Formaldehyde, dipole moment localized orbital

To illustrate these energy levels, Figure 2.1 shows formaldehyde as an example, with all the possible transitions. The n —> n transition deserves further attention upon excitation, an electron is removed from the oxygen atom and goes into the n orbital localized half on the carbon atom and half on the oxygen atom. The n-n excited state thus has a charge transfer character, as shown by an increase in the dipole moment of about 2 D with respect to the ground state dipole moment of C=0 (3 D). 

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. 

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