Raman Spectroscopy

Light Scattering

1. Raleigh molecule

2. Tyndall particle with size 1 – 1,5 x

"for his work on the scattering of light and for the discovery of the effect named after him"

The Nobel Prize in Physics 1930

Sir Chandrasekhara Venkata Raman

Sir C.V. Raman during a lecture at the Raman Research Institute in Bangalore, circa 1959.

Scattering of Radiation:

C. V. Raman discovered that the wavelength of a small fraction of radiation scattered by certain molecules differs from that of the incident beam. The shifts in wavelength depend on the chemical structure of the molecules responsible for the scattering.

Raman Scattering

Raman Scattering:

Results from the same type of quantized vibrational changesthat are associated with IR absorption.

The difference in wavelength between the incident and scatteredradiation corresponds to the mid-IR region.

The Raman scattering spectrum and IR absorption spectrum of a compound often closely resemble each other. But there are enough differences in the kinds of groups that are IR and Raman active that the techniques are complementary.

Energy level diagram

Raman Spectrum

Raman Spectroscopy• Number of peaks related to degrees of freedom

DoF = 3N - 6 (bent) or 3N - 5 (linear) for N atoms

• Energy related to harmonic oscillator

• Selection rules related to symmetry

Rule of thumb: symmetric=Raman active, asymmetric=IR active

Raman: 1335 cm–1

IR: 2349 cm–1

IR: 667 cm–1

CO2

or c

2k(m1m2)m1m2

Raman + IR: 3657 cm–1

Raman + IR: 3756 cm–1

RamanRaman + IR: 1594 cm–1

H2O

CO2 Vibration

Raman active and inactive

Types of Scattered Radiation:

Rayleigh – wavelength is exactly same as excitation source and is significantly more intense than other two types.

Stokes – Raman scattering at longer wavelengths (lowerenergy) than excitation wavelength. (Same as in fluorescence.)

Anti-Stokes – Wavelengths shifted to higher energies (shorter wavelengths). Generally much less intense than Stokes lines, so Stokes part of the Raman spectrum is usually used.

The magnitude of Raman shifts are independent of thewavelength of excitation.

Example of Raman Spectrum of CCl4

Source: Skoog, Holler, and Nieman, Principles of Instrumental Analysis, 5th edition, Saunders College Publishing.

Raman scattering differs from Rayleigh by E, the energy of the firstvibrational level of the ground state.Thus the Raman frequency shift and the IR absorption peak frequency are identical.

Source: Skoog, Holler, and Nieman, Principles of Instrumental Analysis, 5th edition, Saunders College Publishing.

Raman Shift

= incoming - scattering (cm-1)

= 1/incoming (cm) – 1/scattering (cm)

Raman is caused by polarizability

Polarizability value decrease by

- increase in electron density

- increase in bond strength

- decrease in bond length

1

Instrumentation

Raman Instrumentation:

Source – general laser. Chose wavelength to avoid absorption (except Resonance Raman), fluorescence, or photodecomposition.

Sample-Illumination Systems – since visible frequencies, can use glass optical components.

Raman Scattering Condition

1. Raman Intensity usually 10-5 – 10-7 times lower than incoming light intensity, so that high intensity light source in needed

2. Raman lines should separate from fluorescence light, so light source in UV wavelength is needed

3. Raman lines close to excitation and also close together, therefore high resolution monochromator is needed

a suitable light source is laser

Outer cavity System

Inner cavity laser

Inner Cavity

Micro cell

Micro cell

Differences between Raman and IR Spectra

Both depend on same vibrational modes. IR requires change in dipole or charge distribution associated with it. Scattering is momentary distortion of electrons followed by reemission ofradiation. The effectiveness of a bond toward scattering depends on its polarizability (ease at which electrons can be distorted from normal position). Therefore Raman requires a change inpolarizability.

Advantage of Raman over IR:

Water does not cause interference in Raman, so Raman spectra can be obtained from aqueous solution.

Also, glass or quartz cells (rather than NaCl or KBr disks) can be used.

Raman versus IR spectra

Depolarization Ratiop 0.75 asymmetric vibration

5. The following Raman data were obtained for CHCl3 with the polarizer of the spectrophotometer set

(1) parallel to the plane of polarization of the laser and

(2) at 90 deg to the plane of the source.

Calculate the depolarization ratio and indicate which Raman peak asymmetrical stretching

No. , cm-1

Relative Intensities

(1) I// (2) I

(a)

(b)

( c )

(d)

760

660

357

258

0.60

8.4

7.9

4.2

0.46

0.1

0.6

3.2

Applications of Raman Spectroscopy:

Not as wide-spread as IR.

