Isoelectronic principle

Systems are said to be isoelectronic (Greek isos, equal) if they have the same number of nuclei and the same munber of electrons. Thus, Na and F are isoelectronic with Ne, and the following constitutes an isoelectronic series  

The isoelectronic principle states that, for small variations in nuclear charge at least, the arrangement of electrons in isoelectronic species is approximately same. 

A simple example of this is provided by the OH ion. This is isoelectronic with the HF molecule, 0 having the same number of electrons as F. Now the bond formula for the HF molecule is 

Aeeording to the isoelectronic principle, therefore, the bond formula of OH can be written 

Interpretation of main-group vaiencies in terms of a simpie modei of main-group atoms 

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The physical, and often the chemical, properties of isoelectronic molecules are closely related (23). The isoelectronic principle can be extended to a comparison of fluoro with isostructural hydroxo com-... 

We now return to the consideration of the other reactions in Table IV. If the parent ion radical decomposes by losing a hydrogen atom a carbonium ion remains. The main characteristic of the carbonium ion is its strongly acidic character and the formation of saltlike complexes.32 This is another extension of the isoelectronic principle, and is illustrated in reactions 4 and 5 of Table IV. The carbonium ion, CH3+, may be envisioned as forming a three-center transition complex... 

The ionic states are highly reactive because of polarization forces and in the solid state the parent ion radical or parent carbonium ion is the main reactant. The reactions of these ions with neutral molecules can be understood on the basis of energetics, the isoelectronic principle, and charge-exchange bonding. 

Recent experimental data on the transition-metal diatomics is used to test the "isoelectronic" principle as applied to molecules with the same number of d + s valence electrons. Electron configurations auid ground states in the first-row transition-metal monocarbonyl molecules (MCO) are examined on the basis of their CO stretching frequencies, ESR evidence, auid theoretical calculations. 

In summarizing this section, one can say that the present meager experimental data are in encouraging support of the application of the "isoelectronic" principle to these diatomics. Both first-row auid/or mixed-row diatomics containing 12, 13, 14, 15, and 18 valence electrons show indications of having E, E, E, E, and... 

The isoelectronic principle, by which the chemistry of many nitrogen compounds can be related to that of their hydrocarbon counterparts can be useful if used with caution, e.g. account must be taken of the differences in the thermochemistry of the reactions involved s . A comparison of the bond dissociation energies in such isoelectronic sequences is instructive in this regard (Table 27) . 

Other empirical concepts have been used to consolidate data. These include the isoelectronic principle, electronegativity correlations, the relationship to absorption spectra, and the effects of single and multiple substitutions of functional groups on molecules. A similar change in the electron affinities is observed when the more electronegative nitrogen atom replaces a CH group in aromatic molecules. Statistical techniques are used to establish these correlations. Because they are empirical and require additional data for their application, the results are less certain. 

Four sets of curves are easily compared for X2, as shown in Figure 9.8. The overall similarity of the curves is striking and supports the isoelectronic principle. When we compare the values of the dimensionless constants for a given state in Table 9.2,... 

The isoelectronic principle is simple but important. Often, species that are isoelectronic possess the same structure and are isostructural. However, if the term is used loosely and... 

The structure of BPO4 is derived from that of Si02 by replaciug alteruate Si atoms by B or P atoms. Explaiu how this description relates to the isoelectronic principle. 

Restricting to ketene imines with substituents Ri (R2) a nd R3 bonded via carbon atoms to the skeleton and lying in perpendicular planes it has been shown (89a) that, as a result of the isoelectronic principle, approximation ansatze for the calculations of chemical shifts of ketene imines (89b) and ketenes (89a,89c,89rf) may be simply deduced from corresponding expressions for al-lenes, that is, from Equations 37 and 38. For ketene imines they have the analytical forms of Equations 70 and 71, for4cetenes of Equations 72 and 73. 

The isoelectronic principle as formulated qualitatively in Ref. 90 has a quantum-theoretical foundation (91). Relations of the expressions (70)-(74) which have been based essentially on algebraic arguments to quantum-theo-retically derived expressions are discussed in Ref. 89a. 

On the basis of the isoelectronic principle interpolating between the C chemical shifts of allene (Z = 6) (6 = 212.6 ppm relative to TMS) and carbon dioxide 204 (Z = 8) (5c = 125 ppm (93)) one would expect the C chemical shift of carbodiimide (203) which cannot be isolated owing to tautomerism with cyanamide H2NC=N to be observed near 169 ppm. If one assumes that the... 

Based on the isoelectronic principle it is clear that carbodiimides, such as dicyclohexylcarbodilmide (205) = +75 ppm in CCI4 relative to NH4 (616)),... 

The results presented so far have revealed that, on the basis of the isoelectronic principle, it is possible to deduce qualitatively and quantitatively relations between molecular properties and substituent effects within a family of isoelectronic molecules. 

The acceptance of the isoelectronic principle as a symmetry principle has important consequences for the investigation of the structures of physical phenomena on the basis of algebraic arguments, as has been shown for the treatments of chemical shifts of allenes, ketene imines, and ketenes that are scalar... 

The electron density distributions in ketenes and ketene imines may be discussed on the basis of the isoelectronic principle in quite the same way as has been done for the chemical shifts in the preceding subsection. 

As during the whole process of stereoisomerization, that is, for each point of the reaction coordinate, all the preceding species are isosteric, the isoelectronic principle is also applicable for such a dynamic process. Therefore, one may expect a linear relation of the barriers of stereoisomerization V for the species 11a, 49, and 43a to the nuclear charges of the atoms X. This, indeed, is observed based upon theoretical barriers on an ab initio STO-4.31G level (105) (K(lla) = 3.80 kcal mole" (105), K(49) = 9.27 kcal mole" (105), K(43a) = 12.84 kcal mole" (105)) (r = 0.9927). 

The isoelectronic principle as a mean to predict—at least qualitatively— trends of molecular properties probably is also applicable to ionization energies or orbital energies, respectively. 

Therefore, with respect to (x) orbitals (or ionizations of corresponding orbitals) allene (11), ketene imine (49), and ketene (43) represent no proper isoelectronic series (Section I1.G.3). And consequently, with respect to the preceding problems the correct application of the isoelectronic principle requires that concerning x ionizations ketene imines should be compared with carbodi-imides and isocyanates. 

Interestingly with respect to these coupling constants is the fact that the ratio of one-bond and two-bond couplings ( y( CX)/V( CX), X = C, N) in these cumulenes seems to be constant (and of the order of five). Such a finding is expected on the basis of the isoelectronic principle. 

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