Ladanyi Branka M, Nugent Severin
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.
J Chem Phys. 2006 Jan 28;124(4):044505. doi: 10.1063/1.2148967.
We present here the results of molecular-dynamics simulation of solvation dynamics in supercritical CO(2) at a temperature of about 1.05T(c), where T(c) is the critical temperature, and at a series of densities ranging from 0.4 to 2.0 of the critical density rho(c). We focus on electrostatic solvation dynamics, representing the electronic excitation of the chromophore as a change in its charge distribution from a quadrupolar-symmetry ground state to a dipolar excited state. Two perturbations are considered, corresponding to different magnitudes of solute excited-state dipoles, denoted as d5 and d8. The d8 solute is more attractive, leading to a larger enhancement in CO(2) clustering upon solute electronic excitation. This has a large impact on solvation dynamics, especially at densities below rho(c). At these densities, solvation dynamics is much slower for the d8 than for the d5 solute. For both solutes, solvation dynamics becomes faster at densities above rho(c) at which solvent clustering diminishes. We show that the slowest solvation time scale is associated with solvent clustering and we relate it to solute-solvent mutual translational diffusion and the extent of change in effective local density resulting from solute electronic excitation.
我们在此展示了在约1.05T(c)的温度下，超临界CO₂中溶剂化动力学的分子动力学模拟结果，其中T(c)为临界温度，且处于一系列密度范围从0.4到2.0临界密度ρ(c)的条件下。我们关注静电溶剂化动力学，将发色团的电子激发表示为其电荷分布从四极对称基态到偶极激发态的变化。考虑了两种微扰，对应于不同大小的溶质激发态偶极，记为d5和d8。d8溶质更具吸引力，导致溶质电子激发时CO₂聚集的增强更大。这对溶剂化动力学有很大影响，尤其是在密度低于ρ(c)时。在这些密度下，d8溶质的溶剂化动力学比d5溶质慢得多。对于两种溶质，在高于ρ(c)的密度下，溶剂化动力学变得更快，此时溶剂聚集减少。我们表明，最慢的溶剂化时间尺度与溶剂聚集相关，并且我们将其与溶质 - 溶剂相互平动扩散以及溶质电子激发导致的有效局部密度变化程度联系起来。
