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O zależnościach pomiędzy aromatycznością i efektem podstawnikowym w układach jednopierścieniowych

Szatyłowicz Halina, Krygowski Tadeusz Marek

Wiadomości Chemiczne

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2019

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[Z] 73, 3-4

243--261

EN

Aromaticity/aromatic and substituent/substituent effects belong to the most commonly used terms in organic chemistry and related fields. They are used for more than a century, and so far are the subject of thousands publications a year. The quantitative description of the aromaticity of planar π-electron cyclic molecules is based on four criteria: (i) they are more stable than their acyclic unsaturated analogues, (ii) bonds have intermediate lengths between those for the single and double ones, (iii) external magnetic field induces π-electron ring current, and (iv) aromatic systems prefer reactions in which the π-electron structure is preserved. conserved. Quantitative characteristics based on these criteria, named as aromaticity indices, allow to relate aromaticity to the substituent effect. This latter can be described using either traditional Hammett-type substituent constants or characteristics based on quantum-chemistry. For this purpose, the energies of properly designed homodesmotic reactions and electron density distribution are used. In the first case, a descriptor named SESE (substituent effect stabilization energy) is obtained, while in the second case – cSAR (charge of the substituent active region), which is the sum of the charge of the ipso carbon atom and the charge of the substituent. The application of these substituent effect descriptors to a set of π-electron systems, such as: benzene, quinones, cyclopenta- and cyclohepta-dienes, as well as some azoles, allowed to draw the following conclusions: (i) The less aromatic the system, the stronger the substituent influences the π-electron system. Highly aromatic systems are resistant to the substituent effect, in line with the organic chemistry experience that aromatic compounds dislike reactions leading to changes in the π-electron structure of the ring. (ii) Intramolecular charge transfer (resonance effect) is privileged in cases where the number of bonds between the electron-attracting and electron-donating atoms is even. These effects are much weaker when this number is odd. Classically, it may be related to traditional para vs meta substituent effects in benzene derivatives. We should note that in electron-accepting groups, such as CN or NO2 (and others), electron-accepting atoms are second counting from Cipso. (iii) In all cases, when the substituent changes number of π-electrons in the ring in the direction of 4N+2, its aromaticity increases, for example electron-donating substituents in exocyclic substituted pentafulvene, or a halogen atom in complexes with heptafulvene.

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Historyczny rozwój koncepcji aromatyczności

Ciesielski A., Krygowski T. M., Cyrański M. K.

Wiadomości Chemiczne

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2015

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[Z] 69, 9-10

789--808

EN

Aromaticity is one of the most important terms used in organic chemistry. It has been called as a “as a cornerstone of heterocyclic chemistry” or “a theoretical concept of immese practical importance”. The concept, in chemical sense, has been introduced by Friedrich August Kekulé von Stradonitz 150 ago. The paper presents the contribution to its development of many outstanding scientists: Emil Erlenmayer, Albert Ladenburg, Adolf von Baeyer, Victor Meyer, Heinrich Limpricht, Artur Hantzsch, Eugen Bamberger, Richard Willstätter, Ernest Crocker, James W. Armit, Robert Robinson, Erich Hückel, Artur Frost, Boris Musulin, Linus Pauling, Kathleen Lonsdale, Eric Clar, Haruo Hosoya, Henry Edward Armstrong, George W. Wheland, Fritz W. London, John Pople, Paul von Ragué Schleyer and others. Aromaticity is defined on the basis of four main criteria: energetic, geometric, magnetic and reactivity. Two modern definitions of the term are presented in chapter 2 (both are given in English).

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Studies on the charge density distribution in p-substituted phenylnitrenium cation

Przybyłek M.

Ekologia i Technika

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2013

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R. 21, nr 5

230--236

EN

Aniline and its derivatives are known to be mutagenic. This activity is caused by the formation of phenylnitrenium cations during aniline oxidation. Reactivity of the positively charged chemical species can be measured by means of s+ substituent constant. In this paper charge density distribution in p-substituted nitrenium cations and reactivity indices such as, hardness η, electronegativity χ , and electrophilicily ω were analyzed. In order to evaluate which parameters are the most appropriate in describing electron density, correlation coefficients with s+ were calculated. The best results were achieved in case of energy of the lowest unoccupied molecular orbital (ELUMO). NBO lone pair orbital energy NBO ELP, and partial charge on the nitrenium nitrogen calculated using NBO method. NBO N charge. Analysis of these parameters showed that values of s+ depends strongly on the charge density on nitrenium nitrogen atom. According to NBO calculations. electron density on C-2 and C-6 (atoms numeration according to Fig. 1) is lower than in case of C-3 and C-5. These facts are consistent with the resonance theory.

PL

Anilina i jej pochodne wykazują właściwości mutageniczne. Ta aktywność wywołana jest powstawaniem kationu fenylonitreniowego podczas utleniania aniliny. W niniejszej publikacji, zbadana została dystrybucja gęstości ładunku w p-podstawionym kationie fenylonitreniowym. Przeanalizowano, wartości indeksów reaktywności, takich jak twardość η, elektroujemność χ, elektrofilowość ω oraz wartości ładunków Mullikena, NBO. CHelpG i APT. Aby stwierdzić które parametry są najlepsze do oceny gęstości elektronowej obliczono współczynniki korelacji dla tych parametrów z wartościami stałych podstawnika sp+. Najlepsze rezultaty uzyskano w przypadku energii poziomu LUMO, energii orbitaluwolnej pary elektronowej i ładunku NBO na nitreniowym atomie azotu. Analizy tych parametrów wykazały, że wartość stałej sp+ jest silnie uzależniona od gęstości ładunku na nitreniowym atomie azotu. Na podstawie obliczeń NBO stwierdzono, że gęstość elektronowa na atomach C-2 i C-6 jest mniejsza niż w przypadku C-3 i C-5. Przedstawione powyżej fakty są zgodne z teorią rezonansu.

