Transcript File - Dr. KHALID SHADID

Islamic University in Madinah
Department of Chemistry
Infrared Spectroscopy
Theory and Interpretation of IR spectra
Prepared By
Dr. Khalid Ahmad Shadid
Spectroscopy and the Electromagnetic Spectrum
2
Basic Theory of IR Absorption
• Infrared: exciting from one vibrational level to another
• UV/Vis: exciting from one electronic level to another
• Microwaves: exciting from one rotational level to another
3
Basic Theory of IR Absorption
• Changes in interatomic vibrations of a molecule are brought
about through the absorption of IR light
bond dissociation
energy
E
E = h
1
2
Zero point energy E 0  h 
r , interatomic distance
4
The IR Spectrum
• The vibrational spectrum of a molecule is a unique physical
property and is characteristic of the molecule
• IR spectrum can be used as for identification by the
comparison of ‘‘unknown’’ spectrum with reference spectra
• IR spectrum can lead to characterization, and possibly even
identification of an unknown samples
• The IR information can indicate:
– Linear or branched backbone
– If chains are (un)saturated
– Aromatic rings in the structure and substitution
– Functional groups
5
IR Spectroscopy
• In IR spectroscopy, there is interaction between molecules
and radiations from the IR region of the EMR spectrum (IR
region = 4000 - 400 cm-1)
• IR radiation causes the excitation of the vibrations of covalent
bonds within that molecule. These vibrations include the
stretching and bending modes
• In practice, it is the polar covalent bonds that are IR "active"
and whose excitation can be observed in an IR spectrum
• Generally, it is convenient to split an IR spectrum into two
approximate regions:
– Functional group region: 4000-1000 cm-1
– Fingerprint region: < 1000 cm-1 (more complex and much
harder to assign)
6
Regions of Frequencies
Wavenumber(cm-1)
Spectral Region
Frequency(Hz)
Near -to
visible- IR
(NIR)
Combination
bands
3.8 x 1014 to 1.2
x 1014
12800 to 4000
0.78 to 2.5
Mid Infrared
Fundmental
bands for
organic
molecules
1.2 x 1014 to 6.0
x 1012
4000 to 200
2.5 to 50
200 to 10
50 to 1000
Far IR
6.0 x 1012 to 3.0
x 1011
Inorganics
organometallics
Wavelength (,m)
After Table 16-1 of Skoog and West, et al. (Chapter 16)
Basic Theory of IR Absorption

8
We need to talk about

IR energy modes (Types of vibration in molecules)

Band positions

Band Intensity
Infrared Energy Modes
• IR energy absorption corresponds to specific modes,
corresponding to combinations of atomic movements,
such as bending (change in bond angle) and stretching
(change in bond length) of bonds between groups of
atoms
• Energy is characteristic of the atoms in the group and their
bonding
• Corresponds to vibrations and rotations
9
Many possible absorptions per molecule exist: stretching, bending,…
Vibrational modes leading to IR absorptions:
Bond length changes
Symmetrical Stretching
Bond angle changes
Asymmetrical Stretching
Bending:
Scissoring
Rocking
‫مقصية‬
‫تارجحية‬
Wagging
‫ذيل الخيل‬
Twisting
‫التوائية او لولبية‬
Bands Position: Hookes' Law
• A vibrating bond In IR can be compared to the physical
model of a vibrating spring system that can be described by
Hooke's Law of harmonic oscillation
• Using the force constant k (which reflects the stiffness of
the spring) and the two masses m1 and m2, then the
equation indicates how the frequency, u, of the absorption
should change as the properties of the system change
12
Bands Position: Hookes' Law
• Hooke’s Law can be used to estimate the wavenumber () of
light that will be absorbed by chemical bonds
•  = 4.12 * (K / )1/2
K is the force constant (in dynes / cm) and for:
single bond: K = 5 x 105 dynes/cm
double bond: K = 10 x 105 dynes/cm
triple bond: K = 15 x 105 dynes/cm
•  = reduced mass
• For example in C=C bond:
 = 4.12 * (10 x 105 / [12 * 12 / (12 + 12)])1/2 = 1682 cm-1
(calculated) compare to experimental value1650 cm-1
13
Factors affecting Absorption Frequency
•
•
•
•
•
•
•
•
Strength of chemical bond
Masses of attached atoms to the bond
Hydrogen bonding
Resonance
Bond angle or ring strain
Hybridization
Polarity of bonds
External factors: eg. state of measurements, conc., temp.,
solvent used etc.
14
Factors Affecting Absorption Frequency:
Bond Strength
• For a stronger bond (larger k value), u increases Compare
(increasing bond strength) :
– CC bonds : C-C (1000 cm-1), C=C (1600 cm-1) and CC
(2200 cm-1)
– CH bonds: C-C-H (2900 cm-1), C=C-H (3100 cm-1) and CCH (3300 cm-1)
• Strength of the chemical bond:
• stronger
.
bond
15
k , 
 (cm 1 ) 
1
2c
m1m2

