KHS ChemistryUnit 3.4 Structural Analysis1 Structural Analysis 2 Adv Higher Unit 3 Topic 4 Gordon Watson Chemistry Department, Kelso High School

KHS ChemistryUnit 3.4 Structural Analysis2 Introduction Structural AnalysisIR & NMR Spectroscopy X-Ray Crystallography This topic continues to explore methods used in the Structural Analysis of organic molecules including IR & NMR Spectroscopy and X-Ray Crystallography.

KHS ChemistryUnit 3.4 Structural Analysis3 Aspects of Spectroscopy Electromagnetic Radiation Spectroscopy was introduced in Unit 1 as a technique that uses the interaction between Electromagnetic Radiation and particles to help determine structure.

KHS ChemistryUnit 3.4 Structural Analysis4 Visible Spectrum 400 nm750 nm Visible Light Longer Wavelength ( )Shorter Wavelength ( ) Higher Frequency (  )Lower Frequency (  ) Higher Energy (E)Lower Energy (E) E = h E = h

KHS ChemistryUnit 3.4 Structural Analysis5 Infra-Red Radiation Visible Light Just below red in the visible region. Wavelengths usually nm. IR Spectroscopywavenumbers nu 1/,cm -1 IR Spectroscopy uses units called wavenumbers ( ), nu, the reciprocal of the wavelength, (1/ ), in centimeters (cm -1 ). UltravioletInfrared Wavenumbersfrequencyenergy Wavenumbers are proportional to frequency and energy. Wavenumbers usually cm -1

KHS ChemistryUnit 3.4 Structural Analysis6 Molecular Vibrations vibration states Molecules have a variety of possible vibration states. Stretchingchanges the distance Some of these states are due to Stretching: which changes the distance between atoms in the molecule. Bendingchanges the angle Some of these states are due to Bending: which changes the angle between atoms in the molecule.

KHS ChemistryUnit 3.4 Structural Analysis7 Stretching & Bending

KHS ChemistryUnit 3.4 Structural Analysis8 Molecular Dipoles Molecular Dipole polar Many of these vibrations can cause a change in the Molecular Dipole - especially if the bond is polar. electrical field electromagnetic The fluctuating electrical field produced can interact with the electric field of electromagnetic radiation frequencyof the radiation frequency of the vibration If the frequency of the radiation matches the frequency of the vibration - then energy will be absorbed. Infra Red Molecular vibrations are relatively low energy - Infra Red

KHS ChemistryUnit 3.4 Structural Analysis9 IR Active IR inactive If a vibration has no effect on the dipole of the molecule then it will be unable to absorb radiation - IR inactive symmetric IR inactive The symmetric stretching of the O—H bonds in water will be IR inactive assymmetric IR active The assymmetric stretching of the O— H bonds in water will be IR active

KHS ChemistryUnit 3.4 Structural Analysis10 Stretching & Bending Stretchinghigher energy Stretching vibrations are of higher energy cm -1 Stretchingabsorb strongly Stretching vibrations tend to absorb strongly to produce large distinct peaks. functional groups cm -1 is where functional groups can be identified. Bendinglower energy Bending vibrations tend to be of lower energy cm -1 Bendingabsorb weakly Bending absorb weakly to produce complex indistinct peaks. fingerprint region cm -1 is the fingerprint region

KHS ChemistryUnit 3.4 Structural Analysis11 Frequencies 1 Alkanes The spectra of Alkanes are among the ‘simplest’ and, since most organic molecules contain alkyl groups, this is effectively the background upon which other functional groups will appear.

KHS ChemistryUnit 3.4 Structural Analysis12 Frequencies 1 Alcoholsaminesbroad strong Alcohols and amines display broad O-H and N-H stretching bands in the region cm -1. The O-H absorbtion is particularly strong.

KHS ChemistryUnit 3.4 Structural Analysis13 Alcohols & Amines

KHS ChemistryUnit 3.4 Structural Analysis14 Frequencies 2 Alkenealkynesharp medium Alkene and alkyne C-H bonds display sharp stretching absorptions in the region cm -1. The bands are of medium intensity

KHS ChemistryUnit 3.4 Structural Analysis15 Alkene & Alkyne

KHS ChemistryUnit 3.4 Structural Analysis16 Frequencies 3 Carbonyl very strongbroad Carbonyl stretching bands occur in the region cm. The bands are generally very strong and can be broad.

KHS ChemistryUnit 3.4 Structural Analysis17 Aldehydes Carbonyl Carbonyl band tends to be at lower end of cm -1 region. A characteristic double peak between cm -1.

KHS ChemistryUnit 3.4 Structural Analysis18 Ketones Carbonyl No Carbonyl band also tends to be at lower end of cm -1 region. No characteristic double peak between cm -1.

KHS ChemistryUnit 3.4 Structural Analysis19 Esters Carbonyl unreliable Carbonyl band tends to be slightly higher in cm -1 region. C—O stretches can sometimes be picked out (unreliable).

KHS ChemistryUnit 3.4 Structural Analysis20 Acids Carbonyl Carbonyl band tends to be typical. Hydroxyleven broader hydrogen bonding dimerisation Hydroxyl band tends to be even broader than usual. This is due to strong hydrogen bonding between molecules - often leads to dimerisation

KHS ChemistryUnit 3.4 Structural Analysis21 Amides Carbonylmuch lower Carbonyl band tends to be much lower than normal. N—H stretches also present.

KHS ChemistryUnit 3.4 Structural Analysis22 Frequencies 4 Aromatic Aromatic stretching bands occur in various places. They are often difficult to pick out.

KHS ChemistryUnit 3.4 Structural Analysis23 Aromatics 1 aromatic sharp bandsfingerprint region Sometimes the best indication that a molecule is aromatic is a reasonable number of sharp bands in the fingerprint region

KHS ChemistryUnit 3.4 Structural Analysis24 Aromatics 2

KHS ChemistryUnit 3.4 Structural Analysis25 Summary of IR Absorptions 1

KHS ChemistryUnit 3.4 Structural Analysis26 Summary of IR Absorptions 2

KHS ChemistryUnit 3.4 Structural Analysis27 Strengths & Limitations IR alone IR alone cannot determine a structure. ambiguous Some signals may be ambiguous. functional group The functional group is usually indentifiable. absence The absence of a signal is definite proof that the functional group is absent. fingerprint Correspondence with a known sample’s IR spectrum confirms the identity of the compound - fingerprint

KHS ChemistryUnit 3.4 Structural Analysis28 End of Topic 4 Structural Analysis 2