# What physical forces give rise to the peculiar bond angle of hydrogen peroxide?

In one of the Periodic Table videos, Prof. Poliakoff shows a model of a hydrogen peroxide molecule and claims that the H-O bonds will always be at right angles to each other. I have a rudimentary intuition for how molecules arrange themselves based on mutual attraction and repulsion - I've solved some Foldit puzzles - but the H2O2 defies that kind of common sense explanation.

I get that there are forces pushing the hydrogens apart and forces pulling them together, but how does that fragile equilibrium of forces result in an exact right angle?

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I strongly suspect that this will be better answered at chemistry.stackexchange.com. The physics answer is that the only forces present are electrostatic interactions. The chemistry description is likely to be more useful. –  Colin McFaul Jul 19 '12 at 2:37
I consider the question sufficiently on-topic to stay here if you want (bonds are quantum mechanics and all that), but I am with @Colin in thinking that you might get better answers on Chemistry. If you want it migrated just flag or comment and we'll ask the Chem mods. –  dmckee Jul 19 '12 at 3:28
The offer to ask the Chemistry mods if they want this remains open...but I won't migrate it to a beta site without your say-so. –  dmckee Jul 21 '12 at 15:30

According to J. Chem. Theory Comput. 1 (2005) 394, it seems it's mainly the combination of hyperconjugation and repulsion between the lone pairs that favours a minimum at around 120°.

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I'm not sure there is an intuitive answer. The Wikipedia article claims it's an interaction between the lowest unoccupied molecular ($\pi^*$) orbitals on the two OH bonds, but this strikes me as arm waving. Experience suggests that arm waving arguments are dangerous because you can rationalise pretty much anything if you try hard enough. –  John Rennie Jul 19 '12 at 7:42