Infrared (IR).

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Presentation transcript:

Infrared (IR)

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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. Chemists can use portions of the EMS to selectively manipulate the energies contained within a molecule, to uncover detailed evidence of its chemical structure and bonding.  

electromagnetic relationships: λv = c λ µ 1/v E = hv E µ v   E = hc/λ E µ 1/λ λ = wave length v = frequency c = speed of light E = kinetic energy h = Planck’s constant (3.99X10-13 kj*s/mol or 9.54 X 10 -14 kcal*s/mol λ c

The IR Region Just below red in the visible region (380 nm-780nm) Wavelengths usually 2.5-25 mm More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters (104/mm = 4000-400 cm-1) Wavenumbers are proportional to frequency and energy Higher wavenumber higher energy

Vibration and rotation Electromagnetic Spectrum X-ray diffractometry Ultraviolet - Visible Infrared Microwave NMR Pi and non-bonding electrons Molecular Vibration and rotation Molecular Rotation nucleus Proton Spin valence electrons Mass Spectrometry

Infrared radiation λ = 2.5 to 17 μm (wavelenght) v = 4000 to 600 cm-1 (frequency)  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

Energy, frequency, and wavenumber are directly proportional to each other. Infrared spectroscopy (IR) measures the bond vibration frequencies in a molecule and is used to determine the functional group What happens when a sample absorbs IR energy? stretching and bending of bonds (typically covalent bonds) Evibration increases momentarily IR -O-H -O (3500 cm-1) —H

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

There are two main vibrational modes : Vibrations 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 There are two main vibrational modes : Stretching - change in bond length (occurs at higher frequencies; 4000-1250 cm1) Stretching vibration

2. Bending - change in bond angle ( occurs at lower frequency; 1400-666 cm1)

Fingerprint of Molecule No two molecules will give exactly the same IR spectrum (except enantiomers). Simple stretching: 1600-3500 cm-1. Complex vibrations: 600-1400 cm-1, called the “fingerprint region.”

IR Spectrum Regions

IR-Active IR-Inactive The bond undergoing the vibration must be polar A polar bond is usually IR-active. The greater the polarity of the bond, the more intense the absorption The bond’s vibration must cause a change in dipole Asymmetrical stretching/bending change the dipole moment of a molecule. IR-Inactive A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

Interpretation of IR Spectrum Use Correlation tables Looking for presence/absence of functional groups A polar bond is usually IR-active A nonpolar bond in a symmetrical molecule will absorb weakly or not at all

Carbon-Carbon Bond Stretching Stronger bonds absorb at higher frequencies: C-C 1200 cm-1 C=C 1660 cm-1 CC 2200 cm-1 (weak or absent if internal) Conjugation lowers the frequency: isolated C=C 1640-1680 cm-1 conjugated C=C 1620-1640 cm-1 aromatic C=C approx. 1600 cm-1

Carbon-Hydrogen Stretching Bonds with more s character absorb at a higher frequency. sp3 C-H, just below 3000 cm-1 (to the right) sp2 C-H, just above 3000 cm-1 (to the left) sp C-H, at 3300 cm-1

Figure 12.15: The four regions of the infrared spectrum: single bonds to hydrogen, triple bonds, double bonds, and fingerprint.

FEATURES OF AN IR SPECTRUM An IR spectrum is a plot of per cent transmittance (or absorbance) against wavenumber (frequency or wavelength). A typical infrared spectrum is shown below on the next slide 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. Different types of groups of atoms (C-H, O-H, N-H, etc…) absorb infrared radiation at different characteristic wavenumbers

Undecane (alkane) wavelenght frequency

1-hexene (alkene) wavelenght frequency

1-hexyne (alkyne)

O-H and N-H Stretching Both of these occur around 3300 cm-1, but they look different Alcohol O-H, broad with rounded tip Secondary amine (R2NH), broad with one sharp spike Primary amine (RNH2), broad with two sharp spikes No signal for a tertiary amine (R3N)

An Alcohol IR Spectrum

An Amine IR Spectrum

Carbonyl Stretching The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1 Usually, it’s the strongest IR signal Carboxylic acids will have O-H also Aldehydes have two C-H signals around 2700 and 2800 cm-1

A Ketone IR Spectrum

O-H Stretch of a Carboxylic Acid This O-H absorbs broadly, 2500-3500 cm-1, due to strong hydrogen bonding

An Aldehyde IR Spectrum

Variations in C=O Absorption Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1 The C=O group of an amide absorbs at an even lower frequency, 1640-1680 cm-1 The C=O of an ester absorbs at a higher frequency, ~1730-1740 cm-1 Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency

An Amide IR Spectrum

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

Summary of IR Absorptions Chapter 12 => =>

PRACTICE Identify the possible functional group or possible functional groups

Identify the functional group Alcohol Ether Ketone Copyright © Houghton Mifflin Company.All rights reserved.

Practice Identify the possible functional group or possible functional groups

Identify the functional group Alcohol Ether Ketone Copyright © Houghton Mifflin Company.All rights reserved.

Index of Hydrogen Defficiency The sum of the number of rings and pi bonds in a molecule The first step in problems to determine structure Calculated from the molecular formula IHD = (Hreference-Hmolecule)/2 The molecular formula of a reference hydrocarbon is CnH2n+2

Calculate the hydrogen deficiency for 1-hexene The molecular formula is C6H12 The molecular formula for the reference hydrocarbon is C6H14 The IHD of 1-hexene is (14-12)/2= 1

Practice Calculate the IHD of isopentyl acetate The molecular formula is C7H14O2

Practice answer Calculate the IHD of isopentyl acetate The molecular formula is C7H14O2 The molecular formula of the reference hydrocarbon is C7H16 IHD = (16-14)/2 = 1