Infrared Spectroscopy

Infrared Spectroscopy

Electromagnetic Radiation
The propagation of electromagnetic radiation in a vacuum is constant for all regions of the spectrum (= velocity of light): c =  ×  1 Å = 10 –10 m 1 nm = 10 –9 m 1 m = 10 –6 m Another unit commonly used is the wavenumber, which is linear with energy: Work by Einstein, Planck and Bohr indicated that electromagnetic radiation can be regarded as a stream of particles or quanta, for which the energy is given by the Bohr equation:

g-rays IR X-rays UV Microwave Radio Visible IR Spectroscopy
Introduction The entire electromagnetic spectrum is used by chemists: 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

An IR Spectrum A plot of % transmittance vs vibrational frequency in wavenumbers (cm-1) λ = wavelength υ = frequency  c = speed of light in a vacuum

Wavenumbers The higher the wavenumber, the shorter the wavelength.

An IR Spectrum from

An IR Spectrum The wavelength of IR radiation is in the micron range (compare to visible light in the nm range). The frequencies of IR radiation are more conveniently expressed by a wavenumber (cycles per cm), than by (cycles per 3 x cm).

Characteristic Vibrational Frequencies of Bonds
Bonds are not rigid but behave like a spring with a mass at either end. Obey Hooke’s Law: F = -kx This gives rise to a characteristic frequency for the vibration:

Characteristic frequency for the vibration: The frequency is affected by the masses of the atoms in the bond the strength of the bond

The lower the mass, the higher the vibrational frequency. Stretching frequencies for bonds to carbon: C-H > C-C > C-N > C-O

The stronger the bond, the higher the vibrational frequency. Stretching frequencies C≡C > C=C > C-C C≡N > C=N > C-N C≡O > C=O > C-O C(sp)-H > C(sp2)-H > C(sp3)-H

Number of Vibrational Frequencies in a Molecule
There are 3n-6 possible vibrational modes in a nonlinear molecule with no symmetry Symmetry reduces the number of possible vibrational modes. Water has 3 possible vibrational modes. Formaldehyde has 6.

The Fingerprint Region is Unique to the Molecule
In addition, the vibration of a particular bond in a molecule affects the whole molecule. The various harmonics of a bond vibration can combine and lead to a number of combinational bands. The intensity of these bands is usually 1/100 the intensity of the main vibrational absorptions. These make up the “fingerprint region.” (occur at <1250 cm-1)

Intensity of IR Absorptions
In order for a vibration mode to absorb in the infrared, the vibrational motion must cause a change in the dipole moment of the bond. The intensity of the IR “peaks” is proportional to the change in dipole moment that a bond undergoes during a vibration. C=O bonds absorb strongly. C=C bonds generally absorb much less.

How to Analyze an IR Spectrum
Pay the most attention to the strongest absorptions: -C=O -OH -NH2 -C≡N -NO2 Pay more attention to the peaks to the left of the fingerprint region (>1250 cm-1).

Pay the most attention to the strongest absorptions. Pay more attention to the peaks to the left of the fingerprint region (>1250 cm-1). Note the absence of certain peaks. Be wary of O-H peaks, water is a common contaminant.

Characteristic IR Wavenumbers
Functional group wavenumber (cm-1) sp3 C-H str ~ sp2 C-H str ~ sp C-H str ~3300 O-H str ~3300 (broad*) O-H str in COOH ~3000 (broad*) N-H str aldehyde C-H str ~2700, ~2800 *The peak is broad when H bonding is extensive. Otherwise, the peak can be sharp.

Characteristic IR Wavenumbers
Functional group wavenumber (cm-1) C=C isolated ~ C=C conjugated ~ C=C aromatic ~1600 C≡N just above 2200 C≡C just below 2200 C=O ester ~ C=O aldehyde, ketone, or acid ~1710 (aldehyde can run 1725) C=O amide

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.

N-O of NO2 absorption This is a distinctive, strong doublet that it pays to know ( & cm-1). Read the scale for the value of the wavenumbers (be able to interpolate), or Read the wavenumbers in the table provided.

IR Spectroscopy Introduction The IR Spectroscopic Process The quantum mechanical energy levels observed in IR spectroscopy are those of molecular vibration We perceive this vibration as heat When we say a covalent bond between two atoms is of a certain length, we are citing an average because the bond behaves as if it were a vibrating spring connecting the two atoms For a simple diatomic molecule, this model is easy to visualize:

Infrared region LIMIT OF RED LIGHT: 800 nm, 0.8 m, 12500 cm-1
LIMIT OF RED LIGHT: 800 nm, 0.8 m, cm-1 NEAR INFRARED: m, cm-1 MID INFRARED: m, cm-1 FAR INFRARED: m, cm-1 Divisions arise because of different optical materials and instrumentation.

