# In which direction does the electron move during an electronic transition?

Let's imagine a molecule and put it in a 3D space. Let's also imagine an electronic transition for this molecule. I know how an electronic transition works, how to value if it is possible (using the rules derived from the symmetry of the molecule and so on). My question is: how do I understand how the electron moves during the transition WITHOUT using the symmetry of the molecule? Can I "see" in which direction the electron moves in the 3D space without using the symmetry?

(I hope I've been clear enough to understand the question! Sorry for my mistakes)

• I don't think you can really talk about the electron going from point A to point B - instead, the probability distribution for electron position evolves from distribution A to distribution B. I have an answer here that goes into a bit more detail (though it discusses atomic transitions). – Kyle Oman Jun 26 '13 at 15:24
• What you described is called transition dipole moment, which is roughly the vector that shows the shift of the electron density. – gigacyan Jun 27 '13 at 11:41

## 2 Answers

You have to consider the molecular orbital as a hole. The molecular orbitals (MOs) are formed from atomic orbitals of each individual atom. Since the number of MOs and AOs are conserved, if two AOs combined, they would make a bonding and antibonding pair of orbitals.

It is then possible, using a computer to plot the electron density function of the MO - this is the probability of finding the electron of a given molecular orbital in a given region of space. In this respect, it isn't possible to say in which direction the transition will occur, only what the probabilities are of finding the electron in two different MOs before and after the transition.

For example, the image here shows that when two px orbitals combine, they form PI bonding and Antibonding orbitals. Technically there is a zero probability of finding the electron in the middle in the antibonding case (this region is called a node).

What is the time scale you are interested in? During spontaneous transitions between metastable states, electron passes from one quasi-stationary state to another. In this case, you have the wave functions of the initial and final states for which it is possible to compute an expectation of the electron position before and after transition.

• I try to explain more clearly. Imagine the molecule CH2CH2 positionated in the 3D space with the double bond on the z axis. Considering the PI->PI* transition, on which of the three axis does the electron move? I know it moves on the z axis, but how do I justify it? – Federica Boccardi Jun 26 '13 at 14:48
• The quantity which shows the axis where the most significant redistribution of electron density occurs could be the transition dipole moment en.wikipedia.org/wiki/Transition_dipole_moment – freude Jun 26 '13 at 15:30
• Of course, and thanks to it and the the symmetry of the molecule I can say if the transition is possible and the axis involved. Do you think it's possible to understand it without the use of the transition dipole moment? – Federica Boccardi Jun 26 '13 at 15:59
• I have no idea to be honest. Do you mean some theoretical feature or experimental technique? – freude Jun 27 '13 at 7:29
• I have some pages that could be useful, but they're in Italian, and I don't think you could read them :( if I find something in English, I'll post an example! – Federica Boccardi Jun 27 '13 at 7:50