Analysing compounds via spectroscopy

Question 1

what does mass spectroscopy give us?

Answer 1

molar mass of compound.

Question 2

describe how all the fragmentation works.

Answer 2

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

Question 3

define: base peak

Answer 3

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

Question 4

what isotopes do we need to look out for?

Answer 4

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

Question 5

what are the parent molecular ions for CH2Br2?

Answer 5

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

Question 6

what does infrared spectroscopy help us identify?

Answer 6

functional groups.

Question 7

describe how infrared spectroscopy works

Answer 7

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.

Question 8

what molecules can absorb IR?

Answer 8

if it changes the overall dipole

Question 9

factors affecting IR absorption?

Answer 9

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

Question 10

IR: X axis and Y axis?

Answer 10

X is wavenumber of the IR frequency absorbed.

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

Question 11

for peaks above 1400

for peaks below 1400

Answer 11

helps us identify fg’s

fingerprint region - specific to a compound.

Question 12

differentiating between OH and COOH?

Answer 12

OH is separated, COOH is not.

Question 13

in order to interact with radio waves, molecules must have

Answer 13

nuclear spin

Question 14

describe nuclear spin

Answer 14

• 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.

Question 15

define: chemical shift

Answer 15

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

Question 16

why is TMS used?

Answer 16

unreactive.

set to 0, allows for standardisation between different spectrometers

Question 17

why do different environmetns have different chemical shifts

Answer 17

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

Question 18

what do we look for in proton spectroscopy?

Answer 18

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

Question 19

what do we look for in carbon-13 spectroscopy?

Answer 19

• Chemical shift

* No peaks = no. C environments