Infrared (IR) Spectroscopy

• Infrared (IR) spectroscopy is a technique used to elucidate the structure of organic compounds
• It provides a spectrum with numerous absorption bands, unlike UV spectroscopy which shows relatively few peaks
• These bands offer information about the structure of the organic compound under investigation

Useful IR Region

• The most useful part of the IR region is from 2.5 μm to 15 μm, where molecular vibrations are easily detected
• The near-IR region spans from 0.8 μm to 2.5 μm
• The far-IR region spans from 15 μm to 200 μm

Units

• IR radiation absorption is expressed in wavelength (λ) or wave number (ν)
• IR spectra of organic molecules are typically plotted as percentage transmittance against wave number
• Wavelengths range from 2.5 μm to 15 μm in the IR region

Wave number calculation

• Wave number (ν) corresponding to 2.5 μm wavelength: 1 / (2.5 x 10^-4 cm) = 4000 cm^-1
• Wave number (ν) corresponding to 15 μm wavelength: 1 / (15 x 10^-4 cm) = 667 cm^-1

Band Intensity

• Band intensity is expressed as absorbance (A) or transmittance (T)
• These values are related as A = log10(1/T)

Vibrations in Molecules

• Atoms in a covalent molecule are not stationary; they rotate and vibrate
• Absorption of IR radiation causes bond stretching or bond angle vibration (bending)
• Bond stretching involves changes in bond distance along the internuclear axis

Energy

• More energy is required to stretch a bond than to bend it
• Stretching absorptions of a bond appear at higher frequencies (higher energy) compared to bending absorptions

Stretching Vibrations

• Diatomic molecules (e.g., H-H, H-Cl) vibrate in one way
• Triatomic molecules (e.g., CO2) have two stretching modes: symmetric and asymmetric

Bending Vibrations

• Scissoring: Atoms around a central atom approach each other
• Rocking: Atoms move in the same direction
• Wagging: Two atoms move above and below the plane with respect to the central atom
• Twisting: One atom moves above and the other below the plane with respect to the central axis
• Scissoring and rocking are examples of plane bending
• Wagging and twisting are examples of out-of-plane bending

IR Absorption

• Bonds like O-H and groups like NH2 or CH3 absorb IR radiation at specific wave numbers
• It leads to quantized, excited stretching and bending vibrational states
• Only vibrations causing dipole moment changes result in absorption bands
• Absorptions depend on the molecular environment of the bond or group
• Specific wavelength absorption indicates a particular bond or group

Vibrational Frequencies

• Stretching vibrational frequency of a bond can be calculated using Hooke's Law: v = (1/2π) * √(K/μ)
• Where μ is the reduced mass m1m2 / (m1 + m2)
• K is the force constant of the bond, a measure of bond strength
• K values for single, double, and triple bonds are approximately 5 x 10^5, 10 x 10^5, and 15 x 10^5 dynes/cm, respectively

SI units

• SI units, the value of K for single, double and triple bonds are approximately 500 Nm-1, 1000 Nm-1 and 1500 Nm-1 respectively

Vibrational Frequency Dependence

• Directly proportional to bond strength (K)
• Inversely proportional to reduced mass (μ)

Bond Strength Example

• C=C bond's stretching frequency is higher than that of a C-C bond because of the higher bond strength
• O-H bonds absorb at a higher frequency than C-C bonds due to the smaller reduced mass

Fundamental Absorption Bands

• Absorption of IR radiation leads to vibrational transitions
• Transition from ground state to first excited state results in strong absorption
• This creates intense bands known as fundamental absorption bands
• The number of possible fundamental absorption bands is determined by the number of vibrational modes available

Band number by molecule type

• Diatomic molecules have one fundamental vibration mode (stretching) will give only one absorption band
• Non-linear polyatomic molecules have 3n-6 modes (n = number of atoms)
• Linear polyatomic molecules have 3n-5 modes (n = number of atoms)

Number of Fundamental Bands

• Organic compounds have many atoms, hence a large number of fundamental bands are expected
• A lower number of bands than expected is often observed because of weak vibrations, overlapping vibrations or vibrations outside the 2.5 μm to 15 μm range

Selection Rules

• Organic compounds have polar groups with specific dipole moment values
• Stretch vibration along the internuclear axis changes the electronic distribution and dipole moment
• Interaction between IR radiation and the oscillating dipole moment leads to energy absorption
• Interaction occurs if the dipole moment differs at the extremes of vibration
• Selection rules dictate which vibrations are effective in causing absorption
• Regarding IR spectrum, only those vibrations effective in causing absorption are not centro-symmetric

