We characterize the concerted motion of phenyl ring rotation and main chain reorientation for single chains of polystyrene hexamer in a systematic manner by defining a multidimensional reaction coordinate for this motion in terms of the internal coordinates of the polymer. Multidimensional reaction paths and the energy profile along these paths are determined for three different chain conformations for both syndiotactic and isotactic polystyrene hexamers. Our approach makes use of adiabatic potential energy surfaces in the "essential" torsional-angle space of the molecule. The reduced dimensionality of this space, compared to the full Cartesian coordinate space, offers improved computational efficiency, allows the use of rigorous search techniques for transition states, and provides a better physical picture of molecular conformations and their transformations. This detailed atomistic study of conformational transitions in single polystyrene chains represents the first step towards characterizing cooperative conformational transitions, as well as other coupled kinetic processes, that occur in bulk polymers.