MOLECULAR-DYNAMICS SIMULATION STUDY OF ASSOCIATIONS IN AQUEOUS-SOLUTIONS OF QUINUCLIDINE

Aqueous solutions of quinuclidine corresponding to 2 and 6 mol %, respectively, are simulated by using a molecular dynamics technique. Both the structural properties and dynamics of the solute and solvent are investigated. The solute molecules form strong hydrogen bonds with the solvent molecules. In the more concentrated solution an aggregation of the solute molecules is observed. The increased structural order of water compared with a pure liquid is found in agreement with other similar systems. The translational motion of water is decreased, whereas the rotational motion is essentially unchanged compared with pure water. The reorientational motion of quinuclidine is clearly anisotropic due to the hydrogen bonds formed with water. The simulated values are also compared with the experimental results obtained from spin relaxation measurements. The translational diffusion coefficients for the solute and solvent molecules are measured with the Fourier-transform pulsed-gradient spin-echo method. In addition, picosecond spectroscopic studies are carried out in order to find experimental evidence of a possible aggregation of quinuclidine molecules in water. These studies show a weak shift in the absorption band in the water solution when the concentration is increased. Also, a dramatic shortening of the fluorescence lifetime is observed. No such behavior, typical of aggregated molecular system, was found in nonpolar solvents. © 1990, American Chemical Society. All rights reserved.