Infrared radiation λ = 2.5 to 50 μm υ = 4000 to 200 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

Molecular spectra There are three basic types of optical spectra that we can observe for molecules: Electronic or vibronic spectra (UV-visible-near IR) (transitions between a specific vibrational and rotational level of one electronic state and a vibrational and rotational level of another electronic state) Vibrational or vibrational-rotational spectra (IR region) (transitions from the rotational levels of one vibrational level to the rotational levels of another vibrational level in the same electronic state) Rotational spectra (microwave region) (transitions between rotational levels of the same vibrational level of the same electronic state)

Vibrational spectra (II): Anharmonic oscillator model
The actual potential energy of vibrations fits the parabolic function fairly well only near the equilibrium internuclear distance. The Morse potential function more closely resembles the potential energy of vibrations in a molecule for all internuclear distances-anharmonic oscillator model. Fig. 12-1

EVIB = ( En+1 – En ) =h× osc
• The energy difference between the transition from n to n+1 corresponds to the energy of the absorbed light quantum • The difference between two adjacent energy levels gets smaller with increasing n until dissociation of the molecule occurs (Dissociation energy ED ) Note: Weaker transitions called “overtones” are sometimes observed. These correspond to =2 or 3, and their frequencies are less than two or three times the fundamental frequency (=1) because of anharmonicity. Typical energy spacings for vibrational levels are on the order of J. from the Bolzmann distribution, it can be shown that at room temperature typically 1% or less of the molecules are in excited states in the absence of external radiation. Thus most absorption transitions observed at room temperature are from the =0 to the =1 level.

The IR Spectroscopic Process There are two types of bond vibration:
IR Spectroscopy Introduction The IR Spectroscopic Process There are two types of bond vibration: 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 asymmetric symmetric H C H C H C H C scissor rock twist wag in plane out of plane

The IR Spectroscopic Process
Infrared Spectroscopy 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

The IR Spectroscopic Process
Infrared Spectroscopy 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

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%

IR source è sample è prism è detector
graph of % transmission vs. frequency => IR spectrum 100 %T v (cm-1)

toluene

IR spectra of ALKANES C—H bond “saturated” (sp3) cm-1 cm-1 -CH -CH “ and 1375 -CH(CH3) “ and 1370, 1385 -C(CH3) “ and 1370(s), 1395 (m)

n-pentane 2850-2960 cm-1 CH3CH2CH2CH2CH3 3000 cm-1 sat’d C-H

n-hexane CH3CH2CH2CH2CH2CH3

2-methylbutane (isopentane)

2,3-dimethylbutane

cyclohexane no 1375 cm-1 no –CH3

IR of ALKENES =C—H bond, “unsaturated” vinyl (sp2) cm-1 RCH=CH & R2C=CH cis-RCH=CHR (v) trans-RCH=CHR C=C bond cm-1 (v)

1-decene 3020-3080 cm-1 C=C 1640-1680 unsat’d C-H
& RCH=CH2 C=C

4-methyl-1-pentene & RCH=CH2

2-methyl-1-butene R2C=CH2

2,3-dimethyl-1-butene R2C=CH2

IR spectra BENZENEs =C—H bond, “unsaturated” “aryl” (sp2) cm-1 mono-substituted , ortho-disubstituted meta-disubstituted , (m) para-disubstituted (m) C=C bond 1500, 1600 cm-1

ethylbenzene 3000-3100 cm-1 Unsat’d C-H 690-710, 730-770 mono-
1500 & 1600 Benzene ring , mono-

o-xylene ortho

p-xylene (m) para

m-xylene meta , (m)

styrene no sat’d C-H 1640 C=C & RCH=CH2 mono

2-phenylpropene Sat’d C-H R2C=CH2 mono

p-methylstyrene para

IR spectra ALCOHOLS & ETHERS
C—O bond (b) cm-1 1o ROH 1050 2o ROH 1100 3o ROH 1150 ethers O—H bond (b) 

1-butanol (b) O-H C-O 1o CH3CH2CH2CH2-OH

2-butanol O-H C-O 2o

tert-butyl alcohol O-H C-O 3o

methyl n-propyl ether no O--H C-O ether

2-butanone  C=O ~1700 (s)

