Topological Ring-Current and Bond-Current Properties of theAltansof CertainK-Factorizable Conjugated Systems Containing “Fixed” Single-Bonds

2015 ◽  
Vol 119 (20) ◽  
pp. 5019-5025 ◽  
Author(s):  
Timothy K. Dickens ◽  
Roger B. Mallion
Keyword(s):  
Ring Current ◽  
Topological Ring ◽  
Conjugated Systems ◽  
Single Bonds

scholarly journals Rotational Barrier and Conjugation: Theoretical Study of Resonance Stabilization of Various Substituents for the Donors NH2 and OCH3 in Substituted 1,3-Butadienes

2019 ◽  
Vol 19 (4) ◽  
pp. 1055
Author(s):  
Ali Hussain Yateem
Keyword(s):  
Density Functional ◽  
Theoretical Study ◽  
Amino Nitrogen ◽  
Conjugated Systems ◽  
Rotational Barriers ◽  
Out Of Plane ◽  
Resonance Stabilization ◽  
The Difference ◽  
Internal Rotational Barriers ◽  
Single Bonds

The barrier to internal rotation around the central C2–C3 single bond of a series of (1E)-monosubstituted 1,3-butadienes and (1E,3E)-1-Y-4-X-disubstituted butadienes, with Y=NH2 or OCH3 and X=NO2, CHO, COOH, CN, CF3, Cl or F, were studied at the density functional w B97X-D/6-31G∗∗ level. The effect of substituents on π-conjugation in disubstituted 1,3-butadienes was studied by correlating the calculated internal rotational barriers with the difference in structural, atomic and molecular properties between the transition state TS and the s-trans conformers. The calculated differences in lengths of C–C, C–NH2 and C–OCH3 single bonds, N-H-N, and C-O-CH3 angles, NH2 out-of-plane angle, natural charges on amino nitrogen and methoxy oxygen, and the maximum electrostatic potential on amino hydrogens, were found to correlate strongly with the rotational barriers. The conjugative interaction was strongly stabilized in the case of strong π-electron acceptors such as NO2 or CHO and is slightly or negligibly affected with Cl and F groups. The resonance stabilization with the remaining acceptors decreases in the order COOH > CN > CF3. Acceptors X maintain their relative order of stabilization for the two donors, and NH2 is more stabilizing. Dominant resonance structures are suggested for highly and negligibly conjugated systems.

scholarly journals Some Observations on Triplet Ground-States in the Context of ‘Topological’ (HLPM) Ring-Currents in Conjugated Systems

2019 ◽  
Vol 92 (4) ◽  
pp. 445-455
Author(s):  
Timothy K. Dickens ◽  
Roger B. Mallion
Keyword(s):  
Ground State ◽  
Topological Ring ◽  
State Problem ◽  
Triplet Ground State ◽  
Conjugated Systems ◽  
Molecular Graphs ◽  
Ring Currents ◽  
Electron Ring ◽  
State Configuration ◽  
Electronic Ground

When the quasi graph-theoretical Hückel–London–Pople–McWeeny (HLPM) approach is used to calculate ‘topological’ π-electron ring-currents and bond-currents in conjugated hydrocarbons, a problem is identified that occurs whenever application of the Aufbau process gives rise to a π-electronic ground-state configuration that is a triplet. This circumstance seems to occur only occasionally and, even when it does, the generally somewhat outré molecular graphs in question appear unlikely to represent extant or viable conjugated systems. The molecular graphs of four examples are used to illustrate this ‘triplet ground-state problem’, only one of which represents a hydrocarbon that has actually been synthesised. It is pointed out that the ‘triplet ground-state problem’ does constitute an intrinsic limitation of the HLPM approach. It is, though, a limitation that is also necessarily inherent in other equivalent (though ostensibly different) methods of calculating magnetic properties due to π-electron ring-currents — methods that are likewise founded on the Hückel molecular-orbital conventions. When a triplet ground-state arises, topological ring-currents and bond-currents cannot be calculated by the HLPM method and its equivalents. Infinite paramagnetism is formally to be predicted in such situations.

Some graph-theoretical aspects of simple ring current calculations on conjugated systems

1975 ◽  
Vol 341 (1627) ◽  
pp. 429-449 ◽  
Keyword(s):  
Unitary Transformation ◽  
Ring Current ◽  
Spanning Trees ◽  
Molecular Graph ◽  
Current Concept ◽  
Close Correspondence ◽  
Conjugated Systems ◽  
Ring Currents ◽  
Secular Equations ◽  
Simple Connected Graph

In this paper we examine the simple theory of π -electron ring currents in conjugated systems (devised originally by London (1937) and extended by People (1958) and McWeeny (1958)), with particular reference to the topological or graph-theoretical aspects of it (all the necessary graph-theoretical ideas and terminology are explained in the text). There is a close correspondence between the adjacency matrix of the graph representing the σ-bond skeleton of the carbon atoms comprising a given conjugated system, and the secular equations which arise in the theory (a relation now well known to be common to all formalisms based on Hückel ‘topological’ molecular orbitals), but in addition we here emphasize that several other graph-theoretical ideas–notably those concerning circuits and spanning trees–specifically underlie the ring current concept. In this connexion, the question of whether any given molecular graph is semi-Hamiltonian or non-Hamiltonian is of prime importance, and it is pointed out that a unitary transformation originally proposed by McWeeny applies to semi-Hamiltonian molecular graphs, whereas one recently devised by Gayoso & Boucekkine can be applied to any simple, connected graph–as also can an explicit ring current formula (based on the London–McWeeny theory) just published by the present author. These ideas are illustrated by some simple numerical calculations, and an example is given of a conjugated system (decacyclene) whose molecular graph is apparently non-Hamiltonian. It is emphasized that although much graph theory is inherent in the ring current concept, the ring current index itself is not a completely topological quantity–even when a purely topological wavefunction (such as the simple Hückel one) has been used to calculate it.

Ring current model of the naphthalene molecule

1997 ◽  
Vol 92 (3) ◽  
pp. 609-617 ◽  
Author(s):  
RICCARDO ZANASI ◽  
PAOLO LAZZERETTI
Keyword(s):  
Ring Current ◽  
Current Model ◽  
Naphthalene Molecule

Investigation of Ring Current/Storm Dynamics

1991 ◽  
Author(s):  
H. L. Collin ◽  
J. B. Cladis ◽  
J. M. Quinn
Keyword(s):  
Ring Current ◽  
Storm Dynamics

Survey of Ring Current Composition During Magnetic Storms

1998 ◽  
Author(s):  
M. Grande ◽  
C. H. Perry ◽  
A. Hall ◽  
J. Fennell ◽  
B. Wilken
Keyword(s):  
Ring Current ◽  
Magnetic Storms
Sign in / Sign up

Export Citation Format

Share Document