Thursday, 15 January 2009

Proton Transfer

This time I will show a few "quantum images". Hydroxybenzoquinoline is kind of a cool molecule because it shows excited state intramolecular proton transfer. After UV excitation the proton is transferred from the oxygen to the nitrogen. This happens on a time scale of about 30 fs because the process is essentially barrierless. This is a time scale where you can do ab-initio molecular dynamics. And with modern LASER pulses you can also examine it experimentally. That's why these molecules have been rather popular recently.

What I want to show here are the orbitals because I think they look kind of cool. What you see are the two highest occupied orbitals (n, π) and the lowest unoccupied orbital (π*). The two important excitations are just between these orbitals leading to the ππ* and nπ* states respectively.

Related to the proton transfer it can be seen that the π* LUMO has more density on the N which increases its basicity to help it catch the proton.

Something else that is interesting is that a non-bonding orbital (coming from MO theory) really looks a lot like the free electron pair you would talk about in VB theory. I was arguing with an analytical chemistry professor once that he should not interchange the terms non-bonding orbital and free electron pair because they come from different approaches. Now I see that he was right but I did not believe him then because he was from analytical chemistry.

Enol (ground state)Keto (ππ* state)
π*
π
n
Just a short summary of the software: the orbitals are actually from semi-empirical DFTB. And the pictures are made with VMD.

2 comments:

Till said...

Hello,
I have recently read a paper from Marcus Elsner et al. in PNAS (http://www.pnas.org/content/105/50/19672.abstract?sid=b2adfe84-c3f9-438e-806f-0b22f7f0afea) where they claim that their DFTB method can reproduce DFT(w. B3-LYP) calculations. They applied this to a small part of a protein. Have you done any other calculation to test the DFTB method for the keto/Enol stuff?

How did you do the dynamics? Newton? Langevin? Wavefunction propagation?

It's not that easy to believe in results when you not know about the used methods :)

Greetings
Till

Felix said...

actually this is not really related to my group's paper where they used TDDFT/B3LYP with Newtonian and wave packet dynamics (http://dx.doi.org/10.1016/j.chemphys.2007.10.021)

we have some good experience with DFTB for modelling solvation in the ground state including hydrogen bonds (that is not really my topic though). here the topic was time-dependent DFTB for excited states and that is a whole different question. TD-DFTB is amazing because it's so fast. but since already TDDFT causes problems in many cases it is even more of an issue here