C9H12 1500 & 1600 benzene C-H unsat’d & sat’d mono
C9H12 – C6H5 = -C3H7 isopropylbenzene n-propylbenzene?

n-propylbenzene

isopropylbenzene isopropyl split

C8H6 C-H unsat’d 1500, 1600 benzene 3300 C8H6 – C6H5 = C2H C-H mono
phenylacetylene

C4H8 Unst’d C=C R2C=CH2 isobutylene CH3 CH3C=CH2

Which compound is this? 2-pentanone 1-pentanol 1-bromopentane 2-methylpentane 1-pentanol

What is the compound? 1-bromopentane 1-pentanol 2-pentanone 2-methylpentane 2-pentanone

In a “matching” problem, do not try to fully analyze each spectrum
In a “matching” problem, do not try to fully analyze each spectrum. Look for differences in the possible compounds that will show up in an infrared spectrum.

IR Spectroscopy The IR Spectrum 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)

High frequencies and high wavenumbers equate higher energy
IR Spectroscopy The IR Spectrum This unit is used rather than wavelength (microns) because wavenumbers are directly proportional to the energy of transition being observed – chemists like this, physicists hate it High frequencies and high wavenumbers equate higher energy is quicker to understand than Short wavelengths equate higher energy This unit is used rather than frequency as the numbers are more “real” than the exponential units of frequency IR spectra are observed for what is called the mid-infrared: cm-1 The peaks are Gaussian distributions of the average energy of a transition

IR Spectroscopy The IR Spectrum In general: 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 Stronger bonds will have higher energy oscillations Triple bonds > double bonds > single bonds in energy Energy/n of oscillation

The IR Spectrum – The detection of different bonds 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 The intensity of an IR band is affected by two primary factors: Whether the vibration is one of stretching or bending Electronegativity difference of the atoms involved in the bond For both effects, the greater the change in dipole moment in a given vibration or bend, the larger the peak. The greater the difference in electronegativity between the atoms involved in bonding, the larger the dipole moment Typically, stretching will change dipole moment more than bending

The IR Spectrum – The detection of different bonds It is important to make note of peak intensities to show the effect of these factors: Strong (s) – peak is tall, transmittance is low (0-35 %) Medium (m) – peak is mid-height (75-35%) Weak (w) – peak is short, transmittance is high (90-75%) * Broad (br) – if the Gaussian distribution is abnormally broad (*this is more for describing a bond that spans many energies) Exact transmittance values are rarely recorded

II. Infrared Group Analysis A. General The primary use of the IR spectrometer is to detect functional groups Because the IR looks at the interaction of the EM spectrum with actual bonds, it provides a unique qualitative probe into the functionality of a molecule, as functional groups are merely different configurations of different types of bonds 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)

II. Infrared Group Analysis A. General The four primary regions of the IR spectrum Bonds to H O-H single bond N-H single bond C-H single bond Triple bonds C≡C C≡N Double bonds C=O C=N C=C Single Bonds C-C C-N C-O Fingerprint Region 4000 cm-1 2700 cm-1 2000 cm-1 1600 cm-1 400 cm-1

Specific groups Alkanes – combination of C-C and C-H bonds Show various C-C stretches and bends between cm-1 (m) C-C bond between methylene carbons (CH2’s) cm-1 (m) C-C bond between methylene carbons (CH2’s) and methyl (CH3) cm-1 (m) Show sp3 C-H between cm-1 (s) cm -1 Octane

Specific groups Alkenes – addition of the C=C and vinyl C-H bonds C=C stretch occurs at cm-1 and becomes weaker as substitution increases vinyl C-H stretch occurs at cm-1 Note that the bonds of alkane are still present! The difference between alkane and alkene or alkynyl C-H is important! If the band is slightly above 3000 it is vinyl sp2 C-H or alkynyl sp C-H if it is below it is alkyl sp3 C-H 1-Octene

Specific groups Alkynes – addition of the C=C and vinyl C-H bonds C≡C stretch occurs between cm-1; the strength of this band depends on asymmetry of bond, strongest for terminal alkynes, weakest for symmetrical internal alkynes (w-m) C-H for terminal alkynes occurs at cm-1 (s) Remember internal alkynes ( R-C≡C-R ) would not have this band! 1-Octyne

