The notorious contamination of cage T is its isomer. Figure shows a partial 1 H NMR spectrum of an equilibrated solution of cage T in D2O at room temperature. In addition to signals originating from T, the spectrum features a set of lower-intensity signals, indicating the presence of a minor species, which we temporarily denote X. Cage T can be purified from X by recrystallization (by diffusing acetone vapor to the aqueous solution of the cage); the 1 H NMR spectrum of the resulting single crystals freshly dissolved in D2O shows pure T (Figure ). However, acquiring the spectrum after several hours at room temperature reveals the presence of 2 mol% X, indicating a spontaneous T→X transformation. Over the following ten weeks, the equilibrium state containing 6 mol% X is reached (cf. Figure ). When the solution containing 2 mol% X is heated at 80 ºC, the fraction of X increases to 12 mol% within two days (Figure ). Decreasing the temperature back to 25 ºC lowers the fraction of X to the equilibrium value of 6 mol% within several weeks (Figure ).