[Astrokel]

"Nitrogen atom in the ammonia molecule actually inverts and oscillates about the plane of hydrogen atoms. " Explain this phenomenon.

My teacher has given us this question.

The general idea is, the N atom behaves as a wave inside the potential well. Although the N atom does not have sufficient energy to cross the potential barrier, it is able to tunnel through the barrier and appear on the other side of the barrier.

However, i have few questions that i don't understand why,

1) Why do ammonia want to tunnel through the barrier? So that it is able to participate in reaction more readily?

2) Inversion and osscilates about the plane of hydrogen atoms  = tunnelled? How so?

3) Does all molecules actually invert and oscillates to tunnel?

Any hints/helps for me to think about it would be good enough, thanks! 

[Borek]

Why do ammonia want to tunnel through the barrier?

It is not that it wants. It can, so it will tunnel now and then.

Inversion and osscilates about the plane of hydrogen atoms  = tunnelled? How so?

Think in terms of activation energy.

Does all molecules actually invert and oscillates to tunnel?

If geometry permits, most likely yes, although probablity of tunneling drops fast with the barrier width.

[Astrokel]

Inversion and osscilates about the plane of hydrogen atoms  = tunnelled? How so?

Think in terms of activation energy.

By inverting and oscillating, ammonia can behave like a wave rather than a molecule, therefore, it is able to tunnel through the potential hill, hence it can achieve the activation energy?

Is potential reaction barrier = activation energy?

Thanks, 

[tamim83]

By inverting and oscillating, ammonia can behave like a wave rather than a molecule, therefore, it is able to tunnel through the potential hill, hence it can achieve the activation energy?

Is potential reaction barrier = activation energy?

I included a picture for visualization.  The only difference here is that the reactants and products should be the same energy. 

The potential energy barrier is equal to the activation energy, yes.  Normally the species has to go over the hill (think of an ball rolling over a potential hill)  With tunneling, the atom can go through the barrier instead, which is easier.  So it would appear to take less energy for the inversion to happen.  Nitrogen is a heavy atom so as tunneling decreases as particle size increases, the effect of tunneling is expected to be on the low side.  Nevertheless, I imagine it does happen. 

[Astrokel]

hey tamim83,!!

Does this mean that ammonia is actually a reactive species, since it is able to tunnel to achieve activation energy.

I came across this link, http://www.springerlink.com/content/50v5381t73474615/

it says reaction barrier is less than activation energy. if reaction barrier means potential energy barrier, isnt it equal to the activation energy?

thanks! 

[tamim83]

The "reaction" is
NH3 --> NH3(inverted)

It is sort of like a change in conformation reaction, only I believe that it is one of the vibrational modes of ammonia so it should happen pretty fast anyhow. 

I am not sure what the article you linked to is about, I would need to read it and I can't get the pdf.  It looks like a different situation.  But tunneling would make it appear that the reaction potential barrier is less than the activation energy that is quoted because the atom is tunneling through the barrier instead of going over it.  I see this in my research alot but in my case it is hydrogen atoms that are tunneling (they tunnel easily since they are small). 

[Borek]

I suppose it all depend on the temperature. In the low temp activation energy can be too high, yet tunneling may allow switch.

[Astrokel]

The "reaction" is
NH3 --> NH3(inverted)

It is sort of like a change in conformation reaction, only I believe that it is one of the vibrational modes of ammonia so it should happen pretty fast anyhow. 

I am not sure what the article you linked to is about, I would need to read it and I can't get the pdf.  It looks like a different situation.  But tunneling would make it appear that the reaction potential barrier is less than the activation energy that is quoted because the atom is tunneling through the barrier instead of going over it.  I see this in my research alot but in my case it is hydrogen atoms that are tunneling (they tunnel easily since they are small). 

"Muonium spin rotation experiments have been performed in a system of benzene in liquid isopentane solutions in the temperature range between 136 K and 295 K. The observed chemical reaction rate constants show a concave curve in an Arrhenius plot. These results are interpreted in terms of to a significant quantum tunnelling effect for muonium reaction in solutions. The oscillating frequency and the width of barrier for such reactions were found to be 400 cm−1 and 2.1 Å respectively by curve-shape analysis. The reaction barrier (6.0 kJ/mol) is found to be less than the activation energy due to viscous flow of the solvent. "


Thank you, the link is about muonium spin experiment, and i have goggled abit about it, it involves muon and electron, which is small in mass, so i guess it can tunnel quite well,