Specific groups Aromatics Due to the delocalization of electrons in the ring, where the bond order between carbons is 1 ½, the stretching frequency for these bonds is slightly lower in energy than normal C=C These bonds show up as a pair of sharp bands, 1500 (s) & cm-1 (m), where the lower frequency band is stronger C-H bonds off the ring show up similar to vinyl C-H at cm-1 (m) Ethyl benzene

Specific groups Aromatics If the region between cm-1 (w) is free of interference (C=O stretching frequency is in this region) a weak grouping of peaks is observed for aromatic systems Analysis of this region, called the overtone of bending region, can lead to a determination of the substitution pattern on the aromatic ring Monosubstituted 1,2 disubstituted (ortho or o-) 1,2 disubstituted (meta or m-) 1,4 disubstituted (para or p-)

Specific groups Unsaturated Systems – substitution patterns The substitution of aromatics and alkenes can also be discerned through the out-of-plane bending vibration region However, other peaks often are apparent in this region. These peaks should only be used for reinforcement of what is known or for hypothesizing as to the functional pattern.

Specific groups Ethers – addition of the C-O-C asymmetric band and vinyl C-H bonds Show a strong band for the antisymmetric C-O-C stretch at cm-1 Otherwise, dominated by the hydrocarbon component of the rest of the molecule Diisopropyl ether

Specific groups Alcohols Show a strong, broad band for the O-H stretch from cm-1 (s, br) this is one of the most recognizable IR bands Like ethers, show a band for C-O stretch between cm-1 (s) This band changes position depending on the substitution of the alcohol: 1° ; 2° ; 3° ; phenol The shape is due to the presence of hydrogen bonding 1-butanol

Specific groups Amines - Primary Shows the –N-H stretch for NH2 as a doublet between cm-1 (s-m); symmetric and anti-symmetric modes -NH2 group shows a deformation band from cm-1 (w) Additionally there is a “wag” band at cm-1 that is not diagnostic 2-aminopentane

Specific groups Amines – Secondary -N-H band for R2N-H occurs at cm-1 (br, m) 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 pyrrolidine

Cyclohexyl carboxaldehyde
Infrared Spectroscopy Specific groups Aldehydes Show the C=O (carbonyl) stretch from cm-1(s) Band is sensitive to conjugation, as are all carbonyls (upcoming slide) Also displays a highly unique sp2 C-H stretch as a doublet, 2720 & 2820 cm-1 (w) called a “Fermi doublet” Cyclohexyl carboxaldehyde

Specific groups Ketones Simplest of the carbonyl compounds as far as IR spectrum – carbonyl only C=O stretch occurs at cm-1 (s) 3-methyl-2-pentanone

Specific groups Esters C=O stretch occurs at cm-1 (s) Also displays a strong band for C-O at a higher frequency than ethers or alcohols at cm-1 (s) Ethyl pivalate

Specific groups Carboxylic Acids: Gives the messiest of IR spectra C=O band occurs between cm-1 The highly dissociated O-H bond has a broad band from cm-1 (m, br) covering up to half the IR spectrum in some cases 4-phenylbutyric acid

Specific groups Acid anhydrides Coupling of the anhydride though the ether oxygen splits the carbonyl band into two with a separation of 70 cm-1. Bands are at cm-1 and cm-1 (s) Mixed mode C-O stretch at cm-1 (s) Propionic anhydride

Specific groups Acid halides Dominant band at cm-1 for C=O (s) Bonds to halogens, due to their size (see Hooke’s Law derivation) occur at low frequencies, where their presence should be used to reinforce rather than be used for diagnosis, C-Cl is at cm-1 (m) Propionyl chloride

Specific groups Amides Display features of amines and carbonyl compounds C=O stretch occurs from cm-1 If the amide is primary (-NH2) the N-H stretch occurs from cm-1 as a doublet If the amide is secondary (-NHR) the N-H stretch occurs at cm-1 as a sharp singlet pivalamide

Specific groups Nitro group (-NO2) Proper Lewis structure gives a bond order of 1.5 from nitrogen to each oxygen Two bands are seen (symmetric and asymmetric) at cm-1 (m-s) and cm-1 (m-s) This group is a strong resonance withdrawing group and is itself vulnerable to resonance effects 2-nitropropane

Specific groups – Hydrocarbons Nitriles (the cyano- or –C≡N group) Principle group is the carbon nitrogen triple bond at cm-1 (s) This peak is usually much more intense than that of the alkyne due to the electronegativity difference between carbon and nitrogen propionitrile

Infrared Spectroscopy

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