Spectroscopy - ChemHub

🔴Infrared (IR)

IR spectroscopy identifies functional groups. Bonds absorb specific IR frequencies (wavenumbers). Every organic functional group has a characteristic absorption region.

Bonds in molecules vibrate at specific frequencies. When IR radiation of the same frequency hits the bond, it is absorbed. The pattern of absorptions is unique to each functional group (and molecule). A spectrum is a plot of transmittance vs. wavenumber (cm⁻¹).

Range / Position Group / Assignment Note

2500-3300 cm⁻¹ O-H (carboxylic acid) Very broad, strong absorption. Distinctive "bump".

3200-3550 cm⁻¹ N-H (amine/amide) Medium, can be single or double peak.

2850-3000 cm⁻¹ C-H (alkane) Multiple medium peaks. Present in almost all organics.

3300 cm⁻¹ O-H (alcohol) Broad, strong peak. Disappears with D₂O exchange.

1700-1750 cm⁻¹ C=O (carbonyl) Strong, sharp peak. Exact position identifies type: aldehyde, ketone, ester, acid.

1620-1680 cm⁻¹ C=C (alkene) Medium strength. Absent in alkanes.

2100-2260 cm⁻¹ C≡C or C≡N Weak-medium. Sharp narrow peak.

1000-1300 cm⁻¹ C-O (ether/ester) "Fingerprint region" - complex, unique to each molecule.

🧲NMR Spectroscopy

Nuclear Magnetic Resonance identifies the environments of specific nuclei (usually ¹H or ¹³C). Each chemically different hydrogen gives a different signal (chemical shift).

Nuclei with odd mass number (like ¹H, ¹³C) behave like tiny magnets. In a strong external field, they precess at a Larmor frequency. Electron density around a nucleus shields it - less electron density = higher chemical shift (downfield). Splitting patterns reveal neighboring H atoms (n+1 rule).

Range / Position Group / Assignment Note

0-2 ppm Alkyl C-H TMS reference at 0 ppm. CH₃, CH₂, CH groups of plain alkyl chains.

2-3 ppm C-H next to C=O Deshielded by adjacent carbonyl. Methyl ketones appear ~2.1 ppm.

3.5-4.5 ppm C-H next to O or N Ethers ~3.4 ppm. Esters ~4.2 ppm. Large deshielding by electronegative atoms.

5-6 ppm Alkene =C-H C=C deshields the proton. Vinyl protons are distinctive.

7-8 ppm Aromatic C-H Benzene ring currents strongly deshield aromatic H. Very diagnostic.

9-10 ppm Aldehyde C-H The most deshielded C-H - both C=O and C-H combined effects.

10-12 ppm Carboxylic O-H Very deshielded. Broad. Exchangeable with D₂O.

⚖️Mass Spectrometry

Mass spec measures the mass-to-charge ratio (m/z) of ionised fragments. It gives the molecular formula and structural clues from fragmentation patterns.

Sample is vaporised and ionised (often by electron bombardment). Ions are accelerated and deflected by a magnetic field - heavier ions deflect less. A detector records the m/z ratio and abundance of each ion. Fragmentation patterns reveal the molecular structure.

Range / Position Group / Assignment Note

M⁺ peak Molecular ion Highest m/z peak (usually). Gives the molecular mass. Not always visible - some molecules fragment completely.

M+1 peak Isotope peak (¹³C) ¹³C is 1.1% abundant - M+1 peak is 1.1% per carbon. Count carbons from this ratio.

M+2 peak Isotope (Br/Cl) Cl: M:M+2 = 3:1. Br: M:M+2 = 1:1. Distinctive isotope patterns immediately reveal halogens.

m/z = 15 CH₃⁺ Loss of methyl from M⁺. Very common fragment.

m/z = 29 CHO⁺ or C₂H₅⁺ Ethyl cation or formyl cation.

m/z = 77 C₆H₅⁺ Phenyl cation - diagnostic for benzene ring.

🌈UV-Vis Spectroscopy

UV-Visible spectroscopy measures how molecules absorb light in the 200-800 nm range. Conjugated systems and transition metal complexes give characteristic absorptions.

Beer-Lambert Law: A = εcl (Absorbance = molar absorptivity × concentration × path length). Useful for quantitative analysis - measuring the concentration of a coloured solution. Also used to study conjugation, and in protein/DNA quantification (A₂₆₀ for nucleic acids, A₂₈₀ for proteins).

Range / Position Group / Assignment Note

<250 nm Simple alkenes, alkynes Isolated double bonds absorb in far UV. Not practically useful.

250-400 nm Conjugated systems More conjugated = longer wavelength absorption. Polyenes and aromatic compounds.

400-700 nm Highly conjugated/d-orbital Visible absorption = the molecule appears coloured. Complementary colour is observed.

λmax varies Transition metal complexes d-d transitions. MnO₄⁻ is purple, CuSO₄ is blue, Cr₂O₇²⁻ is orange.

🌈 Spectroscopy

🔬 Spectroscopic Methods