Notes

Outline

IR Spectroscopy Web Assignment

1. What is infrared (IR) absorption spectroscopy? IR spectroscopy is the use of light in the mid-infrared range to excite the bonds in a compound in order to learn about it by seeing which frequencies are absorbed.

2. Describe the two main uses of IR spectroscopy. IR spectroscopy is used to confirm the identity of a substance or to learn about an unknown substances structure.

3. Why must there be a change in dipole moment in order for a molecule to absorb infrared radiation? The dipole moment is the difference between the pull of the bonds in a molecule.   If the pull is the same they cancel each other out. If it is different then the change can be detected as absorbtion at that frequency.

4. Describe the basics of the dispersive and Fourier-transform spectrometers. Disperse IR spectrometers use a diffraction grating in a monochromator to disperse the different wavelengths of light. Dispersive IR spectrometers have largely been replaced with FTIR instruments. They find some use in specific applications, such as monitoring a single IR wavelength to measure the kinetics of a fast reaction.

4. Describe the basics of the dispersive and Fourier-transform spectrometers. Most modern IR absorption instruments use Fourier-transform techniques with a Michelson interferometer. To obtain an IR absorption spectrum, one mirror of the interferometer moves to generate interference in the radiation reaching the detector.

4. Describe the basics of the dispersive and Fourier-transform spectrometers. Since all wavelengths are passing through the interferometer, the interferogram is a complex pattern. The absorption spectrum as a function of wavenumber (cm-1) is obtained from the Fourier transform of the interferogram, which is a function of mirror movement (cm).

4. Describe the basics of the dispersive and Fourier-transform spectrometers. This design does not have the reference cell of a dispersive instrument, so a reference spectrum is recorded and stored in memory to subtract from the sample spectrum.

5. What are used as light sources for IR and as IR detectors? Common light sources are Nernst glowers, or glowbars. IR detectors consist of a semiconductor such as PbS or liquid-nitrogen-cooled HgCdTe (also called MCT).

6. What is the basic design of a monochromator?  Find a drawing or         make  one yourself. A typical monochromator design It consists of the diffraction grating (dispersing element), slits, and spherical mirrors. Scanning is done by rotating the grating.

7. What is meant by Fourier-Transform? Fourier-transform is the representation of the entire absorption spectrum of a sample as the sum of sine waves of different frequencies. The smallest frequency can be no smaller than twice the highest frequency of the data. Advantages the simultaneous recording of all frequencies  no physical slit needed for resolution in spectra  improved signal-to-noise ratio.

8. What is a Michelson Interferometer?  How does it work? The purpose of an interferometer is similar to that of a filter or monochromator, i.e., to isolate a specific portion of the electromagnetic spectrum. Interferometers are not dispersive instruments, but use interference to selectively transmit a certain wavelength.

8. What is a Michelson Interferometer?  How does it work? A light source shines onto a fixed mirror across a beam splitter at a 45-degree angle. The beam of light deflected to the side hits a moving mirror and is reflected. This creates interference among the wavelengths entering the detector.

9. What region of the spectrum is used in IR spectroscopy? The mid-infrared region of the spectrum, between 2.5 mm and 25 mm is used in IR spectroscopy.

10. Absorption of infrared radiation affects molecules in what ways? Absorption of infrared radiation changes the vibration speed of molecules and their bond energy.  This causes them to bend and stretch differently.

11. What is meant by “wave number”,  what is the unit, and how does it relate     to wavelength? In IR spectroscopy the wavelength is measured in wave numbers. The unit for this is cm-1 and is equal to 1/wavelength in cm. cm/4000 = 2.5 mm

12. Interpret the following functional group correlation chart. Alcohols and amines display strong broad O-H and N-H stretching bands in the region 3400-3100 cm-1. The bands are broadened due to hydrogen bonding and a sharp 'non-bonded' peak can often be seen at around 3400 cm-1.

12. Interpret the following functional group correlation chart. Alkene and alkyne C-H bonds display sharp stretching absorptions in the region 3100-3000 cm-1. The bands are of medium intensity and are often obscured by other absorbances in the region (i.e., -OH).

12. Interpret the following functional group correlation chart. Triple bond stretching absorptions occur in the region 2400-2200 cm-1. Absorptions from nitriles are generally of medium intensity and are clearly defined. Alkynes absorb weakly in this region unless they are highly asymmetric; symmetrical alkynes do not show absorption bands.

12. Interpret the following functional group correlation chart. Carbonyl stretching bands occur in the region 1800-1700 cm-1. The bands are generally very strong and broad.  Carbonyl compounds which are more reactive in nucleophilic addition reactions are generally at higher wave number than simple ketones and aldehydes  amides are the lowest, absorbing in the region 1700-1650 cm-1.

