STUDIES OF SOLUTION STRUCTURE AND DYNAMICS BY MOLECULAR SPECTROSCOPY

MICHAEL WALTER BALK, Purdue University

Abstract

Information about the motional properties of the sodium ion in liquid pyridine was obtained from an experimental measurement of the sodium ion velocity time correlation function (vcf). The experimental vcf was obtained from the high resolution digital far infrared spectrum of the pyridine solution of sodium tetracarbonylcobaltate. The motional properties of the sodium ion in its pyridine environment were characterized by successfully modeling the experimental vcf with the molecular timescale generalized Langevin equation (MTGLE) theory developed by S. A. Adelman. The high accuracy required in the far infrared spectrum necessitated two developments in infrared instrumentation. First, a computer-spectrometer interactive system was developed which is able to respond to the spectral data as it is being acquired in order to produce an absorbance spectrum with (a) improved accuracy in the absorbance measurements and (b) a constant noise level such that each spectral point has the same standard deviation. Second, a vacuum tight, solvent resistant, pathlength invariant sample cell was developed for high resolution far infrared spectroscopy of nonaqueous solutions. The sodium ion vcf is characterized by a series of damped oscillations which decay nearly to zero in one picosecond. The oscillations result from the "vibrational-like" motion of the sodium ion inside its pyridine cage. The approximate backscattering time is 0.09 picosecond. The damping results from a loss of energy from the sodium ion to its environment, called the heatbath. The MTGLE theory has been used to describe these properties in a precise and realistic manner. Information about the structure in the sodium ion environment is required for a proper understanding of the dynamics. This information was obtained indirectly from an infrared study of the spectrum in the 5(mu) region due to the C-O stretch of the tetracarbonylcobaltate anion. Cryptands are bicyclic ethers which encage alkali ions. A cryptand was added to the solution to modify the normal mix of structures (sites) present. In this case, the spectrum of pure site I resulted. This technique revealed that the undoped pyridine solution consisted of 98% site I (free ions and solvent separated ion pairs) and only 2% of site II (a contact ion pair site). This established the validity of the model in which the sodium ion translates in a cage composed of pyridine molecules.

Degree

Ph.D.

Subject Area

Chemistry

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