Introduction to NMR Flashcards
What is NMR?
Nuclear magnetic resonance, NMR it is resonance transition between magnetic energy levels, happening when atomic nuclei are immersed in an external magnetic field and applied an electromagnetic radiation with specific frequency. It can provide detailed information about the structures of a molecule in solution phase.
It is mainly used for 1H and 13C but can be used on many other nuclei with a magnetic nuclei.
What is needed for NMR to work?
Nuclear spin is needed giving the atom magnetic properties.
What is nuclear spin?
Nuclear spin is type of angular moment, it arises from the unpaired proton and neutron in the nucleus and is shown by the spin quantum number I.
- 1/2 spin = an odd number of neutrons and even number of protons or an even number of protons and an even number of neutrons.
- 1 spin = odd number of protons and neutrons.
What is the nuclear magnetic moment?
All spinning bodies in this case the nuclei has angular momentum s with all charged particles if the nucleus is moved in a loop it will generate a magnetic field this is called the magnetic moment an it denoted by μ. It can be related to the angular momentum using
μ=γI.
γ = gyromagnetic ration (a proportionality constants unique to each nuclei)
What is angular momentum?
Angular momentum is the intrinsic properties of an object to spin without moving anywhere (electrons rotating around the nucleus). Protons and neutrons both have angular moments of 1/2 and each contributes to the total angular moment of a nuclei. It is also called the nuclear spin and it denoted by I.
How do you calculate the allowed nuclear spin states associated with the nuclear spin (I)?
2I + 1
in the absence of a magnetic foil these spin Staes are all degenerate.
e.g.
if I = 1/2 (1H) (2x1/2) + 1 = 2 so there are two degenerate spin states of +1/2 and -1/2
if I = 1 (2H) (2x10+1 =42 h 3 so their are three spin states go +1 , 0 , -1
What are the nuclear energy levels in the presence of a magnetic field?
In the presence of a magnetic field the nuclear spin (I) can take up different orientations (calculated using (2I+1), with each orientation at a different energy level
How do you calculate the energy separation of the two states of a nucleus with a spin of 1/2?
In an applied magnetic field (B0) the energy of each spin state is given as
E = γB0m(h/2π)
so for I = 1, you have m = +1/2 and m = -1/2 leading to two energy levels (2X1/2+1). the difference in energy is then
+1/2γB0(h/2π) - 1/2γB0(h/2π)
γB0(h/2π) = vh v = γ(B0/2π)
What is chemical shift?
Chemical shift is the local magnetic field experienced by a nuclei, it is denoted by δ a reported in ppm.
What does the values of the chemical shift depend on?
although the same magnetic field is applied to the small the effective magnetic field (Beff) experienced by each nucleus is different due to the motion of electrons surrounding the nucleus. They can either shield the nuclei or deshield it by changing electron densities.
How do you calculated the effective magnetic field?
Beff = B0 (1- σ)
σ = shielding constant
What is the integration?
This is the relative intensities of peaks within an NMR spectrum. It indicates how many nuclei are found in each chemical environment. It is found by measuring the area under each peak.
How do you use 2nI +1 equation to determine multiplicity?
