On the Harmonic Oscillator Model of Electron Delocalization (HOMED) Index and Its Application to Heteroatomic Π-Electron Systems
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Planar Cyclopenten‐4‐Yl Cations: Highly Delocalized Π Aromatics
Angewandte Research Articles Chemie How to cite: Angew.Chem. Int. Ed. 2020, 59,18809–18815 Carbocations International Edition: doi.org/10.1002/anie.202009644 German Edition: doi.org/10.1002/ange.202009644 Planar Cyclopenten-4-yl Cations:Highly Delocalized p Aromatics Stabilized by Hyperconjugation Samuel Nees,Thomas Kupfer,Alexander Hofmann, and Holger Braunschweig* 1 B Abstract: Theoretical studies predicted the planar cyclopenten- being energetically favored by 18.8 kcalmolÀ over 1 (MP3/ 4-yl cation to be aclassical carbocation, and the highest-energy 6-31G**).[11–13] Thebishomoaromatic structure 1B itself is + 1 isomer of C5H7 .Hence,its existence has not been verified about 6–14 kcalmolÀ lower in energy (depending on the level experimentally so far.Wewere now able to isolate two stable of theory) than the classical planar structure 1C,making the derivatives of the cyclopenten-4-yl cation by reaction of bulky cyclopenten-4-yl cation (1C)the least favorable isomer.Early R alanes Cp AlBr2 with AlBr3.Elucidation of their (electronic) solvolysis studies are consistent with these findings,with structures by X-raydiffraction and quantum chemistry studies allylic 1A being the only observable isomer, notwithstanding revealed planar geometries and strong hyperconjugation the nature of the studied cyclopenteneprecursor.[14–18] Thus, interactions primarily from the C Al s bonds to the empty p attempts to generate isomer 1C,orits homoaromatic analog À orbital of the cationic sp2 carbon center.Aclose inspection of 1B,bysolvolysis of 4-Br/OTs-cyclopentene -
Introduction to Aromaticity
Introduction to Aromaticity Historical Timeline:1 Spotlight on Benzene:2 th • Early 19 century chemists derive benzene formula (C6H6) and molecular mass (78). • Carbon to hydrogen ratio of 1:1 suggests high reactivity and instability. • However, benzene is fairly inert and fails to undergo reactions that characterize normal alkenes. - Benzene remains inert at room temperature. - Benzene is more resistant to catalytic hydrogenation than other alkenes. Possible (but wrong) benzene structures:3 Dewar benzene Prismane Fulvene 2,4- Hexadiyne - Rearranges to benzene at - Rearranges to - Undergoes catalytic - Undergoes catalytic room temperature. Faraday’s benzene. hydrogenation easily. hydrogenation easily - Lots of ring strain. - Lots of ring strain. - Lots of ring strain. 1 Timeline is computer-generated, compiled with information from pg. 594 of Bruice, Organic Chemistry, 4th Edition, Ch. 15.2, and from Chemistry 14C Thinkbook by Dr. Steven Hardinger, Version 4, p. 26 2 Chemistry 14C Thinkbook, p. 26 3 Images of Dewar benzene, prismane, fulvene, and 2,4-Hexadiyne taken from Chemistry 14C Thinkbook, p. 26. Kekulé’s solution: - “snake bites its own tail” (4) Problems with Kekulé’s solution: • If Kekulé’s structure were to have two chloride substituents replacing two hydrogen atoms, there should be a pair of 1,2-dichlorobenzene isomers: one isomer with single bonds separating the Cl atoms, and another with double bonds separating the Cl atoms. • These isomers were never isolated or detected. • Rapid equilibrium proposed, where isomers interconvert so quickly that they cannot be isolated or detected. • Regardless, Kekulé’s structure has C=C’s and normal alkene reactions are still expected. - But the unusual stability of benzene still unexplained. -
Aromaticity Sem- Ii
