SECTION B

Question 1

What is the formula to convert wavelength from nm to cm-1?

Answer 1

Question 2

What effect does shielding have on NMR frequency?

Answer 2

Shielding and NMR frequency are inversely proportional (highly deshielded nuclei are shifted to higher chemical shift values).

Question 3

What is the formula for calculating IR vibration frequency of a bond?

Answer 3

Question 4

What can an IR peak atr 1670 cm-1 mean?

Answer 4

Either a C=O stretching or OH bending. To distinguish them, we must take into account tht the C=O peak is usually very strong and sharp.

Question 5

How does a multiplet pattern arise in NMR?

Answer 5

It arises due to magnetic coupling between the spins of protons that are on adjacent carbons in a structure and that are in magnetically non equivalent environments.

Multiplet patterns follow the (n+1) rule, where n is the number of 1H nuclei on the neighbouring carbon.

More detail explanation from lecture notes on how doublets arise:

When proton (a) undergoes its resonance and flips its spin, the other nucleus could be in either “up” or “down” states. These will cause the magnetic field felt by proton (a) to be very slightly different, so two closely spaced NMR lines are observed (i.e., a “doublet” for proton (a), rather than a single line.

The same will happen for the proton (Hb) resonance.

The amount of the splitting will depend on the strength of the magnetic interaction between the two protons : this is the magnitude of the coupling constant (J).

Question 6

What does an absorption in the UV-vis range correspond to?

Answer 6

Electronic transitions (an electron absorbs energy provided by the electromagentic radiation an jumps from its ground state to a higher energy level).

Usually, for organic compounds these transitions are between pi and pi* M.Os.

Question 7

What colours do these wavelengths correspond to?

i. 400nm
ii. 500nm
iii. 700nm

Answer 7

Question 8

True or False:

Longer wavelength = higher energy

Answer 8

False. Wavelength and energy are inversely proportional.

E=h*c/λ

.

Question 9

What does an IR peak at 3200cm-1 correspond to?

Answer 9

H-bonded OH or NH.

Question 10

Why does H-bonding lower the IR vibration frequency of OH groups?

Answer 10

• Forming the H-bond reduces the strength of the bond, i.e reduces K, which is directly proportional to frequency (see formula).
• Frequency also decreases due to the fact that the second atom that the H-bonnding H is attached to starts to move, causing the reduced mass to increase slightly. Higher reduced mass, lower frequency (see formula).

Question 11

Why do NMR signals from protons attached to O or N always appear as broad singlets?

Answer 11

• The magnetic coupling from which the multiplet arises does not occur across heteroatoms such as O or N.
• Protons attached to heteroatoms may be easily exchanged and can H-bond. The chemical exchange which is also responsible for the broadening of the signal, is mediated and acceleratd by traces of water, acid, and/or base. When the exchange is very rapid on the NMR timescale, the spin states of protons on the heteroatom are averaged out and the signal appears as a broad singlet.
• The rate of chemical exchange can be slowed down by cooling, removing all traces of the aforementioned substances.

Question 12

Do O2 and N2 have dipole moments?

Answer 12

No, as they are homonuclear diatomic molecules:

• Bond vibrations in e.g H2O or CO2 do cause a change in the electric dipole moment, resulting in a fluctuating electric field that can interact with IR light and thus can give rise to absorption bands in the IR spectrum.
• In O2 and N2, the electric dipole moment remains =0 during the vibration.

Question 13

What causes the broadening of an OH NMR peak?

Answer 13

Intermolecular and intramolecular H-bonding + chemical exchange.

Question 14

What is the formula for isotopoc substitution?

Answer 14

Question 15

Small unsaturated hydrocarbons absorb____________.

Answer 15

Entirely in the UV region, λ<400

Question 16

Non-bonded electrons on a heteroatom give rise to________.

Answer 16

n to pi* transitions in the 200-400nm range, but weaker than the pi to pi* transitions observed in e.g carotene

Question 17

What is the formula for degrees of unsaturation?

What does this result tell us?

Answer 17

0: no pi bonds, no rings
1: 1 pi bond or 1 ring

etc.

Question 18

What is the de Broglie relationship?

