Showing papers by "Thomas Bally published in 2006"

Cyclobutadiene: the antiaromatic paradigm?

Abstract: There is probably no other single molecule that has fascinated experimental and theoretical chemists so consistently over the past 40 years as has cyclobutadiene (CBD). On average, 16 publications which deal, in one form or another, with the parent compound C4H4 and 40 which deal with cyclobutdienes in general have appeared in every year of this time span, and there are probably many more on derivatives and metal complexes. There is no sign that interest in CBD is about to wane, as the highest number of papers per year appeared in 2000 and 2002, and in the last five years two entire thematic issues of Chemical Reviews, both with many references to CBD, were dedicated to issues of aromaticity/antiaromaticity and localization/delocalization, that is, topics that are central to the debate on CBD. Recently the first direct and reasonably accurate determination of the enthalpy of formation of CBD has been achieved by the group of Kass. Thus they have provided a solid foundation for the discussion on the thermochemical consequences of antiaromaticity, and I take this occasion to reflect on recent developments in this field and to point out the pitfalls with which the resulting discussion is fraught. In spite of the great activity documented in the opening paragraph, many myths surround this fascinating molecule, most of which relate to the extent of delocalization of its p electrons and/ or the degree of antiaromaticity. It is widely accepted (and hardly disputed) that benzene, with its six cyclically delocalized p electrons, represents the paradigm of aromaticity. In view of the famous H ckel 4n/4n+ 2 electron rule, one is tempted to rush to the conclusion that CBD, with its four cyclically delocalized p electrons, is therefore the paradigm of antiaromaticity. However, this conclusion is not as straightforward as it might seem, because in benzene the two Kekul resonance structures contribute with equal weights to the D6h equilibrium geometry, whereas in CBD, with its rectangular structure having strongly localized single and double bonds, this is obviously not the case. Thus the notion of “bond delocalization” does not have the same meaning in the two compounds, and one of the issues of the continuing debate is to what extent this affects the properties that are associated with (anti)aromaticity. Another issue is of course how exactly one should or should not assess these properties. I will comment herein on some aspects of this debate. Various criteria have been proposed over time to judge whether a molecule is aromatic or not (it is perhaps worth recalling that the term “aromatic” itself implies that a pleasant odor must at one time have been considered as an important criterion; thus, by the same token, antiaromatic compounds should smell unpleasantly), and it was generally assumed that several of these criteria have to be met for a molecule to qualify for this distinction. Two types of characteristic traits of aromaticity have emerged as “beacon” features. The first relates to magnetic properties, in particular the anisotropy of the diamagnetic susceptibility and its exaltation, which distinguishes aromatic compounds from polyenes. Following suggestions by Pauling and Londsdale, London proposed in 1937 a very appealing model that accounts for these effects in terms of ring currents that are induced by the external magnetic field in the system of cyclically delocalized p electrons. There has been a vigorous debate in recent years about whether and to what extent these ring currents contribute to the deshielding or shielding of protons attached to aromatic or antiaromatic rings, respectively, as manifested in H NMR spectra. At the focus of this debate stand the “nuclear independent chemical shifts” (NICS values) that can be computed at any point in the space surrounding a molecule. It has become clear that the original proposal to calculate NICS values at the center of (anti)aromatic rings does not provide a valid measure of currents induced in the cycle of p electrons. However, it seems that the sign and magnitude of the p component of the magnetic-shielding (or NICS) tensor at some distance (usually 1 ) above the ring provides a good indication of the direction and size of the ring current, even in systems that have only s electrons. However, I will focus herein on the other class of “beacon” properties that are characterictic of (anti)aromaticity, namely thermochemical and/or kinetic ones (the two often being mixed up). Clearly, benzene enjoys a special thermodynamic stability which expresses itself, for example, in the fact that its (mono)hydrogenation is endothermic by 22 kJmol 1 (Scheme 1), in contrast to that of hexatriene, which is exothermic by about 114 kJmol 1 (the exact number depends on the choice of conformations). As there is no evident reason why one or the other of the conjugated dienes that result from these hydrogenations should suffer from any [*] Prof. T. Bally Chemistry Department University of Fribourg 1700 Fribourg (Switzerland) E-mail: thomas.bally@unifr.ch Homepage: http://www-chem.unifr.ch/tb/bally/ Published in "Angewandte Chemie International Edition 45(40): 6616 6619, 20 which should be cited to refer to this work.

Matrix isolation and computational study of the photochemistry of p-azidoaniline.

Abstract: The photochemistry of p-azidoaniline was studied in argon matrices in the absence and presence of oxygen. With the help of quantum chemical calculations we were able to characterize the triplet p-aminophenylnitrene as well as the cis- and trans-p-aminophenylnitroso oxides. It was found that the latter two isomers can be interconverted by selective irradiation and that they are ultimately converted into p-nitroaniline. Although restricted wavefunctions of the nitroso oxides are unstable, CASSCF calculations turned up no evidence for the claimed diradical character of these compounds. Also we found no evidence for dioxaziridines as intermediates of the conversion of the nitroso oxides to p-nitroaniline.

Anthralin: primary products of its redox reactions.

