[MarianaM (OP)]

Hariharan asks...

Most molecules are in singlet ground state which is a higher energy state. A state is more stable. So nature would prefer an arrangement which gives lower energy. Accordingly molecules should prefer to be in triplet state which is of lower energy than singlet energy ground state. Why this is so? Could you please explain?

Can you help?

[chiralSPO]

Molecules adopt the lowest energy state they can, subject to laws of thermodynamics (zero point energy and boltzmann distribution) and QM (Pauli exclusion principle). (sometimes it can take a while for a metastable state to decay into the most stable state--especially when a change in spin state is involved)

The energy of an electronic state involves contributions from electron-nucleus interactions (energy shell, n), electron angular momenta (L and m_L) and electron-electron interactions (electrostatic repulsion and exchange energy).

For molecules with an odd number of electrons, there will always be at least one unpaired electron (doublet).

For molecules with an even number of electrons the choice beetween singlet and triplet (or higher) will be determined by the interplay of the aforementioned contributions. Typically differences in orbital energy due to n, L, and m_L are sufficiently large to favor the singlet state. However, if there are multiple orbitals with the same energy (degenerate) or very close, then the energy due to exchange can lead to triplet (or higher) states being favored. (essentially, if there are multiple electrons in degenerate orbitals, it is slightly favorable for their spins to align, leading to a high spin state)

Subtle effects in molecular geometry or composition (breaking or forming degeneracies) can thus lead to more stable high spin states, or more stable low spin states.

O₂ is an example of a simple molecule that is more stable in the triplet state (having two electrons in two . degenerate π* orbitals--singlet oxygen can be produced by either chemical or photochemical methods, and is incredibly reactive).

Ni(II) atoms have 8 d electrons, which favor a singlet state when the ligands around the Ni ion are in a square planar arrangement, but favor a triplet state when the ligands adopt a tetrahedral or octahedral arrangement. Elements near the middle of their respective rows in the periodic table an form very high spin states because they have half-filled orbitals (like Fe(III) which can adopt a sextet state with five parallel unpaired electrons, or Gd(III) which can have an octet state with seven parallel unpaired electrons)