7r Energy

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

The DRE calculation scheme takes into account only two types of CC bonds, not accounting for the fact that the energy of an acyclic polyene depends on its branching. The 7r-energy of a branched acyclic polyene Ev (BP) is related to the energy of an isomeric linear polyene E (LP) by (75JCP3399)... 

Table 2 Total 7r-Energies and Delocalization Energies of the Naphthyridines2...

We will now consider how the MO approach can be used to understand these differences in excitation energy. The 7r-energy levels and electronic configurations for delocalized and localized 1,3-butadiene are shown in Figure 21-10 (also see Section 21-4). Because the double bonds are so far apart, the ir-... 

The 7r energy levels, along with the molecular orbital wavefunctions, are pic-torially displayed in Fig. 7.1.12. Since there are six n electrons in benzene, orbitals a2u and e % are filled, giving rise to a 1 /V ig ground state with the total n energy... 

The separation of 7r-7r energy levels in the BDHC complexes is comparable with that in the corrinoid complexes. The extent of electronic perturbation on the 7rand 7r levels by the axial ligands is nearly the same for the two complexes, judging from the extent of band shifting. The red shift is increased by the axial ligands as follows ... 

These potentials are affected by two primary factors the size of the negative charge on the equatorial ligand and the tt energy level relative to the 4p or 3d (Co) orbital (the 7r energy level is affected by... 

Like the uncondensed compounds, 2-aminobenzimidazoles exist in the amino forms. Evidence comes from the NH2 bands in the IR (solid state) and comparison of the UV spectra with those of fixed models. A Ki value of 160 in 50% aqueous ethanol has been calculated for 2-amino-l-methylbenzimidazole. Calculations of the 7r-energies of each tautomer also favour the amino form for this compound and for linear and angular 2-aminonaphthimidazoles. Even in the structure (67), where hydrogen bonding might be expected to favour an imino tautomer (68), the amino form persists (Scheme 22). 

Wagner claims that this structure con-esponds to HNg, but no consideration was given to the effect of the H atom. He carried out similar calculations on various pseudohalide anions and was able to show that the pA of the corresponding acids varies linearly with the difference in the 7r-energy between the ions and their acids. Linear correlations were also found betw een (a) calculated bond orders and stretching force constants, (b) calculated charge densities and n.m.r. chemical shifts. 

X-Substituted Benzenes. As before (p. 49), we use the benzyl anion as a model for X-substituted benzenes like anisole. The three lowest-energy orbitals (Fig. 3-10) are, like those of styrene, very similar to those of benzene. The HOMO is ij/A, and this, like the corresponding orbital in the allyl system, has nodes on the alternate atoms. For this reason, it is a non-bonding orbital, and its 7r-energy is zero. 

A series of chlorotricarbonyl rhenium(I) complexes based on benzathiazole derivatives have been reported [34]. One of the chlorotricarbonyl rhenium(l) complexes, shown in Fig. 38.3c, exhibits a solar energy efficiency of around 1.43-1.76% [44]. Compared to other rhenium(l) 2,2 -bipyridine complexes, these were found to have a redshifted absorption because the heterocyclic ligands have lower jr-to- 7r energy levels [44]. 

This correction sets the 7r-energy of methyl cation to be 0 kcal mol , ... 

The resonance energy of the allyl cation was calculated via the force field method by comparing the 7r-energies of the allyl cation (obtained by the MMP2 SCF routine) and ethene, which also has two jr-electrons, but has no corresponding resonance stabilization. Table 3 lists the MMP2 7r-energies. 

The difference in the MMP2 7r-energies of the allyl cation and ethene, 53.2 kcal mo ", can be attributed to the resonance... 

The change in 7r-energy on intramolecular union between two positions i and / in an alternant hydrocarbon (AH) (e.g. Fig. 2.4(a)) might be expected to depend on the coefficients of the occupied orbitals at the point of union. These coefficients are reflected in the calculated rr-bond order between atoms / and / in the original structure. The energy change is given by rule I. 

Since there are two electrons in the new orbital, the total 7r-energy change,... 

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