IR spectroscopy

INFRARED SPECTROSCOPY

Introduction to Infrared Spectroscopy

  • Infrared spectroscopy (IR) is a powerful analytical technique used to identify functional groups and molecular structures.

Learning Objectives

  • Understanding Basic Concepts: Grasp the chemistry and physics underlying infrared spectra measurement.

  • Analysis Skill: Learn to analyze infrared spectra for identifying molecular functional groups.

  • Structural Determination: Utilize infrared spectroscopy to deduce structures of unknown molecules.

Spectroscopy Basics

  • Definition: Spectroscopy involves measuring the interaction of electromagnetic radiation with matter.

  • Components: The process consists of electric and magnetic waves, represented by a light beam traveling in a specific direction.

The Electromagnetic Spectrum

  • Spectrum Characteristics:

    • Increasing Wavelength (m): Ranges from gamma rays (10^-14 m) to radio waves (10^6 m).

    • Increasing Frequency (Hz): The visible spectrum is between 400-700 nm.

    • Xray

      • If it hits a high electron density you will get a large diffraction spot

      • closest we can see to a picture of a molecule

    • UV vis. will not be useful for alkene or aliphatic alcohol (sp3)

    • When IR hits the molecule it makes it stretch and IR is useful to determining functional groups

  • Regions: Different regions of the spectrum include UV, IR, microwaves, and radio waves.

Vibrational Modes of Molecules

  • Types of Vibration:

    • Symmetrical and asymmetrical stretching.

    • In-plane and out-of-plane motions: scissoring rocking, wagging, and twisting.

Regions of the IR Spectrum

  • Key wavenumbers and associated functional groups:

    • C=N, C≠C, C-O, C=O, N-H, O-H, C-H observed within specific ranges of 400-4000 cm^-1.

    • high wavenumber is high energy

      • we use “wavenumber” because it directly relates to energy

      • differentiate between functional groups because where they appear

Characteristic Alcohols and Amines Stretching Vibrations

  • Alcohols (e.g., 2-Butanol)

    • O-H bond around 3300 cm^-1.

      • broad

  • Amines (e.g., Butan-2-amine)

    • N-H bond observed around 3300 cm^-1 and 3200 cm^-1

      • points not broad

IR Spectra of Amines (1°, 2°, and 3°)

  • Distinct Features: Comparison of peak patterns for primary (NH2), secondary (NHR), and tertiary (NR2) amines.

    • Primary and secondary amines exhibit N-H signals.

      • 1° = 2 stretches

      • 2° = 1 stretch

    • Tertiary amines do not show N-H signals.

Characteristic C-H Stretching Vibrations

  • Stretch Types:

    • sp, sp2, sp3 C-H stretching distinctions noted at different wavenumbers

      • sp1 = ~3300 cm^-1

      • sp 2 = ~3100 cm^-1

      • sp3= ~2900 cm^-1

Characteristic Carbonyl C=O Stretching Vibrations

  • Functional Impact: The nature of the neighboring atoms and groups affects the C=O stretching frequencies:

    • Acid chlorides (1815-1790 cm^-1), esters (1750-1735 cm^-1), aldehydes (1740-1725 cm^-1), ketones (1720-1708 cm^-1), and amides (1680-1630 cm^-1).

Factors Affecting C=O Vibration Frequency

  • Influencing factors include:

    • Hydrogen bonding effects (decreases frequency).

      • weaken the bonds

    • Electron-withdrawing groups (increase frequency).

    • Ring strain and conjugation influences.

    • higher strain higher wavenumber

12. Effect of Conjugation on C=O Vibration Frequency

  • Major peaks visible in the spectrum reflect changes due to conjugation (e.g., frequencies at 1718, 1359, 1169 cm^-1).

13. Characteristic C=C Stretching

  • Analysis of C=C stretches in different configurations: (E)-oct-2-ene, (Z)-oct-2-ene, etc.

14. Characteristic Aromatic Stretching

  • Overtones observed in the stretching of chemical bonds within aromatic rings (e.g., propylbenzene).

15. Identifying Compounds Using IR Spectrum

  • Determining molecular structure based on observed peaks and fingerprint regions in the IR spectrum.