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Strukturalne konsekwencje wiązania wodorowego

Krygowski T.M., Szatyłowicz H.

Wiadomości Chemiczne

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2011

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[Z] 65, 11-12

953-974

EN

Hydrogen bonding belongs to the most important chemical interactions in life and geochemical processes as well as in technologies, that is documented in many review articles [1-10], monographs [11-17] and numerous publications. Figure 1 presents how "popular" are studies concerning hydrogen bonds (the term H-bond/bonding/bonded in a title, key-words or in abstract) in the last decade. First information about H-bond formation appeared at the end of XIX and a few other at beginning of XX centuries [19-24]. Most common definition of H-bonding stems from Pauling [27], whereas the newest IUPAC definition was published very recently [26]. Most frequently H-bonding is experimentally described by geometry parameters [28, 32] - results of X-ray and neutron diffraction measurements, but NMR and IR/Raman spectroscopies are also in frequent use. Characteristic of interactions by H-bonding is usually discussed in terms of energies [29-31], with use of various quantum chemical theories [54-57] and applications of various models as AIM [35, 41, 42, 45-48] and NBO [43, 44] which allowed to formulate detailed criteria for H-bond characteristics [35, 48]. H-bonds are classified as strong, mostly covalent in nature [7, 29, 34], partly covalent of medium strength [35] and weak ones, usually non-covalent [7, 29, 34, 35]. Theoretical studies of H-bonding mainly concern equilibrium systems, however simulation of H-bonded complexes with controlled and gradually changing strength of interactions [61-71] are also performed. The latter is main source of data referring to effect of H-bonding on structural properties: changes in the region of interactions, short and long-distance consequences of H-bonding. Application of the model [61] based on approaching hydrofluoric acid to the basic center of a molecule and fluoride to the acidic one, (Schemes 2 and 3) allows to study changes in molecular structure of para-substituted derivatives of phenol and phenolate [62, 64] in function of dB…H, or other geometric parameter of H-bond strength (Fig. 2). It is also shown that CO bond lengths in these complexes is monotonically related to H-bond formation energy and deformation energy due to H-bond formation [65]. Alike studies carried out for para-substituted derivatives of aniline and its protonated and deprotonated forms [77, 78, 81] give similar picture (Fig. 3). AIM studies of anilines [77, 78] lead to an excellent dependence of logarithm of electron density in the bond critical point and geometric parameter of H-bond strength, dB…H presented in Figure 4. Substituents and H-bond formation affect dramatically geometry of amine group [66] in H-bonded complexes of aniline as shown by changes of pyramidalization of bonds in amine group (Fig. 5). Some short- and long-distance structural consequences of H-bonding are shown by means of changes in ipso angle (for amine group) in the ring and ipso-ortho CC bond lengths (Fig. 6). Moreover, the mutual interrelations are in line with the Bent-Walsh rule [84, 86]. Changes of the strength of H-bonds in complexes of p-substituted aniline and its protonated and deprotonated derivative are dramatically reflected by aromaticity of the ring66 estimated by use of HOMA index [87, 88] (Fig. 7), where strength of H-bonding is approximated by CN bond lengths. Scheme 4 presents application of the SESE [91] (Substituent Effect Stabilization Energy) for description in an energetic scale joint substituent and H-bond formation effects.

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Ilościowe kryteria aromatyczności [Cz.2 art.: Aromatyczność : podstawowe pojęcia współczesnej chemii organicznej (Wiad.Chem. 2000, Nr 5-6)]

Cyrański M.K., Krygowski T.M.

Wiadomości Chemiczne

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2000

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[Z] 54, 7-8

533-564

EN

Aromataic character is manifested in p-electron systems by particular physicochemical properties: an increase of stability, averaging of bond lengths, particular magnetic properties and chemical reactivity preffering retention of the p-electron structure. These properties are used for definitions of quantative measures of aromaticity (indices of aromaticity). In principle they do not always predict the aromatic character in a uniform way. Additionally each of the used criteria is biased by some inadequacies or lack of generality. The energetic criterion defined as resonance energy or aromatic stabilisation energy measures the total aromaticity and strongly depends on the choice of reference states and/or reactions,). The same is true for the magnetic criterion - exaltation of the diamagnetic susceptibility. Geometric parameters seem to be the most general and may be used for estimation of both local and global aromatic character. Each of the criteria may be used providing a proper reference state can be defined. Application of variously defined indices of aromaticity is critcally discussed.

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Aromatyczność - podstawowe pojęcia współczesnej chemii organicznej

Cyrański M.K., Krygowski T.M.

Wiadomości Chemiczne

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2000

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[Z] 54, 5-6

357-369

EN

Aromaticity is one of the most often used terms in chemistry. It is not a single property, but a multidimensional phenomenon which can be defined only by convention. Various typical characteristics of aromaticity not always occur to be equivalent. As a ground for these definitions it is usually accepted that aromatic compounds are those cyclic p-electron systems which exhibit the following properties: (i) They are more stable than the non-cyclic analogues; (ii) Their bond lengths are intermediate between the typical double and single bonds; (iii) They exhibit special magnetic properties: in the external field the p-electron ring current is induced. Sometimes an additional criterion is postulated: the aromatic systems react in the way to retain their p-electron structure. Most often it means that the substitution is preffered over the addition reaction. While the first three criteria may be transformed into quantitative parameters called aromaticity indices, reactivity can be used only in a qualitative way.