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1 Last Updated: Generation of the EPR ignal

2 MR and EPR are similar in a way that the amount of absorption energy required to for a transition between atomic or molecular states. pectroscopy generally measures the energy difference ( E) based on the relationship with the absorption of electromagnetic radiation: E = where h is Plank s constant and is the frequency of the radiation. E As the frequency when the frequency is swept or varied and absorption occurs this indicates the energy difference between states. The frequency range for MR signals are typically in the MHz region, however for EPR radiation occurs in the GHz range. Absorption For EPR these energy differences arise from the interaction of unpaired electrons in the sample with an external magnetic field ( ). When the moment of the electron ( ) aligns with and against the direction of it results in the states having the lowest and highest energy, respectively. This is known as the Zeeman Effect. Frequency

3 The energy of the electron spin state is calculated by: E = g B M s where g is the g factor, B is the Bohr magneton and M is the electron spin state designates at ±½ when it is parallel or antiparallel. Thus the energy difference between the two states can be calculated as E = = g B When the two states are not in a magnetic field, they both have the same energy. As the magnetic field increases, the energies diverge linearly. This provides two methods to obtain a spectrum 1. Vary the magnetic field strength while keeping the electromagnetic radiation frequency constant 2. Vary the electromagnetic radiation frequency while the magnetic field strength is kept constant. Method 1 is commonly used due to limitations to the microwave electronics. A signal arises when the magnetic field is tuned to match the energy state difference with the energy of the applied radiation.

4 The field at which a signal appears does not serve well as a finger print for identifying compounds since spectra can be acquired at various frequencies. Typically the g factor is used as a measure for identification since the value is related to the magnetic field and independent of the microwave frequency. g 0 ote the g factor is inversely proportional to the magnetic field. The g factor can provide some useful information but it does not provide much information about the molecular structure. ince nuclei also have a magnetic moment/local magnetic field, an magnetic interaction between the electron and the nuclei can occur. This interaction is called a hyperfine interaction. This can provide a lot of information such as the identity and number of atoms that make up the molecule as well as their distances from the unpaired electrons. The magnetic moment of the nucleus produces a magnetic field (B I ) at the electron which opposes or adds to. When B I adds to B 0, a lower magnetic field is required to observe a signal. The opposite is true when B I opposes.

5 EPR pectrum When a nucleus is present, such as a spin ½ nucleus, the EPR absorption signal splits into two signals which are each B1 away from the original peak. If a second nucleus is present then the peaks will split further resulting in a total of four peaks. Generally for spin ½ nuclei, 2 EPR signals are observed. B I B I 14 e 1 H eepr.cm.utexas.edu As an example, a spectrum of pyrazine radical anion shows splitting by the coupling with two equivalent 14 nuclei (I = 1) which gives a quintet which is then further split into quintets due to the coupling to four equivalent hydrogens. The integrated signal intensity is proportional to the concentration. The signal, however, is greatly affected by the microwave power. When the power is too high, the signal is saturated causing the intensity to decrease and the lines to broaden. The signal is unsaturated when the signal intensity decreases by the square root of the microwave power.