In 1921, The Stern–Gerlach experiment demonstrated that the spatial orientation of angular momentum is quantized. It demonstrated that atomic-scale systems have intrinsically quantum states and properties. They observed that a beam of silver atoms splits into two lines when it is subjected to a magnetic field. While the line splitting in optical spectra, first found by Zeeman in 1896, could be explained by the angular momentum of the electrons, the s-electron of silver could not be subject to such a momentum, not to mention that an azimuthal quantum number l=12could not be explained by classical physics. At that time, quantum mechanics was still an emerging field in physics and it took another three years until this abnormal Zeeman effect was correctly interpreted by the joint research of Uhlenbeck, a classical physicist, and Goudsmit, a fellow of Paul Ehrenfest. They postulated a so-called ‘spin’, a quantized angular momentum, as an intrinsic property of the electron. This research is the foundation of electron paramagnetic resonance spectroscopy (EPR) which is based on the transitions between quantized electron states of the resulting magnetic moment.
In 1941, Yevgeny Zavoisky, a Soviet scientist, tried to detect nuclear magnetic resonance in organic compounds. Although he possessed a sensitive enough detection system to a obtain resonance spectrum , when measuring the spatial homogeneity of the magnetic field, the accuracy of such measurements were not sufficient to produce a complete spectrum. Instead, starting from 1943, Zavoisky focused on electron paramagnetic resonance (EPR), which is much less demanding for the spatial homogeneity of magnetic field. Nonetheless, it requires much more sensitive detection electronics, but Zavoisky had proficient experience in this area. In particular, he had replaced the thermal detection discovered by C. J. Gorter by a much more sensitive electronic technique of grid current. And in 1944, EPR signals were—for the first time—detected in salts, including manganese sulfate, copper sulfate, and hydrous copper chloride. The results proved to be groundbreaking and were initially not accepted by peer Soviet scientists .His results were determined to be verifiable when Zavoisky visited Moscow, assembled an EPR spectrometer from scratch and reproduced his results front of peers.
The fundamental principle of EPR, not surprisingly, rests on the electron. Every electron has a magnetic moment and spin quantum number with magnetic components. The magnetic moment of an electron is a quantity that determines the rotational force it will experience in an external magnetic field. In further detail, the term magnetic moment refers to a system's magnetic dipole moment. The spin quantum number of an electron is s = 12 while the magnetic moment can either be ms=12. In the absence of an external magnetic field the two electron spin states are degenerate. When an atom or molecule and an unpaired electron is subject to a magnetic field, the spin of the unpaired electron can become aligned either in the same direction or in the opposite direction of the incident field. These two possible electron alignments have different energies (i.e. are no longer degenerate) and are linearly proportional to the applied magnetic field intensity. This is called the Zeeman effect. In the presence of an external magnetic fieldB0, the electron's magnetic moment aligns itself either parallel (ms=12) or antiparallel to the field(ms=-12). Each alignment has an energy determined by the Zeeman effect: The energy ends up equalling E=μBB0gemswhere ge is the electron's “g-factor”: “a constant that relates the observed magnetic moment μ of a particle to its angular momentum, equalling 2.0023 for free electrons.” μBis the Bohr magneton: the magnetic moment of an electron caused by either its spin angular momentum.
Since the difference between the two magnetic spin quantum numbers equals 12-(-12)=1, the separation between the lower and the upper states of unpaired free electrons is E=B0geB. This equation implies that the splitting of the energy levels is proportional to the external magnetic field's strength.
An electron can move between the two energy levels by absorbing or emitting a photon of energy hv to obey resonance, E=hv. This results in the “fundamental equation of EPR spectroscopy”:hv=geBB0. Theoretically, this equation permits a innumerable integration of photon frequency and magnetic field values, but the practically all of EPR measurements are made with microwave frequencies in the 9,000–10,000 MHz region, with fields correlating to about 0.35 Tesla or 3500 Gauss. In addition, EPR spectra can be generated by either deviating the magnetic field while keeping the frequency incident on the sample constant or vice versa. As custom, the frequency is kept fixed. Continuing, large samples of of paramagnetic centers, such as free radicals, are exposed to photons at a constant f(9,000-10,000). By increasing the aforementioned external magnetic field, the gap between the upper and lower energy states is widened until it is equivalent to that of the incident microwaves. At this point the unpaired electrons can move between their two quantum spin states. Due to the Maxwell-Boltzmann distribution, there are typically more electrons in the lower energy state.As electrons jump to the higher energy state, there is often a net absorption of energy. It is this absorption that is monitored and converted into a EPR spectrum.
EPR spectra is usually directly measured as the first derivative of the absorption. This is accomplished by using field modulation. An negligible oscillating magnetic field is implemented on the external magnetic field at a frequency of 100 kHz. By detecting the amplitude, the first derivative can be determined. By using phase sensitive detection, only signals with an equal modulation are detected. This results in the lessening of the probability of noise affecting the spectrum.
In actual systems, electrons are infrequently solitary, but instead are associated with one or many nuclei. There are a couple of influential consequences of this: An unpaired electron’s angular momentum can fluctuate, causing a change in the value of its g-factor, differing from the initial ge. This is especially cardinal for chemical systems with transition metal ions. In addition, the magnetic moment of a nucleus with nuclear spin will affect any unpaired free electrons associated with such atom. This leads to the phenomenon of “hyperfine coupling”. analogous to J-coupling in NMR, hyperfine coupling is the splitting of the EPR resonance spectrum into doublets, triplets and multiplets.
EPR spectroscopy is used in various applications in science for the detection and identification of free radicals and paramagnetic centers. Relevant applications include F-centers: a type of crystallographic defect in which an anionic vacancy in a crystal is filled by one or more unpaired electrons. EPR is a sensitive method for studying both radicals formed in chemical reactions and the reactions themselves. For example, when ice is decomposed by high-energy radiation exposure, radicals such as OH, H, and HO2 are produced. These radicals can be further identified and analysed by EPR. Both organic and chemical radicals can be detected by EPR in electrochemical systems and in materials exposed to ultraviolet light. In most cases, the reactions that produce the radicals and the succeeding reactions of the radicals are of interest to spectroscopers, while in other cases EPR is used to provide information on a radical's composition, geometry and the orbital of the unpaired electron. EPR spectroscopy is also used in geology and archaeology as a carbon dating tool. It can be applied to a wide range of materials such as carbonates, sulfates, phosphates, and silicates.
EPR can also be used in medical and biological applications. Although free radicals are reactive, and therefore rarely occur in high concentrations in nature, special reagents have been developed to “spin-label” molecules. These reagents are particularly useful in organic systems. Such designed non reactive radical molecules can attach to specific sites in a cell, and EPR spectra can then identify the environment of these spin labels. Spin-labeled fatty acids have been extensively used to study “dynamic organisation of lipids in cell membranes, lipid-protein interactions and temperature of transition of gel to liquid crystalline phases.”
F-center in NaCl