Bonding and Antibonding Molecular Orbitals?

When two molecule of water attract each other because of a hydrogen bond, do they create molecular orbitals (one bonding and one antibonding?

• Answer:

  Bonding and antibonding orbitals are part of molecular orbital theory. Even in a single molecule of water, there are bonding and antibonding orbitals. Perhaps, of more interest to you is the interactions occurring between antibonding and bonding orbitals between water molecules. Let us first take a look at a water molecule first before exploring interactions between molecules. In looking at a water molecule we know that the central oxygen has two lone pairs and 1 bond to each of the two hydrogens. Looking at the number of bonds formed, it becomes clear the number of electron domains or steric number is 4. Drawing the molecular orbital diagram for water, we see that we have 2, 1s orbitals from hydrogen, and a 2s and 3, 2p orbitals from oxygen. The 2s and 3, 2p orbitals hybridize to form 4, sp3 orbitals. About 75% p orbital character, and 25% s orbital character. We would then form our bonds. There would be two sigma orbitals and two corresponding sigma star antibonding orbitals. Our 2 leftover 2p orbitals lie somewhere in the middle between the bonding and antibonding orbitals. Oxygen has 6 electrons, hydrogen has 1 each. Filling in the molecular orbital diagram we can see that water is in fact diagmagnetic. Of importance, the HOMO is the sp3 hybrid orbitals containing our lone pairs, and the LUMO is the 2 sigma star antibonding orbitals. In combing two water molecules, we expect hydrogen bonding to occur. Hydrogen bonding occurs between the hydrogen and oxygen atoms, however, what is hydrogen bonding? Wikipedia defines hydrogen bonding as "A hydrogen bond is the https://en.wikipedia.org/wiki/Electromagnetism attractive interaction between https://en.wikipedia.org/wiki/Chemical_Polarity https://en.wikipedia.org/wiki/Molecules, in which https://en.wikipedia.org/wiki/Hydrogen (H) is bound to a highlyhttps://en.wikipedia.org/wiki/Electronegativity atom, such as https://en.wikipedia.org/wiki/Nitrogen (N), https://en.wikipedia.org/wiki/Oxygen (O) or https://en.wikipedia.org/wiki/Fluorine (F). The name hydrogen bond is something of a misnomer, as it is not a true bond but a particularly strong https://en.wikipedia.org/wiki/Intermolecular_force#Dipole.E2.80.93dipole_interactions, and should not be confused with a https://en.wikipedia.org/wiki/Covalent_bond." However, how are the electrons "truly" interacting? That is why we look at our LUMO and HOMO frontier orbitals. Knowing that when our HOMO reacts, it is called a nucleophile and our LUMO reacts which is called an electrophile. In this case we have already identified our nucleophile HOMO's on both water molecules: that is the lone pairs on an anionic atom. Our electrophile LUMO's are sigma star antibonding orbitals on each of the water molecules. We can use the arrow pushing convention to show the transfer of electrons from HOMO's to LUMO's. Thus, our electrons from the lone pairs on the sp3 orbitals are transferred over to our electrophile sigma star antibonding orbitals, or antibonding OH bonds, creating the hydrogen bond. Edit: I'm not really sure about this... No one has really given a concrete answer about hydrogen bonding, much less so with bonding in two water molecules. It's simply described as an electronegative atom with a hydrogen and a lone pair are necessary to create the bond. It is an intermolecular force with the electron density much more weakly shared than an ionic or covalent bond. I hope someone can explain this more quantitatively, as I do not have any background in physical chemistry.