Infrared Spectroscopy Dr AKM Shafiqul Islam School of Bioprocess Engineering University Malaysia Perlis

Introduction Spectroscopy is an analytical technique which helps determine structure It destroys little or no sample The amount of light absorbed by the sample is measured as wavelength is varied

Infrared spectroscopy is very useful for obtaining qualitative information about the molecules. But molecule must possess certain properties in order to undergo absorption.

IR Spectroscopy The presence and also the environment of functional groups in organic molecule can be identified by infrared (IR) spectroscopy. Infrared spectroscopy is nondestructive. Moreover, the small quantity of sample needed, the speed with which spectrum can be obtained, the relatively low cost of the spectrometer, and I wide applicability of the method combine to make infrared spectroscopy one the most useful tools available to the organic chemist

THE ELECTROMAGNETIC SPECTRUM high Frequency (n) low high Energy low MICRO- WAVE X-RAY ULTRAVIOLET INFRARED RADIO FREQUENCY Nuclear magnetic resonance Vibrational infrared Ultraviolet Visible 2.5 mm 15 mm 1 m 5 m 200 nm 400 nm 800 nm BLUE RED short Wavelength (l) long

Types of Energy Transitions in Each Region of the Electromagnetic Spectrum REGION ENERGY TRANSITIONS X-ray Bond-breaking UV/Visible Electronic Infrared Vibrational Microwave Rotational Radio Frequency Nuclear and (NMR) Electronic Spin

Principles IR Spectroscopy Energy: E=h where:  is the frequency in hertz In IR, frequency is commonly expressed as wave numbers ( , in Reciprocal cm, or cm-1) Where

Principles IR Spectroscopy Absorption of radiation in this region by a typical organic molecule results in the excitation of vibrational, rotational, and bending modes, while the molecule itself remains in its electronic ground state. Molecular asymmetry is a requirement for excitation by infrared radiation and fully symmetric molecules do not display absorbance in this region unless asymmetric stretching or bending transitions are possible. Symmetric stretch Assymmetric stretch Symmetric bending

Principles IR Spectroscopy For the purpose of routine organic structure determination, the most important absorptions in the infrared region are the simple stretching vibrations. For simple systems, these can be approximated by considering the atoms as point masses, linked by a “spring” having a spring constant k and following Hooke’s Law.

Principles IR Spectroscopy Using this simple approximation, the equation shown in below can be utilized to approximate the characteristic stretching frequency (in cm-1) of two atoms of mass m1 and m2, linked by a bond with a spring constant k: Where =m1m2/(m1+m2) , also called “reduced mass”

Absorption of Infrared Radiation Only bonds which have significant dipole moments will absorb infrared radiation. Dipole is the polar covalent bond in which a pair of electron is shared unequally. For absorption occur, there must be a charge in the dipole moment (polarity) of the molecule. A diatomic molecule must have a permanent dipole in order to absorb, but larger molecule do not.

DIPOLE MOMENTS Bonds which do not absorb infrared include: Symmetrically substituted alkenes and alkynes Many types of C-C Bonds Symmetric diatomic molecules H-H Cl-Cl

Molecular Vibrations Light is absorbed when radiation frequency = frequency of vibration in molecule Covalent bonds vibrate at only certain allowable frequencies Associated with types of bonds and movement of atoms Vibrations include stretching and bending

IR Instrumentation Light source: Nichrome wire that glows when an electrical current is passed through; Interferometer: no monochrometer Detector: thermocouple detector, whose output voltage varies with changes caused by varying levels of radiation striking the detector.

IR Instrumentation

Infrared Spectroscopy (IR) No two molecules of different structure will have exactly the same natural frequency of vibration, each will have a unique infrared absorption pattern or spectrum. Two Uses: IR can be used to distinguish one compound from another. Absorption of IR energy by organic compounds will occur in a manner characteristic of the types of bonds and atoms in the functional groups present in the compound; thus, infrared spectrum gives structural information about a molecule. The absorptions of each type of bond (N–H, C–H, OH, C–X, C=O, C–O, C–C, C=C, C≡C, C≡N, etc.) are regularly found only in certain small portions of the vibrational infrared region, greatly enhancing analysis possibilities.

Infrared Spectroscopy (IR) The Infrared Spectrum A plot of absorption intensity (% Transmittance) on the y-axis vs. frequency (wavenumbers) on the x-axis.

