[lespaul]

I know molecules that tend to be IR active (ie. show absobance) tend not to be Raman active - and vice versa.

But what about UV-Vis - are there any general correlations between say IR (ftir, mir, nir - or Raman) and its ability to absorb in the UV spectrum?

Would a molecule that absorbs in the IR spectrum (or Raman) also absorb in the UV?

I know it depends on the molecule, but just looking for generalities here.

Thanks

[Arkcon]

I don't expect you'll find a  very strong correlation between IR and UV/Vis for molecules.  IR absolution depends on atomic interactions with the IR wavelengths.  UV/Vis depends on (generally) conjugated double bonds. 

[Wastrel]

There is a correlation between IR and Raman because they are different ways of stimulating the same transition.  A transition, say a vibration between two atoms may be in Mid-IR, Mid-Raman, both or neither according to selection rules but however it's done it's the same vibration.

Regular spectroscopy with NIR/Vis/UV is dominated by direct higher energy events, for example electron excitation in the molecule.

[Arkcon]

There is a correlation between IR and Raman because they are different ways of stimulating the same transition.  A transition, say a vibration between two atoms may be in Mid-IR, Mid-Raman, both or neither according to selection rules but however it's done it's the same vibration.

Can you give an example of a UV-Vis response to a typical vibrational transition?

[Wastrel]

When light passes through a substance some light is scattered at the same frequency, Rayleigh scattering.  There is a smaller probability that a photon will couple in such a way as to change the vibrational state of a molecule - Raman scattering.  If the molecule excites then the photon will be lower in energy (Stokes emission) and if it deexcites the photon will be higher in energy (Anti-Stokes emission).  The probability depends on the population of excited states.  This works with NIR, Vis and UV with UV providing the strongest signal, but also the most easily interfered with.

If the light used has a narrow bandwidth then the spectra of the scattered light is the Raman spectrum of the compound.  To put a few bits of flesh on these bones, for green light the Stokes spectrum corresponding to the 400 to 4000 cm^-1 range is about 200nm and the Raman scattered light may be 4 orders of magnetude below the Rayleigh scattered light.