BASIC: For mapping It is recognized as molecular one of the mostNuclear Magnetic structures & powerful Resonance learning how techniques forspectroscopy is a molecules chemical NDT. function & relate analysis. to each other.
The study of absorption of radiofrequencyradiation by nuclei in amagnetic field is called Nuclear Magnetic Resonance.
UTILITIES & IMPORTANCE The importance of this The utility of this is for structural technique is reflected in the characterization arises because efforts that have been made to different atoms in a moleculeextent its applicability to smaller experience slightly different & smaller sample sizes. magnetic fields.
PRINCIPLES The angular Nuclear momentum associated with the spin of theparticle would be Spin an integral or a half-integralmultiple of h/2pi. Elementary particles such as electrons or a nucleus are known to behave as if they rotate about an axis & thus have the property of spin.
Nuclear Energy Levels Since a nucleus possesses a charge, its spin gives rise to a magnetic field that is analogous to the field produced when an electric current is passed through a coil of wire. When a spinning nucleus is placed in a strong uniform magnetic field(H), the field exerts a torque upon the nuclear magnet.
Resonance Conditions When an alternating radiofrequency field, superimposed over the stationaryIn general, the energy difference magnetic field, rotates at exactly the frequency of an energy level, the nuclei b/w states is given by: will be provided enough energy to undergo a transition from a lower energy level to a higher energy level.
RELAXATION PROCESSWhen nuclei in the higher energy However, the emission is state return to the lower state, insignificant because theemission of radiation takes place. probability of re-emission of There are two major R.P- photons varies with the cube of the frequency. At radio- 1.Spin-lattice(longitudinal) frequencies, re-emission is 2.Spin-Spin(transverse) negligible.
It is expressed The Chemical as: shift The difference b/w the field necessary for resonance in the sample & in some arbitrarily chosen reference compound is called the ‘Chemical Shift’.
TYPES Continuous-Wave NMR SpectroscopyA CW-NMR spectrometer consists of a control console, magnet, & two orthogonal coils of wire that serve as antennas for radiofrequency radiation. One coil is attached to an rf generator & serves as a transmitter.The other coil is the rf pick-up coil & is attached to the detection electronics.
Fourier Transform NMR SpectroscopyIt use a pulse of rf radiaiton which causes nuclei in a magnetic field to flip into the higher energy alignment. Due to Heisenberg uncertainty principle, thefrequency width of the rf pulse is wide enough to simultaneously excite nuclei in environment. All the nuclei will re-emit rf radiation at their respective resonance frequencies, creating an interference pattern in the resulting rf emission vs. time, known as FID(Free Induction Decay)
The key parts of the system are:-A magnet which produces a magneitc field in the range 10,000-25,000 gauss, Radio-frequency transmitting system, The signal amplifier & detector, A display device, A non-magnetic sample holder Computer workstation
ELECTRON SPIN RESONANCE SPECTROMETERS It involves detecting the detection of a physical It is the study of magnetic phenomenon of dipoles of electronic absorption of origin by applying, usuallyelectromagnetic radiation fixed microwave in the microwave region frequencies to a sample by paramagnetic species residing in a varying that are subjected to an magnetic field. external magnetic field.
INSTRUMENTATION FOR E.S A source of radiation with which to excite the sample. An electron energy analyzer,It contains the following An electron detector components: A read-out system, & A high vacuum system
INSTRUMENTATION FOR X-RAY SPECTROMETRY THE IMPORTANT PARTS ARE:- X-ray generating equipment, Collimator, Monochromators, and Detectors