Abstract
We have characterised the stable polymorphic forms of two drug molecules, indomethacin (1) and nifedipine (2) by ^13^C CPMAS NMR and the resonances have been assigned. The signal for the CCl carbon of indomethacin has been studied as a function of applied magnetic field, and the observed bandshapes have been simulated. Variable‐temperature ^1^H relaxation measurements of static samples have revealed a T~1ρ~ minimum for indomethacin at 17.8 °C. The associated activation energy is 38 kJ mol^−1^. The relevant motion is probably an internal rotation and it is suggested that this involves the COCH~3~ group. Since the two drug compounds are potential candidates for formulation in the amorphous state, we have examined quench‐cooled melts in detail by variable‐temperature ^13^C and ^1^H NMR. There is a change in slope for $T_{1}^{H}$ and $T_{1\rho}^{H}$ at the glass transition temperature (T~g~) for indomethacin, but this occurs a few degrees below T~g~ for nifedipine, which is perhaps relevant to the lower real‐time stability of the amorphous form for the latter compound. Comparison of relaxation time data for the crystalline and amorphous forms of each compound reveals a greater difference for nifedipine than for indomethacin, which again probably relates to real‐time stabilities. Recrystallisation of the two drugs has been followed by proton bandshape measurements at higher temperatures. It is shown that, under the conditions of the experiments, recrystallisation of nifedipine can be detected already at 70 °C, whereas this does not occur until 110 °C for indomethacin. The effect of crushing the amorphous samples has been studied by ^13^C NMR; nifedipine recrystallises but indomethacin does not. The results were supported by DSC, powder XRD, FTIR and solution‐state NMR measurements. Copyright © 2005 John Wiley & Sons, Ltd.