Structure Determination:
Structure Determination: Infrared Spectroscopy
Chapter 12 - Part 2
12.5 Spectroscopy of the Electromagnetic Spectrum Radiant energy is proportional to its frequency
(cycles/s = Hz) as a wave (Amplitude is its height) Different types are classified by frequency or
wavelength ranges
Spectroscopy of the Electromagnetic Spectrum γ-rays (gamma rays)- greatest energy and highest
frequency. Emitted from some radioactive nuclei. Can cause biological damage. X-rays- lower in energy than γ -rays. Can cause biological damage in high doses. Ultraviolet (UV) light- can cause sunburn and even skin cancer. Visible light- speaks for itself ! Infrared radiation (IR) - heat! Microwaves- we cook with them and also used in radar. Radio waves- lowest frequency. Radio and TV transmissions and NMR spectroscopy.
1
Properties of Electromagnetic Radiation The mode of propagation of electromagnetic
radiation is the ______.
The wave is characterized by its wavelength
(__), frequency (__), and amplitude.
Frequency (ν) units: s-1 or ________ The intensity of a wave is proportional to the
square of its amplitude.
Electromagnetic radiation travels at constant
velocity in a vacuum: 3.00 x 1010 cm/s (speed of light).
Properties of Electromagnetic Radiation Wavelength x Frequency = Speed λ(cm) x ν(s-1) = c (cm/s)
Electromagnetic Energy Thanks to Max Planck
and Al Einstein:
E = Energy of 1 photon (1 quantum) h = Planck’s constant (6.62 x 10-34 J·s = 1.58 x 10-34cal·s) ν = Frequency (s-1) λ = Wavelength (cm) c = Speed of light (3.00 x 1010 cm/s)
2
Electromagnetic Energy The energy of a photon varies directly with
the frequency ν and inversely with the wavelength λ High frequencies and short wavelengths = ________ Low frequencies and long wavelengths = ________
Electromagnetic Energy Quick calculations:
E=
N A hc
λ
=
Absorption Spectra Organic compound exposed to electromagnetic
radiation, can absorb energy of only certain wavelengths (unit of energy) Transmits, energy of other wavelengths. Changing wavelengths to determine which are absorbed and which are transmitted produces an ________ Energy absorbed is distributed internally in a distinct and reproducible way (See Figure 12-11)
3
12.6 Infrared Spectroscopy of Organic Molecules IR region lower energy than visible light (below red –
produces heating as with a heat lamp)
________ to ________ region used by organic
chemists for structural analysis
IR energy in a spectrum is usually measured as
wavenumber (cm-1), the inverse of wavelength and proportional to frequency Specific IR absorbed by organic molecule related to its structure
12.6 Infrared Spectroscopy of Organic Molecules The IR spectrum covers the range from
4000 cm-1 to 400 cm-1 This represents energy ranges from
48.0 - 4.80 kJ/mol (11.5 - 1.15 kcal/mol).
4
Convert the following: 3.10 µm to cm-1 5.85 µm to cm-1 2250 cm-1 to µm 970 cm-1 to µm Calculate the energy (kJ)/mol for the first two
wavelengths.
