Introduction to Spectroscopy
What is Spectroscopy?
⚗️ Spectroscopy is a technique used for structural elucidation of molecules. It is based on the interaction of atoms or molecules with electromagnetic radiation.
Key Principles:
- Different molecules absorb specific wavelengths of electromagnetic radiation
- Absorption causes transitions between energy states
- The absorption pattern provides a “fingerprint” of the molecule
- Different techniques probe different molecular properties
UV-Visible
Electronic transitions
200-800 nm
IR Spectroscopy
Vibrational transitions
4000-625 cm⁻¹
NMR Spectroscopy
Nuclear spin transitions
Radio frequency
Mass Spectrometry
Mass-to-charge ratio
Molecular mass determination
Learning Strategy:
Focus on understanding what each technique reveals about molecular structure and how they complement each other in structural elucidation.
Index of Hydrogen Deficiency (IHD)
Calculating Degrees of Unsaturation
🧮 The IHD indicates the total number of rings and π-bonds in a molecule.
Where: C = # of Carbons, N = # of Nitrogens, H = # of Hydrogens, X = # of Halogens
IHD Calculator
Example: Ethene (C₂H₄)
IHD = (2×2 + 2 – 4 – 0)/2 = (4 + 2 – 4)/2 = 2/2 = 1
This confirms one double bond in ethene.
Example: Benzene (C₆H₆)
IHD = (2×6 + 2 – 6 – 0)/2 = (12 + 2 – 6)/2 = 8/2 = 4
This indicates one ring and three double bonds in benzene.
IR Spectroscopy
Functional Group Identification
📊 IR spectroscopy identifies functional groups through their characteristic absorption frequencies.
| Functional Group | Wavenumber (cm⁻¹) | Intensity |
|---|---|---|
| O-H (alcohols/phenols) | 3200-3500 | Broad, strong |
| N-H | 3300-3500 | Medium |
| ≡C-H (alkyne) | ~3300 | Strong |
| C-H (alkane) | 2850-2960 | Medium-strong |
| C=O (carbonyl) | 1630-1780 | Very strong |
| C=C (alkene) | 1620-1680 | Variable |
| C-O (alcohol/ether) | 1050-1150 | Strong |
IR Spectrum of Ethanol (C₂H₅OH)
Memorization Tip:
Fingerprint Region: Below 1500 cm⁻¹ – unique to each compound but difficult to interpret.
Functional Group Region: Above 1500 cm⁻¹ – used for identification of functional groups.
UV-Visible Spectroscopy
Electronic Transitions and Color
🎨 UV-Vis spectroscopy studies electronic transitions and predicts color based on absorbed wavelengths.
400-435 nm
435-480 nm
480-490 nm
490-500 nm
500-560 nm
560-580 nm
580-700 nm
Color Prediction Rule:
The color observed is complementary to the color absorbed.
If a compound absorbs violet light (400-435 nm), it will appear yellow-green.
Example: [Ti(H₂O)₆]³⁺
This complex absorbs yellow and green light (500-560 nm), so it appears violet.
Color Predictor
NMR Spectroscopy
¹H NMR (Proton NMR)
🔍 ¹H NMR provides information about hydrogen environments in molecules.
Proton NMR of Ethyl Chloride (CH₃-CH₂-Cl)
Key NMR Concepts:
- Chemical Shift (δ): Measured in ppm, indicates electronic environment
- Integration: Area under peaks shows relative number of protons
- Spin-Spin Splitting: Follows n+1 rule for adjacent protons
- TMS Reference: Tetramethylsilane at 0 ppm
Multiplicity Rules (n+1 Rule)
- 0 neighbors → Singlet (1 peak)
- 1 neighbor → Doublet (2 peaks)
- 2 neighbors → Triplet (3 peaks)
- 3 neighbors → Quartet (4 peaks)
In ethyl chloride: CH₂ has 3 neighbors → Quartet, CH₃ has 2 neighbors → Triplet
¹³C NMR (Carbon NMR)
🔬 ¹³C NMR provides information about carbon environments in molecules.
Predicting Number of ¹³C NMR Peaks
- Ethanol (CH₃-CH₂-OH): 2 peaks (CH₃ and CH₂)
- Acetone ((CH₃)₂C=O): 2 peaks (CH₃ and C=O)
- Benzene (C₆H₆): 1 peak (all carbons equivalent)
- Butane (CH₃-CH₂-CH₂-CH₃): 2 peaks (CH₃ and CH₂)
Chemical Shift Ranges (¹³C NMR):
- Carbonyl (C=O): 160-220 ppm
- Aromatic carbons: 120-140 ppm
- Alkene carbons: 100-150 ppm
- C bonded to O: 50-90 ppm
- Alkyl CH₂, CH₃: 0-50 ppm
Practice Exercises
Quick Navigation
- Introduction
- IHD Calculator
- IR Spectroscopy
- UV-Vis Spectroscopy
- NMR Spectroscopy
- Practice Exercises