On the LT 3.2 mm probe the RF amplitude of all the pulses was 60 kHz, which was calibrated with the nutation behaviour of glycine-( 13 C 2 , 15 N, 2,2-d 2 ). The 2 H shifts were referenced externally to deuterated glycine at 3.70 ppm. For frequency-swept pulses the same parameters were used as on the 2.5 mm probe with the exception of the RF field amplitude. The sample temperature for the variable temperature experiments was varied from 96 to 305 K and determined by measuring the 79 Br spin-lattice relaxation time constant (T 1 ) relaxation times of a small amount of KBr (99%, Sigma) mixed with the sample. On the 4.0 mm probe the RF amplitude of all the pulses was 42 kHz which was measured by the nutation behaviour of deuterated ethanol (C 2 H 5 OD). The 2 H shifts were referenced externally to C 2 H 5 OD at 2.50 ppm. For short high-power adiabatic pulses the tanh/tan 29 pulse scheme was employed with 5 MHz sweep width and a 100 µs pulse length. The sample temperature for the variable temperature experiments was calibrated based on the 207 Pb shift in Pb(NO 3 ) 2 (99%, Sigma). Additional experimental solid-state NMR details can be found in the supplementary material.