Formaldehyde, dipole moment compounds

The extinction coefficient, e = 20, is very small. The low intensity is typical for most n- jr transitions of carbonyl compounds and can be explained on the basis of the local symmetry. In the case of formaldehyde, which has Cjv symmetry, an electronic transition from the n orbital (b2) into the jt MO (b ) is dipole forbidden. This is no longer true for carbonyl compounds of lower symmetry such as acetaldehyde (CJ. Here the n orbital is still essentially of p character, so that the overlap density is approximately p Py and still has practically no dipole moment. 

---