Dominant Role of the pi Framework in Cyclobutadiene

The extrinsic antiaromaticity of archetypal cyclobutadiene (CBD) is addressed with particular emphasis on the sigma-pi separability problem. The destabilization energy E(d)(CBD) of CBD is obtained by appropriate homodesmotic reactions involving the open chain zigzag, polyene(s). It is shown that E(d)(CBD) does not depend on the electron correlation and the zero-point vibrational energy contributions, since they are small and of the opposite sign. Consequently, they cancel in the first approximation. Further, it turns out that E(d)(CBD) can be estimated accurately enough with a very modest cc-pVDZ basis set at the Hartree-Fock (HF) level. The extrinsic antiaromatic destabilization E(ean)(CBD) of CBD is deduced after extracting the angular strain energy estimated to be 32 kcal/mol. The resulting E(ean)(CBD) value of 52 kcal/mol is in excellent agreement with the experimental thermodynamic data. If the E(ean)(CBD) is estimated relative to two isolated C=C double bonds, then it assumes 38 kcal/mol, which is roughly 10 kcal/mol per one pi electron. It is, therefore, safe to state that extrinsic antiaromaticity of CBD is larger than its angular strain. Although the sigma and pi electrons are coupled by a mutual Coulomb interaction V-ee(sigmapi), several attempts of their decoupling is made by using three partitioning schemes: stockholder, equipartition, and standard pi-electron theory recipe. The latter allocates the V-nn and V-ee(sigmapi) terms to the sigma- and pi-electron frameworks, respectively. The nuclear repulsion term V-nn is dissected into sigma and pi components in the former two partitioning schemes by using stockholder criterion. It appears that the extrinsic antiaromatic destabilization E(ean)(CBD) is determined by the pi-electron framework according to all three partitioning models

A Novel Approach in Analyzing Aromaticity by Homo- and Isostructural Reactions: An ab Initio Study of Fluorobenzenes

The influence of fluorine substitutions on the stability of benzene is examined by using the Hartree-Fock (HF) and MP2 models. It is conclusively demonstrated that homodesmotic reactions based on the open-chain zigzag polyenes are unsatisfactory. A comparison of the intramolecular interactions of educts and products shows that they are not well balanced. Hence, these reactions should be abandoned in discussing aromaticity. A much better vehicle for exploring aromaticity is provided by homostructural reactions, which employ cyclic monoene and diene as reference model compounds. Their heavy atoms are enforced to assume planar geometries to enable sigma/pi separation. The HF/cc-pVTZ calculations show that extrinsic aromaticity of benzene B DeltaE(ease)(B)() arises both from the sigma- and pi-contributions. They are -14.8 and -23.1 in kcal/mol, respectively, if the stockholder energy partitioning scheme is employed. This result implies that both the sigma- and pi-frameworks contribute to the aromatic stabilization of B, the latter being more important. The total aromatic stabilization DeltaE(ease)(B)() is -37.9 kcal/mol. Schleyer's indene-isoindene isomerization approach also strongly indicates that the decisive factor in determining the aromatic stability of the benzene moiety is the pi-electron framework. The origin of extrinsic aromaticity is identified as the increased nuclear-electron attraction of both sigma- and pi-electrons, if 1,3-cyclohexadiene is used as a gauge compound. Further, by using a system of isostructural reactions, it is conclusively demonstrated that fluorobenzenes exhibit a remarkable additivity of the substituent effects, as far as the stability of multiply substituted benzenes is concerned. This additivity rule is so accurate that it enables delineation of the fluorine repulsions and the aromaticity defect DeltaE(AD). It appears that the DeltaE(AD) values increase upon sequential fluorine substitution at the next nearest (vicinal) position thus making multiply fluorinated benzenes less stable

