Long-range proton hyperfine splittings in the epr spectra of cyclohexyl and alkylcyclohexyl radicals: ring inversion in the cyclohexyl radical
Long-range proton hyperfine splittings in the epr spectra of cyclohexyl and alkylcyclohexyl radicals: ring inversion in the cyclohexyl radical
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环己基和烷基环己基自由基的 EPR 谱中的长程质子超精细分裂：环己基自由基中的环反转
DOI：
10.1039/p29920002025
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发表时间：
1992
期刊：
影响因子：
--
通讯作者：
A. Steel
中科院分区：
文献类型：
--
作者：
B. P. Roberts;A. Steel
在低温下，通过电子顺磁共振（EPR）光谱对液态环丙烷中的环己基自由基进行了研究，并且首次观测到了来自γ -质子的超精细分裂。考虑到β -、γ -和δ -质子交换，对温度相关的EPR光谱进行了计算机模拟，从而为该自由基中椅式 - 椅式相互转化提供了一个新的阿累尼乌斯速率表达式[方程（A）]（θ = 2.303 RT kJ mol⁻¹）。
log₁₀(kₑₓₕ / s⁻¹) = (13.5 ± 0.3) - (23.5 ± 1.1) / θ （A）
4 - 叔丁基环己基自由基由于赤道位的叔丁基而构象锁定，其EPR光谱没有表现出与温度相关的线形效应，也没有可分辨的来自轴向δ -质子的分裂。顺 - 3,5 - 二甲基环己基自由基也被锁定在椅式构象中，其EPR光谱显示出来自两个轴向γ -质子和赤道位δ -质子的远程分裂。从头算和半经验（AM1）分子轨道计算预测环己基自由基在Cα处有一个略微非平面的平衡构型；Hα倾向于处于赤道位置，但平面化的能量代价非常小。
The cyclohexyl radical has been studied by EPR spectroscopy in liquid cyclopropane at low temperatures and hyperfine splittings from the γ-protons have been observed for the first time. The temperature-dependent EPR spectra have been computer simulated, taking account of β-, γ- and δ-proton exchange, to afford a new Arrhenius rate expression [eqn. (A)] for chair-to-chair interconversion in this radical (θ= 2.303 RT kJ mol–1). log10(kexch/s–1)=(13.5 ± 0.3)–(23.5 ± 1.1)/θ(A)The 4-tert-butylcyclohexyl radical is conformationally locked by the equatorial tert-butyl group and its EPR spectrum does not exhibit temperature-dependent lineshape effects or resolvable splitting from the axial δ-proton. The cis-3,5-dimethylcyclohexyl radical is also locked in a chair conformation and its EPR spectrum shows long-range splittings from two axial γ-protons and from the equatorial δ-proton. Ab initio and semi-empirical (AM 1) molecular orbital calculations predict a slightly non-planar equilibrium configuration at Cα in the cyclohexyl radical; Hα prefers equatorial placement, but the energetic cost of planarisation is very small.