B.Tech. Spectroscopy

Introduction to Spectroscopy

  • Spectroscopy: The science that studies the interactions between radiation (energy in wave or particle form) and matter.

  • Radiation: Emission or transmission of energy in waves or particles through space or a medium.

  • Matter: Substance with mass that occupies space, composed of particles.

Types of Radiation

Electromagnetic Radiation

  • Consists of photons including:

    • Radio waves

    • Microwaves

    • Infrared

    • Visible light

    • Ultraviolet

    • X-rays

    • Gamma rays (γ)

Particle Radiation

  • Involves particles with non-zero rest energy:

    • Alpha radiation (α)

    • Beta radiation (β)

    • Proton and neutron radiation

Acoustic Radiation

  • Includes ultrasound, sound, and seismic waves which depend on a physical transmitting medium.

Properties of Electromagnetic Radiation

  • Can be described as waves with:

    • Wavelength (λ): Linear distance between successive wave maxima or minima (measured in meters).

    • Frequency (ν): Number of oscillations of the electric field vector per unit time (measured in Hertz, Hz).

    • Velocity (v): Speed of wave propagation (measured in m/s).

    • Amplitude: Height of the wave.

  • Light's unique property: Travels through a vacuum, unlike sound waves that require a medium, and travels nearly a million times faster than sound.

Dual Nature of Radiation

  • The wave model falls short for absorption/emission processes, allowing for:

    • Treatment of electromagnetic radiation as discrete packets (photons).

    • The concepts of wave and particle are complementary.

Fundamental Equations and Terms

  • Velocity equation:

    • v = λν

  • Energy of a photon:

    • E = hν

  • Planck's constant (h): 6.626 x 10^-34 Js

Interaction of Electromagnetic Radiation with Matter

  • Conditions for Interaction: Matter with electric and magnetic effects interacts with electromagnetic radiation.

  • Changes in Energy Types: Caused by the absorption/emission of electromagnetic radiation, affecting:

    • Translational Energy: Overall movement of a molecule (not quantized).

    • Rotational Energy: Spinning motion about axes (quantized).

    • Vibrational Energy: Molecule vibrations (quantized).

    • Electronic Energy: Energy transitions between electronic levels (quantized).

Spectroscopies

Types of Spectroscopy based on Energy Changes

  • Microwave Spectroscopy: Measures rotational energy changes.

  • Infrared (IR) Spectroscopy: Measures vibrational energy changes.

  • UV/Visible Spectroscopy: Measures electronic energy changes.

Absorption and Emission Spectra

  • Absorption Spectrum: Resulting from the absorption of electromagnetic radiation by compounds.

  • Emission Spectrum: Emitted radiation when absorbed energy is re-emitted.

Beer-Lambert Law for UV-Visible Spectroscopy

  • Formula: A = log(I₀/I) = εcl

  • Parameters:

    • A: Absorbance

    • Io: Intensity of incident light

    • I: Intensity of transmitted light

    • ε: Molar absorptivity

    • c: Concentration of the sample

    • l: Path length of the sample

Limitations of Beer-Lambert Law

  • Not obeyed in:

    • Keto-enol tautomers

    • Presence of fluorescent compounds

    • Solute-solvent complexes.

Spectrometric Methods

Instruments

  • Spectrophotometer: Device measures percentage transmittance of light radiation.

    • Components: Light source, monochromator, detector, amplifier, recording devices.

    • Light sources: Tungsten filament lamp (visible) and hydrogen-deuterium discharge lamp (UV).

Cells

  • Transparent Material Choices:

    • Silica cells or quartz for UV region; glass not usable.

Data Interpretation

Absorption Bands

  • Expectation vs Reality: Expected sharp peaks, actual broad absorption bands due to vibrational and rotational motions.

Electronic Transitions Process

  • Absorption leads to electronic excitation, moving electrons from ground state to excited state via transitions like σ → σ*, n → σ*, and π → π*.

Color and Chromophores

  • Compounds absorbing 400-800 nm light appear colored, determined by absorbed wavelengths.

  • Chromophores: Groups responsible for color, can undergo transitions affecting absorbance properties.

    • Types: π electrons versus n electrons.

  • Auxochromes: Groups that enhance color without acting as chromophores themselves.

Influencing Factors on Absorbance

Factors to Consider

  1. Conjugation: Longer wavelength absorption due to double bond conjugation.

  2. Temperature: Affects vibrational and rotational states and sharpness of absorption bands.

  3. Solvent Effects: Solvent polarity affects absorption maximum; 95% ethanol preferred as a solvent.

Woodward-Fieser Rules

  • Method for calculating absorption wavelength maxima ( ( λ_{max} )) for conjugated systems and carbonyl compounds using base values and increments for various substituent effects.

Infrared Spectroscopy

Importance

  • Provides insight into organic compound structure through molecular vibrations detected as absorption bands. Creates a spectrum rich in details unlike UV.

Instrumentation and Techniques

  • IR spectroscopy uses Nernst glowers or silicon carbide rods for infrared radiation. Involves splitting light into sample and reference beams and measuring differences in intensities.

Summary of Key Points

  • NMR Spectroscopy: Examines nuclei in strong magnetic fields using radiofrequency energy absorption, correlating structures to chemical shifts in the spectrum.

  • Chemical Shifts: Observed shifts influenced by the electronic environment, with tetramethylsilane serving as a reference standard.