How many lines in epr

In EPR spectrometers a phase-sensitive detector is used. This results in the absorption signal being presented as the first derivative. So the absorption maximum corresponds to the point where the spectrum passes through zero. This is the point that is used to determine the center of the signal. As mentioned earlier, an EPR spectrum is obtained by holding the frequency of radiation constant and varying the magnetic field. This is known as the field for resonance. As spectra can be obtained at a variety of frequencies, the field for resonance does not provide unique identification of compounds.

The proportionality factor, however, can yield more useful information. For a free electron, the proportionality factor is 2. For organic radicals, the value is typically quite close to that of a free electron with values ranging from 1. For transition metal compounds, large variations can occur due to spin-orbit coupling and zero-field splitting and results in values ranging from 1.

In addition to the applied magnetic field, unpaired electrons are also sensitive to their local environments. Frequently the nuclei of the atoms in a molecule or complex have a magnetic moment, which produces a local magnetic field at the electron.

The resulting interaction between the electron and the nuclei is called the hyperfine interaction. Hyperfine interactions can be used to provide a great deal of information about the sample including providing information about the number and identity of nuclei in a complex as well as their distance from the unpaired electron. This interaction expands the previous equation to:.

The energy difference between the two absorptions is equal to the hyperfine coupling constant. It is important to note that if a signal is split due to hyperfine interactions, the center of the signal which is used to determine the proportionality factor is the center of the splitting pattern. So for a doublet, the center would be half way between the two signals and for a triplet, the center would be the center of the middle line.

The number of lines which result from the coupling can be determined by the formula:. It is important to note that this formula only determines the number of lines in the spectrum, not their relative intensities. As a result, the EPR spectrum shows seven lines with relative intensities of EPR Spectrum of benzene radical anion. If an electron couples to several sets of nuclei, then the overall pattern is determined by first applying the coupling to the nearest nuclei, then splitting each of those lines by the coupling to the next nearest nuclei, and so on.

An example of this can be seen in the radical anion of pyrazine. EPR Spectrum of pyrazine radical anion. As NMR spectroscopy does not usually provide useful spectra for paramagnetic compounds, analysis of their EPR spectra can provide additional insight. Then what is the fingerprint of a molecule?

For organic radicals, the g value is very close to ge with values ranging from 1. For transition metal complexes, the g value varies a lot because of the spin-orbit coupling and zero-field splitting.

Usually it ranges from 1. For instance, the g value of Cu acac 2 is 2. To determine the g value, we use the center of the signal. The value of g factor is not only related to the electronic environment, but also related to anisotropy.

Another very important factor in EPR is hyperfine interactions. Additional information can be obtained from the so-called hyperfine interaction. The nuclei of the atoms in a molecule or complex usually have their own fine magnetic moments. Such magnetic moments occurrence can produce a local magnetic field intense enough to affect the electron. Such interaction between the electron and the nuclei produced local magnetic field is called the hyperfine interaction. Then the energy level of the electron can be expressed as:.

In which a is the hyperfine coupling constant, m I is the nuclear spin quantum number. Hyperfine interactions can be used to provide a wealth of information about the sample such as the number and identity of atoms in a molecule or compound, as well as their distance from the unpaired electron. Table 1. Bio transition metal nuclear spins and EPR hyperfine patterns [3]. The rules for determining which nuclei will interact are the same as for NMR. For isotopes which have even atomic and even mass numbers, the ground state nuclear spin quantum number, I , is zero, and these isotopes have no EPR or NMR spectra.

For isotopes with odd atomic numbers and even mass numbers, the values of I are integers. For example the spin of 2 H is 1. For isotopes with odd mass numbers, the values of I are fractions. Here are more examples from biological systems:. Table 2. Bio ligand atom nuclear spins and their EPR hyperfine patterns [3]. N is the number of equivalent nuclei and I is the spin. We can see 8 lines from the EPR spectrum. When coupling to a single nucleus, each line has the same intensity.

When coupling to more than one nucleus, the relative intensity of each line is determined by the number of interacting nuclei. The spectrum looks like this:. If an electron couples to several sets of nuclei, first we apply the coupling rule to the nearest nuclei, then we split each of those lines by the coupling them to the next nearest nuclei, and so on. The EPR spectra have very different line shapes and characteristics depending on many factors, such as the interactions in the spin Hamiltonian , physical phase of samples, dynamic properties of molecules.

To gain the information on structure and dynamics from experimental data, spectral simulations are heavily relied.