On the extension of the huckel rule to polycyclic nonalternant conjugated hydrocarbons article pdf available in canadian journal of chemistry 5411. An alternant hydrocarbon is any conjugated hydrocarbon system which does not possess an oddmembered ring. An aromatic hydrocarbon or arene or sometimes aryl hydrocarbon is a hydrocarbon with sigma bonds and delocalized pi electrons between carbon atoms forming a circle. Pdf chemical and electronic properties of polycyclic. In alternant hydrocarbons, the bonding and antibonding orbitals occur in pairs. Thirdorder nonlinear optical properties of asymmetric nonalternant openshell condensedring hydrocarbons. Scribd is the worlds largest social reading and publishing site. The optical properties and chemistry of alternant and nonalternant hydrocarbons vary widely with respect to each other. Nonalternant aromaticity and partial double bond in. Aromaticity and substituent effects are among the most im. Aromaticity of nonalternant annulenoannulenes and of. Wu and colleagues describe the challenging synthesis and physical characterization of a macrocyclic octaradicaloid and decaradicaloid.
Disjoint molecular orbitals in nonalternant conjugated. We investigate the role of kekule and nonkekule structures in the radical character of alternant polycyclic aromatic hydrocarbons pahs using thermallyassistedoccupation density functional theory taodft, an efficient electronic structure method for the study of large groundstate systems with strong static correlation effects. One goes to the hydrogen atom, and one each to the two neighboring carbons. Aromaticity in benzenoid and nonbenzenoid compounds, alternant and nonalternant hydrocarbons, huckels rule, annulenes, antiaromaticity, homoaromaticity. In its simplest form, resonance delocalization of azulene yields an. Most frequently, nonbranded products at affordable prices get negative attention in this respect, but products from renowned companies may contain pahs as well. Many of the compounds have a sweet scent, hence the term aromatic. Furthermore, non alternant hydrocarbons and their derivatives have a number of physiological proper ties such as antipyretic and antiinflamatory.
Double bond fixation in nonalternant nelectron systems 1221 discussion for alternant hydrocarbons secondorder bond fixation becomes important only for the higher members of the series. Jul 26, 2016 we investigate the role of kekule and nonkekule structures in the radical character of alternant polycyclic aromatic hydrocarbons pahs using thermallyassistedoccupation density functional. If and only if the molecule in question meets the criteria for being either aromatic or antiaromatic i. Yale university 1991 submitted to the department of chemistry in partial fulfillment of the requirements. Pdf a structural bridge between alternant and nonalternant. Alternant hydrocarbons display three very interesting properties. The clarsextet and annulene models of aromaticity are applicable to benzenoid and nonbenzenoid conjugation, respectively, and are thus seldom used in conjunction. In this letter, we derive a simple rule that governs the occurrence of qi in weakly coupled dimer stacks of both alternant and nonalternant polyaromatic hydrocarbons pahs and extends the tada. Balaban department of organic chemistry, the polytechnic, bucharest, roumania abstract a brief historical tabellar survey reveals two intensive research periods in aromaticity, closely following the two major theoretical breakthroughs kekules formula, and huckeis rule. Revisit to the electron density, bond order, and aromaticity haruo hosoya, maki shobu, keiko takano, and yukari fujii department of chemistry, ochanomizu university, bunkyoku, tokyo 112, japan abstractspatial distribution of electrons in a molecule is analyzed by several different approaches, i. Anomalous effect of nonalternant hydrocarbons on carbocation and carbanion electronic configurations. High temperature behavior of alternant and nonalternant polycyclic aromatic hydrocarbons. The influence of the substituents on the proton chemical shifts in the benzene ring has also been investigated for many years and again there is still no quantitative calculation of these effects.
Thirdorder nonlinear optical properties of asymmetric non alternant openshell condensedring hydrocarbons. Ideas of aromaticityantiaromaticity to explain the stability of cyclic, conjugated systems having a closedshell singlet configuration was first. On the relations between aromaticity and substituent effect. Alternant and nonalternant hydrocarbons delocalized chemical.
