Molecular systems benzene

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

Friestad and co-workers recently demonstrated that N-acyl hydrazones were excellent radical acceptors in the presence of a chiral Lewis acid [84], Valerolactam-derived hydrazone 117 proved to be the optimal substrate for enantioselective radical additions. Upon further optimization it was found that Cu(OTf )i and f-bulyl bisoxazoline ligand 96 gave the best yields and ee s (Scheme 31). Interestingly, a mixed solvent system (benzene dichloromethane in a 2 1 ratio, respectively) in the presence of molecular sieves (4 A) were necessary to achieve high yields and selectivities. 

Valence tautomers, benzene oxide 1 and oxepine 2 (Equation 1), as well as relative tautomeric systems, benzene sulfide-thiepine and o-xylene-2,7-dimethyloxepine, have been studied by a post-Hartree-Fock (HF) ab initio QCISD(r)/6-31G //MP2/6-31G method. In particular, the enthalpy calculated for a benzene oxide-oxepine system is 0.59 kj moF1 <1997PCA3371>. The calculated molecular orbital (MO) energies are in linear relationship to those from the photoelectron (PE) spectra <1996JCF1447>. Barrier to tautomerization for a benzene oxide-oxepine system is 29.4 kj mol-1. Protonation stabilizes the oxide form versus the oxepine <1997PCA3371>. 

For various metallabenzenes with Fe, Re, Ru, Rh, Pd, Os, Ir, and Pt, free energies for the conversion into energetically favored cyclopentadienyl semisandwich systems as well as the fairly high activation barriers have been calculated by DFT methods [296], Also, DFT calculations involving the four occupied n molecular orbitals (benzene has only three such occupied orbitals) provide an understanding of the slight deviation of the metal from the plane of the aromatic ring [284],... 

In practice, we have utilized it for the analysis of molecular systems comprised of 3 to 5 fused benzene rings. Our discussion in this document is limited to the following compounds phenanthrene, anthracene, fluoranthene, pyrene, benz(a)anthracene, chrysene, benzo(e)pyrene, benzo(a)pyrene, and dibenz(a,h)anthracene. The structures for these compounds are presented in Table I. It is important to note that the method has also been adapted to the determination of several other PAH compounds (e.g., benzo(c)phenanthrene, perylene, 3-methylcholanthrene, carbazole, 7H-dibenzo(c,g)carbazole, and indeno(l,2,3-cd)pyrene). 

There are molecular systems exhibiting 7r-bond fixation patterns that are entirely opposite to that induced by the Mills-Nixon effect [82,83,67]. Typical examples of this kind are provided by benzoborirene 33 and benzocyclopropenyl cation 34 (Fig. 19) These compounds represent extended 7r-systems relative to benzene itself since they encompass now empty 7r-orbitals at B and C+ atoms, respectively. The structural parameters offered by HF/6-31G [82] and MP2(fc)/6-31G [43] models are given in Table 10. Both molecules are planar. A salient feature of the aromatic CC bonds is their stretching relative to benzene at ortho and para positions. In contrast, meta bonds are more localized and shortened. Another striking property is a pronounced delocalization within the three-membered ring (aromatic pattern involving 27t electrons) as easily visualized by the resonance structures shown in Scheme 4. The same resonance mechanism is operative in benzocyclopropenyl cation. 

Br0nsted and Colmant studied the vapour pressures, at 18 °C, of solutions in which the two components had widely different molecular dimensions (e.g. benzene +n-butyl sebacate). In these systems only the solvent has an appreciable vapour pressure. From measurements of this vapour pressure it is possible to determine the activity coefficient of the solvent. For the system benzene + i-butyl sebacate, the activity coefficients of the benzene can be expressed very accurately by an equation of the form... 

In spite of the lack of a unique and precise definition, aromaticity is one of the most frequently used concepts in (organic) chemistry. This phenomenon, which is classically associated with a cyclic tt-electron delocalization, results in a stabilization of the molecular system considered. Benzene is the archetype of the phenomenon of aromaticity. Thus, a question of interest is to what extent the amino substituent influences the electron delocalization in the ring. There are several criteria to evaluate the aromaticity, including the geometry-based (HOMA), energy-based (ASE), magnetism-based (NICS) and electronic delocalization (PDI) models. Recent theoretical evaluations65 of these parameters... 

The polarizable continuum model (PCM) by Tomasi and coworkers [77-79] was selected to describe the effects of solvent, because it was used to successfully investigate the effect of solvent upon the energetics and equilibria of other small molecular systems. The PCM method has been described in detail [80]. The solvents and dielectric constants used were benzene (s = 2.25), methylene chloride (g = 8.93), methanol (g = 32.0), and water (g = 78.4). Full geometry optimizations were carried out for the discrete and PCM models. To simultaneously account for localized hydrogen bonding and bulk solvation effects, PCM single-point energy calculations have been conducted on stationary points of the acrolein and butadiene reaction with two waters explicitly... 

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