[Rabn]

You need to keep in mind that every wave is a particle and every particle is a wave.  They are one and the same.  Every part of an atom is essentially delocalized.   nitrogen nuclei, being higher in electronegativity than hydrogen, will occupy an electron more than the hydrogen, the resultant difference in charge between the nuclei will cause them to drift around.  You have to visualize the system in as dynamic a fashion as possible in order to really understand what's going on. To understand why it inverts...since the electrons are being shared, and the electronegativity difference is large, there will be a point in time when the region opposite the lone pair of electrons is sufficiently devoid of electrons, because the 3 hydrogen atoms happen to be occupying the electrons at that time, that the nitrogen nuclei becomes polarized and the lone pair will be sufficiently attracted to the electron deficient region which causes the molecule flip its orientation.  It is likely that any atom with a lone pair could become polarized enough to cause it to flip but if the electronegativity difference between the atom with the lone pair and its ligands is not sufficiently large, the flip will not occur.

[Astrokel]

\when the region opposite the lone pair of electrons is sufficiently devoid of electrons, because the 3 hydrogen atoms happen to be occupying the electrons at that time, that the nitrogen nuclei becomes polarized 

hey rabn,

i don't get this part and also i find it hard to imagine it. Would you mind explaining it again?

thanks,

[Rabn]

Due to the difference in electronegativity, the nitrogen atom will occupy the bonding electrons moreso than the hydrogen atoms.  Since the bonding electrons are always moving we can assume that there will be a time when 4 of the bonding electrons are occupied by the nitrogen; this would cause two of the hydrogens to be positively charged and the lone pair, being very mobile, would be attracted to those hydrogen atoms and move toward them.  However, the bonding electrons would also be moving toward those same nuclei.  The bonding electrons would get to the hydrogen atoms first, and since the lone pair was already accelerated toward the hydrogen atoms they would move into the region between the 3 hydrogen "legs".  It then becomes an energy struggle and it is no doubt easier for all 3 hydrogen atoms to "flip" to the other side, leaving the lone pair electrons to occupy the region where the hydrogen atoms were, than to re-accelerate the lone pair electrons in such a way that they return to their original position.

[Astrokel]

perfect! thank you so much rabn for the molecular explanation(cookie for you), i understand it now, so thats what is called quantum tunnelling.   

[tamim83]

Due to the difference in electronegativity, the nitrogen atom will occupy the bonding electrons moreso than the hydrogen atoms.  Since the bonding electrons are always moving we can assume that there will be a time when 4 of the bonding electrons are occupied by the nitrogen; this would cause two of the hydrogens to be positively charged and the lone pair, being very mobile, would be attracted to those hydrogen atoms and move toward them.  However, the bonding electrons would also be moving toward those same nuclei.  The bonding electrons would get to the hydrogen atoms first, and since the lone pair was already accelerated toward the hydrogen atoms they would move into the region between the 3 hydrogen "legs".  It then becomes an energy struggle and it is no doubt easier for all 3 hydrogen atoms to "flip" to the other side, leaving the lone pair electrons to occupy the region where the hydrogen atoms were, than to re-accelerate the lone pair electrons in such a way that they return to their original position.

I have huge reservations with this explanation.  Mostly because you are treating the electrons like particles scurrying around the molecule.  The electrons are standing waves on the nuclei of the molecule.  The umbrella vibration of ammonia goes through a planar transition state.  The reason why there is a barrier at all is because planar ammonia has a lot less overlap than pyramidal overlap.  It is the hydrogen 1s orbitals that forms a bonding orbital with the 2p orbital of nitrogen.  Overlap is increased if the molecule is pyramidal.   If there is enough energy to get over the barrier, it will invert.  Also, it is more likely that the hydrogens are moving, not the nitrogen.  Because nitrogen is much heavier.  Hydrogen atoms tunnel easily and are the ones that will go through the barrier rather than over it.  Replace the hydrogens with deuteriums and the tunneling will decrease becasue D is heavier than H. 

An alternate explaination can be made with hybrid orbitals.  The nitrogen is changing from sp3 to sp2 (TS) back to sp3 during the inversion, which still means the ammonia molecule is going through a planar transition state. 

The point I am trying to make is that it is the overlap of orbitals that is changing and the electrons are not really moving much.  Electrons drive the inversion but not by scurrying.  The change in overlap is what causes the atoms to move.  Also, if the energy is there, it happens.