1. Introduction to spectroscopy 1-infrared spectroscopy

2. Spectroscopy
At this point in the course, we have learned many organic reactions that can be used to synthesize organic molecules.
How do you determine whether the product that you have obtained in an organic reaction is the product that you had expected? Also, how would you determine the structure of unknown organic compounds that are isolated from natural sources?

3. Definition Spectroscopy:
It is an analytical technique concerned with studying of the interaction of electromagnetic radiation and matter in order to determine the structure.

4. Types of Spectroscopy Infrared spectroscopy (IR)
measures the bond vibration frequencies in a molecule and is used to determine the functional group
Mass spectrometry (MS)
fragments the molecule and measures the masses
Nuclear magnetic resonance spectroscopy (NMR)
detects signals from hydrogen atoms and can be used to distinguish isomers

5. electromagnetic relationships:λ c
Wavelength (λ): Crest-to-crest distance between waves
Frequency (v): Number of cycles per second
number of cycles/sec or s-1
or Hertz (Hz)

6. electromagnetic relationships:
Electromagnetic radiation displays the properties of both particles and waves
The particle component is called a photon
The energy (E) component of a photon is proportional to the frequency. Where h is Planck’s constant and n, is the frequency in Hertz (cycles per second)
E = hn
The speed of light, c, is constant, the frequency, n, is
inversely proportional to how long the oscillation is, or wavelength: c
___
n =
c = 3 x 1010 cm/s l hc
___
 E = hn = l

7. Electromagnetic (EM) Spectrum
Frequency, n in Hz
~1019
~1017
~1015
~1013
~1010
~105
Wavelength, l
~.0001 nm
~0.01 nm
10 nm
1000 nm
0.01 cm
100 m
Energy (kcal/mol)
> 300
300-30
300-30
~10-4
~10-6
g-rays
X-rays UV IR
Microwave
Radio
nuclear excitation (PET)
core electron excitation (X-ray cryst.)
electronic excitation (p to p*)
molecular vibration
molecular rotation
Nuclear Magnetic Resonance NMR (MRI)
Visible

8. High frequency (u) Short wavelength (l)
= High energy
Low frequency (u)
Long wavelength (l)
= Low energy

9. The Spectrum and Molecular Effects

10. Absorption Spectra
Organic compound exposed to electromagnetic radiation, can absorb energy of certain wavelengths.
Changing wavelengths to determine which are absorbed and which are transmitted produces an absorption spectrum
Energy absorbed is shown as dips in spectrum
Infrared Absorption of Ethyl Alcohol CH3CH2OH

11. Infrared Spectroscopy

12. Infrared radiation 1. what type of bonds are present
IR is used to tell:
1. what type of bonds are present
2. some structural information

13. Infrared Spectroscopy
Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared is energetic enough to excite molecular vibrations to higher energy levels.
The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of organic and organometallic molecules. The high selectivity of the method makes the estimation of an analyte in a complex matrix possible.

14. The IR Spectroscopic Process
As a covalent bond oscillates – due to the oscillation of the dipole of the molecule – a varying electromagnetic field is produced
The greater the dipole moment change through the vibration, the more intense the EM field that is generated

15. The IR Spectroscopic Process
When a wave of infrared light encounters this oscillating EM field generated by the oscillating dipole of the same frequency, the two waves couple, and IR light is absorbed
The coupled wave now vibrates with twice the amplitude
IR beam from spectrometer
“coupled” wave
EM oscillating wave
from bond vibration

16. Dipole Changes During Vibrations Absorption of infrared radiation is thus confined largely to molecular species that have small energy differences between various vibrational and rotational states. In order to absorb infrared radiation, a molecule must undergo a net change in dipole moment as a consequence of its vibrational or rotational motion.
The dipole moments is determined by the magnitude of the charge difference and the distance between the two occurs during the vibration or rotation of homonuclear species such as O2, N2, or Cl2; consequently centers of

17. Types of Molecular Vibrations:
charge. No net change in dipole moment, such compounds cannot absorb in the infrared
A polar bond is usually IR-active
A nonpolar bond in a symmetrical molecule will absorb weakly or not at all
Types of Molecular Vibrations:
Vibrations fall into the basic categories of stretching and bending. A stretching vibration involves a continuous change in the interatomic distance along the axis of the bond between two atoms. Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting.

18. Different Types of Vibration
Symmetric Stretch                Asymmetric Stretch
Scissoring                             Rocking
Wagging Twisting

19. Stretch – Vibration or oscillation along the line of the bond
Bend – Vibration or oscillation not along the line of the bond H C H C
symmetric
asymmetric H C H C H C H C
twist
wag
rock
scissor
out of plane
in plane

20. What is the Infrared? Just below red in the visible region
Wavelengths usually  m
More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters ( cm-1)
Wavenumbers are proportional to frequency and energy
Energy required to distort covalent bonds (stretching, compression, wagging, etc.)
Light that is absorbed shows up as a peak in absorbance or a dip in transmittance.

