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Rev. 96,Koos, W., Elementy Chemii Kwantowej Sposobem Niematematy-cznym Wyozone. PWN, , W., Sadlej. mechaniki kwantowej, i nie ma on swojego odpowiednika w mechanice czy wi ˛azania chemiczne z obcymi atomami, typu wodór lub tlen. . curriculum and a suggestion of the program coordinator, Dr. Koos Mars, to submit. , 67, (1) Koos, W. Chemia Kwantowa (Quantum Chemistry) (in Polish );(32) Cyranski, M. K.; Schleyer, P. v. R.; Krygowski, T. M.; Jiao.

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Published on May View 1. Some Other Remarks on Aromaticity Valence Bond Approach Linear Polyenes as Reference Systems Mono- or Multidimensional Perspective of and subsequent Ph. Department of Warsaw University. In he was holder of a Polish5. In he received the6. Acknowledgments Grabowski Reward for outstanding activity in the field of physical organic7.

In he was the elected secretary of the Executive Committee of the Polish Chemical 1. Delocalization versus Localization Society. His main scientific interests are the structural chemistry of small organic compounds, crystallography, molecular modeling ab initioIn quantum mechanics the state of an electron isstructural aspects of the aromatic character of -electron systems, and, defined within the frame of probabilistic theory.

He has published more than electrons of an isolated atom are confined to the space 60 original papers and 10 reviews.

His hobby is classical music. Following the Heisenberg uncertainty prin-and magnetic susceptibilities, which can be expressed ciple even at this very basic atomic level the electronsusing additive schemes of empirically devised bond are delocalized with an uncertainty in their positions, parameters. These parameters are transferrable which is inversely proportional to the uncertaintybetween molecules.

Dewar divided molecular prop- of their momenta. Both valence and the inner occupying specific delocalized molecular orbitals. It is worth mentioning that some properties are simply suggest that many of the molecules behave associated with inner electrons, some with valenceas if the bonds in them were localized.

Within the electrons, and some with unoccupied orbitals. Manyconcept of bond localization, Cremer3 clarified that properties can be rationalized in this way, such as the electrons or bonds are localized if the properties heats of formation, dipole moments, polarizabilities, of the molecule can be expressed in terms of bond In turn, they are considered to be makes comparisons difficult.

Aromaticity separated bonds, or separated fragments. Different topological modes of the electron delocalization in- Historically, the chemical consequences of in- clude i ribbon delocalization of either – or -elec- creased stability were the main features that distin- trons i.

Cyclic -electron aromatic compounds were rec-However, changes in structure are not always paral- ognized from an early stage13,14 as systems in whichleled by changes in reactivity or other physicochem- the nonadditivity of collective properties was theical properties. This review critically evaluates various models and concepts in the quantitative estimation2.

Energetic Criterion of cyclic -electron delocalization from an energeticThe thermodynamic stability of the system is perspective as applied to aromatic systems, which are enhanced with respect to a structurally analogous regarded as the cornerstone of modern organic chem- model system most often an acyclic systemwhich istry.

In most physical energy for typical aromatic systems. For instance, the ionization potential of an atom We might liken this problem to a meal to which or molecule is defined as the energy required to some spice was added: Two kinds of ionization energies can be of the food and significantly influences its other distinguished: Despite its leading role, aromatic effectswhether the resulting ion is in its ground vibrational are not an easy task to separate out and quantify,state or possesses the vibrational energy determined for two main reasons: In chemistry the situation is slightly meal to meal ; and ii the weights of the spicy meal different s in most cases, a given energetic charac- and its spice-free model are very similar, whichteristic needs comparison with some molecular 3.


Then itof experimental data. Another problem is that for more complex interactions it is very difficult to find a reference state 2.

Sandrogreco Energetic Aspects Of Cyclic Pi Electron Delocalization Evaluation Of The

Geometric Criterion that would ideally represent a particular phenom- Cyclic bond lengths tend to be intermediate be- enon, which is free from any contamination of tween those typical for single and double bonds. Importantly, this property is a cooperative effect The quantitative measure of the effect is the of both the and orbitals. In the case of ben- resonance energy16 and more generally the aromatic zene, its regular shape is actually a result of the stabilization energy ASE.

The the difference in geometry between kwnatowa system with criterion has often been used to explore various full -electron delocalization and a comparable model hypotheses concerning the chemical behavior of nonaromatic Kekule-like structure with kooos systems considered to be aromatic. It has been shown10m,24,68 that, among papers by Pauling and Sherman13 and then Kistia- the indices based on geometry, the harmonic oscil- kowsky et al. Nonaromatic Kekule structures36 and realsummation and R is an empirical constant chosen to structure of benzene.

In valence bond theory systems e. The individual bond lengths are represented ergy40,42,43 are the operational criteria for the extentby Ri. Even if it is wellHNdCH2. HOMA supports this maticity.

In such as AJ and I6 show incorrectly that the centralring is more aromatic than the outer ones. Again, thebond elongation factor not present in the definitionsof AJ and I6 is of fundamental importance.

The model has been successfully applied to thedescription of cyclic -delocalization in a variety ofcarbo, and heterocyclic systems,31,71,92,93 ben-zenoid hydrocarbons,87, and their aza deriva-tives,98 nonbenzenoid nonalternant hydrocar-bons and their heteroanalogues, sphericalsystems, and Y aromatics. The case of dibenzo def,i naphtho 1,8,7-v,w,x pyran-threne illustrates this point well.

