14 research outputs found
Why 1,2‑quinone derivatives are more stable than their 2,3‑analogues?
In this work, we have studied the relative stability
of 1,2- and 2,3-quinones. While 1,2-quinones have
a closed-shell singlet ground state, the ground state for
the studied 2,3-isomers is open-shell singlet, except for
2,3-naphthaquinone that has a closed-shell singlet ground
state. In all cases, 1,2-quinones are more stable than their
2,3-counterparts. We analyzed the reasons for the higher
stability of the 1,2-isomers through energy decomposition
analysis in the framework of Kohn–Sham molecular orbital
theory. The results showed that we have to trace the origin
of 1,2-quinones’ enhanced stability to the more efficient
bonding in the π-electron system due to more favorable
overlap between the SOMOπ of the ·C4n−2H2n–CH·· and
··CH–CO–CO· fragments in the 1,2-arrangement. Furthermore,
whereas 1,2-quinones present a constant trend with their elongation for all analyzed properties (geometric,
energetic, and electronic), 2,3-quinone derivatives present a
substantial breaking in monotonicity.European
Union in the framework of European Social Fund through the Warsaw
University of Technology Development Programme. O.A. S., H.
S. and T.M. K
The electron density of delocalized bonds (EDDB) applied for quantifying aromaticity
In this study the recently developed electron density of delocalized bonds (EDDB) is used to define a new measure of aromaticity in molecular rings. The relationships between bond-length alternation, electron delocalization and diatropicity of the induced ring current are investigated for a test set of representative molecular rings by means of correlation and principal component analyses involving the most popular aromaticity descriptors based on structural, electronic, and magnetic criteria. Additionally, a qualitative comparison is made between EDDB and the magnetically induced ring-current density maps from the ipsocentric approach for a series of linear acenes. Special emphasis is given to the comparative study of the description of cyclic delocalization of electrons in a wide range of organic aromatics in terms of the kekulean multicenter index KMCI and the newly proposed EDDBk indexThe research was supported in part by the Faculty of Chemistry
at Jagiellonian University (grant K/DSC/001469, DS), Foundation
for Polish Science (FNP START 2015, stipend 103.2015, DS),
National Science Centre, Poland (NCN SONATA, grant 2015/17/
D/ST4/00558, DS) as well as the PL-Grid Infrastructure of the
Academic Computer Centre CYFRONET with the calculations
performed on the cluster platform ‘‘Prometheus’’. MS thanks for
the support of the Ministerio de Economa y Competitividad of
Spain (Project CTQ2014-54306-P), Generalitat de Catalunya (project
number 2014SGR931, Xarxa de Refere`ncia en Qumica Teo`rica i
Computacional, and ICREA Academia prize), and European
Fund for Regional Development (FEDER grant UNGI10-4E-801
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Cooperativity effect in noncovalent interactions of selected molecular complexes stabilised by hydrogen and halogen bonds
Among various so-called weak interactions, a halogen bond [8 and references
therein] is currently probably one of more explored by researchers. This is due to the
fact that it has several properties in common with the hydrogen bonding, and thus,
similarly as already well characterised H-bond, it may have a crucial role in different
physical, chemical, and biological processes. This bond is formed due to stabilising
interactions between a region of positive charge located on a surface of the halogen
atom and the other atomic center possessing the electron charge surplus (e.g. a lone
pair) [8]. The region of positive charge appears on the halogen atom surface due
to deformation of its electron cloud resulting in its ellipsoidal shape with the short
axis opposite the covalent bond and the long axis in the perpendicular direction
[11]. This results in a particular distribution of local charges on the atomic surface,
as shown in Figure 1. As a consequence the halogen atom may exhibit a dual character,
acting as either electron charge donor or acceptor, depending on the type of
interaction and the direction of the appearing interactomic contact. A good example
of such situation is shown in Figure 2. Thus, one may consider the situation when
two interactions are formed simultaneously and the halogen atom acts as an electron
charge donor and acceptor at the same time. For such situation the synergism
of both interactions may strengthen complexation. In order to analyze that case,
various representative complexes were investigated [13, 17, 18, 20, 21] by means
of many-body interaction approach [5, 6]. In general, it appears that as distinct to
hydrogen bond [2–4], the synergism is rather weak, with some exceptions for iodine
atom due to stronger halogen bonds formed by that atomic centre [13, 17, 18]. In the
case of halo-amine tetramers [21] the additional stabilising effect derived from back
bonding of π type was found – for the first time for a halogen bond
Lack of Cooperativity in the Triangular X<sub>3</sub>Halogen-Bonded Synthon?
