2ESR Spectroscopy Electron Spin Resonance Spectroscopy Also called EPR SpectroscopyElectron Paramagnetic Resonance SpectroscopyNon-destructive techniqueApplicationsOxidation and reduction processesReaction kineticsExamining the active sites of metalloproteinsElectron spin resonance (ESR) spectroscopy, also referred to as electron paramagnetic resonance (EPR) spectroscopy, is a versatile, nondestructive analytical technique which can be used for a variety of applications including: oxidation and reduction processes, biradicals and triplet state molecules, reaction kinetics, as well as numerous additional applications in biology, medicine and physics.
3What compounds can you analyze? Applicable for species with one or more unpaired electronsFree radicalsTransition metal compoundsUseful for unstable paramagnetic compounds generated in situElectrochemical oxidation or reductionHowever, this technique can only be applied to samples having one or more unpaired electrons.
4Energy TransitionsESR measures the transition between the electron spin energy levelsTransition induced by the appropriate frequency radiationRequired frequency of radiation dependent upon strength of magnetic fieldCommon field strength 0.34 and 1.24 T9.5 and 35 GHzMicrowave regionAs we know, spectroscopy is the measurement and interpretation of the energy difference between atomic or molecular states. The absorption of energy causes a transition of an electron from a lower energy state to a higher energy state. In EPR spectroscopy the radiation used is in the gigahertz range. Unlike most traditional spectroscopy techniques, in EPR spectroscopy the frequency of the radiation is held constant while the magnetic field is varied in order to obtain an absorption spectrum.
5How does the spectrometer work? Shown is a block diagram for a typical EPR spectrometer. The radiation source usually used is called a klystron. Klystrons are vacuum tubes known to be stable high power microwave sources which have low-noise characteristics and thus give high sensitivity. A majority of EPR spectrometers operate at approximately 9.5 GHz, which corresponds to about 32 mm. The radiation may be incident on the sample continuously (i.e., continuous wave, abbreviated cw) or pulsed. The sample is placed in a resonant cavity which admits microwaves through an iris. The cavity is located in the middle of an electromagnet and helps to amplify the weak signals from the sample. Numerous types of solid-state diodes are sensitive to microwave energy and absorption lines then be detected when the separation of the energy levels is equal or very close to the frequency of the incident microwave photons. In practice, most of the external components, such as the source and detector, are contained within a microwave bridge control. Additionally, other components, such as an attenuator, field modulator, and amplifier, are also included to enhance the performance of the instrument.
6What causes the energy levels? When an electron is placed within an applied magnetic field, Bo, the two possible spin states of the electron have different energies. This energy difference is a result of the Zeeman effect. The lower energy state occurs when the magnetic moment of the electron is aligned with the magnetic field and a higher energy state where m is aligned against the magnetic field. The two states are labeled by the projection of the electron spin, MS, on the direction of the magnetic field, where MS = -1/2 is the parallel state, and MS = +1/2 is the antiparallel state.Resulting energy levels of an electron in a magnetic fieldEbsworth, E. A. V.; Rankin, David W. H.; Cradock, Stephen Structural Methods in Inorganic Chemistry; CRC Press: Boca Raton, 1987.
7SpectraLike most spectroscopic techniques, when the radiation is absorbed, a spectrum is produced similar to the one on the left. 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.When phase-sensitive detection is used, the signal is the first derivative of the absorption intensity
8Describing the energy levels Based upon the spin of an electron and its associated magnetic momentFor a molecule with one unpaired electronIn the presence of a magnetic field, the two electron spin energy levels are:E = gmBB0MSg = proportionality factor mB = Bohr magnetonMS = electron spin B0 = Magnetic fieldquantum number(+½ or -½)So for a molecule with one unpaired electron in a magnetic field, the energy states of the electron can be defined as:E = gmBBoMS = ±1/2gmBBowhere g is the proportionality factor (or g-factor), mB is the Bohr magneton, Bo is the magnetic field, and MS is the electron spin quantum number. From this relationship, there are two important factors to note: the two spin states have the same energy when there is no applied magnetic field and the energy difference between the two spin states increases linearly with increasing magnetic field strength.
9Proportionality Factor Measured from the centerof the signalFor a free electronFor organic radicalsTypically close to free-electron valueFor transition metal compoundsLarge variations due to spin-orbit coupling and zero-field splittingAs mentioned earlier, an EPR spectrum is obtained by holding the frequency of radiation constant and varying the magnetic field. Absorption occurs when the magnetic field “tunes” the two spin states so that their energy difference is equal to the radiation. 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 For organic radicals, the value is typically quite close to that of a free electron with values ranging from For transition metal compounds, large variations can occur due to spin-orbit coupling and zero-field splitting and results in values ranging from
10Proportionality Factor MoO(SCN)52-1.935VO(acac)21.968e-2.0023CH32.0026C14H10 (anthracene) cation2.0028C14H10 (anthracene) anion2.0029Cu(acac)22.13acac = acetylacetonateAtherton, N. M. Principles of Electron Spin Resonance; Ellis Horwood: Chichester,
11Hyperfine Interactions EPR signal is ‘split’ by neighboring nucleiCalled hyperfine interactionsCan be used to provide informationNumber and identity of nucleiDistance from unpaired electronInteractions with neighboring nucleiE = gmBB0MS + aMsmIa = hyperfine coupling constantmI = nuclear spin quantum numberMeasured as the distance between the centers of two signalsIn 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:E = gmBBoMS + aMSmIwhere a is the hyperfine coupling constant and mI is the nuclear spin quantum number for the neighboring nucleus.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.
