Internal rotation of fluorinated butane compounds: The maximum hardness principle and carbon–carbon rotational barriers

A density functional study of the internal rotation, about the central carbon–carbon bond, of butane, 1,1,1,3,3-pentafluorobutane (PFB) and perfluorobutane (PerFB), has been investigated. The bond length, torsional potential energy and hardness profiles were obtained using the B3LYP density functional method with the basis set 6-311G. The maximum hardness principle (MHP) is only verified for butane. It was also found that for butane and PerFB there is a reciprocal relationship between the central carbon–carbon bond length variations and the hardness profile, being the agreement for butane excellent. This could provide an alternative approach for studying the MHP.