Vibrational study of phase transition in N-benzyl methyl ammonium dihydrogenmonophosphate monohydrate

Raman spectra (100-3200 cm−1) of polycrystalline samples of C6H5CH2NH2CH3H2PO4·H2O were studied at temperature ranging from 77 to 350 K. By comparison with homologue compounds and calculated wavenumbers, an assignment of the observed bands is proposed. The thermal evolution of Raman spectra reveals an order-disorder phase transition at about 120 K involving the hydrogen bonds coupled with the distortion of the organic cations. The line at 244 cm−1assigned to τ(CH3) shows a remarkable evolution across the transition. Therefore, a careful analysis of the thermal evolution of this mode was performed using Porto model. The activation energy, obtained from the plot of Δν1/2 versus temperature, is Ea = 1.27 kcal mol−1. This value is of the same order of magnitude as the one found for homologue hydrogen bonded compounds.