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I.R. Spectroscopy C.I. 6.4. Molecular Spectroscopy Energy possessed by molecules is quantised. When a molecule interacts with radiation there can be changes.

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Presentation on theme: "I.R. Spectroscopy C.I. 6.4. Molecular Spectroscopy Energy possessed by molecules is quantised. When a molecule interacts with radiation there can be changes."— Presentation transcript:

1 I.R. Spectroscopy C.I. 6.4

2 Molecular Spectroscopy Energy possessed by molecules is quantised. When a molecule interacts with radiation there can be changes in electronic, vibrational or rotational energy. (C.I. 6.2) These changes depend on the frequency of the radiation. Analysis of the energy needed to change from one energy level to another forms basis of molecular spectroscopy.

3 Infrared Spectroscopy Substances exposed to radiation from frequency range Hz to Hz (wavelengths 2.5μm -15μm) causing vibrational energy changes in the molecule these absorb infrared radiation of specific frequencies. point is to identify functional groups in the molecule

4 Remember c = λv from this equation we can get the reciprocal of the wavelength ( 1 / λ ) this is a direct measure of the frequency

5 the reciprocal is described as the wavenumber it is the wavenumber, measured in cm -1 that is recorded on an infrared spectrum wavenumber ( 1/λ) / cm -1 wavelength ( λ) / μm frequency (v) / Hz x x 10 14

6 Bond deformation SIMPLE diatomic molecules can only vibrate one way, by stretching. H Br For these molecules there is only one vibrational infrared absorption.

7 Bond deformation More complex molecules have more possible deformations OCO symmetric stretch

8 Bond deformation OCO OCO asymmetric stretch

9 Bond deformation OCO

10 O C O

11 O C O

12 O C O

13 OCO

14 O C O

15 O C O

16 O C O bending

17 Bond deformation Frequencies are different for each molecule Energy required for vibration depends on strength of bond Weaker bonds requiring less energy.

18 Simple version  Sample placed in ir spectrometer  Subjected to ir radiation  Molecule absorbs energy  Molecule bonds starts to undergo different types of vibration (stretching, bending etc.)  This produces different signals that the detector records as ‘peaks’ on the spectrum.

19 Important … When an ir spectrum is obtained we do not try to explain the whole thing, simply look for one or two signals that are characteristic of different bonds.

20 O-H bond stretch C-H bond stretch C-O bond stretch

21 O CC H H H H H H O-H bond stretch 3670 cm -1 C-O bond stretch 1050 cm -1 C-H bond stretch cm -1

22 Interpreting the spectra! Usually match a particular bond to a particular absorption region. The precise position of the peak depends on the bond environment, so only wavenumber regions can be quoted.

23 absorption intensity  The strongest (more intense) absorptions occur when a large change in bond polarity associated with the vibration.  e.g. C=O bonds will give more intense absorptions than C=C bonds.

24 Some typical absorptions Below 1500cm-1 the ir spectrum can be quite complex This region is characteristic of a particular molecule Hence known as ‘fingerprint region’ Absorption range / cm -1 Bonds responsibleExamples Single bonds to HO-H, C-H, N-H Triple bonds C ≡ C, C ≡ N Double bondsC=C, C=O Below 1500variousC-O, C-X


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