m1  m2
k

Factors Affecting Absorption
Frequency: Masses of Atoms
• Masses of the attached atoms to the bond
• For heavier atoms (larger m value), u decreases compare
(increasing reduced mass):
– C-H (3000 cm-1)
– C-C (1000 cm-1)
– C-Cl (800 cm-1)
– C-Br (550 cm-1)
heavier atoms
 , 
– C-I (500 cm-1)
C O

 (cm 1 ) 
1
2c
k


m1m2
m1  m2
~
3000cm-1
C S
1100cm-1
R F
R Cl
C H
650cm-1
R Br

16
Factors Affecting Absorption
Frequency: Hydrogen Bonding
• H-bonding
• For example: free OH is observed at 3600cm-1 while Hbonded –OH is observed at 3400cm-1
18
Factors Affecting Absorption
Frequency: Resonance





Resonance: electronic factors
Conjugation lowers the energy to vibrate bond
isolated ketones: 1710 cm-1
a,b-unsaturated ketones: 1690 cm-1
a,b,g,d-unsaturated ketones: 1675 cm-1
20
Factors Affecting Absorption
Frequency: Bond Strain
• Internal factors: Bond angle or ring strain
22
Factors Affecting Absorption Frequency:
Hybridization
•
•
•
•
•
Hybridization:
Bonds are stronger in the order sp > sp2 > sp3
C-H (sp): 3300 cm-1
C-H (sp2): 3100 cm-1
C-H (sp3): 2900 cm-1
23
Factors Affecting Absorption
Frequency: Polarity of Bond
• The more polar a chemical bond is, the higher the intensity of
the band
• Low dipole moments results in a weak bands
C
C
H
C
C
C
C
C
• Low dipole moments results in a weak bands
C
N
C
C
O
O
O
H
• Band intinsity is qualitatively described as: very strong (vs),
strong (s), medium (m), weak (w) and variable (var).
24
Band intensity
 Symmetrical bonds have no dipole moments and thus no IR
bands observed in the spectrum (ie. infrared inactive)
26
Band intensity
• Absorption other than fundamental modes of vibration
– overtones: exactly 2x or 3x of a fundamental frequency
– combination bands where freq. = (1  2). For example:
aromatics between 1600 - 2000 cm-1
– coupling: interaction between 2 vibrating groups in close
proximity; e.g.:
27
Band intensity
• An IR spectrophotometer is made of an IR light source, a sample
container, a prism to separate light into different wavelengths,
a detector, and a recorder (to produces the infrared spectrum)
28
Spectroscopy and 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
29
Infrared Spectroscopy of Organic Molecules
• Organic compounds when exposed to electromagnetic
radiation, can absorb energy of only certain wavelengths (unit
of energy)
– Transmits, energy of other wavelengths.
• IR region lower energy than visible light (below red –
produces heating as with a heat lamp)
• 2.5  106 m to 2.5  105 m region used by organic chemists
for structural analysis
• IR energy in a spectrum is usually measured as wave number
(cm-1), the inverse of wavelength and proportional to
frequency
30
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)
31
• 2000-1500 cm-1 double bonds
(stretching)
– C=O 1680-1750
– C=C 1640-1680 cm-1
• Below 1500 cm-1 “fingerprint”
region
