Carbon-13 spin–lattice relaxation in strongly associated molecules: Carboxylic acids

¹³C spin–lattice relaxation times determined for the protonated carbons of carboxylic acids and methyl esters give indications of solution dimerization with the free acids. Since isopthalic and fumaric acids have two carboxyl functions they are able to polymerize in solution. Unlike the case for molecular aggregation due to weak hydrogen bonding in solution (e.g. alcohols, phenols), the ¹³C T₁ values of mono carboxylic acids are not significantly affected by dilution to c. 10^?2 M. Variable temperature T₁ measurements of both the mono and dibasic acids gave activation energies for molecular reorientation of the order of 2 kcal mol^?1, considerably lower than E_a for hydrogen bonded alcohols and comparable with E_a for the unassociated methyl esters of propionic and benzoic acids.