Explain why the dipole moment of 4-chloronitrobenzene (2.69 D) is less than that of nitrobenzene (3.99 D), and the dipole moment of p-nitroanisole (1-methoxy-4-nitrobenzene, 4.92 D) is greater than that of nitrobenzene, even though the electronegativities of chlorine and oxygen are about the same.