Introduction to Infrared (IR) Spectroscopy lec 2

What is Infrared Spectroscopy?

The analysis of infrared light interacting with a molecule.

What are the three main regions of the infrared spectrum?

Near-IR, Mid-IR, and Far-IR.

Which has higher energy and frequency: Near-IR or Far-IR?

near ir

How do stronger bonds affect IR vibrations?

Stronger bonds vibrate at higher wavenumbers.

What are the three IR regions based on wavenumber?

• Near-IR: 13,000 – 4,000 cm⁻¹

• Mid-IR: 4,000 – 400 cm⁻¹

• Far-IR: 400 – 33 cm⁻¹

Which IR region is most widely used and why?

Mid-IR (4,000 – 400 cm⁻¹) because it corresponds to vibrational energy changes and is most useful for chemical analysis.

What are the subregions of Mid-IR?

• Functional group region: 4,000 – 1,500 cm⁻¹ (stretching vibrations)

• Fingerprint region: 1,500 – 500 cm⁻¹ (bending vibrations)

What type of molecules is the Far-IR region useful for?

Molecules containing heavy atoms (e.g., inorganic compounds), but it requires specialized techniques.

What is the main use of IR spectroscopy?

To identify unknown compounds by detecting functional groups and unique fingerprints.

How does IR light interact with molecules?

It causes vibrational and rotational changes, not electronic transitions.

What occurs when radiant energy matches a specific molecular vibration?

Absorption occurs.

What is the term for the characteristic vibration of bonds?

Natural frequency of vibration.

What are the two criteria for a molecule to absorb IR radiation?

1. Correct wavelength of radiation (matches natural frequency)
2. Change in dipole moment

What happens to a molecule’s vibration when IR radiation is absorbed?

The amplitude of molecular vibration changes.

When is a molecule IR active?

When its vibration causes a change in its electric dipole moment.

What is an electric dipole?

When a molecule has a slight positive and slight negative charge on its atoms.

Which non-polar diatomic molecules are not IR active?

N₂, H₂, and O₂ (they do not absorb IR radiation).

What is a dipole moment?

The vector sum of all bond dipole moments in a molecule.
Memo tip: “All bonds team up to make the molecule’s dipole superpower.”

How do you determine the resultant dipole moment from bond vectors?

Draw vectors from the same starting point, complete the parallelogram, and the diagonal is the resultant.
Memo tip: “Parallelogram hack = total dipole.”

Why does CO₂ in the ground state have no dipole moment?

Symmetrical C=O bonds cancel each other out → no net dipole → IR inactive.
Memo tip: “CO₂ is the perfect symmetrical snob.”

When is CO₂ IR-active?

A: During asymmetrical stretching when C=O bonds are different lengths → net dipole forms.
Memo tip: “Mess it up asymmetrically → it finally talks to IR.”

What are the two types of molecular vibrations?

1. Stretching (change in bond length)
2. Bending (change in bond angle)
Memo tip: “Stretch to length, bend to angle.”

Which bonds are IR-active in reality?

Polar covalent bonds. Stronger bonds → stronger absorption.
Memo tip: “Polar = party time for IR.”

Typical energy ranges for stretching and bending vibrations?

Stretching: 4,000 – 1,250 cm⁻¹ (high energy)

Bending: 1,400 – 666 cm⁻¹ (low energy)
Memo tip: “Stretch high, bend low.”

Types of CH₂ group vibrations?

• Stretching: Symmetrical & Antisymmetrical

• Bending: Scissoring, Rocking, Wagging, Twisting
  Memo tip: “CH₂ loves to dance in six moves.”

Why is H₂O IR-active?

Non-linear structure → uneven electron distribution → net dipole moment ≠ 0.
Memo tip: “Bent water = chatty with IR.”

Stretching & bending modes for H₂O?

Symmetrical stretching, asymmetrical stretching, bending (scissoring)
Memo tip: “Three moves = IR groove.”

Why is CO₂ symmetrical stretch IR-inactive?

Linear molecule → dipoles cancel out → no net change in dipole.
Memo tip: “Line it up perfectly = silent to IR.”

What units are used in UV/Vis vs IR spectroscopy?

• UV/Vis → wavelength in nm

• IR → wavenumber in cm⁻¹
  Memo tip: “UV = long names (nm), IR = short names (cm⁻¹).”

How does wavenumber relate to energy?

Directly proportional — higher wavenumber = higher energy.
Memo tip: “Big number = big energy.”

In IR, what is measured instead of absorbance?

Transmittance %.
Memo tip: “IR sees how much light sneaks through, not how much is soaked up.”

What happens to transmittance when IR is absorbed?

It decreases, forming a peak.
Memo tip: “Peak = energy eaten.”

What does each IR peak show?

A functional group in the molecule.
Memo tip: “Each bump = one group’s vibe.”

Peak at 1650–1670 cm⁻¹ means?

A C=C (alkene) bond is present.
Memo tip: “1600s = double bond drama.”

Peaks at 1600 & 1500 cm⁻¹ mean?

An aromatic ring (benzene) is present.
Memo tip: “Twin peaks = benzene bling.”

Why is IR spectroscopy useful?

For identifying functional groups.
Memo tip: “IR = group detector.”

How to identify groups using IR?

Compare vibrational frequencies with an IR databank.
Memo tip: “Compare peaks, find your match.”

What are functional groups?

Specific atom groups responsible for characteristic chemical reactions.
Memo tip: “They give molecules their attitude.”