The effects of shear strain on n-butane liquids and glasses are examined with molecular simulations. The simulations focus on the nonlinear mechanical properties and the transport properties of these systems. These properties are analyzed in terms of the changes in the potential energy landscape with strain, which is quantified by the curvature of the energy minima and the height, position, and curvature of barriers between minima. Shear strain is shown to cause local minima to disappear and barrier heights to decrease to zero, which renders the system mechanically unstable and forces the system toward alternate local minima. These disappearances of energy minima and barriers lead to plastic deformation in mechanically stressed n-butane glasses, and enhanced diffusion and molecular alignment in flowing n-butane liquids.