Electron spin and chemical bonding

[chroot]

I am relaying this question from rtharbaugh1, who originally posted it to a thread in the Quantum Physics forum:

https://www.physicsforums.com/showthread.php?s=&postid=98040

Originally posted by rtharbaugh1
I first read that paired electrons in an atomic shell must have opposite spin. Does this mean that a chemical bond will not form between, say, Na+ and Cl-, unless the two atoms have electrons of opposite spin? Wouldn't this mean that two such atoms brought sufficiently close together would only form a bond half the time? Can we manipulate single atoms of sodium and chlorine to test this idea? Has it been done?

Can any of you chemistry nerds answer this?

- Warren

 

[Chemicalsuperfreak]

There is no chemical bond between chlorine and sodium. That is to say it is only electrostatic attraction, an ionic bond as opposed to a covalent bond. The pauli exclusion principle applies to electrons that occupy the same orbital, such as in a covalent bond.

If you bring an atom of sodium near a atom of chlorine, the sodium will give its electron to chlorine, resulting in the sodium cation and the chloride ion, which bind ionically.

But that skirts the question. If you bring a hydrogen atom, a radical, together with another hydrogen atom, will they only react half the time? Is this a better question? What do you think will happen?

 

[Mike H]

The typical train of thought (as I understood it) is that the total spin of the reactants must be equal to the products in order for a reaction to occur. The well worn example is that of recombination after radical pair generation by photolysis/radiolysis - there needs to be a conversion from the triplet state into the singlet state for the diamagnetic starting product to be formed.

 

[rtharbaugh1]

spin states and bonding

Originally posted by Chemicalsuperfreak

If you bring a hydrogen atom, a radical, together with another hydrogen atom, will they only react half the time? Is this a better question? What do you think will happen?

Thanks for not leaving me with a skirted question. I appreciate the correction, and agree that the H-H covalent bond is a better example of the process I am curious about.

As for what I think will happen, I suspect that the hydrogen atoms will rotate in 3space until the electron spins match up to give the lowest energy state. Then they will bond together in that spatial relationship. I imagine this process as being similar to a pair of macroscopic magnets sufficiently close together re-aligning to match their magnetic fields. In fact, I would speculate that magnetism in general may have much to do with spin states.

There may be some small factor of energy taken to effect the rotation, and that may cause some small delay in the bonding of some atoms with certain others, but given the very small mass of an atom, I doubt if the rotation delay could be measured.

Thanks for the answer. I will watch for your reply about the magnetizm thing.

Richard

 

[Bystander]

Search "Woodward-Hoffman rules;" there are a couple decent sites on the first page of the list, 224 hits = 127 actual sites.