Nuclear Magnetic Relaxation Study of Formic Acid. Isotope Effect and Carbon-13 Relaxation

Abstract

The transient recovery after adiabatic inversion has been observed separately for the magnetizations of the ${}^{12}\mathrm{C}–\mathrm{H}{\mathrm{,\hspace{0.17em}}}^{13}\mathrm{C}–\mathrm{H}$ , and $\mathrm{O}–\mathrm{H}$ protons of formic acid. A large difference was observed between the decay constants for the ${}^{12}\mathrm{C}–\mathrm{H}$ and ${}^{13}\mathrm{C}–\mathrm{H}$ protons. It has been concluded that this difference is due to (1) flip of the ¹³C spin resulting in an exchange of the two ${}^{13}\mathrm{C}–\mathrm{H}$ proton signals, and (2) intramolecular dipolar coupling between the ¹H and ¹³C spins. The above results were used along with the nuclear Overhauser effect data to estimate the ¹³C spin‐lattice relaxation time. It was also shown that the difference in relaxation times between the ${}^{12}\mathrm{C}–\mathrm{H}$ and ${}^{13}\mathrm{C}–\mathrm{H}$ protons is primarily due to the intramolecular dipolar coupling between the ¹H and ¹³C spins.