AROMATICITY SEM- II In 1931, German chemist and physicist Sir Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties .This is a very popular and useful rule to identify aromaticity in monocyclic conjugated compound. According to which a planar monocyclic conjugated system having ( 4n +2) delocalised (where, n = 0, 1, 2, .....) electrons are known as aromatic compound . For example: Benzene, Naphthalene, Furan, Pyrrole etc. Criteria for Aromaticity 1) The molecule is cyclic (a ring of atoms) 2) The molecule is planar (all atoms in the molecule lie in the same plane) 3) The molecule is fully conjugated (p orbitals at every atom in the ring) 4) The molecule has 4n+2 π electrons (n=0 or any positive integer Why 4n+2π Electrons? According to Hückel's Molecular Orbital Theory, a compound is particularly stable if all of its bonding molecular orbitals are filled with paired electrons. - This is true of aromatic compounds, meaning they are quite stable. - With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level (the number of subsequent energy levels is denoted by n), leaving all bonding orbitals filled and no anti-bonding orbitals occupied. This gives a total of 4n+2π electrons. - As for example: Benzene has 6π electrons. Its first 2π electrons fill the lowest energy orbital, and it has 4π electrons remaining. These 4 fill in the orbitals of the succeeding energy level. The criteria for Antiaromaticity are as follows: 1) The molecule must be cyclic and completely conjugated 2) The molecule must be planar. -
Aromaticity in Polyhydride Complexes of Ru, Ir, Os, and Pt† Cite This: DOI: 10.1039/C5cp04330a Elisa Jimenez-Izala and Anastassia N
PCCP View Article Online PAPER View Journal r-Aromaticity in polyhydride complexes of Ru, Ir, Os, and Pt† Cite this: DOI: 10.1039/c5cp04330a Elisa Jimenez-Izala and Anastassia N. Alexandrova*ab Transition-metal hydrides represent a unique class of compounds, which are essential for catalysis, organic i À i À synthesis, and hydrogen storage. In this work we study IrH5(PPh3)2,(RuH5(P Pr3)2) ,(OsH5(PPr3)2) ,and À OsH4(PPhMe2)3 polyhydride complexes, inspired by the recent discovery of the s-aromatic PtZnH5 cluster À anion. The distinctive feature of these molecules is that, like in the PtZnH5 cluster, the metal is five-fold Received 23rd July 2015, coordinated in-plane, and holds additional ligands attheaxialpositions.Thisworkshowsthattheunusual Accepted 17th September 2015 coordination in these compounds indeed can be explained by s-aromaticity in the pentagonal arrangement, DOI: 10.1039/c5cp04330a stabilized by the atomic orbitals on the metal. Based on this newly elucidated bonding principle, we additionally propose a new family of polyhydrides that display a uniquely high coordination. We also report the first www.rsc.org/pccp indications of how aromaticity may impact the reactivity of these molecules. 1 Introduction many catalytic cycles, where they have either been used as catalysts or invoked as key intermediates.15,16 Aromaticity in Aromaticity is a very important concept in the contemporary general is associated with high symmetry, stability, and specific chemistry: the particular electronic structure of aromatic reactivity. Therefore, the stability of transition-metal hydride systems, where electrons are delocalized, explains their large complexes might have an important repercussion in the kinetics energetic stabilization and low reactivity. -
Bsc Chemistry
Subject Chemistry Paper No and Title Paper 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No and Module 3: Hyper-Conjugation Title Module Tag CHE_P1_M3 CHEMISTRY PAPER No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 3: Hyper-Conjugation TABLE OF CONTENT 1. Learning outcomes 2. Introduction 3. Hyperconjugation 4. Requirements for Hyperconjugation 5. Consequences and Applications of Hyperconjugation 6. Reverse Hyperconjugation 7. Summary CHEMISTRY PAPER No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No. 3: Hyper-Conjugation 1. Learning Outcomes After studying this module you shall be able to: Understand the concept of hyperconjugation. Know about the structural requirements in a molecule to show hyperconjugation. Learn about the important consequences and applications of hyperconjugation. Comprehend the concept of reverse hyperconjugation. 2. Introduction In conjugation, we have studied that the electrons move from one p orbital to other which are aligned in parallel planes. Is it possible for electron to jump from p orbital to sp3 orbital that are not parallelly aligned with one another? The answer is yes. This type of conjugation is not normal, it is extra-ordinary. Hence, the name hyper-conjugation. It is also know as no-bond resonance. Let us study more about it. 3. Hyperconjugation The normal electron releasing inductive effect (+I effect) of alkyl groups is in the following order: But it was observed by Baker and Nathan that in conjugated system, the attachment of alkyl groups reverse their capability of electron releasing. They suggested that alkyl groups are capable of releasing electrons by some process other than inductive. -
