Infrared Spectroscopy
Lecture 6
Infrared Spectroscopy
Chemistry 618B
13C-NMR
Spectroscopy
Each nonequivalent 13C gives a different signal Low abundance means weak signals C-C splitting is insignificant (1.1%) C-H splitting is big and complex so it is “turned off” by “decoupling” Range of Chemical Shifts is large compared to H Some Coupling info can be recovered by DEPT Integrals of 13C spectra are not useful except under very special circumstances Chemistry 618B
The DEPT method
DEPT uses a complex series of pulses in both the 1H and 13C ranges, with the result that CH3, CH2, and CH signals exhibit different phases; – signals for CH3 and CH carbons are recorded as positive signals (odd numbers of H) – signals for CH2 carbons are recorded as negative signals (even numbers of H) – quaternary carbons give no signals in the DEPT method (zero H) Chemistry 618B
Unknown Empirical formula C4H9, MW =114 120 115
25.6
110
25.60
105
Positive in DEPT
100 95 90 85 80
0.00 TMS
75 70 65 60 55 50 45 40 35 30 25
Disappears in DEPT
35.0
20 15
35.00
10 5 0
70
60
50
40
30
20
10
0
-10
Chemistry 618B
Infrared Spectroscopy
Infrared Spectroscopy
Vibrational IR spectral region covers – 2.5 x 10-6 m (2.5 micrometers) to 2.5 x 10-5 m (25 µm) – Human hair is about 50 µm in diameter
Absorption of IR radiation in this region causes bonds to change from a lower vibrational energy level to a higher one
Chemistry 618B
Infrared Frequency Scale
IR radiation is commonly expressed in wavenumbers Wavenumber: the number of waves per centimeter, cm-1 (read reciprocal centimeters or Kysers) Expressed in wavenumbers, the vibrational IR extends from ~4000 cm-1 to ~400 cm -1 10,000 µm• cm -1 2.50 µm
= 4000 cm-1
10,000 µm• cm -1 25.0 µm
= 400 cm-1
(ν- )
Chemistry 618B
Transmittance (%)
The IR Chart 100 2.5
0
4000
Micrometers
Wavenumber (cm -1 )
20
400
Please “know” this Chemistry 618B
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 for most covalent bonds are from 2 to 10 kcal/mol (8.4 to 42 kJ/mol) Chemistry 618B
Molecular Vibrations
For a molecule to absorb IR radiation, the bond undergoing vibration – must be polar (change dipole moment)
Covalent bonds that do not meet this criterion are said to be IR inactive – the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, do not absorb IR radiation because they are not polar bonds H2
N2
CO2
?????? Chemistry 618B
Molecular Vibrations
For a simple harmonic oscillator, the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring
ν =
1
2 πc
NK µ
N = Avogadro’s number c = velocity of light K = force constant, a measure of the bond strength µ = the reduced mass Chemistry 618B
Mr Hooke says:
The position (frequency) of absorption of a stretching vibration depends on – the strength of the vibrating bond (direct) and – the masses of the atoms (inverse)
The stronger the bond and the lighter the atoms connected by that bond, the higher the frequency (wavenumber) of the vibration
The intensity of absorption depends primarily on the polarity of the vibrating bond Chemistry 618B
Chloroform and Deuteriochloroform
CDCl3 CHCl3
the two major differences in these spectra are (1) the disappearance of the C-H stretching (3020 cm-1) and bending (1220 cm-1) in deuterated compound and (2) a shift to the right about 20 cm-1 relative to the CHCl3. The first is caused simply by the lack of C-H bonds in CDCl3. The second is illustrative of this property that heavier atoms (deuterium vs. hydrogen) will cause attached bonds to absorb at lower frequencies. Chemistry 618B
IR Group Correlation Tables
Characteristic IR absorptions for some of the functional groups we deal with most often Bond
Frequency (cm-1 )
O-H
3200-3650
strong and broad
N-H
3100-3500 2850-3300
medium
C-H C=O C=C C-O
Intensity
1630-1810 1600-1680
medium to strong strong weak
1050-1250
strong Chemistry 618B
Hydrocarbon Vibration Alkane C-H stretching CH 2 bending CH 3 bending Alkene C-H C=C Alkyne C-H C C
Frequency (cm -1)
Intensity
2850 - 3000 1450 1375 and 1450
strong medium weak to medium
stretching stretching
3000 - 3100 1600 - 1680
weak to medium weak to medium
stretching stretching
3300 2100-2250
medium to strong weak Chemistry 618B
Practice, Practice, Practice……… Chemistry 618B
Chemistry 618B
Chemistry 618B
Chemistry 618B
Summary
Practice carbon nmr correlation charts (C=O) Know how to read/interpret DEPT spectra IR measures vibrational transitions Can be described by classical oscillator theory – Frequency proportional to [ bond strength/mass]1/2
Characteristic Group Frequencies – OH and C=O are particularly easy to identify
Know how to read the chart…cm-1?? Practice at the online sites…nmr, IR..Excellent!! Chemistry 618B
Infrared Spectroscopy
