Department of Chemistry and Centre for Laser, Atomic and Molecular Sciences, University of New Brunswick, Fredericton, New Brunswick, Canada.
J Phys Chem A. 2009 Oct 22;113(42):11435-42. doi: 10.1021/jp9076646.
The rotational effects of the CH(3) and CF(3) 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 CH(3) and CF(3) HFCCs of both radicals are found to obey the McConnell relation during rotation. The two CH(3) groups of the DMNO* do not gear with each other, but the rotation of the first CH(3) group induces only a small rocking effect ( approximately 7 degrees ) in the second group. However, in TFMNO*, the fluorine atoms from different CF(3) 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 CF(3) 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 CF(3) groups during rotational averaging. Stable configurations at specific CF(3) group orientations have a large overlap with the NO pi-electron cloud because the lobes of the hybridized p(sigma)(F(2)), p(sigma)(F(3)), p(sigma)(F(5)), and p(sigma)(F(6)) orbitals along the F-C bonds have cylindrical symmetry and are of the correct phases for hyperconjugation to occur. The calculated TFMNO* C(1)-N and C(2)-N bond orders range from 0.91 to 0.95 as the CF(3) 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 (19)F 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 (14)N nucleus. This increases the magnitude of the (14)N HFCC and decreases the (19)F 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 (14)N HFCC and decreased (19)F HFCCs, which is in agreement with experiment.
使用 UB1LYP 杂化密度泛函方法研究了 CH(3)和 CF(3)基团在二甲氧基氮氧化物（DMNO*）和三氟甲氧基氮氧化物（TFMNO*）中的转动效应对电子结构和核超精细耦合常数（HFCCs）的影响。发现这两种自由基的 CH(3)和 CF(3)HFCCs 在旋转过程中都符合 McConnell 关系。DMNO的两个 CH(3)基团彼此不同步，但第一个 CH(3)基团的旋转仅在第二个基团中引起微小的摇摆效应（约 7 度）。然而，在 TFMNO中，来自不同 CF(3)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成为两个互锁齿轮，一个驱动另一个。因此，两个 CF(3)基团都进行了完全旋转。据我们所知，这是第二个研究的“齿轮”示例。超共轭稳定化也是影响 CF(3)基团在旋转平均过程中 HFCCs 大小的主要因素。在特定 CF(3)基团取向的稳定构型与 NO π 电子云有很大的重叠，因为沿着 F-C 键的杂化 p(sigma)(F(2))、p(sigma)(F(3))、p(sigma)(F(5))和 p(sigma)(F(6))轨道的 lobes 具有圆柱对称性，并且处于正确的相位发生超共轭。当 CF(3)基团旋转时，计算得到的 TFMNOC(1)-N 和 C(2)-N 键级范围为 0.91 至 0.95。因此，C-N 键基本上是单键。这与约 50 cm(-1)的低旋转能垒相结合，解释了为什么在 163-297 K 范围内，（19）F 超精细分裂的 EPR 强度是六个等效快速旋转氟原子的特征。TFMNO的面外 NO 振动在（14）N 核中诱导额外的 s 特征。这增加了（14）N HFCC 的大小，并减小了（19）F HFCCs。随着温度的升高以及由于第一激发面外振动态的混合，NO 振动幅度也增加。这导致（14）N HFCC 增加和（19）F HFCCs 减少，这与实验结果一致。
