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The  2 H-NMR solid echo spectra of biphenyl molecules as guests in the mesopores of neat and silylated SBA-15 have been measured as a function of temperature. At low temperatures typical  2 H-Pake patterns with parameters of (Q  zz  =132 kHz, corresponding to Q  cc  =176 kHz) and ( η =0.04) are observed. All samples exhibit a strong reduction of the melting point from the bulk value of 342.4 K to values between 222 K and 229 K, depending on both the pore diameter and the surface state and a glass like behavior of the biphenyl molecules in the melting regime. Employing the Roessler  two-phase model  of the modeling of glass-transitions by  2 H-solid state NMR the distribution of activation energies for the rotational motions has been determined. At temperatures closely below the glass-transition temperature deviations from a static Pake pattern of an aromatic deuteron are observed, which indicate a pre-melting motion of biphenyl, which could be caused by C 2 -ring flips of the phenyl rings.

Melting of Low Molecular Weight Compounds in Confinement Observed by 2 H-Solid State NMR: Biphenyl, a Case Study