brightsky's Chem Thread

[Patches]

Could someone explain the difference between the vibration of bonds in IR and in UV-Vis? Or have I completely misunderstood it? Cheers

[Mao]

Could someone explain the difference between the vibration of bonds in IR and in UV-Vis? Or have I completely misunderstood it? Cheers

IR absorption causes bonds to vibrate. UV-Vis absorption causes electrons to jump up and down (think: atomic shells, but applied to molecules)

[lzxnl]

Well if you want...think atomic orbitals that have overlapped as well

[brightsky]

radiation from the visible/ultraviolet region of the electromagnetic spectrum has enough energy to promote the electrons in atoms to a higher electronic energy level. so in uv-vis spec, you shoot uv-vis radiation at the sample; if the energy of a particular photon corresponds exactly with the energy difference between two possible allowed energy states, then the sample will absorb that photon and then re-emit it, leading to a reduction in the intensity of the incident beam, which is then measured by an electronic detector.

on the other hand, radiation from the ir region of the ems does not have sufficient energy to promote electrons to higher energy levels. rather ir radiation brings about changes in the bonds of a molecule. you will recall that covalent bonds are not rigid, but areconstantly bending and stretching in various ways. stretching and bending results in vibrations. if you shoot ir radiation at a molecule, provided that the molecule is ir-active (e.g. not a homonuclear diatomic molecule like H2 or O2) and there is at least one photon whose energy corresponds to the energy difference betwen two allowed vibrational energy states, then the molecule will absorb. crucially, in ir spec, the molecule does not re-emit, but rather 'uses up' the energy. this too leads to a reduction in the intensity of the incident beam, which again can be measured.

in short, uv-vis spec has nothing to do with vibrational modes. that is exclusive to ir-spec. uv-vis spec is about electrons.

[lzxnl]

His "infra red active" means a vibration which changes the overall dipole moment of the molecule. If you had diatomic oxygen, for instance, bending or twisting the bond wouldn't do anything as the molecule would still be neutral. That's why molecules like fluorine, nitrogen, oxygen and hydrogen aren't infra red active.
Similarly, with CO2, the absorption of energy that corresponds to symmetrically stretching the C=O bonds is IR inactive because the dipole moment is still zero.

[Mao]

then the sample will absorb that photon and then re-emit it

...

in short, uv-vis spec has nothing to do with vibrational modes. that is exclusive to ir-spec. uv-vis spec is about electrons.

1. not always true
2. not always true

You will learn more about this in 2nd and 3rd year chemistry.

[Mao]

His "infra red active" means a vibration which changes the overall dipole moment of the molecule. If you had diatomic oxygen, for instance, bending or twisting the bond wouldn't do anything as the molecule would still be neutral. That's why molecules like fluorine, nitrogen, oxygen and hydrogen aren't infra red active.
Similarly, with CO2, the absorption of energy that corresponds to symmetrically stretching the C=O bonds is IR inactive because the dipole moment is still zero.

Yes, that's right, but that's not helpful for VCE students. Considering most have not learnt how light and electric fields interact, the invocation of dipole moment doesn't really shed any insight, yet.

[brightsky]

Yes, that's right, but that's not helpful for VCE students. Considering most have not learnt how light and electric fields interact, the invocation of dipole moment doesn't really shed any insight, yet.

nah if i recall aright, the heinemann textbook talks about the concept of dipole moment.

1. not always true
2. not always true

You will learn more about this in 2nd and 3rd year chemistry.

would you be able to elaborate a little further? the post is tantalizing.

[Mao]

nah if i recall aright, the heinemann textbook talks about the concept of dipole moment.

That's horrendous. Introducing a concept as an explanation then not explaining it at a fundamental level, and expecting people to accept it simply because it's a "one level deeper", is one of the worst things to do in science education. This is why I refrain from using higher level concepts as explanations here.

would you be able to elaborate a little further? the post is tantalizing.

I don't really have time at the moment, but I can give you a few key-words if you want to endeavour in some googling:

1:
fluorescence
phosphorescence
non-radiative relaxation

2:
vibronic coupling
Born-Oppenheimer approximation
Frank-Condon principle

[Patches]

Thanks all - some notes someone gave me said 'UV or Visible light excites electrons in the bonds of appropriate samples', not sure what they mean by that I guess.

[brightsky]

Thanks Mao.

Also, why is it that if a pure solid is contaminated, its melting point goes down? Similarly, why is it that if a pure liquid is contamined, its boiling point goes up? I read somewhere that entropy/disorder gives rise to the colligative properties of a solution...but I'm as yet unaware of the full particulars...

Would appreciate some enlightenment.

[brightsky]

Also, what are the state symbols for the molecules involves in the formation of ethanoic anhydride:

2 CH3COOH --> (CH3C=O)2O + H2O

i'm guessing they're all liquids?

[Mao]

Also, why is it that if a pure solid is contaminated, its melting point goes down? Similarly, why is it that if a pure liquid is contamined, its boiling point goes up? I read somewhere that entropy/disorder gives rise to the colligative properties of a solution...but I'm as yet unaware of the full particulars...

Impurities in a solid causes defects in the crystal, so it's easier to break up the crystal (i.e. lower melting point)

I don't think that claim re:liquids is necessarily true. There isn't a theory that fully explains the behaviour of molecular liquids. You appear to be seeking explanations that don't yet exist.

[brightsky]

How are amino acid zwitterions actually formed? Which mechanism is more accurate?

a. The carboxyl group is deprotonated and the amino group protonated by some foreign species.
b. The original amino acid molecule undergoes some kind of intramolecular acid-base reaction, wherein the amino group deprotonates the carboxyl group, resulting in an 'internal transfer' of a proton.

Thanks!

[Alwin]

How are amino acid zwitterions actually formed? Which mechanism is more accurate?

a. The carboxyl group is deprotonated and the amino group protonated by some foreign species.
b. The original amino acid molecule undergoes some kind of intramolecular acid-base reaction, wherein the amino group deprotonates the carboxyl group, resulting in an 'internal transfer' of a proton.

Thanks!

(b) Internal transfer.

Can't find a simple explanation, but I'm sure someone on AN will (I'd put my money on Mao )
Here is a quick overview tho: http://www.chemguide.co.uk/organicprops/aminoacids/acidbase.html