'Relativistic Quantum Chemistry' - Say whut?

[bodykey]

'Relativistic Quantum Chemistry' -- Say whut?

So I just found out that one of my co-workers (well, someone who works in the same building with me but not directly with me) has a PhD in this field, he took like the last two weeks off for Christmas vacation, but when he explained to me some of what he was talking about it really interested me.

Now, I pretty much fully get the gist of relativity, time dilation, space-time warping/bending and the effects of gravity. I kind of have a grasp on quantum physics as well, but that's a little deeper than I can go. I know jack didly on chemistry.

But here's the thing, what he's saying is that in heavy elements where the mass of the element is strong, it causes the electrons to spin around the nucleus at very high speeds to prevent the atoms from collapsing in upon themselves. When this happens, the electrons run near the speed of light, and actually create changes in the chemical properties of the elements, thus creating stranger and different outcomes.

So, from my little mind trying to grasp this, I have absolutely no grasp on mathematics, so anything I'm reading here looks like Chinese backwards, I'm good at understanding the theory and the concepts though which is what I'm asking for here.

How in the world does an electron going faster change the chemistry of an atom? Furthermore, how does relativity affect this, I mean...are there two different definitions for the same word in this situation? Because my only knowledge of what happens in a relativistic state is that space and time are warped, there is an introduction of time dilation and in some cases of extreme gravitational pull you create a black hole...and that's pretty much the gist of it.

Can someone explain this is non-math terms so that I can understand it? This is totally new and foreign to me but it's astounding to me. :)

Thanks!

 

[stevendaryl]

So I just found out that one of my co-workers (well, someone who works in the same building with me but not directly with me) has a PhD in this field, he took like the last two weeks off for Christmas vacation, but when he explained to me some of what he was talking about it really interested me.

Now, I pretty much fully get the gist of relativity, time dilation, space-time warping/bending and the effects of gravity. I kind of have a grasp on quantum physics as well, but that's a little deeper than I can go. I know jack didly on chemistry.

But here's the thing, what he's saying is that in heavy elements where the mass of the element is strong, it causes the electrons to spin around the nucleus at very high speeds to prevent the atoms from collapsing in upon themselves. When this happens, the electrons run near the speed of light, and actually create changes in the chemical properties of the elements, thus creating stranger and different outcomes.

So, from my little mind trying to grasp this, I have absolutely no grasp on mathematics, so anything I'm reading here looks like Chinese backwards, I'm good at understanding the theory and the concepts though which is what I'm asking for here.

How in the world does an electron going faster change the chemistry of an atom? Furthermore, how does relativity affect this, I mean...are there two different definitions for the same word in this situation? Because my only knowledge of what happens in a relativistic state is that space and time are warped, there is an introduction of time dilation and in some cases of extreme gravitational pull you create a black hole...and that's pretty much the gist of it.

Can someone explain this is non-math terms so that I can understand it? This is totally new and foreign to me but it's astounding to me. :)

Thanks!

Well relativity makes several small changes to the quantum mechanics of atoms and molecules, even for ordinary electron speeds. But it does seem strange to me that there could be enough of a difference between relativistic and nonrelativistic effects to warrant having a separate field of relativistic quantum chemistry.

But Wikipedia lists some relativistic effects that are important in chemistry:

http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#Color_of_gold_and_caesium

 

[cgk]

Well relativity makes several small changes to the quantum mechanics of atoms and molecules, even for ordinary electron speeds. But it does seem strange to me that there could be enough of a difference between relativistic and nonrelativistic effects to warrant having a separate field of relativistic quantum chemistry.

The changes are only small for light elements. They are *massive* for later ones. Why this requires special handling will become obvious if you consider what kind of energies the core electrons of a, say, uranium atom are exposed to. Also non-scalar relativistic effects, like spin-orbit coupling, become very important in higher elements (their strength goes approximately with Z^4, where Z is the atomic number).

One of the most important applications of relativistic quantum chemistry is, however, the development and adjustment of effective core potentials (ECPs, "pseudopotentials"). These are very often used to replace the relativistic core electrons by an effective non-relativistic description in standard quantum chemistry methods (for both computational and accuracy reasons---good relativistic ECPs are normally much more accurate than non-relativistic all-electron treatments of non-light elements).

 

[bhobba]

One thing that needs to be mentioned is Pauli's Exclusion principle, which requires relativistic QM to explain, is very important in Chemistry.

Thanks
Bill

 

[Naty1]

Oh no, more quantum math nobody knows how to interpret:



http://en.wikipedia.org/wiki/Effective_Core_Potential

The very high accuracy of these "semiempirical pseudopotentials" in fitting optical and photoemission spectra of many semiconductors has never been equaled, not even by the most elaborate "first principles" calculations; this very high accuracy of simple single-electron models remains a puzzle today.

I wonder if the interpretation is more closely that the "nodeless pseudo-wavefunctions" [referred to in wiki] of the electron effectively propagates more quickly rather than

...electrons to spin around the nucleus at very high speeds

That description from the OP sure sounds like an attempt to use a classical picture to describe more complicated quantum phenomena.

 

[mfb]

The nonrelativistic binding energy of the innermost electrons can be written as
$$E=\frac{1}{2}m_e c^2 \alpha^2 Z^2$$ with the fine-structure constant $\alpha \approx \frac{1}{137}$. As soon as Z gets comparable to 1/α, the binding energy gets comparable to the rest energy of the electrons, and relativistic effects get important - they change the orbitals and energy levels (and things like spin-orbit coupling, see cgk's post) significantly.