Density Function Theory on the Clar Structures of Polybenzenoid
Hydrocarbons and BN Analogues
Abstract: Clar structure of polybenzenoid hydrocarbons (PBHs) have attracted
considerable interest of both theoretical and experimental chemists since it observed in
the 1950s. And they have been widely applied in electronics and materials sciences.
However, whether existence Clar structure of BN-acenes (the carbon atoms of PBHs
are replaced by alternating boron and nitrogen atoms) is not very clear. Thus, clarifying
this issue is urgent. Here, we present thorough density functional theory (DFT)
calculations on aromaticity and thermodynamic energies for a series of Clar structures
of BN analogues. We choose electron localization function (ELFπ), nucleus
independent chemical shift (NICS) values and six-center bond as indices to determine
the aromaticity of BN heptacenes. Our results reveal that, in comparison with the Clar
structures of PBHs, the aromaticity and of BN-acenes are greatly reduced and even
have nonaromaticity. However, we found there are good linear relationships between
the [n]acenes number (n = 3-7) and relative Gibbs free energies between isomers of
containing one π-sextet and most π-sextet. And for different isomers, with the more
number of π-sextets, the more stable regardless of PBHs or BN analogues.
Key words: Clar structure; ELFπ; NICS; aromaticity; multicenter bonds
Introduction
Since Clar structure was observed in the 1990s and relative experiments report that
relative stabilities and UV/Vis absorption maxima of heptabenzenoid isomers,[1,2] in
addition, the maximum number of π-sextet was the most stable and had the most blueshifted UV/Vis absorption (λ = 295 nm) obtained from the experimental spectra.[2] Clar
structure have attracted increasing interest from both experimentalists and theoreticians.
Over the past decades, Eric Clar and other experimentalists prepared numerous new
polybenzenoid hydrocarbons (PBHs).[3] Especially graphene separated from graphite in
2004s,[4] it has been widely researched and applied as its special electron properties.[510]
However, in comparison with PBHs, the Clar structure of BN analogues relatively
unknown. As we know, BN analogues used as precursors for polymers and ceramics,[11]
several experiments had reported about borazanaphthalene.[12,13] In addition, there are
many theoretical reports about the aromaticity of borazine[14-21] and linear BNacenes.[16,22] While the aromaticity and stability of different π-sextets of BN heptacenes
have no report. Therefore, it is necessary to take measures to figure out these questions.
Here we present comprehensive density functional theory (DFT) calculations on the
aromaticity[23]and stability of BN-acenes (n=7).
As we know, aromaticity is an important concept associated with exceptional
geometric, energetic and magnetic properties[24] and several criterias have been
proposed as the defining characteristic of aromaticity. Avoiding calculation deviation,
we choose electron localization function (ELFπ),[25] nucleus independent chemical shift
(NICS)[24] values and multicenter bonds[26] as indices to determine the aromaticity of
BN heptacenes. According to Organic Chemistry textbooks[27] may be found in which
a compound is said aromatic meaning only “that its π electrons are delocalized over the
entire ring and that it is stabilized by the π-electron delocalization”. So all the
parameters are indicated π aromaticity of six member ring. For multicenter bonds, all
values are six-center π bond orders and all atomic numbers clockwise input with
Multiwfn package[28]. Commonly, we choose benzene as range standard, when the
larger the value, the greater the scale of aromaticity. At present, there are several
reports[26,29] about the method to measure aromaticity in PBHs, while for BN analogues
has little coverage. In this work, the second criterion based on topological analysis
properties of electron density, while π contribution to the electron localization function
(ELFπ) could perfectly display the aromatic degree of the molecular systems as was
discussed by Krygowski[30]. ELF could be interpreted as a measure of Pauli repulsion
in the atomic or molecular space and reflects the probability of finding opposite spin
electron pairs. The theoretical basis of this index is that the ELF value at bifurcation
point measures interaction between adjointing ELF domains, the larger value means
electrons have better delocalization between these domains, which is commonly
recognized as the nature of aromaticity. The ELFπ is bounded between 0 and 1. The
values close to one means that electrons are greatly localized, however the values are
very small indicated Pauli repulsion is large between electron pairs. The process is
conveniently by variation the bifurcation diagram.[31] According to Krygowski[30] and
Santos[32] reports, for traditional π aromatic molecules, the ELFπ values about 0.64 to
0.91 means these are π bonds of a hexagon are of delocalized. If the isovalues are larger
than 0.91 or small than 0.64 should be classified as double or single bond, respectively.
The span of bifurcation values ([ΔBV(ELFπ)])[33] also showed in figures to help us
better understand of the π-electron delocalization of aromatic rings (the threshold used
herein is ≤ 0.20). To the best of our knowledge, for the index of familiar NICS, a
negative value indicates aromatic character while a positive value indicates
antiaromatic character. Our results demonstrate that in comparison with the Clar
structures of PBHs, the aromaticity and of BN-acenes are greatly reduced and even
have nonaromaticity. But we found there are good linear relationships between the
[n]acenes number (n = 3-7) and relative Gibbs free energies between isomers of
containing one π-sextet and most π-sextet. And for different isomers, with the more
number of π-sextets, the more stable regardless of PBHs or BN analogues, which is
known to provide valuable insights into the grapheme structures and lay the foundation
for its application in our daily life.
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