Molecular dynamics in solid L-adrenaline by proton NMR.

Proton NMR measurements of the spectrum, second moment, spin-lattice relaxation time T1 and dipolar relaxation time T1D were carried out on polycrystalline L-adrenaline at 14 and 25 MHz between 55 and 400 K. Between 70 K and 250 K relaxation is dominated by C3 reorientation of the single methyl group in each molecule, characterized by an activation energy 8.3+/-0.3 kJ/mole. Below 70 K tunnelling assisted relaxation is significant, characterized by an excitation energy of 1.9+/-0.2 kJ/mole. Above 250 K an additional molecular motion becomes significant, with activation energy above 28 kJ/mole, attributed to conformational motion of the methylene group in the ethylamine side chain.