Study of substituent effects on rotational isomers and monomer-dimer equilibria of the 3-carboxy group in 4-substituted (R)-2-(3,4-methylenedioxyphenyl)-6-isopropyloxy-2H-chromen-3-carboxylic acids in dilute CCl4 and CHCl3 solutions by FTIR spectroscopy

FTIR spectra of the title carboxylic acids (I-III) with 4-substituents (H, CH3 or C6H5) and their related compounds IV-VI with 4-(substituted phenyl) groups were measured in dilute CCl4 and CHCl3 solutions. The concentration dependence of FTIR spectra of I-IV was also measured in these solutions. Th ese spectra were subjected to curve analysis in order to quantitatively ide ntify the rotational isomers of 3-carboxy group attributable to steric hind rance of the 4-substituents. For I, II and III-VI, two, four and five nu (C =O) bands were observed for their carboxy groups, respectively, indicative of monomer-dimer equilibrium and two and three kinds of rotational isomers for II and III-VI, respectively. Compounds III-VI were found to form intra- molecular hydrogen bonds between the trans-type of the 3-carboxy group and the pi -electrons in the 4-benzene ring. We have worked out a method to est imate the association constant (K) of complicated monomer-dimer equilibria such as II-VI. The K values of I-IV decrease remarkably in the order of H ( I), C6H5, (III), CH3 (II) and C6H4-p-OCH3 (IV) in CCl4 and I, II, III and I V in CHCl3; these orders are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.