Infrared spectroscopy (IR)

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Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional group.

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Infrared Spectroscopy

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Spectroscopy is an instrumentally aided studies of the interactions between matter (sample being analyzed) and energy (any portion of the electromagnetic spectrum , EMS ) EMS refers to the seemingly diverse collection of radiant energy, from cosmic rays to X-rays to visible light to microwaves, each of which can be considered as a wave or particle traveling at the speed of light.

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=> EMS and Molecular Effects

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Energy (E) E = h n = hc / l = hc n ’ where h is Planck’s constant, c is the speed of light, n is frequency or the number of vibrations per second and l is the wavelength Wavenumber ( n ’) n ’ = 1/ l given in cm -1 Period (P) P = 1/ n the time between a vibration Energy, frequency, and wavenumber are directly proportional to each other.

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Method Abbrev. Energy used Units Ultraviolet-Visible Spectroscopy UV-Vis ultraviolet-visible nm Infrared Spectroscopy IR infrared mm or cm -1 Nuclear Magnetic Resonance NMR radio frequencies Hz Mass Spectroscopy MS electron volts amu The four most common spectroscopic methods used in organic analysis are: What actually happens to the sample during an analysis? {How do the sample and energy “interact”?}

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What happens when a sample absorbs UV/Vis energy? excitation of ground state electrons (typically p and n electrons) E electronic increases momentarily What happens when a sample absorbs IR energy? stretching and bending of bonds (typically covalent bonds) E vibration increases momentarily UV/Vis p p * sample p  p * transition IR - O - H - O ( 3500 cm - 1 ) — H ( 200 nm) Matter/Energy Interactions

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Infrared spectroscopy ( IR ) measures the bond vibration frequencies in a molecule and is used to determine the functional group

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Just below red in the visible region Wavelengths usually 2.5-25 m m More common units are wavenumbers, or cm -1 , the reciprocal of the wavelength in centimeters (10 4 / m m = 4000-400 cm -1 ) Wavenumbers are proportional to frequency and energy The IR Region The IR region is divided into three regions: the near, mid, and far IR. The mid IR region is of greatest practical use to the organic chemist.

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Molecular Vibrations and IR Spectroscopy

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Molecules are made up of atoms linked by chemical bonds. The movement of atoms and chemical bonds like spring and balls (vibration)

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What is a vibration in a molecule? Any change in shape of the molecule- stretching of bonds, bending of bonds, or internal rotation around single bonds Vibrations There are two main vibrational modes : Stretching - change in bond length (higher frequency) Stretching vibration

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Stretching Types Symmetric Asymmetric Bending Types In-plane ( Scissoring ) 2. Bending - change in bond angle (lower frequency) Out-plane ( Twisting )

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Modes of vibrations Stretching: change in bond distance. Occurs at higher energy: 4000-1250 cm  1 . -CH 2 - H 2 O Bending: change in bond angle. Occurs at lower energy: 1400-666 cm  1 .

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More complex types of stretching and bending are possible Can a vibration change the dipole moment of a molecule? Infrared active vibrations (those that absorb IR radiation) must result in a change of dipole moment Asymmetrical stretching/bending and internal rotation change the dipole moment of a molecule. Asymmetrical stretching/bending are IR active. Symmetrical stretching/bending does not. Not IR active

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Fundamental Vibrations (Absorption Frequencies) A molecule has as many as degrees of freedom as the total degree of freedom of its individual atoms. Each atom has 3 degree of freedom ( x , y , z ) three Cartesian coordinate axes. A molecule of n atoms therefore has 3n degrees of freedom. Non linear molecules (e.g. H 2 O) Vibrational degrees of freedom or Fundamental Vibrations = 3n – 6 Symmetrical Stretching ( υ s OH) 3652 cm -1 Asymmetrical Stretching ( υ as OH) 3756 cm -1 Scissoring ( δ s HOH) 1596 cm -1

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For linear molecule (e.g. CO 2 ) : Vibrational degrees of freedom or Fundamental Vibrations = 3n – 5 Symmetrical Stretching ( υ s CO 2 ) 1340 cm -1 Asymmetrical Stretching ( υ as CO 2 ) 2350 cm -1 Scissoring (bending out of the plane of the paper) ( δ s CO 2 ) 666 cm -1 Scissoring (bending in the plane of the paper) ( δ s CO 2 ) 666 cm -1

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The total number of observed absorption bands is generally different from the total number of fundamental vibrations. It is reduced because some modes are not IR active and a single frequency can cause more than one mode of motion to occur. Conversely, additional bands are generated by the: appearance of overtones (integral multiples of the fundamental absorption frequencies), combinations of fundamental frequencies, differences of fundamental frequencies, coupling interactions of two fundamental absorption frequencies, coupling interactions between fundamental vibrations and overtones or combination bands (Fermi resonance).

