g Hold one methyl group constant and rotate other methyl group around the central C-C bond. Does this bond rotation generate a new molecule or isomer

This bond rotation generates a new isomer

Explanation:

This process described in the question is called stereoisomerism. This is a process in which molecules have the same molecular formula and bonding but differ in there three-dimensional structure in space. These three-dimensional spacial orientation can be changed by turning a bond around a fixed point making the functional change position in space. This rotation does not bring about a new molecule but a new isomer through a process known as stereoisomerism.