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H3 spectroscopic techniques

Work through molecular orbitals, UV/visible, infrared, NMR and mass spectrometry in order, with diagrams and focused practice.

  • GCE A-Level H3 Chemistry 2027
  • 40 lessons
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Before you begin

Molecules have quantised energy levels, and each kind of spectroscopy detects a different kind of level. This topic starts with molecular orbitals, photons and energy levels, then covers UV–visible, infrared and NMR spectroscopy and mass spectrometry, and how to use their evidence to identify a compound.

Be comfortable with: atomic orbitals, σ and π bonds, organic functional groups and names, and logarithms and concentration in mol dm⁻³.

Learning goals
  • Distinguish atomic and molecular orbitals
  • Classify bonding, antibonding and nonbonding orbitals
  • Distinguish molecular orbitals with sigma and pi symmetry
  • Explain discrete molecular-orbital energy levels
  • Apply LCAO to homonuclear diatomic molecules
  • Apply LCAO to benzene and linear polyenes
  • Construct and interpret diatomic MO diagrams, including HOMO and LUMO
  • Construct and interpret pi-MO diagrams for benzene and polyenes
  • Relate electromagnetic radiation, photons and E = hf
  • Compare electronic, vibrational, rotational and nuclear energy quantisation
  • Explain photon absorption and emission as energy-level transitions
  • Relate electronic transitions and chromophores to UV/visible absorption
  • Predict UV/visible absorption from a chromophore
  • Explain conjugation, energy gaps and longer-wavelength absorption
  • Use the Beer–Lambert law in concentration calculations
  • Plan quantitative analysis using UV/visible spectroscopy
  • Describe stretching vibrations
  • Describe bending vibrations
  • Predict IR absorption count and vibrations for simple molecules
  • Identify characteristic functional-group IR absorptions
  • Suggest structures from IR spectra
  • Predict characteristic IR absorptions from structure
  • Relate polyatomic-gas IR absorption to the greenhouse effect
  • Explain nuclear spin
  • Explain energy absorption in NMR
  • Interpret chemical shift
  • Account for deuterated solvents and labile protons
  • Determine proton equivalence and signal count
  • Use peak integration to count protons
  • Interpret first-order spin–spin splitting and multiplicity
  • Use the delta scale and TMS reference
  • Explain electronegativity and inductive effects on shielding
  • Explain anisotropic effects on chemical shift
  • Explain hydrogen-bonding effects on chemical shift
  • Explain mass-spectrometric ionisation and fragmentation
  • Interpret mass-to-charge ratio
  • Identify a molecular-ion peak
  • Interpret M+1, M+2 and M+4 isotope patterns
  • Suggest major fragment ions without rearrangement
Syllabus statements covered
  • understand basic molecular orbital (MO) theory, involving: — atomic and molecular orbitals
  • understand basic molecular orbital (MO) theory, involving: — bonding, anti-bonding and non-bonding orbitals
  • understand basic molecular orbital (MO) theory, involving: — molecular orbitals with σ and π symmetry
  • understand that molecular orbitals represent discrete electronic energy levels in molecules (see also 1.1 (e)(ii))
  • apply linear combination of atomic orbitals (LCAO) principles to obtain the shape and relative energies of molecular orbitals in the following: — simple homonuclear diatomic molecules such as H2, O2, and F2
  • apply linear combination of atomic orbitals (LCAO) principles to obtain the shape and relative energies of molecular orbitals in the following: — benzene and linear polyenes (molecular orbitals of π symmetry only) [quantitative treatment of LCAO is not required]
  • construct and interpret molecular orbital diagrams, and identify the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the following: — simple homonuclear diatomic molecules such as H2, O2, and F2
  • construct and interpret molecular orbital diagrams, and identify the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the following: — benzene and linear polyenes (molecular orbitals of π symmetry only) [knowledge of orbital mixing between orbitals of the same symmetry is not required]
  • understand the following in relation to the fundamental principles of spectroscopy: — properties of electromagnetic radiation - the electromagnetic spectrum (with range of wavelengths for different types of radiation used in spectroscopy) - the photon as a discrete packet (quantum) of electromagnetic energy - the relationship between wavelength, frequency and speed of light, including the use of the equation, E = hf