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The intensities of overtone, combination, and difference bands are less than those of the fundamental bands. The combination and blending of all the factors thus create a unique IR spectrum for each compound.

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The theoretical no. of fundamental vibrations will seldom be observed because overtones (multiples of a given frequencies) and combination tones (sum of two other vibrations) increase the no. of bands . Other phenomena reduce the no. of bands including: Fundamental frequencies that fall outside the 4000-400 cm -1 region. Fundamental bands that are too weak to be observed. Fundamental bands that are so close that they coalesce. The occurrence of a degenerate band from several absorptions of the same frequency in highly symmetrical molecules. The failure of certain fundamental vibrations to appear in the IR because of the lack of change in molecular dipole.

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Which of the following atoms or molecules will absorb IR radiation: I—Cl H 2 N 2 Cl 2 Why?

IR-Active and Inactive:

IR-Active and Inactive A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

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How does the mass influence the vibration? H 2 I 2 MM =2 g/mole MM =254 g/mole The greater the mass - the lower the wavenumber ( ύ) For a vibration at 4111 cm -1 (the stretch in H 2 ), how many vibrations occur in a second? 120 x 10 12 vibrations/sec or a vibration every 8 x 10 -15 seconds! 120 trillion vibration per second!!!!

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Hooke’s Law ύ = The vibration frequency (cm -1 ) c = Velocity of light (cm/s) f = force constant of bond (dyne/cm) M 1 and M 2 are mass (g) of atom M 1 and M 2

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The relative contributions of bond strength is also considered in vibrational frequencies. In general functional groups that have a strong dipole give rise to strong absorptions in the IR.

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= 1 2 p c n K m m = m 1 m 2 m 1 + m 2 n = frequency in cm -1 c = velocity of light K = force constant in dynes/cm m = atomic masses SIMPLE HARMONIC OSCILLATOR > > multiple bonds have higher K’s m = reduced mass ( 3 x 10 10 cm/sec ) THE EQUATION OF A This equation describes the vibrations of a bond. where

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= 1 2 p c n K m larger K, higher frequency larger atom masses, lower frequency constants 2150 1650 1200 C=C > C=C > C-C = C-H > C-C > C-O > C-Cl > C-Br 3000 1200 1100 750 650 increasing K increasing m

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Frequency decreases with increasing atomic mass. Frequency increases with increasing bond energy . Stretching Frequencies

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Instrumentation schematic diagram of a double beam double-grating infrared spectrophotometer 1. Radiation source 2. Monochromator 3. Solvents, sample cells, samples 4. Readout / Recorder

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black-body heat sources

Discriminator:

Discriminator Most IR today use an interferometer, where the spectral encoding takes place, instead of a monochromator . The Michelson interferometer is a multiplex system with a simple design: a fixed mirror, a moving mirror, an optical beam splitter.

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Schematic diagram of the inside of an FTIR spectrometer showing the position of the Michelson interferometer

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The source radiation hits the beam splitter from where some of the light is reflected to the moving mirror and some is transmitted to the fixed mirror. The mirrors reflect the light back to the beam splitter, some of which recombines and goes on to the detector. The key point of the moving mirror is to generate a difference in the optical paths of the two paths of light separated by the beam splitter; consequently, one is slightly out of phase from the other since it travels a slightly different distance.

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The recombined light produces an interference spectrum of all the wavelengths in the beam before passing through the sample. In other words, the sample sees all the wavelengths simultaneously and the interference pattern changes with time as the mirror is continuously scanned at a linear velocity.

