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Flashcards in Analysing compounds via spectroscopy Deck (19):
1

what does mass spectroscopy give us?

molar mass of compound.

2

describe how all the fragmentation works.

in the ionisation chamber, high energy electrons knock off an electron from the sample, forming the molecular ion [..]+ .
these are nustable, due to umpaired electron, so form fragment ions

3

define: base peak

produced by the most abundant ion. Relative intensity set to 100%, others are measured relative to it.

4

what isotopes do we need to look out for?

Br (79Br, 81Br) and Cl (35Cl, 37Cl) - due to high abundance unlike CHO.

5

what are the parent molecular ions for CH2Br2?

[CH279Br2]+, [CH281Br2]+, [CH279Br81Br]+

6

what does infrared spectroscopy help us identify?

functional groups.

7

describe how infrared spectroscopy works

Amount of energy required to move from one vibration energy level to the next = the amount of energy contained in the IR region of the ES.
When a molecule absorbs IR, its bonds bend and stretch, causing vibration.

8

what molecules can absorb IR?

if it changes the overall dipole

9

factors affecting IR absorption?

weaker bond/bigger molar mass means absorbs IR of a lower frequency, and a lower energy/frequency vibration.

10

IR: X axis and Y axis?

X is wavenumber of the IR frequency absorbed.
Y axis is the absorbance. 100% is when all IR light passes though, reduces when absorbed.

11

for peaks above 1400
for peaks below 1400

helps us identify fg's
fingerprint region - specific to a compound.

12

differentiating between OH and COOH?

OH is separated, COOH is not.

13

in order to interact with radio waves, molecules must have

nuclear spin

14

describe nuclear spin

• Only occurs for nuclei (pl. nucleus) with an odd number of protons and/or neutrons (eg. 1H (proton) and 13C)
• The odd number of nuclear particles causes nuclei to behave like bar magnets
o In the presence of an magnetic field, magnets (hence these nuclei) line up in the same direction as the field (requires less energy) or in the opposite direction (requires more).
o Inside the NMR machine, nuclei are usually lined up in the same direction as the field. The energy from the radio wave transmitter flips the nuclei into their high energy state. Over time they flip back into their lower energy spin state, which releases energy which is represented in the NMR spectrum.

15

define: chemical shift

difference in energy needed to change the spin state in a sample, compared to that needed to change the spin state of tetramethyl silane.

16

why is TMS used?

unreactive.
set to 0, allows for standardisation between different spectrometers

17

why do different environmetns have different chemical shifts

because they experience different nuclear shielding (from electrons). thus they absorb/emit different energies, resulting in different chemical shift

18

what do we look for in proton spectroscopy?

chemical shift
no peaks = no hydrogen envs
rrelative peak areas = number of hydrogen atoms in each env
number of peaks = n +1

19

what do we look for in carbon-13 spectroscopy?

• Chemical shift
• No peaks = no. C environments