Spectrometry infra merah dr. Mohammad Masykuri, M. Si. Program studi s2 pendidikan sains

SPECTROMETRY INFRA MERAH

Dr. Mohammad Masykuri, M.Si.

PROGRAM STUDI S2 PENDIDIKAN SAINS

UNIVERSITAS SEBELAS MARET

Email: [e-mail]

Website: https://mmasykuri.wordpress.com/

Praktikum IPA Terpadu

Introduction

• Spectroscopy is an analytical technique which helps determine structure.
• It destroys little or no sample.
• The amount of light absorbed by the sample is measured as wavelength is varied.

Types of Spectroscopy

• Infrared (IR) spectroscopy measures the bond vibration frequencies in a molecule and is used to determine the functional group.
• Mass spectrometry (MS) fragments the molecule and measures the masses.
• Nuclear magnetic resonance (NMR) spectroscopy detects signals from hydrogen atoms and can be used to distinguish isomers.
• Ultraviolet (UV) spectroscopy uses electron transitions to determine bonding patterns.

Electromagnetic Spectrum

• Examples: X rays, microwaves, radio waves, visible light, IR, and UV.
• Frequency and wavelength are inversely proportional.
• c = ln, where c is the speed of light.
• Energy per photon = hn, where h is Planck’s constant.

The Spectrum and Molecular Effects

The IR Region

• Just below red in the visible region.
• Wavelengths usually 2.5-25 mm.
• More common units are wavenumbers, or cm-1, the reciprocal of the wavelength in centimeters.
• Wavenumbers are proportional to frequency and energy.

Molecular Vibrations

Covalent bonds vibrate at only certain allowable frequencies.

Stretching Frequencies

• Frequency decreases with increasing atomic weight.
• Frequency increases with increasing bond energy.

Vibrational Modes

Nonlinear molecule with n atoms usually has 3n - 6 fundamental vibrational modes.

Fingerprint of Molecule

• Whole-molecule vibrations and bending vibrations are also quantitized.
• No two molecules will give exactly the same IR spectrum (except enantiomers).
• Simple stretching: 1600-3500 cm-1.
• Complex vibrations: 600-1400 cm-1, called the “fingerprint region.”

IR-Active and Inactive

• A polar bond is usually IR-active.
• A nonpolar bond in a symmetrical molecule will absorb weakly or not at all.

An Infrared Spectrometer

FT-IR Spectrometer

• Uses an interferometer.
• Has better sensitivity.
• Less energy is needed from source.
• Completes a scan in 1-2 seconds.
• Takes several scans and averages them.
• Has a laser beam that keeps the instrument accurately calibrated.

Carbon-Carbon Bond Stretching

• Stronger bonds absorb at higher frequencies:
• C-C 1200 cm-1
• C=C 1660 cm-1
• CC 2200 cm-1 (weak or absent if internal)
• Conjugation lowers the frequency:
• isolated C=C 1640-1680 cm-1
• conjugated C=C 1620-1640 cm-1
• aromatic C=C approx. 1600 cm-1

Carbon-Hydrogen Stretching

Bonds with more s character absorb at a higher frequency.

• sp3 C-H, just below 3000 cm-1 (to the right)
• sp2 C-H, just above 3000 cm-1 (to the left)
• sp C-H, at 3300 cm-1

An Alkane IR Spectrum

An Alkene IR Spectrum

An Alkyne IR Spectrum

O-H and N-H Stretching

• Both of these occur around 3300 cm-1, but they look different.
• Alcohol O-H, broad with rounded tip.
• Secondary amine (R2NH), broad with one sharp spike.
• Primary amine (RNH2), broad with two sharp spikes.
• No signal for a tertiary amine (R3N)

An Alcohol IR Spectrum

An Amine IR Spectrum

Carbonyl Stretching

• The C=O bond of simple ketones, aldehydes, and carboxylic acids absorb around 1710 cm-1.
• Usually, it’s the strongest IR signal.
• Carboxylic acids will have O-H also.
• Aldehydes have two C-H signals around 2700 and 2800 cm-1.

A Ketone IR Spectrum

An Aldehyde IR Spectrum

O-H Stretch of a Carboxylic Acid

This O-H absorbs broadly, 2500-3500 cm-1, due to strong hydrogen bonding.

Variations in C=O Absorption

• Conjugation of C=O with C=C lowers the stretching frequency to ~1680 cm-1.
• The C=O group of an amide absorbs at an even lower frequency, 1640-1680 cm-1.
• The C=O of an ester absorbs at a higher frequency, ~1730-1740 cm-1.
• Carbonyl groups in small rings (5 C’s or less) absorb at an even higher frequency.

An Amide IR Spectrum

Carbon – Nitrogen Stretching

• C - N absorbs around 1200 cm-1.
• C = N absorbs around 1660 cm-1 and is much stronger than the C = C absorption in the same region.
• C  N absorbs strongly just above 2200 cm-1. The alkyne C  C signal is much weaker and is just below 2200 cm-1 .

A Nitrile IR Spectrum

Summary of IR Absorptions

Strengths and Limitations

• IR alone cannot determine a structure.
• Some signals may be ambiguous.
• The functional group is usually indicated.
• The absence of a signal is definite proof that the functional group is absent.
• Correspondence with a known sample’s IR spectrum confirms the identity of the compound.

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