Lecture slides – Wed Sept 8 (more MOs)

Slides for today’s lecture are located here.

Several of you came back with excellent questions after lecture today and I want to share some of this material with the rest of the class.
One question asked for clarification of the roles of atomic orbitals, hybrid orbitals, and molecular orbitals. To answer this, I took ammonia, NH3, as an example.

My goal in using the MO model is to find an orbital that describes each electron pair in NH3. There are three NH bond pairs and one N lone pair. Since there are 4 electron domains around N, it is appropriate to use sp3 hybrid orbitals for this atom. To do this, I imagine N going through the following changes in electron configuration:

1. I start with the standard electron configuration one expects for an isolated N atom, i.e., (2s)2 (2px)1 (2py)1 (2pz)1 (sorry, but its inconvenient to put super- and subscripts into these web pages)
2. There is no need to move electrons between orbitals, so I proceed to mix the four atomic orbitals and replace them with sp3 hybrids. This makes the electron configuration: (sp3 #1)2 (sp3 #2)1 (sp3 #3)1 (sp3 #4)1.

The next step is to notice which N orbital will describe the lone pair. I can see from step #2 that sp3 #1 is occupied by two electrons, so this is the lone pair orbital.

The final step is to construct bonding and antibonding MOs for the NH bonds. There are only two elecrons in each bond so only the bonding MO will be occupied. Each BMO is constructed by combining overlapping N and H orbitals, i.e., N sp3 with H 1s.

To summarize, the process is find the appropriate valence orbitals and electron configuration for each atom. Valence orbitals that contain 2 electrons are used to describe lone pairs. Valence orbitals that contain 1 electron are overlapped with comparable orbitals on other atoms to make bonding MOs.

The second question concerned pictures of sp3 orbitals vs. pictures of BMO built from sp3 orbitals. The lecture slides contain pictures of all four BMO for CH4, but they do not contain any pictures of “isolated” sp3 orbitals. Fortunately, you can find drawings of sp3 orbitals in Loudon. The difference between the two pictures is fairly small; just take the BMO from the lecture slide and shrink the lobe that extends over H so that it stays closer to C and you have a nice picture of an sp3 orbital.

This person also asked if sp3 orbitals only appear in molecular orbitals or if they ever appear on their own. The answer to this can be seen from the NH3 model described above. A hybrid orbital can describe a lone pair so hybrid orbitals are not limited to service as components of MOs.