IR ABSORPTION RANGE
The typical IR absorption range for covalent bonds is 600 - 4000 cm-1. The graph shows the
regions of the spectrum where the following types of bonds normally absorb. For example
a sharp band around 2200-2400 cm-1 would indicate the possible presence of a C-N or a C-C
triple bond.
Graphics source: Wade, Jr., L.G. Organic Chemistry, 5th ed. Pearson Education Inc., 2003
Regions of the Infrared Spectrum
35
Regions of the Infrared Spectrum
36
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
37
12.8 Infrared Spectra of Hydrocarbons
•
38
C-H, C-C, C=C, C  C have characteristic peaks
– absence helps rule out C=C or C  C
Infrared Spectra of Some Common Functional
Groups
• n-Alkanes
- look for stretching and bending of C–H and C–C bonds
• C–C bends: ca. 500 cm–1 (out of spectral window)
• C–C stretches: 1200–800 cm–1, weak bands not of
value for interpretation (fingerprint)
More characteristic
• C–H stretches: occurs from 3000 - 2840 cm–1
CH3: 2962 cm–1, asymmetrical stretch
2872 cm–1, symmetrical stretch
CH2: 2926 cm–1, asymmetrical stretch
2853 cm–1, symmetrical stretch
• C–H bends:
CH3: ca. 1375 cm–1
CH2: ca. 1465 cm–1
39
n-Alkanes
• n-Hexane CH3(CH2)4CH3
CH3 (s)
CH3 (as)
CH3 (s)
CH3 (as)
40
C-H
Stretches
C-H
Bends
Finger printing
C10H22
C12H26
41
The IR of
C10H22
and
C12H26
are
Similar
but Not
Identical
Unconjugated Alkenes
• Linear alkenes:
– C=C–H stretch: ≥ 3000 cm-1
– C=C–H bending in the range 1000-650 cm-1
– C=C stretch: moderate to weak at 1680-1600 cm-1
42
Unconjugated Alkenes
• Example: 1-Hexene
43
Cyclic Alkenes
• The C=C stretch is sensitive to ring strain (size)
44
Conjugated alkenes
• often conjugation moves C=C stretch to lower frequencies and
increases the intensity
• The alkene bond stretching vibrations in alkenes without a
center of symmetry, e.g. 1-methylbutadiene, gives to two C=C
stretches
• For symmetrical molecules, e.g. butadiene, only the asymmetric
stretch is observed
Me
–1
1650 cm (as)
1600 cm–1 (s)
45
1600 cm–1 (as)
Conjugated alkenes
• 2-methylbutadiene
C-H stretch
3090 cm–1
46
Symmetrical
C=C stretch
1640 cm–1
(weak)
Asymmetrical
C=C stretch
1598 cm–1 (strong)
Out of plane
C=C–H bends
990, 892 cm–1
Alkynes
• C C–H bend: 700-610 cm-1: broad, strong
• C C–H stretch: 3333–3267 cm-1, strong and narrow (as
compared to OH or NH)
• C C stretch: weak absorption at 2260-2100 cm-1
– not observed for symmetrical alkynes
– terminal alkynes (R-C C-H) absorptions are stronger
than internal (R-C C-R) absorptions
– Disubstituted or symetrically substituted triple bonds give
either no absorption or weak absorption
47
Terminal Alkynes
• Example: 1-octyne
Alkyne
CC stretch
2119 cm–1
Alkyne
C-H stretch
3310 cm–1
48
Alkyne
C-H bend overtone
1260 cm–1
Alkyne
C-H bend
630 cm–1
Aromatic Rings
• C=C-H stretch occurs at value 3100-3000 cm-1
• C=C-H out of plane bending occurs at 900–690 cm-1
