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The spin densities of the radical anions from 4-methylcatechol, 4-carboxylic catechol, and 3-tiicthylcatechol were calculated by means of the I N D O method with the geometrical parameter adjusting method. The observed proton hyperfine coupling constants were exactly reproduced.

Abstract-The reaction indices of π-electronic structures of the diuretics, xanthine, thiazide and triazine derivatives were calculated. An attempt was made to explain the potency of the diuretic action in the light of quantum chemistry. The reaction indices of xanthine derivatives, the first frontier superdelocalizability at the substi tuent occupying the 7-position of the highest occupied orbital and the second frontier superdelocalizability at the 3-position of the occupied orbital show fairly good cor relations with the diuretic activities. The reaction index S_r”^(E) is defined as: S_{r”= fr¹/λ1+fr²/λ2 where subscripts 1 and 2 stand for the highest occupied (or the lowest unoccupied) orbital, and the next highest occupied (or the next lowest unoccupied) orbital, fr is the electron density of the corresponding orbital and λi (i=1, 2) is the coefficient of energy in the i-th molecular orbital ε=α+λ1β, α and β being the stand ard Coulomb and resonance integrals. The diuretic activities of thiazide derivatives are in good parallel with the reaction indices, the π-electron densities at the 2-position, at the 4-position and at the 7-position. The potency of carbonic anhydrase inhibi tion of thiazide derivatives and sulphonamide derivatives however are fairly well parallel with the reaction indices, the π-electron density of the lowest unoccupied orbital and the formal charge at the position of amino group or at the position of nitrogen atom in sulphonamide.The diuretic activities of triazine derivatives show good correlation with the reaction indices, the first frontier superdelocalizability of the lowest unoccupied orbital at the 1-position (carbon atom) and the π-electron density of the unoccupied orbital at the 4-position (nitrogen atom). The positions of these diuretics, which are assumed to play the important role in diuretic action, are the positions in the group =N-?? -N=.As the values of the Coulomb integral and the resonance integral of trill uoromethyl group (CF3) and the Coulomb integral of carbon atom adjacent to this pseudo-atom, α 2.0β, 1.0β and α-0.1β are employed.Although many studies have been made of the mechanism of action of diuretics, there have been few quantitative studies done on the relation of chemicalphysical characters of diuretics to the potency of the diuretic action. It is difficult to comprehend the potency of the diuretic action from only the chemical configurations of the diuretics.The mechanism of diuretic action of the xanthine derivatives is still uncertain. It has been assumed that the diuresis resulting from the ingestion per os of xanthine diuretics is due to a renal tubular effect (1, 2). For example, aminophylline has been considered to inhibit the enzyme systems in the renal tubule necessary for reabsorption of sodium and chloride with a r...}

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