[Sam (NG)]

I have a question regarding the molecular orbitals of water.  I have to show the molecular orbitals of water as derived from s and p orbitals that are responsible for the "lone pair character" in water.  Now, i have the electronic structure of water as 1a₁²2a₁²1b₂²3a₁²1b₁².  I derived this from the Character table for C_2v, finding the symmetry adapted linear combinations for the 2 hydrogen 1s orbitals.  The orbitals have been arranged in energy order according to data from lsbu.ac.uk/water (a reliable source).


I know for definite that the 1b₁ orbital (mainly p_x character) is not involved in the sigma bonds and is responsible for the "lone pair" effect on oxygen.  However, having constructed the MO correlation diagram according to the rules in "Group Theory for Chemists", i see that the other non-bonding orbital is 2a₁, which is not responsible for lone pair character.  The lone pair character comes from 3a₁, but according to the MO diagram (and other data) this orbital is also involved in bonding.  How can i explain that this is the orbital responsible for lone pair character when it is involved in bonding?

The 3a₁ orbital is made from p_z and the two hydrogen s orbitals.  Theoretically the 2s on oxygen is also involved, if i mixed this in, this results in 3 mixed MOs of a₁.  Does this mean that the middle orbital (3a₁) is non-bonding and appears as such (i think this diagram looks more correct, esp. if you consider the energies of the orbitals shown at the side)?


On a side note, would the fact that removal of electrons from these orbitals by XPS does not cause dissociation of the molecule be evidence that the electrons are in fact the lone pair and not bonding electrons?

[tamim83]

Hey,

The 3a₁ is a bonding orbital, it just happens to be the orbital where one lone pair of electrons resides.  So one water lone pair is bonding and one lone pair is in a nonbonding orbital.  This is proved by the photoelectron spectrum for water, 2 types of lone pair electrons. 

I don't know about the 3a₁ electrons being nonbonding, they could be slightly nonbonding due to overlap issues (I don't think the p_z orbital overlaps that well with the b₁ orbital from the H₂ fragment for bent water)

[Sam (NG)]

Hey,

The 3a₁ is a bonding orbital, it just happens to be the orbital where one lone pair of electrons resides.  So one water lone pair is bonding and one lone pair is in a nonbonding orbital.  This is proved by the photoelectron spectrum for water, 2 types of lone pair electrons. 

I don't know about the 3a₁ electrons being nonbonding, they could be slightly nonbonding due to overlap issues (I don't think the p_z orbital overlaps that well with the b₁ orbital from the H₂ fragment for bent water)

Thanks for the reply, i figured it out eventually.  The 3a₁ are "slightly" bonding, if that makes sense. Using the photoelectron spectrum you see that when you remove electrons from the 3a₁ you get a change in the frequency of vibrations in the water molecule (weakening O-H bonds).  However, it does not cause dissociation of the water molecule, so they are "non-bonding" enough to be a "lone pair" instead of a bonding pair.

Basically, they don't fit into a specific category.