Isoelectronic surface

Figure 1.10 Isoelectronic surface of the total electron density of ammonia 2 and Irenzene 3. Note the lack of lone pairs or a it-clond. (See insert for color representation of this figure.)...

The electron density distribution is a four-dimensional function (the number of elearons at a given point (x,y,z)), which is difficult to visually represent. Figures 1 and 2, respectively, show a three-dimensional isoelectronic surface of benzene and a contour plot of the elearon density p(r) in the molecular plane of benzene. Both representations show only gross features of the density. In particular, the total electron density distribution is dominated by the core electrons and appears simply as an aggregate of slightly distorted spheres... 

Figure 1 Isoelectronic surface of benzene. Note the general lack of features for example, no it cloud and no atomic shell structure.

Figure 2-125. Different isovalue-based surfaces of phenylalanine a) isoelectronic density b) molecular orbitals (HOMO-LUMO) c) isopotential surface and d) isosurface of the electron cryo-microscopic volume of the ribosome of Escherichia coii.

Electron density surfaces can also be used to uncover trends and build qualitative descriptions. For example, size surfaces for the isoelectronic molecules, methyl anion, ammonia and hydronium cation show a marked decrease in overall size. 

The OJ ion is a 19-electron radical and is isoelectronic with AB2-type radicals such as SOJ and NO2- which have been observed on surfaces. In these ions, the energy levels are well separated (Fig. 21) and because they are not significantly perturbed by the surface crystal field, the g tensor can be used to fingerprint the species (96). Comparison between the g tensor... 

This is supported by comparison with the isoelectronic ion SO3, which has an isotropic g tensor at 2.0036 in K2CH2(S03)2 (376), C103, which exhibits an axial g tensor (gL = 2.008, g = 2.007) in NH4C104 (377), and POr (gL = 2.001, gn = 1.999) in Na2HP03-5H20 (378). However, on surfaces there are only two reports of 25-electron radicals adsorbed on surfaces. SO3 adsorbed at the surface of MgO gives an isotropic signal with... 

We will now compare the N2 system to the much more studied isoelectronic CO molecule adsorbed on Ni(100). Like N2, CO adsorbs in a c(2 x 2) overlayer structure on Ni(100), occupying on-top sites with the carbon end down with a C—Ni distance of 1.73 A, see Chapter 1 for details. However, the adsorption energy of 1.2 eV [63] is much higher in comparison to that of N2. It is therefore very interesting to see how the difference in electronegativity of the carbon and oxygen atoms influences the surface-chemical bond in comparison to the isoelectronic N2. 

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