Disjoint molecular orbitals in nonalternant conjugated diradial hydrocarbons. The observed isomers are nonaromatic, due to nonplanarity, not. A structural bridge between alternant and nonalternant. Nonalternant hydrocarbons invariably feature at least one ring constructed with an odd number of carbon atoms, whereas alternant hydrocarbons have no such. Bonding weaker than covalent eda complexes, addition compounds, crown ether complexes and cryptates, inclusion.
In this letter, we derive a simple rule that governs the occurrence of qi in weakly coupled dimer stacks of both alternant and non alternant polyaromatic hydrocarbons pahs and extends the tada. Nonalternant aromaticity and partial double bond in petroleum. Chemical and electronic properties of polycyclic aromatic hydrocarbons. In this unit, you will learn more about hydrocarbons. The gasoline and diesel fuel that are used in cars, trucks, and. Macrocyclic polyradicaloids with unusual superring structure. These non alternant molecules provide a bridge between alternant and non alternant hydrocarbons.
Distribution of alternant and non alternant polycyclic. Effects of diradical character, asymmetricity, and exchange interaction. Distribution of alternant and non alternant polycyclic aromatic hydrocarbons in sediments and clams of the pialassa baiona lagoon ravenna, italy. A simple set of trimethylenesubstituted even, fullypbonded, non alternant monocycles is shown to have several key features in common with acyclic, even alternant polyenes at the huckel level. On the relations between aromaticity and substituent. Physical, chemical, and some biological properties of strained ring hydrocarbons. Azulene has several unique characteristics that distinguish it from its aromatic bicyclic c 10h 8 isomer, naphthalene. They have a relatively low solubility in water, but are highly lipophilic. Whether in mouse pads, toys, or bathing shoes polycyclic aromatic hydrocarbons pahs are ubiquitous.
Aromaticity of nonalternant annulenoannulenes and of corannulenes. Other authors consider pahs to start with the tricyclic species phenanthrene and anthracene. Synthetic approaches to cyclopentaaphenalene by katherine lin lee b. Antiaromaticity is a characteristic of a cyclic molecule with a. Both alternant and nonalternant pah are present in petroleum molecules, and the significance of nonalternant hydrocarbons in disrupting electron distribution of aromatic hydrocarbons was revealed by quantifying local aromaticity using nucleusindependent chemical shift nics, complemented by the qualitative prediction with the clars sextet. Cyclic even alternant hydrocarbons, australian journal of chemistry on deepdyve, the largest online rental service for scholarly research with thousands of academic publications available at your fingertips. Recently, new kekulenerelated molecules, septulene, which is a nonalternant benzenoid hydrocarbon, and a tetracyclopentatetraphenylene tcptp derivative belonging to nonalternant nonbenzenoid hydrocarbons, were synthesized. Pseudoazulenes as isoelectronic analogues of nonbenzenoid. Non alternant hydrocarbons are considerably richer in the range of their aromatic behaviour that the alternants, and thus they represent a much greater challenge to any model of aromaticity. It has alternating double and single bonds between carbon atoms forming rings. It can be observed that all these compounds contain a benzene ring. A structural bridge between alternant and nonalternant hydrocarbons.
The obtained hose values were found to be in a good correlation with the re values obtained by hess and schaad for alternant hydrocarbons and nonalternant species, with correlation coefficients, cc, 0. Atomic displacements due to spinspin repulsion in conjugated alternant hydrocarbons. Aromatic hydrocarbon simple english wikipedia, the free. This article presents theoretical and experimental aspects of kekulenerelated molecules focusing on. Aromatic hydrocarbons are circularly structured organic compounds that contain sigma bonds along with delocalized pi electrons. Role of kekule and nonkekule structures in the radical. Jan 18, 2008 read a huckellevel kineticstabilitybased approach to aromaticity. The ring of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible hydrocarbon ring, benzene.
By definition, polycyclic aromatic hydrocarbons have multiple cycles, precluding benzene from being considered a pah. They are also used as the starting materials for manufacture of many dyes and drugs. An aromatic hydrocarbon, or arene, is a hydrocarbon ring compound. A few examples of aromatic hydrocarbons are provided below. A simple set of trimethylenesubstituted even, fullypbonded, nonalternant monocycles is shown to have several key features in common with acyclic, even alternant polyenes at the huckel level. In contrast, aliphatic hydrocarbons lack this delocalization. Interplay of aromaticity and antiaromaticity in ndoped nanographenes.
