Complex molecular dynamics of (CH3NH3)5Bi2Br11 (MAPBB) protons from NMR relaxation and second moment of NMR spectrum

► Cations in MAPBB undergo a complex stochastic motion. ► These are: tunneling and over the barrier jumps in a triple potential. ► Jumps between two equilibrium sites and isotropic rotation. ► The spectral densities were derived and applied to the T1 and M2 equations. ► The tunnelling splitting of the CH3 and NH3 groups was estimated.