Periodic trends in bond dissociation energies. A theoretical study

Bond dissociation energies (BDEs) of all possible A-X single bonds involving the first- and second-row atoms, from Li to Cl, where the free valences are saturated by hydrogens, have been estimated through the use of the G3-theory and at the B3LYP/6-311+G(3df,2pd)//B3LYP/6-31G(2df,p) DFT level of theory. BDEs exhibit a periodical behavior. The A-X (A = Li, Be, B, Na, Mg, Al, and Si) BDEs show a steady increase along the first and the second row of the periodic table as a function of the atomic number Z(X). For A-X bonds involving electronegative atoms (A = C, N, O, F, P, S, and Cl) the bond energies achieve a maximum around Z(X) = 5. The same behavior is observed when BDEs are plotted against the electronegativity χ(X) of the atom X. Thus, for A-X bonds (A = Li, Be, B, Na, Mg, Al, Si), the BDEs for a fixed A increases, grosso modo, as the electronegativity differences between X and A increase, with some exceptions, which reflect the differences in the relaxation energies of the radicals produced upon the bond cleavage. A similar trend, albeit less pronounced, is found for single A-X bonds, where A = C, N, O, F, P, S, and Cl. However, there is an additional feature embodied in the enhancement of the strength of the A-boron bonds due to the ability of boron to act as a strong electron acceptor. The trends in bond lengths and charge densities at the bond critical points are in line with the aforementioned behavior. © 2005 American Chemical Society.Peer Reviewe

Clar's sextet rule is a consequence of the sigma-electron framework

A number of condensed PAHs are examined to identify the underlying reasons governing empirical Clar's rule taking benzene as a limiting case. It is found that the so-called Clar's structures are the only minima on the MP2(fc) potential energy hypersurfaces, meaning that other conceivable valence isomers are nonexistent. The influence of the electron correlation energies to the stability of Clar's structures is substantial with predominating influence of the sigma-electrons. However, the contributions arising from the sigma- and pi-electron correlation energies are approximately the same, if Clar's structures are compared with some artificial pi-electron localized or graphite-like delocalized planar systems. Analysis of the Hartree-Fock (HF) energies provides a compelling evidence that the origin of stability of Clar's structures lies in a decrease of the positive T, V(ee) and V(nn) energy terms relative to some characteristic virtual "delocalized" or "localized" model geometries. Partitioning of the mixed and terms in the sigma- and pi-type contributions, by using the stockholder (SHR), equipartitioning (EQP) and standard pi (SPI) schemes, unequivocally shows that the driving force leading to Clar's structures are more favorable sigma-type interactions. All these conclusions hold for the archetypal benzene too, which could be considered as a limiting Clar system. Finally, the boundaries of Clar's hypothesis and some common misconceptions are briefly discussed. Perusal of the geometric parameters and pi-bond orders reveals that there are no benzene rings completely "vacant" or "fully occupied" by the pi-electrons, envisaged by Clar in his picture of condensed benzenoid compounds. Instead, there are six-membered rings with higher and lower total pi-electron density. The bond length anisotropy of the former rings is smaller. It is concluded that Clar's proposition is a useful rule of thumb providing qualitative information on the stability of the PAH systems, which in turn should not be overinterpreted

The Origin of Aromaticity: Important Role of the Sigma Framework in Benzene

The physical nature of aromaticity is addressed at a high ab initio level. It is conclusively shown that the extrinsic aromatic stabilization energy of benzene E(ease)B, estimated relative to its linear polyene counterpart(s), is very well-reproduced at the Hartree-Fock (HF) level. This is a consequence of the fact that the contributions arising from the zero-point vibrational energy (ZPVE) and electron correlation are rather small. More specifically, they yield together 2.0 kcalmol(-1) to the destabilization of benzene. A careful scrutiny of the HF energies by virial theorem shows further that the kinetic energies of the sigma and pi electrons E(T)HF(sigma) and E(T)HF(pi) are strictly additive in the gauge linear zig-zag polyenes, which also holds for their sum Et(T)HF This finding has the important corollary that E(ease)B is little dependent on the choice of the homodesmic reactions involving zig-zag polyenes. A detailed physical analysis of the sigma- and pi-electron contributions to extrinsic aromaticity requires explicit introduction of the potential energy terms Vne, Vee, and Vnn, which signify Coulomb interactions between the electrons and the nuclei. The Vee term involves repulsive interaction Vee(sigmapi) between the sigma and pi electrons, which cannot be unequivocally resolved into sigma and pi contributions. The same holds for the Vnn energy, which implicitly depends on the electron density distribution via the Born-Oppenheimer (BO) potential energy surface. Several possibilities for partitioning Vee(sigmapi) and Vnn terms into sigma and pi components are examined. It is argued that the stockholder principle is the most realistic, which strongly indicates that E(ease)B is a result of favorable sigma-framework interactions. In contrast, the pi-electron framework prefers the open-chain linear polyenes

Gas-phase and liquid-phase basicities of two Diguanidinium paralytic shellfish poisons

NRC publication: Ye