Mass spectrometry, IR and NMR · Grade 11
Spectroscopy identifies molecules from the way they respond to energy. A mass spectrometer gives the relative molecular mass and fragments of a molecule; infrared spectra show which bonds (and so which functional groups) are present; NMR spectra show how many different environments the carbon and hydrogen atoms are in, and how they are arranged.
Key facts
- In a mass spectrum, Ar = Σ(m/z × abundance) ÷ Σ(abundance). For a compound, the molecular ion peak M⁺ (furthest right, apart from a small M+1 peak) gives Mr; other peaks are fragments such as CH₃⁺ (15), C₂H₅⁺ (29) and CH₃CO⁺ (43).
- One chlorine atom gives M and M+2 peaks in a 3 : 1 ratio (³⁵Cl and ³⁷Cl); one bromine atom gives them in a 1 : 1 ratio. High-resolution mass spectrometry gives precise masses that tell apart formulas with the same whole-number mass.
- Infrared absorptions (cm⁻¹): O–H in alcohols 3230–3550 (broad), O–H in acids 2500–3000 (very broad), C=O 1680–1750, C–H 2850–3100, N–H 3300–3500. Below about 1500 cm⁻¹ is the fingerprint region.
- In ¹³C NMR, each different carbon environment gives one peak; carbons that are the same by symmetry give one peak.
- In ¹H NMR, the number of peaks is the number of hydrogen environments, and the area of each peak is proportional to the number of hydrogens in it.
- Splitting follows the n + 1 rule: a peak is split into n + 1 lines by n hydrogens on the neighbouring carbon atoms (singlet, doublet, triplet, quartet). O–H hydrogens do not usually split, and disappear when D₂O is added.
- TMS, Si(CH₃)₄, is the reference at δ = 0; samples are dissolved in solvents such as CDCl₃ that have no ¹H atoms.