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The result of the sample absorbing radiation is a spectrum in the time domain called an interferogram . Fourier transformation (FT) converts this very complex signal to the frequency domain. Combined FTIR and Raman spectrometers based on the Michelson interferometer are commercially available. Depending on the spectral regions of interest required, it can be operated with a single calcium fluoride (CaF 2 ) beam splitter or a combination of materials with an automatic changer.

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As with IR, both dispersive and FT–Raman instruments exist. The main advantage of FT–Raman is the ability to change from using visible to NIR excitation with an associated reduction in broad-band fluorescence. Secondly, an FTIR instrument can be configured to carry out FT–Raman experiments without significant effort. However, Raman instruments that do not employ an interferometer are still commonly used owing to their high radiation throughput and lower cost.

Interferogram:

Interferogram The interferogram displays the interference pattern and contains all of the spectrum information. A Fourier transform converts the time domain to the frequency domain with absorption as a function of frequency.

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Solvents Must be transparent in the region studied: no single solvent is transparent throughout the entire IR region 2. Water and alcohols are seldom employed to avoid O-H band of water . 3. Must be chemically inert (does not react with substance or cell holder). CCl 4 , CS 2 , or CHCl 3 ; may be used but we should consider its IR spectrum SOLVENTS, CELLS, SAMPLES Cells NaCl or KCl cells may be used (moisture from air and sample should be avoided: even with care, their surfaces eventually become fogged due to absorption of moisture) Very thin (path length = 0.1 to 1.0 mm) Sample concentration = about 0.1 – 10%

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Samples 1. Solid KBr disk (1 mg solid sample + 100 mg KBr pressed into a disk) Mull : 1 mg solid sample suspended in Nujol (heavy liquid hydrocarbon) 2. Liquid Neat (thin film of liquid between two NaCl plates solution in CCl 4 and put in special NaCl cells. 3. Gas IR spectrum is obtained directly by permitting the sample to expand into an evacuated special cells.

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Special sampling techniques for solids: Hard materials can be compressed between two parallel diamond faces to produce a thin film as diamond transmits most of the mid IR. Diffuse reflectance: that is used to obtain IR spectral information on the surface of a sample. Here reflected radiation instead of transmitted radiation is measured. Attenuated total reflectance (ATR): Another technique in which the sample is brought into close contact with the surface of a prism made of a material with a high refractive index, e.g. sapphire. A light beam approaching the interface from the optically denser medium at a large enough angle of incidence is totally reflected. However, the beam does penetrate a small distance into the optically thinner medium (the sample) . If the sample absorbs IR radiation, an IR spectrum can be obtained. By changing the angle of incidence, depth profiling can be achieved. The film (∼200 nm) is made thin enough to allow light to pass through to the adjoining aqueous phase.

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optically denser medium optically thinner medium Diagram of an attenuated total reflectance (ATR) cell

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Identification of functional groups on a molecule – this is a very important tool in organic chemistry Spectral matching can be done by computer software and library spectra Since absorbance follows Beer’s Law, can do quantitative analysis Use of IR spectra

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Applications Infrared spectrometry can be used for testing emissions . For example, IR has been reported for determining carbon monoxide and nitrogen oxide and other trace gases, remote sensing of volcanic gases and trace analysis of halocarbons in the atmosphere, among many other applications. IR is also employed in the beverage industry for monitoring alcohol, sugar and water content in drinks, and sugars, fibres and acidity in juices. In the food industry , IR spectrometry is very useful for determining protein, oil, ash, moisture and particle size in flour, for following fermentation and microbiological reactions and for the study of microorganisms in food products.

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An IR spectrum is a plot of per cent transmittance (or absorbance) against wavenumber (frequency or wavelength). A typical infrared spectrum is shown below. A 100 per cent transmittance in the spectrum implies no absorption of IR radiation. When a compound absorbs IR radiation, the intensity of transmitted radiation decreases. This results in a decrease of per cent transmittance and hence a dip in the spectrum. The dip is often called an absorption peak or absorption band. FEATURES OF AN IR SPECTRUM Different types of groups of atoms (C-H, O-H, N-H, etc…) absorb infrared radiation at different characteristic wavenumbers .