– intense bands, strongly coupled to adjacent hydrogens on
the ring
– position and number of bands gives information about the
ring substitution pattern
• C=C stretch occurs in pairs: 1600-1585; 1500-1400 cm-1
• C=C out of plane ring bending: 600-420 cm-1
49
Aromatic rings
• Example: Toluene
Overtone bands
2000-1650 cm–1
Aromatic
C-H Stretches
3087, 3062,
3026 cm–1
Aromatic C-H in plane bends
1300-1000 cm–1
Aromatic C-C Stretches
1600-1585; 1500-1400 cm–1
50
Aromatic
C-H out of
Plane bends
728 cm–1
out of
plane
ring
bending
428 cm–1
694 cm-1
Alcohols and phenols
• The value is strongly dependent on hydrogen-bonding
– Free non-hydrogen bonded
OH
not H-bonded
O-H groups absorb strongly in
even when 'neat'
too hindered
1
the 3700-3600 cm- range
– H-bonded O-H band is broad at 3400-3300 cm-1
• C-O-H bending appears at 1440-1220 cm-1 as broad and weak
peak often obscured by CH3 bending
• C-O (alcohol) stretching at 1260-1000 cm-1. Used to assign
primary secondary and tertiary alcohols)
• C-O (phenol) stretching at 1800-1260 cm-1
51
C–O stretching Vibrations of Alcohols
• Primary alcohol: 1050-1085 cm-1
• Secondary alcohol: 1085-1125 cm-1
• Tertiary alcohol: 1125-1200 cm-1
OH
1073 cm-1
OH
1110 cm–1
OH
52
1202 cm–1
Ethers
• C–O–C stretching bands are very prominent (1300-1000 cm-1)
due to strong dipole moment. C=O and O-H must be absent to
ensure C-O stretch is not due to ester or alcohol
• aliphatic ethers:
– strong band due to asymmetrical stretching, 1150-1085
cm-1 (usually 1125 cm-1)
– weak band due to symmetrical stretching (lower freq)
• Alkyl aryl ethers:
– asymmetrical stretch at 1275-1200 cm-1
– symmetrical stretch at 1075-1020 cm-1
• Vinyl alkyl ethers:
– asymmetrical stretch at 1225-1200 cm-1
– symmetrical stretch at 1075-1020 cm-1
53
C=O In Aldehydes
• C=O stretch appears at 1740-1725 cm1 for normal aliphatic
aldehydes
• Conjugation of C=O with a,b C=C: 1700-1680 cm1 for C=O
and 1640 cm1 for C=C
• Conjugation of C=O with phenyl: 1700-1660 cm1 for C=O
and 1600-1450 cm1 for the ring
• C-H stretch of aldehyde H ( in CHO): show pair of weak
bands at 2860-2800 cm1 and 2760-2700 cm1
54
C=O in Ketones
• C=O stretch: 1720-1708 for aliphatic ketones
• Conjugation of C=O with a,b C=C: 1700-1675 cm-1 for C=O and
1644-1617 cm1 for C=C
• Conjugation of C=O with phenyl: 1700-1680 cm-1 for C=O and
1600-1450 cm-1 for the ring
• In strained rings, interaction with the adjacent C-C bonds
increases the frequency of C=O stretching
55
C=O in Ketones
• 2-Heptanone
56
C=O in Carboxylic Acids
• O-H stretch usually very broad (H-bonding) occurs at
3400-2400 cm1 and often overlaps the C-H absorptions
• C=O stretch, strong broad, occurs at 1730-1700 cm1
• C-O stretch occurs in the range 1320-1210 cm1 in medium
intensity
57
C=O in Carboxylic Acids
• Hexanoic acid
Acids and alcohols
are well
distinguished
58
C=O in Esters




