Anomalous proton spin-lattice relaxation in organic compounds containing methyl groups

Temperature measurements of proton spin-lattice relaxation time performed for acetates ((CH3COO)(2)Ba, (CH3COO)(2)Cd, and (CH3COO)(2)Ca) and acetyl halides ((CH3CO)(2)O, CH3COBr and CH3COCl) are fitted to a Haupt equation. It is impossible to fit the temperature dependence of T-1 protons using the BPP equation. Of importance is the assumption that complex C-3 molecular motion of methyl protons takes place. An understanding of the correlation functions of complex C-3 reorientation allow for the calculation of the relaxation time, T-1, and the second moment of the NMR resonance. The spectral densities are calculated applying Woessner theory of complex motion and assuming a tunneling correlation time implemented from solving the Schrodinger equation. The acceptance of the tunneling correlation time resulting from the Schrodinger equation elucidates the reduction in the second moment at 0 K. The fitting leads to an excellent agreement between the experimental results of T-1 temperature dependences and theoretical equations. From this, the tunneling splitting and motional parameters of methyl groups were estimated. The highest value of T-1 minimum in the temperature dependence for (CH3COO)(2)Ba indicates the highest value of tunneling splitting. The smallest value, which is close to the T-1 minimum value predicted by the BPP theory for CH3COCl, shows a lack or significantly reduced tunneling splitting. The tunneling jumps disappear at T-tun temperature, which was estimated for all studied compounds. The limitations on the use of Haupt's equation are presented. The presented approach differs from the other theoretical approaches and these differences are evaluated.