Figure 3 illustrates the Figure 2. Aromaticity indices for benzene, [6]radialene,24 and perylene. The reason for this shortcom- ing is that these geometry-based models are defined solely on the basis of the variance of bond lengths82 or equivalent bond orders.

Acidbase properties of selected flavonoid glycosides

Aromatic character of dibenzo def,i naphtho- It haswhich the geometry-based descriptor of aromaticity been shown that it is possible to separate in anHOMA may be very helpful for interpreting the analytical way the bond ,wantowa factor the so-electronic structure of the system. Two pyrrole rings with NH groups are 3 has been also extended kqantowa hetero–electron systemsclearly more aromatic HOMA 0. Map of -current density in free-base porphyrin. The diamagnetic circulation is shown anticlockwise.

Acidbase properties of selected flavonoid glycosides – [PDF Document]

Re- printed with permission from ref First of all, as in the case of energetic descriptors, the reference nonaromatic model systems are arbi- trarily chosen. Jwantowa is clear when one analyzes the variety of systems used as references for single and double bonds see Table 1. Obviously the geometry-based aromaticity indices, if they are to be applied in a reliable way, need a reliable molecular geometry. However, different ex- perimental techniques such as gas-phase electron diffraction ED ,72, microwave spectroscopy MWX-ray and neutron diffraction,75, and occasionally infrared or Raman spectroscopy, as well as theo- Figure 4.

Reprinted with permissionmolecular geometries chemka in a slightly different from ref Using ipsocentric current-density mapping tech-corresponding DFT optimized molecular structures. Although some details differ, theirset geometries using smaller ones. Depending on the acidity of phenol, the experi- mental CC bond lengths in the benzene fragment can vary from 1.

Reference values of HOMA are 0. Reprinted with permission from ref Copyright American Chemical Society. The problem of intermolecular interactions affect- ing bond length alternation and hence aromaticity is very complex and is discussed in detail in another review in this issue by Krygowski et al.


Finally, apart from the variations in geometry- based indices caused by the source of the geometrical data, another very important point should be con- sidered. Bond length equilization or alternation does not always correctly indicate the changes in -electron delocalization!

Recently, Fowler et al. Scatter plots of the mean kwzntowa parameters a, b, c versus the acidity of phenols: Number of data points, n Copyright Polish Chemical tetrakis klos cyclooctatetraene D4htet- Society. Despite dependencies between the pKa of complexes of sub- a similar degree of strong bond length alternation in stituted phenols with oxygen and nitrogen bases the COT moieties R 0. Following and kos mean bond kqantowa of the ring for enumera- the London expectations of paratropic ring currents tion of the bonds see Figure 7.

Relaxation of cyclooctatetraene to its tub-shaped equilibrium geometry c has relatively little effect on the bond length alternation R 0.

The -current density map drawn in the kwnatowa plane of the molecule shows localized circulations from the four Figure 7.

Labeling of geometric parameters used in Figure 6. In the case of tetrakis cyclobutadieno cy- clooctatetraene dbond length alteration is exactly Similar analysis based on the optimized geometriesthe same as in the case of bicyclo[2. Maps of -current density in a tetrakis bicyclo[2. Except for c the nuclear positions are projected in the plotting plane. In c the median plane was used as the plotting plane.

The current densities induced by a unit magnetic field acting along the principal axis are plotted in a plane 1 above that of the central ring. The diamagnetic circulation is shown anticlockwise and the paramagnetic one clockwise. The electronic specifically as a result of the conjugation of thecharacter of the clamping groups, rather than their clamping group with the central ring.

Obviously, therole as geometrical constraints, is the decisive factor differences observed in these current maps are not ain whether the central ring current is retained or not. Strong bond length alternation in a ring may be a Instead, the switch from localized to delocalized symptom of either a rigid saturated clamp without behavior is due to the difference between the elec- significant effect on ring current or a strongly inter- tronic structures of the 16 -electron conjugated acting unsaturated clamp which, by pushing the tetrakis cyclobutadieno cyclooctatetraene and the 8 HOMO and LUMO apart, quenches the current.

Similar conclu- Very recently, Sola et al. Despite a sub- distances were replaced by relevant delocalization stantial alternation of the CC bond lengths of theindices, in both cases derived from Baders atoms in benzene ring in tris bicyclo[2,1,1]hexeno benzene a molecules AIM theory. Magnetic Criterion is retained, whereas in the case of tris benzocyclo- butadieno benzene c Figure 10 and tris cyclobuta- An external magnetic field perpendicular to the dieno benzene d Figure 10where the bond lengthmolecular plane induces a diatropic ring current.

This alternation in the central ring is comparable, no current gives rise to a secondary field, which is diamagnetic -electron ring current is observed. Now, the nucleus-independent chemical as in the case of cyclooctatetraene, the differences in shift NICS is often considered to be a better current maps are evidently not a consequence of criterion. Maps of -current density in a tris bicyclo[2.

The nuclear positions are projected into the plotting plane, the current densities are plotted 1 above the molecular plane. Copyright Royal Society of Chemistry. External magnetic field inducing an internalthe F values to benzene.

Recently, also Schleyer26, proposedsystem is needed, in practice the anisotropy of mag- that compounds which exhibit significantly exaltednetic susceptibility is difficult to apply as a measure diamagnetic susceptibility are aromatic.