PO2- Symmetric-Stretching Raman Line and Molecular Aggregation States of Barium Dialkyl Phosphates

Article abstract of DOI:10.1021/ja00384a026

The Raman spectra of barium dialkyl phosphates having various chain lengths were investigated.Dipentyl phosphate (DPP) was found to form a liquid crystal that is stable at room temperature.Calorimetric measurements of the DPP liquid crystal exhibited a phase transition at 60 deg C with ΔH = 1.7 kcal/mol.The PO2^- symmetric stretching Raman line of DPP was observed at 1106 cm^-1 for the crystal and at 1075 cm^-1 for an aqueous solution.For the liquid crystal, two lines were observed at 1096 and 1075 cm^-1.The relative intensities of these lines changed sharply at the phase-transition temperature but were essentially constant below and above the transition point.On the other hand, the intensity of the diester OPO symmetric-stretching Raman line as well as that of the accordion vibration was hardly altered by the phase transition.This suggested little structural change in the phosphoalkyl group upon phase transition and the coexistence of two types of aggregation structures with different Ba^2+...PO2^- interaction modes in the liquid crystal.The PO2^- symmetric-stretching Raman lines of dibutyl phosphate were observed at 1090 and 1068 cm^-1, and their relative intensities changed significantly upon micellization.The all-trans form of the alykl group was favored in the micelle state of dibutyl phosphate.It was concluded that the cation-phosphate interaction mode is important for investigation of the aggregation structure of dialkyl phosphates and is characterized by the PO2^- symmetric-stretching Raman lines.The freqiencies of the accordion vibrations for mono- and dialkyl phosphates fell on identical straight lines when they were plotted against the reciprocal of the carbon number.