IR

[remo14]

does atomic mass of the atom determine the 1/cm value not electronegativity?

[Mao]

does atomic mass of the atom determine the 1/cm value not electronegativity?

They are related. The frequency (which is the same as the wavenumber) is calculated from the force constant and the masses. Electronegativity cannot be used directly to calculate the force constant, but I would imagine it may have some effect on IR bands.

[halfbloodprincess]

while we're on the topic of IR.... does ethane absorb IR radiation? or propane?

[Tonychet2]

yes C-H bond will be present and the fingerprint region

[charmanderp]

And C-C! (:

[Tonychet2]

ahhhhhhh yes u got me :p lol

[Mao]

yes C-H bond will be present and the fingerprint region

The C-H bond shows strong absorption at ~2950 cm^-1, this peak is present in all C-H containing compounds.

[halfbloodprincess]

yeah I understand that, but i'm just a bit confused because i thought only molecules with dipole moments will absorb IR? does ethane have a dipole moment? 

[Mao]

yeah I understand that, but i'm just a bit confused because i thought only molecules with dipole moments will absorb IR? does ethane have a dipole moment? 

That's talking about bond dipole moments, not overall (molecular) dipole moment. If you consider the C-H bond by itself, it has a dipole moment.

[sin0001]

Why don't N2 (nitrogen gas), and also molecules without dipole moments in general, absorb infared radiation?

[Mao]

Why don't N2 (nitrogen gas), and also molecules without dipole moments in general, absorb infared radiation?

There is no bond dipole moment.