IR-Spectroscopy

IR-Spectroscopy

IR region IR-Spectroscopy The part of electromagnetic radiation between the visible and microwave regions 0.8 m to 50 m (12,500 cm-1-200 cm-1) is called IR region Most interested region in Infrared Spectroscopy is between 2.5m-25 m

(4,000cm-1-400cm-1), which corresponds to vibrational frequency of molecules Interaction of IR with molecules Only molecules containing covalent bonds with dipole moments are infrared active Only the infrared radiation with the frequencies matching the natural vibrational frequencies of a bond (the energy states of a molecule are quantitised) is absorbed

Absorption of infrared radiation by a molecule rises the energy state of the molecule increasing the amplitude of the molecular rotation & vibration of the covalent bonds Rotation - Less than 100 cm-1 (not included in normal Infrared Spectroscopy) Vibration - 10,000 cm-1 to 100 cm-1 The energy changes through infrared radiation absorption is in the range of 840 KJ/mol 1 Spectroscopy Application Atoms

IR-Spectroscopy in a molecule are constantly in motion There are two main vibrational modes: Stretching - (symmetrical/asymmetrical) change in bond length - high frequency Bending - (scissoring/stretch/rocking/twisting) change in bond angle - low freq. The

rotation and vibration of bonds occur in specific frequencies Every type of bond has a natural frequency of vibration, depending on the mass of bonded atoms (lighter atoms vibrate at higher frequencies) the stiffness of bond (stiffer bonds vibrate at higher frequencies) the force constant of bond (electronegativity) the geometry of atoms in molecule The same bond in different compounds has a slightly different vibration frequ.

Functional groups have characteristic stretching frequencies. Infrared Spectroscopy Characteristics of an IR Spectrum In an IR spectrometer, light passes through a sample. Frequencies that match the vibrational frequencies are absorbed, and the remaining light is transmitted to a detector. An IR spectrum is a plot of the amount of transmitted light versus its wavenumber. Frequencies in IR spectroscopy are reported using a unit ~ called wavenumber (): = 1/

Wavenumber is inversely proportional to wavelength and is reported in reciprocal centimeters (cm1). Let us now consider the IR spectrum of 1-propanol, CH3CH2CH2OH. Infrared Spectroscopy Characteristics of an IR Spectrum1-Propanol 4 Infrared Spectroscopy Characteristics of an Infrared Spectrum The IR spectrum is divided into two regions: the functional group region (at 1500 cm-1), and the fingerprint region (at < 1500 cm-1). Figure 13.8

Comparing the functional group region and fingerprint region of two compounds 5 Infrared Spectroscopy Characteristics of an Infrared Spectrum The y-axis is % transmittance: 100% transmittance means that all the light shone on a sample is transmitted and none is absorbed. 0% transmittance means that none of the light shone on the sample is transmitted and all is absorbed. Each peak corresponds to a particular kind of bond, and each bond type (such as OH and CH) occurs at a characteristic frequency. Wavenumber, frequency and energy decrease from left to

right. Where a peak occurs is reported in reciprocal centimeters (cm-1). Infrared Spectroscopy IR Absorptions Bonds absorb in four predictable regions of an IR spectrum. Figure 13.10 Summary: The four regions of the IR spectrum 7 Spectroscopy Application IR Spectrum

Principal Correlation Chart OH NH CH CN CC C=O C=C CO XH region3800 - 2600 OH, NH, CH (sp, sp2, sp3) stretches triple bond 2400 - 2000 CC, CN, C=C=C stretches

double bond 1900 - 1500 C=O, C=N, C=C stretches fingerprint1500 - 400 many types of absorptions 1400 - 900 C-O, C-N stretches 1500 - 1300 CH in-plane bends, NH bends 1000 - 650 CH out-of-plane (oop) bends 3600 cm 3500 cm-1 3000 cm-1 2250 cm-1 2150 cm-1 1715 cm-1 1650 cm-1

1100 cm-1 -1 Dispersive (Double Beam) IR Spectrophotometer IR Source Region freq. (cm-1) what is found there?? Prism or Diffraction Lenz Grating

Split Beam Slit Air Sample Photometer Recorder 8 Infrared Spectroscopy

IR Absorptions 9 Infrared Spectroscopy IR Absorptions Even subtle differences that affect bond strength affect the frequency of an IR absorption. The higher the percent s-character, the stronger the bond and the higher the wavenumber of absorption. Infrared Spectroscopy IR Absorptions For a bond to absorb in the IR, there must be a change in dipole moment during the vibration.