Sc-Homoaromaticity
This is a repository copy of Modern Valence-Bond Description of Homoaromaticity. White Rose Research Online URL for this paper: https://eprints.whiterose.ac.uk/106288/ Version: Accepted Version Article: Karadakov, Peter Borislavov orcid.org/0000-0002-2673-6804 and Cooper, David L. (2016) Modern Valence-Bond Description of Homoaromaticity. Journal of Physical Chemistry A. pp. 8769-8779. ISSN 1089-5639 https://doi.org/10.1021/acs.jpca.6b09426 Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. This is indicated by the licence information on the White Rose Research Online record for the item. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ Modern Valence-Bond Description of Homoaromaticity Peter B. Karadakov; and David L. Cooper; Department of Chemistry, University of York, Heslington, York, YO10 5DD, U.K. Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, U.K. Abstract Spin-coupled (SC) theory is used to obtain modern valence-bond (VB) descriptions of the electronic structures of local minimum and transition state geometries of three species that have been con- C sidered to exhibit homoconjugation and homoaromaticity: the homotropenylium ion, C8H9 , the C cycloheptatriene neutral ring, C7H8, and the 1,3-bishomotropenylium ion, C9H11. -
Synthesis and Reactivity of Cyclopentadienyl Based Organometallic Compounds and Their Electrochemical and Biological Properties
Synthesis and reactivity of cyclopentadienyl based organometallic compounds and their electrochemical and biological properties Sasmita Mishra Department of Chemistry National Institute of Technology Rourkela Synthesis and reactivity of cyclopentadienyl based organometallic compounds and their electrochemical and biological properties Dissertation submitted to the National Institute of Technology Rourkela In partial fulfillment of the requirements of the degree of Doctor of Philosophy in Chemistry by Sasmita Mishra (Roll Number: 511CY604) Under the supervision of Prof. Saurav Chatterjee February, 2017 Department of Chemistry National Institute of Technology Rourkela Department of Chemistry National Institute of Technology Rourkela Certificate of Examination Roll Number: 511CY604 Name: Sasmita Mishra Title of Dissertation: ''Synthesis and reactivity of cyclopentadienyl based organometallic compounds and their electrochemical and biological properties We the below signed, after checking the dissertation mentioned above and the official record book(s) of the student, hereby state our approval of the dissertation submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Chemistry at National Institute of Technology Rourkela. We are satisfied with the volume, quality, correctness, and originality of the work. --------------------------- Prof. Saurav Chatterjee Principal Supervisor --------------------------- --------------------------- Prof. A. Sahoo. Prof. G. Hota Member (DSC) Member (DSC) --------------------------- -
Structure, Stability, and Substituent Effects in Aromatic S-Nitrosothiols: the Crucial Effect of a Cascading Negative Hyperconjugation/Conjugation Interaction
