NMR and NQR study of polymorphism in carbamazepine

Four polymorphic forms of carbamazepine have been simultaneously investigated by ¹H NMR and ¹⁴N NQR. The results show that the proton spin-lattice relaxation time and the ¹⁴N NQR spectra can be used to differentiate between various polymorphic forms. Spontaneous transformations from Form II to Form III and from Form IV to Form III have been investigated through their influence on the ¹⁴N NQR spectrum and the proton NMR signal and spin-lattice relaxation. The ¹⁴N NQR spectra prove that in the observed polymorphic forms of carbamazepine the hydrogen bonded dimers of carbamazepine molecules are the basic elements of the crystal structure. The dimers are centrosymmetric in Forms III and IV and in metastable polymorphic form occurring during the transformation of Form IV to Form III. Two non-equivalent molecular positions are observed in Form II with the occupation ratio 1:1 and in Form I with the occupation ratio either 2:1 or 3:1. The ¹⁴N NQR data are related to the published crystal structures. Possible reasons for the mismatch of the X-ray and NQR data for Forms I and II are discussed.