1. Structural Information
Infrared
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Structural Information
The most useful part of the infrared spectrum for the detection and determination of organic species is from 2.5 to 15 m in wavelength which corresponds to a wavenumber range of 4000 to 667 cm-1. 3 1

2. Infrared Chemists tend to use wavenumbers not wavelengths.
Wavenumbers are directly proportional to energy.
A higher wavenumber corresponds to a higher energy. 3 2

3. Infrared Infrared Absorption
Molecules excited to a higher energy state
A quantised process
A molecule only absorbs selected frequencies
Corresponds to energy changes of the order 8-40 kJ/mole 3 4

4. Infrared Infrared Absorption
The absorbed energy corresponds to the VIBRATIONAL frequencies of the molecule.
Not all bonds in a molecule are capable of absorbing infrared energy.
Only those bonds which contain a DIPOLE MOMENT.
A bond must therefore present an electrical dipole which is changing as a function of time at the same frequency as the incoming radiation. 3 5

5. Infrared Vibrational Modes Simplest types Many vibrational modes
Stretching
Bending 3 8

6. Infrared 3 8

7. Infrared 3 8

8. Infrared Bond Properties
A diatomic molecule can be considered as two vibrating masses connected by a spring.
The bond distance continually changes but an equilibrium or average bond distance can be defined.
This behaviour is described as harmonic oscillation.
The natural frequency of vibration of a bond is derived from Hooke’s law for vibrating springs. 3 9

9. Infrared Bond Properties
The total amount of energy is proportional to the frequency of the vibration and for a harmonic oscillator is determined by the force constant (K) of the spring and the masses m1 and m2 of the two bonded atoms.
The reduced mass,  of the system is given by:
 = m1m2 / m1 + m2 3 10

10. Infrared
Stronger bonds have a larger force constant and vibrate at higher frequencies. 3 12

11. Infrared
Bonds between atoms of higher masses vibrate at lower frequencies. 3 12

12. Infrared Instrumentation Sample Preparation
Dispersive Spectrometers
Fourier Transform Spectrometers
Sample Preparation
Correlation Charts and Tables 3 13

13. Spectrum Analysis
The method of spectral analysis is dependent upon the information you have available.
General Rules
Identification of Functional Groups
Molecular Formulae and Hydrogen Deficiency
Full Spectral Interpretation 3 15

14. Spectrum Analysis 3 15

15. Spectrum Analysis
How to approach the analysis of an infrared spectrum.
Or - what you can tell at a glance!
Looking for functional groups. 3 15

16. DO NOT WORRY ABOUT SUBTLETIES.
Spectrum Analysis
Look for a few major functional groups - C==O, OH, NH, C==C and C==C - which are conspicuous.
Do not try to make a detailed analysis of the CH absorptions near 3000cm-1.
DO NOT WORRY ABOUT SUBTLETIES. 3 15

17. Spectrum Analysis 1. Is a carbonyl group present?
A strong absorption in the region cm-1
Often the strongest in the spectrum.
You can’t miss it!
2. If a carbonyl is present check the following types, if it is absent go to 3. 3 15

18. Spectrum Analysis ACIDS: is OH also present?
broad absorption near cm-1.
AMIDES: is NH also present?
medium absorption near 3400 cm-1.
ESTERS: is C--O also present?
strong intensity absorption near cm-1. 3 15

19. Spectrum Analysis ANHYDRIDES:two C==O near 1810 and 1760 cm-1.
ALDEHYDES: is aldehyde C--H present?
two weak absorptions near 2850 and cm-1.
KETONES: the proceeding five choices have been eliminated. 3 15

20. Spectrum Analysis 3. If carbonyl absent then check:
Alcohols and phenols
broad absorption near cm-1 and strong intensity absorption near cm-1.
Amines
medium absorption near 3400 cm-1.
Ethers
strong intensity absorption near cm-1. 3 15

21. Spectrum Analysis 4. Double Bonds A weak absorption near 1650 cm-1.
5. Triple Bonds
A weak, sharp absorption near 2150 cm-1.
6.Hydrocarbons
None of the preceeding found
Major absorptions in CH region near 3000 cm-1. 3 15

22. RESIST THE IDEA OF TRYING TO INTERPRET EVERY PEAK.
Spectrum Analysis
RESIST THE IDEA OF TRYING TO INTERPRET EVERY PEAK.
IT IS NOT
POSSIBLE! 3 15

23. Spectrum Analysis Molecular Formulae Ethane
Derived from Empirical Formulae
Ethane
Empirical formula CH3
Molecular mass 30
Molecular formula CH3CH3 3 15

24. Spectrum Analysis The Index of Hydrogen Deficiency
The number of  bonds or rings a molecule contains.
From a comparison of the molecular formula and that of a corresponding acyclic saturated compound.
The difference in numbers of hydrogens between these formulae divided by two gives the index of hydrogen deficiency. 3 15

25. Spectrum Analysis Hydrogen Deficiency Alkane: CnH2n+2
Alkene or cycloalkane: CnH2n
Alkyne: CnH2n-2
N, P, As, etc: +1
O, S, Se, Te: no change
Halides: -1 3 15

26. Spectrum Analysis The Index of Hydrogen Deficiency
One - a double bond or a ring but not both.
Two - a triple bond, 2 double bonds, two rings or a combination of both.
Four - a ring and three double bonds for example benzene. 3 15

27. Spectrum Analysis
Other examples
Chloral Hydrate
Nicotine 3 15

28. How to approach the analysis of an infrared spectrum.
Spectrum Analysis
How to approach the analysis of an infrared spectrum.
A more detailed look. 3 15