## Abhisar one year ago Can anyone teach me how to write configuration for hetero-atomic molecules using MOT. Eg. $$\sf H_2O$$

1. Abhisar

@Kainui

2. Abhisar

@JFraser

3. Abhisar

I am doing it like $$\sigma 1s^2 ~\sigma^*1s^2~\sigma 2s^2~\sigma ^*2s^2 (\pi 2p_x^1 \pi2p_y^1)$$

4. Abhisar

But according to this there are 2 unpaired electrons and thus water should be para-magnetic but water is actually diamagnetic.

5. Abhisar

@aaronq

6. anonymous

Your problem is here in the $$\sigma ^*2s$$ orbital's placement. It's actually higher in energy than the $$\pi$$ orbitals. $$\sigma 1s^2 ~\sigma^*1s^2~\sigma 2s^2~\sigma ^*2s^2 (\pi 2p_x^1 \pi2p_y^1)$$ So the correct placement should be: $$\sigma 1s^2 ~\sigma^*1s^2~\sigma 2p_z^2(\pi 2p_x^2 \pi2p_y^2)~\sigma ^*2p_z^0$$ Which has only paired electrons, thus diamagnetic like you're looking for. Also note that I changed them from s to $$p_z$$. Remember these $$\sigma$$ molecular orbitals came from two p orbitals not s orbitals! Here's the difference in a picture! |dw:1439133799765:dw|

7. anonymous

I think my picture got cut off so here I found one on the internet: http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/graphics/fig8_32.gif http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/graphics/fig8_30.gif See how both combine to give $$\sigma$$ orbitals bonding and antibonding?

8. Abhisar

ok, will you write the configuration similarly for $$\sf B_2$$ ?

9. Abhisar

I am sure that for homonuclear molecules like $$\sf N_2, B_2$$ energies for molecular orbitals are in order $$\sigma 1s <\sigma^*1s<\sigma 2s<\sigma ^*2s <(\pi 2p_x\approx~\pi2p_y)$$

10. anonymous

$$B_2$$ is different than what you might expect depending on what you know. It obeys what's called "light bonding scheme" where the two orbitals are flipped relative to other homonuclear diatomic molecules found in the same period as $$B_2$$ And here's a pic of the MO diagram in case you were curious: http://img.sparknotes.com/figures/8/83ce1fb7be648ede5785ea60c96b495c/b2correlate.gif Writing it out should follow from this, I started typing it and got lost in all the pi's and sigmas and subscripts so I'll just let you figure that out and I can check you, but you can see where all the electrons are placed. Also, since the lower orbitals don't contribute to bonding, usually people omit them in case you were looking for the 1s orbitals.

11. Abhisar

Yes that's what I am confused, the scheme followed by heteronuclear molecules like HF and H2O seems to be different. I want to learn how to write their configuration.

12. anonymous

You are correct, to add to what you've written: $$\sigma 1s <\sigma^*1s<\sigma 2s<\sigma ^*2s <(\pi 2p_x\approx~\pi2p_y) < \sigma2p_z <(\pi 2p_x^*\approx~\pi2p_y^*) < \sigma2p_z^*$$

13. anonymous

Well I think this is a good picture showing the differences: http://chemwiki.ucdavis.edu/ @api/deki/files/10245/MO_diff_Diagram.jpg?size=bestfit&width=384&height=288&revision=1 I got it from here: http://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Pictorial_Molecular_Orbital_Theory/How_to_Build_Molecular_Orbitals I sort of hesitate to give any solid answers here because you have to realize that these are just models and not entirely true, so I have forgotten some of the specific details, but how to combine heteronuclear diatomics and how they mix rely on the electronegativity differences, electrons will tend towards for instance oxygen, which is because that's a lower potential energy state as opposed to hydrogen. --Don't read the next part lol-- In reality you sorta have to determine these with more sophisticated mathematics using wavefunctions and/or actual experiments... or other hand wavy sounding reasoning. For instance water's real MO diagram will look like this http://www.brynmawr.edu/chemistry/Chem/sburgmay/chem231/MOpics/H2Omos.jpg

14. Abhisar

Thanks for the help, I really appreciate it :)

15. Abhisar

I actually kinda get it now, this video is a good explanation https://www.youtube.com/watch?v=estiedAlXII

16. anonymous

Cool yeah I have to review this topic I'm trying to get into graduate school but I'm sorta practicing inorganic and physical chemistry right now, but I really love organic chemistry so if you have any more fun questions I really need to practice and look stuff up haha

17. Abhisar

Sure, I'll keep it coming as I encounter them. Thanks for the help again :)