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No two molecules will give exactly the same IR spectrum (except enantiomers) Simple stretching: 1600-3500 cm -1 Complex vibrations: 400-1400 cm -1 , called the “fingerprint region” Baseline Absorbance/Peak IR Spectrum

Infrared Spectrum of Propanal (CH3CH2CHO) :

Infrared Spectrum of Propanal (CH 3 CH 2 CHO)

Describing IR Absorptions:

Describing IR Absorptions IR absorptions are described by their frequency and appearance. Frequency ( n ) is given in wavenumbers (cm -1 ) Appearance is qualitative: intensity and shape conventional abbreviations: vs very strong s strong m medium w weak br broad sh sharp OR shoulder

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In general, the IR spectrum can be split into four regions for interpretation: 4000  2500 cm -1 : Absorption of single bonds formed by hydrogen and other elements e.g. O  H , N  H , C  H 2500  2000 cm -1 : Absorption of triple bonds e.g. C ≡ C , C ≡ N 2000  1500 cm -1 : Absorption of double bonds e.g. C = C , C = O 1500  400 cm -1 : This region often consists of many different, complicated bands. This part of the spectrum is unique to each compound and is often called the fingerprint region . It is rarely used for identification of particular functional groups.

Summary of IR Absorptions:

Summary of IR Absorptions

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BASE VALUES (+/- 10 cm -1 ) These are the minimum number of values to memorize. O-H 3600 N-H 3400 C-H 3000 C N 2250 C C 2150 C=O 1715 C=C 1650 C O ~1100 large range

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O-H STRETCH

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Typical Infrared Absorption Regions O-H 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

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O-H 3600 cm -1 (alcohol, free) O-H 3300 cm -1 (alcohols & acids, H-bonding) 3600 3300 H-BONDED FREE broadens shifts The O-H stretching region

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HYDROGEN-BONDED HYDROXYL Many kinds of OH bonds of different lengths and strengths This leads to a broad absorption. Longer bonds are weaker and lead to lower frequency . Hydrogen bonding occurs in concentrated solutions ( for instance, undiluted alcohol ). “Neat” solution.

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“FREE” HYDROXYL Distinct bond has a well-defined length and strength. Occurs in dilute solutions of alcohol in an “inert” solvent like CCl 4 . Solvent molecules surround but do not hydrogen bond. The “free” hydroxyl vibrates without interference from any other molecule.

Cyclohexanol:

Cyclohexanol O-H H-bond C-H C-O CH 2 ALCOHOL neat solution ,

Butanoic Acid:

Butanoic Acid O-H H-bond C-H C=O CH 2 C-O CARBOXYLIC ACID neat solution

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CARBOXYLIC ACID DIMER Strong hydrogen bonding in the dimer weakens the OH bond and leads to a broad peak at lower frequency.

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N-H STRETCH

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Typical Infrared Absorption Regions N-H 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

The N-H stretching region:

The N-H stretching region Primary amines give two peaks Secondary amines give one peak Tertiary amines give no peak symmetric asymmetric N-H 3300 - 3400 cm -1

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NH 2 NH 2 scissor CH 2 CH 3 PRIMARY AMINE aliphatic 1-Butanamine

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NH 2 Ar-H -CH 3 benzene Ar-H PRIMARY AMINE aromatic 3-Methylbenzenamine

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NH benzene Ar-H CH 3 SECONDARY AMINE N -Ethylbenzenamine

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no N-H benzene CH 3 Ar-H Ar-H -CH 3 TERTIARY AMINE N,N -Dimethylaniline

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C-H STRETCH

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Typical Infrared Absorption Regions C-H 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O * We will look at this area first

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C-H aldehyde, two peaks (both weak) ~ 2850 and 2750 cm -1 3000 divides UNSATURATED SATURATED C-H sp stretch ~ 3300 cm -1 C-H sp 2 stretch > 3000 cm -1 C-H sp 3 stretch < 3000 cm -1 The C-H stretching region BASE VALUE = 3000 cm -1

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3000 -C-H =C-H 3100 3300 =C-H = 2900 2850 2750 -CH=O (weak) increasing CH Bond Strength sp 3 -1s sp 2 -1s sp-1s increasing frequency (cm -1 ) aldehyde increasing s character in bond increasing force constant K STRONGER BONDS HAVE LARGER FORCE CONSTANTS AND ABSORB AT HIGHER FREQUENCIES CH BASE VALUE = 3000 cm -1

Hexane:

