Nucleus shielding

The first ionization energy of Li is found experimentally to be 5.363 eV. If the electron in the second shell (n = 2) is assumed to move in a central field of an effective nuclear charge, Zejy, consisting of the nucleus and the other electrons, by how many units of charge is the nucleus shielded by the other electrons Assume that the ionization energy can be calculated from Bohr theory. 

Nucleus shielding and the configuration of outer electrons. In general, d electrons do not shield the nucleus as well as 5 or / electrons hence the polarizability of atoms with d electrons is less than that of similarly sized atoms with s or p electrons. 

We have seen that the magnetic field R, required to obtain the resonance crmdition for nucleus i at a particular irradiating rf field (Ri) is not equal to the applied static field Rq, but is instead R, = Ro(l — a) [see Eq. (20.9)], where the nuclear screening constant, a, depends on the chemical structural environment of nucleus /. The local electron density in the vicinity of the nucleus shields it from the applied field Bo by producing small local magnetic fields (diamagnetic currents). Any structural feamre that alters the electronic environment of a nucleus will affect its screening constant a and lead to an alteration in its resonance frequency or chemical shift 5,. 

When full, isotropic averaging is achieved, most of the interactions between a nucleus and its environment are averaged to zero. One exception to this is the amount by which the applied magnetic field is modified by the electrons circulating around the nucleus (shielding), which is averaged to a finite (non-zero) value. This value depends on the... 

If the nucleus were large, then orbitals of different / would have different orbital energies. This explains the energy differences for the s,p,d,... levels, because the outer shell electrons move in the field of the nucleus shielded by the inner shell electrons (thus, in a field of something that can be seen as a large pseudo-nucleus). 

If the excited level Ej lies closely below the ionization limit, the molecule M (Ej ) can be ionized by an external electric DC field (Fig.6.20a). This method is particularly efficient if the excited level is a long-lived highly excited Rydberg state. The required minimum electric field can readily be estimated from Bohr s atomic model, which gives a good approximation for atomic levels with large principal quantum number n. The ionization potential for the outer electron at the mean radius r from the nucleus is determined by the Coulomb field of the nucleus shielded by the inner electron core. 

There is a physical motivation for choosing this variation function. As an electron moves about in the helium atom, there is some probability that the other electron will be somewhere between the first electron and the nucleus, shielding the first electron from the full nuclear charge and causing it to move as though the nucleus had a smaller charge. A value of Z smaller than 2 should produce a better approximate energy value than the value of -74.8 eV in Eq. (19.1-8). 

In the second region (14 electrons), increased 7t-eleetron cloud of the heteroaromatic nucleus shields the positive charge of the eationic azide, and the quantum yield remains the same on passing from the neutral azide to cation. This effect appears in azidoacridine. It is noteworthy that invariability of the (p value ean testify indirectly to invariability of the rate eonstants of photophysical processes (emission, internal eonversion, and intersystem erossing, see formula 1) on going from the neutral to eationic azide. 

Equation (9.8) shows that the ionization energy ( j) is a linear function of (Zgff) and the straight-line graphs (equations 9.6 and 9.7) show that is a linear function of Z. We are on the right track. However, we must now take into account that we are considering one-electron systems with a nucleus shielded by a closed shell. 

---