Squid-Nmr Study of Methyl Quantum Tunneling in a Series of Carboxylic-Acids

Quantum tunneling frequencies of methyl groups at low temperature can be measured directly by low-field nuclear magnetic resonance (NMR) using our dc-SQUID (superconducting quantum interference device) spectrometer. From NMR spectra recorded in magnetic fields below 20 mT, tunneling frequencies between 140 and 250 kHz were measured for a homologous series of carboxylic acids at 4.2 K. For solids with equivalent crystal structures, we observe an exponential correlation between the tunneling frequency and the enthalpy of fusion; from this correlation we obtain information about the contribution of intermolecular interactions to the energy barrier for methyl rotation.