Marquette University e-Publications@Marquette Chemistry Faculty Research and Publications Chemistry, Department of 10-23-2014 Structure, Stability, and Substituent Effects in Aromatic S-Nitrosothiols: The Crucial Effect of a Cascading Negative Hyperconjugation/Conjugation Interaction Matthew Flister Marquette University, [email protected] Qadir K. Timerghazin Marquette University, [email protected] Follow this and additional works at: https://epublications.marquette.edu/chem_fac Part of the Chemistry Commons Recommended Citation Flister, Matthew and Timerghazin, Qadir K., "Structure, Stability, and Substituent Effects in Aromatic S-Nitrosothiols: The Crucial Effect of a Cascading Negative Hyperconjugation/Conjugation Interaction" (2014). Chemistry Faculty Research and Publications. 360. https://epublications.marquette.edu/chem_fac/360 Marquette University e-Publications@Marquette Chemistry Faculty Research and Publications/College of Arts and Sciences This paper is NOT THE PUBLISHED VERSION; but the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation below. Journal of Physical Chemistry : A, Vol. 18, No. 42 (October 23, 2014): 9914–9924. DOI. This article is © American Chemical Society Publications and permission has been granted for this version to appear in e-Publications@Marquette. American Chemical Society Publications does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from American Chemical Society Publications. Structure, Stability, and Substituent Effects in Aromatic S-Nitrosothiols: The Crucial Effect of a Cascading Negative Hyperconjugation/Conjugation Interaction Matthew Flister Department of Chemistry, Marquette University, Milwaukee, Wisconsin Qadir K. Timerghazin Department of Chemistry, Marquette University, Milwaukee, Wisconsin Abstract Aromatic S-nitrosothiols (RSNOs) are of significant interest as potential donors of nitric oxide and related biologically active molecules. -
Contribution of Quantum Chemistry to the Study of Dienes and Polyenes
The Chemistry of Dienes and Polyenes. Volume 1 Edited by Zvi Rappoport Copyright ¶ 1997 John Wiley & Sons, Ltd. ISBN: 0-471-96512-X CHAPTER 1 Contribution of quantum chemistry to the study of dienes and polyenes V. BRANCHADELL, M. SODUPE, A. OLIVA and J. BERTRAN´ Departament de Qu´ımica, Universitat Autonoma` de Barcelona, 08193 Bellaterra, Spain Fax: (34)35812920; e-mail: [email protected] I. INTRODUCTION ..................................... 2 II. SURVEY OF THEORETICAL METHODS .................... 2 III. GROUND STATE STRUCTURE AND VIBRATIONAL SPECTRA .... 4 A. Butadiene ........................................ 4 1. Geometry ...................................... 4 2. Vibrational frequencies and force field ................... 5 3. Conformational equilibrium .......................... 6 B. Trienes and Tetraenes ................................ 7 1. Geometries and conformations ........................ 7 2. Vibrational frequencies and force constants ................ 9 C. Longer Polyenes .................................... 9 IV. EXCITED STATES .................................... 10 A. Butadiene ........................................ 11 B. Hexatriene ........................................ 13 C. Octatetraene ...................................... 14 D. Longer Polyenes .................................... 14 V. MOLECULAR ELECTRIC PROPERTIES ..................... 15 VI. CHEMICAL REACTIVITY .............................. 17 A. The Diels Alder Reaction ............................. 17 1. Reaction mechanism .............................. -
Effect of Hyperconjugation on Ionization Energies of Hydroxyalkyl Radicals
J. Phys. Chem. A 2008, 112, 9965–9969 9965 Effect of Hyperconjugation on Ionization Energies of Hydroxyalkyl Radicals Boris Karpichev, Hanna Reisler, Anna I. Krylov, and Kadir Diri* Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089-0482 ReceiVed: June 14, 2008; ReVised Manuscript ReceiVed: August 01, 2008 On the basis of electronic structure calculations and molecular orbital analysis, we offer a physical explanation of the observed large decrease (0.9 eV) in ionization energies (IE) in going from hydroxymethyl to hydroxyethyl radical. The effect is attributed to hyperconjugative interactions between the σCH orbitals of the methyl group in hydroxyethyl, the singly occupied p orbital of carbon, and the lone pair p orbital of oxygen. Analyses of vertical and adiabatic IEs and hyperconjugation energies computed by the natural bond orbital (NBO) procedure reveal that the decrease is due to the destabilization of the singly occupied molecular orbital in hydroxyethyl radical as well as structural relaxation of the cation maximizing the hyperconjugative