SpectroscopyMaster Class

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Master Class

Spectroscopy

To understand how chemists obtain and interpret the following three types of spectra.

Aims of this session

Spectroscopy

Mass spectroscopy

NMR spectroscopy IR spectroscopy

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Mass spectrometers can be used as an analytical tool to measure the relative molecular mass (RMM) of a compound.

Mass spectrometers can beused as an analytical tool tomeasure the relativeatomic mass (RAM) of anelement (and its isotopes).

Mass Spectrometry

Spectroscopy

An analytical technique which uses the differences between the mass (& charge) of ions as its basis.

35Cl and 37Cl75.8% 24.2%

35·5

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www.specs.com

Mass Spectrometer : Basic Structure

Spectroscopy

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Mass Spectrometry

Spectroscopy

A mass spectrometer can also create charged fragments from compounds and so can provide information about chemical structure.

When the vapour is ionized, electrons are lost to form positive ions.

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Mass Spectrometer : Different masses

Spectroscopy

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Mass Spectrometry: Fragments

Spectroscopy

A mass spectrum contains peaks which correspond to particular fragments. Stable fragments create larger peaks.

Certain fragments are easily identifiable:

CH3+=

C2H5+=

M+=

15

29heaviestpeak

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Mass Spectrometry: Paired up

Spectroscopy

A powerful and widely used method is to couple Gas Chromatography with Mass Spectrometry (GC/MS).

A mixture of compounds are firstly separated (GC step) and then analysed (MS step).

Compound x

Compound y

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Spectroscopy

Spectroscopy & EMR

Robert Wilhelm Bunsen

1855

Gustav Kirchhoff

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Spectroscopy

Spectroscopy & EMR

An electron in a lower orbital receives energy - in this case by absorbing light. It is then promoted to a higher energy orbital.

An excited state electron will eventually lose energy (emitted as light) and fall back to the lower orbital. This is known as relaxation.

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Hotspot: Play triangle

AbsorptionEmission

Spectroscopy

Spectroscopy & EMR

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Spectroscopy

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Hotspots: Beam, Prism, Slider, Autoscan

IR Spectrometry

Spectroscopy

An analytical technique which uses the differences between bonds (& electron levels) as its basis.

Bonding electrons absorbing IR cause the bonds to deform.

Typical changes to bonds include…

Stretching

Bending

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Spectroscopy

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Hotspots: Assigned absorption bands

Particular useful technique for helping to identify organic functional groups.

IR Spectrometry

Spectroscopy

OHstretch

COstretch

CH3deformation

CHstretch

OHbend

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wavenumbers

IR Spectrometry

Spectroscopy

Different of IR can be associated with the deformation of particular bonds.

wavelengths

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An analytical technique which uses the differences between the magnetic property of nuclei as its basis.

NMR Spectrometry

Spectroscopy

RF coil (generator)

Powerfulmagnet

Sample tube

Coolant

RF coil (detector)

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– this makes them behave like tiny magnets. They will match or oppose an external field.

– this makes them behave like tiny magnets.

Both Protons

Protons possess spin

NMR Spectrometry: Spin states

Spectroscopy

Magnetic field

Proton B

Proton A E

Proton B

Proton A

Radio waves are required to ‘flip’ the nuclei. This technique is important for 1H, 13C, 19F and 31P nuclei.

Energy

N

S

N

S

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A low-energy nuclei (aligned with the applied field) will jump to a high energy spin state when given a pulse of RF.

NMR Spectrometry: Relaxation & detection

Spectroscopy

Againstfield

Withfield

When the magnetic field is removed, the nuclei revert back to their original state.

RF signal coil detector coilcan be the same coil!

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Induced signal

We can either fix the field strength and vary the radio waves until the nuclei flip … or… we use one particular radio wave and vary the magnetic field.

The energies of the two spin states relate to the magnetic field.

NMR Spectrometry: Energy levels

Spectroscopy

E

Proton B

Proton A

Strongfield

E

Proton B

Proton A

Weakfield

We can either fix the field strength and vary the radio waves until the nuclei flip … or…

Both protons Both protons

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The opposing field variesaccording to the nearbybonds and nuclei.

This is called the chemical shift phenomenon and causes a difference in the energy spin states for nuclei.

E

Not all nuclei experiencethe same strength ofexternal magnetic field.

NMR Spectrometry: Shift

Spectroscopy

E

CH

C

OH

HH

E

E

E

In a magnetic field electronscirculate. This creates anopposing magnetic field.

E

E

E

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10 0 2 4 6 8

CH3 CH3 Si CH3

CH3

Chemical shift

For ethanal, its two types of hydrogen nuclei will produce different signals during NMR.

NMR Spectrometry: Assigning peaks

Spectroscopy

C

H

C

OH

HH

The area under each peak relates to the number of each type of hydrogen.

TMS

Used to calibrateThe signals

Area = 65

Area = 22

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Analysis of many organic compounds has enabled chemists to create tables of chemical shifts…

NMR Spectrometry: Spectra

Spectroscopy

The peaks themselves contain additional information that relates to how neighbouring hydrogens interact in 3D.

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Together these three methods of spectroscopy form a powerful tool for the chemist - identifying functional groups, bonds and the 3D structure of compounds.

Combing Techniques

Spectroscopy

Mass spectroscopy

NMR spectroscopy IR spectroscopy

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Master Class

Spectroscopy

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