MD simulations told us about the underlying mechanisms. And also show that what is called "breathing motion" is actually a complex rearrangement of interactions that have to be re-shuffled such that the ring can rotate. T-dependent MAS NMR told us how the crystal restricts such motions.
Aromatic rings can rotate by 180° -- so-called ring flips -- but to do so their surrounding needs to make some space. These collective dynamic modes are sometimes called "breathing motions", a poorly defined term that has been around for 45 years. But what is this? And does it exist in crystals?