Conformational Preferences of cis-1,3-Cyclopentanedicarboxylic Acid and Its Salts by ^1H NMR Spectroscopy: Energetics of Intramolecular Hydrogen Bonds in DMSO

Abstract

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 (^3JHH) obtained in solution to their theoretical counterparts. Geometries used for ^3JHH 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 (Δδ_H = 7.43 ppm) observed at −60 °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 × 10^7) was significantly larger than the value in water (2 × 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 1A was ∼3.1 kcal mol^(−1), which is ∼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 1A and 2A suggests that 1,3-diaxial conformers are more favored for cyclopentane than for cyclohexane rings.