Talk:Molecular orbital diagram

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The molecular orbit diagram for oxygen shows the sigma2p orbitals below the pi2p orbitals. My copy of 'Chemistry' by R Chang shows them being the other way round, and explains that this is because of mixing between the 2p and 2s orbitals. Would a more qualified chemist like to review this? Nigel D (talk) 20:57, 20 November 2007 (UTC)

The diagram is correct. The cross over point is around dinitrogen and that is the tricky one. Mixing between 2s and 2p depends on the energy difference. It is greatest when the energy difference is small. That difference increases going from B to F so the lowering of energy of the "2s" MOs and raising of the energy of the "2P" MOs decreases and the "2p" MO is lower for O2. I put "2s" and "2p" in quotes because they each have a bit of the other mixed in, in the MOs, due to the mixing. I have not seen Chang for a while, but it looks as if it technically wrong. However it really does not matter as both the 2p sigma and the 2p bonding pi are occupied and he may just be using the same order of MOs for simplicity. --Bduke (talk) 22:19, 20 November 2007 (UTC)

Thanks Bduke.. You are right and Chang is right. I had misread the notes accompanying Changs diagram. Nigel D (talk) 22:31, 20 November 2007 (UTC)

"In diboron the 1s and 2s electrons do not participate in bonding but the single electrons in the 2p orbitals occupy the 2πpy and the 2πpx MO's resulting in bond order 1." Relative to the paragraph before this explanation, shouldn't the orbitals actually be the 2πpy and 2πpz orbitals? It had explained that, in its orientation of the molecule, the x orbital only makes sigma bonds. Ryan M 2:50, 11 January 2009 (PU) —Preceding unsigned comment added by (talk)

The diagram Molecular_Orbitals_CO2.jpg has an error: the label MO6 is used twice, with MO7 missing. The MO6 next to the MO5 should be MO7. (talk) 19:08, 20 April 2010 (UTC)

Agreed. Will fix. --Vaughan Pratt (talk) 05:40, 23 June 2010 (UTC)

Question re table of MO energies[edit]

A few days ago editor added the note "(methods/basis?)" at the beginning of the Section MO energies overview.

First note that this sort of question/comment belongs on this talk page, and not on the article page where it is distracting, so I will remove it today.

The question is however a reasonable one, so I have looked at the 2005 Journal of Chemical Education article cited in the heading of Table 1, and was disappointed to find that the authors have not really answered it. Instead they just quote the orbital energies from two older publications which are Cade P.E. and Wahl A.C., Atm. and Nuc. Data Tables 1974, 13, 339-389 (for the MO orbital energies); and Clementi E. and Roetti C., Atm. and Nuc. Data Tables 1974, 14, 177 (for the basis orbital energies). Can someone access these 1974 tables and summarize the methods and the basis set for inclusion in a note to this article? Dirac66 (talk) 01:38, 25 March 2014 (UTC)

Actually the 2005 article by Harrison and Lawson does describe the method as Hartree-Fock-Roothaan theory, as supposed to various semi-empirical methods. I will insert that information. Dirac66 (talk) 02:46, 25 March 2014 (UTC)
I am not clear what you are wanting here. I think that the Cade P.E. and Wahl A.C. stuff and the Clementi E. and Roetti C. stuff is also in papers by these authors in more accessible journals like J Chem Phys. I can find them if you want. --Bduke (Discussion) 03:24, 25 March 2014 (UTC)
The original note asked for the basis set used in the calculations, which help to define the accuracy of the results. STO, double-zeta, STO-3G, ... ? I think that is the most relevant information missing. Dirac66 (talk) 11:08, 25 March 2014 (UTC)

Quick comments on latest edits for water molecule[edit]

  1. The b1 and b2 symmetry labels are not the same in all books. I think the way you have it now (b1 perpendicular to the molecular plane) follows the majority of authors, but not all.
  2. The orbital numbering should include the core orbital as for the other molecules in the article. For water 1a1 = O1s, and the valence shell orbitals are 2a1, 1b2, 3a1 and 1b1.
  3. The photoelectron spectrum is limited to 3 peaks if the photon source is the He I emission at 21.2 eV which is the most common source. However the 4th peak for 2a1 can be seen also with a higher energy source, usually He II (He+) near 40 eV. Levine, Quantum Chemistry (4th edn. Prentice-Hall 1991, p.475) lists 5 peaks: core 1a1 at 539.7 eV which requires an X-ray source, 2a1 at 36.7 eV, and the 3 you have listed which he has at 18.5 eV, 14.7 eV and 12.6 eV.
  4. Since the article title is MO diagram, we really should have a diagram for the water molecule as for all the other molecules in the article. Levine has a diagram on p.473. Dirac66 (talk) 02:33, 21 May 2015 (UTC)
I have found a French wikipedia picture which is also used on the Japanese wikipedia. Has minimal French in the image so I think it would be understandable to English people who don't understand French (like me). At least better than no picture. However the orbital numbering, given the image, can't be changed without a subsequent change to the image (perhaps I'll manually trace the image to SVG if I have time because the auto tracing is bad. Then that image would be language-neutral and have the orbital numbering as you've recommended).--Officer781 (talk) 07:37, 21 May 2015 (UTC)
Created a simplified but hopefully more accurate SVG image of the diagram. In the new diagram the bonding and antibonding hydrogen MO's have an energy gap and the oxygen 2p orbitals start at the same energy. Small admixture is marked by smaller dotted lines which more accurately reflects reality compared to a more "equal" mixing of the 2s and 2pz orbitals in the 2a1 level as the older image seems to imply. Do check it out and if there's any errors let me know. Thanks!--Officer781 (talk) 09:55, 21 May 2015 (UTC)
Good work - the diagram for H2O looks good now. In fact it is more clearly labelled than the diagrams for the first-row diatomics. Dirac66 (talk) 10:58, 21 May 2015 (UTC)

MO diagram and ionic bond[edit]

MO modelling is only valid when the atomic orbitals have comparable energy; when the energies differ greatly the mode of bonding becomes ionic.

Does this imply that MO diagram is only applicable for covalent bond? But I found some sources ( state that MO diagram can be employed for ionic compounds too? (Concho4chan (talk) 02:31, 12 July 2017 (UTC))

The word modelling is poorly chosen. Any molecule including NaCl can be modelled by molecular orbital theory or other forms of approximate quantum mechanics, and yes, MO diagrams can be drawn. However when the atomic orbitals energies differ greatly as in NaCl, the orbitals turn out to be mostly on one atom or other as your source says, so that there is no significant covalent bonding and the bond is best described as ionic. I will revise this sentence in the article. Dirac66 (talk) 18:54, 12 July 2017 (UTC)