Reorientational dynamics of organophosphate glass formers - A joint study by 31P NMR, dielectric spectroscopy and light scattering

We study molecular reorientation in the glass formers triethyl-, tripropyl-, and m-tricresyl phosphate by measuring ³¹P NMR spectra, relaxation (T₁ and T₂), stimulated echo decays and two-dimensional spectra over a large temperature range (130-370 K). The results are compared to those from dielectric spectroscopy (DS) and depolarized light scattering (LS). While the time constants τα of the primary (α-) process in the range 10^-11-100 s well agree, the stretching of the reorientational correlation function is probe dependent, i.e., the rank-two functions (NMR and LS) essentially agree whereas the rank-one function (DS) is less stretched. The very similar 2D spectra recorded as a function of mixing time demonstrate that the reorientational mechanism does not significantly vary among the super-cooled liquids. A model of combining large- and small-angle reorientation allows for reproducing the 2D spectra and may be taken as generic for the dynamics in viscous molecular liquids. Pronounced secondary (β-) processes do not only effect the NMR relaxation but can be identified directly in the time domain by the stimulated echo technique. This becomes possible due to its broad time window (10 μs-100 s). Thus, applying the different 1D and 2D techniques makes ³¹P NMR well suited to probe molecular reorientation over a wide dynamic range.