Introduction to IR Spectroscopy

Infrared (IR) Spectroscopy is a widely used analytical technique that provides information about the molecular structure of compounds by studying their vibrational modes. It works by measuring the absorption of infrared radiation, which causes changes in the vibrational energy of molecules.

  • Wavelength Range: The infrared region of the electromagnetic spectrum typically spans from 700 nm to 1 mm, but the most commonly used range in IR spectroscopy is 4000 cm⁻¹ to 400 cm⁻¹.
  • Principle: Molecules absorb infrared radiation at specific wavelengths corresponding to the vibrational modes of their bonds, leading to an increase in vibrational energy. The frequencies at which absorption occurs are characteristic of specific bonds and functional groups in a molecule.

 

Theory of IR Spectroscopy

  • Molecular Vibrations: Molecules can vibrate in different ways depending on the type of bonds and atoms in the molecule. When infrared radiation is passed through a sample, certain wavelengths are absorbed if they match the natural frequency of vibration of the molecule.
  • Vibrational Energy Levels: Molecules have quantized vibrational energy levels. When a molecule absorbs infrared radiation, it moves from one vibrational energy level to a higher one.
    • Selection Rule: For a vibration to be IR active (i.e., for a molecule to absorb IR radiation), there must be a change in the dipole moment of the molecule during vibration. Non-polar molecules or symmetric molecules like diatomic oxygen (O₂) or nitrogen (N₂) do not absorb infrared radiation because their dipole moment does not change during vibration.

 

Modes of Molecular Vibrations

Molecules can exhibit different types of vibrations that correspond to different modes:

  • Stretching Vibrations: This involves the change in the bond length between two atoms.
    • Symmetric Stretching: Both bonds expand and contract together.
    • Asymmetric Stretching: One bond expands while the other contracts.
  • Bending Vibrations: This involves the change in the bond angle between three atoms.
    • In-plane Bending: Bending occurs in the plane of the molecule.
    • Out-of-plane Bending: Bending occurs outside the plane of the molecule.

Each vibrational mode is associated with a specific frequency, and these frequencies depend on the mass of the atoms and the strength of the bond.

 

Sample Handling in IR Spectroscopy

  • Sample Preparation: Samples in IR spectroscopy must be prepared in a form that allows interaction with infrared radiation. There are several methods of sample preparation:
    1. Liquid Samples: Liquids are typically placed between two transparent sodium chloride (NaCl) or potassium bromide (KBr) windows. These materials are transparent to infrared radiation.
    2. Solid Samples: Solids can be analyzed in two main forms:
      • KBr Pellet Method: A small amount of the sample is ground with dry KBr and pressed into a transparent pellet.
      • Nujol Mull Method: The sample is mixed with a small amount of mineral oil (Nujol) and placed between the IR-transparent windows.
    3. Gas Samples: Gases can be analyzed by placing them in a gas cell with windows transparent to IR radiation.
  • Preparation Considerations:
    • The sample must be homogenous and free from contaminants.
    • Moisture in samples can interfere with IR measurements, especially for water-sensitive compounds.

 

Instrumentation of Dispersive and Fourier-Transform IR Spectrometers

There are two main types of IR spectrometers: Dispersive IR spectrometers and Fourier-transform IR (FTIR) spectrometers.

Dispersive IR Spectrometer

  • The Dispersive IR Spectrometer works by dispersing infrared light into its component wavelengths using a monochromator or prism.
  • Components:
    1. Light Source: Typically, a Globar (silicon carbide) or tungsten filament is used, emitting infrared radiation.
    2. Monochromator: A device that disperses the incoming infrared radiation into its component wavelengths. A rotating grating or prism is used to select the specific wavelength of light.
    3. Sample Holder: The sample is placed in a beam of IR light, and the transmitted light is collected for analysis.
    4. Detector: A detector such as a thermocouple or pyroelectric detector measures the amount of transmitted infrared light.
    5. Readout System: The absorption spectrum is displayed on a monitor or recorded by the instrument.