21. Absorption of Infrared light in the wavelength range of 2
Absorption of Infrared light in the wavelength range of 2.5x10-6 m to 25x 10-6 m (2.5x10-4 cm to 25x 10-4 cm)=(2.5 m to 25 m) causes bond vibrations
Specific wavelengths (or frequencies) can be correlated with specific functional groups
Wave Number => (cm-1) => proportional to energy n
Remember
Energy is inversely proportional to wavelength but proportional to wavenumber n

22. Number of possible modes (1) If Linear molecule: 3N – 5
Vibrational Modes
Number of possible modes
(1) If Linear molecule: 3N – 5
Example: Carbon dioxide is a linear molecule and thus has 3 x 3 – 5 = 4 modes
(2) If Nonlinear molecule: 3N – 6
Example : Triatomic molecule such as water, sulfur dioxide, or nitrogen dioxide have 3 x 3 – 6 = 3 vibrational modes

23. Infrared Instruments An infrared spectrophotometer is;
an instrument that passes infrared light through an organic molecule and produces a spectrum that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis.
* In infrared spectra the absorption peaks point downward because the vertical axis is the percentage transmittance of the radiation through the sample.
Absorption of radiation lowers the percentage transmittance value. Since all bonds in an organic molecule interact with infrared radiation, IR spectra provide a considerable amount of structural data.

24. How does it work? A molecule is exposed to IR.
It starts to undergo distortions based on the types of bonds present.
The spectrum represents the presence of chemical groups (Functional groups).

25. The IR Spectrum
Each stretching and bending vibration occurs with a characteristic frequency as the atoms and charges involved are different for different bonds
The y-axis on an IR spectrum is in units of % transmittance
In regions where the EM field of an osc. bond interacts with IR light of the same n – transmittance is low (light is absorbed)
In regions where no osc. bond is interacting with IR light, transmittance nears 100%
The x-axis of the IR spectrum is in units of wavenumbers, n, which is the number of waves per centimeter in units of cm-1 (Remember E = hn or E = hc/l)

26. How are these interpreted?
The spectrum is divided into two distinct sections.
4000cm-1 to 1300cm-1 is the functional group region.
1300cm-1 – 400cm-1 is the fingerprint region.
Bands in these areas can be used as evidence of certain functional groups.
Fingerprint region tends to have small bands, that are difficult to individually resolve.

27. Summary of IR Absorptions
Bonds to H
Triple bonds
Double bonds
Single Bonds

28. CC 2200 cm-1 (weak or absent if internal)
The IR Spectrum – The detection of different bonds
1-Lighter atoms will allow the oscillation to be faster – higher energy. This is especially true of bonds to hydrogen – C-H, N-H and O-H
2-Stronger bonds will have higher energy oscillations
Triple bonds > double bonds > single bonds in energy
C-C cm-1
C=C cm-1
CC cm-1 (weak or absent if internal)

29. 3-As opposed to chromatography or other spectroscopic methods, the area of a IR band (or peak) is not directly proportional to concentration of the functional group producing the peak
4-The intensity of an IR band is affected by two primary factors:
a.Whether the vibration is one of stretching or bending
b. Electronegativity difference of the atoms involved in the bond

30. c. For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak.
d. The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment, Hence polar bonds such as O-H and C=O give more intense absorptions than non-polar C-H, C-C and C=C bonds.
e. Typically, stretching will change dipole moment more than bending

31. Infrared Group Analysis
Since most “types” of bonds in covalent molecules have roughly the same energy, i.e., C=C and C=O bonds, C-H and N-H bonds they show up in similar regions of the IR spectrum
Remember all organic functional groups are made of multiple bonds and therefore show up as multiple IR bands (peaks)
Infrared Group Analysis

32. How to Analyze an IR Spectrum; Look for what’s there and what’s not there
C-H absorption
The wavenumber will tell you sp3(C-C), sp2(C=C), sp (C≡C) and perhaps aldehyde.
Carbonyl (C=O) absorption
Its presence means the compound is an aldehyde, ketone, carboxylic acid, ester, amide, anhydride or acyl halide.
Its absence means the compound cannot be any of the carbonyl-containing compounds.
O-H or N-H absorption
This indicates either an alcohol, N-H containing amine or amide, or carboxylic acid.
C≡C and C≡N absorptions
Be careful: internal triple bonds often do not show up in IR spectra.
C=C absorption
Can indicate whether compound is alkene or aromatic.

33. EXAMPLE
2-aminopentane
Amines - Primary
Shows the –N-H stretch for NH2 as a doublet between cm-1 (symmetric and anti-symmetric modes)

34. EXAMPLE
pyrrolidine
Amines – Secondary
N-H band for R2N-H occurs at cm-1 as a single sharp peak weaker than –O-H
Tertiary amines (R3N) have no N-H bond and will not have a band in this region

35. Examples of Infrared Spectra

36. O-H Stretch of a Carboxylic Acid
This O-H absorbs broadly, cm-1, due to strong hydrogen bonding

37. Carbonyl Stretching
Exact absorption characteristic of type of carbonyl compound
The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1
Usually, it’s the strongest IR signal
Carboxylic acids will have O-H also
Aldehydes have two C-H signals around 2700 and 2800 cm-1

38. Practice
Q: Identify the chemical species represented in this IR spectra?

39. Types of peaks V-shape peak Broad U-shape peak
-N—H bond for 2o amine (R2N—H)
Broad U-shape peak
-O—H bond
W-shape peak
-N—H bond for 1o amine (RNH2)
Sharp & strong peak C=O at1710 cm-1

40. 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

41. Web References