Nuclear Magnetic Resonance Lab Report

Introduction of Nuclear Magnetic Resonance Nuclear Magnetic Resonance (NMR) spectroscopy is a type of absorption spectroscopy that involves the distribution of molecules in magnetic field externally which causes splitting of spin state into states with unequal energy. It is also a quantitative and qualitative technique to determine the content and purity of sample. For known compounds, NMR can analyse the mixture quantitatively while for unknown compounds, it can be used to evaluate the basic structure which in turn determine the physical properties of molecules for instance solubility and conformational changes (1).

Principle

Nucleus has spin and they are electrically charged particles. In absence of a magnetic field, they oriented randomly.

Inductive effect-when the molecules possess of electronegative atoms which has the electron withdrawing ability may reduce the electron cloud density resulting in downfield effect and delta value is greater.

2. Van der Waals force-when the protons are surrounded by bulky groups which tend to repel, resulting in de-shielding of proton.

3. Anisotropic effect-anisotropy means non-uniform. Different compounds have different distribution of electrons. For example, in alkene, induced magnetic field generated by the circulation of electrons is diamagnetic around carbon atom and paramagnetic to protons. These causes the protons to experience higher field strength and thus resonate at lower applied field.

4. Hydrogen boding-de-shielding and downfield shift is dependent upon the strength of H-bonding. The shifting is affected by concentration and also temperature. Electronegative atoms such as Fluoride which attach to the hydrogen bonding proton induces the attraction of electrons towards F resulting in the reduced in electron density around the proton.

There are three types of decoupling:

• Homo-nuclear Decoupling is decoupling of spin when nuclei irradiated by radio frequency (RF) are isotopes to the nuclei being analysed.

• Hetero-nuclear Decoupling is decoupling of spin when the nuclei being (RF) irradiated are different isotope with the nuclei being observed.

• Off Resonance Decoupling is compromising decoupling technique in which when the sample is irradiated with a second RF it will preserve one bond couplings and eliminate more than one bond couplings by up-field and down-field.

Nuclear Overhauser Effect

NOE is the space effect that involves the transfer of nuclear spin polarization from one nuclei to another via cross-relaxation which causes the change in intensity of spin. For example, when a nucleus is excited by radiofrequency the stimulation will be transferred to its neighbour via relaxation occurs between the two nuclei and so the intensity of the nucleus which is equilibrium changes and may increase up to 50%. As the physical distance between the equilibrium nucleus and the excited nucleus gets smaller, the change increases.

Signal of