Hexane CH stretching vibrations ALKANE includes CH 3 sym and asym CH 2 sym and asym CH bending vibrations discussed shortly

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C-H BENDING

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CH 2 bending ~ 1465 cm -1 CH 3 bending (asym) appears near the CH 2 value ~ 1460 cm -1 CH 3 bending (sym) ~ 1375 cm -1 THE C-H BENDING REGION

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Scissoring Wagging Rocking Twisting Bending Vibrations ~1465 cm -1 ~720 cm -1 ~1250 cm -1 ~1250 cm -1 in-plane out-of-plane METHYLENE GROUP BENDING VIBRATIONS

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CH 3 CH 2 1465 1460 1375 asym sym METHYLENE AND METHYL BENDING VIBRATIONS these two peaks frequently overlap and are not resolved C-H Bending, look near 1465 and 1375 cm -1

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CH 3 CH 2 1465 1460 1375 asym sym 1370 1380 1370 1390 METHYLENE AND METHYL BENDING VIBRATIONS geminal dimethyl t-butyl (isopropyl) two peaks two peaks The sym methyl peak splits when you have more than one CH 3 attached to a carbon. ADDITIONAL DETAILS FOR SYM CH 3 one peak

Hexane:

Hexane CH stretch CH 2 bend CH 3 bend CH 2 rocking ALKANE

1-Hexene:

1-Hexene =CH CH C=C CH 2 CH 3 bend CH ALKENE

Toluene:

Toluene C=C benzene CH 3 Ar-H Ar-H AROMATIC

1-Hexyne:

1-Hexyne C=C = =C-H = C-H CH 2 , CH 3 ALKYNE

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C 10 H 22 C 12 H 26 Similar But Not Identical Fingerprinting

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C N AND C C STRETCH

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Typical Infrared Absorption Regions C=N C=C = = 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

The triple bond stretching region:

The triple bond stretching region C N 2250 cm -1 C C 2150 cm -1 = = = = The cyano group often gives a strong, sharp peak due to its large dipole moment. The carbon-carbon triple bond gives a sharp peak, but it is often weak due to a lack of a dipole. This is especially true if it is at the center of a symmetric molecule.

Propanenitrile:

Propanenitrile C=N = NITRILE BASE = 2250

1-Hexyne:

1-Hexyne C=C = =C-H = ALKYNE BASE = 2150

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C=O STRETCHING

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Typical Infrared Absorption Regions C=O 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

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This region stretches from about 1800 to 1650 cm -1 - RIGHT IN THE MIDDLE OF THE SPECTRUM The base value is 1715 cm -1 (ketone) The bands are very strong !!! due to the large C=O dipole moment. C=O is often one of the strongest peaks in the spectrum THE CARBONYL STRETCHING REGION

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2-Butanone KETONE C=O C-H overtone 2x C=O CH bend BASE = 1715 1715 C=O

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1690 1710 1715 1725 1735 1800 1810 and 1760 BASE VALUE acid chloride ester aldehyde carboxylic acid amide ketone anhydride ( two peaks ) EACH DIFFERENT KIND OF C=O COMES AT A DIFFERENT FREQUENCY C=O IS SENSITIVE TO ITS ENVIRONMENT THESE VALUES ARE WORTH LEARNING all are +/- 10 cm -1

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1.225 A 1.231 A 1.235 A 1.248 A acid chloride ester ketone amide C=O BOND LENGTHS IN CARBONYL COMPOUNDS shorter longer 1780 cm -1 1735 cm -1 1715 cm -1 1680 cm -1

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Ketones are at lower frequency than Aldehydes because of the second electron-donating alkyl group. Acid chlorides are at higher frequency than ketones because of the electron-withdrawing halide. Esters are at higher frequencies than ketones due to the electron-withdrawing oxygen atom. This is more important than resonance with the electron pair on the oxygen. Amides are at lower frequencies than ketones due to resonance involving the unshared pair on nitrogen. The electron-withdrawing effect of nitrogen is less important than the resonance. SUMMARY Note the electronegativity difference, O versus N, weights the two factors (resonance/ e-withdrawal) differently in esters than in amides. Acids are at lower frequency than ketones due to H-bonding.

