1. Vibrational Spectroscopy
Dr Jacqueline Stair
Lecture 3

2. types of vibrational spectroscoy
Mid-infrared
( cm-1)
(2.50 to 40.0 microns)
Infrared
NIR
Raman
NIR-IR
Near-infrared
( cm-1)
(0.78 to 2.50 microns)
Laser radiation

3. Fig. 2. Electromagnetic Radiation Spectrum

4. 1. Infrared Spectroscopy
In IR Spectroscopy there is interaction b/w molecules and radiation from IR region of the EMR spectrum( IR region cmˉ¹)
IR radiation causes the excitation of the vibration of the covalent bonds of that molecules. These vibration may appear as bending (change in bond angle) or stretching (change in bond length) modes.
Molecules that absorb IR radiation must have a change in dipole moment (unequal sharing of electrons).

5. IR spectrum can be used for characterization, and even identification of unknown samples when compared with reference spectra.
IR spectroscopy can be very sensitive to determination of functional groups.
Applied for both qualitative and quantitative analysis of complex mixtures of similar compounds of both solids and liquids.
Vibrations with no change in dipole moment will not absorb IR e.g. O2, N2, Br2.

6. h= Planks constant
v= frequency of photons

7. Change in Dipole Moment
Chemical dipole moment  :
 = d
is the charge
d is the distance between atoms.

8. for non-linear molecule
Vibrational modes
for non-linear molecule
3n-6
O=C=O
for a linear molecule
3n-5
Formadehyde vibrational modes

10. Samples NaCl or AgCl Salt Plates Vis-IR ATR Crystal KBr Salt Disc
UV-Vis-IR

11. Attenuated total-reflectance (ATR) IR spectroscopy
Samples are placed onto a crystal that is transparent in the IR.
Characterisation of coatings rather than bulk material.

12. Infrared Spectroscopy
FTIR: Fourier Transform Infrared Spectroscopy

13. Fourier Transform (FT)
The Fourier transform is a mathematical operation that decomposes a signal into its constituent frequencies. Thus the Fourier transform of a musical chord is a mathematical representation of the amplitudes of the individual notes that make it up. The original signal depends on time, and therefore is called the time domain representation of the signal, whereas the Fourier transform depends on frequency and is called the frequency domain representation of the signal. The term Fourier transform refers both to the frequency domain representation of the signal and the process that transforms the signal to its frequency domain representation.
FTIR and interferograms
Interference
pattern
Wavelength
components λ1 λ2
λ1+2

14. Vibrational Bands

15. IR correlation table Bond Type of compound Frequency range (cm_1)
Intensity
C - H
Alkanes
Strong
Alkenes, aromatic
Medium
O - H
Alcohols, phenols
Variable
N - H
Amines, amides
C=C
Alkenes
Aromatic ring
C - N
Nitriles
C - O
Alcohols, ethers, carboxylic acids
strong
C = O
Aldehydes, ketones, carboxylic acids

16. Background Spectrum O-H stretch H-O-H bending O=C=O Asymmetric stretch
The first thing you notice about the background spectrum is there are strong IR peaks near 3800, 2400,1600 cm-1. These peaks are due to the O-H stretch of H2O, the asymmetric stretch of CO2 and the H-O-H bending of H2O, respectively. The H2O and CO2 are present in the air, which fills the spectrometer. The H2O bands consist of many sharp peaks. These peaks are the vibrational transitions between different rotational states of H2O. With a higher resolution instrument, the CO2 band shows similar rotational splitting, but the lines are closer together. Rotational splitting is only observed in the gas phase. Passing a steady stream of nitrogen though the spectrometer can eliminate the H2O and CO2 bands. This is called purging the spectrometer.
O-H
stretch
H-O-H
bending
O=C=O
Asymmetric stretch

17. Spectrum of
The Final Spectra At this point your spectrum should look similar to the one shown below. The background has been removed, and the major peaks are labeled with their wavenumbers. As you can see on the example spectrum of ethyl acetate, the major peaks at cm-1 are C-H stretches, and the peaks at 1447 and 1374 cm-1 are C-H bends. The ester carbonyl (C=O) peak is clearly visible at 1735 cm-1, and the major peak at 1243 cm-1 is a (C=O)-O stretch. You are now ready to print your spectrum. Left-mouse click the "Print" button (3rd button from the left). Your spectrum should print now.

