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Bchem 471 > Raman Spec > Flashcards

Raman Spec Flashcards

1
Q

What is scattering dependent on

A

Can occur at any wavelength

Depends on polarizability (aplha) of the scattering molecule

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2
Q

What is polarizability

A

Describes how easy the electron cloud of a molecule can be distorted from its original shape by an external field, ion, or dipole

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3
Q

Slide 2 equation of dipole

A

In notes

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4
Q

What are the two types of scattering and what do they mean

A

Elastic (Rayleigh):

wavelength doesn’t change

Amount of scattering depends on the wavelength

  1. Inelastic scattering:

Molecule changes vibrational or rotational levels

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5
Q

Slide 3 equation of wavelength inverse and energy of vibrational transition

Amount of scattering is inversely proportional to wavelength to the power of 4

A

In notes

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6
Q

What is Raman spec

A

A small fraction of the scattered light has a diff wavelength than the incident light

The 1 on 10mill photons will show a shift bc of vibrations and rotations during the interaction with light

The spectrum of this shifted light is Raman spectrum

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7
Q

What is Raman scattering

A

Scattering of light with the change in frequency/ wavelength

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8
Q

During scattering What do the shifts in wavenumber depend on

A

The chemical structure of the molecules responsible for the scattering

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9
Q

What components are included when light is scattered from a molecule

A

The Rayleigh scattering (frequency not changing)

The Raman stokes (red) shift , longer wavelength

The Raman antistokes (blue shift), shorter wavelength

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10
Q

On a excitation plot, the Ir absorption levels are from

A

Ground to first level

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11
Q

Describe Rayleigh scattering energy level change

A

Goes up from ground to first virtual energy state then back down to ground

Comes down at same energy diff (not change in frequency or wavelength)

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12
Q

Stokes Raman scattering energy level change

A

Starts at ground goes up to first virtual energy level

Then down to first energy level (excited energy level) not ground

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13
Q

Anti Stokes Raman scattering energy level change

A

Starts at first energy level (excited) goes up to first virtual energy level

Then down to ground

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14
Q

What is special about the energy of Raman shifted photons

A

They can be either higher or lower in energy depending on the vibrational state of the molecule

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15
Q

In what wavelength region does Raman scattering occur

A

Since it occurs with vibrational and rotational levels

The Raman scatted radiation occurs in the mid infrared region (so in ir spec)

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16
Q

Comparing IR and Raman spec

A

Resemble each other

But since some groups are more IR active and other groups are more Raman active

The spectra compliment each other (meaning they each give diff useful info of the same molecule, you can better detect something with one method rather than the other)

17
Q

Which type of scattering causes the most scattering

Least

A

Rayleigh

Antistokes

18
Q

Describe what the stocks and antistokes bands look like compared to Rayleigh

A

Since Rayleigh doesn’t change it’s in the middle

The anti stokes is at a lower wavelength (so higher wave number) than Rayleigh

stokes is higher wavelength (lower wave number)

The distance between the stokes and anti stokes is symmetric (same value but negative or positive compared to reighleigh

19
Q

In comparison to Rayleigh stokes would have what sign wave number

Antistokes

A

Stokes is increase wavelength so lower wave number

Negative

Antistokes is lower wave length so increased wave number

Positive

20
Q

What type of scattering is usually measured and collected

A

Stokes raman scattering

21
Q

What is the light source of a Raman spec

A

Laser

Which is monochromatic

Intense

narrow

22
Q

Explain how a Raman spectrophotometer works

A

Laser shines light

A dichromic beam splitter shines light on the sample

Than light scatters off the sample and onto the detector that is above the sample (this is the Raman light)

23
Q

How can you change the Raman spectrophotometer analysis to get scattering at different parts of the sample

A

Can move the sample and the lens to get scattering at diff parts of the sample

24
Q

What is different in the peaks of Raman spec vs or spec

A

Raman has positive (facing up) peaks

Ir is plotted as transsmittance so the peaks are negative

25
Q

What is the fingerprint region of a sample

A

A pattern of bands to identify the molecules in the sample

26
Q

What is an advantage to Raman spec

A

Water is not an interferent in Raman spec

It is in IR

27
Q

Disadvantage of Raman spec

How can they be overcome

A

Weak signals (can be overcome by strong lasers)

Interference by fluorescence of the sample (or the impurites in the sample)

28
Q

In a beta solenoid amyloid, what is its amide 11 intensity in comparison to the parallel in register and the alpha syn

A

The amide II scattering intensity is higher in beta solenoid than the other two

29
Q

What is the technique called to enhance the signal in Raman spec

How does it work

What is the limitations

A

Resonance Raman spectroscopy

When the wavelength of the light is close to an absorption peak, the Raman signal gets more intense

This only works for Raman bands that are coming from a chromophore (ex. so they can’t happen with iron atoms in hemoglobin)

30
Q

What is the technique called to enhance the signal in Raman spec

How does it work

What is the limitations

A

Resonance Raman spectroscopy

When the wavelength of the light is close to an absorption peak, the Raman signal gets more intense

This only works for Raman bands that are coming from a chromophore (ex. so they can’t happen with iron atoms in hemoglobin)

31
Q

In resonance Raman spec, how can you change the concentrations of the sample

How does this help

A

You can use lower concentrations of the sample

This means that all of the peaks that aren’t enchanted by the resonance Raman spec would be weak and won’t interfere with the analysis

32
Q

What is an example of applying resonance Raman spectroscopy

A

Checking the different absorbances of bacteriorhodopsin when it has different forms

Every time it goes to a different compound, it has a different maximum absorption

If they wanted to just study one compound they would know to keep it at a specific temperature (less than 120 degrees) and measure it at that point

33
Q

What is the max absorbance of bacteriophodopsin

A

570nm

34
Q

What is the experimental setup of the analysis of bacteriorhodopsin

A

The sample goes onto a copper tip , and it is cooled to 77K through liquid nitrogen

The copper tip keeps the sample cold

The sample spins and each time different part of it is exposed so that it can then be recooled

The sample gets cold in the dark to not trap intermediates of the sample (and we’re starting with the original bacteriorhodopsin)

35
Q

What can you explain about the bacteriophodopsin spectra

A

When just the probe is on, the only get the spectra of bacteriorhodopsin

When both pump and probe, they get both bacteriorhodopsin and it’s intermediate K since it’s a steady state mixture

When they do a difference spectrum of bacteriorhodopsin and it’s intermediate K, the only get the spectra of K

36
Q

Why is the fluoresce in the bacteriorhodopsin pump and probe less than in just the probe

A

Because in just the probe BR is fluorescent

In the pump and prove you have BR and K but K isn’t fluorescent so the overall fluorescence goes down

37
Q

What did they learn about the bacteriophodopsin experiment

A

The found the the BR was all trans

Then when absorbing light, one of the bonds in the retinal in bacteriorhodopsin shifted to cis

This forms K which is cis

38
Q

What is surface enhanced Raman spec

A

The sample is adsorbed on a rough metal surface

Can get 6-12 order of magnitude of intensity in the spec (10^6-10^12)

Makes use of electromagnetic fields

Bchem 471 (13 decks)

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