Energy towards 32-electron rule

In Fig. 1 there is indicated the division of the nine outer orbitals into these two classes. It is assumed that electrons occupying orbitals of the first class (weak interatomic interactions) in an atom tend to remain unpaired (Hund s rule of maximum multiplicity), and that electrons occupying orbitals of the second class pair with similar electrons of adjacent atoms. Let us call these orbitals atomic orbitals and bond orbitals, respectively. In copper all of the atomic orbitals are occupied by pairs. In nickel, with ou = 0.61, there are 0.61 unpaired electrons in atomic orbitals, and in cobalt 1.71. (The deviation from unity of the difference between the values for cobalt and nickel may be the result of experimental error in the cobalt value, which is uncertain because of the magnetic hardness of this element.) This indicates that the energy diagram of Fig. 1 does not change very much from metal to metal. Substantiation of this is provided by the values of cra for copper-nickel alloys,12 which decrease linearly with mole fraction of copper from mole fraction 0.6 of copper, and by the related values for zinc-nickel and other alloys.13 The value a a = 2.61 would accordingly be expected for iron, if there were 2.61 or more d orbitals in the atomic orbital class. We conclude from the observed value [Pg.347]

The treatment of atoms with more than one electron (polyelectronic atoms) requires consideration of the effects of interelectronic repulsion, orbital penetration towards the nucleus, nuclear shielding, and an extra quantum number (the spin quantum number) which specifies the intrinsic energy of the electron in any orbital. The restriction on numbers of atomic orbitals and the number of electrons that they can contain leads to a discussion of the Pauli exclusion principle, Hund s rules and the aufbau principle. All these considerations are necessary to allow the construction of the modern form of the periodic classification of the elements. 

Thus, the chemical interconversion for equal electronic parity channels has four separated aspects i) activation via molding of reactants ii) population of TS rovibrational quantum states iii) population of reactants molded into configurations covered by the TS, and iv) relaxation towards products in their ground states. All such changes are submitted to energy and angular momentum conservation rules. 

Another significant deviation from known rules has been observed in the rather low affinity of K+ with the 1,10-diaza crown 18-C-6 with only AG=10 kJ/mol (in methanol). It has been shown that the free energy of binding AG in crown ether and ciyptand complexes usually is an additive function of number and electron donicity of the host donor atoms which are in contact with the metal ion.[17] Molecular mechanics calculations suggest the reduced affinity with the diazacrown to be due to the N-lone pairs in pseudoaxial position, pointing away from the metal ion (Figure 4). This has led to experiments with the N-methyl substituted crown here the N-alkyl substituents would clash which each other inside the macrocycle, therefore a pseudoequatorial lone pair orientation towards the cation is enforced, and the stability of the complex indeed returns to the normal scale with an increase to 29.5 kJ/mol.[ 18]... 

A unique treatment of cyclopropane has been advanced by Dewar, who introduced the concept of a aromaticity, which explains some of the anomalous chemical and physical properties of cyclopropane. The notion of cr-conjugation implies that three cr-bonds form a cyclic system of six electrons thus cyclopropane is aromatic by the (4n -1- 2) rale. This explanation well accounts for the strain energy of cyclopropane. The actual value (27.5 kcal mol ) is much lower than the predicted value of 104 kcal mol (1 cal = 4.2 J), calculated from the C—C—C bending force constants obtained spectroscopically. - A similar comparison for cyclobutane (antiaromatic by the above notion and the 4n rule) underestimated the strain energy. o -Aromaticity also accounts for such observations as H NMR chemical shifts and the reactivity of cyclopropane toward electrojAiles. 

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