Faculty of Biological Sciences, University of Zielona Góra, Szafrana 1, 65-516 Zielona Góra, Poland.
Solid State Nucl Magn Reson. 2009 Nov;36(3):144-50. doi: 10.1016/j.ssnmr.2009.09.002. Epub 2009 Oct 4.
Molecular dynamics of a polycrystalline sample of (CH(3))(4)PSb(2)Br(9) (PBA) has been studied on the basis of the T(1) (24.7 MHz) relaxation time measurement, the proton second moment of NMR and the earlier published T(1) (90 MHz) relaxation times. The study was performed in a wide range of temperatures (30-337 K). The tunnel splitting omega(T) of the methyl groups was estimated as of low frequency (from kHz to few MHz). The proton spin pairs of the methyl group are known to perform a complex internal motion being a resultant of four components. Three of them involve mass transportation over and through the potential barrier and are characterized by the correlation times tau(3) and tau(T)of the jumps over the barrier and tunnel jumps in the threefold potential of the methyl group and tau(iso) the correlation time of isotropic rotation of the whole TMP cation. For tau(3) and tau(iso) the Arrhenius temperature dependence was assumed, while for tau(T)--the Schrödinger one. The fourth motion causes fluctuations of the tunnel splitting frequency, omega(T), and it is related to the lifetime of the methyl spin at the energy level. The correlation function for this fourth motion (tau(omega) correlation time) has been proposed by Müller-Warmuth et al. In this paper a formula for the correlation function and spectral density of the complex motion made of the above-mentioned four components was derived and used in interpretation of the T(1) relaxation time. The second moment of proton NMR line at temperatures below 50K is four times lower than its value for the rigid structure. The three components of the internal motion characterized by tau(T), tau(H), and tau(iso) were proved to reduce the second moment of the NMR line. The tunnel jumps of the methyl group reduce M(2) at almost 0K, the classical jumps over the barrier reduce M(2) in the vicinity of 50K, while the isotropic motion near 150K. Results of the study on the dynamics of CH(3) groups of TMP cation based on the second moment measurements were correlated with those based on T(1) time measurements.
(CH(3))(4)PSb(2)Br(9) (PBA) 的多晶样品的分子动力学已经基于 T(1) (24.7 MHz) 弛豫时间测量、质子 NMR 二阶矩和早先发表的 T(1) (90 MHz) 弛豫时间进行了研究。研究在很宽的温度范围内（30-337 K）进行。估计甲基基团的隧道分裂 ω(T) 为低频（从 kHz 到几 MHz）。已知甲基基团的质子自旋对执行复杂的内部运动，是四个分量的结果。其中三个涉及质量运输跨越和穿过势垒，并由跨越势垒的跳跃和甲基基团的三倍势垒中的隧道跳跃的相关时间 tau(3) 和 tau(T)以及整个 TMP 阳离子的各向同性旋转的相关时间 tau(iso)来表征。对于 tau(3) 和 tau(iso)，假设了 Arrhenius 温度依赖性，而对于 tau(T)——Schrödinger 依赖性。第四个运动导致隧道分裂频率 ω(T)的波动，并且与甲基自旋在能级上的寿命有关。Müller-Warmuth 等人提出了第四个运动的相关函数（tau(omega) 相关时间）。在本文中，推导出了由上述四个分量组成的复杂运动的相关函数和谱密度公式，并用于解释 T(1)弛豫时间。在低于 50 K 的温度下，质子 NMR 线的二阶矩比刚性结构的二阶矩低四倍。由 tau(T)、tau(H)和 tau(iso)表征的内部运动的三个分量降低了 NMR 线的二阶矩。甲基基团的隧道跳跃几乎在 0 K 时降低 M(2)，经典的跨越势垒的跳跃在 50 K 附近降低 M(2)，而各向同性运动则在 150 K 附近降低 M(2)。基于二阶矩测量的 TMP 阳离子 CH(3)基团动力学研究结果与基于 T(1)时间测量的结果相关联。
