The conformational populations of cis-1,3-cyclopentanedicarboxylic acid (1) and its mono- and dianion were established in DMSO solution by comparing the vicinal proton-proton coupling constants ((3)J(HH)) obtained in solution to their theoretical counterparts. Geometries used for (3)J(HH) theoretical estimation (using Karplus-type equations) were obtained from optimized structures at the B3LYP/6-31G-(2d,2p) level. The diacid (1) adopted many conformations, whereas the ionized species (1A mono- and 1B dianion) assumed single conformations. A downfield chemical shift of 19.45 ppm (Delta delta(H) = 7.43 ppm) observed at -60 degrees C was indicative of intramolecular hydrogen bonding in 1A, which was later corroborated by determining the ratio of the first (K-1) to the second (K-2) ionization constants. K-1/K-2 in DMSO (1.3 x 10(7)) was significantly larger than the value in water (2 x 10). In addition, K-1/K-E = 200 (where K-E is the acidity constant of the monomethylester of 1) was greater than the intramolecular hydrogen bonding threshold value of 2. The calculated intramolecular hydrogen bond strength of IA was similar to 3.1 kcal mol(-1), which is similar to 2.7 kcal mol(-1) more stable than the values for cis-1,3-cyclohexanedicarboxylic acid (2A). Thus, the relative energies of intramolecular hydrogen bonding in the monoanions IA and 2A suggests that 1,3-diaxial conformers are more favored for cyclopentane than for cyclohexane rings.
