Spectroscopy I: Unit Review

Introduction

This unit has introduced four techniques that together allow chemists to identify almost any small organic molecule. Each technique uses a different interaction between matter and energy, and each answers a different structural question. This review draws the threads together: what each method measures, the key numbers to remember, and how the methods combine into one reliable strategy for structure determination.

Core explanation

The shared foundation. Molecules have quantised energy levels. Absorption occurs only when the energy of a photon, E = hν, matches a gap between levels. Different regions of the electromagnetic spectrum probe different gaps: radio waves for nuclear spin, infrared for vibrations, and UV-visible for electronic transitions. Mass spectrometry is different: it does not use absorption but measures the mass-to-charge ratio of ions.

Infrared spectroscopy — functional groups. Bonds vibrate like springs; stronger bonds and lighter atoms vibrate at higher wavenumbers. Only vibrations that change the dipole moment absorb. Key bands:

Bond Wavenumber / cm⁻¹ --- --- O–H (alcohol, broad) 3200–3550 O–H (acid, very broad) 2500–3300 N–H 3300–3500 C–H 2850–3100 C≡N, C≡C 2100–2260 C=O 1680–1750 C=C 1620–1680

The fingerprint region below 1500 cm⁻¹ identifies a compound by matching.

UV-visible spectroscopy — conjugation and concentration. Electrons are promoted, mainly π → π and n → π , in chromophores. Extended conjugation narrows the energy gap and moves λmax to longer wavelength, eventually into the visible region, producing colour; we see the complementary colour. The Beer–Lambert law, A = εcl, links absorbance to concentration, and calibration curves make UV-visible spectroscopy a key quantitative tool.

NMR spectroscopy — the carbon–hydrogen framework. Nuclei such as ¹H and ¹³C behave as tiny magnets in a strong field and absorb radio waves at frequencies that depend on their electronic environment. The chemical shift, measured in ppm from TMS, reflects shielding: electronegative atoms, carbonyl groups and aromatic ring currents deshield nuclei. In ¹H NMR, integration gives relative numbers of protons and splitting by the n+1 rule gives the number of neighbouring protons. ¹³C NMR counts carbon environments over a range of about 0–220 ppm. Deuterated solvents avoid solvent signals, and a D₂O shake identifies OH and NH protons.

Mass spectrometry — mass and fragments. Molecules are ionised, often to radical cations, and separated by m/z. The molecular ion gives Mᵣ; high resolution gives the exact molecular formula. The M+1 peak estimates the carbon count, and M+2 reveals Cl (3:1) or Br (1:1). Fragmentation favours stable cations — tertiary carbocations, acylium, oxonium and iminium ions — and the base peak is the most abundant ion.

The combined strategy. Formula (MS) → degree of unsaturation → functional groups (IR) → carbon environments (¹³C NMR) → hydrogen environments and connectivity (¹H NMR) → check against fragments.

Step-by-step reasoning

To choose a technique, ask what you need to know:

1. What is the Mᵣ or formula? Use mass spectrometry. 2. Which functional groups are present? Use IR. 3. How are the atoms connected? Use NMR. 4. How much is present, or how conjugated is it? Use UV-visible.

Visual explanation

Picture a single molecule, such as ethyl ethanoate, at the centre of four arrows. The IR arrow labels its C=O and C–O bonds; the NMR arrow labels the quartet, singlet and triplet; the mass spectrometry arrow labels Mᵣ 88 and the fragment at 43; the UV-visible arrow notes only a weak absorption because it is not conjugated.

Real-world analogy

The four techniques are like four specialists examining a building. A surveyor measures its size (mass spectrometry), an electrician identifies the fittings (IR), an architect draws the floor plan (NMR), and a lighting engineer measures how it reacts to light (UV-visible). Together they describe the building completely.

Real-world example

When a pharmaceutical company registers a new drug, its structure is supported by a full data set: high-resolution mass spectrum, IR spectrum, ¹H and ¹³C NMR spectra and UV-visible data. The same techniques are then used routinely to confirm identity and purity of every manufactured batch.

Why?

Why use several techniques rather than one very powerful one? Each technique has blind spots: IR cannot give Mᵣ, mass spectrometry often cannot separate isomers, and NMR may not reveal a functional group directly. Their weaknesses rarely overlap, so combining them gives confident, cross-checked answers.

Common misconception

"NMR alone can solve every structure." NMR is powerful, but without a molecular formula you cannot be sure how many protons an integration ratio represents, and groups with no hydrogen atoms, such as C=O or C≡N, can be invisible in ¹H NMR.

Worked example

Question: A compound C₂H₄O₂ shows a very broad IR band from 2500 to 3300 cm⁻¹ and a band at 1710 cm⁻¹. Its ¹H NMR spectrum shows a singlet (3H) near 2.1 ppm and a broad singlet (1H) near 11.5 ppm. Identify it.

Reasoning: DBE = (4 + 2 − 4) ÷ 2 = 1, accounted for by C=O. The IR bands indicate a carboxylic acid. The 3H singlet is CH₃ next to C=O with no neighbouring protons; the signal near 11.5 ppm is the acid OH.

Answer: Ethanoic acid, CH₃COOH.

Quick check

1. Which technique would you use to measure the concentration of a coloured complex ion in solution? Answer: UV-visible spectroscopy (colorimetry), applying the Beer–Lambert law with a calibration curve.

Exam focus

Expect multi-part questions that give data from several techniques. Learn the core data tables, show each deduction linked to its evidence, and finish with a structure that fits the molecular formula and every spectrum. Use correct units: cm⁻¹, nm, ppm and m/z.

Advanced insight

The next level of spectroscopy extends each technique: two-dimensional NMR maps connectivity directly, tandem mass spectrometry fragments selected ions to sequence proteins, and Raman spectroscopy detects vibrations that change polarisability, complementing IR. The underlying principles — quantised energy, selection rules and ion stability — remain the same.

Summary

IR identifies functional groups from bond vibrations; UV-visible spectroscopy probes conjugation and measures concentration through A = εcl; NMR reveals the carbon–hydrogen framework through chemical shift, integration and splitting; mass spectrometry gives Mᵣ, formula and fragments. Combined in a logical order with the degree of unsaturation, they allow confident structure determination.

Practice questions

1. Which technique detects vibrations and why must the dipole moment change? Answer: IR spectroscopy; the oscillating dipole must interact with the electric field of the radiation for energy to be absorbed. 2. A compound shows a molecular ion at m/z 72 and IR absorption at 1715 cm⁻¹, with no O–H band and no aldehyde C–H bands. Its ¹H NMR spectrum shows a singlet (3H), a quartet (2H) and a triplet (3H). Identify it. Answer: Butanone, CH₃COCH₂CH₃: a ketone of Mᵣ 72 with a CH₃CO singlet and an ethyl group. 3. Why does β-carotene appear orange while ethene is colourless? Answer: β-Carotene has an extended conjugated system that lowers the π → π energy gap so it absorbs blue light; ethene absorbs only in the far ultraviolet. 4. State what the M+2 peak reveals when it is equal in height to the M peak. Answer: The compound contains one bromine atom, because ⁷⁹Br and ⁸¹Br are present in roughly equal amounts.