[magikarpet]

Hi guys, I've got my Organic chem final coming up and I'm working through the past papers. There's one question on computational chemistry that we only briefly touched on for two lectures, but it comes up every year for 20 marks so I'd like to understand exactly what I'm supposed to be answering with.

"Determine the reaction mechanism (i.e., the activation barrier and the
reaction enthalpy) for the formation of HF from the concerted addition of H2 to F2.
Describe the process by which you will achieve this. Include specific details on the number
and type of the calculations that will be carried out, as well as the computational method
that you would employ."

What does concerted addition of H2 to F2 mean? Is that simply just asking for H2 + F2 = 2HF? I understand that you need to first pick a computational method, such as DFT with the M06 functional, which should work well as the system is small and has relatively few atoms. And we can therefore use the 6-311++G(d,p) basis sets. At this point I'm lost... I have notes on optimization of minima and transition states but I don't understand where this ties in. I can think of how to hit 5 marks at most...

[Hunter2]

I think the reaction is a kind of chain reaction.

F₂ => 2 F*

F* + H₂ => HF + H*

H* + F₂ => HF + F*

and so one until all flourine and hydrogen is consumed.

Stop reaction H* + F* => HF

Similar it works with chlorine but need light.

http://chemed.chem.purdue.edu/demos/main_pages/10.7.html

With your computer modeling I cant help.

[Irlanur]

First of all, the question is unprecise or even wrong, because it is unclear whether you should determine the reaction mechanism (if this is even possible by any means...) or whether you should investigate the proposed mechanism (a concerted reaction). I assume it is the second.

Concerted means in one step, which is the basically the exact opposite of a chain reaction.

Now you need to calculate the activation barrier and the reaction enthalpy. let's start with the second one. How would you calculate that? I don't mean with which program/functional/whatever, but which energies of which exact species do you have to determine?

[DeanC]

Hi guys, I've got my Organic chem final coming up and I'm working through the past papers. There's one question on computational chemistry that we only briefly touched on for two lectures, but it comes up every year for 20 marks so I'd like to understand exactly what I'm supposed to be answering with.

"Determine the reaction mechanism (i.e., the activation barrier and the
reaction enthalpy) for the formation of HF from the concerted addition of H2 to F2.
Describe the process by which you will achieve this. Include specific details on the number
and type of the calculations that will be carried out, as well as the computational method
that you would employ."

What does concerted addition of H2 to F2 mean? Is that simply just asking for H2 + F2 = 2HF? I understand that you need to first pick a computational method, such as DFT with the M06 functional, which should work well as the system is small and has relatively few atoms. And we can therefore use the 6-311++G(d,p) basis sets. At this point I'm lost... I have notes on optimization of minima and transition states but I don't understand where this ties in. I can think of how to hit 5 marks at most...

Concerted reactions happen in one step, as opposed to step-wise. The easiest way to think of it is all bonds are broken and formed at the same time.