Block 2 - Spectroscopy

Question 1

Types of spectroscopy

Answer 1

Mass spectroscopy (MS)
Infrared spectroscopy (IR)
Ultraviolet-visible spectroscopy (UV-VIS)
Nuclear magnetic resonance spectroscopy (NMR)

Question 2

Molecular spectroscopy

Answer 2

IR, UV-Vis and NMR are forms of spectroscopy collectively known as molecular spectroscopy
Depend on interaction of molecules with radiation of specific energy

Question 3

Energy, frequency, and wavelength relationship

Answer 3

Energy directly proportional to frequency

Energy inversely proportional to wavelength

Question 4

Mass spectrometry allows…

Answer 4

The determination of the mass of individual ions derived from compounds in the gas phase

Question 5

Mass spectrometry - how to read the molar mass

Answer 5

The highest m/z (where z = 1, so m) value on the graph is the molar mass

Question 6

Mass spectrometry - limitation(s)

Answer 6

Unable to distinguish between compounds with same molecular formula
Unable to distinguish between different molecular formulae that have same integer mass

Question 7

IR spectroscopy - units

Answer 7

1/cm

Question 8

UV radiation is of…

Answer 8

Appropriate energy to raise electrons in some molecules (generally those with pi e-) from lower energy bonding (or non-bonding (lone pairs)) molecular orbitals to higher energy anti-bonding molecular orbitals

Question 9

UV-VIS radiation causes…

Answer 9

The transition of an e- from a lower energy (ground state) to a higher energy (excited state) level

Question 10

Conjugation

Answer 10

Pi electron systems (double bonds) that are linked together

Double bonds are only conjugated when there is only ONE single bond between the double bonds, i.e. double-single-double

Question 11

Conjugated vs non-conjugated systems

Answer 11

Exhibit different UV-VIS absorbance
ΔE for electron excitement is smaller in conjugated systems, so absorption is observed at longer wavelengths than for non-conjugated systems

Question 12

Bonding molecular orbital

Answer 12

When 2 atoms share 2e- (one molecular orbit between 2 atoms)

Question 13

UV-VIS - sigma bonds

Answer 13

Compounds containing only sigma bonds are generally transparent to UV-VIS light - won’t see an absorption

Question 14

UV-VIS - number of double bonds?

Answer 14

Can’t tell how many C=C or C=O bonds there are; only one absorption seen

Question 15

UV-VIS: non-conjugated C=C bond

Answer 15

Shows an absorption at ~170nm due to π-π* electron transition

Question 16

UV-VIS: non-conjugated C=O bond

Answer 16

Shows an extra absorption (weak) at 280nm due to a n-π* electron transition
i.e. has absorption at 170nm and 280nm

Question 17

Conjugated double bond - minimum absorption

Answer 17

200nm

Question 18

The greater the number of double bonds in conjugation…

Answer 18

The smaller the ΔE and the larger the max wavelength

Question 19

For a particular compound at a specified wavelength…

Answer 19

Absorbance is directly proportional to concentration and path length

Question 20

Beer’s Law

Answer 20

A = ɛ b c
Where
A = absorbance (no unit)
ɛ = molar absorptivity (L mol-1 cm-1)
b = path length (cm)
c = concentration (mol L-1)

Question 21

Effectiveness of molecule absorbing light and absorbance

Answer 21

The more ‘effective’ a molecule is at absorbing light at a wavelength, the greater the absorbance

Question 22

Bonding vs non-bonding orbitals - energy

Answer 22

Bonding orbitals lower in energy than anti-bonding orbitals

Question 23

UV-VIS - least to most energy bonds

Answer 23

C=C: σ < π < π* < σ*

C=O: σ < π < n(on-bonding) < π* < σ*

Question 24

What happens when nuclei with spin are placed in a magnetic field

Answer 24

They align themselves with (parallel to) or against (anti-parallel to) the external magnetic field
Nuclei aligned with the magnetic field have slightly lower energy than those aligned against the field

Question 25

Nuclei magnetic field - irradiation

Answer 25

On irradiation with radio frequency of appropriate energy, energy is absorbed and the nuclear spin flips from the lower energy state to the higher energy state

Question 26

NMR absorptions

Answer 26

Known as chemical shifts Measured in 𝛿 (delta) units (ppm) TMS = 0 --> ignore

Question 27

0𝛿 - close and far

Answer 27

Closer to 0: shielded (C-H or C-C) | Further from 0: de-shielded (bonded to more electronegative atoms)

Question 28

What Cs or Hs give rise to different signals

Answer 28

Each unique C or H will give rise to different signals

Question 29

Chemical range for 13C NMR

Answer 29

Generally 𝛿0 - 210 𝛿0 - 90: sp3 carbon 𝛿100 - 210 (or sometimes higher): sp2 carbon

Question 30

13C NMR: intensity of signal

Answer 30

Generally not an indication of the number of Cs giving rise to a signal

Question 31

Interpreting 1H NMR - absorption signals

Answer 31

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

Question 32

Chemical shift range for 1H NMR

Answer 32

Generally 𝛿0 - 15, with most signals observed in 𝛿0 - 10

Question 33

NMR: signals for H bonded to sp2 carbon

Answer 33

Signals are at higher 𝛿 values

Question 34

NMR: C and H link to deshielding

Answer 34

If C experiences less electron density around it, then the H bonded to the C will also experience less --> more de-shielded

Question 35

Splitting pattern for absorption signals (H)

Answer 35

Splitting of an H absorption signal occurs when there are vicinal Hs for said H

Question 36

Vicinal Hs

Answer 36

Neighbouring Hs | Signal for one H is split by vicinal Hs and vice versa

Question 37

Number of H signals

Answer 37

In general, an absorption signal is split into N+1 lines, where N is the number of vicinal Hs The line intensities within the split signal accord with Pascal's triangle