Spectroscopy

Question 1

de Broglie relationship

Answer 1

λ = h / p
p = momentum = mv

Question 2

c =

Answer 2

νλ

ν = frequency (units s^-1)

Question 3

ṽ =

Answer 3

Wavenumber

1 / λ

Question 4

E =

Answer 4

hν = hc / λ = (kB)T = RT

For molar energies multiply by 6.02 x 10^23

Question 5

Rough wave numbers in cm^-1 of electromagnetic waves

Answer 5

10^-4 - 10^-1 radio waves
10^-1 - 10^1 microwaves
10^1 - 10^4 infrared
10^4 - 10^4.5 visible
10^4.5 - 10^6 ultra-violet
10^6 - 10^10 x-rays
10^10 - 10^14 γ-rays

Question 6

Type of electromagnetic wave used in each spectroscopy technique

Answer 6

NMR - radio waves
ESR - microwaves
Molecular rotation - microwaves, infrared
Molecular vibration - infrared, visible
Electronic energies - visible, UV, X-rays
Nuclear energies - γ-rays

Question 7

Explain absorption and emission

Answer 7

Moleules in an excited state can emit light. Molecule can go from excited state to ground state and vice versa. The probability of these 2 processes happening is the same in an electromagnetic wave. So overall amount of light absorbed depends on difference in population of the 2 energy levels

Question 8

What is ε, the molar extinction coefficient

Answer 8

Constant for a particular molecule at a certain wavelength

Has units mol^-1 dm^3 cm^-1

Question 9

How to determine ε

Answer 9

A plot of A vs. c

Question 10

Electron in a box model, larger box means…

Answer 10

larger charge separation –> stronger interaction with the electric field –> larger ε

Question 11

Explain electronic spectroscopy

Answer 11

It involves the moving of electrons between molecular orbitals - changing the electronic structure
The part of the molecule that absorbs light is called the chromophore
- organic molecules, mostly involves n, π or π* orbitals
- inorganic molecules, often involves transfer of electrons between d-orbitals

Question 12

Uses of electronic spectroscopy

Answer 12

Transitions are usually broad, so little use for identifying molecules, but can follow trends in electronic structure
Very useful for measuring the concentration of a known chromophore or following the course of a reaction

Question 13

Complementary colours

Answer 13

violet yellow
blue orange
red green
Compounds that absorb colour on the left appear colour on the right

Question 14

Vibrational spectroscopy can be modelled by…

Answer 14

The Simple Harmonic Oscillator

Question 15

Units of force constant k

Answer 15

Nm^-1

Question 16

ve (vibrational frequency of 2 masses connected by a spring) =

Answer 16

(1/2π) sqrt(k/μ)

Question 17

Relationship between force constants and bond strength

Answer 17

In general:
triple bonds > double bonds > single bonds
stronger bond = higher k

Question 18

No of vibrational modes in a molecule

Answer 18

If there are n-atoms, there are 3n - 6 vibrational modes

Except for linear molecules –> 3n - 5

Question 19

Rough IR absorptions for functional groups

Answer 19

O-H broad, 3500
C-H spiky, 3000
C≡N very strong, 2400
C≡C weak, 2300
C=O very strong, 1800
C=N 1500
Aromatic 2 bands at about 1500 and 1600
C=C 1650
NO2 1500

Question 20

Explain nuclear spin

Answer 20

Many nuclei have a quantum mechanical property known as spin, characterised by spin quantum number I

Spin gives magnetic moment μ = γsqrt(I(I+1)ħ)
ħ = h/2π, γ = magnetogyric ratio of the nucleus

In an applied magnetic field B the magnetic moment can align in 2I + 1 ways with respect to the field

Question 21

Nuclear spin energy levels

Answer 21

In a magnetic field B, the magnetic moment can only take certain orientations with respect to B
The orientation is described by a second quantum number m(l) = -l, -l+1,….,l
The energy of these states is given by E = -γm(l)ħB
Selection rule Δm(l) = ±1

Question 22

How does NMR work?

Answer 22

Need a big magnet

• very high field strength
• superconducting magnet

Sample placed in magnet

RF transmitter/receiver to obtain spectra

Question 23

Frequency of NMR, ν(NMR) =

Answer 23

γB / 2π

Question 24

Chemical shift 𝛿x =

Answer 24

(νx - νref) ν0 x 10^6 ppm
vref = ν of solvent
ν0 = operating ν

Question 25

2 mass spectrum methods and what they are used for

Answer 25

MALDI - polymers and biological macromolecules

ESI - biomolecules, but often leads to multiply charged ions

Question 26

No of rings + double bonds for CxHyNzOn =

Answer 26

C - 0.5H + 0.5N + 1

Question 27

Ratio of carbon isotopes

Answer 27

12 - 98.9%

13 - 1.1%

Question 28

Ratio of chlorine isotopes

Answer 28

35 - 75%

37 - 25%

Question 29

Ratio of bromine isotopes

Answer 29

79 - 50%

81 - 50%

Question 30

IR: molecules of greater mass…

Answer 30

have a lower frequency and so lower absorption

Question 31

Converting g mol^-1 into kg

Answer 31

divide by 1000, and divide by 6.02x10^23 mol^-1

reduced mass must be in kg