IR spectroscopy 2025

IR Spectroscopy Overview

• IR spectroscopy utilises photons with longer wavelengths than UV-Vis radiation, which contains insufficient energy for electronic transitions in molecules.

Electromagnetic Spectrum and Energy

• IR radiation features wavelengths ranging from 900 nm to 1,000,000 nm, which corresponds to thermal energies.

• The electromagnetic spectrum is categorised based on wavelength, energy, and molecular effects:

  • Gamma Rays: 10^-9

  • X-Rays: 10^-7

  • UV Radiation: electronic transitions around 10^-5 to 10^-4

  • Infrared Radiation (IR): 10^-4 for molecular vibrations

  • Microwave: Rotational motion

  • Radio Waves: 102 for nuclear spin transitions

Molecular Vibrations

• Molecules can be conceptualised as springs, wherein specific bonds have unique "stretching" frequencies that correspond to IR radiation absorption.

• The equilibrium position of each bond allows for specific vibrations under IR absorption.

Bond Behavior in IR Spectroscopy

• Stretching of Covalent Bonds: Bonds may undergo symmetric or asymmetric stretching. Additionally, bonds may bend in motions termed "scissoring"

• Vibrational Modes: Common modes include:

  • Symmetric Stretching

  • Antisymmetric Stretching

  • Rocking

  • Wagging

  • Twisting

  • Scissoring

IR Spectrum Characteristics

• IR absorption is molecule-specific, with each molecule possessing distinct IR absorption wavelengths.

• For example, the ethanol molecule registers molecular vibrations at approximately 1511.77 cm-1.

Spectrum Translation

• The IR spectrum visually represents absorption modes. Each peak indicates where IR is absorbed:

  • Y-axis: Absorption of IR radiation

  • X-axis: Frequency of radiation in wavenumbers.

Spectrum Transmission Modes

• The most prevalent form of spectrum is in transmission mode, which is frequently plotted against frequency in wavenumbers.

• Wavenumber is expressed as the reciprocal of the wavelength in centimeters (cm).

Vibrational Modes of Methanol and Other Compounds

• Different observations of vibrational modes in methanol highlight bends and stretches that correlate with distinctive peaks on the spectrum.

• Complex molecules reveal varied peaks due to specific bends and stretches, structured into four discrete regions:

  • Single C-X Bonds: 600-1400 cm-1

  • Double Bond Region: C=C, C=O, and C=N around 1600-1800 cm-1

  • Triple Bond Region: 1800-2600 cm-1

  • Single X-H Bonds: 2500-4000 cm-1

Fingerprint Region

• The fingerprint region (<1500 cm-1) is characterized by many overlapping peaks, making it complex to interpret, yet unique to specific molecules and effective for identification against a database of spectra.

Application in Organic Structure Determination

• IR spectroscopy is critical in identifying organic compounds, determining bond types (single, double, triple), identifying functional groups, and distinguishing isomers.

• Emphasis lies on data interpretation as opposed to memorization due to the complexity and potential overlap of spectral data.

Peak Characteristics

• Peaks above 1500 cm-1 identify functional groups, aiding in structure determination. These include:

  • C-H Stretch: 3400-2700 cm-1

  • C-O Stretch: 1300-1000 cm-1

  • C=O Stretch: Typically around 1700 cm-1

  • N-H and O-H Stretch: Characteristic strong and broad peaks in 3500-3100 cm-1 range.

Band Classification and Shapes

• Bands in IR spectroscopy can be classified as:

  • Weak (w): occupies about a third of the y-axis

  • Medium (m): covers approximately half

  • Strong (s): dominates the y-axis.

• Bands can be narrow (sharp and pointed) or broad (wider and smooth).

Practical Examples

• Carboxylic acids show specific absorption patterns around 1710 cm-1 for C=O and about 3400 cm-1 for O-H stretches, indicative of functional groups.

• Interpretation of the IR spectra often requires recognition of patterns and understanding of chemical features.

Advanced Applications of IR Spectroscopy

• IR is a crucial tool in forensic science to identify organic compounds such as accelerants, fibers, drugs, and explosives by analyzing spectra of unknown samples.

• It's utilized for qualitative analysis, establishing the presence or absence of functional groups, and creating molecular fingerprints for compound identification.

Summary of IR Spectroscopy Information

• Provides qualitative data regarding compound presence, often paired with other techniques to elucidate a comprehensive profile of the molecular structure.

• Concentration calculations are less common compared to techniques such as mass spectrometry or UV-Vis.