Functional Groups

• Functional groups in organic compounds are non-centro-symmetric
• Functional groups in organic compounds respond to IR spectroscopy

Absorption Band Position

• The position of the absorption band is closely related to the bond/functional group in the molecule
• A specific bond/group can be characterized based on the nature and position of the absorption band
• The absorption band at 3 μm (3333 cm-1) indicates the presence of a hydroxyl group

Variation

• The position of a particular band/group may vary because it depends on the molecule's structure
• The IR spectrum can be divided into two regions for interpretation

Region 1

• Absorption bands in the region of 2.5 to 8 μm (4000 to 1250 cm-1) are associated with the vibrational states of different bonds
• This identifies the types of bonds present

Region 2

• Absorption bands in the region of 8 μm (less than 1250 cm-1) are associated with complex vibrational and rotational energy changes
• This region, known as the fingerprint region, helps establish compound identity
• No two closely related compounds have identical IR spectra

Absorption Band Intensity

• The intensity of the absorption band depends on the magnitude of the dipole moment or molecule polarity
• Greater polarity leads to a more intense absorption spectrum
• e.g. Carbonyl compounds (carbon and oxygen bond) are highly polar, giving rise to strong absorption band

Factors Influencing Vibrational Frequency

• Hooke's law helps calculate the approximate frequency of absorption
• The calculated frequency differs from the experimental value because of the molecule's structure and neighboring groups

Electronic Effects

• Electronic effects (inductive and resonance) influence stretching vibrational frequencies of C=O and C=C bonds
• Both effects typically work together, but one may dominate
• They can increase or decrease the force constant

Inductive Effect

• The inductive effect of nearby groups (+I or -I) alters the force constant
• Alkyl groups (+I effect) reduce the force constant, shifting absorption to lower wave numbers
• Atoms like Cl or Br (-I effect) increase the force constant, shifting absorption to higher wave numbers

Resonance Effect

• Conjugation in unsaturated hydrocarbons or aromatic rings decreases the force constant
• Conjugation shifts the absorption to lower wave numbers

Hydrogen Bonding

• In compounds with hydrogen bonding (phenols, carboxylic acids, amines), the force constant decreases
• This shift absorption to lower wave numbers.
• Hydrogen bonding can be intermolecular or intramolecular

Hydrogen Bonding types

• Intermolecular hydrogen bonding yields broader bands
• Intramolecular hydrogen bonding presence does not depend upon concentration

Effect of Hydrogen Bonding on Molecules

• Hydrogen bonding can increase the O-H bond length, weakening the bond and decreasing the force constant
• Leads to lower absorption frequency and a broad band

Carboxylic acids

• In solution, they exist as dimers due to hydrogen bonds
• Their O-H stretching frequencies appear at lower wave numbers (2700-2500 cm-1)
• Hydrogen bonding also lowers the C=O stretching vibration
• N-H bond's the shift to the strength of the stretch is much less than that of the O-H bond

Recording IR spectra

• Recording of IR spectra involves a spectrophotometer
• A sample of the substance is exposed to a beam of infrared radiation
• The relative intensity is then plotted

Spectrum output

• The transmission (% absorbance) is plotted versus wave number (or wavelength)
• Percent transmission is normally preferred
• A common source of radiation uses the output of a combination of yttrium and erbium

Process

• Yttrium and erbium radiation sources are heated electrically to ~1775 K
• The sample containers, known as cells, and the optical components of the spectrophotometer are made from salts (NaCl)
• Glass is opaque in the IR spectrum

Samples state

• Samples can be gaseous, liquid or in solution
• Solids tend to scatter radiation if placed to directly on the beam

Solvents for Samples

• Chloroform (CHCl3) or carbon disulfide (CS2) are used as solvents for the solids
• Even liquid paraffin may be used for the purpose

Sample Preparation

• Solids can be mixed with potassium bromide and converted to pellet form
• Very small amount of sample is used during this process

Water

• Water gives strong bands when in the IR spectrum
• These strong absorption bands can cause interference with the analysis of other compounds

Group Frequencies

• Group frequencies are used to identify functional groups present in an organic compound
• Correlation tables presents expected values in the IR spectrum
• With the help of this data, the identity of these known organic compounds ca be determined