N,N-dimethyl-selenobenzamide: two solid phases and low-temperature phase change

Two polymorphs (I: mp 49.0-50.0 degrees C; II: mp 80.0-82.0 degrees C) of N ,N-dimethyl-selenobenzamide, (CH3)(2)NC(Se)Ph, have been observed. Both I a nd II can be prepared separately using the same reaction under different co nditions. The phase change from phase I to phase II occurs at low temperatu res and this has been confirmed by solid state NMR (C-13), powder X-ray, an d single-crystal structural studies. The single-crystal X-ray diffraction s tudy reveals that the lower melting point form (phase I) crystallizes in th e triclinic space group P-1 with two conformations in the unit cell, while the higher melting point form (phase II) crystallizes in the monoclinic spa ce group P2(1)/c with one conformation in the unit cell. Theoretical calcul ations on model clusters using the Universal Forcefield (UFF) show that the total energy of phase I (triclinic form) is 5.9 kcal per mol molecule high er than that of phase II (monoclinic form). Also, the immersion energy whic h is due to non-bonding interactions, namely Van der Waals and Coulombic (e lectrostatic) terms, has been calculated using UFF. The Van der Waals terms were very similar in the two crystalline forms (triclinic: - 43.1 kcal mol (-1); monoclinic: - 44.8 kcal mol(-1)), but the Coulombic terms were signif icantly different (triclinic: -14.0 kcal mol(-1); monoclinic: -31.5 kcal mo l(-1)) and favor the monoclinic form. The triclinic form (phase I) is a kin etically favored metastable phase and upon cooling it changes to the monocl inic form (phase II), a thermodynamically stable phase. (C) 1999 Elsevier S cience Ltd. All rights reserved.