I NFRARED (IR).???????????? Spectroscopy “seeing the unseeable” Using electromagnetic radiation as a probe to obtain information about atoms and molecules

I NFRARED (IR)

?????????????

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: v = c 1/v E = hvE v E = hc/E 1/ = wave length v = frequency c = speed of light E = kinetic energy h = Plancks constant (3.99X10 -13 kj*s/mol or 9.54 X 10 -14 kcal*s/mol c

Just below red in the visible region (380 nm-780nm) Wavelengths usually 2.5-25 m More common units are wavenumbers, or cm -1, the reciprocal of the wavelength in centimeters (10 4 / m = 4000-400 cm -1 ) Wavenumbers are proportional to frequency and energy Higher wavenumber higher energy The IR Region

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

Infrared radiation = 2.5 to 17 m (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) E vibration increases momentarily IR -O-H-O-H -O-O ( 3500 cm - 1 ) HH

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

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 : 1.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 )

F INGERPRINT OF M OLECULE 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 S PECTRUM R EGIONS

IR-A CTIVE 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 bonds vibration must cause a change in dipole Asymmetrical stretching/bending change the dipole moment of a molecule. IR-I NACTIVE A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

I NTERPRETATION OF IR S PECTRUM 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

C ARBON -C ARBON B OND S TRETCHING 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

C ARBON -H YDROGEN S TRETCHING Bonds with more s character absorb at a higher frequency. sp 3 C-H, just below 3000 cm -1 (to the right) sp 2 C-H, just above 3000 cm -1 (to the left) sp C-H, at 3300 cm -1

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 FEATURES OF AN IR SPECTRUM

U NDECANE ( ALKANE ) wavelenght frequency

1- HEXENE ( ALKENE ) wavelenght frequency

1- HEXYNE ( ALKYNE )

27 O-H AND N-H S TRETCHING Both of these occur around 3300 cm -1, but they look different Alcohol O-H, broad with rounded tip Secondary amine (R 2 NH), broad with one sharp spike Primary amine (RNH 2 ), broad with two sharp spikes No signal for a tertiary amine (R 3 N)

28 A N A LCOHOL IR S PECTRUM

29 A N A MINE IR S PECTRUM

30 C ARBONYL S TRETCHING The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm -1 Usually, its the strongest IR signal Carboxylic acids will have O-H also Aldehydes have two C-H signals around 2700 and 2800 cm -1

31 A K ETONE IR S PECTRUM

32 O-H S TRETCH OF A C ARBOXYLIC A CID This O-H absorbs broadly, 2500-3500 cm -1, due to strong hydrogen bonding

33 A N A LDEHYDE IR S PECTRUM

34 V ARIATIONS IN C=O A BSORPTION 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 Cs or less) absorb at an even higher frequency

35 A N A MIDE IR S PECTRUM

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

Cha pter 12 37 S UMMARY OF IR A BSORPTIONS =>

PRACTICE Identify the possible functional group or possible functional groups

P RACTICE Identify the possible functional group or possible functional groups

I NDEX OF H YDROGEN D EFFICIENCY 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

C ALCULATE THE HYDROGEN DEFICIENCY FOR 1- HEXENE The molecular formula is C 6 H 12 The molecular formula for the reference hydrocarbon is C 6 H 14 The IHD of 1-hexene is (14-12)/2= 1

P RACTICE Calculate the IHD of isopentyl acetate The molecular formula is C 7 H 14 O 2

P RACTICE ANSWER Calculate the IHD of isopentyl acetate The molecular formula is C 7 H 14 O 2 The molecular formula of the reference hydrocarbon is C 7 H 16 IHD = (16-14)/2 = 1

Documents

I NFRARED (IR).???????????? Spectroscopy “seeing the unseeable” Using electromagnetic radiation as a probe to obtain information about atoms and molecules