Böhmer Till, Richter Timo, Gabriel Jan Philipp, Zeißler Rolf, Weigl Peter, Pabst Florian, Blochowicz Thomas
Institute for Condensed Matter Physics, Technical University of Darmstadt, 64289 Darmstadt, Germany.
Glass and Time, IMFUFA, Department of Science and Environment, Roskilde University, Roskilde, Denmark.
J Chem Phys. 2023 Aug 7;159(5). doi: 10.1063/5.0160894.
We investigate the reorientation dynamics of four octanol isomers with very different characteristics regarding the formation of hydrogen-bonded structures by means of photon-correlation spectroscopy (PCS) and broadband dielectric spectroscopy. PCS is largely insensitive to orientational cross-correlations and straightforwardly probes the α-process dynamics, thus allowing us to disentangle the complex dielectric relaxation spectra. The analysis reveals an additional dielectric relaxation contribution on time scales between the structural α-process and the Debye process. In line with nuclear magnetic resonance results from the literature and recent findings from rheology experiments, we attribute this intermediate contribution to the dielectric signature of the O-H bond reorientation. Due to being incorporated into hydrogen-bonded suprastructures, the O-H bond dynamically decouples from the rest of the molecule. The relative relaxation strength of the resulting intermediate contribution depends on the respective position of the hydroxy group within the molecule and seems to vanish at sufficiently high temperatures, i.e., exactly when the overall tendency to form hydrogen bonded structures decreases. Furthermore, the fact that different octanol isomers share the same dipole density allows us to perform an in-depth analysis of how dipolar cross-correlations appear in dielectric loss spectra. We find that dipolar cross-correlations are not solely manifested by the presence of the slow Debye process but also scale the relaxation strength of the self-correlation contribution depending on the Kirkwood factor.
我们通过光子相关光谱法（PCS）和宽带介电谱法研究了四种具有截然不同氢键结构形成特性的辛醇异构体的重新取向动力学。PCS对取向交叉相关性基本不敏感，能直接探测α过程动力学，从而使我们能够解开复杂的介电弛豫谱。分析揭示了在结构α过程和德拜过程之间的时间尺度上存在额外的介电弛豫贡献。与文献中的核磁共振结果以及流变学实验的最新发现一致，我们将这种中间贡献归因于O - H键重新取向的介电特征。由于被纳入氢键超结构中，O - H键与分子的其余部分动态解耦。所得中间贡献的相对弛豫强度取决于羟基在分子中的各自位置，并且在足够高的温度下似乎消失，即恰好在形成氢键结构的总体趋势降低时。此外，不同辛醇异构体具有相同偶极密度这一事实使我们能够深入分析偶极交叉相关性在介电损耗谱中是如何出现的。我们发现偶极交叉相关性不仅通过缓慢的德拜过程的存在来体现，而且还根据柯克伍德因子缩放自相关贡献的弛豫强度。
