Spin‐lattice NMR relaxation and second moment of NMR line in solids containing CH3 groups

Abstract

Complex proton dynamics in solids containing methyl groups are studied over a wide temperature range. Temperature dependences of the proton T1 relaxation time and the second moment of NMR lines are analyzed. General rules describing the temperature dependencies have been formulated from an analysis of the data. The second moments, M2, are correlated with those based on T1 measurements. The C3 jumps over the barrier cause a minimum in T1 below the liquid nitrogen temperature in the presence of tunnelling splitting, or a little above 77 K when tunnelling splitting equals zero. The slope of the high‐ temperature side of this minimum permits evaluation of the activation energy, which can be used to characterize the tunnelling correlation time. The T1 is temperature independent in the lowest temperature range, indicating that the tunnelling correlation time assumes a constant value. The tunnelling splitting was estimated as the best fit parameter from the T1 temperature dependence. The high tunnelling splitting is responsible for T1 values which are high and independent from the Larmor frequency. The tunnelling jumps reduce the rigid lattice second moment at zero Kelvin. The second reduction is caused by over the barrier C3 jumps of methyl protons. The tunnelling jumps cease above a temperature Ttun. The occurrence of the minimum T1 above liquid nitrogen temperature, as well as the reduction of the second moment M2 are due to slowest motion of protons, which can be the isotropic motion of molecule. This minimum is well described by the Woessner method of calculating the correlation function of complex motion. © 2015 Wiley Periodicals, Inc. Concepts Magn Reson Part A 44A: 214–225, 2015.