Why Use Spectroscopy?
Imagine you've run a chemical reaction and isolated a product — but how do you know what it is? You can't see individual molecules. Spectroscopy provides the answer: by shining electromagnetic radiation on a compound and measuring how it absorbs or responds, chemists can determine molecular structure, identify functional groups, and count atoms.
The three most important spectroscopic techniques in organic chemistry are: - Infrared (IR) spectroscopy: identifies functional groups - Nuclear Magnetic Resonance (NMR) spectroscopy: determines carbon skeleton and H environments - Mass Spectrometry (MS): measures molecular weight and fragmentation pattern
Together, these three techniques can often completely characterize an unknown organic compound.
Infrared (IR) Spectroscopy
How It Works
IR spectroscopy measures which frequencies of infrared radiation a molecule absorbs. When IR light passes through a sample, bonds absorb energy at specific frequencies that match their vibrational frequencies (stretching and bending).
The IR spectrum is a plot of % transmittance (or absorbance) vs. wavenumber (cm⁻¹, typically 4000–400 cm⁻¹). A "dip" in transmittance is called a band or absorption.
The energy of an IR absorption depends on: - Bond strength: stronger bonds vibrate at higher frequency (higher wavenumber) - Atomic mass: heavier atoms vibrate at lower frequency
Interpreting an IR Spectrum
The functional group region (4000–1500 cm⁻¹) contains diagnostic absorptions from specific functional groups:
| Wavenumber (cm⁻¹) | Bond/Group | Interpretation |
|---|---|---|
| 3200–3550 | O–H (broad) | Alcohol or carboxylic acid |
| 2500–3300 | O–H (very broad) | Carboxylic acid |
| 3300–3500 | N–H | Amine or amide |
| 2850–3000 | C–H | Alkane (below 3000) |
| 3000–3100 | =C–H | Alkene (above 3000) |
| ~3300 | ≡C–H | Terminal alkyne (sharp) |
| 2100–2260 | C≡C, C≡N | Triple bond |
| 1630–1680 | C=C | Alkene |
| ~1715 | C=O | Ketone |
| ~1725 | C=O | Aldehyde |
| ~1710 | C=O | Carboxylic acid |
| ~1735 | C=O | Ester |
| ~1680 | C=O | Amide |
The fingerprint region (1500–400 cm⁻¹) is complex and unique to each molecule — like a molecular fingerprint. It's used to confirm identity by comparison with a reference spectrum.
Practical Tips
- A broad, strong O–H absorption around 3300 cm⁻¹ strongly suggests an alcohol or carboxylic acid.
- A sharp, strong C=O near 1715 cm⁻¹ without broad O–H signals suggests a ketone.
- Absence of C=O rules out aldehydes, ketones, acids, esters, and amides.
- The 2850–3000 cm⁻¹ region (alkyl C–H) is almost always present.
Nuclear Magnetic Resonance (NMR) Spectroscopy
NMR is the most powerful tool for determining organic structure. It exploits the magnetic properties of certain nuclei — primarily ¹H (proton NMR) and ¹³C NMR — when placed in a strong magnetic field and irradiated with radiofrequency radiation.
The Chemical Shift (δ, in ppm)
Nuclei in different chemical environments absorb at different frequencies. The chemical shift (δ) is measured in parts per million (ppm) relative to a standard (TMS, tetramethylsilane, set to 0 ppm).
Electron-withdrawing groups (halogens, carbonyls, oxygens) pull electron density away from nearby nuclei, deshielding them → larger δ (further downfield).
Typical ¹H chemical shifts:
| Proton Environment | δ (ppm) |
|---|---|
| TMS (reference) | 0 |
| –CH₃ (alkyl) | 0.9–1.0 |
| –CH₂– (alkyl) | 1.2–1.4 |
| –C≡C–H | ~2.5 |
| –C–H (next to C=O) | 2.0–2.5 |
| –O–CH₃ | 3.3–3.5 |
| –CH₂–X (halide) | 3.0–3.5 |
| =C–H (vinyl) | 4.6–5.7 |
| Ar–H (aromatic) | 7.0–8.5 |
| –CHO (aldehyde) | 9.5–10.5 |
| –COOH | 10–12 |
| –OH, –NH | variable (2–5) |
Integration
The area under each NMR signal is proportional to the number of equivalent protons causing that signal. Relative integrations reveal the ratio of different proton types. For example, if two signals have areas in a 3:1 ratio, one represents 3H and the other 1H (or 6H:2H, etc.).
