H NMR and C NMR

Question 1

What is NMR?

Answer 1

it is a technique that allows the chemical environment of certain atomic nuclei to be observed - structure determination

1 - in the same molecule = allows compounds tp be identified
2 - in adjacent molecules

Question 2

How many atoms can H NMR look at?

Answer 2

it can look at one atom type at a time - C, H, F, N
it does not result in a single peak as each atom in the molecule experience a slightly different magnetic field due to its chemical environment

Question 3

What is nuclear spin?

Answer 3

all nuclei have spin and spin can be in a number of directions
spin is represented by m/
there is no energy difference between spins

Question 4

What are the different energy levels for nuclei spin?

Answer 4

it can be represented by the formula 2i + 1
nuclear of 1/2 is the most important for NMR as it gives 2 energy levels of +1/2 and -1/2
NMR only measures elements with a spin of 1/2 as it shows 2 peaks - cannot measure more than 2 peaks

Question 5

Why don’t elements with a nuclear spin of 0 show up on H NMR spectra?

Answer 5

a nuclei with a spin of 0 has only one possible spin as 2i+1 give 1
the spin cannot flip between energy levels so cannot be measured by NMR spectra

Question 6

How does a magnet affect nuclei spin?

Answer 6

nuclei spin behaves like a magnet and can flip between energy states/levels

absence of magnetic field - no energy difference between directions of spin (+1/2 and -1/2)
presence of magnetic field - energy differences

nuclear magnetism aligned with the field = low energy state (alpha spin state) - +1/2
nuclear magnetism aligned against the field = high energy state (beta spin state) - -1/2

Question 7

How does H NMR instrumentation work?

Answer 7

nuclei is placed in a magnetic field
initially most align with the magnetic field, Bo = low energy state
radio frequency radiation is fired at the nuclei - excites/promotes it to a higher energy state = against magnetic field
radiation firing is stopped and as nuclei come back to lower energy state they release energy/frequency they absorbed

NMR measure the exact frequency of this energy (absorbance vs frequency)

Question 8

What is resonance frequency?

Answer 8

puts nuclei in resonance with the field

frequency of radiation needed to excite the nuclei from the lower energy state to the higher energy state - flip the spin state

depends on the magnetic field strength - the bigger the difference between them the more resonance frequency is required to flip the nuclei spin

Question 9

What does the energy change depend on?

energy change - when nuclei flips back from higher state to lower state

Answer 9

magnetic field - the larger the Bo, the more sensitive the environment will be and the are detailed the spectra will be

nuclear magnetism - fixed for each nuclei
- depends on electrons density (shielding/deshielding)

Question 10

Why does electron density affect nuclear magnetism and energy change?

Answer 10

electrons have their own spin and magnetic field
electron magnetic field affects the strength of the magnetic field felt by the nuclei

electron magnetic field opposes the magnet’s magnetic field

Question 11

What is shielding and deshielding?

Answer 11

Shielding
- H atom is surrounded by high electron density
- electron magnetic field opposes the magnet’s magnetic field
- nuclei is hidden/shielded from the magnetic field and feels a weaker/smaller magnetic field
- nuclei absorbs less radio frequency radiation
= smaller energy change and smaller resonance frequency

Deshielding
- H atom is linked to an electronegative atom
- electron density is pulled away from the nuclei
- nuclei is deshielded and is exposed to the magnetic field = larger Bo is felt
- nuclei absorbs more radio frequency radiation
= larger energy change and larger resonance frequency

Question 12

What is a Fourier transform used for?

Answer 12

it is used to process the high resolution NMR spectra

energy released by nuclei when going back to lower energy state is called free induction decay (signal released)
Fourier transform applies a mathematical function to identify all the frequencies and convert it into a NMR spectra

Question 13

What are the advantages and disadvantages of H NMR?

Answer 13

advantage - does not destroy the sample
disadvantage - insensitive method
- needs a concentrated sample = needs mg not mcg

MS needs smaller sample than NMR

Question 14

What is chemical shift in H NMR?

Answer 14

H spectra covers 0 - 10 ppm
some can appear above or below this range

difference in frequency between the resonance frequency of the observed proton and the reference tetramethylsilane

shows different environments in a compound

Question 15

Why is ppm used to measure chemical shift?

Answer 15

allows us to compare spectra run on different spectrometers

Question 16

How does shielding/deshielding affect chemical shift?

Answer 16

shielded nuclei 
- upfield chemical shift 
- closer to TMS = closer to 0 ppm
- to the right 
deshielded
- downfield chemical shift
- further away from TMS = closer to 10 ppm
- to the left

Question 17

Why is a reference used in H NMR?

Answer 17

tetramethylsilane
- each H atom is chemically equivalent = in the same environment
- all resonate to the same frequency
acts as a reference point to show the difference between other chemical shifts

Question 18

What is integration in H NMR?

Answer 18

area under each peak is proportional to the number of H atoms/protons producing that peak
it is a ratio

1 - can measure the height of the peaks
2 - ratio given

Question 19

What is multiplicity in H NMR?

Answer 19

shows the splitting pattern of group peaks

uses the n+1 to show multiplicity

Question 20

What are the splitting patterns?

Answer 20

single - 1
double - 1:1
triplet - 1:2:1
quartet - 1:3:3:1
quintet - 1:4:6:4:1

can use pascals triangle to find them

Question 21

How can the splitting pattern for equivalent protons be found?

Answer 21

use the n+1 rule

count the number of H atoms on the neighbouring atoms and add 1

Question 22

What is the coupling constant?

Answer 22

occurs because the magnetic field of adjacent protons influences the magnetic field the protons feels

difference between two sub peaks in a peak - distance between the lines in Hz

equivalent protons have the same coupling constant
non-equivalent protons have different coupling constants