Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on February 14, 2020, 03:00:32 AM
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Why is :CH2 foundamental state a triplet ?
My textbook speaks about a hund's rule for stabilty...I have to put the electrons in different orbital with parallel spin
But here I have an electron on a sp2 orbital and the other on a p orbital at different energy
Isn't Hund's rule for orbital at the same energy level?
Thanks
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Simplistically, yes, but when orbitals are close together you have to consider the energy balance between promoting an electron to a higher orbital and pairing the spins.
If the energy difference between the orbitals is EO, and the spin-pairing energy is EP, then the difference between the singlet and triplet states is EO - EP. Whether this is positive or negative depends on the relative magnitudes of EO and EP. Evidently in carbene this balance favours the triplet.
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And don't forget that hybridization is a construct invented to make it easier for us to visualize bonding but the experimental evidence is that it does not occur. So your idea that triplet CH2 has one electron in a p orbital and one in a sp2 orbital is not accurate.
Experimentally the ground state is triplet, but not by much (~38 kJ/mol kcal/mol). There have been a lot of calculations on this simple system trying to understand why triplet is below singlet. Triplet has wider bond angles (>125 degrees) than the singlet (~105 degrees). At typical temperatures, there is a small but significant population of the singlet state.
Any pi donor substituents push the singlet state lower than the triplet.