IR Chemistry
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Transcript of IR Chemistry
Spectroscopy“seeing the unseeable”
Using electromagnetic radiation as a probe to obtain information about atoms and molecules that are too small to see.
Electromagnetic radiation is propagated at the speed of light through a vacuum as an oscillating wave.
electromagnetic relationships:
λυ = c λ ∝ 1/υ
E = hυ E ∝ υ
E = hc/λ E ∝ 1/λ
λ = wave length
υ = frequency
c = speed of light
E = kinetic energy
h = Planck’s constant
λ
c
some electromagnetic radiation ranges
Approx. freq. range Approx. wavelengths
Hz (cycle/sec) meters
Radio waves 104 - 1012 3x104 - 3x10-4
Infrared (heat) 1011 - 3.8x1014 3x10-3 - 8x10-7
Visible light 3.8x1014 - 7.5x1014 8x10-7 - 4x10-7
Ultraviolet 7.5x1014 - 3x1017 4x10-7 - 10-9
X rays 3x1017 - 3x1019 10-9 - 10-11
Gamma rays > 3x1019 < 10-11
Provides information about the vibrations of functional groups in a molecule
Infrared Spectroscopy
Therefore, the functional groups present in a molecule can be deduced from an IR
spectrum
Infrared radiation
λ = 2.5 to 17 μm
υ = 4000 to 600 cm-1
These frequencies match the frequencies of covalent bond stretching and bending vibrations. Infrared spectroscopy can be used to find out about covalent bonds in molecules.
IR is used to tell:
1. what type of bonds are present
2. some structural information
IR source è sample è prism è detector
graph of % transmission vs. frequency
=> IR spectrum
4000 3000 2000 1500 1000 500
v (cm-1)
100
%T
0
E2
E2
∆E = hνE2
E1
∆E = hν
Frequency
Intensity
νFrequency
Intensity
ν
Absorbance Emission
7
The functional group concept of organic chemistry
IR spectroscopy can identify functional groups!
Some Important and Characteristic Infrared Absorption Frequencies and Wavelengths for Some Common Stretching Motions
Atom Group Typical of Frequency (cm-1) Wavelength (µ) O-H (free) Alcohols (dilute) 3550-3650 cm-1 2.8 µ O-H (H bonded) Alcohols
(concentrated) Carboxylic acids
3200-3400 cm-1 3.0 µ
C C H Acetylene (CH) 3300 cm-1 3.0 µ C C H Benzene (CH),
Ethylene (CH) 3010-3100 cm-1 3.3 µ
C_____C_____H Ethane (CH) 2950-3000 cm-1 3.5 µ C C Acetylene 2100-2260 cm-1 4.5 µ C N Nitriles 2000-2300 cm-1 4.5 µ C O Carbonyl 1650-1750 cm-1 5.5- 6.0 µ C C Alkene 1620-1680 cm-1 6.0 µ C C Alkane 600-1500 cm-1 6.7-17 µ C O Alcohols,
Ethers 1000-1300 cm-1 10-7.7 µ
In general we will only be using the data in an IR spectrum for stretching vibrations which have energies higher than 1620 cm-1. Although the bands at lower energy are known and assigned, the region below 1620 cm-1 is very congested with single bond stretches of two heavy atoms (see C-C and C-O in table) and C-H bends and are beyond the scope of what we want to do.
IR spectra of ALKANES
C—H bond “saturated”
(sp3) 2850-2960 cm-1
+ 1350-1470 cm-1
-CH2- + 1430-1470
-CH3 + “ and 1375
-CH(CH3)2 + “ and 1370, 1385
-C(CH3)3 + “ and 1370(s), 1395 (m)
IR of ALKENES
=C—H bond, “unsaturated” vinyl
(sp2) 3020-3080 cm-1
+ 675-1000
RCH=CH2 + 910-920 & 990-1000
R2C=CH2 + 880-900
cis-RCH=CHR + 675-730 (v)
trans-RCH=CHR + 965-975
C=C bond 1640-1680 cm-1 (v)
IR spectra BENZENEs
=C—H bond, “unsaturated” “aryl”
(sp2) 3000-3100 cm-1
+ 690-840
mono-substituted + 690-710, 730-770
ortho-disubstituted + 735-770
meta-disubstituted + 690-710, 750-810(m)
para-disubstituted + 810-840(m)
C=C bond 1500, 1600 cm-1
IR spectra ALCOHOLS & ETHERS
C—O bond 1050-1275 (b) cm-1
1o ROH 1050
2o ROH 1100
3o ROH 1150
ethers 1060-1150
O—H bond 3200-3640 (b) «
nn--Hexane: CHHexane: CH33CHCH22CHCH22CHCH22CHCH22CHCH33
11--Hexene: CHHexene: CH22=CHCH=CHCH22CHCH22CHCH22CHCH33
1650 cm1650 cm--11
3100 cm3100 cm--11
??
IR: Masses, Atoms and Springs
F = force, restoring back to equilibrium position
k = characteristic stretching constant
x = displacement from the equilibrium position
A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds).
As a first approximation use Hooke’s Law
F = -kx
ν =1
2πk
mr
mr =m1m2
m1 + m2
IR Stretching Frequencies of two bonded atoms:
ν = frequencyk = spring strength (bond stiffness)mr = reduced mass (~ mass of largest atom)
What Does the Frequency, ν, Depend On?
