| Abstract | The 1-methyl-, 1-ethoxy-, and 1-chlorocyclopropyl radicals have been observed by low-temperature EPR spectroscopy in "frozen" configurations in which the ring hydrogens that are syn and anti to the unpaired electron's orbital have different hyperfine splittings. The a H(syn)/a H(anti) ratios are 1.5 (CH 3), 1.8 (EtO), and 1.9 (Cl), all considerably lower than the ratio of ca. 3.3 found by Kawamura et al. 25 for methyl-substituted 1-fluorocyclopropyl radicals. The out-of-plane angles of the 1-substituent have been calculated from measured a 13Ca values to be 22.5° (cyclopropyl), 22.9° (CH 3), 29.1° (EtO), and 5.8° (Me 3Si). These angles are considerably smaller than those that have been calculated for some of these radicals by ab initio and other methods. Variable-temperature EPR spectroscopy on 1-methylcyclopropyl yields the following Arrhenius equation for its inversion: log (k inv/s -1) = (13.1 ± 0.3) - (3.1 ± 0.2)/2.3RT kcal/mol. For 1-ethoxycyclopropyl the rate constant for rotation about the C-OEt bond can be represented by log (k rot/s -1) = (12.5 ± 0.2) - (5.8 ± 0.2)/2.3RT. The barrier to inversion of this radical is ≥9 kcal/mol. The 1-chlorocyclopropyl radical could only be observed at very low temperatures. |
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