Driver G W, Huang Y, Laaksonen A, Sparrman T, Wang Y-L, Westlund P-O
Department of Chemistry, Umea University, SE 90187 Umea, Sweden.
Phys Chem Chem Phys. 2017 Feb 15;19(7):4975-4988. doi: 10.1039/c6cp05801a.
Proton/fluoride spin-lattice (T) nuclear magnetic relaxation dispersion (NMRD) measurements of 1-butyl-3-methyl-1H-imidazolium hexafluorophosphate, [Cmim][PF], have been carried out using high field spectrometers and a fast-field-cycling instrument at proton Larmor frequencies ranging from 10 kHz to 40 MHz, at different temperatures. The NMRD profiles are interpreted by means of a simple relaxation model based on the inter- and intra-ionic dipole-dipole relaxation mechanism. Using an atomic molecular-ion dynamic simulation at 323 K the relevant spin dipole-dipole (DD) correlation functions are calculated. The results indicate that the NMRD profiles can be rationalized using intra- and inter-ionic spin DD interactions, however, anions are mainly modulated by ionic reorientation because of temporary correlations with cations, where modulation by translational diffusion plays a minor role. Reorientational dynamics of charge-neutral ion couples (i.e. [Cmim][PF]) and [Cmim] ions are in the nano-second (ns) time range whereas the reorientation of [PF] is characterized by a reorientational correlation time in the pico-second (ps) regime. Based on the NMRD profiles we conclude that the main relaxation mechanism for [PF] is due to fast internal reorientational motion, a partially averaged F-F intra- and F-H inter-ionic DD coupling as the anion resides in close proximity to its temporary oppositely charged cation partner. The F-T-NMRD data display a ns dispersion which is interpreted as being due to correlated reorientational modulations resulting from the H-containing charge-neutral ion couple [Cmim][PF]. The analysis of ionicity is based on the free anion fraction, f, and it increases with temperature with f → 1 at the highest temperatures investigated. The fraction is obtained from the H-F NMRD profiles as correlated-non-correlated dynamics of the ions. The analysis of T relaxation rates of C, H, F and P at high fields cannot generally give the fraction of ions but is consistent with the interpretation based on the NMRD profiles with relaxation contributions due to DD-intra and -inter, CSA-intra (and -inter for C), including spin rotation for P. The investigation has led to a description of the mechanics governing ion transport in the title ionic liquid via identification of transient correlated/non-correlated ion dynamics.
使用高场光谱仪和快速场循环仪器，在10 kHz至40 MHz的质子拉莫尔频率范围内、不同温度下，对1-丁基-3-甲基-1H-咪唑鎓六氟磷酸盐（[Cmim][PF₆]）进行了质子/氟化物自旋晶格（T₁）核磁共振弛豫色散（NMRD）测量。通过基于离子间和离子内偶极-偶极弛豫机制的简单弛豫模型来解释NMRD谱图。利用323 K下的原子分子离子动力学模拟计算了相关的自旋偶极-偶极（DD）相关函数。结果表明，NMRD谱图可以通过离子内和离子间的自旋DD相互作用来合理解释，然而，由于与阳离子的临时相关性，阴离子主要受离子重取向的调制，其中平移扩散的调制作用较小。电荷中性离子对（即[Cmim][PF₆]）和[Cmim]离子的重取向动力学处于纳秒（ns）时间范围，而[PF₆]的重取向以皮秒（ps） regime中的重取向相关时间为特征。基于NMRD谱图，我们得出结论，[PF₆]的主要弛豫机制是由于快速的内部重取向运动，当阴离子与其临时带相反电荷的阳离子伙伴紧密相邻时，F-F离子内和F-H离子间的DD耦合部分平均化。F-T NMRD数据显示出纳秒级的色散，这被解释为是由于含H的电荷中性离子对[Cmim][PF₆]引起的相关重取向调制。离子性分析基于自由阴离子分数f，并且它随温度升高而增加，在所研究的最高温度下f → 1。该分数是从H-F NMRD谱图中作为离子的相关-不相关动力学获得的。在高场下对C、H、F和P的T₁弛豫率的分析通常不能给出离子分数，但与基于NMRD谱图的解释一致，其中弛豫贡献来自DD-内和-间、CSA-内（以及C的CSA-间），包括P的自旋旋转。通过识别瞬态相关/不相关离子动力学，该研究得出了对标题离子液体中离子传输控制机制的描述。