Inorganics and Biologicals – since can use aqueous solutions.

Organics – advantage for double-bond stretching vibrationsthat are weak IR absorption bands. Therefore more informationabout olefins than IR.

Application

1. Inorganic compound, better than IR since water is used as solvent and vibration of metal-ligand exist between 100 – 700 cm-1, the vibrations are commonly IR inactive

2. Organic compound, complement of IR

3. Biology, small sample, water does not affect

What are the Applications of Raman Spectroscopy?

1. Chemical Systems e.g., identification, analysis, and characterization of organics, inorganics, including solvents, petrochemicals, carbon materials and films.

3. Pharmaceuticals e.g., identification and analysis

of drug constituents, critical additives, excipients, drugs of abuse, quality control of the purity and quality of pharmaceuticals.

2. Chemical Processes e.g., following polymer formulations and polymerization processes, measuring/quantification of multiple components in a mixture (including solvent mixture and aqueous solutions) in real time, identification of organic contaminants, following immediate and final product of a hydrogenation reaction, predicting the morphological properties of polymers.

What are the Applications of Raman Spectroscopy?

4. Polymers and Plastics e.g., quality control of

incoming/outgoing products, identification of contaminants during manufacturing, real time monitoring of polymerization reactions, multivariate analysis/Chemometrics to predict physical properties such as birefringence, crystallinity, glass transition temperature, crystallization temperature etc.

5. Biological and Medical Systems e.g., measuring the

concentrations of total protein and biological analytes in blood and serum, determining metabolic concentrations, measuring blood and tissue oxygenation, molecular-level cancer (cervical, lung, etc.) and cardiovascular disease (such as atherosclerosis) diagnosis.

What are the Applications of Raman Spectroscopy?

6. Food Products

e.g.,measuring the fatty acid unsaturation in food oils, detecting bacteria/contaminants in food products, identification of additive drugs in nutraceuticals and fruit drinks.

7. Forensic applications e.g., identification and analysis of explosives, inflammables, toxic solvents, additive drugs, drugs of abuse, bio-warfare agents, and inks from documents.

8. Materials, Semiconductors, Gemological, Geological, Archaeological, and Environmental Sciences, etc.

Problems1. At what wavelengths in nm would the Stokes and anti-Stokes Raman lines

for carbon tetrachloride ( = 218, 314, 459, 762, and 790 cm-1) appear if the source was

(a) a helium/neon laser (632.8 nm)? (b) an argon ion laser (488.0 nm)?2, Compare the relative intensities of one of the CCl4 Raman lines when each

of the two excitation sources described in Problem 1 is used.3. Under what circumstances would a helium/neon laser be preferable to an

argon ion laser as a Raman source?4. For viibrational states, the Boltzmann equation can be written as

N1/N0 = e-E/kT

where No and N1 are the populations of the lower and higher energy states respectively, AE is"the energy difference between the states, k is Boltzmann's constant, and T is the temperature in °K.

For temperatures of 20 and 40°C, calculate the ratios of the inten sities of the anti-Stokes and Stokes lines for CCl4 at

(a) 218 cm-1. (b) 459 cm-1. (c) 790 cm-1.

Problem

1 Should the nitrogen molecule absorb in the IR? Should it be Raman-active? Why?

2 Which of the following vibrational modes in the molecules given will be IR-active, Raman-active, or both: (a) symmetric stretch of O2, (b) asymmetric stretch of CO2, (c) bending mode in H20, (d) twisting mode in C2H4?

3 The IR spectrum of carbon tetrachloride contains one fundamental absorption peak at 790 cm-1. The Raman spectrum of CCl4 shows fundamental vibration peaks at wave number shifts of 218, 314, 459, and 790 cm-1. Explain the absence of the first three peaks in the IR spectrum.

4 In the Raman spectrum of a compound obtained with He-Ne laser excitation lines are observed at 650.0 661.5, and 670.3 nm. Calculate the wavenumber shift () of each line.

5 Why is Raman spectroscopy considered to be superior to IR spectroscopy for the analysis of many organosulfur compounds such as sulfides, disulfides, and thiophenes?

Answer

1. No; yes. 2. (a) Raman-active, (b) IR-active, (c) both,

(d) neither. 5.Sulfur is a highly polarizable substituent

atom, so it produces a high intensity Raman signal. Sulfur-carbon bonds do not produce an especially strong dipole, however, so the corresponding IR bands are weak.