D. S. Alavi

Rotational dielectric friction on a generalized charge distribution

The dielectric friction on a solute molecule reorienting in a liquid solution is computed by modeling the solute as a rigid collection of point charges rotating within a spherical cavity in a dielectric continuum. Such a calculation on an extended charge distribution is a logical progression from existing theories, which treat only single point charges or point dipoles. It is shown how a more realistic charge distribution can change the calculated friction coefficient by several orders of magnitude, and the generalized theory is applied to rotational diffusion data for three phenoxazine dyes in dimethylsulfoxide.

A test of continuum models for dielectric friction. Rotational diffusion of phenoxazine dyes in dimethylsulfoxide

Rotational diffusion times for three mechanically similar phenoxazine dyes with differing electronic properties were measured. These studies probe the importance of dipole/dipole and ion/dipole couplings to the friction. The experimental results are compared with the predictions of available continuum theories of dielectric friction. Future directions for a more realistic model of dielectric friction are discussed.

The influence of wave vector dependent dielectric properties on rotational friction. Rotational diffusion of phenoxazine dyes

The rotational diffusion of three mechanically similar phenoxazine dyes possessing distinct electrical properties was studied in isopropanol. The results, along with previously presented results from other polar solvents, were analyzed in terms of continuum theories for rotational dielectric friction. It was found that a continuum based theory can account for the observed rotational relaxation dynamics, but only with realistic modeling of the solute charge distribution (not a point dipole), and by accounting for both frequency and wave vector dependences of the solvent dielectric properties.

Optically heterodyned polarization spectroscopy. Measurement of the orientational correlation function

Polarization spectroscopy has been developed as a useful method for the investigation of molecular reorientation in both liquid phase solutions and in the gas phase. This technique has the advantage of measuring a single particle orientational correlation function directly but the disadvantage of averaging over rotation in all electronic states. Described and characterized herein is a variant of this technique, optically heterodyned polarization spectroscopy, which is able to disentangle various contributions to the signal and determine the rotational relaxation of the solute molecule in different electronic states independently. This work also demonstrates the measurement of the normalized value of the orientational correlation function at time zero, r(0), without extensive normalization of laser parameters. Lastly, various technical advantages of the optically heterodyned method are discussed.

Dielectric Continuum Models of Solute/Solvent Interactions

Electrostatic interactions between a solute and its environment are quite significant and can significantly modify the solute molecule’s dynamics. In the case of chemical reactions, polarity effects of the medium can significantly alter reaction rates and the relative yields of different reaction products. However these interactions are not quantitatively understood for molecular systems. This failing is clearly indicated by the prevalence of semiempirical measures of these important interactions [1]. A focus of recent work from our group has been to better quantitate these important interactions. The methodology has been to first experimentally measure the rotational diffusion of rigid solute compounds in order to empirically determine the friction experienced when a molecule moves on molecular length and time scales. Secondly electrostatic models of the solute/solvent coupling have been explored and compared to the experimentally determined friction coefficients. The result of this effort has been to show the necessity of including the extended nature of the solute molecule’s charge distribution when modeling the friction. It has been found that point source models may underestimate the magnitude of the friction by a hundred times for medium sized molecules (a displaced volume of a few hundred cubic angstroms). Furthermore the experimental results show that structural aspects of the solvent medium can be quite important to the solute molecule’s dynamics. It is clear however that over a wide regime that continuum models of the solvent work fairly well in describing what is characteristically molecular behaviour. The following discussion focusses on these results in more detail.

Rotational Diffusion of Phenoxazine Dyes: Characterization of Molecular Friction

Relative contributions of mechanical, dielectric, and specific solute/solvent interactions to molecular friction in liquids are evaluated by measuring solute reorientation times for phenoxazine dyes.