Dispersive IR Spectrometer Diagram:

 

+--------------------+     +------------------+                    +------------------+

|   Light Source     | --> |   Monochromator   | --> |   Sample Holder   |

+--------------------+     +------------------+                     +------------------+

                                                             |

                                                            V

                                              +------------------+

                                                |    Detector      |

                                              +------------------+

                                                             |

                                                            V

                                                +------------------+

                                              |   Readout System |

                                                 +------------------+

 

 

Fourier-Transform IR (FTIR) Spectrometer

  • FTIR spectrometers use a Michelson interferometer to collect all wavelengths of infrared light simultaneously and then apply a mathematical process called Fourier Transform to convert the data into an IR spectrum.
  • Components:
    1. Interferometer: The key component of FTIR. It splits the incoming light into two beams, which are recombined after traveling different optical paths. The interference pattern that results is used to obtain all wavelengths at once.
    2. Sample Holder: Similar to dispersive IR, the sample is placed in the infrared beam.
    3. Detector: Commonly used detectors are the mercury cadmium telluride (MCT) or deuterated triglycine sulfate (DTGS) detectors.
    4. Readout System: Fourier Transform is applied to the interferogram (the raw data) to produce the final IR spectrum.

FTIR Spectrometer Diagram:

+--------------------+     +-------------------+     +------------------+

|    Light Source    | --> |  Interferometer    | --> |  Sample Holder   |

+--------------------+     +-------------------+     +------------------+

                                                             |

                                                            V

                                              +------------------+

                                                |    Detector      |

                                              +------------------+

                                                              |

                                                             V

                                               +------------------+

                                              | Readout System |

                                                +------------------+

 

Factors Affecting Vibrational Frequencies

Several factors affect the vibrational frequencies of molecules in infrared spectroscopy:

  1. Bond Strength: Stronger bonds (e.g., triple bonds like C≡C) generally absorb at higher frequencies, whereas weaker bonds (e.g., single bonds like C–H) absorb at lower frequencies.
  2. Atomic Mass: Lighter atoms (e.g., hydrogen) tend to vibrate at higher frequencies than heavier atoms (e.g., carbon or oxygen).
  3. Bond Order: Higher bond order (e.g., C≡O) leads to higher frequencies compared to lower bond orders (e.g., C–O).
  4. Resonance and Conjugation: Molecules with conjugated double bonds or resonance structures often exhibit absorption at lower frequencies due to delocalization of electrons.
  5. Electron-withdrawing or Electron-donating Groups: Substituents that withdraw electrons (e.g., NO₂) or donate electrons (e.g., NH₂) can shift the absorption frequencies of functional groups.

 

Applications of IR Spectroscopy

IR spectroscopy is used in various fields for both qualitative and quantitative analysis:

  1. Functional Group Identification: IR spectroscopy is an essential tool for identifying functional groups within a molecule. Specific bond stretching and bending frequencies provide insights into the presence of alcohols, amines, aldehydes, ketones, and more.
  2. Organic Synthesis: It helps in confirming the structure of synthesized organic compounds by comparing the observed IR spectrum with known reference spectra.
  3. Pharmaceutical Analysis: IR spectroscopy is used to identify and analyze pharmaceutical compounds, detect impurities, and ensure the quality of drugs.
  4. Polymer Analysis: Used to characterize polymers and to monitor polymerization processes, checking the presence of functional groups like esters, acids, and amides.
  5. Environmental Monitoring: It is employed in detecting pollutants and environmental contaminants such as volatile organic compounds (VOCs) in air, water, and soil samples.
  6. Forensic Analysis: IR spectroscopy can be used to analyze trace evidence such as drugs, fibers, and paints in forensic investigations.
  7. Food and Beverage Industry: It is used for quality control, determining ingredients, or detecting adulterants in food products.