2Molecular 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.
3Infrared Spectroscopy Substances exposed to radiation from frequency range 1014 Hz to 1013 Hz(wavelengths 2.5μm -15μm)causing vibrational energy changes in the moleculethese absorb infrared radiation of specific frequencies.point is to identify functional groups in the molecule
4Rememberc = λvfrom this equation we can get the reciprocal of the wavelength (1/λ)this is a direct measure of the frequency
5the reciprocal is described as the wavenumber it is the wavenumber, measured in cm-1 that is recorded on an infrared spectrumwavenumber (1/λ) / cm-1wavelength (λ) / μmfrequency (v) / Hz1000200030004000102.52.5 x 10131.0 x 1014
6Bond deformationSIMPLE diatomic molecules can only vibrate one way, by stretching.BrHFor these molecules there is only one vibrational infrared absorption.
7Bond deformation O C O symmetric stretch More complex molecules have more possible deformationsOCOsymmetric stretch
17Bond deformation Frequencies are different for each molecule Energy required for vibration depends on strength of bondWeaker bonds requiring less energy.
18Simple version Sample placed in ir spectrometer Subjected to ir radiationMolecule absorbs energyMolecule bonds starts to undergo different types of vibration (stretching, bending etc.)This produces different signals that the detector records as ‘peaks’ on the spectrum.
19Important …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.
21H H C C O H H H H C-H bond stretch 3010 -2850 cm-1 C-O bond stretch O-H bondstretch3670 cm-1OHHHH
22Interpreting 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.
23absorption intensityThe 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.
24Some typical absorptions Absorption range / cm-1Bonds responsibleExamplesSingle bonds to HO-H, C-H, N-HTriple bondsC≡C, C≡NDouble bondsC=C, C=OBelow 1500variousC-O, C-X‘can use acid to catalyse’ hyperlink takes to slide 11‘carboxylate salt’ hyperlink takes to slide 12Below 1500cm-1 the ir spectrum can be quite complexThis region is characteristic of a particular moleculeHence known as ‘fingerprint region’