Carbon-13 Relaxation Time Measurements in Formic Acid

Abstract

The variations in carbon‐13 nuclear magnetization with time have been determined for formic acid under proton‐coupled and proton‐decoupled conditions by the adiabatic rapid passage technique. The time‐dependent variations for proton‐coupled carbon‐13 magnetization are nonexponential, while the nonequilibrium, proton‐decoupled carbon‐13 magnetization follows an exponential relationship. By applying the theoretical equations to the experimental results, spin–lattice relaxation times for the spin‐decoupled case were calculated for both the formyl proton and carbon‐13 nuclei as well as for the internuclear process. It is demonstrated that the carbon‐13 relaxation process is dominated by the mutual relaxation of formyl protons and carbonyl carbons through the intramolecular dipole–dipole mechanism. Various means of extracting relaxation data with and without spin‐decoupling equipment are discussed for the four‐line two‐spin system.