CH407/CH507 INTERPRETATIVE SPECTROSCOPY

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1 Dr.D. SPICER 3. EECTRON SPIN RESONANCE (ESR) SPECTROSCOPY. 3.1 Background Electron spin resonance (ESR), also known as Electron Paramagnetic Resonance (EPR), has been used for about the last 50 years to study species which contain unpaired electrons (paramagnetic). Such species can include organic and main group radicals and transition metal compounds with unpaired electrons. ESR can be thought of as broadly complementary to NR spectroscopy in that ESR deals with paramagnetic species, while NR deals primarily with diamagnetic species. In ESR it is the measurement of electron spin interacting with the external magnetic field, while in NR it is the interaction of the nuclear spin with the magnetic field which is key. et us consider what happens when an electron is placed in a magnetic field. In the absence of a magnetic field the two spin states (m s = ±½) are degenerate. When an external field is applied, the degeneracy of the spin states (m s ) is lifted, and thus two electron spin energy levels occur, with energies given by: E = g B B o m s Where g is called the g-factor (= for a free electron), B is the Bohr magneton, B o is the magnetic field strength and m s is the electron spin quantum number. This situation can be described by the simple graph (Figure 3.1, below). E m s = 1/2 A single transition, as shown by the vertical arrow, can occur from the m s = - ½ spin state to the m s = ½ spin state if the system is irradiated with electromagnetic radiation of appropriate energy. m s = -1/2 Figure 3.1 The splitting of electron spin states in a magnetic Field. B o 3.1 The EPR Experiment The vast majority of EPR instruments are based on the general design shown in the schematic diagram on the right. icrowaves are generated by the klystron tube and the power level adjusted with the attenuator. The circulator directs the microwaves to the cavity where the sample is mounted. At resonance, a portion of the microwaves are absorbed by the sample. The microwaves are reflected back from the cavity and detected by a diode detector and any Figure 3.2 Schematic Diagram of an ESR Spectrometer.

2 Dr.D. SPICER excess microwaves reflected from the diode are absorbed by the load. The diode is mounted to lie in the direction of the electric vector of the microwaves and generates a direct current proportional to the microwave power reflected from the cavity. In principle the absorption of microwaves by the sample could be detected by the decrease in current. In practice, such a dc measurement is too noisy. The solution is to introduce a small amplitude field modulation. A small oscillating magnetic field is superimposed on the fixed field by means of small coils built into the cavity walls. At the resonance point the result is to sweep the field back and forth across the resonance and the detected diode current is now an ac current. Amplification using a frequency sensitive amplifier eliminates much of the noise. The detected ac signal is the change in sample absorption and so what is observed is the first derivative of the absorption curve. ost commonly, EPR spectra are run at one of two fixed frequencies with an associated field strength: X-Band 0.33T / 9GHz Q-Band 1.4 T / 32GHZ Figure 3.3 The Derivative Spectrum 3.2 Isotropic EPR Spectra. An isotropic EPR spectrum, ie that of a paramagnetic molecule tumbling freely in solution, can contain several types of useful chemical information: i) g-value (parallels chemical shift in NR). ii) The hyperfine coupling pattern provides information on the number and types of magnetic nuclei with which the unpaired electron interacts iii) The spacing of the lines and the centre of gravity of the spectrum yield the hyperfine coupling constants (A) and the g-values, which are related to the way in which the electron spin density is distributed in the molecule. iv) The integrated spectral intensity is proportional to the concentration of radicals in the sample v) The spectral linewidths are related to the rate of rotation and the rate of fluxional processes Hyperfine Coupling As observed in NR spectroscopy, coupling can be observed to nuclei with I 0. The energies of the resulting spin states are given by the equation: E = g B B o m s + Am s m I Where A is the hyperfine coupling constant (broadly equivalent to J in NR) and m I is the nuclear spin quantum number. In the same way as in NR spectroscopy, coupling to n nuclei with nuclear spin I leads to 2nI+1 lines in the spectrum. Consider the effect of the coupling of a nucleus of spin I = 3/2 to an electron in a magnetic field. The energy levels are given by substituting values of s ( = ± ½ ) and m I (= -3/2, -1/2, 1/2, 3/2) into the above equation. The energy levels obtained are shown in the figure below:

3 Dr.D. SPICER E m I +3/2 +1/2-1/2-3/2 m s = +1/2-3/2-1/2 +1/2 +3/2 B m s = -1/2 Figure 3.4 The effect of Coupling to a Nuclear Spin I = 3/2 As seen previously, the electron spin energy levels are split by the magnetic field. In addition there is further splitting due to the four possible values of m I. Since the coupling is to a nucleus of spin I = 3/2, there should be 2n(I + 1) lines (ie 2.1.(3/2 +1) = 4 lines). The four transitions are shown on the diagram. There are only four transitions because, as in all forms of spectroscopy, there are selection rules. In this case the selection rules state that Δm s = 1 and Δm I = 0. Note that the energy of each transition is the same (fixed microwave frequency) but that they occur at different fields (B is scanned). 3.3 Anisotropy Isotropic spectra are obtained from solutions and from species with cubic (tetrahedral or octahedral) symmetry. In less symmetric species, and particularly in solid state samples, we see the effects of anisotropy. We have considered the g-factor to be a scalar quantity, but in fact it is a symmetric tensor. Such a tensor can be diagonalised to give three principle g-factors, g xx, g yy and g zz. In solution these are averaged by molecular motion to give a single isotropic g-factor, and in systems with cubic symmetry, g xx = g yy = g zz. For a system with axial symmetry two of the terms are the same giving rise to two distinct g-factors, g and g. For lower symmetries, all three terms are inequivalent. et us consider the case of distortion in octahedral transition metal complexes. A pure octahedral complex will give a simple isotropic spectrum. However, if we compress or extend one axis (eg Jahn-Teller distortion), put different ligands on one axis, or remove ligands from one axis (ie go to a square planar complex) then we generate a system with axial symmetry. This is often referred to as a tetragonal distortion, and gives rise to a spectrum in which two g-factors are observed, g and g. This is shown schematically below.

4 Dr.D. SPICER Compression or elongation of axis (exaggerated) ' ' Figure 3.5 Tetragonal Distortion in Octahedral Complexes and the Resulting Spectral Form Further distortion or change of the ligand set, destroying the axial symmetry gives rise to what is known as a rhombic distortion, characterised by three unique g-factors, g xx, g yy and g zz (NB sometimes the symbols g x, g y and g z are used). Again this is shown schematically below. ' ' ' '' ' '' Figure 3.6 Rhombic Distortion in Octahedral Complexes and the Resulting Spectral Form 3.4 Other Observations. It should be noted that ESR is a remarkably sensitive technique. Samples with concentrations as low as 10-9 can be detected in favourable cases. This is in contrast to NR which ideally requires samples of at least ESR is also versatile, in that a range of sample types can be studied. It is best for magnetically dilute systems, where the paramagnetic species is surrounded by diamagnetic material, such that there is no interaction of neighbouring electron spins. Solutions are most widely used and give the more easily interpreted spectra. Frozen solutions (often referred to as glasses) are also widely used. Spectra from powdered solids can be more complex, but are OK. For the most detailed studies, ESR from single crystals, or from doped crystals (paramagnetic ions doped at low concentrations in a crystalline, diamagnetic host matrix) enables the precise orientation of the molecules in the field, giving access to a great deal of information regarding the electronic structure of the molecule.

5 Dr.D. SPICER Interpretation of ESR spectra at a superficial level is fairly straightforward. Approximate values of g and A can be extracted, although second order effects and anisotropy can make it difficult to obtain accurate parameters. The g-values are indicative of the electron environment. The magnitude of the hyperfine coupling constant A is usually a good indicator of where an electron is located within the molecule in question. In transition metal complexes, the unpaired electron is usually located in a metal based orbital. Consequently, if the metal has a non-zero nuclear spin, the value of A will be large. Coupling to other nuclei will be much smaller. There are now sophisticated computer programmes readily available which can simulate the spectra and give a far more detailed quantum mechanical analysis.