Vibrational and thermodynamic properties and molecular motions in the incommensurate crystal of morpholinium tetrafluoroborate studied by 1H NMR

Dynamics of molecules in morpholinium tetrafluoroborate, [NH2(C2H4)(2)O][BF4], has been studied in a wide temperature range by means of AC calorimetry, H-1 NMR and infrared spectroscopy. Simultaneously, DFT (density functional theory) calculations of the molecular structure and normal vibration frequencies have been performed by using various functionals for initial structural data of the low temperature phase. Infrared spectra of polycrystalline [NH2(C2H4)(2)O][BF4] have been analyzed in a frequency range 4000-400 cm (1). Substantial changes in a temperature evolution of internal modes of both morpholinium cations and [BF4] anions are due to the freezing of these moieties motions below 117 K. A dynamic non-equivalence of protons involved in N-H center dot center dot center dot F and N-H center dot center dot center dot O hydrogen bonds has been detected. Heat capacity anomalies around structural phase transitions detected by AC calorimetry (117 and 153 K) have been evaluated and described. A molecular mechanism of the phase transitions is discussed on the basis of presented results. (C) 2011 Elsevier B. V. All rights reserved.