A second classification among pahs is the division into alternant and nonalternant pahs. Winter department of chemistry, iowa state university, 1608 gilman hall, ames, ia 50010, usa. In alternant pahs, the carbon atoms can be assigned as starred s and unstarred u with each scarbon having only u neighbours and vice versa figure 2. The clarsextet and annulene models of aromaticity are applicable to benzenoid and non benzenoid conjugation, respectively, and are thus seldom used in conjunction. A second classification among pahs is the division into alternant and non alternant pahs. Zahradnik mo calculations for nonalternant systems it may be seen clearly that during the past 150 years the temporal density of discoveries and research in the field of aromaticity was far from uniform. Breakdown of interference rules in azulene, a nonalternant.
An alternant hydrocarbon is any conjugated hydrocarbon system which does not possess an. Aromaticity in benzenoid and non benzenzoid compounds, alternant non alternant hydrocarbons, huckle rule,energy level of pmolecular orbitals, annulenes, anti aromaticity,homo aromaticity,pmo approach. Anomalous effect of nonalternant hydrocarbons on carbocation and. Thirdorder nonlinear optical properties of asymmetric non. By comparison, the isomeric aromatic hydrocarbon, naphthalene, which, like azulene, is an aromatic molecule with 10. Polycyclic aromatic hydrocarbons problem chemicals. Pdf on the extension of the huckel rule to polycyclic non. Polycyclic aromatic hydrocarbons pahs constitute a large class of organic.
Aromatic hydrocarbons can be divided into alternant and. Macrocyclic polyradicaloids with unusual superring. Coronoid aromatic molecules, containing a macrocyclic array of contiguously fused rings, are one class of molecules in which both of these descriptions are of high significance. Most of the pahs with low vapour pressure in the air are adsorbed on particles. Bond distortions in nonalternant hydrocarbons takeshi nakajima department of chemistry, faculty of science, tohoku university. In an alternate hydrocarbons, the mo energy levels are symmetrically paired above the e.
They exhibit the longsoughtafter annulenewithinanannulene superring structure and unusual global aromaticity with individual inner and outer rings following huckels rule in the singlet state and bairds rule in the triplet state, respectively. Naphthalene, with its two rings, is considered the simplest polycyclic aromatic hydrocarbon by us epa and cdc for policy contexts. Article in journal of chemical information and computer sciences 435. The term aromatic was assigned before the physical mechanism determining aromaticity was discovered. Each carbon atom in the hexagonal cycle has four electrons to share. For such systems it is possible to undertake a starring process, in which the carbon atoms are divided into two sets. Double bond fixation in non alternant nelectron systems 1221 discussion for alternant hydrocarbons secondorder bond fixation becomes important only for the higher members of the series. They are also referred to as arenes or aryl hydrocarbons. Alternant and nonalternant hydrocarbons delocalized. Realworld reading link if you have ridden in a car or a bus, you have used. These nonalternant molecules provide a bridge between alternant and nonalternant hydrocarbons. Both alternant and nonalternant pahs are present in petroleum molecules, and the significance of nonalternant hydrocarbons in disrupting electron distribution of aromatic hydrocarbons was revealed by quantifying local aromaticity using nucleusindependent chemical shift, complemented by the qualitative prediction with clars sextet theory. Aromatic hydrocarbons free download as powerpoint presentation.
Chapter pdf available september 2011 with 991 reads how we measure reads. Thus, you can well understand the importance of hydrocarbons in your daily life. Nonalternant hydrocarbons invariably feature at least one. Aromaticity in benzenoid and non benzenoid compounds, alternant and non alternant hydrocarbons, huckels rule, energy level of molecular orbitals, annulenes, antiaromaticity, aromaticity, homo aromaticity, pmo approach. Anomalous effect of nonalternant hydrocarbons on carbocation. High temperature behavior of alternant and nonalternant. Naphthalene is an alternant and azulene a nonalternant hydrocarbon. Aromaticity in benzenoid and non benzenzoid compounds, alternant nonalternant hydrocarbons, huckle rule,energy level of pmolecular orbitals, annulenes, antiaromaticity,homoaromaticity,pmo approach. Realworld reading link if you have ridden in a car or a bus, you have used hydrocarbons.
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