Nuclear Magnetic Resonance Spectroscopy (NMR)

 

Nuclear Magnetic Resonance Spectroscopy
(NMR)

Nuclear Magnetic Resonance

¨  Nuclear magnetic resonance is the most powerful physicochemical tool to determine the organic structures of molecules.

¨  NMR Spectroscopy is the study of molecular structure determination by recording the interaction of radiofrequency (Rf) electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field.

Basis of NMR

¨  NMR is used in determination of molecular structures as well as the content and purity os samples.

¨  The most widely used method of NMR is Proton (1H) NMR in analytical chemistry.

¨   The protons present in the molecule will behave differently depending on the surrounding chemical environment, making it possible to elucidate their structure.

NMR Principal

¨  As per the NMR principal most of the nuclei are exhibiting spin and all the nuclei are charged electrically.

¨  A nucleus with an odd atomic number or an odd mass number has a nuclear spin.

¨  The spinning charged nucleus generates a magnetic field.

¨  When placed in an external field these spinning protons act like bar magnets.

¨  When an external magnetic field is applied energy transfer from base energy state to higher energy state occur.

¨  The magnetic fields of the spinning nuclei will align either with the external field, or against the field.

¨  The wavelength at which energy transfer occur coincide with the radio waves.

¨  Emission of energy at the same frequency occur when the spin comes back to the base state.

¨  Processing of NMR spectrum for the concerned nuclei is measured by the signals that matches this transfer.

¨  The number of signals shows how many different kinds of protons are present.

¨  The location of the signals shows how shielded or deshielded the proton is.

¨  The intensity of the signal shows the number of protons of that type.

¨   Signal splitting shows the number of protons on adjacent atoms.

Working of NMR

¨  1^st of all sample is placed within magnetic field.

¨  The nuclei sample is excited with the help of radio waves within the magnetic field to generate NMR signals.

¨  A detector sensitive to radio waves is used to detect these signals.

¨  The intramolecular magnetic filed surrounding the molecules changes the resonance frequency of the atoms in the molecule.

¨  These changes in the resonance frequency of atom gives detain about the functional groups and structure of the molecule.

¨  Reaction state, structure of molecule, chemical environment and dynamics of a molecule are determined in this way by applying this technique.

Chemical Shift

¨  A nuclei that exhibit charge when do spinning it generates magnetic field, this magnetic field results magnetic moment.

¨  This magnetic moment is directly proportional to the spin.

¨  When an external magnetic field is applied it results in two spin states

¨  Up spin

¨  Down spin

¨  Among these up spin and down spin one aligns with the external magnetic filed and the other one opposes it.

¨  Difference between the resonant frequency of the spinning charged nuclei i.e proton and the signals of the reference molecule characterize the chemical shift.

¨  Magnetic shielding

¨  If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.

¨  But protons are surrounded by electrons that shield them from the external field.

¨  Circulating electrons create an induced magnetic field that opposes the external magnetic field.

¨  Magnetic field strength must be increased for a shielded proton to flip at the same frequency.

¨  Nuclear magnetic resonance chemical change is one of the most important properties usable for molecular structure determination.

¨   1H and 13C are the most widely used nuclei that are detected by NMR

¨  Beside the above, 15N (nitrogen 15), 19F (fluorine 19), are also used.

¨  Chemical shift is measured in (ppm)parts per million.

¨  Chemical shift δ = νsample - ν reference

¨                                                 ν _reference

¨  V sample = absoulte frequency of sample

¨  V reference = absoulte frequency reference

¨  Magnetic field B₀ would be same for both reference and sample

¨  numerator is usually expressed in hertz

¨   and the denominator in megahertz 

¨  Chemical Shift δ is expressed in ppm

¨  The detected frequencies (in Hz) for ¹H, ¹³C, and ²⁹Si nuclei are usually referenced against TMS (tetramethylsilane), TSP (Trimethylsilylpropanoic acid), or DSS (Sodium trimethylsilylpropanesulfonate).

¨  TMS, TSP and DSS have a chemical shift of zero if chosen as the reference.

¨  NMR signal observed at a frequency 300 Hz higher than the signal from TMS, where the TMS resonance frequency is 300 MHz, has a chemical shift of:

¨   

¨  δ =    300Hz        =   1 x 10^-6  = 1ppm

¨           300 x 10⁶

Instrumentation

¨  Sample holder: Sample holder is 8.5 cm long and 0.3 cm in diameter glass tube.

¨  Magnetic coils : Magnetic coil generates magnetic field whenever current flows through it.

¨  Permanent magnet : Permanent magnet provides a homogenous magnetic field at 60 – 100 MHZ.

¨  Sweep generator: It modifies the strength of the magnetic field which is already applied.

¨  Radiofrequency transmitter: It generates a powerful and short pulse of the radio waves.

¨  Radiofrequency: It supports detecting receiver radio frequencies.

¨  RF detector: It determines unabsorbed radio frequencies.

¨  Recorder: It records the NMR signals which are received by the RF detector.

¨  Readout system: Computer system records the data.

Applications

¨  NMR is used in the structural determination of organic and inorganic compounds.

¨  Microstructure determination of polymer chain.

¨  Determination of physical and chemical properties of atoms.

¨  In medicine it is used in MRI, Tumors, tissue perfusion studies and angiography.