1. Infrared Spectroscopy & MASS SPECTROMETRY
Unit 15

2. Introduction
Spectroscopy is a technique used to determine the structure of a compound.
Most techniques are nondestructive (it destroys little or no sample).
Absorption spectroscopy measures the amount of light absorbed by the sample as a function of wavelength.

3. Types of Spectroscopy
Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group.
Mass spectrometry (MS) fragments the molecule and measures their mass. MS can give the molecular weight of the compound and functional groups.
Nuclear magnetic resonance (NMR) spectroscopy analyzes the environment of the hydrogens in a compound. This gives useful clues as to the alkyl and other functional groups present.
Ultraviolet (UV) spectroscopy uses electronic transitions to determine bonding patterns.

4. Electromagnetic Radiation
Electromagnetic radiation: light and other forms of radiant energy
Wavelength (): the distance between consecutive peaks on a wave
Frequency (): the number of full cycles of a wave that pass a given point in a second
Hertz (Hz): the unit in which radiation frequency is reported; s-1 (read “per second”)

5. Electromagnetic Radiation
Common units used to express wavelength

6. Molecular Spectroscopy
Molecular spectroscopy: the study of which frequencies of electromagnetic radiation are absorbed or emitted by a particular substance and the correlation of these frequencies with details of molecular structure
we study three types of molecular spectroscopy

7. The IR Region
From right below the visible region to just above the highest microwave and radar frequencies .
More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters.
Wavenumbers are proportional to frequency and energy.

8. Infrared Spectroscopy
The vibrational IR extends from 2.5 x m (2.5 m) to 2.5 x m (25 m)
the frequency of IR radiation is commonly expressed in wavenumbers
wavenumber : the number of waves per centimeter, with units cm-1 (read reciprocal centimeters)
expressed in wavenumbers, the vibrational IR extends from 4000 cm-1 to 400 cm -1

9. Fingerprint Region of the Spectrum
No two molecules will give exactly the same IR spectrum (except enantiomers).
Fingerprint region is between 600–1400 cm-1, and has the most complex vibrations.
The region between 1600–3500 cm-1 has the most common vibrations and we can use it to get information about specific functional groups in the molecule.

10. The Infrared Spectrometer

11. IR Spectrum of Alkanes
An alkane will show stretching and bending frequencies for C—H and C—C only.
The C—H stretching is a broad band between 2800– cm-1, a band present in virtually all organic compounds.
In this example, the importance lies in what is not seen, i.e., the lack of bands indicates the presence of no other functional group.

12. IR Spectrum of Alkenes
The most important absorptions in the 1-hexene are the C═C stretch at 1642 cm-1, and the unsaturated stretch at 3080 cm-1.
Notice that the bands of the alkane are present in the alkene.

13. IR Spectrum of Alkynes

14. Infrared Spectroscopy
IR spectrum of 3-methyl-2-butanone

15. Molecular Vibrations
Atoms joined by covalent bonds undergo continual vibrations relative to each other
The energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed
The energies associated with transitions between vibrational energy levels correspond to frequencies in the infrared region, 4000 to 400 cm-1

16. Molecular Vibrations For a molecule to absorb IR radiation
the bond undergoing vibration must be polar and
its vibration must cause a periodic change in the bond dipole moment
Covalent bonds which do not meet these criteria are said to be IR inactive
the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, are IR inactive because they are not polar bonds

17. Molecular Vibrations
For a nonlinear molecule containing n atoms, there are 3n -6 allowed fundamental vibrations
For even a relatively small molecule, a large number of vibrational energy levels exist and patterns of IR absorption can be very complex
The simplest vibrational motions are bending and stretching

18. Molecular vibrations
Fundamental stretching and bending vibrations for a methylene group

19. Molecular Vibrations
Consider two covalently bonded atoms as two vibrating masses connected by a spring
the total energy is proportional to the frequency of vibration
the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring
K = a force constant, which is a measure of the bonds’ strength; force constants for single, double, and triple bonds are approximately 5, 10, and 15 x 105 dynes/cm
m = reduced mass of the two atoms, (m1m2)/(m1 + m2), where m is the mass of the atoms in grams

20. Molecular Vibrations
From this equation, we see that the position of a stretching vibration
is proportional to the strength of the vibrating bond
is inversely proportional to the masses of the atoms connected by the bond
The intensity of absorption depends primarily on the polarity of the vibrating bond

21. Correlation Tables
Characteristic IR absorptions for the types of bonds and functional groups we deal with most often

22. Hydrocarbons

23. Alkanes
IR spectrum of decane

24. Alkenes
IR spectrum of cyclohexene

25. Alkynes
IR spectrum of 1-octyne

26. Aromatics
IR spectrum of toluene

27. Alcohols
IR spectrum of 1-hexanol

28. Ethers
IR spectrum of dibutyl ether

29. Ethers
IR spectrum of anisole

30. Amines
IR spectrum of 1-butanamine

31. IR of Molecules with C=O Groups

32. IR of Molecules with C=O Groups

33. Aldehydes and Ketones
IR spectrum of menthone

34. Carbonyl groups
The position of C=O stretching vibration is sensitive to its molecular environment
as ring size decreases and angle strain increases, absorption shifts to a higher frequency
conjugation shifts the C=O absorption to lower frequency O O O O
1715 cm -1
1745 cm -1
1780 cm -1
1850 cm -1 O O O H
1717 cm -1
1690 cm -1
1700 cm -1

35. Carboxylic acids
IR spectrum of pentanoic acid

36. Esters
IR of ethyl butanoate

37. Summary of IR Absorptions

38. Strengths and Limitations
IR alone cannot determine a structure.
Some signals may be ambiguous.
The functional group is usually indicated.
The absence of a signal is definite proof that the functional group is absent.
Correspondence with a known sample’s IR spectrum confirms the identity of the compound.

39. Mass Spectrometry
Molecular weight can be obtained from a very small sample.
A beam of high-energy electrons breaks the molecule apart.
Destructive technique, the sample cannot be recovered.
The masses of the fragments and their relative abundance reveal information about the structure of the molecule.

40. Mass Spectrometer

41. Separation of Ions
A beam of electrons causes molecules to ionize and fragment.
The mixture of ions is accelerated and passes through a magnetic field, where the paths of lighter ions are bent more than those of heavier atoms.
By varying the magnetic field, the spectrometer plots the abundance of ions of each mass.
The exact radius of curvature of an ion's path depends on its mass-to-charge ratio, symbolized by m/z. In this expression, m is the mass of the ion (in amu) and z is its charge.
The vast majority of ions have a +1 charge, so we consider their path to be curved by an amount that depends only on their mass.

42. Fragmentation
The reactions that take place in a mass spectrometer are unimolecular, that is, they do not involve collisions between molecules or ions. This is true because the pressure is kept so low (10-6 torr) that reactions involving bimolecular collisions do not occur
We use single-barbed arrows to depict mechanisms involving single electron movements
The relative ion abundances, as indicated by peak intensities, are very important

43. The Mass Spectrum
In the spectrum, the tallest peak is called the base peak and it is assigned an abundance of 100%. The % abundance of all other peaks are given relative to the base peak.
The molecular ion (M+) corresponds to the mass of the original molecule.

44. Gas Chromatography–Mass Spectrometry (GC–MS)
The gas chromatograph column separates the mixture into its components.
The mass spectrometer scans mass spectra of the components as they leave the column.