CH04 - IR, NIR and Raman Spectroscopy

Question 1

Range of wavelenths for source in Raman spectroscopy?

Answer 1

NIR (800-2500nm or 4000-10000cm-1) and Vis (400-800nm)

Question 2

Range of wavelengths in Raman spectrum?

Answer 2

Same as in Mid-IR (400-4000 cm^-1) since scattered radiation results from molecule relaxing to ground vibrational state.

Question 3

3 ways in which Raman spectroscopy complements mid-IR spectroscopy?

Answer 3

1. Weak absorption lines in mid-IR may appear as strong lines in Raman spectroscopy (e.g Cl-Cl are IR incative, but raman active).
2. Spectrae for a given molecule does not depend on the sample’s physical form (e.g powder or pill).

3.Raman can differentiate between substances with little chemical difference like isomers.

Question 4

How is polarizability related to the selection rule for raman activity?

Answer 4

To be raman active vibration must be accompanied by a change in polarizability

Question 5

Range of Wavenumbers for mid-IR?

Answer 5

400-4000cm^-1

Question 6

Range of Wavenumbers for near IR (NIR)?

Answer 6

4000-100000 cm^-1

Question 7

NIR Absorption lines result from ________ or ____ bands of fundamental vibrational transitions in mid-IR.

Answer 7

overtones, combination

Question 8

What is NIR spectroscopy?

Answer 8

A spectroscopic method which relies on absorption of radiation in the NIR range (4000-10000 cm^(-1))

Question 9

What is a fundamental vibrational transition?

Answer 9

The term that describes the Δn=±1 change between vibrational energy states E = 0 to E = 1.

Question 10

What are overtones?

Answer 10

Transitions that appear with decreasing intensity at fundamental vibrational frequencies Δn=±2,±3… etc

Question 11

Fundamental transition bands appear as ____ bands in IR spectrum.

Answer 11

Intense

Question 12

Overtone transition bands appear as ____ bands in IR spectrum.

Answer 12

Weak

Question 13

The fundamental absorption bands in mid IR are used for _______analysis, whereas absorption bands in NIR are used for _______ analysis.

Answer 13

qualitative, quantiative

Question 14

Condition for NIR activity?

Answer 14

(dμ_e)/dx≠0 (same as in mid IR).

Question 15

Disadvantages of NIR compared to mid IR?

Answer 15

1. Low signal intensity
2. Limited structural information (The bands that occur in this region are generally due to OH, NH, and CH bonds).

Question 16

Advantages of NIR compared to mid IR (list 6)?

Answer 16

1. Tungsten halogen light source provides intense and stable lighting over the entire NIR range.
2. Fused silica and quartz can be used for lenses and sample holders (those materials absorb mid IR so cannot be used there - mid IR uses water-soluble salts instead).
3. NIR spectroscopy uses PbS detectors, which do not require cooling by liquid nitrogen.
4. No need\few need for sample preparation (solvents or reagents not required). Enables faster analysis with less interference and nondestructive process monitoring.
5. High dynamic concentration range.
6. Shorter wavelength enables deep sample penetration.

Question 17

Possible effect of isotopic abundance on IR spectrum?

Answer 17

Elements with common isotopes can form doublet absorption bands with intensity ratio corresponding to relative abundance.

E.g. HCl vibration band may appear as a doublet with intensity ratio 1:3 (due to Cl)

Question 18

Primary alchohols create C-C→O bands in _______ cm^-1

Answer 18

1048 cm^-1

Question 19

S=C=S gives a strong absorption line in _____ cm^-1.

Answer 19

1500cm^-1

Question 20

Quantitative analysis in IR range is performed using _______ law.

Answer 20

Beer’s law (A=abc, where a and b are constants, c is determined).

Question 21

Sample holders and optics in mid IR analysis are made from____ or ____ which are _____ soluble.

Answer 21

KBr, NaCl (since quartz and fused silica absorb mid IR). These salts are water soluble.

Question 22

Even at electronic ground state, a molecule still has nonzero ______ energy.

Answer 22

Vibrational (because E_vib=h_bar*ν(n+1/2))

Question 23

In Raman and Rayleigh scattering, electrons are exited to ______ states.

Answer 23

Virtual (thus it is a “Scattering” and not an “Absorption”)

Question 24

“An elastic scattering process that occurs when photons of monochromatic radiation
with energy strikes a molecule, and the scattered radiation contains photons of the same energy as that of the incident radiation”

Answer 24

rayleigh scattering

Question 25

The process by which the molecule gains energy during the inelastic Raman scattering collisions at the expense of the incident photons, resulting in Raman scattered photons of lower energy than that of the incident photons

Answer 25

stokes raman scattering

Question 26

The process by which the photon gains energy during the inelastic Raman scattering collisions at the expense of the molecule, resulting in Raman scattered photons of higher energy than that of the incident photons,

Answer 26

anti-stokes raman scattering