Infrared Energy Modes IR energy absorption corresponds to specific modes,
corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes” Energy is characteristic of the atoms in the group and their bonding Corresponds to vibrations and rotations
12.7 Interpreting Infrared Spectra Most functional groups absorb at about the same
energy and intensity independent of the molecule they are in Characteristic higher energy IR absorptions in Table 12.1 can be used to confirm the existence of the presence of a functional group in a molecule IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region; 1300 to 625 cm-1) See samples in Figure 12-13
5
Regions of the Infrared Spectrum 4000-2500 cm-1 N-H, C-
H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H 2500-2000 cm-1 C≡C and C ≡ N (stretching)
2000-1500 cm-1 double
bonds (stretching) C=O 1680-1750 C=C 1640-1680 cm-1 Below 1500 cm-1 “fingerprint” region
6
Differences in Infrared Absorptions Molecules vibrate and rotate in normal modes, which
are combinations of motions (relates to force constants) Bond stretching dominates higher energy modes Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H For two heavy atoms, stronger bond requires more energy: C ≡ C, C ≡ N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen
12.8 Infrared Spectra of Hydrocarbons ≡ C have characteristic peaks absence helps rule out C=C or C ≡ C
C-H, C-C, C=C, C
12.9 Infrared Spectra of Some Common Functional Groups Spectroscopic behavior of functional group is
discussed in later chapters Brief summaries presented here
7
IR: Alcohols and Amines O–H 3400 to 3650 cm−1
Usually broad and intense
N–H 3300 to 3500 cm−1
Sharper and less intense than an O–H
1-butanol
butylamine
8
IR: Aromatic Compounds Weak C–H stretch at 3030 cm−1 Weak absorptions 1660 - 2000 cm−1 range Medium-intensity absorptions 1450 to 1600 cm−1 See spectrum of phenylacetylene, Figure 12.15
IR: Carbonyl Compounds Strong, sharp C=O peak 1670 to 1780 cm−1 Exact absorption characteristic of type of carbonyl
compound 1730 cm−1 in saturated aldehydes 1705 cm−1 in aldehydes next to double bond or aromatic ring
C=O in Ketones 1715 cm−1 in six-membered ring and acyclic ketones 1750 cm−1 in 5-membered ring ketones 1690 cm−1 in ketones next to a double bond or an
aromatic ring
9
C=O in Esters 1735 cm−1 in saturated esters 1715 cm−1 in esters next to aromatic ring or a
double bond
Carboxylic Acids 1700 - 1725 cm-1 (strong)
O R
OH 2500 - 3300 cm-1 (very broad and intense)
10
Pentanoic acid
Identify the functional groups in compounds that are responsible for the following absorptions:
A compound with a strong absorption at 1710 cm-1 A compound with a strong absorption at 1540 cm-1 A compound with a strong absorption at 1720 cm-1 and at
2500-3100 cm-1
How might you use IR spectroscopy to distinguish between the following pairs of isomers? CH3CH2OH and CH3OCH3 Cyclohexane and 1- hexene
11
Last Slide Chapter 12 Part 2
12
Chapter 12 - Part 2
12.5 Spectroscopy of the Electromagnetic Spectrum Radiant energy is proportional to its frequency
(cycles/s = Hz) as a wave (Amplitude is its height) Different types are classified by frequency or
wavelength ranges
Spectroscopy of the Electromagnetic Spectrum γ-rays (gamma rays)- greatest energy and highest
frequency. Emitted from some radioactive nuclei. Can cause biological damage. X-rays- lower in energy than γ -rays. Can cause biological damage in high doses. Ultraviolet (UV) light- can cause sunburn and even skin cancer. Visible light- speaks for itself ! Infrared radiation (IR) - heat! Microwaves- we cook with them and also used in radar. Radio waves- lowest frequency. Radio and TV transmissions and NMR spectroscopy.
1
Properties of Electromagnetic Radiation The mode of propagation of electromagnetic
radiation is the ______.
The wave is characterized by its wavelength
(__), frequency (__), and amplitude.
Frequency (ν) units: s-1 or ________ The intensity of a wave is proportional to the
square of its amplitude.
Electromagnetic radiation travels at constant
velocity in a vacuum: 3.00 x 1010 cm/s (speed of light).
Properties of Electromagnetic Radiation Wavelength x Frequency = Speed λ(cm) x ν(s-1) = c (cm/s)
Electromagnetic Energy Thanks to Max Planck
and Al Einstein:
E = Energy of 1 photon (1 quantum) h = Planck’s constant (6.62 x 10-34 J·s = 1.58 x 10-34cal·s) ν = Frequency (s-1) λ = Wavelength (cm) c = Speed of light (3.00 x 1010 cm/s)
2
Electromagnetic Energy The energy of a photon varies directly with
the frequency ν and inversely with the wavelength λ High frequencies and short wavelengths = ________ Low frequencies and long wavelengths = ________
Electromagnetic Energy Quick calculations:
E=
N A hc
λ
=
Absorption Spectra Organic compound exposed to electromagnetic
radiation, can absorb energy of only certain wavelengths (unit of energy) Transmits, energy of other wavelengths. Changing wavelengths to determine which are absorbed and which are transmitted produces an ________ Energy absorbed is distributed internally in a distinct and reproducible way (See Figure 12-11)
3
12.6 Infrared Spectroscopy of Organic Molecules IR region lower energy than visible light (below red –
produces heating as with a heat lamp)
________ to ________ region used by organic
chemists for structural analysis
IR energy in a spectrum is usually measured as
wavenumber (cm-1), the inverse of wavelength and proportional to frequency Specific IR absorbed by organic molecule related to its structure
12.6 Infrared Spectroscopy of Organic Molecules The IR spectrum covers the range from
4000 cm-1 to 400 cm-1 This represents energy ranges from
48.0 - 4.80 kJ/mol (11.5 - 1.15 kcal/mol).
