# Symmetry Adapted molecular orbitals of Formaldehyde

I am currently trying to find the Symmetry Adapted linear Combinations (SALCs) of Formaldehyde $$\ce{H2C=O}$$, and use the 1s orbitals on the hydrogen atoms, and 2s, 2px, 2py and 2pz orbitals on the carbon and oxygen atoms to construct molecular orbitals. However I am finding some issues.

I understand how to work out the SALCs of the hydrogen S orbitals, which is $$A_1 = H_{1s} + H_{2s}$$ and $$B_1 = H_{1s} - H_{2s}$$. However, I don't understand how figure out the SALCs of the orbitals belonging to oxygen and carbon. I can only guess the combinations by drawing them, and not through calculation, like I can with the hydrogen S orbitals as these can interchange. This isn't the case with the oxygen and carbon orbitals.

Is there a way of finding this through calculation? Or do I just draw out all the possible combinations?

• SALCs are only relevant for atoms that are symmetry-related. The two H atoms in CH2O are related by symmetry (interchangeable via C2 or a mirror plane). The C and the O aren't related at all. – orthocresol May 8 at 13:58
• @orthocresol Thank you so much! I realise there isnt a delete button for my obvious question – Eurus delielio May 8 at 14:02
• No worries. It's not stupid at all. You can most certainly leave it up. – orthocresol May 8 at 14:46
• There is an example calculation here chemistry.stackexchange.com/questions/118342/… – porphyrin May 8 at 14:54
• If you want to know how the individual orbitals transform, the "linear" and "quadratic" part of your character table can help. For example, for the O orbital that forms the pi bond (the $2p_y$), the $C_{2v}$ character table tells you that it is $B_2$. Similarly the C $2p_y$ orbital is $B_2$. The $2p_z$ orbitals are $A_1$ and the $2p_x$ are $B_1$. You can use these symmetry labels to determine which orbitals may mix to form molecular orbitals. – levineds May 8 at 19:14