2-Butanone:

KETONE C=O C-H overtone CH bend BASE = 1715 2-Butanone 1719 x 2 = 3438 overtone of strong C=O peak 3438

Nonanal:

ALDEHYDE C=O CHO CH bend BASE = 1725 Nonanal 3460

Dodecanoyl Chloride:

ACID CHLORIDE C=O C-H CH bend BASE = 1800 Dodecanoyl Chloride 3608

Ethyl Butanoate:

ESTER C=O C-O C-H BASE = 1735 Ethyl Butanoate 3482

2-Methylpropanoic Acid:

CARBOXYLIC ACID C=O O-H C-H C-O BASE = 1710 2-Methylpropanoic Acid

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CARBOXYLIC ACID DIMER Strong hydrogen-bonding in the dimer weakens the O-H and C=O bonds and leads to broad peaks at lower frequencies. lowers frequency of C=O and also of O-H RECALL

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AMIDE C=O NH 2 C-H CH bend BASE = 1690 two peaks sym / asym Propanamide

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C=C STRETCHING ALKENES AROMATICS

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Typical Infrared Absorption Regions C=C 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

The C=C stretching region:

The C=C stretching region C=C double bond at 1650 cm -1 is often weak or not even seen. C=C benzene ring shows peak(s) near 1600 and 1400 cm -1 , one or two at each value - CONJUGATION LOWERS THE VALUE . When C=C is conjugated with C=O it is stronger and comes at a lower frequency.

1-Hexene:

1-Hexene ALKENE oops C=C =C-H C-H aliphatic C-H bend

Toluene:

Toluene AROMATIC benzene oops benzene C-H C=C

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Typical Infrared Absorption Regions C-O 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

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C - O STRETCHING

The C-O stretching region:

The C- O stretching region The C - O band appears in the range of 1300 to 1000 cm -1 Look for one or more strong bands appearing in this range! Ethers, alcohols, esters and carboxylic acids have C - O bands

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ETHER C-O BASE = 1100 CH 2 CH 3 bending C-H Dibutyl Ether

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AROMATIC ETHER benzene oops C-O C-H aromatic BASE = 1100 Anisole

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ALCOHOL BASE = 3600 BASE = 1100 OH C-O CH 2 bend C-H Cyclohexanol

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CARBOXYLIC ACID OH CH C=O C-O 2-Methylpropanoic Acid

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ESTER CH C=O C-O Ethyl Butanoate

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N = O STRETCHING

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Typical Infrared Absorption Regions N-O 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

The N=O stretching region:

The N = O stretching region N= O stretching -- 1550 and 1350 cm -1 asymmetric and symmetric stretchings Often the 1550 cm -1 peak is stronger than the other one

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NITROALKANE N=O N=O C-H gem-dimethyl 2-Nitropropane

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Typical Infrared Absorption Regions C-Cl 2.5 4 5 5.5 6.1 6.5 15.4 4000 2500 2000 1800 1650 1550 650 FREQUENCY (cm -1 ) WAVELENGTH ( m m) O-H C-H N-H C=O C=N Very few bands C=C C-Cl C-O C-N C-C X=C=Y (C,O,N,S) C N C C N=O N=O *

The C-X stretching region:

The C-X stretching region C - Cl 785 to 540 cm -1 , often hard to find amongst the fingerprint bands!! C - Br and C - I appear outside the useful range of infrared spectroscopy. C - F bonds can be found easily, but are not that common.

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Often used as a solvent for IR spectra. When it is used, spectra show C - Cl absorptions. C-Cl Carbon Tetrachloride

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C-Cl oops benzene C=C benzene ring combination bands Chlorobenzene

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=C-H OUT OF PLANE BENDING

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H H H OUT-OF-PLANE BENDING above below PLANE (OOPS) H ALKENES BENZENES also with

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Monosubstituted cis -1,2- trans -1,2- 1,1- Trisubstituted Tetrasubstituted Disubstituted 10 11 12 13 14 15 1000 900 800 700 cm -1 s s m s s s m =C-H OUT OF PLANE BENDING ALKENES

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10 11 12 13 14 15 1000 900 800 700 cm -1 Monosubstituted Disubstituted ortho meta para Trisubstituted 1,2,4 1,2,3 1,3,5 BENZENES m s s s s s s s s s m m m m OOPS RING H’s combination bands

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