The electron-pair origin of antiaromaticity: Spectroscopic manifestations

Abstract

ABSTRACT:It is shown that the antiaromatic character of certain conjugated cyclic hydrocarbons is due to the presence of an even number of distinct electron pairs in the Ž . system such as, but not necessarily electrons . In these systems, the ground state is Ž . constructed from an out-of-phase combination of two valence bond VB structures, and its equilibrium geometry is necessarily distorted along the coordinate that interchanges these structures. If a new symmetry element appears during the transition between the two structures, the ground electronic state at the symmetric point transforms as one of the nontotally symmetric irreducible representations of the point group. The conjugate excited state, formed from the in-phase combination of the same two structures, transforms as the totally symmetric representation of the group and is strongly bound. Its structure is similar to that of the ground state at the symmetric point, and the energy separation between the two states is small compared to that of conjugated cyclic hydrocarbons having an odd number of distinct electron pairs. Motion along the ''Kekule-type'' vibrational mode on the excited-state potential surface is very similar tó motion along the reaction coordinate connecting the two distorted structures on the ground-state surface. It is characterized by a significantly higher vibrational frequency compared to frequencies of similar modes in ground-state molecules. These qualitative predictions are supported by quantum chemical calculations on cyclobutadiene, cyclooctatetraene, and pentalene.

Valence bond descriptions of benzene and cyclobutadiene and their counterparts with localized bonds

“…This suggests that the system is an important factor in the determination of the geometry, which can be understood in terms of the model given by Shaik et al [17][18][19][20][21][22][23]. Zilberg and Haas [25] stated that the geometric distortion of cyclobutadiene is a fundamental property of 4n electron ring systems. This is not the case according to our calculations.…”

“…Recently, Zilberg and Haas [25] investigated the VB model from a fundamental perspective by examining the interaction between the Kekulé structures of systems with 4n and 4nϩ2 electrons. Using this approach, they found that the sign and the absolute size of the interaction alternate between the systems.…”

Comment on “The electron‐pair origin of anti‐aromaticity: Spectroscopic manifestations”

“…For the two VB functions used in Ref. 1, we find that the ground state is always a superposition with positive sign (at least in the simple semi‐empirical model employed), independent of whether the compound is aromatic or anti‐aromatic.…”

“…This Comment refers to the article “The Electron‐Pair Origin of Anti‐aromaticity: Spectroscopic Manifestations” by Shmuel Zilberg and Yehuda Haas 1. In that article, the electronic ground state of aromatic and anti‐aromatic compounds is expressed in terms of only two valence bond (VB) functions that correspond to the classical Kekulé structures.…”

“…In Ref. 1 it is proposed that the electronic ground state of aromatic molecules is a symmetric superposition of the two VB functions, while in anti‐aromatic molecules the two functions contribute with different sign. Similar claims have been made by other investigators 2, 3, often only for single substances.…”