VT SS 2 H NMR spin-echo measurements of crystalline 1-R-d4 in the 295-333K temperature range indicated a weak temperature-dependence of the signal line shape which can be attributed to the low frequency or even close to static librations of the deuterated PP moiety (Figure ). The presence of such relatively slow flipping motion of the PP moiety can be explained by its tight packing within the crystal structure and by a larger number of short intermolecular contacts compared to the Ph a and Ph b moieties. At the same time, experimental 2 H NMR line shapes obtained for 1-R-d20 display more profound changes with temperature elevation (Figure , black lines), suggesting the presence of more significantly thermally activated dynamic processes involving the NHC phenyl moieties (Ph a and Ph b ). It should be noted that in this case, the phenyl moieties contain two different types of deuterium atoms contributing to the 2 H NMR spectra (Figure ): two pairs of deuterium atoms at ortho-and meta-positions (cone angle = ±60°) and one deuterium at the para-position (cone angle = 0°, which should not affect the line shape with temperature change). The obtained VT 2 H NMR spinecho experimental patterns of 1-R-d20 can not be accurately fitted using a single dynamic mode for Ph-d5 moiety. This is probably due to Ph a and Ph b being non-equivalent in the crystal structure and experiencing different crystal packing, suggesting the presence of two types of thermal motion. Based on these premises, we used two modes of motion in the 2 H NMR simulations (Figure , red lines; for the simulation details, see Figure ): a faster 180° 2-fold jumping model (rotation 1) and a slower 90° 4-fold motion with 4:1 populations (rotation 2; Figure ). Since the Ph a is displaying generally larger thermal ellipsoids in SCXRD structures at all temperatures, we presume that it can exhibit a faster 180° 2-fold motion, while the slower 90° 4-fold jumping motion may be attributed to Ph b . In the latter case, the crystal packing environment of Ph b is also more suitable for the presence of 4-fold 90° motion.