Symmetrical nonpolar bonds do not absorb in the IR. This type of vibration is said to be IR inactive. Infrared Spectroscopy IR Absorptions in Hydrocarbons Hexane has only C-C single bonds and sp3 hybridized C atoms. Therefore it has only one major absorption at 3000-2850 cm-1. 12 Infrared Spectroscopy IR Absorptions in Hydrocarbons 1-Hexene has a C=C and Csp2-H, in addition to sp3 hybridized C atoms. Therefore, there are three major absorptions: C sp2-H at 3150-3000 cm-1;

Csp3-H at 3000-2850 cm-1; C=C at 1650 cm-1. 13 Infrared Spectroscopy IR Absorptions in Hydrocarbons 1-Hexyne has a CC and Csp-H, in addition to sp3 hybridized C atoms. Therefore, there are three major absorptions: C sp-H at 3300 cm-1; Csp3-H at 3000-2850 cm-1; CC at 2250 cm-1. 14 Infrared Spectroscopy IR Absorptions in Oxygen Containing Compounds The OH group of the alcohol shows a strong absorption at 36003200 cm-1. The peak at ~3000 cm-1 is due to sp3 hybridized CH

bonds. 15 Infrared Spectroscopy IR Absorptions in Oxygen Containing Compounds The C=O group in the ketone shows a strong absorption at ~1700 cm-1. The peak at ~3000 cm-1 is due to sp3 hybridized CH bonds. 16 Infrared Spectroscopy IR Absorptions in Oxygen Containing Compounds The ether has neither an OH or a C=O, so its only absorption above 1500 cm-1 occurs at ~3000 cm-1, due to sp3 hybridized CH bonds.

17 Infrared Spectroscopy IR Absorptions in Nitrogen Containing Compounds The NH bonds in the amine give rise to two weak absorptions at 3300 and 3400 cm-1. 18 Infrared Spectroscopy IR Absorptions in Nitrogen Containing Compounds The amide exhibits absorptions above 1500 cm-1 for both its NH and C=O groups: NH (two peaks) at 3200 and 3400 cm -1; C=O at 1660 cm-1. 19

Infrared Spectroscopy IR Absorptions in Nitrogen Containing Compounds The CN of the nitrile absorbs in the triple bond region at ~2250 cm-1. 20 Infrared Spectroscopy Worked example The infrared spectrum below is for one of the molecules shown. Identify the molecule and explain how you arrived at your choice. 21 Interpretation of the spectra

Looking in the region above 1500 cm1, we can identify a band in the region 28403100 cm1 that is due to a CH bond and an absorption in the region 17001750 cm1 that is due to the C=O bond. This eliminates molecules A and B, as neither of these contains a C=O bond. C is a carboxylic acid, which would be expected to have a very broad OH band in the region 24003400 cm 1. This band is not present in the spectrum, so the spectrum cannot be for molecule C. This means that we are left with molecule D. Molecule D contains CH and C=O bonds and should also give rise to a band in the region 10001300 cm 1 as it contains a CO bond; we can see from the spectrum that there is a band 22 in this region. IR-Spectroscopy Use of Infra-Red spectroscopy

Spectroscopy Application IR spectroscopy can be used to distinguish one compound from another. No two molecules of different structure will have exactly the same natural frequency of vibration, each will have a unique infrared absorption spectrum. A fingerprinting type of IR spectral library can be established to distinguish a compounds or to detect the presence of certain functional groups in a molecule.

Obtaining structural information about a molecule Absorption of IR energy by organic compounds will occur in a manner characteristic of the types of bonds and atoms in the functional groups present in the compound Practically, examining each region (wave number) of the IR spectrum allows one identifying the functional groups that are present and assignment of structure when combined with molecular formula information. The known structure information is summarized in the Correlation Chart 23

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