59
C=O stretch appears in range 1750-1735 for normal
aliphatic esters
Conjugation of C=O with a,b C=C: 1740-1715 cm1 for C=O
and 1640-1625 cm1 for C=C
Conjugation of C=O with phenyl: 1740-1715 cm1 for C=O
and 1600-1450 cm1 for the ring
C-O stretch in two or more bands, one stronger and
broader than the other, occurs at 1300-1000 cm1
O
C=O in Esters
O
1763 cm–1
1199, 1164, 1145 cm–1 C–O
60
C=O in Amides
• C=O stretch appears in range 1680-1630 cm1
• N-H stretch in primary amides (NH2) gives two bands near
3350 and 3180 cm1. Secondary amides have one band ca.
3300 cm1
• N-H bending occurs at ca. 1640-1550 cm1 for primary and
secondary amides
O
O
NH
61
O
NH
NH
C=O in Amides
O
Et
N
H
H
1662
(II)
cm-1 (I)
O
H3C
62
N
H
CH3
-1 (II)
1565
cm
1655 cm-1 (I)
Acid Chlorides
• C=O stretch appears in range 1810-1775 cm1 in unconjugated
chlorides. Conjugation lowers the frequency to 1780-1760 cm1
• C-Cl stretch occurs in the range 730-550 cm1
63
Alkyl Halides
• C-F: Stretch (strong) at 1400-1000 cm-1
• C-Cl: Stretch (strong) in aliphatic chlorides at 785-540 cm-1
• C-Br: Stretch (strong) in aliphatic bromides at 650-510 cm-1. Aryl
bromides absorb between 1075 and 1030 cm-1
• C-I: Stretch (strong) in aliphatic iodides at 600-485 cm-1
• CH2-X bending at 1300 -1150 cm-1
64
Anhydrides
• C=O stretch always has two bands at 1830-1800 cm1 and 17751740 cm1 with variable relative intensities
• C-O stretch (multiple bands) occurs in the range 1300-900 cm1
O
O
O
O
O
O
O
O
alkyl
O
alkyl
1818; 1750 cm–1
65
R
–1
1775; 1720 cm
R
1865; 1782 cm–1
Amines
• N-H stretch ca. 3500-3300 cm-1
• Primary amine have two bands, secondary amines have one
band while tertiary amines have no N-H stretch
• N-H bend in primary amines results in a broad band ca. 16401560 cm-1. Secondary amines absorbs near 1500 cm-1
• C-N stretch occurs near 1350-1000 cm-1
66
Amines
• Example: Propylamine
67
Others
• Nitrile: C Nmedium intensity, sharp absorption at 2250 cm1
• Imine -C=N- at ca. 1690 -1640 cm-1
• Aliphatic nitro compounds: strong asymmetrical stretch at 16001530(s) cm-1 and medium symmetrical stretch 1390 -1300(s) cm1
• Aromatic nitro compounds: strong asymmetrical stretch at 1550
-1490(s) cm-1 and strong symmetrical stretch 1355 -1315(s) cm-1
68
Remember…..
• The absence of an absorption band can often provide more
information about the structure of a compound than the
presence of a band
• Be careful to avoid focusing on selected absorption bands and
overlooking others
• Look for absorption bands in decreasing order of importance
69
Analysis of IR spectrum: What To Do!
• Look if a C=O group is present (1820-1660 cm-1)
• If present look for:
– Is O-H also present? (3400-2400 cm-1): acid
– Is N-H also present? (ca. 3400 cm-1): amide
– Is C-O also present? (1300-1000 cm-1): Ester
– Two C=O present? (1810-1760 cm-1): anhydride
– Is aldehyde C-H present? (2840 & 2720 cm-1): aldehyde
– If all of the above absent, then think of ketone!
• If C=O is absent:
– Is O-H (3400-3300 cm-1) and C-O (1300-1000 cm-1) present?
Alcohol C-O-H
– Is N-H (~3400 cm-1) present? amine
70
– Is C-O (1300-1000 cm-1) present? ether
Analysis of IR spectrum: Remember….
• Double bonds/aromatic rings:
– C=C (~1650 cm-1)
– aromatic C=C (1600-1450 cm-1)
– vinyl C-H (>3000 cm-1)
• Triple bonds
– C,N triple bond (~2250 cm-1)
– C,C triple bond (~2150 cm-1)
– acetylenic C-H (~3300 cm-1)
• Nitro groups
– N-O(1600-1530 & 1390-1300 cm-1)
• Hydrocarbons
71
Remember…..
• The C-H absorption(s) 3100 - 2850 cm-1
• Absorption above 3000 cm-1 indicates C=C, either alkene or
aromatic
• Confirm the aromatic ring by finding peaks at 1600 and 1500
cm-1 and C-H out-of-plane bending to give substitution patterns
below 900 cm-1
• Confirm alkenes with an absorption at 1640-1680 cm-1
• C-H absorption between 3000 and 2850 cm-1 is due to aliphatic
hydrogens
• If the main absorptions are approximately 2935 and 2860 cm-1
and there are also absorptions at 1470 and 720 cm-1 then the
compound probably contains a long linear aliphatic chain
72
Remember…..
• Hydroxy or Amino groups appear at 3650–3250 cm-1 while the
C-H stretch of a terminal alkyne (acetylene) exhibits a relatively
narrow absorption at 3300 cm-1
• If the main absorption band in the area is broad, the compound
probably contain a hydroxyl or amino group. For -NH2 a doublet
will be observed
• The C-O absorption between 1080 and 1300 cm-1. These peaks
are normally rounded like the O-H and N-H peak and are
prominent. Carboxylic acids, esters, ethers, alcohols and
anhydrides all containing this peak.
• A methyl group may be identified with C-H absorption at 1380
cm-1. This band is split into a doublet for isopropyl(gemdimethyl) groups
73
Good Luck!