Spectroscopy

Question 1

What are the 4 types of spectroscopy?

Answer 1

Mass spectrometry (MS)
Infrared spectroscopy (IR)
UV-Visible spectroscopy (UV-VIS)
Nuclear magnetic resonance spectroscopy

Question 2

What is mass spectrometry?

Answer 2

Examination of ions (fragments) derived from molecules that allow the calculation of molecular mass

Question 3

What is infrared spectroscopy?

Answer 3

Determines the presence or absence of particular functional groups in a molecule

Question 4

What is UV-Vis spectroscopy?

Answer 4

Determines the presence of pi-bond electrons in a molecule as well as the presence of any conjugation in the structure of the compound

Question 5

What is NMR?

Answer 5

Provides information about the carbon and hydrogen framework of a molecule

Question 6

What is molecular spectroscopy?

Answer 6

IR, UV-VIS, NMR - depend on the interaction of molecules with radiation of specific energy

Question 7

What is the relationship between energy and frequency? Provide an equation

Answer 7

E= h x v

where
h is plancks constant
v is frequency of radiation

Hence energy is directly proportional to frequency of radiation

Question 8

What is the relationship between energy and wavelength? Provide an equation

Answer 8

c = v x lambda

where
c is speed of light
v is frequency of radiation
lambda is wavelength

therefore sub into E = hv

E = hc/lambda

Therefore energy is inversely proportional to wavelength of radiation

Question 9

How does mass spectrometry work?

Answer 9

A molecule is bombarded with high energy electron beam -> this causes expulsion of an electron from the molecule creating a positively charged molecular ion (radical cation)

The radical cation formed is fragmented forming other positive ions and neutral fragments

A plot of relative ion abundance vs m/z forms the mass spectrum (mass of ion/charge of ion (+1))

Question 10

How is the molecular mass of the molecule determined from the mass spectra?

Answer 10

The molecular ion (radical cation) must be the ion of the highest m/z hence the highest m/z value corresponds to the molecular mass of the parent ion

Question 11

How does infrared spectroscopy work?

Answer 11

Absorption occurs when the infrared radiation frequency matches the frequency of bond vibration (specific to the type of bond)

Question 12

How is the functional groups determined from IR spectra?

Answer 12

The relative masses of atoms involved and bond strengths determine the position of IR absorption displayed on spectrum.

Hence specific bonds will show absorption at specific ranges of the spectra

Question 13

What are the strengths and weaknesses of MS

Answer 13

Only information about molecular mass - cannot determine between isomers

Question 14

What are the strengths and weaknesses of IR spectroscopy?

Answer 14

Fast identification of functional groups

Compounds containing same functional groups will show very similar spectra
No indication on number of functional groups present

Question 15

How does UV-Visible spectroscopy work?

Answer 15

UV-VIS radiation raises electrons in molecules (with pi bonds) from lower energy bonding and non-bonding molecular orbitals to higher energy anti-bonding molecular orbitals

The wavelength at which maximum absorption occurs is considered

Question 16

What is a conjugated pi-system?

Answer 16

When the molecule contains a pi bond - sigma bond - pi bond

Question 17

What is the wavelength range of UV?

Answer 17

200-400nm

Question 18

What is the wavelength range of Visible light?

Answer 18

400-800nm

Question 19

What is an anti-bonding MO?

Answer 19

All bonding orbitals have an anti-bonding orbital

e.g. pi - bonding orbital
pi star - anti-bonding orbital

Question 20

What are the types of non-conjugated functional groups that show absorption on UV-VIS spectra?

Answer 20

C=C 170-180nm

C=O 170-180nm 280nm

Question 21

What absorptions are seen for C=C (non-conjugated system) and why?

Answer 21

The excitation of an electron from the pi bonding MO to the anti-bonding MO shows absorption at 170-180nm

Question 22

What absorptions are seen for C=O (non-conjugated system) and why?

Answer 22

The excitation of an electron from the lone pair on oxygen to anti-bonding pi orbital shows weak absorption at 280nm

The usual absorption at 170-180nm from excitation of pi orbital electron is also seen.

Question 23

Does UV-VIS provide info on the number of pi bonds?

Answer 23

No

Question 24

What is the relationship between absorption and wavelength for conjugated systems compared to non-conjugated systems?

Answer 24

Absorption occurs at longer wavelengths for conjugated systems as the energy gap between the molecular orbitals are small

Usually >200nm

Question 25

What is the relationship between number of double bonds in conjugation and wavelength?

Answer 25

The greater number of double bonds in conjugation = the smaller the gap in energy between molecular orbitals = larger the longest wavelength value

Question 26

What is Beer's Law

Answer 26

A = Ebc where A is absorbance b is path length (cm) c is concentration (mol L-1) E is molar absorptivity (L mol-1 cm-1) Hence absorbance is directly proportional to concentration and path length

Question 27

What is the relationship between amount of conjugation and molar absorptivity?

Answer 27

No relation

Question 28

How does NMR work?

Answer 28

Nuclear magnetic resonance provides information about the carbon and hydrogen frameworks of molecules. A magnetic field is applied to a number of nuclei causing excitation from a lower to higher energy spin state

Question 29

What is the factor that affects the energy required for a flip in nuclei spin?

Answer 29

Shielding of protons due to electrons

Question 30

What does shielding do to the chemical shift value?

Answer 30

Shielding decreases chemical shift

Question 31

What is the chemical shift range for 13C NMR spectrum?

Answer 31

delta 0-220

Question 32

What is the chemical shift range for sp3 hybridized carbon atoms?

Answer 32

0-90 delta

Question 33

What is the chemical shift range for sp2 hybridised carbon atoms?

Answer 33

100-220 delta units

Question 34

How does shielding change?

Answer 34

Shielding decreases when electronegative groups of atoms are bonded to the carbon as it pulls electrons away from the molecule

Question 35

With proton NMR what are the 4 components of spectra?

Answer 35

Number of absorption signals Position of absorption signals Relative areas under absorption signals Splitting pattern for absorption signals

Question 36

What information does the number of absorption signals provide?

Answer 36

Number of H nuclei in different environment

Question 37

What information does the position of absorption signals provide?

Answer 37

The amount of shielding of the proton nuclei

Question 38

What information does the relative area under absorption signals provide?

Answer 38

The number of protons in the same environment

Question 39

What does the splitting pattern for absorption signals provide?

Answer 39

The number of neighbouring hydrogens - each neighbouring proton will cause the signal to split once