The primary advantage of EPR over nuclear magnetic resonance (NMR) in the study of defects is that of sensitivity. This higher sensitivity is mainly a result of the larger quantum of energy absorbed when an electron rather than a nucleus is flipped in a given magnetic field.

Electron Spin Resonance (ESR), often called Electron Paramagnetic Resonance (EPR), is similar to Nuclear Magnetic Resonance (NMR), the fundamental difference being that ESR is concerned with the magnetically induced splitting of electronic spin states, while NMR describes the splitting of nuclear spin states.

The EPR spectrum of a free electron, there will be only one line (one peak) observed. But for the EPR spetrum of hydrogen, there will be two lines (2 peaks) observed due to the fact that there is interaction between the nucleus and the unpaired electron.

The electromagnetic radiation used in NMR typically is confined to the radio frequency range between 300 and 1000 MHz, whereas EPR is typically performed using microwaves in the 3 – 400 GHz range. In EPR, the frequency is typically held constant, while the magnetic field strength is varied.

The g-value of a free electron is 2.0023, and the g-values of most free radicals are very close to this value, since the unpaired electron has very little orbital contribution to the magnetic moment. (carbon-based radicals, spin-orbit coupling very small).

In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard in a magnetic field. The variations of nuclear magnetic resonance frequencies of the same kind of nucleus, due to variations in the electron distribution, is called the chemical shift.

δ

δ = (fsamp − fref ) / fref The chemical shift (δ) is therefore a small number, expressed in units of parts per million (ppm).

Chemical shift is associated with the Larmor frequency of a nuclear spin to its chemical environment. Tetramethylsilan[TMS;(CH3)4Si] is generally used for standard to determine chemical shift of compounds: δTMS=0ppm.

Chemical shifts are reported on the horizontal axis of the spectrum. Most protons absorb between 0-12 ppm. All these values are measured relative to the position of a reference peak at 0 ppm on the δ scale due to tetramethylsilane (TMS). TMS is a volatile inert compound that gives a single peak at 0 ppm.

It is often convienient to describe the relative positions of the resonances in an NMR spectrum. For example, a peak at a chemical shift, δ, of 10 ppm is said to be downfield or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is upfield or shielded with respect to the peak at 10 ppm.

Tetramethylsilane became the established internal reference compound for 1H NMR because it has a strong, sharp resonance line from its 12 protons, with a chemical shift at low resonance frequency relative to almost all other 1H resonances. Thus, addition of TMS usually does not interfere with other resonances.

Transcranial magnetic stimulation (TMS) is a noninvasive procedure that uses magnetic fields to stimulate nerve cells in the brain to improve symptoms of depression. TMS is typically used when other depression treatments haven’t been effective.

The (n+1) Rule, an empirical rule used to predict the multiplicity and, in conjunction with Pascal’s triangle, splitting pattern of peaks in 1H and 13C NMR spectra, states that if a given nucleus is coupled (see spin coupling) to n number of nuclei that are equivalent (see equivalent ligands), the multiplicity of the …

Uses in NMR spectroscopy Because of its high volatility, TMS can easily be evaporated, which is convenient for recovery of samples analyzed by NMR spectroscopy. Because all twelve hydrogen atoms in a tetramethylsilane molecule are equivalent, its 1H NMR spectrum consists of a singlet.

The hydrogen nuclei in TMS are highly shielded because silicon has a low electronegativity. It gives one strong sharp peak because it is caused by the combined effect of 12 equivalent hydrogen atoms. (They are joined on exactly the same things in exactly the same way.)

PMR

The NMR phenomenon relies on the interaction of the nuclei of certain atomic isotopes with a static magnetic field. This magnetic field makes the possible spin-states of the nucleus differ in energy, and using NMR techniques the spins can be made to create observable transitions between the spin states.

The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. If an external magnetic field is applied, an energy transfer is possible between the base energy to a higher energy level (generally a single energy gap).

ultraviolet and visiblelight radiowaves infrared radiation What is measured in this spectroscopic method? the vibrational frequency of a bond the frequency that causes a nucleus to flip its spin the absorption.

Proton nuclear magnetic resonance (proton NMR, hydrogen-1 NMR, or 1H NMR) is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules.

The Pascal’s triangle is a graphical device used to predict the ratio of heights of lines in a split NMR peak.

Consequently, this isotope is highly responsive to NMR measurements. Furthermore, 19F comprises 100% of naturally occurring fluorine. The only other highly sensitive spin 1⁄2 NMR-active nuclei that are monoisotopic (or nearly so) are 1H and 31P. The 19F NMR chemical shifts span a range of ca.

Phosphorus is commonly found in organic compounds and coordination complexes (as phosphines), making it useful to measure 31P NMR spectra routinely. The only other highly sensitive NMR-active nuclei spin ½ that are monoisotopic (or nearly so) are 1H and 19F.

19F is one of the most important nuclei for NMR spectroscopy  19F has a nuclear spin of 1/2 and a high magnetogyric ratio, which means that this isotope is highly responsive to NMR measurements. Furthermore, 19F comprises 100% of naturally-occurring fluorine.

C NMR spectroscopy is much less sensitive to carbon than 1H NMR is to hydrogen since the major isotope of carbon, the 12C isotope, has a spin quantum number of zero and so is not magnetically active and therefore not detectable by NMR.

Because different shielding from the electron density of different groups in the molecule, the resonance appeared in different frequencies, hence a spectrum is recorded. Carbon NMR is very interesting for chemists. Unfortunately, 12C is not paramagnetic.

Any nucleus with an odd number of protons and/or neutrons is NMR active. Next to hydrogen, the most commonly studied nucleus is C−13. In 1H-NMR we observe signals arising from hydrogen nuclei and we infer the presence of the carbon atoms to which they are attached. In 13C-NMR we observe carbon atoms directly.

Subatomic particles (electrons, protons and neutrons) can be imagined as spinning on their axes. In many atoms (such as 12C) these spins are paired against each other, such that the nucleus of the atom has no overall spin. However, in some atoms (such as 1H and 13C) the nucleus does possess an overall spin.