ELECTRON SPIN RESONANCE MOHAMMAD IMRAN AZIZ Assistant Professor PHYSICS DEPARTMENT SHIBLI NATIONAL COLLEGE, AZAMGARH (India). ELECTRON SPIN RESONANCE SPECTROSCOPY 1. INTRODUCTION •The spin of an electron and its associated magnetic moment are the basis of ESR spectroscopy •The technique can only be applied to species having one or more electrons [e.g., free radicals, biradicals and other triplet states, and many transition metal compounds] •The technique is particularly valuable for the study of unstable paramagnetic species generated in situ by electrochemical oxidation/reduction. 2. BASIC PRINCIPLE In the presence of a magnetic field B, a molecule or ion having one e- has two electron-spin energy levels given by, Ems = geBBms, where ms = ½, B is the Bohr magneton and g is a proportionality factor, equal to 2.00232 for a free electron E = E - E = geBB [for radicals g ~ ge while for transition metal compounds g = 2 to 4]. When the resonance condition [h = geBB] is satisfied, strong absorption of the radiation (microwave frequency) occurs. •Just as in NMR, the spin magnetic moment interacts with the local magnetic field and so, the resonance condition is given by the modified equation, h = geBBlocal = geB(1-)B = gBB where g = (1-) ge = g-factor of the radical or complex. •Most commonly, fields of 0.34 and 1.24T (1T = 104 Hz) are used, with corresponding frequencies of 9.5 and 35 GHz, which are in the microwave region. Numerical illustration-1 The center of an ESR spectrum of methyl radical occurred at 329.4 mT in a spectrometer operating at 9.233 GHz. What is the g-value? [h = 6.626093 X 10-34 Joules/sec; Bohr Magneton (B) = 9.274026 X 10-24 Joules/Tesla; Thus, h/B = 71.4448 mT/GHz] Use the equation, h = gB g = h/B = (71.44X 9.233)/329.4 = 2.0024 Numerical illustration-2 Calculate the magnetic field at which a methyl radical (g = 2.0024) comes into resonance in a spectrometer operating at 9.468 GHz. [h = 6.626093 X 10-34 Joules/sec; Bohr Magneton (B) = 9.274026 X 10-24 Joules/Tesla; Thus, h/B = 71.4448 mT/GHz] Use the equation, h = gB = (h/B)(/g) = (71.44X 9.468)/2.0024 = 337.8 mT 3. RELAXATION PROCESSES AND SIGNAL INTENSITIES Relaxation processes: (a) Spin-lattice relaxation: The absorbed microwave energy is transferred from the spin system to its surroundings and (b) Spin-spin relaxation: The absorbed microwave energy is transferred from the spin system to the adjacent spin. If the relaxation time is long, the population of the upper state will increase during observation and the signal intensity will saturate or decrease in intensity. If the relaxation time is short, then by the uncertainty principle, the resonance lines must be wide. This is the case with transition metal complexes, where the spectral lines are observed at liq. N2 or liq. He temperatures only. 4. LAY OUT AND PRESENTATION OF THE SPECTRUM A typical ESR spectrometer consists of the following layout: Radiation Source: This consists of a Klystron source which generates microwave frequency Cavity or Sample Chamber: The cavity size is so chosen that a standing wave is set up and the location of the cavity coincides with a region of uniform magnetic field. Detection and Recording System: The detection system utilizes a small-amplitude magnetic field modulation and a phase-sensitive detector as a means of reducing noise. Presentation of Spectrum: As in NMR, the ESR spectrum can be represented by plotting intensity against the strength of the applied field, but ESR spectra are generally presented as derivative curves (i.e., the slope of the absorption is plotted against the magnetic field strength). Much greater sensitivity can be achieved by this detection method if the line shape is broad. 5. THE g VALUE The g value is the proportionality constant in the basic equation, h = g B. [For in kG, and in MHz, g = 0.71446 X / where is the fixed frequency of the microwave radiation and is the magnitude of the static field at resonance.] •If the electron spin is the only source of magnetism, then ge = 2.0023. Fortunately, g can be measured with great accuracy (usually > 0.001) and hence small deviations from 2.0023 will help to characterize the species. •The g value is the unique property of the molecule as a whole and is independent of any electron-nuclear hyperfine interactions that may be present. •When the unpaired electron is in an orbital that is far removed from other levels, g will be close to the spin-only value of 2.0023.