Spectroscopy is the study of interaction of electromagnetic radiation with matter. It is a powerful analytical technique for the determination of structure of molecules.

Billions of compounds have been synthesized to date, each differing from one another based on the arrangement of atoms, bond positions and the type of functional groups present in their structures.

Traditionally, identification of compounds relied on chemical tests, but spectroscopy has now superseded conventional methods for various reasons:

• It is easier and takes very little time to analyze a sample
• It requires a very small amount of substance
• It provides more reliable information about chemical molecules

Sunlight consists of a wide range of electromagnetic waves including radio waves, microwaves, infrared radiations, visible radiations, ultraviolet radiations, etc.

When electromagnetic radiations interact with molecules, some rays are absorbed while others are transmitted. The wavelength and frequency of absorbed light provide valuable information about the molecular structure.

Increasing Wavelength ← → Increasing Energy

Infrared spectroscopy is used to detect the type of bonds and functional groups present in molecules. IR frequency is expressed in wavenumber (cm⁻¹). The most useful IR range lies between 4000-670 cm⁻¹.

IR Regions:

• Near IR: 12000-4000 cm⁻¹
• Middle IR: 4000-200 cm⁻¹
• Far IR: 200-10 cm⁻¹

When molecules absorb IR radiation, it leads to increased intensity of vibrations. These vibrations can be:

• Bond Stretching: Bond length increases or decreases
• Bond Bending: Bond angle between atoms changes

IR Spectrum Regions:

• Functional Group Region (4000-1500 cm⁻¹): Identification of functional groups
• Fingerprint Region (1500-500 cm⁻¹): Unique pattern for compound identification

Applications:

• Identification of functional groups in organic molecules
• Detection of impurities in samples
• Quality control in pharmaceuticals and food industry

UV-Vis spectroscopy is used to determine the presence of double and triple bonds as well as conjugated systems in molecules.

Regions:

• UV region: 200-400 nm
• Visible region: 400-800 nm

When molecules absorb UV-Vis radiation (200-800 nm), electronic transitions occur:

• σ → σ* transition: Requires very high energy (beyond UV-Vis range)
• π → π* transition: Occurs in molecules with double/triple bonds or aromatic rings (180-320 nm)
• n → π* transition: Occurs when double/triple bonds are connected to heteroatoms like N, O, S (200-500 nm)
• n → σ* transition: Occurs in saturated molecules with heteroatoms like alkyl halides, alcohols (150-300 nm)

Applications:

• Determination of double bonds, triple bonds, aromatic systems, and heteroatoms
• Determining concentration of unknown compounds using Beer-Lambert’s law

NMR spectroscopy involves nuclei of certain elements like ¹H (proton) that exhibit random spin. When an external magnetic field is applied, nuclear spins align in two ways:

• Low energy state: Aligned with the magnetic field
• High energy state: Aligned against the magnetic field

When subjected to radio frequency radiation in a strong magnetic field, nuclei absorb energy and flip to high energy state. This absorption is detected as NMR signals.

¹H NMR Graph Parameters:

• x-axis: Chemical shift (ppm relative to TMS)
• y-axis: Absorption intensity
• Peaks: Splitting pattern (singlet, doublet, triplet, quartet)

Ethanol (C₂H₅OH) ¹H NMR:

• CH₃ protons: Triplet at 1.1-1.3 ppm
• CH₂ protons: Quartet at 3.5-4 ppm
• OH proton: Broad singlet at 4-5 ppm

Applications: Drug analysis, material science, forensic analysis, structural determination of organic compounds.

These techniques are used to identify elements in various samples including metal compounds.

Atomic Absorption Spectroscopy:

• Sample exposed to wide range of light
• Atoms absorb specific wavelengths corresponding to energy needed for electron transitions
• Absorbed wavelengths appear as dark lines on bright background
• Used to identify specific elements

Atomic Emission Spectroscopy:

• Electrons excited by external energy source (heat or electricity)
• When excited electrons return to ground state, they emit light of specific wavelengths
• Emitted light appears as bright lines on dark background
• Each element has unique emission spectrum

Mass spectroscopy is used to determine molecular mass and structure of compounds.

Process:

• Sample is vaporized and bombarded with high-energy electrons
• Molecules are ionized to form molecular ions
• Ions pass through magnetic field and follow curved path
• Detector records mass-to-charge ratio (m/z)

Mass Spectrum of Ethanol: Shows peaks at m/z values corresponding to molecular ion and fragments.

Applications:

• Determination of molecular mass of unknown compounds
• Identification and purification of drugs and pharmaceutical products
• Structural analysis of organic compounds