Kinetics, Electrochemical and Optical Investigation of Bipyridilium Herbicide and Related Compounds

Abstract

It was the purpose of the present project to carry out some electrochemical, kinetics and optical studies on two important biologically relevant compounds: N,N’-dimethyl, 4,4’-bipyridilium dichloride (and homologues) and 1-ehtyl 4-carbomethoxy pyridinium iodide. The former which is called paraquat (or methyl viologen) is known herbicide while the later is a biological model compound for NAD. It was hoped that the present study would directly help in understanding the mechanism of the biological reactions in which the two compounds ( and homologues) are either directly involved (e.g. paraquat as herbicide) or act as model compound. The studies encompassed a large variety of subject matter, e.g. determining (reversible) E1/2 protonation reaction, solvation free energies, ion pair phenomenon etc. Through electrochemical studies (i) the existence and stability of the pyridinyl anion and methyl viologen “neutral” were established the disproportion equilibrium constant for the reduction processes for pyridinium, bispyridilium and 1:n:1 type pyridinium compounds were obtained. The highest reduction product of 1:3:1 were investigated (some of the higher reduction products were never reported). Kinetics, i.e. protonation reactions of the higher reduction products of pyridinium and bipyridilium compounds were investigated in various solvents and on various electrodes. From these electrochemical and kinetic investigation it was proposed that the higher reduction product of methyl viologen, now named methyl viologen neutral, might be taking part in the herbicidal activity if methyl viologen (paraquat). The electrochemical data gave E1/2’s of a series of pyridinium and bispyridilium compounds, which were correlated with the molecular orbital energies (in w-technique prescri9ption). The solvation of ions had to be taken into consideration for some compounds. (From these data the electron affinities of the charged species and free redical could be calculated, which had never been reported before). The solvochromic property of 1-ethyl 4-carbomethoxy pyridinium iodide was utilized in developing a method for the evaluation of solvation free energies of transfer of anions. The method was found to be as good as Pleakoc fo forrocene method, Through the conductance studies of pyridinium iodide in solvents like acetonitrile, dimethyl formamide, acetone, and methanol gave the equilibrium constant for the disassociation of the ion pair into free ions was found to be 0 10^-3-10^-2. In chloroform no disassociation was observed. Magnetic resonance studies gave interesting and useful information. First, NMR studies on such a class of compounds have not been carried out before. Through the chemical shifts of the pyridinium ring protons in various solvents the equilibrium constants for the charge transfer complex (or very tight ion pair) going to loose ion pair, were obtained. This was also the first time such equillibria were studied. The equilibrium constant varied from 0.5 in chloroform to 10 in formamide. It was established that through PyI doesn’t dissociate into ions in chloroform, it exist in CT comples and loose ion pair from. This way we established that charge transfer complex and loose ion pair are two different moieites. Also through NMR studies exchange equilibrium between a CT complex and loose ion pair was studied. Through these NMR studies it was also found that contrary to earlier report PyI does not exist as CT complex in water. A new z-value (a solvent polarity parameter) for water was proposed. Also it was found that the chemical shift of PyI varied linearly with the z-value of solvent and hence the chemical shift can itself be utilized as solvent polarity parameter. In the process of carrying out the project two new analytical technique were developed (i) the PyI method for solvation free energies of transfer (ii) the Mohammad- Nicholson- Shain method for studying kinetics of reactions of reactive intermediates like free redicals, redical anion and dianions. The later method is very elegent, simple rapid and theoretically sound for studying fast reactrions.