We have investigated 44 crystal structures, found in the Cambridge Structural Database, containing the X3 synthon (where X = Cl, Br, I) in order to verify whether three type II halogen-halogen contacts forming the synthon exhibit cooperativity. A hypothesis that this triangular halogen-bonded motif is stabilized by cooperative effects is postulated on the basis of structural data. However, theoretical investigations of simplified model systems in which the X3 motif is present demonstrate that weak synergy occurs only in the case of the I3 motif. In the present paper we computationally investigate crystal structures in which the X3 synthon is present, including halomesitylene structures, that are usually described as being additionally stabilized by a synergic interaction. Our computations find no cooperativity for halomesitylene trimers containing the X3 motif. Only in the case of two other structures containing the I3 synthon a very weak or weak synergy, i.e. the cooperative effect being stronger than -0.40 kcal mol-1, is found. The crystal structure of iodoform has the most pronounced cooperativity of all investigated systems, amounting to about 10% of the total interaction energy
Aromaticity Induced by Electric Field: The Case of Polycalicenes
Local and global π-electron
delocalization occurring in planar
poly-1,7-[<i>N</i>]calicenes is investigated with use of
10 aromaticity measures based on different physical properties. Systematic
change of aromatic character is observed along chains of connected
calicene units. Multidimensionality of the aromaticity phenomenon
is studied with use of principal component analysis (PCA). The structural
characteristics are compared with the properties of the isolated calicene
molecule exposed to external electric fields of various intensities.
Interrelations between the value of electric field applied and physical
properties of the calicene molecule are discussed in the context of
calicene unit affected by its surroundings in polycalicene chains.
The patterns of global π-electron delocalization are described
in graph theory terminology, and interconnections between local and
global aromaticity in these systems are established
Fluoride Anions: Unexploited but Effective Halogen Bond Acceptors
Due to their high electron density, fluoride anions can be considered the most effective halogen bond (HaB) acceptors among the halides. However, under common experimental conditions, F- uncommonly acts as HaB acceptor, expectedly as it is present in hydrated form. Herein we report that under specific crystallization conditions a hydrogen bond-free F- functioning as donor of electron density can be obtained, with the formed HaBs constituting the driving force of the observed crystal packings. Computations confirm the strength of these HaBs compared to analogous interactions involving other halides
Aromaticity of acenes : the model of migrating -circuits
The concept of migrating Clar's sextet is extended to explain the local aromaticity trends in linear acenes predicted by different aromaticity criteria from theoretical calculations as well as from experimental data. The electron density of delocalized bonds is used to assess the link between resonance and reactivity and to rationalize the constant-height AFM image of pentacen
The role of the long-range exchange corrections in the description of electron delocalization in aromatic species
In this article, we address the role of the long-range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange-correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long-standing debate in the literature. The presented results indicate that the noncorrected exchange-correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteriaThe research was supported in part by the Foundation for Polish Science (FNP START 2015, stipend 103.2015, DS), National
Science Centre, Poland (NCN SONATA, grant 2015/17/D/ST4/ 00558, DS) as well as the PL-Grid Infrastructure of the Academic Computer Centre CYFRONET, with the calculations performed on cluster platforms Zeus and Prometheus. MS thanks for the support of the Ministerio de Economía y Competitividad of Spain (Project CTQ2014-54306-P), Generalitat de Catalunya (project number 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, and ICREA Academia prize), and European Fund for Regional Development (FEDER grant UNGI10-4E-801
The role of the long-range exchange corrections in the description of electron delocalization in aromatic species
In this article, we address the role of the long-range exchange corrections in description of the cyclic delocalization of electrons in aromatic systems at the density functional theory level. A test set of diversified monocyclic and polycyclic aromatics is used in benchmark calculations involving various exchange-correlation functionals. A special emphasis is given to the problem of local aromaticity in acenes, which has been a subject of long-standing debate in the literature. The presented results indicate that the noncorrected exchange-correlation functionals significantly overestimate cyclic delocalization of electrons in heteroaromatics and aromatic systems with fused rings, which in the case of acenes leads to conflicting local aromaticity predictions from different criteriaThe research was supported in part by the Foundation for Polish Science (FNP START 2015, stipend 103.2015, DS), National
Science Centre, Poland (NCN SONATA, grant 2015/17/D/ST4/ 00558, DS) as well as the PL-Grid Infrastructure of the Academic Computer Centre CYFRONET, with the calculations performed on cluster platforms Zeus and Prometheus. MS thanks for the support of the Ministerio de Economía y Competitividad of Spain (Project CTQ2014-54306-P), Generalitat de Catalunya (project number 2014SGR931, Xarxa de Referència en Química Teòrica i Computacional, and ICREA Academia prize), and European Fund for Regional Development (FEDER grant UNGI10-4E-801