A structural powder diffraction study of two polymorphic forms of nortriptyline hydrochloride.

Nortriptyline hydrochloride, a tricyclic antidepressant, appears in two different polymorphic forms, only one of which (hereafter, form β) has been previously characterized by single-crystal analysis. Form β is monoclinic, P2(1)/c, with a = 5.070(2), b = 34.088(5), c = 9.976(1) Å, and β = 90.74(2)°. A second crystalline form (the α form) has now been characterized by structural powder diffraction methods (using both laboratory and synchrotron radiation diffraction data). Form α crystallizes in the monoclinic P2/c space group, a = 9.99126(6), b = 5.10021(3), c = 34.1636(1) Å, and β = 98.684(6)°. The thermodynamic relationship between the two forms has been determined by differential scanning calorimetry analysis and variable-temperature thermodiffractometric experiments, revealing that the two forms are monotropically related and form α is more stable. Both phases are characterized by a sequence of hydrogen-bonded N-protonated molecules, which, in the two crystalline environments, adopt the same conformation. The difference between the two crystals can be traced back to the supramolecular arrangement characterized by one-dimensional chains, built by homochiral molecules (for conformationally driven chirality) in the α form, and by enantiomeric ones in the β form. This observation nicely explains why, upon heating, solid-solid interconversion between the two forms does not occur.