IB Chemistry

Spectroscopic Techniques

WORKBOOK

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Core Topic 11.3: Spectroscopic techniques (SL)

Syllabus Content

Essential Idea: Analytical techniques can be used to determine the structure of a compound, analyse the composition of a substance or determine the purity of a compound. Spectroscopic techniques are used in the structural identification of organic and inorganic compounds.

Spectroscopic techniques Nature of science:

Improvements in instrumentation—mass spectrometry, proton nuclear magnetic resonance and infrared spectroscopy have made identification and structural determination of compounds routine. (1.8)

Models are developed to explain certain phenomena that may not be observable—for example, spectra are based on the bond vibration model. (1.10)

Understandings:

• The degree of unsaturation or index of hydrogen deficiency (IHD) can be used to determine from a molecular formula the number of rings or multiple bonds in a molecule.

• Mass spectrometry (MS), proton nuclear magnetic resonance spectroscopy (1H NMR) and infrared spectroscopy (IR) are techniques that can be used to help identify compounds and to determine their structure.

International Mindedness

• Monitoring and analysis of toxins and xenobiotics in the environment is a continuous endeavour that involves collaboration between scientists in different countries.

TOK:

• Electromagnetic waves can transmit information beyond that of our sense perceptions. What are the limitations of sense perception as a way of knowing??

Application and skills:

• Determination of the IHD from a molecular formula.

• Deduction of information about the structural features of

a compound from percentage composition data, MS, 1H

NMR or IR.

Utilization:

• IR spectroscopy is used in heat sensors and remote sensing in physics.

• Protons in water molecules within human cells can be detected by magnetic resonance imaging (MRI), giving a three-dimensional view of organs in the human body.

Syllabus and cross-curricular links: • Topic 1.2—determination of the

empirical formula from percentage composition data or from other experimental data and determination of the molecular formula from both the empirical formula and experimental data.

• Topic 2.1—the nuclear atom • Topic 5.3—bond enthalpies

Guidance:

• The electromagnetic spectrum (EMS) is given in the data booklet in section 3. The regions employed for each technique should be understood.

Aims: Aim 7: Spectral databases could be used here.

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• The operating principles are not required for any of these methods.

• The data booklet contains characteristic ranges for IR

absorptions (section 26), 1H NMR data (section 27) and specific MS fragments (section 28).

• For 1H NMR, only the ability to deduce the number of different hydrogen (proton) environments and the relative numbers of hydrogen atoms in each environment is required. Integration traces should be covered but splitting patterns are not required.

Aim 8: The effects of the various greenhouse

gases depend on their abundance and their

ability to absorb heat radiation.

Textbook References

Brown, C. and Ford, M. 2014, Higher Level Chemistry : Pearson Baccalaureate / Pearson Education Limited

Brown, C. and Ford, M. 2014, Standard Level Chemistry : Pearson Baccalaureate / Pearson Education Limited

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An Introduction to Spectroscopy

o Light carries energy in the form of tiny particles known as photons. o Each photon has a discrete amount of energy, called a quantum. o Light has wave properties with characteristic wavelengths and frequency. o The energy of photons is related to the frequency (n) and wavelength (l) of the light

through the two equations:

&

The relationship between light and energy

• Spectroscopy is the study of the way light (electromagnetic radiation) and matter interact.

• There are a number of different types of spectroscopic techniques and the basic principle shared by all is to shine a beam of a particular electromagnetic radiation onto a sample and observe how it responds to such a stimulus; allowing scientists to obtain information about the structure and properties of matter.

• When matter absorbs electromagnetic radiation the change which occurs depends on the type of radiation, and therefore the amount of energy, being absorbed.

• Absorption of energy causes an electron or molecule to go from an initial energy state (ground state) to a high energy state (excited state) which could take the form of the increased rotation, vibration or electronic excitation.

• By studying this change in energy state scientists are able to learn more about the physical and chemical properties of the molecules.

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Summary of the techniques and their Molecular effects

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Index of Hydrogen Deficiency (IHD)

• Index of Hydrogen Deficiency: IHD is a measure of the degree of unsaturation in a molecule.

• For a Hydrocarbon: CxHy is given by the following relationship

The following Rules apply whey calculating IHD for non-hydrocarbon compounds:

• Oxygen and sulfur atoms do not alter the value of the IHD.

• Halogens are treated like hydrogen atoms, for example, CH2Cl2 has the same IHD value as CH4.

For each nitrogen atom, add one to the number of carbons and one to the number of hydrogen.

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TEST YOURSELF

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Using Mass Spectroscopy to identify the structure of Organic molecules

The Molecular Ion Peak

• The molecular ion peak is the peak produced by the unipositive ion formed by the loss of the loss of one electron from a molecule of the compound.

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Example 1 – mass spectrum of propane

Fragmentation patterns

o In mass spectrometry, molecules are ionized, using high energy electrons and these molecular ions subsequently undergo fragmentation.

o The above mass spectrum of propanone demonstrates that there are lots of peaks other than the molecular ion peak.

o These are called the fragmentation pattern – these arise because a molecule can break apart into smaller fragments when it is bombarded by high-energy electrons.

o The molecular ion peak along with its fragmentation pattern allows chemists to piece together its structure.

o The mass spectrum may also show a peak with a mass one unit higher than the molecular ion – an (M+1)+ peak. This is caused by the presence of an atom of 13C in some molecules.

o Fragmentation patterns can be very complex, as molecules can undergo fragmentation in many different ways and rearrangement of fragments can also occur.

o See the mass spectrum of propanoic acid shown below.

