Conrotatory motion maintains C2 symmetry, and disrotatory motion
maintains Cs symmetry. We may therefore draw an orbital energy
correlation diagram, in which the orbitals of butadiene are
classified under Cs symmetry (a',a'',a',a''), on the lhs, and
under C2 symmetry (b,a,b,a), on the rhs. The bonds of
cyclobutene which are involved ( ) are shown in the middle.
Lines are then drawn connecting the cyclobutene orbitals under
Cs symmetry (a',a',a'',a''), and under C2 symmetry (a,b,a,b).
The diagram clearly shows that the disrotatory path is favoured
photochemically and the conrotatory path is favoured thermally.
We shall use C2v symmetry with the axes as shown. For H2CO, we
can form CH bonding orbitals (a1 and b2), the O lone pair
(2pyO=b2) and the CO orbital (b1). Thus the electronic
configuration is 1a121b222b221b12. For H2, the orbital has a1
symmetry. For CO, the bonding orbital has a1 symmetry, and the
orbitals have b1+b2 symmetry. Thus the separated molecules have
electronic configuration 1a122a121b221b12.