  • understand the following in relation to the fundamental principles of spectroscopy: — the quantisation of energy in relation to - electronic, vibrational and rotational energy levels - nuclear energy levels in applied magnetic field
  • understand the following in relation to the fundamental principles of spectroscopy: — energy level transitions associated with the absorption and emission of photons with energy matching the energy gap
  • explain that ultraviolet/visible absorption in organic molecules requires electronic transitions (σ→σ*, n→σ*,π→π*, n→π* transitions; forbidden and allowed transitions) between energy levels in chromophores which contain a double or triple bond, a delocalised system, or a lone pair of electrons [detailed knowledge of instrumentation is not required]
  • predict whether a given organic molecule will absorb in the ultraviolet/visible region by identifying the chromophore
  • explain qualitatively how increasing conjugation in an organic molecule decreases the gap between energy levels and hence shifts the absorption towards longer wavelength
  • use the Beer–Lambert law, absorbance = lg(Io / I) = εcl, where ε is taken merely as a constant characteristic of the substance concerned, to calculate the concentration of a given species (either organic or inorganic) in solution
  • apply ultraviolet/visible spectroscopy to quantitative analysis of a given species (either organic or inorganic) in solution
  • explain the origin of IR spectroscopy in simple molecules in terms of: — stretching vibrations
  • explain the origin of IR spectroscopy in simple molecules in terms of: — bending vibrations [detailed knowledge of instrumentation is not required]
  • predict the number of IR absorptions for a given simple molecule (e.g. CO2 or SO2), and identify the molecular vibrations which give rise to them
  • identify characteristic IR absorptions in the IR spectrum of a compound which may contain different functional groups [absorptions of common functional groups will be provided in the Data Booklet]
  • suggest structures for a compound from its IR spectrum
  • predict the characteristic IR absorptions that will be present in the IR spectrum of a compound, given its structure
  • describe qualitatively, in terms of their IR absorption, the role of polyatomic gases (e.g. CO2, H2O, CHF3) in the greenhouse effect
  • outline the basic principles of NMR with reference to: — nuclear spin
  • outline the basic principles of NMR with reference to: — the process of absorption of energy [quantitative calculations of transitional energy are not required; detailed knowledge of instrumentation is not required]
  • understand the following features and use them in the interpretation and prediction of 1H NMR spectra: — chemical shift
  • understand the following features and use them in the interpretation and prediction of 1H NMR spectra: — deuterated solvents in the identification of labile protons
  • understand the following features and use them in the interpretation and prediction of 1H NMR spectra: — the number of 1H NMR signals: equivalent and non-equivalent protons
  • understand the following features and use them in the interpretation and prediction of 1H NMR spectra: — peak area (integration) and proton counting
  • understand the following features and use them in the interpretation and prediction of 1H NMR spectra: — spin-spin splitting: first order spin-spin coupling; multiplicity
  • explain the use of the δ scale with tetramethylsilane (TMS) as the reference
  • explain the factors affecting chemical shift: — electronegativity: inductive effect of substituents, including shielding and deshielding effects
  • explain the factors affecting chemical shift: — anisotropic effects
  • explain the factors affecting chemical shift: — hydrogen bonding
  • outline the basic principles of mass spectrometry, with reference to: — ionisation and fragmentation
  • outline the basic principles of mass spectrometry, with reference to: — mass/charge ratio, m/z [detailed knowledge of instrumentation is not required]
  • understand the following features and use them in the interpretation and prediction of mass spectra: — molecular ion peak
  • understand the following features and use them in the interpretation and prediction of mass spectra: — isotopic abundances including the use of (M+1) peak caused by 13C and (M+2) and (M+4) peaks for the identification of halogen compounds
  • understand the following features and use them in the interpretation and prediction of mass spectra: — major fragment ions [fragment ions obtained from rearrangements are not included]

How the ideas connect

  1. Molecular orbitals and energy levels
  2. UV–visible spectroscopy
  3. Infrared spectroscopy
  4. NMR spectroscopy
  5. Mass spectrometry
Explore the lessons

Lessons

Work through them in order.

