Chemistry, Application of Spectroscopy Complete Notes Unit-1

 A. VIBRATIONAL SPECTROSCOPY

Explanation (Easy + M.Sc. Level)

Vibrational spectroscopy studies the vibrational motion of molecules when they absorb IR or Raman radiation. Molecules like AB₂, AB₃, etc., have different shapes (linear, bent, trigonal planar), and vibrational modes depend on symmetry.

Raman vs IR

IR requires change in dipole moment

Raman requires change in polarizability

Used for studying bonding modes of ambidentate ligands (SCN⁻, NO₂⁻, SO₄²⁻, urea) and ionic equilibrium in solutions.

Very Short (1-liners)

1. Vibrational spectroscopy studies bond vibrations.

2. IR active vibration needs dipole moment change.

3. Raman active vibration needs polarizability change.

4. Bent molecules show asymmetric stretching.

5. AB₂ (linear) has 2 fundamental vibrations.

Short Notes

Modes of Vibration

Stretching (symmetric, asymmetric) and bending (scissoring, rocking, wagging, twisting) are basic vibrational modes.

Bonding of Ambidentate Ligands

SCN⁻ (thiocyanate), NO₂⁻, SO₄²⁻ show different IR/Raman peaks based on coordination through N/O/S.

Long Answer

Vibrational Spectroscopy in Study of Molecular Structure

Explain:

symmetry of AB₂ (linear/bent)

vibrations depend on mass and bond strength

Raman helps identify symmetric modes

Ambidentate ligand coordination shifts IR peaks

Example: SCN⁻:

N-bonded: 2050 cm⁻¹

S-bonded: 2100 cm⁻¹

Used to determine structure, bonding mode, and ion equilibria.

MCQs

1. IR active vibration requires:

(B) Change in dipole moment

2. Raman active vibration requires:

(C) Change in polarizability

3. Linear AB₂ has:

(A) 2 vibrations

4. SCN– S-bonded peak is at:

(D) 2100 cm⁻¹

5. Which studies polarizability?

(C) Raman spectroscopy

B. ELECTRON SPIN RESONANCE (ESR) SPECTROSCOPY

Explanation

ESR studies species with unpaired electrons. Magnetic field splits spin states (Zeeman splitting). Microwave radiation causes transitions.

Used for atoms, transition metal ions, and radicals like PH₄, F₂, BH₃.

1-liners

1. ESR is also called EPR.

2. Required species: unpaired electron.

3. ESR uses microwave radiation.

4. Splitting occurs due to Zeeman effect.

5. g-tensor gives information about electronic environment.

Short Notes

Hyperfine Coupling

Interaction between electron spin and nuclear spin; creates multiple ESR lines.

Spin–Orbit Coupling

Gives information on metal complexes: direction of distortion, ligand field, electron density.

Long Answer

Describe:

Principles

Zeeman effect

Hyperfine splitting formula

ESR of transition metals with one unpaired electron

Radicals PH₄, BH₃ ESR patterns

MCQs

1. ESR uses:

(C) Microwave radiation

2. ESR active species must have:

(A) Unpaired electron

3. Hyperfine splitting arises from:

(B) Nuclear spin

4. g value of free electron:

(D) 2.0023

5. Zeeman effect relates to:

(C) Magnetic field

C. MOSSBAUER SPECTROSCOPY

Explanation

Mossbauer spectroscopy is based on recoil-free gamma-ray absorption. It gives information on oxidation state, bonding, spin, and coordination number.

Common nuclei: Fe-57, Sn-119.

1-liners

1. Mossbauer effect requires recoil-free gamma rays.

2. Most used isotope: Fe-57.

3. Isomer shift indicates electron density at nucleus.

4. Quadrupole splitting shows asymmetry of electron cloud.

5. Sn²⁺ and Sn⁴⁺ show different Mossbauer shifts.

Short Notes

Isomer Shift

Measures s-electron density → oxidation state difference.

Quadrupole Splitting

Arises from electric field gradient → crystal symmetry.

Applications

Fe²⁺ vs Fe³⁺ identification, bonding in complexes, spin states.

Long Answer

Explain:

Mossbauer effect

Isomer shift equation

Quadrupole splitting

Bonding and structure in Fe²⁺/Fe³⁺ complexes

Mossbauer parameters of Sn²⁺ and Sn⁴⁺

MCQs

1. Mossbauer spectroscopy uses:

(D) Gamma rays

2. Most used isotope:

(A) Fe-57

3. Isomer shift gives info on:

(C) Electron density

4. Quadrupole splitting arises from:

(B) Electric field gradient

5. Sn²⁺ and Sn⁴⁺ differ due to:

(A) Isomer shift

D. UV–VISIBLE SPECTROSCOPY

Explanation

Studies electronic transitions in molecules (π → π*, n → π*). Beer–Lambert law relates absorbance to concentration.

1-liners

1. UV range: 200–400 nm.

2. Visible: 400–800 nm.

3. n → π* transition is weaker.

4. π → π* is intense.

5. Beer–Lambert law: A = εbc.

Short Notes

Effect of substituents: auxochromes cause bathochromic shift (red shift).

Conjugation decreases energy gap → λmax increases.

Long Answer

Discuss:

UV principles

Electronic transitions

Solvent effect

Conjugation and shifts

Applications in aromatic and carbonyl compounds

MCQs

1. n → π* transition occurs in:

(A) Carbonyl compounds

2. Beer–Lambert law relates absorbance to:

(B) Concentration

3. Red shift means:

(D) λmax increases

4. UV uses light from:

(C) 200–800 nm

5. π → π* intensity is:

(B) High