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Valence Bond Theory tells us that each of the “sp3” (in reality, 44% s character) oxygen line pairs in water can act as electron donors, but observing the MO diagram for water tells us that the 2 non-bonding orbitals are of vastly different energies. (One of the “lone pairs” has bonding character too.) As such, how does water even form 2 electron-donating hydrogen bonds?

Mithoron
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ANZGC FlyingFalcon
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  • It is not like one orbital on the diagram corresponds to one lone pair. – Ivan Neretin Feb 15 '19 at 10:20
  • I know that. The MO diagram of water has one “strictly nonbonding” (HOMO; b1 symmetry) and one “mostly nonbonding” (a1 symmetry, formed from 2s of oxygen) that has almost all electron density on the oxygen due to the energy difference. However, hydrogen bonding is often shown as the oxygen bonding to 2 hydrogens (Google it up and you’ll know what I mean), so I was inquiring about that. – ANZGC FlyingFalcon Feb 15 '19 at 11:27
  • Then why would you say that one of the lone pairs has such-and-such character, which implies that the lone pairs are different, which in fact they aren't? – Ivan Neretin Feb 15 '19 at 11:35
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    @IvanNeretin The two lone pairs of water are of slightly different energy i.e. those two lone pairs are not exactly similar. Those two lone pairs reside in different (non-degenerate) orbitals having different symmetries. In fact, photo-electron spectrum of water also tells about the difference of the two lone pairs. They are not exactly similar. – Soumik Das Feb 15 '19 at 12:32
  • Thanks, Somnik Das. I don’t think Ivan is referring to the same thing as us. – ANZGC FlyingFalcon Feb 15 '19 at 12:38
  • Indeed, there is an ambiguity here. The two pairs of electrons sitting on different molecular orbitals are different all right. The two protruding clouds of high electron density are similar. If you were referring to "lone pairs" in the former sense, then you may disregard my previous comments. – Ivan Neretin Feb 15 '19 at 13:00
  • This website has a number of references to studies of the electronics and orbital interactions of hydrogen bonding in liquid water: http://www1.lsbu.ac.uk/water/h2o_orbitals.html Might be a good place to start. – Andrew Feb 15 '19 at 15:32
  • How exactly hydrogen bonding in water looks like in actual liquid is fully known. AFAIK some percent of H2O molecules does accept second h-bond, but definitely not all of them. – Mithoron Feb 15 '19 at 21:40
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    related https://chemistry.stackexchange.com/questions/50906/are-the-lone-pairs-in-water-equivalent https://chemistry.stackexchange.com/questions/69370/how-many-hydrogen-bonds-are-formed-by-water-and-by-hf – Mithoron Feb 15 '19 at 21:42
  • The assumption seems to strong . The lone pairs are there and this suffices. What makes me puzzling now is if the various H-binds can be treated as the same or not once formed. But shouldn't be, at least in principle, I think. Is where photo-electron spectroscopy should be valuable. – Alchimista Feb 16 '19 at 09:00

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