) are affected by first-, second-, and third-order quadrupolar interactions, contributing to the (ultra-)wide powder patterns expected for full (CT and ST) quadrupolar environments of In and 209 Bi NMR. While second-order perturbation theory is sufficient for many quadrupolar NMR studies, numerous examples have emerged demonstrating the significance of higher-order effects. The rule of thumb for the breakdown of second-order perturbation theory and the emergence of third-order effects is when the quadrupolar frequency  Q given by: