H NMR and C NMR Flashcards
What is NMR?
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
How many atoms can H NMR look at?
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
What is nuclear spin?
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
What are the different energy levels for nuclei spin?
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
Why don’t elements with a nuclear spin of 0 show up on H NMR spectra?
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
How does a magnet affect nuclei spin?
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
How does H NMR instrumentation work?
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)
What is resonance frequency?
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
What does the energy change depend on?
energy change - when nuclei flips back from higher state to lower state
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)
Why does electron density affect nuclear magnetism and energy change?
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
What is shielding and deshielding?
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
What is a Fourier transform used for?
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
What are the advantages and disadvantages of H NMR?
advantage - does not destroy the sample
disadvantage - insensitive method
- needs a concentrated sample = needs mg not mcg
MS needs smaller sample than NMR
What is chemical shift in H NMR?
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
Why is ppm used to measure chemical shift?
allows us to compare spectra run on different spectrometers
How does shielding/deshielding affect chemical shift?
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
Why is a reference used in H NMR?
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
What is integration in H NMR?
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
What is multiplicity in H NMR?
shows the splitting pattern of group peaks
uses the n+1 to show multiplicity
What are the splitting patterns?
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
How can the splitting pattern for equivalent protons be found?
use the n+1 rule
count the number of H atoms on the neighbouring atoms and add 1
What is the coupling constant?
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
