IR

Infrared Spectroscopy (IR)

Definition and Purpose

  • Spectroscopy: Study of the absorption and emission of light and other radiation by matter.

  • Infrared (IR) Spectroscopy: An analytical method to identify the structure of a compound using light in the infrared region of the electromagnetic spectrum.

  • IR Spectrum: A plot indicating the specific wavenumbers (frequencies) at which the molecule absorbs light, allowing identification of functional groups based on molecular vibrations.

Classification of Radiation

  • Types of Radiation:

    • Radiofrequency (low energy)

    • Infrared

    • Ultraviolet

    • X-ray (high energy)

  • Wavelength Ranges:

    • Radio waves: >1m

    • Infrared: 2.5 to 17 µm

    • Ultraviolet: 10 to 400 nm

    • Visible light: 400 to 800 nm

Analysis of IR Spectrum

Fingerprint Region

  • <1300 cm^-1: Known as the fingerprint region, unique for every compound.

Strengths and Limitations

  • Strengths:

    • Identifies functional groups present in the compound.

  • Limitations:

    • Cannot solely determine the structure of a compound.

    • Some signals may be ambiguous; absence of signal confirms absence of a functional group.

    • Comparison with known samples (e.g., SDBS database) can confirm identity.

Functional Groups and Their IR Absorption

Functional Group

Wavelength Range (cm^-1)

Signal Strength

Alcohol O-H

3000-3500

Strong, Broad peak

Carboxylic Acid O-H

2500-3500

Very Broad peak

Amine N-H

3200-3500

Broad, half size as O-H

Alkyl -C-H

2850-3000

Sharp, medium

Alkenes C=C

1600-1680

Sharp, medium

Aromatic C=C

1450-1600

Sharp, medium

Carbonyl C=O

1650-1850

Sharp, strong

Specific Functional Groups

  • Alcohols: O-H stretch around 3000-3500 cm^-1.

  • Carboxylic Acids: O-H stretch around 2500-3500 cm^-1; have very broad peaks.

  • Amines: N-H stretch at 3200-3500 cm^-1; primary amines show 2 signals, secondary have 1.

Unique Features and Vibrational Characteristics

Aldehydes and Fermi Doublet

  • Fermi Doublet: Unique to aldehydes with C=O absorption at ~1710 cm^-1.

  • Key Identifiers: Carboxylic acids show O-H absorption, while aldehydes also show two C-H bands at ~2700-2800 cm^-1.

IR Activity of Vibrations

  • IR Active Bonds: A bond can absorb IR radiation only if its dipole moment changes when vibrating.

  • Examples: C=C bonds can be symmetrical or asymmetrical, impacting IR activity.

Identifying IR Signals

Structures and Associated IR Signals

  • Csp3-H (alkyl): 2850-3000 cm^-1

  • C=O (Carbonyl): 1650-1850 cm^-1

  • Csp2-H (alkene): 3000-3100 cm^-1

Practice Identifying Functional Groups

  • Steps:

    1. Identify the functional group.

    2. Determine the IR signals and range.

    3. Locate peaks within that range.

  • Example: For Csp3-H (alkyl C-H) expect a signal around 2850-3000 cm^-1 and for O-H around 3000-3500 cm^-1.