Internuclearseparation
Heavy atomsand /or
weak bonds
Internuclearseparation
Light atomsand/or
strong bonds
X Y X Y
X Y X Y
X YX Y
PE
r re re
a b
c d
F = dPE/dr
Vibrations, potential energy and motion
IR Stretching Frequencies: What Do they Depend On?
Directly on the strength of the bonding between the two atoms (ν ~ k)
Inversely on the reduced mass of the two atoms (v ~ 1/m)
Expect: ν will increase with increasing bond strength (bond order) and decreasing mass
Examples of stretching frequencies and correlations with bond strengths (bond order)
CC--CC
C=CC=C
C C
Bond strength*
350350
600600
840840
Bond order
11
22
33
νν
2200 cm2200 cm--11
1600 cm1600 cm--11
1000 cm1000 cm--11
*In kJ/mol*In kJ/mol For same reduced mass!For same reduced mass!
Quantum mechanics: The frequency (ν) depends on the energy gap between vibrational levels
E = hν = hc/λ (cm-1)
The natural frequency (8.67 x 1013 s-1) is absorbed selectively
Only the natural frequency will be absorbed
The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency ν can be transferred to the
atoms involved in the vibration
The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon
What does the absorption intensity depend on?
Dipole Moment Must Change during for a vibration to be “IR active”!
In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole
moment changes.
Which of the vibrations are “IR” active”?
What is the intensity of an IR signal of: O2 or N2 or H2?
Ans: In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of
the molecule
O2, N2 and H2 DO NOT ABSORB IR LIGHT!
The are not “Greenhouse” gases
Does O=C=O absorb IR light?
Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light
vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light
No dipoleNo dipolegeneratedgenerated
Dipole Dipole generatedgenerated
Which should have a higher stretching frequency, CO, CO+, or CO-? Why?
Ans: The higher the bond order, the stronger the bond and the higher the frequency for the IR stretch.
Bond order: CO = 3, CO+ = 5/2, CO- = 5/2
CO will have the higher stretching frequency
CO+ and CO- will have similar, lower frequencies
CO = (σ)2(π)4 CO+ = (σ)2(π)3 CO- = (σ)2(π)4(π∗)1
region of infrared that is most useful lies between2.5-16 µm (4000-625 cm-1)
depends on transitions between vibrational energy states
stretching
bending
Infrared Spectroscopy: further review
Stretching Vibrations of a CH2 Group
Symmetric Antisymmetric
Bending Vibrations of a CH2 Group
In plane In plane
Bending Vibrations of a CH2 Group
Out of plane Out of plane
2000200035003500 30003000 25002500 1000100015001500 500500
Wave number, cmWave number, cm--11
Infrared Spectrum of Hexane
CHCH33CHCH22CHCH22CHCH22CHCH22CHCH33
CC——H stretchingH stretchingbendingbending bendingbending
bendingbending
2000200035003500 30003000 25002500 1000100015001500 500500
Wave number, cmWave number, cm--11
Infrared Spectrum of 1-Hexene
HH22C=CHCHC=CHCH22CHCH22CHCH22CHCH33
HH——CCC=CC=C——HH
C=CC=C
HH22C=CC=C
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
sp C—H 3310-3320
sp2 C—H 3000-3100
sp3 C—H 2850-2950
sp2 C—O 1200
sp3 C—O 1025-1200
Infrared Absorption Frequencies
Structural unit Frequency, cm-1
Stretching vibrations (multiple bonds)
Infrared Absorption Frequencies
C C 1620-1680
—C N
—C C— 2100-2200
2240-2280
Structural unit Frequency, cm-1
Stretching vibrations (carbonyl groups)
Aldehydes and ketones 1710-1750
Carboxylic acids 1700-1725
Acid anhydrides 1800-1850 and 1740-1790
Esters 1730-1750
Amides 1680-1700
Infrared Absorption Frequencies
C O
Structural unit Frequency, cm-1
Bending vibrations of alkenes
Infrared Absorption Frequencies
CH2RCH
CH2R2C
CHR'cis-RCH
CHR'trans-RCH
CHR'R2C
910-990
890
665-730
960-980
790-840
Structural unit Frequency, cm-1
Bending vibrations of derivatives of benzene
Monosubstituted 730-770 and 690-710
Ortho-disubstituted 735-770
Meta-disubstituted 750-810 and 680-730
Para-disubstituted 790-840
Infrared Absorption Frequencies
2000200035003500 30003000 25002500 1000100015001500 500500
Wave number, cmWave number, cm--11
Infrared Spectrum of tert-butylbenzene
HH——CC
ArAr——HH
MonsubstitutedMonsubstitutedbenzenebenzene
CC66HH55C(CHC(CH33))33
Structural unit Frequency, cm-1
Stretching vibrations (single bonds)
O—H (alcohols) 3200-3600
O—H (carboxylic acids) 3000-3100
N—H 3350-3500
Infrared Absorption Frequencies: functional groups
2000200035003500 30003000 25002500 1000100015001500 500500
Wave number, cmWave number, cm--11
Infrared Spectrum of 2-Hexanol
HH——CC
OO——HH
OHOH
CHCH33CHCH22CHCH22CHCH22CHCHCHCH33
2000200035003500 30003000 25002500 1000100015001500 500500
Wave number, cmWave number, cm--11
Infrared Spectrum of 2-Hexanone
HH——CC
C=OC=O
OO
CHCH33CHCH22CHCH22CHCH22CCHCCH33