IR

[Studyinghard]

sweet. crosses fingers for chemical shift definition XD

[Studyinghard]

Now that we're on the topic. how does the IR wavenumber relate to its actual process. the more bonds vibrating the higher the wavenumber?

[olly_s15]

Now that we're on the topic. how does the IR wavenumber relate to its actual process. the more bonds vibrating the higher the wavenumber?

the wavenumber is the reciprocal of the wavelength required for vibration of the bond

[Martoman]

higher wavenumber -> lower wavelength -> higher frequency -> more energy to vibrate bonds.


Bonds are vibrating all the time. Heavier ones will vibrate less (coz their big and fat) and thus have smaller wave numbers. The absorbance corresponds to EXACTLY the right frequency to which the vibrating, scissoring is taking place.

Also stronger bond ie double or triple MORE energy required so HIGHER wavelength.

[Studyinghard]

Oh so like for the highest wavenumber which is 3350–3500 the least energy needed to vibrate bonds because they are as you say not as big and fat?

[olly_s15]

Oh so like for the highest wavenumber which is 3350–3500 the least energy needed to vibrate bonds because they are as you say not as big and fat?

this is a simplification but yeah sorta

think of it like this - wavenumber is reciprocal of wavelength. so if the wavenumber is 33500-3500 (large) then it's reciprocal which is wavelength is small (hence small energy to vibrate bond)

[Martoman]

nonononononono

Big fat -> LESS energy.

Higer wavenumber -> MORE energy.

These guys are fit and limber. They have more energy than fat slobs.

[olly_s15]

nonononononono

Big fat -> LESS energy.

Higer wavenumber -> MORE energy.

These guys are fit and limber. They have more energy than fat slobs.

i think we're contradicting each other

[Toothpaste]

Yep here's a summary

• A stronger bond will vibrate at higher frequencies (wavenumber)
• Greater atom mass = lower frequency
• Also usually a "stretch" > "bend" and single bond > triple bond > double bond

You also need a dipole to see the absorption in the infared spectrum. I.e. can't have but you can have .

[Studyinghard]

nonononononono

Big fat -> LESS energy.

Higer wavenumber -> MORE energy.

These guys are fit and limber. They have more energy than fat slobs.

i think we're contradicting each other

exactly, you'z just contradicted each other now im all :S!.

[olly_s15]

nonononononono

Big fat -> LESS energy.

Higer wavenumber -> MORE energy.

These guys are fit and limber. They have more energy than fat slobs.

i think we're contradicting each other

exactly, you'z just contradicted each other now im all :S!.

i think the flaw in my explanation is that i said small wavelength = small energy but that isn't true isssss it

[Studyinghard]

Okay I think its like this.

The stronger the bond in the molecule ( so lets take covalent bonding ) the bigger the wavenumber needed to vibrate the bonds. yes?

[olly_s15]

Okay I think its like this.

The stronger the bond in the molecule ( so lets take covalent bonding ) the bigger the wavenumber needed to vibrate the bonds. yes?

yes.

strong bond = high energy = high frequency = low wavelength = high wavenumber

[Studyinghard]

BUT how come primary amines are highest wavenumber. they have hydrogen bonding :S

[olly_s15]

well then that is a strong bond hence high wavenumber as indicated by my previous post