Ethane H 3 C−CH 3 (n = 8) has 4 degrees of rotational freedom: two about axes that are perpendicular to the central bond, and two more because each methyl group can rotate independently about that bond, with negligible resistance. Therefore, the number of independent deformation modes is 3 n − 7, which gives f = 3 + 4 + 2(3 n − 7) = 6n − 7 = 41.