Steric Hindrance in the Solute-Solvent Interactions of 2-Substituted 4,5-Dimethoxypyridazin-3(2H)-ones: An Infrared Spectral and Theoretical Study

Infrared spectra for a series of 2-substituted 4,5-dimethoxypyridazin-3(2H)-ones (1) were measured in hexane (n-C6H14)-CHCl3 and CH3CN-D2O binary mixtures. The solute-solvent interactions of pyridazinones 1 were studied by correlation of v(C=O) values with mole fractions of the less polar components of binary mixtures. The different solute-solvent interacting species were distinguished and compared with those for 3-methyl-4-phenylthiazol-2(3H)-one (2) and previously investigated 1-methylpyrrolidin-2-one (3). The steric effects of the substituents adjacent to the C=O group amplified by the geometrical requirements of the six-membered pyridazin-3(2H)-one ring appeared to be the main factor governing the existence of individual hydrogen-bonded species. The solute-solvent interactions are sterically most restricted in pyridazinones 1, while in thiazolones 2 they are freer and almost comparable to those in the sterically least hindered 1-methylpyrrolidin-2-ones 3. The above results were confirmed by inspection of calculated semiempirical AM1 and PM3 and ab initio 6-31G* geometrical data for methyl derivatives 1h, 2 and 3.