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as level chemistry > 2.8 instrumental analysis > Flashcards

2.8 instrumental analysis Flashcards

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1
Q

what can mass spectroscopy be used to identify?

A

different elements and find the relative isotopic masses of elements

(it can also be used to identify the molecular mass of a molecule and fragment ions from the molecule)

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2
Q

what are the three ways that can be used together to determine the molecular formula of an unknown molecule?

A
  1. mass spectroscopy
  2. infrared spectroscopy
  3. NMR spectra
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3
Q

how does a mass spectra work? what are the 5 stages?

A
  1. vapourisation - sample turned into a gas
  2. ionisation - the atom (or molecule) is ionised by knocking one or more electrons off to give a positively charged ion. for a molecule, the removal of electrons will cause bonds to break and the molecule to fragment
  3. acceleration - the positive ions are accelerated between a set of charged plates. the ions are repelled by the first plate and attracted to the second. a hole in the second plate allows the ions to exit into the next chamber, all with the same kinetic energy
  4. deflection - the ions are then deflected by a magnetic field according to their mass to charge ratio (m/z). the smaller the m/z ratio, the more they are deflected
  5. detection - the beam of positive ions passing through the machine is detected electrically
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4
Q

how do you calculate the relative atomic mass from a mass spectrum?

A

(% abundance x m/z) + (% abundance x m/z) + …. / total abundance

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5
Q

on a mass spectra, what does the last peak tell us?

A

the Mr

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6
Q

what are absorptions measured in?

A

wavenumber (cm^-1)

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7
Q

when is % transmittance found on the y-axis of an IR spectrum?

A

when the detector is set to calculate how much energy passed through the molecule at each frequency

100% at the top and 0% at the bottom

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8
Q

what is an example of a practical application of an infrared spectroscopy?

A

modern breathalysers in police stations are attached to an infrared spectrometer

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9
Q

what are some tips when reading an IR spectrum?

A
  • ignore peaks below 1500 (fingerprint region)
  • peaks travel downwards, not upwards
  • match up the peaks to the corresponding number on the data sheet
  • (look at peaks below 50% transmittance first, then if need more info, look at all peaks)
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10
Q

what is the last peak on the mass spectrum called?

A

the molecular ion peak
(it tells us the molecular mass (Mr) of the species)

(all the other peaks before this peak are fragments of the sample molecule)

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11
Q

why do mass spectrums have lots of tiny peaks?

A

the sample is never 100% pure

avoid trying to assign every single peak - look at the bigger ones

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12
Q

what species is responsible for the molecular ion peak? e.g

A

[compound]^+

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13
Q

what does NMR stand for?

A

nuclear magnetic resonance spectroscopy

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14
Q

. H H H
I I I
H - C - C - C - H
I I I
H H H
how many hydrogen environments does it have?

A

2
left 3 and right 3, middle 2

left 3 identical to each other as bonded to same carbon
left 3 and right 3 in the same environment as symmetrical

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15
Q

. H H H H H
I I I I I
H - C - C - C - C - C - Cl
I I I I I
H H H H H
how many hydrogen environments?

A

5 environments
left 3, next 2, next 2, next 2, next 2

they are not identical as different distances from Cl

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16
Q

features of NMR spectras:

A
  • hydrogen NMR spectra have an x-axis that runs backwards from 0 around 12 (has units ppm)
  • carbon -13 NMR spectra also run backwards and have an x axis that has a scale running from 0 to around 200ppm
  • every NMR spectrum has a zero point - this is used as a standard or reference substance. the molecule responsible for this is TMS (tetramethylsilane)
  • this can be ignored when looking at the NMR spectra
  • as with IR spectra, WJEC expect you to match the peaks to a fragment of a molecule
17
Q

^1H NMR spectra:

A
  • identify the number of hydrogen environments in the molecule. each hydrogen environment produces one peak
  • the closer the hydrogen atoms are to electronegative atoms, the further downfield the signal produced is. this is because electronegative elements deshield the nuclei of the hydrogen atoms
  • the PEAK AREA is the number of hydrogen producing that peak
  • in hydrogen NMR, the scale goes from 0-10 but backwards
  • if you are unsure, look for symmetry within the molecule. if the molecule is symmetrical, this will reduce the number of environments
  • using numbers from the data sheet will give you a guide as to what groups are responsible for what peaks - don’t use the numbers rigidly
  • size of peak needs to be in ratio
18
Q

1H NMR of methoxymethane:
H3C - O - CH3

A

. H H
I I
H - C - O - C - H
I I
H H

1 environment
H
I
C - O so at 4 (HC-O = 3.3-4.3)
peak 6 high (bc 6 hydrogens in environment)

19
Q

does peak height matter in 13C NMR spectras?

A

no

20
Q

13C NMR spectra:

A
  • identical principles to hydrogen NMR but with carbon atoms
  • each peak represents a different carbon environment
  • scale runs from 0-200
  • use data sheet
21
Q

if the peak height is different in a 13C NMR spectra, what does it mean?

A

nothing - ignore it

22
Q

what is the point of NMR spectras?

A
  • powerful analytical technique to identify what you have
  • can identify isomers
23
Q

what happens when infrared radiation is directed at a compound?

A

bonds within the molecule absorb specific frequencies of radiation

24
Q

what happens when specific frequencies of radiation are absorbed by bonds in a molecule?

A

the bonds vibrate. this causes the bond to stretch or bend

25
Q

what can be identified using peaks on an infrared spectrum? how?

A

the functional groups within a molecule
- by comparing the peaks to known values in a data book

26
Q

what is the fingerprint region on an infrared spectrum?

A
  • the region below 1500
  • that contains a complicated pattern of absorptions. every compound has a unique fingerprint region
27
Q

what is a carbon environment?

A

the atoms, or groups of atoms, that a carbon atom is bonded to

28
Q

what can be deduced from a carbon-13 NMR spectrum?

A
  • number of different carbon environments = no. of peaks on the spectrum
  • types of carbon environments = compare the chemical shift values of the peaks to the data book to identify the carbon environments
29
Q

how to name the compound from a 13-C NMR spectra?

A
  1. determine the number of carbon environments
  2. use the data sheet to assign chemical shift values to 13-C environments
  3. draw all possible structural isomers. identify the number of 13-C environments in each structure
30
Q

the molecular ion has the largest m/z NOT THE LARGEST PEAK

A

as level chemistry (16 decks)

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