Spectrometry and Spectroscopy summary

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NOCKHARDY
NUCLEAR MAGNETIC RESONANCE - NMR
K NOTES SPECTROSCOPY · produces information about hydrogen atoms in molecules
AT A GL AN CE · a liquid sample is spun in a magnetic field
A LEVEL CHEMISTRY · hydrogen atoms in different environments respond differently to the field
· each different environment produces a different signal at a different chemical shift
· tetramethylsilane provides the reference signal at = 0
· Infra-red spectroscopy · the area under each peak (signal) is proportional to the number of hydrogens
· Nuclear magnetic resonance (NMR) spectroscopy
· Mass spectrometry H H H 1-BROMOPROPANE
H C C C Br
H H H
INFRA RED 3
2 2 TMS
· chemical bonds vibrate at different frequencies
· the frequency is found by detecting when molecules absorb radiation
· the frequency of any absorption is affected by adjoining atoms
· spectra are complex due to the many vibrations occurring
· correlation charts may be used to assign peaks
· each molecule gives a unique spectrum MASS SPECTROMETRY
· characterisation of a substance involves comparison of spectra
· organic molecules tend to have a lot of C-C and C-H bonds · the process is similar to that used to gather information about isotopes
· spectra will thus have many peaks in the 1400 cm-1 to 800 cm-1 range · gaseous molecules are ionised, accelerated and deflected
· this region of the spectrum is referred to as the "fingerprint" region · the amount of deflection depends on m/z - mass/charge ratio
· typical absorptions occur at about 3500 cm-1 for O-H bonds · each molecule undergoes fragmentation into smaller particles
1750 cm-1 for C=O bonds · each peak in the spectrum is due to a charged fragment with a certain m/z value
· the final peak is due to the molecular ion which has the largest m/z value
Wavelength (microns)
2.5 3 4 5 6 7 8 9 10 12 15 · the m/z value of the molecular ion is used to calculate the relative molecular mass
.00 .00
.10 .10
100 43
OCTANE C8H18
.20 .20
80
60
Absorbance
.40 .40
29 71 85
57
40 Molecular ion
+
.80 .80 M
Relative abundance
20 114
4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 10 20 30 40 50 60 70 80 90 100 110 120
-1 m/z
Frequency , cm
© J. L. HOPTON 1999

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