Direct measurement of chemical exchange events in the crystalline polycarbonate monomer 4,4'- isopropylidenediphenol (bisphenol A, BPA) via 2D C-13 solid-state NMR reveals slow, large-amplitude aromatic ring reorientations. X-ray diffraction, however, indicates a static crystalline structure. Experiments with multiple exchange times show that ring flips occur in all of the three unique conformers found in the crystalline unit cell of this compound, but in specific cases, two of the three unique molecules actually switch conformations. Kinetic analysis of the exchange data indicates that the average rate constant k(ex) = 0-01 s(-1) for ring flips and conformer interchange at room temperature. Differential scanning calorimetry and variable-temperature powder diffraction studies indicate a systematic volume expansion that accompanies this motion but no first-order phase transition. All room-temperature exchange events may be quenched at 213 K, at least on the time scale (up to several seconds) probed in this work. Simulation of the potential-energy surface of BPA molecules reveals that the lowest-energy pathway for ring flips (maximum energy of 1.9 kcal/mol) involves +/-110degrees flips of one ring coincident with +/-70degrees flips of the second. The mechanism of the ring dynamics in the crystalline monomer is unique relative to those previously reported for the polycarbonate in that the collective motion of both rings in a monomer unit is 180degrees, whereas single-ring flips of 180degrees occur in the polymer.