1.1 Basic Principles of Spectroscopy

  1. Distinguish atomic and molecular orbitalsExplain how atomic orbitals combine into molecular orbitals that belong to the whole molecule.
  2. Classify bonding, antibonding and nonbonding orbitalsClassify a molecular orbital as bonding, antibonding or nonbonding from its phase and nodes.
  3. Distinguish molecular orbitals with sigma and pi symmetryLabel a molecular orbital σ or π by how it behaves when rotated about the bond axis.
  4. Explain discrete molecular-orbital energy levelsRead an MO diagram as a set of allowed energies and fill it using the Aufbau, Pauli and Hund rules.
  5. Apply LCAO to homonuclear diatomic moleculesCombine atomic orbitals of matching symmetry into bonding and antibonding orbitals for H₂, O₂ and F₂.
  6. Apply LCAO to benzene and linear polyenesWork out the π molecular orbitals that p orbitals form across buta-1,3-diene and benzene.
  7. Construct and interpret diatomic MO diagrams, including HOMO and LUMOBuild and fill MO diagrams for H₂, O₂ and F₂ to find bond order, unpaired electrons, HOMO and LUMO.
  8. Construct and interpret pi-MO diagrams for benzene and polyenesBuild π-MO diagrams for conjugated molecules from their nodes, then fill them to find the HOMO and LUMO.
  9. Relate electromagnetic radiation, photons and E = hfUse E = hf to link frequency, wavelength and photon energy across the electromagnetic spectrum.
  10. Compare electronic, vibrational, rotational and nuclear-spin levelsCompare the sizes of electronic, vibrational, rotational and nuclear-spin energy gaps in a molecule.
  11. Explain photon absorption and emission as energy-level transitionsDecide whether a transition absorbs or emits a photon, and match the photon's energy to the level gap.

1.2 Ultraviolet/visible Spectroscopy

  1. Relate electronic transitions and chromophores to UV/visible absorptionIdentify the chromophore and the electronic transition behind a UV/visible absorption.
  2. Predict UV/visible absorption from a chromophorePredict from a structure whether a compound absorbs in the UV or visible region.
  3. Explain conjugation, energy gaps and longer-wavelength absorptionExplain why extending conjugation narrows the HOMO–LUMO gap and moves absorption to longer wavelength.
  4. Use the Beer–Lambert law in concentration calculationsUse the Beer–Lambert law, A = εcl, to calculate concentration from absorbance.
  5. Plan quantitative analysis using UV/visible spectroscopyPlan a UV/visible determination, from choosing the wavelength to using a calibration graph.

1.3 Infrared (IR) Spectroscopy

  1. Describe stretching vibrationsRecognise stretching vibrations as changes in bond length, and explain why they absorb infrared.
  2. Predict stretching frequency with a supplied bond modelUse reduced mass and bond stiffness in a supplied harmonic model, keeping angular frequency, frequency and wavenumber distinct.
  3. Describe bending vibrationsRecognise bending vibrations as changes in bond angle, and explain when they absorb infrared.
  4. Predict IR absorption count and vibrations for simple moleculesWork out the vibrations of CO₂ and SO₂ and predict how many infrared absorptions each gives.
  5. Identify characteristic functional-group IR absorptionsIdentify functional groups from the position, shape and intensity of infrared bands.
  6. Suggest structures from IR spectraCombine a molecular formula with an infrared spectrum to suggest a structure.
  7. Predict characteristic IR absorptions from structurePredict the main infrared absorptions a given structure will show.
  8. Relate polyatomic-gas IR absorption to the greenhouse effectExplain why carbon dioxide and water absorb Earth's infrared radiation while nitrogen does not.

1.4 Nuclear Magnetic Resonance (NMR) Spectroscopy

  1. Explain nuclear spinExplain how a proton's spin gives two energy states in a magnetic field.
  2. Explain energy absorption in NMRExplain why radio waves are absorbed when their energy matches the gap between spin states.
  3. Interpret chemical shiftUse chemical shift to identify the environment of each set of protons.
  4. Account for deuterated solvents and labile protonsExplain why spectra use deuterated solvents, and how D₂O exchange reveals OH and NH protons.
  5. Determine proton equivalence and signal countCount proton environments from symmetry to predict how many signals a spectrum shows.
  6. Use peak integration to count protonsTurn integration areas into a whole-number ratio of protons and check it against the formula.
  7. Interpret first-order spin–spin splitting and multiplicityUse the n + 1 rule to link a signal's splitting to the number of neighbouring protons.
  8. Use the delta scale and TMS referenceUse TMS and the δ scale to report chemical shifts that do not depend on the instrument.
  9. Explain electronegativity and inductive effects on shieldingExplain how nearby electronegative atoms deshield protons and move their signals downfield.
  10. Explain anisotropic effects on chemical shiftExplain how circulating π electrons shift the signals of protons on benzene rings and double bonds.
  11. Explain hydrogen-bonding effects on chemical shiftExplain why OH and NH signals move with concentration and hydrogen bonding.