interactions. The stabilization is achieved due to the contraction of the CO and CC bonds, whereas large changes in torsional angles bear little effect on the total hyperconjugation energies and, consequently, IEs. 1. Introduction bond strengths21 and conformations20 upon substitutions, the vibrational spectra and structures of hydrocarbon radicals,22,23 Hydroxyalkyl radicals are important intermediates in combus- 1,2 the trends in electronically excited states in alkylperoxy radi- tion and atmospheric processes. The most studied of these 24 3-12 cals, and more. There is also a strong evidence that the radicals are the hydroxymethyl and 1-hydroxyethyl radicals. -
Conjugated Molecules
Conjugated Molecules - Conjugated molecules have alternating single and multiple (i.e. double or triple) bonds. Example 1: Nomenclature: 2,6-dimethylhepta-2,5-diene This molecule is not conjugated because it does not have alternating single and multiple bonds (the arrangement of the bonds starting from C2 is double, single, single and double). Between the two double bonds, there is a saturated center (C4) and two intersecting single bonds. Example 2: Nomenclature: 2,5-dimethylhexa-2,4-diene This molecule is conjugated because the arrangement of the bonds starting at C2 is double, single, and double. They are alternated. Example 3: Nomenclature: (2E)-hept-2-en-5-yne note: (2E) is a stereochemical identifier. The letter “E” indicates the arrangement of the double bond (where E usually refers to trans and Z refers to cis). The number “2” indicates the position of the double bond. This molecule is not conjugated because the single and multiple bonds are not alternated. Example 4: Nomenclature: (2Z)-hex-2-en-4-yne This molecule is conjugated because there is an alternation of multiple and single bonds (double, single, and triple starting from C2). More examples: note: the term conjugation refers to parts of the molecule. If you can find one conjugated system within the molecule, that molecule is said to be conjugated. Example: In this molecule, the double bond A is not conjugated. However, since double bond B is conjugated with double bond C, the molecule is said to be conjugated. Special Nomenclature: The Letter "S" stands for “single” and indicates that we are talking about the conjugated double bonds. -
Orbital Interactions in Metal Dimer Complexes
4884 Orbital Interactions in Metal Dimer Complexes P. Jeffrey Hay, Jack C. Thibeault, and Roald Hoffmann* Contribution from the Department of Chemistry and Materials Science Center, Cornell University, Ithaca. New York 14853. Received January 9, 1975 Abstract: A molecular orbital analysis shows that the antiferromagnetic contributions to magnetic coupling, favoring a low- spin ground state for a dimer containing two weakly interacting metal centers, can be analyzed in terms of pairwise interac- tions of dimeric molecular orbitals, with the square of the splitting in energy between the members of a pair being a measure of the stabilization of the low-spin state. The effect of geometrical distortions, electronegativity, and variation of substituents on the magnetic interaction in dimeric systems is examined in detail for singly bridged L,M-X-ML, (n = 3,4, 5); Cu~C16~- and other doubly bridged species where the bridging ligands are halogens, OR, pyridine N-oxides, oxalate, squarate; and the acetate bridged dimers Cu~(RC00)4.The emphasis is on d9 Cu(I1) dimers, but other transition metal systems are also ana- lyzed. Transition metal complexes containing more than one mentslv'O,'iwhich seek to extend such analyses to the cases metal atom with unpaired electrons can generally be cate- involving molecular, rather than atomic bridging species, gorized according to their magnetic behavior into three with special interest in molecular dimers. Within this latter main groups depending on the strength of the metal-metal context this paper will attempt to provide a broader theoret- interaction. In the noninteracting type the magnetic proper- ical framework for the analysis of superexchange interac- ties of the dimer (or polymer) are essentially unchanged tions.