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The mass spectrum of ethanol

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C-C bond in ethanol can break-up in different ways as shown below. Only charged species will show up as peaks in the IR spectrum as neutral species are not affected by electric and magnetic fields.

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Common peaks in mass spectra

The fragmentation patterns can be very complex for some molecules but you are not expected to know them.

These are common peaks that IB focuses on. There is NO NEED to memorise them as the

Section 28 of the IB data book gives these data.

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TEST YOURSELF

1. Examine the mass spectrum for benzene carboxylic acid and identify the ions responsible for each of the major peaks.

2. The diagram below shows the mass spectrum of propanal.

Write equations to explain the strong peaks observed at m/z 29 and 57.

3. A compound gives a mass spectrum with peaks at m/z = 77 (40%), 112 (100%), 114 (33%) and essentially no other peaks. Identify the compound.

4.

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5.

6.

7.

Infrared Spectroscopy (IR)

• Infra-red spectroscopy depends on the fact that infrared radiation is absorbed by certain molecular bonds and causes them to vibrate.

• There are several different types of vibrations that cause absorptions in the infrared region.

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• Most common of these vibrations are bending and stretching of bonds.

• Bending and stretching of water molecules are shown in the following diagram.

• Only the stretches that are associated with a changing dipole moment for the molecules will give rise to absorption bands in the IR spectrum.

Although, CO2 is a symmetrical molecule, and so it has no overall charge, it absorbs strongly in IR region when it undergoes asymmetric stretch which causes an uneven distribution of charge.

This gives the CO2 molecule a temporary dipole moment, enabling it to absorb IR.

• Different bonds absorb IR of different wavelengths and frequencies. E.g. C=O absorbs IR of different wavelength to O-H bond.

• Functional groups in organic molecules are identified by their position of their absorption bands (measured by a wavenumber range) on the infrared absorption spectrum.

• This information is provided in Section 26 of IB data booklet.

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Infrared spectrum of propanone

Factors that affect vibrations

o The type of vibrations

Note the axis labels:

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o Strength of bonds

o Mass of atoms

TEST YOURSELF

1. The IR spectrum shown below is for one of the compounds in the table given below. Identify the compound and explain how you arrive at your choice.

2. Which of the following compounds will have an IR absorption band in the 1700 – 1750

cm-1 regions?

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3. Predict the IR absorption bands and the bonds responsible for them in the region above 1500 cm -1 for the following molecules.

(a) Propane (b) Propan-2-ol (c) Propene (d) Propanoic acid.

4. Three by modes of vibrations of the carbon dioxide molecule are shown below. Complete the table by indicating if each mode would be IR active or IR inactive.

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NMR Spectroscopy

• NMR has applications in all fields of the experimental sciences. It is the single most powerful technique available to chemists for studying the composition, structure and function of molecules.

• NMR spectroscopy depends on whether H atoms in a molecule are equivalent to each or not. (ie. Whether they have the same identical chemical environments).

• In the NMR spectrum, there is one signal for each set of equivalent H atoms, with the intensity of each signal being proportional to the number of equivalent H atoms it represents.

The principle of NMR

o NMR technique relies on a combination of nuclear physics and chemistry to determine structure of molecules.

o The nuclei of atoms with an odd number of protons such as 1H, 13C, and 9F spin and behave like tiny bar magnets.

o When placed in an external magnetic field, some of these nuclei will line up with an applied filed and if they have sufficient energy, some will line up against it.

o Energy in the radio frequency range of the EMS can be used to cause a hydrogen nucleus to change its orientation relative to the applied magnetic field.

o It is these changes in energy state that occur in nuclear magnetic resonance (NMR) spectra.

o When analyzing NMR spectra, absorption due to 1H nuclei (often called proton) is used.

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Solvents and Calibration

o 1H NMR spectra are recorded using a solution of the sample.

o If the solvent contains any 1H atoms, then these will appear in the spectrum (which can hide signals due to H atoms in the sample).

o As a result, the solvents such as CCl4 and deuterated solvents such as CDCl3, are used.

o To calibrate the spectrum, a small quantitiy of tetramthylsilane (TMS) is added to samples as this produces a single signal (peak) providing an internal standard to which other peaks are compared.

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Chemical Shift

o The position of the NMR signal relative to this standard is called the chemical shift of the proton.

o The size of the chemical shift depends on what other atoms/groups are near the H, e.g. the more electronegative atoms there are near the H, the greater the chemical shift.

o Data booklet: 1H NMR data is provide in section 27.

Typical chemical shifts compared to TMS of some common 1H environments

TEST YOURSELF

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NOTES

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Atom Notes

NOTES

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1

For you to do: Completed

Textbook Questions 26 – 29, pp.367 -268 (SL)

Textbook Questions 30 pp.369 (SL)

Textbook questions 31-35 p 372 (SL)

Textbook questions 36-40 p 374 (SL)

Textbook questions 41-44 pp. 379 (SL)

End of chapter Practice questions 1- 11 pp 381 -385 (SL)