7: Modern analytical techniques 1

Question 1

What is a mass spectrum produced by?

Answer 1

Mass spectrometer

Question 2

What are mass spectra used for?

Answer 2

Showing relative isotopic abundances to calculate relative atomic mass

Determining molecular masses of compounds

Question 3

What is a molecular ion denoted by?

Answer 3

M⁺_(g)

Question 4

How does a molecular ion form?

Answer 4

Molecule is bombarded with electrons which removes an electron from the molecule

Question 5

Where is the molecular peak found?

Answer 5

Peak with the highest m/z value

(ignoring any small peaks to the right - M+1)

Question 6

What is the M+1 peak?

Answer 6

Caused by the presence of ¹³C

Very small as it is not very abundant

Question 7

What is on the y-axis of a mass spectra?

Answer 7

Abundance of ions

%

Question 8

What is the on the x-axis of a mass spectra?

Answer 8

Mass/charge ratio

Question 9

What is the typical charge of molecules or fragments in a mass spectra?

Answer 9

+1

Question 10

How do fragments of compounds form in a mass spectra?

Answer 10

Bombarding of electrons make then break up into fragments

Question 11

What is a fragmentation pattern?

Answer 11

Fragments which are the ions which show up on the mass spectrum

Question 12

Why are fragmentation patterns useful?

Answer 12

Allows for the identification of molecules and their structure

Question 13

When a molecule fragments in mass spectrometry what forms?

Answer 13

Molecule → positive ion + free radical

Only positive ion is identified

Question 14

Learn these common fragment molecular masses

Answer 14

CH₃⁺ - 15

C₂H₅⁺ - 29

OH⁺ - 17

Question 15

What should you assume about the m/z value?

Answer 15

Same as the mass of the ion that made it

As charge is +1

Question 16

How can you tell two different compounds with the same molecular formula using mass spectrometry?

Answer 16

Produce different fragmentation patterns

Question 17

What occurs in infrared spectroscopy?

Answer 17

Infrared raditation is passed through a sample of the chemistry

Transmission of the light is measured

Question 18

What absorbs IR radiation in molecules?

Answer 18

Absorbed by covalent bonds in molecules

This increases their vibrational energy

Question 19

Why can IR be used to differentiate between different bonds?

Answer 19

Bonds between different atoms absorb different frequencies of radiation

Bonds in different places absorb different frequencies

Question 20

What is the peak for C=O in infrared spectroscopy?

Answer 20

Strong and sharp absoption at about 1700 cm^-1

Question 21

What is on the x and y axis from infrared spectroscopy?

Answer 21

x axis - wavenumber (cm^-1)

y axis - transmittance (%)

Question 22

What does a peak on an infrared spectra show you?

Answer 22

At which frequency the radiation is being absorbed

Question 23

What is wavenumber?

Answer 23

1/wavelength in cm

This is proportional to the frequency of the radiation

Question 24

When is infrared spectroscopy most used?

Answer 24

To determine if a reaction has occured

Because it shows which functional groups are present