Q.3. (a) Discuss briefly the theory of ESR spectroscopy, spin-spin splitting and its qualitative applications
Ans.3. (a) Electron Spin Resonance It is a branch of absorption spectroscopy in which radiation having frequency in the microwave region is absorbed by paramagnetic substances to induce transitions between magnetic energy levels of electrons with unpaired spins. The magnetic energy splitting is done by applying a static magnetic field.
The electron spin resonance phenomenon is shown by atoms having an odd number of electrons, ions having partly filled inner electron shells and other molecules that carry angular momentum of electronic origin. The main interest of electron spin resonance lies in the study of free radicals having unpaired electrons. These electrons remain after the hemolytic fission of a covalent bond which is brought about by the irradiation of the sample with the ultraviolet or gamma radiation.
The electron spin resonance (ESR) is another type of magnetic resonance. It is concerned with the magnetic behavior of spinning electrons. The ESR spectrum like NMR, results from transition from one spin state to the other state of an electron. Each spin state has its energy level, the transition occurs under resonance condition in a magnetic field. However ESR differs from NMR in that the transition in ESR is induced by radiation of microwave frequency rather than by radio wave.
Qualitative application
1. Structural Determination:The ESR technique cannot be applied to determine molecular structure because the information obtained from the superfine structure is mostly about the extent of electron delocalization and Fermi contact interaction. It does not tell us about the arrangement of the atoms in the molecule although the symmetry of the molecule can be sometimes deduced from the sets of equivalent nuclei. However, in certain cases, ESR is able to provide useful information about the shape of the radicals. Eg- determination of methyl radical, it may have one of the following structures: planar or tetrahedral.
2. Study of Inorganic Compounds:ESR is very successful in the study of inorganic compounds. Some examples are asfollows:
(a) An interesting example is that [NO(SO3)2]2- yields a triplet in its ESR spectrum in chloroform. This arises from the interaction between the spin of the unpaired electron and the spin of a 14N nucleus (I = 1), confirming that this electron is mainly localized on the nitrogen atom.
(b) Another interesting example is sodium trimesitylborate whose ESR spectrum in tetrahydrofuran shows four peaks. This indicates that the odd electron in the [(BC6H2Me3)3]- ion couples with the spin of 11B for which I = 3/2.
(c) Determination of oxidation state of a metal: the ESR spectrum of Cu (II) complex gives a spectrum signal whereas that of Cu(I) complex system gives a much reduced signal. This difference between the signals for two oxidation states of copper has been successfully used to determine the state the copper in many complexes and biological compounds. For eg: copper is found to be divalent in copper protein complexes whereas it is found to be monovalent in some biologically active copper complexes.
Spin-spin splitting: In the simplest case we expect to see a single peak for each type of proton in a molecule. But consider what happens if a proton that we are looking at (HA) is near another non-equivalent proton (HB). In half of the molecules the HA proton will be adjacent to an HB aligned with the field and in the other half the HA proton will be adjacent to an HB aligned against the field. Thus, half the HA's in the sample will feel a slightly larger magnetic field than they would in the absence of HB and half will feel a slightly smaller magnetic field. Thus, we will observe two absorptions for the HA proton. (Of course we would also observe the same thing for HB.) This splitting of the HA resonance into two peaks is termed "Spin-spin coupling" or "spin-spin splitting" and the distance between the two peaks (in Hz) is called the "coupling constant" (Usually represented by the symbol J). The spin-spin coupling is transmitted through the electrons in the bonds and so depends on the bonding relationship between the two hydrogen’s.
Rules for splitting of proton signals:
In general the following set of rules may be found very helpful in predicting or interpreting the splitting of proton signals.
(1). Splitting of a proton signal is caused only by neighbouring or vicinal protons (i.e. Protons on adjacent carbon atoms) which are not equivalent to the proton under consideration. Non-equivalent protons are, of course, those protons which have different chemical shifts. For instance, in the spectrum of CH2Cl-CH2Cl, there is no splitting as the vicinal protons are equivalent to one another.
(2). Splitting of one proton by another on the same carbon very rarely comes across because such protons are generally equivalent each to other.
At the same, the mutual splitting of protons, separated by more than two atoms is also very uncommon. For instance, let us consider the compound 1-bromo-2, 2-dimethylpropane. In this compound there is no splitting of signals as the non-equivalent protons are separated by more than two carbon atoms.
(3). the number of peaks (N) into which a proton signal is split up is equal to one more than the number of vicinal protons (n). That is:
Thus, The NMR signal due to a proton is split into:
A doublet (with peak intensities 1:1) by one vicinal proton
A triplet (with peak intensities 1:2:1) by two vicinal protons.
A quartet (with peak intensities 1:4:4:1) by three vicinal protons
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