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CHEM170 > Spectroscopy > Flashcards

Spectroscopy Flashcards

(60 cards)

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1
Q

What’s the absorption of a O-H stretch?

A

3500 - 3600

Broad with H bonding - sharp if no H bonding

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2
Q

What’s the absorption of a N-H stretch?

A

3000 - 3500

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3
Q

What’s the absorption of a sp3/sp2 hybridised C-H stretch?

A

Sp3: broad 3100 - 2700

Sp2: weak 3100 - 2700

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4
Q

What’s the absorption of a C≡C stretch?

A

2200 - usually weak

If terminal alkyne shows peak at 3300

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5
Q

What’s the absorption of a C≡N stretch?

A

2200 - usually weak

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6
Q

What’s the absorption of a X=Y=Z stretch?

A

2200 - 1900 - strong

Allene, isocyanate, azide, diazo

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7
Q

What’s the absorption trend of a aldehyde? What confirms it?

A

1750 - 1690

Aldehyde is confirmed by C-H peak at 3100 - 2700

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8
Q

What does a aldehyde’s absorption have that an ketone’s doesn’t?

A

Aldehyde is confirmed by C-H peak at 3100 - 2700

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9
Q

What’s the absorption peak of a ketone stretch?

A

1750 - 1690

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10
Q

What’s the absorption of an ester, what confirms this?

A

1750 - 1735

Confirmed by C-O peak at 1300 - 1000

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11
Q

What’s the absorption trend of a carboxylic acid, what confirms this?

A

1760 - 1710

Confirmed by O-H peak at 3500 - 2500 and C-O peak at 1300 - 1000

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12
Q

What’s the absorption patter of an amide stretch, what confirms this?

A

1700 - 1690

Confirmed by N-H peak at 3500 - 3000

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13
Q

What’s the absorption of C=C and C=N?

A

1650 - 1500 - usually weak

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14
Q

What makes the C=C absorption peak stronger?

A

Stronger if conjugated

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15
Q

What’s the absorption of a terminal ≡C-H stretch?

A

3300

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16
Q

What the absorption pattern of an aromatic C=C stretch?

A

1600 - variable strength and usually associated with peaks in fingerprint region

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17
Q

What’s the absorption of a NO2 stretch?

A

1550 - strong

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18
Q

What’s the absorption of a CO2 stretch?

A

2350 - path length imbalance not sample

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19
Q

What’s the absorption of a NO2 / -SO2 stretch?

A

1350 - Strong / strong with a peak at 1150 also

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20
Q

What’s the absorption of a P=O stretch?

A

1300 - 250

Strong

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21
Q

What’s the absorption of a C-O stretch? What groups show this?

A

1300 - 1000

Strong - alcohol ether ester or acid

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22
Q

What’s the absorption of a C-H aromatic bend?

A

850 - 700

Indicates substitution pattern

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23
Q

What’s the absorption pattern of a C-Cl stretch?

A

800 - 750

Strong, but is obscured by CHCl3

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24
Q

How much substance can be used to conduct mass spec?

A

1ng can be sufficient

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25
What ways are there to ionise a substance for mass spec?
Electron impact Chemical ionisation Fast atom bombardment Matrix assisted laser desorption ionisation time-of-flight (MALDI-TOF) Electrospray
26
What ionisation processes are hard / soft for mass spec?
Electron impact is hard Chemical ionisation is soft Fast atom bombardment is soft MALDI-TOF is soft Electro spray is soft
27
What does hard/soft ionisation technique mean?
Hard means that the molecule is fragmented a lot Soft rarely produces fragmented ions
28
Where does electron come from to produce ionisation?
Typically from a lone pair
29
Why would electrospray be used?
When looking into proteins/polypeptides, molecules with a large MW, since its soft you are able to analyse the structure better Droplets further desolvate, allows for solids and liquids to be analysed
30
What equations give time for Time-of-Flight?
K = 1/2mv^2 Time = length of flight tube / velocity Rearrange for v and sub into t=d/v
31
What is the nitrogen rule?
Odd number of Ns will have an odd molecular weight Even number of N, or lack of, will give even molecular weight
32
What is a label bond?
A weak bond which is easily broken - such as between a heteroatom
33
What is the m/z of a tropylliuym ion?
M/z = 91 Results in m/z at 65 (5 membered ring)
34
What’s the isotope ratio of bromine?
1:1
35
What is an acyllium ion?
Where the bond is broken adjacent to the carbonyl group, C=O triple bond + methyl
36
What’s the mass of a typical acyllium ion?
43 m/z
37
What do tropyllium ions form from?
From monosubstituted benzene rings
38
What do alcohols typically lose?
Water
39
What do esters typically lose?
Carboxylic acids
40
What do halides lose?
HX
41
What happens in 2 bond cleavages?
Only one molecule sustains the +ve charge
42
What is the McLafferty rearrangement?
Where carbonyl derivatives with a gamma H undergo a characteristic fragmentation Proton is transferred to carbonyl fragment
43
What type of stretch yields a higher frequency?
Stretch > frequency than bend
44
What is needed to observe infrared spectroscopy?
A change in dipole moment
45
What increases vibrational frequency?
When bond strength increases When reduced mass decreases
46
How many vibrations for non-linear molecules?
3n-6
47
How many vibrations for linear molecules?
3n-5
48
What general techniques should be followed to analyse IR spectra?
Begin at higher wavenumbers Note strongest absorptions and correlate them Note absence of peaks in important areas Do not attempt to correlate all peaks, especially in fingerprint region
49
What’s the use of TMS?
A reference material, always calibrates at 0.0ppm
50
What is the coupling constant in H NMR?
It is the distance between the peaks of a splitting pattern
51
At what ppm does CDCl3 occur?
7.2ppm
52
How does coupling across a double bond affect coupling?
Double bond limits rotation Coupling constant for Cis < Trans
53
How do heteroatoms affect coupling constants?
EWG reduce J coupling constant
54
What is roofing?
The difference in chemical shift is smaller than the coupling constant, then a ‘roofing’ effect will be observed Peaks begin to ‘lean in’ to each other - interior peaks become larger and anterior peaks become smaller,
55
Why use deuterated solvents?
So that solvent peak is very small, reduces interference. Lock solvents - solvent locks magnetic field of molecules and prevents it from drifting.
56
What can move OH,NH and SH peaks in NMR?
Concentration and temperature 0.5ppm to 4.0ppm
57
Where’re the expected peaks for phenols?
4.0 - 7.5ppm
58
Where’re the expected peaks for carboxylic acids?
10 - 13.2ppm They exist as dimers
59
What is {decoupling}?
Where the coupling patterns have been turned off Simplifying the spectra
60
How is decoupling done?
Extra pulse of radio frequency radiation is supplied while the C13 spectrum is acquired All the H1 are then in the +1/2 spin state and do not show coupling to the C13s

CHEM170 (2 decks)

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