How to Keep Up with Molecular Orbitals for Intro Chemistry

[Cruikshank]

I am a professional math and science tutor, primarily focused on math and physics. I used to tutor organic chemistry but gave it up because I didn't want to study orgo an hour per week for the rest of my life. I like general chemistry and have no trouble with most topics, but the way of teaching atomic orbitals, hybrid orbitals, and molecular orbitals is evolving so rapidly, I find it almost impossible to decipher professors' lecture notes or students' homework problems. Is there a recommended resource that keeps up with the changes?

 

[DrClaude]

but the way of teaching atomic orbitals, hybrid orbitals, and molecular orbitals is evolving so rapidly, I find it almost impossible to decipher professors' lecture notes or students' homework problems. Is there a recommended resource that keeps up with the changes?

I'm not aware of any change in the last few decades. Do you have an example? Or a recent textbook that is causing you problems?

 

[DrClaude]

Maybe @DrDu can chime in.

 

[DrDu]

I fear I am also not familiar with recent general chemistry textbooks.

The transfer of knowledge from theoretical chemistry into introductory textbooks has been exceedingly slow. If in the beginning theoretical chemistry was based mainly on qualitative valence bond type discussions, which became standard curriculum for a long time due to Linus Paulings influential books. Since the 1960's, theoretical chemists tried to promote the teaching of molecular orbital theory, mainly due to the relative ease with which quantitative calculations can be performed. In introductory texts, both concepts are often not clearly separated, probably due to limited understanding of the matter on behalf of the authors. Today, computers are sufficiently powerful to perform quantitative valence bond calculations, too, and, as it should be, in the end both MO and VB methods give the same results. Most routine calculations on both organic and inorganic compounds use density functional theory these days and even less covered in introductory texts.
There must be some readable modern quantum chemistry books out there, but alas, I am not familiar with them.

 

[Cruikshank]

Thank you both for your replies, As I learned it, there are atomic orbitals such as 1s, 2s, 2p, etc for each atom. One system for adjusting orbitals due to the presence of other atoms is the "hybrid atomic orbitals": sp, sp², sp³, sp³d, and sp³d² (and I've heard that sp³d³ exists for IF₇.) The other system is molecular orbitals, sigma bonds, sigma* bonds, and so forth. My general chemistry text only covered homonuclear diatomic orbitals for periods 1 and 2. My understanding was that s's and p's would merge into an equal number of hybrids, or an equal number of MOs. I did not learn how to convert hybrids to MOs, or how many orbitals of which kinds to use while ignoring the rest. I have never seen a coherent explanation of the "shaded figure 8" pictures that I am guessing are meant to depict p orbitals in or out of phase and making various more elaborate MOs, to describe resonance--and what I have seen from students' notes is inconsistent--which of the options to use, where the nodes are, etc. I am really lacking a criterion for identifying which of the myriad possibilities are relevant. I've seen "MO" diagrams with hybrid orbitals in them! My overall impression is that the names for things are changing, the level of detail demanded is changing, which set of orbitals are relevant to teach is changing, and the models themselves are changing. I have recently seen *ionic bonding* described with sigmas, and a vague rule that orbitals can hybridize/form MOs when they are "similar" in energy level. I can vaguely follow the gist of all this, but when I teach it, my students complain that that isn't how their instructor teaches it, we have language difficulties, and I can't figure out the problems because there always seems to be one or two crucial details of their system that I don't have.
I hope that clarifies a bit where I'm having trouble and what I'm looking for. One more specific example: last week a student brought me a setup with 3 carbons forming a p-system and had a diagram with the figure 8s. I can imagine 8 different ways to shade those orbitals, and even accounting for symmetry I don't see all of them being used. Further, in one state, the middle figure 8 wasn't shaded at ALL. So are there now 64 shading possibilities? You can see my frustration.

 

[DrDu]

I have been looking around a little bit. These series of lectures seem to be both quite quite sound and modern on one hand and explaining the basic theoretical concepts on the other:
http://www.nyu.edu/classes/tuckerman/adv.chem/lectures/notes.html

 

[DrDu]

One system for adjusting orbitals due to the presence of other atoms is the "hybrid atomic orbitals": sp, sp², sp³, sp³d, and sp³d² (and I've heard that sp³d³ exists for IF₇.)

Hybrid orbitals involving d-orbitals are no longer considered relevant for the description of bonding in main group elements like IF7.