In this paper we present our results from a molecular dynamics study of n-octane liquids confined between planar bcc solid surfaces. The systems studied were wide enough to develop a bulklike region throughout the middle portion of the film and two well-separated interfaces. Our work focused on segmental dynamics and relaxation of "adsorbed" octane molecules. In particular, we investigated the role of architectural and dynamical features peculiar to short chain molecules (almost fixed bend angles and restricted torsional rotations) on the dynamics of "adsorbed" chains. We found that the relaxation of octane molecules exhibits the same qualitative trends as those observed in molecular simulations of generic "bead-spring" oligomer films. The most important effect is the dramatic slow down of rotational motions (up to a factor of 1000) for chains adsorbed on strongly physisorbing surfaces (adhesion energy per segment of 1-2 kT). Despite the qualitative similarities with bead-spring chains, the dynamics of realistic short hydrocarbon chains are affected much more strongly by the interfacial, environment than their bead-spring counterparts. These stronger effects originate largely from the suppression of torsional angle transitions inside the extremely dense first layer (in cases of strong physisorption). The frequency of torsional transitions was found to be correlated directly with the amount of "free volume" available inside the crowded first layer.