12. Interpret the following functional group correlation chart. Carbon-carbon double bond stretching occurs in the region around 1650-1600 cm-1. The bands are generally sharp and of medium intensity. Aromatic compounds will typically display a series of sharp bands in this region.

12. Interpret the following functional group correlation chart. Carbon-oxygen single bonds display stretching bands in the region 1200-1100 cm-1. The bands are generally strong and broad. Many other functional groups have bands in this region which appear similar.

13. What is the basic content of an IR spectrum? An IR spectrum shows where the infrared radiation was absorbed. These are the peaks.  This is where the bond energy was changed and the bonds began to bend or stretch. Everywhere else is frequencies that were not absorbed.

14. What is the “fingerprint” region, why is it called that, and how is it used? The region between 1400 and 400 cm-1 is less useful in terms of assigning peaks to specific bonds The complex patterns that show up in this region are sometimes useful in identifying molecules The region below 1400 cm-1 is therefore called the "fingerprint region" This region is used to determine the identity of the substance being tested.

15. Label the peaks marked with an “X” as the type of vibration. 3200 cm-1 -is OH hydrogen bond (alcohol or phenol) stretching 3000 cm-1 -is  carboxylic acid stretching 2100 cm-1  -is alkyne stretching 1680 cm-1  -alkene stretching

16. Why do IR peaks have different intensities and/or different frequencies? The intensity of an IR absorption is proportional to: The strength of the dipole moment change associated with the vibration The number of such bonds in the molecule

17. What is conjugation, and how does it affect the IR frequencies? A conjugated system is a molecular entity whose structure may be represented as a system of alternating single and multiple bonds When carbonyls (or other multiple bonds) are in conjugation with another double or triple bond a resonance form can be drawn in which the carbonyl oxygen bears a negative charge. The contribution of this resonance form reduces the double bond character of the carbonyl. This, in turn, reduces the force constant, k, which means that the conjugated carbonyl will absorb at a lower frequency.

18. What is hydrogen bonding, and how does it affect the IR frequencies? Hydrogen bonding is when there is an OH group attached to the compound. When the hydrogen of a hydroxyl group is involved in a hydrogen bond a resonance form can be drawn in which the oxygen bears a negative charge (this breaks the O-H bond itself). The contribution of this resonance form reduces the single bond character of the hydroxyl bond. This, in turn, reduces the force constant, k, which means that the hydrogen bonded hydroxyl will absorb at a lower frequency.

19. Describe the two types of stretching vibrations, and the four types of bending vibrations. The two types of stretching are symmetrical and asymmetrical. In symmetrical stretching the bonds stretch with the same strength at the same time. In asymmetrical stretching the bonds stretch at different times.  This causes a change in the dipole moment. The four types of bending are scissoring, rocking, wagging and twisting.

20. What is Hookes’ Law, and what in      the world does it have to do with IR spectroscopy? To help understand IR, it is useful to compare a vibrating bond to the physical model of a vibrating spring system.  The spring system can be described by Hooke's Law.

20. What is Hookes’ Law, and what in the world does it have to do with IR spectroscopy? Consider a bond and the connected atoms to be a spring with two masses attached. Using the force constant k (which reflects the stiffness of the spring) and the two masses m1 and m2, then the equation indicates how the frequency, u, of the absorption should change as the properties of the system change.

21. What are the steps to follow in the analysis of an infrared spectrum? First determine where there are peaks. Then you can use a chart to see what bonds and bond vibrations those peaks represent. http://www.whitworth.edu/academic/department/chemistry/classes/chem/ch278/handouts/RefInfo/IRfreq.htm To determine the identity of a substance you should compare the fingerprint region you have with the fingerprint regions of known substances to find a match.

References http://www.whitworth.edu/academic/department/chemistry/classes/chem/ch278/lectures/ir/IR00.htm http://www.chem.ucla.edu/~webspectra/irintro.html http://chipo.chem.uic.edu/web1/ocol/spec/IR.htm http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/irspec1.htm http://www.chem.ucalgary.ca/courses/351/Carey/Ch13/ch13-ir-1.html http://wwwchem.uwimona.edu.jm:1104/spectra/irvibs.html

References http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM http://spectroscopy.lbl.gov/FTIR-Martin/ http://www.ijvs.com/volume5/edition5/section1.htm http://www.sciences.univ-nantes.fr/physique/enseignement/english/michp.html http://www.chem.orst.edu/ch361-464/ch362/irinstrs.htm http://www.mee-inc.com/ftir.html http://www.photometrics.net/ftir.html http://www.psrc.usm.edu/macrog/irabs.htm