²H-Solid-State NMR Study of Benzene-d₆ Confined in Mesoporous Silica SBA-15

Benzene- d 6 confined in the hexagonal ordered cylindrical pores of mesoporous silica SBA-15 (pore diameter 8.0 nm) was studied by low-temperature 2 H-solid-state NMR spectroscopy in the temperature range between 236 and 19 K and compared to bulk benzene- d 6 . The solid-state spectra of the bulk benzene- d 6 exhibit quadrupolar Pake patterns at high and low temperatures, and in the intermediate temperature regime the typical line shape changes caused by rotational jumps around the 6-fold axis. At all temperatures the benzene molecules are characterized by a single rotational correlation time. For benzene- d 6 confined in SBA-15, however, these exchange dominated line shapes are not found. At all temperatures below the freezing point the spectra of benzene in the silica show the coexistence of two states with temperature-dependent intensity ratios. This behavior is the result of a Gaussian distributions of activation energies for the rotational jumps inside the pores. For the solid I−solid II (fast 6-fold jump to slow 6-fold jump) transition the center of the distribution is at 40 K (6.0 kJ/mol) with a width of 19.5 K (2.9 kJ/mol). For the liquid−solid I (liquidlike to fast 6-fold jump) transition the center of the distribution is at 204 K (30.6 kJ/mol) and the width is 15 K (2.2 kJ/mol). From the pore volume and the filling factor, a thickness of four molecular layers of this surface phase is estimated.