prepared by x-irradiation of the corresponding isobutyl halides in an adamantane matrix at 77$sup 0$K and their conformations were determined by analysis of their EPR spectra. The radicals are nonplanar at the radical site, the fluoro and chloro radicals trans eclipsed and the bromo and iodo radicals gauche staggered with respect to the relative orientation of the carbon halogen bond and the direction of the singly occupied orbital. Vibration-rotation motions about the favored conformation are much larger for the fluoro radical than for the others. The rate of interconversion of the inequivalent methylene protons is approximately 1.5 x 10$sup 9$ sec$sup -1$ for the bromo radical at 202$sup 0$K while it is too slow to measure for the iodo radical at the same temperature. The barrier to interconversion has a lower limit of 3 kcal/mol for the bromo radical and higher than that for the iodo radical. The halogen and proton hfsc in gauss and the g values for the XCH$sub 2$C(CH$sub 3$$)$$sub 2$ radicals are: $sup 19$F = 103.7, CH$sub 2$ = 10.4, CH$sub 3$ = 23.3, g = 2.0030 at 214 K; $sup 35$Cl = 19.5, CH$sub 2$ = 6.3, CH$sub 3$ = 21.1, g = 2.0042 at 215 K; $sup 81$Br = 6.7, CH$sub 2$ = 21.4, 42.7, CH$sub 3$ = 21.4, g = 2.0010 at 202 K; $sup 127$I = 7.0, CH$sub 2$ = 21.9, 43.8, CH$sub 3$ = 21.9, g = 2.0009 at 208 K. The fluoro radical decays to nonradical products above 318$sup 0$K, the chloro radical converts to 2-methyl allyl radical above 306$sup 0$K, the bromo radical converts to tert-butyl radical by exchange with a matrix proton (or deuteron) at 209$sup 0$K as does the iodo radical above 225$sup 0$K. Photolysis with a Xe lamp converts the bromo and iodo radicals to nonradical products in less than the experimental time constant of 0.3 sec. The hypothesis is put forward that the nonplanarity and high barrier to rotation observed explain the retention of stereochemical configuration in reactions involving $beta$-chloro, $beta$-bromo, and $beta$-iodo alkyl radicals. (auth)
- Research Organization:
- Univ. of Connecticut, Storrs
- Sponsoring Organization:
- USDOE
- NSA Number:
- NSA-33-002668
- OSTI ID:
- 4166116
- Journal Information:
- J. Am. Chem. Soc., v. 97, no. 21, pp. 5986-5992, Journal Name: J. Am. Chem. Soc., v. 97, no. 21, pp. 5986-5992; ISSN JACSA
- Country of Publication:
- United States
- Language:
- English
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