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Physical Biochemistry Raman Spectroscop y

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A form of vibrational spectroscopy

An electric field will distort the molecular orbital

This is a weak effect that grows with the square of intensity

A strong electric field can induce an electric dipole moment

A varying EM field will result in the induced dipole varying

EMR has an oscillating electric field which will create an oscillating electric dipole moment, which can scatter

EMR.

Raman Scattering:

Scattered photons can be emitted in any direction, and may have adifferent frequency to the excitation photon. If the scattered photon has

the same frequencyas the excitation photon, then Rayleigh scattering is

occurring. It is an elastic interaction; there is no non-kinetic transfer of

energy between molecule and photon. v sc § v ex

Raman Scattering is an inelastic interaction; energy is transferred between the molecule and the photon. v sc

v ex . Stokes lines (Stokes scattering) ± Energy of molecule increases: v sc < v ex Anti-Stokes lines ± Energy of photon increases: v sc > v ex

Energy Level Diagrams:

At room temperature, most molecules will be is the v = 0 or 1 vibrational energy states.

In scattering, electrons are not actually promoted to a higher energy state, but we can use virtual energy

states. They do not correspond to an actual absorption.

A pictorial representation ±

not the actual process.

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Physical Biochemistry Raman Spectroscop y

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Raman Spectra:

Transitions between vibrational / rotational levels will lead to spectral lines on either side of the excitation

line.

The spectra on the Stokes side will be more intense than that of the anti-Stokes side. Stokes scattering is

more likely as there will be more molecules in the v = 0 state than v = 1 at room temperature.

Vibrational spectra is centred at the wavelength of the source (Raleigh)

Raman Versus IR: Aqueous solutions can be used Any wavelength can be used Requires variation in polarisability(some normal modes that are not visible in IR might be visible in

Raman) e.g. O2 ± has variation in polarisation, so can be used in Ramen, but could not be used in IR.Homonuclear diatomic molecules are good examples of this.

Some normal modes are invisible to both Ramen and IR.

Both Ramen and IR data is presented as transmission (a negative curve). 100 - %Abs.

A comparison of IR and Ramen spectra is useful, as some bonds will only be visible in one or the other.

Raman is often more simple (fewer peaks) and therefore easier to interpret.

Raman Spectrometer:

High intensities are required, as this is a weak effect; provided by laser illumination that is

focused onto the specimen.

Light from the illuminated spot on the sample is

collected using a lens, then scattered using a

monochromator. FT techniques cannot be used.

Resonance Raman scattering can also be used.(Needed?)