Spin-Spin Splitting (Coupling)
Non-equivalent adjacent protons interact through bonds, splitting each other's signals into multiplets. The n+1 rule: a proton with n neighboring non-equivalent protons is split into n+1 peaks.
- 0 neighbors → singlet (s)
- 1 neighbor → doublet (d)
- 2 neighbors → triplet (t)
- 3 neighbors → quartet (q)
The coupling constant (J) is the distance between split peaks in Hz. Typical vicinal coupling: J ≈ 6–8 Hz. This splitting pattern is the key to assigning which protons are adjacent to each other.
Example — ethanol (CH₃CH₂OH): - CH₃: split by 2 neighboring CH₂ protons → triplet at ~1.2 ppm - CH₂: split by 3 neighboring CH₃ protons → quartet at ~3.7 ppm - OH: singlet (exchange broadening) at ~2–5 ppm
¹³C NMR
¹³C NMR provides one signal per unique carbon environment. It's run as a broadband decoupled spectrum (all H-C couplings removed), giving sharp singlets. Carbon chemical shifts range from 0–220 ppm: - Carbonyl carbons (C=O): 160–220 ppm - Aromatic carbons: 110–160 ppm - Alkene carbons: 100–150 ppm - Alkyne carbons: 60–90 ppm - Alkyl carbons: 0–50 ppm
The number of signals tells you how many chemically distinct carbon environments exist — a key symmetry indicator.
Mass Spectrometry (MS)
How It Works
In mass spectrometry, the sample is vaporized and ionized — typically by electron bombardment (EI) or electrospray ionization (ESI). The ions are accelerated through a magnetic or electric field and separated by their mass-to-charge ratio (m/z).
The molecular ion peak (M⁺) gives the molecular weight of the compound. High-resolution MS can determine the molecular formula precisely (e.g., C₄H₈O vs. C₃H₄O₂ — same nominal mass, different exact masses).
Fragmentation Patterns
The molecular ion often fragments into smaller pieces. The fragmentation pattern is fingerprint-like — characteristic of specific structural features:
- Loss of 15: loss of CH₃ (methyl group present)
- Loss of 18: loss of H₂O (alcohol)
- Loss of 29: loss of CHO (aldehyde) or C₂H₅
- Loss of 45: loss of OC₂H₅ (ethyl ester)
- Loss of 77: loss of C₆H₅ (phenyl ring)
- McLafferty rearrangement: γ-hydrogen transfer in ketones/esters gives characteristic peaks
Base Peak and Relative Abundance
The base peak is the most intense peak in the mass spectrum (assigned 100% relative abundance). The molecular ion peak intensity decreases with increased branching (more stable carbocations form more easily from branched structures).
Isotope Patterns
The M+1 and M+2 peaks arise from naturally occurring isotopes. Carbon-13 (1.1% natural abundance) contributes to M+1 intensity proportional to the number of carbons. Bromine (⁷⁹Br/⁸¹Br, ~1:1 ratio) gives a distinctive M and M+2 peak of equal height — immediately recognizable as bromine-containing compound. Chlorine (³⁵Cl/³⁷Cl, ~3:1 ratio) gives M and M+2 peaks in 3:1 ratio.
Putting It All Together: Structure Determination Workflow
A systematic approach to structure determination from spectral data:
- MS → Molecular formula: What is the molecular weight? Any halogens (Cl/Br isotope patterns)?
- Degrees of unsaturation: Calculate (2C + 2 + N – H – X)/2. Each = 1 ring or π bond. 4 suggests an aromatic ring.
- IR → Functional groups: Is there a carbonyl? O–H? N–H? What type (ester, acid, aldehyde)?
- ¹H NMR → Proton environments: How many types of protons? Integration ratios? Splitting patterns?
- ¹³C NMR → Carbon skeleton: How many carbon environments? Any carbonyl carbons (>150 ppm)?
- Piece together the structure: Combine all information into a consistent structure.