Dephasing of electron spin echoes for nitroxyl radicals in glassy solvents by non-methyl and methyl protons
Dephasing of electron spin echoes for nitroxyl radicals in glassy solvents by non-methyl and methyl protons
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DOI：
10.1080/00268979809483256
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发表时间：
1998-12-20
影响因子：
1.7
通讯作者：
Lindgren, M
中科院分区：
文献类型：
--
作者：
Zecevic, A;Eaton, GR;Lindgren, M
在II K和40 K的玻璃态溶剂中的硝酰基自由基的0.1-0.4 mM溶液的双脉冲电子自旋回波的退相主要由溶剂中的核自旋决定。低于约70 K时，4-氧代-2，2，6，6-四甲基哌啶-1基（tempone）和Fremy盐（过丁胺二磺酸盐）的失相速率相同。在不含甲基的溶剂中，回波移相的速率常数随着质子浓度的增加而线性增加。回波衰减曲线的形状可以用基于由于具有30 +/-5Hz M-1的速率常数的质子触发器引起的失相的模型来模拟。在相同的总质子浓度的电子自旋回波衰减率是更快的溶剂中含有甲基比溶剂中没有甲基。几个观察结果表明，溶剂中的甲基对回波移相的影响是由于量子力学隧道效应：（a）在11 K和40 K之间，回波退相近似与温度无关，（B）甲基质子的表观自旋翻转速率W-m与甲基质子的浓度无关，（c）甲基质子的W-m的估计值以受阻质子<较少受阻质子<芳香族甲基的顺序增加，这是降低经典旋转势垒并因此增加隧穿频率的顺序，和（d）由于CF 3基团中的氟引起的退相与非甲基质子的退相相当。这些结果表明，硝酰基自旋回波退相可以用来探测自由基的质子环境。
Dephasing of two-pulse electron spin echoes for 0.1-0.4 mM solutions of nitroxyl radicals in glassy solvents at II K and 40 K is dominated by nuclear spins in the solvent. Below about 70 K the rate of dephasing is the same for 4-oxo-2,2,6,6-tetramethylpiperin-1yl (tempone) and for Fremy's salt (peroxylaminedisulphonate). In solvents that do not contain methyl groups the rate constant for echo dephasing increases linearly with increasing proton concentration. The shape of the echo decay curve could be simulated with a model based on dephasing due to proton flip-flops with a rate constant of 30 +/- 5 Hz M-1. At the same total proton concentration the electron spin echo decay rate is faster in solvents that contain methyl groups than in solvents without methyl groups. Several observations indicate that the effect of the methyl groups in the solvent on the echo dephasing is due to quantum mechanical tunnelling: (a) the echo dephasing is approximately independent of temperature between 11 K and 40 K, (b) the apparent spin flip-flop rate, W-m, for the methyl protons is independent of the concentration of methyl protons, (c) the estimated values of W-m for the methyl protons increase in the order hindered aliphatics < less hindered aliphatics < aromatic methyls, which is the order of decreasing barriers to classical rotation and therefore increasing tunnelling frequency, and (d) dephasing due to fluorines in CF3 groups is comparable with that for non-methyl protons. These results indicate that the nitroxyl spin echo dephasing can be used to probe the proton environment of the radical.