The dielectric response of chloromethylsilyl and dichloromethylsilyl dipolar rotors on fused silica surfaces

We have measured the dielectric response of monolayer films of surface mounted chloromethyl- and dichloromethylsilyl dipolar rotors on fused silica at frequencies in the I kHz range and temperatures from 4 to 300 K. The torsional potentials, calculated from molecular mechanics, show an asymmetrical three-fold barrier to rotation with a barrier height sufficient to hinder motion of the rotor at experimental temperatures. A broad distribution of barrier heights is observed experimentally, consistent with calculated results showing that the intrinsic barrier of the rotor is modified by interactions with the underlying substrate. For a series of samples with differing concentrations of the rotor, the observed signal strength varies in proportion to the rotor coverage measured by Auger spectroscopy; however, the absolute strength of the signal is about three times larger than expected.