Abstract: One-electron reduction significantly enhances the ability of anthralin, 1, to act as a hydrogen atom donor. On annealing of an MTHF glass in which the radical anion of anthralin, 1•-, is generated radiolytically, this species decays mainly by loss of H• to give the anthralyl anion, 2-. On the other hand, radicals formed on radiolysis of matrices that are suitable for the generation of radical anions or cations are capable to abstract H• from anthralin to give the anthralyl radical, 2•. Both 2- and 2• are obtained simultaneously by mesolytic cleavage of the radical anion of the anthralin dimer. Contrary to general assumptions, the anthralyl radical is found to be much more reactive toward oxygen than the anion. All intermediates are characterized spectroscopically and by reference to quantum chemical calculations. Attempts to generate the radical cation of anthralin by X-irradiation of an Ar matrix containing anthralin led also to significant formation of its radical anion, i.e., anthralin acts apparently ...

Experimental and theoretical study of 2,6-difluorophenylnitrene, its radical cation, and their rearrangement products in argon matrices.

Abstract: 2,6-Difluorophenylnitrene was reinvestigated both experimentally, in Ar matrices at 10 K, and computationally, by DFTand CASSCF/ CASPT2 calculations. Almost-pure samples of both neutral rearrangement products (the bicyclic azirine and the cyclic ketenimine) of a phenylnitrene were prepared and characterized for the first time. These samples were then subjected to X-irradiation in the presence of CH 2 Cl 2 as an electron scavenger, which led to ionization of the neutral intermediates. Thereby, it was shown that only the phenylnitrene and the cyclic ketenimine yield stable radical cations, whereas the bicyclic azirine decays to both of these compounds on ionization. The cyclic ketenimine yields a novel aromatic azatropylium-type radical cation. The electronic structure of the title compound is discussed in detail, and its relation to those of the iso-π-electronic benzyl radical and phenylcarbene is traced.

The Dewar benzene radical cation and its ring-opening reaction.

Abstract: The radical cation of Dewar benzene, 1*+, has been generated and observed by optical spectroscopy in cryogenic matrices. 1*+ distinguishes itself by a charge resonance band at 600 nm, very similar in shape and position to that observed for the related radical cation of norbornadiene. This coincidence indicates that in ground-state 1*+ the odd electron is also located in a pi-MO. The energy of the charge resonance transition, which is very sensitive to the dihedral angle between the four-membered rings in 1*+, is predicted consistently too low by TD-DFT and CASPT2. Probably this angle is too large in the B3LYP and CASSCF geometries. As 1*+ can be observed at 77 K, it must be separated by a barrier of at least 7-8 kcal/mol from its very exothermic decay to the radical cation of benzene, 2*+. An analysis shows that the ring-opening of 1*+ is a multistep process involving two avoided crossings between potential surfaces of different symmetry and electronic nature. Owing to the orbital symmetry-forbidden nature of the process, the energy of 1*+ starts by increasing steeply on stretching the central C-C bond, but then the system undergoes a crossing to a 2A1 surface which leads adiabatically to an excited state of 2*+. Therefore, another avoided crossing must be transited before the molecule can decay on the ground-state surface of 2*+. The rearrangement of 1*+ to 2*+ is an example of a "pseudodiabatic" thermal reaction that transits between potential surfaces representing very different electronic structures.

Radical cations from dicyclopropylidenemethane and its octamethyl derivative.

Abstract: The radical cations of dicyclopropylidenemethane (2) and its octamethyl derivative (2-Me8) are prone to rearrangements into those of (2-methylallylidene)cyclopropane (2a) and its octamethyl derivat...

Oxidative rearrangements of tricyclic vinylcyclobutane derivatives.

Abstract: Three tricyclic vinylcyclobutanes (3-methylenetricyclo[5.3.0.0(2,6)]decanes 1-3) have been subjected to ionization by photoinduced electron transfer in solution and by X-irradiation in Ar matrices. All three compounds undergo oxidative cycloreversion; the cleavage of the four-membered ring, however, occurs in a different direction depending on the presence of a methyl group in position 6 of the tricyclic framework. In those derivatives, cycloreversion is found to lead to 1-methyl-8-methylene-1,6-cyclodecadiene radical cations (5.+ from 1, 8.+) from 2) which upon back electron transfer yield two different hydrocarbons (6 from 5.+, 9 from 8.+), depending on the configuration around the endocyclic double bonds of the respective cyclodecadiene derivative. In the absence of a methyl group on C6, the cycloreversion leads to a radical cation complex between 1-methylenecyclopent-2-ene and cyclopentene (12.+) which appears to revert to 3 on back electron transfer. The intermediate radical cations 5.+, 8.+, and 12.+ have been identified and characterized by UV/Vis and IR spectra in Ar matrices. The mechanism of their formation is elucidated by quantum chemical calculations.

The Interplay and Synergism of Experiment and Quantum Chemistry in Research on Reactive Intermediates

Abstract: Three examples from the work of the author's research group on reactive intermediates are used to illustrate how fruitful interaction between experimental and computational chemistry may lead to the solution of problems that neither of the two methods could have solved without the other. It is shown that computational chemistry has indeed become an indispensable tool for the verification of spectroscopic assignments, the elucidation of reaction mechanisms, and the understanding of the factors that drive chemistry.