Formaldehyde

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The localized orbitals in your best Lewis structure can interact strongly. A filled bonding or lone pair orbital can act as a donor and an empty or filled bonding, antibonding, or lone pair orbital can act as an acceptor. These interactions can strengthen and weaken bonds. For example, a lone pair donor->antibonding acceptor orbital interaction will weaken the bond associated with the antibonding orbital. Conversly, an interaction with a bonding pair as the acceptor will strengthen the bond.

Strong electron delocalization in your best Lewis structure will also show up as donor-acceptor interactions. Interactions greater than 20 kJ/mol for bonding and lone pair orbitals are listed below.
The interaction of the second lone pair donor orbital, 8, for O2 with the antibonding acceptor orbital, 59, for C1-H3 is 114. kJ/mol.
The interaction of the second lone pair donor orbital, 8, for O2 with the antibonding acceptor orbital, 60, for C1-H4 is 114. kJ/mol.

The orbital energies are given in eV, where 1 eV=96.49 kJ/mol. Orbitals with very low energy are core 1s orbitals. More antibonding orbitals than you might expect are sometimes listed, because d orbitals are always included for heavy atoms and p orbitals are included for H atoms. Up spins are shown with a ^ and down spins are shown as v.

The total electronic energy is a very large number, so by convention the units are given in atomic units, that is Hartrees (H). One Hartree is 2625.5 kJ/mol. The energy reference is for totally dissociated atoms. In other words, the reference state is a gas consisting of nuclei and electrons all at infinite distance from each other. The electronic energy includes all electric interactions and the kinetic energy of the electrons. This energy does not include translation, rotation, or vibration of the molecule.
Total electronic energy = -114.5456831412 Hartrees « mai multe referate din Chimie