In slide #4, we can see that the value of the quadrupole term goes from -2 to +1 when \alpha = 1, leading to a value of the lowest energy of the monopole energy plus twice \alpha in slide #6. By changing the value of \alpha to -1, we will get the same figure in slide #4 but inverted. Thus, the lowest energy will be the monopole energy plus \alpha. However, since \alpha is multiplying the whole quadrupole term, this term becomes zero for \alpha = 0 (no contribution from the quadrupole term).