Spectroscopy 2 Flashcards

1
Q

Raman Spectroscopy (1)

A

When radiation passes through a transparent medium, most of it is elastically scattered (Rayleigh scattering).

This is where light is scattered at the same frequency to the incident radiation.

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2
Q

Raman Spectroscopy (2)

A

Around one in a million photons are inelastically scattered (Raman scattering). This is where light is scattered at a different frequency to the incident radiation.

This often uses high-powered lasers, which ensure that more photons are scattered inelastically. Lasers are a monochromatic source so the wavelength can be tuned to avoid fluorescence.

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3
Q

Raman Spectroscopy (3)

A

A decrease in frequency is known as Stokes emission. An increase in frequency is known as anti-Stokes emission.

Typically we see Stokes emission, given that anti-Stokes emission requires the molecule to be in an excited state initially.

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4
Q

Raman Spectroscopy (4)

A

Bond vibration leads to a change in frequency.

The difference between the incident and scattered ration will fall in the IR region and so provides information on molecular structure.

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5
Q

RS Selection Rules

A

Raman Spectroscopy is a complementary technique, with different selection rules to IR, showing different vibrations.

Gross selection rule:
There must be a change in molecular polarizability on vibration.

Specific selection rule:
The change in vibrational energy level must be Δν = ±1.

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6
Q

Motion of Molecules on RS

A

There are different ways in which molecules may move:
Symmetric stretch
Asymmetric stretch
Wagging
Scissoring

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7
Q

Raman Spectroscopy - Benefits

A

Generally little sample prep is required.
Can handle any state of sample.
Preferable to FTIR for inorganic analysis.

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8
Q

Raman Spectroscopy - Limitations

A

Often more expensive than FTIR.
Less detailed information due to the low numbers of photons that scatter inelastically.

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9
Q

UV-Vis Spectroscopy (1)

A

This type of spectroscopy shows electronic excitation of a molecule.

It can be very useful for determining concentration of coloured solutions containing organic compounds with a high level of pi-conjugation.

Relies on the Beer-Lambert Law:
A = εcl

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10
Q

UV-Vis Spectroscopy (2)

A

Limited to detecting molecules with strong UV-Vis absorbance.

If a sample is too concentrated, there will be a flattening of the peak, and accurate calculations are impossible.

Matrix mathematics is required for mixtures, which is doable but complicated.

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11
Q

UV-Vis Spectroscopy (3)

A

Can be used as a presumptive test, and is useful to determine instrument settings for confirmatory testing.

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12
Q

Fluorescence Spectroscopy

A

This is complementary to UV-Vis spectroscopy.

Some molecules absorb in the UV region and then emit light at a longer wavelength, often in the visible region.

It is very useful for organic molecules.

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13
Q

Nuclear Magnetic Resonance Spectroscopy (NMRS) (1)

A

Nuclei have an intrinsic property called “spin” - when placed in a magnetic field, the nuclei either align with or against the field.

Applying a radio frequency to the sample will cause the spins to be excited to the higher spin state.

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14
Q

Nuclear Magnetic Resonance Spectroscopy (NMRS) (2)

A

Not all nuclei are NMR active, they must have an overall spin that is non-integer.

The most common forms of NMR use spin -1/2 nuclei.

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15
Q

Nuclear Magnetic Resonance Spectroscopy (NMRS) (3)

A

Depending on which atoms they are bonded to, nuclei respond differently and have a “chemical shift”, meaning we can identify functional groups much like in IR and Raman spectroscopy.

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16
Q

Nuclear Magnetic Resonance Spectroscopy (NMRS) (4)

A

Interestingly, if we apply a gradient to the magnetic field across our sample, we can image it very precisely.

This is how an MRI scanner works.