Answer 18

It expresses the particle-wave duality of light: each “particle” has a pilot wave, termed a photon, with a wavelength related to the mass and particle velocity

Question 19

What is the Beer-Lambert law?

Answer 19

Also A=log(Io/It)

Question 20

Equation to convert A to %T?

Answer 20

A= 2 -log%T

Question 21

What is the Larmor frequency and what is its formula?

Answer 21

When placed in a magnetic field, charged particles will precess about the magnetic field. In NMR, the charged nucleus, will then exhibit precessional motion at a characterisitc frequency known as the Larmor Frequency. The Larmor fequency is specific to each nucleus.

Question 22

What is the formula for chemical shift?

Answer 22

Question 23

What is the IR frequency of the C-H stretch in aliphatic hydrocarbons?

How does this value change in unsaturated hydrocarbons?

Answer 23

~3000 cm-1

Unsaturated: shifted to higher frequencies (>3000cm-1)

Question 24

c = ?

Answer 24

c = λ*v

Question 25

What is Planck’s relation?

Answer 25

E = h*ν

In atoms, electrons orbit around the nucleus (in molecules, electrons orbit around several atomic nuclei).

• The electron energy is quantized : it is related to the radius of the orbit.
• The energy is related to the principal quantum number n. n can take values 1, 2, 3, 4, etc

(1s, 2s, 2p, 3s, 3p, 3d orbitals etc ….)

• If the atom absorbs UV-visible light the electron can jump to a higher energy orbit.
• The energy jump is related to the frequency and wavelength of light absorbed by Planck’s relation : ΔE = h ν = hc / λ

Question 26

Which electronic transitions involve

a) Valence (bonding) electrons
b) tightly bound electrons
c) weakly bound electrons

Answer 26

a) UV-Vis transitions
b) X-ray transitions
c) Transitions in the IR

Question 27

UV-Vis is useful to study…. (5)

Answer 27

• pi-pi* transitions in aromatic organic molecules
• determine chain lengths in conjugated hydrocarbons
• study C=C vs. C=O unsaturated species
• Determine presence of chromphores
• determine the presence of organometallic, transition metal compounds: absorb light in the UV-Vis range due to d-d orbital transitions

Question 28

What gives rise to UV-Vis absorption bands?

Answer 28

Electron jumps between molecular orbitals

Question 29

In what region of the electromagnetic spectrum are light energies associated with bond vibrational frequencies found?

Answer 29

IR

Question 30

Are atoms in molecules static?

Answer 30

No, they vibrate around their equilibrium positions

Question 31

How does IR work?

Answer 31

The atoms in molecules are not static - they vibrate around their equilibrium positions.
• The vibrational frequencies correspond to light energies (E = hν) in the infrared region of the spectrum.

• Some of the vibrations cause a change in electric dipole moment of the molecule.
• This results in a fluctuating molecular electric field associated with the vibration.
• This “resonates” with the fluctuating electric field of the light wave, causing absorption of the light energy at each vibrational frequency which results in a peak at that frequency in the IR spectrum.

Question 32

How can IR be used for functional group analysis?

Answer 32

Functional groups will always have approximately the same IR peaks in different molecules, so they can be used to work out features of the structure of an “unknown” molecule

Question 33

How does NMR work?

Answer 33

• Nuclei like 1H, 13C have a property called “spin”.
• The spin results in the nucleus behaving like a tiny magnet, with N-S poles.
• If the molecule is placed in an external magnetic field, the spins line up with the field direction.
• If the sample is irradiated with light (electromagnetic radiation), it can cause the spins to “flip” (the interaction occurs with the fluctuating magnetic field of light).
• The spin flips have a particular energy, that depends on

(a) the size of the external field;
(b) the type of nucleus;
(c) the local environment (structure, bonding) of each atom.

• The spin flip energies occur in the radio wave region of the electromagnetic spectrum.
• We use radio transmitters and receivers to generate, detect the spin flip energies - an NMR spectrum.
• Different atoms (nuclei) within a given molecule “resonate” at different positions.
• They can also interact with other magnetic nuclei in different ways, to produce characteristic patterns.

Question 34

For NMR, in which region of the electromagnetic spectrum do spin flip energies occur?

Answer 34

The spin flip energies occur in the radio wave region of the electromagnetic spectrum.