4
Convert the following: 3.10 µm to cm-1 5.85 µm to cm-1 2250 cm-1 to µm 970 cm-1 to µm Calculate the energy (kJ)/mol for the first two
wavelengths.
Infrared Energy Modes IR energy absorption corresponds to specific modes,
corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes” Energy is characteristic of the atoms in the group and their bonding Corresponds to vibrations and rotations
12.7 Interpreting Infrared Spectra Most functional groups absorb at about the same
energy and intensity independent of the molecule they are in Characteristic higher energy IR absorptions in Table 12.1 can be used to confirm the existence of the presence of a functional group in a molecule IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region; 1300 to 625 cm-1) See samples in Figure 12-13
5
Regions of the Infrared Spectrum 4000-2500 cm-1 N-H, C-
H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H 2500-2000 cm-1 C≡C and C ≡ N (stretching)
2000-1500 cm-1 double
bonds (stretching) C=O 1680-1750 C=C 1640-1680 cm-1 Below 1500 cm-1 “fingerprint” region
6
Differences in Infrared Absorptions Molecules vibrate and rotate in normal modes, which
are combinations of motions (relates to force constants) Bond stretching dominates higher energy modes Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H For two heavy atoms, stronger bond requires more energy: C ≡ C, C ≡ N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen
12.8 Infrared Spectra of Hydrocarbons ≡ C have characteristic peaks absence helps rule out C=C or C ≡ C
C-H, C-C, C=C, C
12.9 Infrared Spectra of Some Common Functional Groups Spectroscopic behavior of functional group is
discussed in later chapters Brief summaries presented here
7
IR: Alcohols and Amines O–H 3400 to 3650 cm−1
Usually broad and intense
N–H 3300 to 3500 cm−1
Sharper and less intense than an O–H
1-butanol
butylamine
8
IR: Aromatic Compounds Weak C–H stretch at 3030 cm−1 Weak absorptions 1660 - 2000 cm−1 range Medium-intensity absorptions 1450 to 1600 cm−1 See spectrum of phenylacetylene, Figure 12.15
IR: Carbonyl Compounds Strong, sharp C=O peak 1670 to 1780 cm−1 Exact absorption characteristic of type of carbonyl
compound 1730 cm−1 in saturated aldehydes 1705 cm−1 in aldehydes next to double bond or aromatic ring
C=O in Ketones 1715 cm−1 in six-membered ring and acyclic ketones 1750 cm−1 in 5-membered ring ketones 1690 cm−1 in ketones next to a double bond or an
aromatic ring
9
C=O in Esters 1735 cm−1 in saturated esters 1715 cm−1 in esters next to aromatic ring or a
double bond
Carboxylic Acids 1700 - 1725 cm-1 (strong)
O R
OH 2500 - 3300 cm-1 (very broad and intense)
10
Pentanoic acid
Identify the functional groups in compounds that are responsible for the following absorptions:
A compound with a strong absorption at 1710 cm-1 A compound with a strong absorption at 1540 cm-1 A compound with a strong absorption at 1720 cm-1 and at
2500-3100 cm-1
How might you use IR spectroscopy to distinguish between the following pairs of isomers? CH3CH2OH and CH3OCH3 Cyclohexane and 1- hexene
11
Last Slide Chapter 12 Part 2
12