Infrared Spectroscopy (IR) Principal Frequency Bands (from left to right in spectrum) OH 3600 cm-1 (Acids - Very Broad, Alcohols - Broad) NH 3300 - 3500 cm-1 (2, 1, 0 peaks – 1o, 2o, 3o) C≡N 2250 cm-1 (Nitrile) C≡C 2150 cm-1 (Acetylene) C=O 1685 - 1725 cm-1 (1715) (Carbonyl) C=C 1650 cm-1 (Alkene); 4 absorptions 1450-1600 (aromatic) CH2 1450 cm-1 (Methylene Group) CH3 1375 cm-1 (Methyl Group) CO 900 - 1100 cm-1 (Alcohol, Acid, Ester, Ether, Anhydride) -CH (Saturated Alkane absorptions on Right side of 3000 cm-1) =C-H (Unsaturated Alkene absorptions on Left side of 3000 cm-1) =C-H (Aromatic absorptions) – Verify at 1667 – 2000 cm-1 ≡C-H (Unsaturated Alkyne absorptions on Left side of 3000 cm-1)

Infrared Spectroscopy (IR) Suggested approach for analyzing IR Spectra Step 1. – Check for the presence of the Carbonyl group (C=O) at 1715 cm-1. If molecule is conjugated, the strong (C=O) absorption will be shifted to the right by ~30 cm-1,i.e., ~1685 cm-1 If the Carbonyl absorption is present, check for: Carboxylic Acids - Check for OH group (broad absorption near 3300-2500 cm-1) Amides - Check for NH group (1 or 2 absorptions near 3500 cm-1) Esters - Check for 2 C-O group (medium absorptions near 1300-1000 cm-1) Anhydrides - Check for 2 C=O absorptions near 1810 and 1760 cm-1 Aldehydes - Check for Aldehyde CH group (2 weak absorptions near 2850 and 2750 cm-1) Ketones - Ketones (The above groups have been eliminated)

Infrared Spectroscopy (IR) Step 2. - If the Carbonyl Group is Absent Check for Alcohols, Amines, or Ethers. Alcohols & Phenols - Check for OH group (Broad absorption near 3600-3300 cm-1 Confirm present of CO near 1300-1000 cm-1 Amines - Check for NH stretch (Medium absorptions) near 3500 cm-1 Primary Amine - 2 Peaks Secondary Amine - 1 Peak Tertiary Amine - No peaks N-H Scissoring at 1560 - 1640 cm-1 N-H Bend at 800 cm-1 Ethers - Check for C-O group near 1300- 1000 cm-1 and absence of OH

Infrared Spectroscopy (IR) Step 3. – Refine the Structure Possibilities by Looking for Double Bonds, Triple Bonds and Nitro Groups Double Bonds - Unsaturated C=C (and C≡C) stretch show absorptions on the left side of 3000 cm-1 Alkene C=C weak absorption near 1650 cm-1 Aromatic C=C (4 absorptions 1450-1650 cm-1) (Verify Aromatic at 1667 – 2000 cm-1) Triple Bonds - R-C ≡ N Nitrile - medium, sharp absorption (stretch) near 2250 cm-1 R – C ≡ C – R Alkyne - weak, sharp absorption (stretch near 2150 cm-1) R – C ≡ C – H Terminal Acetylene (stretch near 3300 cm-1) Nitro Groups - Two strong absorptions 1600 – 1500 cm-1 and 1390 - 1300 cm-1

Infrared Spectroscopy (IR) Step 3 (Con’t) Aromatic Ring Absorptions Aromatic unsaturated C=C bonds show an absorption on the left side of 3000 cm-1, but the aromaticity must be verified in the overtone region (1667 – 2000 cm-1) and the out-of-plane (OOP) region (900 - 690 cm-1) 4 Medium to strong absorptions in region 1650 - 1450 cm-1 Many weak combination and overtone absorptions appear between 2000 and 1667 cm-1 The relative shapes and numbers (1 - 4) of the overtone absorptions can be used to tell whether the aromatic ring is monosubstituted or di-, tri-, tetra-, penta-, or hexa-substituted. Positional (ortho (o), meta (m), para (p)) isomers can also be distinguished. Note: A strong carbonyl absorption can overlap these overtone bands, making them unusable.

Infrared Spectroscopy (IR) Step 3 (Con’t) Aromatic Ring Absorptions (Con’t) The unsaturated =C-H “Out-of-Plane (OOP) bending absorptions in the region 900 – 690 cm-1 can also be used to determine the type of ring substitution. The number of absorptions and their relative positions are unique to each type of substitution. Although these absorptions are in the “Fingerprint” region they are particularly reliable for rings with Alkyl group substitutions. They are less reliable for Polar substituents.

Infrared Spectroscopy (IR) Step 4. If none of the above apply then the compound is most likely a: Hydrocarbon or Alkyl Halide Generally, a very simple spectrum Hydrocarbons - Check for saturated Alkane absorptions just on the right side of 3000 cm-1