Spectroscopy – IR, UV-Vis, NMR Fundamentals engineering chemistry notes by Mohan Dangi (Gold medalist)

Unit Spectroscopy – IR, UV-Vis, NMR Fundamentals engineering chemistry notes by Mohan Dangi (Gold medalist)

Spectroscopy studies how matter interacts with electromagnetic radiation. IR, UV-Vis, and NMR are essential analytical techniques for identifying functional groups, electronic structure, and molecular environments.

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1. Infrared (IR) Spectroscopy

1.1 Principle and Instrumentation

IR spectroscopy measures absorption of infrared light (4000–400 cm⁻¹) by molecular vibrations.

• Source: Globar or Nernst lamp
• Monochromator: Prism or grating to select wavenumbers
• Sample Compartment: Liquid cell (NaCl windows) or KBr pellet for solids
• Detector: Thermocouple, pyroelectric detector (DTGS), or Mercury Cadmium Telluride (MCT)

1.2 Vibrational Modes and Frequency Ranges

Key bond stretches and bends occur in characteristic wavenumber ranges:

Functional Group	Stretch/Bend	Wavenumber (cm−1)
O–H (alcohols/phenols)	Stretch	3200–3600 (broad)
N–H (amines, amides)	Stretch	3300–3500 (sharp)
C=O (carbonyls)	Stretch	1650–1750
C≡N (nitriles)	Stretch	2210–2260
C=C (alkenes)	Stretch	1600–1680
C–H (alkanes)	Stretch	2850–2960

1.3 Sample IR Spectrum Interpretation

1. Identify broad O–H band near 3400 cm⁻¹ for alcohols.
2. Locate strong C=O band at ~1700 cm⁻¹ for ketones/esters.
3. Confirm C–H stretches below 3000 cm⁻¹.

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2. Ultraviolet-Visible (UV-Vis) Spectroscopy

2.1 Principle and Instrumentation

UV-Vis spectroscopy measures absorption of 190–800 nm light by electronic transitions.

• Light Sources: Deuterium lamp (190–400 nm), tungsten lamp (400–800 nm)
• Monochromator: Diffraction grating to select wavelength
• Sample Cell: Quartz cuvette for UV, glass cuvette for visible
• Detector: Photomultiplier tube (PMT) or photodiode array (PDA)

2.2 Electronic Transitions and Chromophores

• π → π* transitions in conjugated systems (alkenes, aromatics)
• n → π* transitions in nonbonding electrons (carbonyls, nitro groups)
• Charge transfer bands in complexes (e.g., metal–ligand)

2.3 Beer–Lambert Law

A = ε b c
where
A = absorbance,
ε = molar absorptivity (L·mol−1·cm−1),
b = path length (cm),
c = concentration (mol/L).

Plotting A vs c yields a straight line through origin if Beer–Lambert holds.

2.4 Quantitative and Qualitative Applications

• Determination of concentration of DNA or protein at 260 nm and 280 nm
• Monitoring reaction kinetics by measuring absorbance change with time
• Characterization of dyes and pigments via λ_max
• Assessment of purity by comparing peak shape and baseline

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3. Nuclear Magnetic Resonance (NMR) Spectroscopy

3.1 Principle and Instrumentation

NMR detects resonance of magnetic nuclei (¹H, ¹³C) in a strong external field (B₀), perturbed by radiofrequency (RF) pulses.

• Magnet: Superconducting magnet (300–900 MHz for ¹H)
• RF Transmitter/Receiver: Generates and detects pulses
• Probe: Contains sample and coil for RF field
• Shimming and Lock: Ensures field homogeneity and frequency stability

3.2 Key Parameters

• Chemical Shift (δ): δ (ppm) = (ν_sample – ν_ref)/ν_ref × 10⁶
• Spin–Spin Coupling (J): Splitting in Hz between peaks due to neighboring nuclei
• Integration: Area under ¹H peaks corresponds to relative proton count
• T₁ and T₂ Relaxation: Time constants for longitudinal and transverse relaxation

3.3 Typical Chemical Shift Ranges (¹H NMR)

Proton Type	Chemical Shift (δ, ppm)
Alkane (R–CH₃, R–CH₂–R)	0.9–2.0
Allylic/Benzylic (C=C–CH)	1.6–2.5
Alcohol (R–OH)	1.0–5.5 (broad)
Aldehyde (R–CHO)	9.0–10.0
Aromatic (Ar–H)	6.0–8.5
Carboxylic acid (R–COOH)	10–13 (broad)
Alkene (C=CH)	4.5–6.5

3.4 NMR Spectrum Interpretation Workflow

1. Count signals: Number of distinct proton environments
2. Check integration: Ratio of integrals → relative proton count
3. Analyze splitting: n+1 rule to find number of adjacent protons
4. Assign chemical shift: Correlate δ value with functional groups
5. Construct structure: Use combination of above data

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4. Comparative Summary

Technique	Wavelength/Region	Transition	Info Obtained
IR	4000–400 cm−1	Vibrational (stretch/bend)	Functional groups, bonding, isomerism
UV-Vis	190–800 nm	Electronic (π→π*, n→π*)	Conjugation, concentration, kinetics
NMR (¹H)	Radiofrequency (MHz)	Spin transitions (I=½)	Detailed structure, environment, dynamics

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5. Essential Equations

• Planck’s relation: E = hν = hc/λ
• Beer–Lambert Law: A = ε b c
• Chemical Shift: δ (ppm) = (ν – ν_ref)/ν_ref × 10^6
• Spin–Spin Splitting: n + 1 peaks for n neighbors

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6. Solved Example – NMR

Problem: A molecule shows three ¹H NMR signals at δ 1.25 (t, 3H), δ 3.70 (q, 2H), δ 7.26 (s, 5H). Propose the structure.

Solution: Triplet integrating to 3H (–CH₃ adjacent to –CH₂–), quartet 2H (–CH₂ adjacent to –CH₃), singlet 5H aromatic → Ethylbenzene (Ph–CH₂–CH₃).

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7. Competitive Exam Focus Points

• Recall IR frequency ranges for major functional groups
• Apply Beer–Lambert law for concentration calculations
• Interpret UV-Vis spectra: understand λ_max shifts and molar absorptivity
• Use ¹H NMR splitting patterns and chemical shifts for structure elucidation
• Distinguish information types from IR, UV-Vis, and NMR

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This comprehensive guide covers IR, UV-Vis, and NMR spectroscopy fundamentals with detailed principles, instrumentation, characteristic data tables, workflow for spectrum interpretation, and solved examples—designed for engineering chemistry and competitive exam preparation.