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?

  • $\begingroup$ 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. $\endgroup$ – orthocresol May 8 at 13:58
  • $\begingroup$ @orthocresol Thank you so much! I realise there isnt a delete button for my obvious question $\endgroup$ – Eurus delielio May 8 at 14:02
  • $\begingroup$ No worries. It's not stupid at all. You can most certainly leave it up. $\endgroup$ – orthocresol May 8 at 14:46
  • 1
    $\begingroup$ There is an example calculation here chemistry.stackexchange.com/questions/118342/… $\endgroup$ – porphyrin May 8 at 14:54
  • $\begingroup$ 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. $\endgroup$ – levineds May 8 at 19:14

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