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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Effects of Restricted Rotations and Dynamic Averaging on the Calculated Isotropic Hyperfine Coupling Constants of the bis-Dimethyl and bis-Di(trifluoromethyl) Nitroxide Radicals
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Effects of Restricted Rotations and Dynamic Averaging on the Calculated Isotropic Hyperfine Coupling Constants of the bis-Dimethyl and bis-Di(trifluoromethyl) Nitroxide Radicals

机译：限制旋转和动态平均对双-二甲基和双-二（三氟甲基）氮氧化物自由基的各向同性超精细耦合常数的影响

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摘要

The rotational effects of the CH3 and CF3 groups on the electronic structure and nuclear hyperfine coupling constants (HFCCs) in dimethylnitroxide (DMNO· ) and ditrifluoro-methynitroxide (TFMNO· ) are investigated using the UB1LYP hybrid density functional method. The CH3 and CF3 HFCCs of both radicals are found to obey the McConnell relation during rotation. The two CH3 groups of the DMNO· do not gear with each other, but the rotation of the first CH3 group induces only a small rocking effect (～7°) in the second group. However, in TFMNO· , the fluorine atoms from different CF3 groups are close enough so that the steric repulsion between them causes them to act as two interlocked gears, where one drives the other. Therefore, both CF3 groups undergo full rotation. To the best of our knowledge, this is only the second example of “gearing” to be studied. Stabilization due to hyperconjugation is also a major factor that affects the magnitudes of the HFCCs of the CF3 groups during rotational averaging. Stable configurations at specific CF3 group orientations have a large overlap with the NO π-electron cloud because the lobes of the hybridized pσ(F2), pσ(F3), pσ(F5), and pσ(F6) orbitals along the F-C bonds have cylindrical symmetry and are of the correct phases for hyperconjugation to occur. The calculated TFMNO· C1-N and C2-N bond orders range from 0.91 to 0.95 as the CF3 groups are rotated. Therefore, the C-N bonds are essentially single bonds. This, in conjunction with the low rotational energy barrier of approximately 50 cm-1, explains why the EPR intensities of the 19F hyperfine splittings, in the range of 163-297 K, are characteristic of six equivalent rapidly rotating fluorine atoms. The TFMNO· out-of-plane NO vibrations induce additional s character at the 14N nucleus. This increases the magnitude of the 14N HFCC and decreases the 19F HFCCs. As the temperature increases and because of mixing of the first excited out-of-plane vibrational state, the NO vibrational amplitudes also increase. This leads to an increased 14N HFCC and decreased 19F HFCCs, which is in agreement with experiment.
机译：利用UB1LYP杂化密度泛函方法研究了CH3和CF3基团对二甲基硝基氧化物（DMNO·）和二三氟甲基甲基硝基氧化物（TFMNO·）中电子结构和核超细偶合常数（HFCC）的旋转作用。发现两个自由基的CH3和CF3 HFCC在旋转过程中服从McConnell关系。 DMNO·的两个CH3组互不啮合，但是第一组CH3的旋转仅在第二组中引起很小的摇摆效应（约7°）。然而，在TFMNO·中，来自不同CF3基团的氟原子足够接近，因此它们之间的空间排斥作用使它们充当两个互锁齿轮，一个驱动另一个齿轮。因此，两个CF3组均进行完整旋转。据我们所知，这只是要研究的“齿轮”的第二个例子。超共轭引起的稳定性也是影响旋转平均过程中CF3组HFCC幅度的主要因素。在特定CF3基团取向上的稳定构型与NOπ电子云有很大的重叠，因为沿着FC键的杂化pσ（F2），pσ（F3），pσ（F5）和pσ（F6）轨道的波瓣具有圆柱对称，是发生超共轭的正确相位。随着CF3基团的旋转，计算出的TFMNO·C1-N和C2-N键序范围为0.91至0.95。因此，C-N键本质上是单键。结合约50 cm-1的低旋转能垒，可以解释为什么19F超细裂隙的EPR强度在163-297 K范围内是六个当量快速旋转的氟原子的特征。 TFMNO·面外NO振动会在14N原子核处引起额外的s特性。这增加了14N HFCC的大小，并减少了19F HFCC。随着温度升高并且由于第一激发的平面外振动状态的混合，NO振动幅度也增大。这导致增加的14N HFCC和减少的19F HFCC，这与实验一致。

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