 Hi, I'm Professor Nesheba and I'm here to tell you a little bit about the molecular orbital picture of Lewis acid-base chemistry. So just to remind you, the stoichiometry of a Lewis acid-base reaction is, there's a base with the lone pair of electrons available to form a bond. It combines with this thing, A, called the acid, and to make the adduct, B, A, that's the product of the Lewis acid-base reaction. I've laid out the start of an MO diagram here. So here's B on the left, here's A on the right, and there's going to be the adduct, B, A in the middle, and you see I've just put a few molecular orbitals. One of those is quite important for forming the dated bond, and I'm just labeling it as psi B. Usually it's the HOMO, the highest occupied molecular orbital of the base, sometimes it might be one below, but usually the HOMO, okay, and the thing to bear in mind is that lone pair of electrons there are the same electrons as in that orbital. So there's the MO picture of the base, and now we have the MO picture of the acid and same kind of idea, but the point about the Lewis acid is that it needs to have some orbitals that don't have any electrons in them, because as you can see from this stoichiometry, it didn't bring any electrons to that bond. So usually it's the LUMO, lowest unoccupied molecular orbital, but it might be one above that. So I'm going to give that symbol psi A for that orbital of the acid and the occupied orbital of the base. I'm going to call it psi B, and now I'm just going to draw kind of a hypothetical set of energy levels for the adduct, and what we think about is that the psi B that orbital interferes constructively to produce a new molecular orbital which has lower energy, and the usual case is also that it would form a destructive interference form an antibonding orbital. We'll call these psi D and psi star D for a dated bond, and so that's the thing is that that MO will interfere with that MO on the acid to produce sort of a bonding MO and an antibonding MO, and the challenge is finding which ones these are just by the appearance of the MOs that went into making it. The other challenge about this is that these are usually inside a forest of other molecular orbitals of B A that are there, but they don't participate in the bonding, when one final point here is that it better be that that MO psi D that results from the constructive interference from the base and the acid needs to have a pair of electrons in it because that pair of electrons is that pair of electrons right there that form the bond.