Unit 4 Spectroscopy

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How does Mass spectroscopy work?
by using the varying mass-to-charge ratios of ions and their varying degrees of defletion in a magnetic field in order to determine their relative abundances
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What are the main peaks on the mass spectrum caused by?
the parent molecule and fragments of it, although some minor peaks could be caused by rearrangements
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What is the peak at the maximum m/z value?
the parent molecular ion, M+.(a radical cation), which has an m/z value equal to the Mr of th molecule
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Even though the parent ion can fragment into a cation and a radical, why is only the the cation detected?
because it can be accelerated by the electric field and detected by the magnetic field.
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Can further fragmntation of the cation take place?
yes.
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How are the heights of the peaks on the mass spectrum shown?
as percentages of the height of the largest (base) peak
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What happens when the fragment is more abundant?
the more stable it is and the easier it will be to form it(this shows the bond broken is weaker)
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What will molecular ions that are two m/z units apart in a 3:1 intensity ratio be due to?
the presence of Chlorine, occuring as roughly 75% 35Cl and 25% 37Cl
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What will molecular ions that are two m/z units apart in a 1:1 intensity ratio be due to?
the presence of Bromine, occuring as roughly 50% 79Br and 50% 81Br
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Where does fragmentation usually occur?
as the weaker bonds break and the stonger bonds remain unbroken
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What are the most common stable ions formed as a result of fragmentation?
CH3(m/z=15) NH2(m/z=16) OH(m/z=17) C2H5(29) C6H5(m/z=77)
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What will alcohols readily eliminate from the parent ion?
water, to form a peak at (M-H2O)+.
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Where do alkanes tend to give peaks?
(M-R)+ where R is an alkyl group
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What cations to ketones tend to form?
stable acylium cations (RCO+)
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How does Infrared spectroscopy work?
by using characteristic absorption wavelengths of photons for different bonds in order to identify different compounds
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What can covelant bonds do?
vibrate by either stetching or bending, and they do this at a resonant frequency for each pattern of vibration
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How does the pattern of vibrations show an absorbtion spectrum?
It corresponds to a specific wavelength in the infrared spectrum which can be absorbedby the bond electrons, thus the intensity of each wavelength of infrared light passingthrough a sample is recorded and compared to that of a blank
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How is the scale of infrared spectrum given?
in terms of the wavenumber v, whih is defined as 1/v. Th units are given in cm-1
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What is the fingerprint region?
the region of spectra below 1500cm-1 which is very complex and is unique to each compound so can be used to compare the compound to the database
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What does the higher wavenumber region aid with?
identifying functional groups
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What absorption pattern does an alcohol give?
a broad absorption around 3300cm-3 due to H-bonding
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What absorption pattern does an O-H acid give?
a very broad absorption peak around 2800cm-1
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What absorption pattern does a C=O give?
a small very sharp absorption at about 1700cm-1
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what absorption patter does CN give?
a small sharp absorption peak around 2250cm-1
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What is the most useful information that can be gained from infrared?
the absence of particular functional groups, as this eliminates possibilities
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What information does NMR give?
the numbers of and positions of hydrogen atoms
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How are the radio frequency waves in NMR created and used?
magnetic fields around nuclei, created by them being charged and spinning, are placed in a magnetic field so they line up with high/low energy. The difference between the states corresponds to radio frequency waves,causing resonance-effects detected
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What does chemical shift depend on? and how is it measured?
the amount of shielding of the protons by electrons(the more shielding, the higher the field strength needed for resonance and the lower the shift value). chemical shift is measured in ppm(parts per million)
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What does the amount of shielding depend on?
the environment of the protons- if they are next to an electronegative atom then there will be deshielding+high shift value(downfield),or if they are next to an atom with low electronegativity there will be more shielding+lower shift value(upfield)
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What is the shift value determined relative to?
a standard of TMS (tetramethylsilane)
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Why is TMS used as a standard?
it's 12 protons in the same environment give a strong signal, Si is less electronegative than C so the signal is upfield of other organic molecules (shift value defined as 0.0ppm), and it is inert, non toxic, cheap and volatile
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How is the number of protons represented?
proportionally to the area under each peak on the NMR spectrum, and this ratio is given by the integration curve
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What extra can high res NMR show?
splitting of individual peaks, due to being adjacent to other protons in a different environment
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If n is the number of adjacent protons, how many splits will there be?
n+1
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What will a triplet with a 1:2:1 ratio show?
the protons are adjacent to a CH2 group
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What will a quartet with a 1:3:3:1 ratio show?
the protons are adjacent to a CH3 group
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What will a triplet and quartet with a integration curves in a 3:2 ratio show?
an ethyl group (CH2CH3)
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What will a singlet show?
no splitting because the protons are not adjacent to any different protons (the H in OH)
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What will a splitting of the splitting pattern (ie, a quartet of triplets) show?
the protons are next to two different types proton
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What is the splitting of peaks called?
the coupling effect
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Why is it often most useful to compare the results of all these spectroscopic analyses(mass spectrometry, infra-red spectroscopy and NMR spectroscopy)?
they give different types of information, but chemical analytical tests should also be used to determine the identity of a compound
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Other cards in this set

Card 2

Front

What are the main peaks on the mass spectrum caused by?

Back

the parent molecule and fragments of it, although some minor peaks could be caused by rearrangements

Card 3

Front

What is the peak at the maximum m/z value?

Back

Preview of the front of card 3

Card 4

Front

Even though the parent ion can fragment into a cation and a radical, why is only the the cation detected?

Back

Preview of the front of card 4

Card 5

Front

Can further fragmntation of the cation take place?

Back

Preview of the front of card 5
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