1.5 Mass Spectrometry

  1. Explain mass-spectrometric ionisation and fragmentationExplain how electron impact forms a radical cation that can break into fragment ions.
  2. Interpret mass-to-charge ratioCalculate m/z for ions with one or more charges.
  3. Identify a molecular-ion peakIdentify the molecular-ion peak and use it to find the relative molecular mass.
  4. Interpret M+1, M+2 and M+4 isotope patternsUse M+1, M+2 and M+4 peaks to count carbon atoms and detect chlorine or bromine.
  5. Suggest major fragment ions without rearrangementAssign formulae to the major fragment peaks from simple bond breaking, without rearrangement.

Practise and check

Recommended nextSpectroscopy principles topic check

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Topic reference

Quick reference

TechniqueWhat it detectsRadiationKey relationships and features
UV–visibleelectrons moving to a higher orbital (σ→σ*, n→σ*, π→π*, n→π*)ultraviolet and visibleA = lg(I₀/I) = ε cl; more conjugation, longer wavelength
infraredvibrations that change the dipoleinfrareddata booklet ranges; 3N − 6 (non-linear) or 3N − 5 (linear) modes
¹H NMRproton spin states in a magnetic fieldradio waveschemical shift δ from TMS; area = number of protons; n + 1 rule
mass spectrometryions, after ionisation and fragmentationnone: ions are sorted by m/zm/z; M+1 ≈ 1.1% per C; Cl gives M : M+2 = 3 : 1; Br gives 1 : 1
QuestionFirst moveWhat to show
Photon energyconvert λ to metresE = hf = hc/λ, per photon
Bond order from an MO diagramfill the levels, using Hund’s rule for degenerate pairs1/2(bonding − antibonding electrons); unpaired electrons: paramagnetic
Concentration from absorbancecheck the units of ε and lc = A/(ε l); multiply by any dilution factor
Number of IR absorptionscount modes from the shaperemove inactive modes; group degenerate ones
Assign an NMR signaluse shift, area and splitting separatelyenvironment, number of protons, neighbouring protons
Halogens from a mass spectrumread the M, M+2, M+4 clusterratio and spacing of the peaks

Core knowledge to remember

  • N atomic orbitals give N molecular orbitals. In-phase overlap gives a bonding orbital, out-of-phase overlap an antibonding orbital with a node between the nuclei.
  • σ orbitals are symmetrical around the bond axis; π orbitals have a nodal plane containing it. For O₂ and F₂: σ2s < σ2s < σ2p < π2p < π2p < σ*2p.
  • A molecule absorbs a photon only if its energy matches the gap between two levels. Gaps: electronic > vibrational > rotational > nuclear spin.
  • A forbidden transition is weak, not impossible. Absorbing in the ultraviolet does not make a compound coloured.
  • A vibration absorbs infrared radiation only if it changes the dipole. That is why CO₂, H₂O and CHF₃ are greenhouse gases but N₂ is not.
  • Electronegative atoms, π electrons and hydrogen bonding deshield protons, moving their signals downfield. O–H and N–H signals disappear after a D₂O shake.
  • The molecular ion contains the whole molecule; the base peak is the tallest peak. Only charged fragments are detected.

Common mistakes

  • Identifying a compound from one piece of evidence, such as a single IR absorption or NMR shift, when other structures still fit.
  • Saying more intense radiation can make up for the wrong frequency.
  • Counting the lines of an NMR multiplet as the number of protons.
  • Taking the base peak, or the peak furthest right, as the molecular ion.
  • Pairing the two π* electrons of O₂ in one orbital.
  • Reading M+2 as a charge of +2.