Chemistry 618B
13C-NMR
Spectroscopy
Each nonequivalent 13C gives a different signal Low abundance means weak signals C-C splitting is insignificant (1.1%) C-H splitting is big and complex so it is “turned off” by “decoupling” Range of Chemical Shifts is large compared to H Some Coupling info can be recovered by DEPT Integrals of 13C spectra are not useful except under very special circumstances Chemistry 618B
The DEPT method
DEPT uses a complex series of pulses in both the 1H and 13C ranges, with the result that CH3, CH2, and CH signals exhibit different phases; – signals for CH3 and CH carbons are recorded as positive signals (odd numbers of H) – signals for CH2 carbons are recorded as negative signals (even numbers of H) – quaternary carbons give no signals in the DEPT method (zero H) Chemistry 618B
Unknown Empirical formula C4H9, MW =114 120 115
25.6
110
25.60
105
Positive in DEPT
100 95 90 85 80
0.00 TMS
75 70 65 60 55 50 45 40 35 30 25
Disappears in DEPT
35.0
20 15
35.00
10 5 0
70
60
50
40
30
20
10
0
-10
Chemistry 618B
Infrared Spectroscopy
Infrared Spectroscopy
Vibrational IR spectral region covers – 2.5 x 10-6 m (2.5 micrometers) to 2.5 x 10-5 m (25 µm) – Human hair is about 50 µm in diameter
Absorption of IR radiation in this region causes bonds to change from a lower vibrational energy level to a higher one
Chemistry 618B
Infrared Frequency Scale
IR radiation is commonly expressed in wavenumbers Wavenumber: the number of waves per centimeter, cm-1 (read reciprocal centimeters or Kysers) Expressed in wavenumbers, the vibrational IR extends from ~4000 cm-1 to ~400 cm -1 10,000 µm• cm -1 2.50 µm
= 4000 cm-1
10,000 µm• cm -1 25.0 µm
= 400 cm-1
(ν- )
Chemistry 618B
Transmittance (%)
The IR Chart 100 2.5
0
4000
Micrometers
Wavenumber (cm -1 )
20
400
Please “know” this Chemistry 618B
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 for most covalent bonds are from 2 to 10 kcal/mol (8.4 to 42 kJ/mol) Chemistry 618B
Molecular Vibrations
For a molecule to absorb IR radiation, the bond undergoing vibration – must be polar (change dipole moment)
Covalent bonds that do not meet this criterion are said to be IR inactive – the C-C double and triple bonds of symmetrically substituted alkenes and alkynes, for example, do not absorb IR radiation because they are not polar bonds H2
N2
CO2
?????? Chemistry 618B
Molecular Vibrations
For a simple harmonic oscillator, the frequency of a stretching vibration is given by an equation derived from Hooke’s law for a vibrating spring
ν =
1
2 πc
NK µ
N = Avogadro’s number c = velocity of light K = force constant, a measure of the bond strength µ = the reduced mass Chemistry 618B
Mr Hooke says:
The position (frequency) of absorption of a stretching vibration depends on – the strength of the vibrating bond (direct) and – the masses of the atoms (inverse)
The stronger the bond and the lighter the atoms connected by that bond, the higher the frequency (wavenumber) of the vibration
The intensity of absorption depends primarily on the polarity of the vibrating bond Chemistry 618B
Chloroform and Deuteriochloroform
CDCl3 CHCl3
the two major differences in these spectra are (1) the disappearance of the C-H stretching (3020 cm-1) and bending (1220 cm-1) in deuterated compound and (2) a shift to the right about 20 cm-1 relative to the CHCl3. The first is caused simply by the lack of C-H bonds in CDCl3. The second is illustrative of this property that heavier atoms (deuterium vs. hydrogen) will cause attached bonds to absorb at lower frequencies. Chemistry 618B
IR Group Correlation Tables
Characteristic IR absorptions for some of the functional groups we deal with most often Bond
Frequency (cm-1 )
O-H
3200-3650
strong and broad
N-H
3100-3500 2850-3300
medium
C-H C=O C=C C-O
Intensity
1630-1810 1600-1680
medium to strong strong weak
1050-1250
strong Chemistry 618B
Hydrocarbon Vibration Alkane C-H stretching CH 2 bending CH 3 bending Alkene C-H C=C Alkyne C-H C C
Frequency (cm -1)
Intensity
2850 - 3000 1450 1375 and 1450
strong medium weak to medium
stretching stretching
3000 - 3100 1600 - 1680
weak to medium weak to medium
stretching stretching
3300 2100-2250
medium to strong weak Chemistry 618B
Practice, Practice, Practice……… Chemistry 618B
Chemistry 618B
Chemistry 618B
Chemistry 618B
Summary
Practice carbon nmr correlation charts (C=O) Know how to read/interpret DEPT spectra IR measures vibrational transitions Can be described by classical oscillator theory – Frequency proportional to [ bond strength/mass]1/2
Characteristic Group Frequencies – OH and C=O are particularly easy to identify
Know how to read the chart…cm-1?? Practice at the online sites…nmr, IR..Excellent!! Chemistry 618B