18. IR Spectra of polymorphs

19. Attenuated total reflectance (ATR)
Includes multiple reflection.
Involves light being reflected internally by transmittance medium.
Requires minimum amount of sample (few milligrams).
No sample preparation (can be measured as received).
Figure Handheld Thermo FTIR and laboratory based Thermo Nicolet 6700 FTIR both equipped with ATR

20. 2. Near-infrared (NIR) Spectroscopy
Spectra arises from overtones and combinations of vibrational modes
Vibration bands are weak and highly overlapping
Advantages: simple sample prep and no glass absorption
Method of choice for on-line monitoring during pharmaceutical processing

21. NIR
Near-infrared (NIR) region is the region situated between the visible and infrared region ( nm).
It is split to two ranges: short wavelength or Herschel range ( nm) and long wavelength ( nm).
The technique is non-destructive and requires no sample preparation (weak absorbance).
It obeys the anharmonic oscillator (Morse law) .
The higher anharmonicity constants are mostly encountered in hydrogen bonding such as: C-H, N-H, O-H and S-H.

22. NIR Counterfeit Detection
Genuine Viagra 100mg reference
Hindgra 100mg (clone)
Kamagra 100mg chewable (clone)
Viagra 50mg suspect
Viagra 25mg suspect

23. NIR second derivative spectra: for easier comparison
Viagra 100mg (red) vs
Authentic Viagra 100mg (blue)
Kamagra 100mg chewable (red) vs
Authentic Viagra 100mg (blue)

24. Authentication of Tanzanian Lamivir sample
Figure 14 PCA scores plot of the SNV-D2 spectra of UK ( ) and Tanzanian ( ) Combivir tablets and generic lamivudine/zidovudine Tanzanian tablets ( ).
Figure 13 SNV-D2 NIR spectra of Combivir Tablets 150/ 300 mg UK authentic (white tablet and blue spectrum) and a test product which proved to be authentic (red tablet and red spectrum) (r = 0.992).

25. Thank you

26. 3. RAMAN
It is based on the scattering of light which occurs as a result of collision between the photons and the sample when the sample is irradiated with a source (monochromatic light).
Unlike NIRS, it can measure the sample regardless of its state whether solid, liquid or films.
Advantage over NIRS is that it is not affected by humidity (water signal is weak) and the Raman signal is proportional to the concentration of the sample.
Disadvantage of Raman spectroscopy is that the Raman signal is often masked by the fluorescence of the sample.

27. Nobel Laureate in Physics
Indian Physicist and
Nobel Laureate in Physics
Chandrashekhara Venkata Raman

28. Raman Spectroscopy
The Raman effect is based on the inelastic scattering of laser light by molecules or crystals.
Used for structural analysis, identification (“fingerprint”), and quantitative measurements
Sensitive to symmetric vibrations such as O2 and N2
Spectra is complimentary to IR spectra

29. Raman Spectroscopy
The Raman effect is based on the inelastic scattering of laser light by molecules or crystals.

30. Raman spectra using green laser light (514.5 nm)
inelastic
higher E
elastic
same E
inelastic
lower E

31. Ideal for polymorph analysis
Minimal sample preparation
Relative intensities are not affected by sampling parameters
Absorption bands are well resolved and very reproducible (minimal interference of other compounds)
Paired with optical microscopy-use of samples sizes down to 1µm

32. Rayleigh and Raman scattering
RAYLEIGH SCATTERING
FLUORESCENCE
Figure 2 Baseline corrected Raman spectrum of Ciproxin tablet (green) measured by the Ahura Truscan (now Thermo) instrument.

33. Ranitidine-HCl
Intensity
Wavenumber (cm-1)