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

The ²H-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 ²H-Pake patterns with parameters of (Qzz = 132 kHz, corresponding to Qcc = 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 ²H-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 C₂-ring flips of the phenyl rings.