12Which nuclei will interact? Selection rules same as for NMREvery isotope of every element has a ground state nuclear spin quantum number, Ihas value of n/2, n is an integerIsotopes with even atomic number and even mass number have I = 0, and have no EPR spectra12C, 28Si, 56Fe, …Isotopes with odd atomic number and even mass number have n even2H, 10B, 14N, …Isotopes with odd mass number have n odd1H, 13C, 19F, 55Mn, …The rules for determining which nuclei will interact are the same as for NMR. For every isotope of every element, there is a ground state nuclear spin quantum number, I, which has a value of n/2, where n is an integer. For isotopes which the atomic and mass numbers are both even, I=0, and these isotopes have no EPR (or NMR) spectra. For isotopes with odd atomic numbers but even mass numbers, the value of n is even leading to values of I which are integers, for example the spin of 14N is 1. Finally for isotopes with odd mass numbers, n is odd, leading to fractional values of I, for example the spin of 1H is ½ and the spin of 51V is 7/2.
13Hyperfine Interactions So a single nucleus with a spin ½ will split each energy level into two, as shown above, and then two transitions (or absorptions) can be observed. The energy difference between the two absorptions is equal to the hyperfine coupling constant.Interaction with a single nucleus of spin ½Ebsworth, E. A. V.; Rankin, David W. H.; Cradock, Stephen Structural Methods in Inorganic Chemistry; CRC Press: Boca Raton, 1987.
14Hyperfine Interactions Coupling patterns same as in NMRMore common to see coupling to nuclei with spins greater than ½The number of lines:2NI + 1N = number of equivalent nucleiI = spinOnly determines the number of lines--not the intensitiesThe coupling patterns that are observed in EPR spectra are determined by the same rules that apply to NMR spectra. However, in EPR spectra it is more common to see coupling to nuclei with spins greater than ½. The number of lines which result from the coupling can be determined by the formula:2NI + 1where N is the number of equivalent nuclei and I is the spin. It is important to note that this formula only determines the number of lines in the spectrum, not their relative intensities.
15Hyperfine Interactions Relative intensities determined by the number of interacting nucleiIf only one nucleus interactingAll lines have equal intensityIf multiple nuclei interactingDistributions derived based upon spinFor spin ½ (most common), intensities follow binomial distributionThe relative intensities of the lines is determined by the number of interacting nuclei. Coupling to a single nucleus gives lines each of equal intensity.
16Relative Intensities for I = ½ 11 : 121 : 2 : 131 : 3 : 3 : 141 : 4 : 6 : 4 : 151 : 5 : 10 : 10 : 5 : 161 : 6 : 15 : 20 : 15 : 6 : 1Relative intensities of splitting patterns observed due to hyperfine coupling with a nucleus with I = ½. The splitting patterns are named similar to those in NMR:2 lines = doublet3 lines = triplet4 lines = quartet5 lines = quintet6 lines = sextet7 lines = septet
17Relative Intensities for I = ½ Computer simulations of EPR spectra for interactions with N equivalent nuclei of spin 1/2.
19Relative Intensities for I = 1 Computer simulations of EPR spectra for interactions with N equivalent nuclei of spin 1.
20Hyperfine Interactions Example:VO(acac)2Interaction with vanadium nucleusFor vanadium, I = 7/2So,2NI + 1 = 2(1)(7/2) + 1 = 8You would expect to see 8 lines of equal intensity
21Hyperfine Interactions EPR spectrum of vanadyl acetylacetonate
22Hyperfine Interactions Example:Radical anion of benzene [C6H6]-Electron is delocalized over all six carbon atomsExhibits coupling to six equivalent hydrogen atomsSo,2NI + 1 = 2(6)(1/2) + 1 = 7So spectrum should be seven lines with relative intensities 1:6:15:20:15:6:1An example is shown by the EPR spectrum of the radical anion of benzene, [C6H6•]-, in which the electron is delocalized over all six carbon atoms and therefore exhibits coupling to six equivalent hydrogen atoms. As a result, the EPR spectrum shows seven lines with relative intensities of 1:6:15:20:15:6:1.
23Hyperfine Interactions EPR spectrum of benzene radical anion
24Hyperfine Interactions Coupling to several sets of nucleiFirst couple to the nearest set of nucleiLargest a valueSplit each of those lines by the coupling to the next closest nucleiNext largest a valueContinue 2-3 bonds away from location of unpaired electronIf 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.
25Hyperfine Interactions Example:Pyrazine anionElectron delocalized over ringExhibits coupling to two equivalent N (I = 1)2NI + 1 = 2(2)(1) + 1 = 5Then couples to four equivalent H (I = ½)2NI + 1 = 2(4)(1/2) + 1 = 5So spectrum should be a quintet with intensities 1:2:3:2:1 and each of those lines should be split into quintets with intensities 1:4:6:4:1An example of this can be seen in the radical anion of pyrazine. Where coupling to two equivalent 14N (I = 1) nuclei gives a quintet with the relative intensities of 1:2:3:2:1 which are further split into quintets with relative intensities of 1:4:6:4:1 by coupling to four equivalent hydrogens.
26Hyperfine Interactions EPR spectrum of pyrazine radical anion
27Conclusions Analysis of paramagnetic compounds Compliment to NMR Examination of proportionality factorsIndicate location of unpaired electronOn transition metal or adjacent ligandExamination of hyperfine interactionsProvides information on number and type of nuclei coupled to the electronsIndicates the extent to which the unpaired electrons are delocalizedAs NMR spectroscopy does not usually provide useful spectra for paramagnetic compounds, analysis of their EPR spectra can provide additional insight. Analysis of the coupling patterns can provide information about the number and type of nuclei coupled to the electrons. The magnitude of a can indicate the extent to which the unpaired electrons are delocalized and g values can show whether unpaired electrons are based on transition metal atoms or on the adjacent ligands.