Electron donating and withdrawing groups

Aside from the fact that I do not believe in the existence of HOMOs and LUMOs, it is sometimes good to know how they work. In particular, I can never remember how electron-donating and withdrawing groups work. Here is how I understand it:

• An electron-donating group adds more electrons to the system and thus increases electron-electron repulsion (or decreases the effective nuclear charge). As a consequence the HOMO and LUMO energies increase.
• An electron-withdrawing group removes electrons and, thus decreases the HOMO and LUMO energies.
• An electron-donating group usually acts through an occupied non-bonding orbital. This is energetically close to the HOMO. Therefore, it has a stronger effect on the HOMO than on the LUMO (at least in organic molecules).
• An electron-withdrawing group acts through a virtual orbital, which interacts more strongly with the LUMO.
• As a consequence, electron-donating and withdrawing groups are both expected to lower the HOMO-LUMO gap in organic molecules.
• Things are different for transition metal complexes. For example an electron-withdrawing fluorine group still lowers orbital energies. But it can affect the HOMO more strongly and increase the overall gap in fluorinated iridium complexes, see this Ref.

Cheap nonadiabatic dynamics simulations II

Staying true to the topic of cheap nonadiabatic dynamics simulations, here is another paper by us: Surface hopping within an exciton picture - An electrostatic embedding scheme that just appeared in JCTC. The idea in this case was to speed up the efficiency of photodynamics simulations by doing computations on individual chromophores and combining the results through a Frenkel exciton model.

Cheap nonadiabatic dynamics simulations

What is the cheapest way to run nonadiabatic dynamics simulations and get results that are at least better than a random number generator? How about parameterising a linear vibronic coupling Hamiltonian using only a single excited-state computation and running surface hopping dynamics with it. This is what we tried in our new paper "Highly efficient surface hopping dynamics using a linear vibronic coupling model" that just appeared in PCCP. And to our surprise, the results were actually a lot better than a random number generator. We could reproduce the main physics of the dynamics of intersystem crossing in SO₂, the presence/absence of ultrafast internal conversion in adenine/2-aminopurine, as well as ultrafast intersystem crossing in 2-thiocytosine. Only for 5-azacytosine we were somewhat off the mark.

Some of the referees were a little bit "not amused" because it almost seems like kind of an unfair trick to run dynamics using such a simple setup. But if it works and if it gives you relevant information about the real world, why should you not do it?

Bound vs Bounded

It's always good to start the day with an arbitrary rant. Here is the one for today (since a lot of people seem to get this wrong). There are two similar words in the English language: bind and bound. "Bind" means "to connect" - and this is what we usually use in chemistry. "Bound" means "to restrict" - and this does not really have a use in chemistry. Now, the tricky bit is that "bound" is also the past tense and past participle of "bind".

This is how you should use it:

• A strongly binds to B.
• A strongly bound to B, yesterday.
• A and B are strongly bound.
• But A and B are not strongly bounded (unless you mean that they are restricted).

If you do want to use "bounded", then use it in a mathematical context:

• The function f(x)=x² is bounded from below.

You are allowed to say that.

By the way, if you want to avoid this whole dilemma, use bond/bonded/bonded.

Move to Loughborough

I just relocated to Loughborough, UK, to work as a lecturer at the Department of Chemistry. It is a busy time but very exciting :)

You can check out my new homepage here. I will move the more "professional" announcements of papers and similar things to my new newsfeed, and I will probably reserve this blog for more personal posts.