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A number of condensed polycyclic aromatic hydrocarbons (PAHs) are investigated to identify the underlying reasons governing empirical Clar's rule taking benzene as a limiting case. The results have shown 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.

A combination of Schwesinger's phosphazene base concept and the idea of the disubstitued 1,8-naphthalene spacer, yields a superbase, 1,8-bis(hexamethyltriaminophosphazenyl)naphhalene (HMPN), which is the most basic representative of 'proton sponges'. The high basicity of HMPN is a consequence of the high energy content of the base in its initial neutral state and the intramolecular hydrogen bonding in the resulting conjugate acid with contributions to proton affinity.

The possible additivity of the exchange-correlation energy (ECE) built in the B3LYP DFT method, which in turn seems to be most popular and widely used density functional approach is discussed. As a final comment, pointing out that it is both surprising and gratifying that a lot of useful information about molecules can be derived from simple facts that are composed of atoms.

The problem of bond-stretch isomerism in benzocyclobutene (1) and benzo[1,2:4,5]- dicyclobutadiene is addressed by single-state as well by two-state theoretical formalisms. It is argued that the strategic substitution of benzol[1,2:4,5]-dicyclobutadiene might lead to a discrimination of two different isomers.

The nondynamical and dynamical correlation energies of pi electrons in a wide variety of planar hydrocarbons is discussed. The nondynamical energy of pi-electron in planar hydrocarbons can be quite accurately calculated by employing a modest basis set.

Results reveal that polyguanidines exhibit very high proton affinities with the linear chain poly-guanides saturation effect depending on the number of guanidine units. A doubly bifurcated heptaguanide shows the largest proton affinity caused by substantial resonance effect of the protonation.

A study extended investigations on the proton affinity of organic compounds by examining cyclopropeneimines and some related systems which might yield very strong bases. It analyzed how the protonation effect is transferred across several planar rings forming extended pi electron networks. The study also aimed to determine whether the effect of aromatization triggered by protonation could be amplified to develop organic bases more powerful than any other in existence. Results are discussed.

Two theoretical models were used to test a hypothesis about Kekule valence isomerism pertaining and the existence of two Kekule valence isomers termed 1a and 1b of cyclobutadieno-p-benzoquinone. Results of the theoretical study show that 1a is 5kcal/mol more stable than 1b. The barrier separating 1a and 1b is highly sensitive to electron correlation as expected. It can be concluded that with a high degree of certainty that 1b isomer does not exist and that cyclobutadieno-p-benzoquinone does not inhibit Kekule isomerism.

The proton and methyl cation affinities of benzene and its di- and polysubstituted analogues were investigated at the MP2(fc)/6-31 G**//HF/6-31G*+ZPE(HF/6-31G*) theoretical level. Experimental results showed that methyl cations of polysubstituted benzenes follow the additivity rule, which is similar to the additivity property of protons in multiply-substituted aromatics. The additivity approach is based on the independent substituent approach and is very suitable due to its high performance, simplicity and intuitive appeal.

Both ab initio theoretical models MP2(fc)/6-31+G**/HF/6-31+G* and MP2(fc)/6-311+G**//HF/6-31+G* offer practical methods of predicting the intrinsic acidity of substituted benzenes. They are the results of a search for an economical theoretical model that can be applied to large compounds but can provide reliable data on acidity. It is also shown that acidities of polysubstituted benzenes adhere to a simple additivity rule, indicating that acidity is directly obtained once the increments for monosubstituents are determined.

The bis-guanidino compound [H.sub.2]C[{hpp}.sub.2] (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) is converted to the monocation and isolated as the chloride and tetraphenylborate salts. Triadic analysis has shown that the dominant term causing the high basicity is the relaxation energy.

Various G3 and G2 methodologies are employed to calculate the Bronsted acidities of several inorganic acids by using the triadic formula. The analysis proves the linear relationship between the proton affinities and the Hammett-Taft constants of these acids.

Hydride affinities (HAs) of the ethynes substituted by a wide range of different substituents are considered by using the density functional theory (DFT) method. A salient structural feature of the hydrides is that substituent groups assume conformations that enable an optimal resonance effect with the CC double bond.

A comment on the paper 'On the limits of highest-occupied molecular orbital-driven reactions: The Frontier Effective-for-Reaction Molecular Orbital Concept' by da Silva et al. remarks that is not necessary to baptize the specific molecular orbitals (MOs) as frontier effective-for-reaction MOs (FERMOs). It is concluded that FERMOs introduce by da Silva et al. have their predecessor and that the single-electron MO picture is not satisfactory in rationalizing (de)protonation processes, but it is a key ingredient of the triadic analysis.

Influence of fluorine substitutions on the stability of benzene is examined by using the Hartree-Fock (HF) and MP2 models. The homodesmotic reactions based on the open-chain zigzag polyenes are unsatisfactory and the result implies that both sigma- and pi-frameworks contribute to the aromatic stabilization of benzene in which the latter is more important.

A brief description on the high level ab initio and density functional theory (DFT) calculations of the bond dissociation energies (BDEs) of single bonds in molecules containing the first- and second-row atoms of the system of elements is presented. The A-X 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).

The extrinsic antiaromaticity of archetypal cyclobutadiene (CBD) is addressed with particular emphasis on the alpha-pi separability problem. The study found that the one-electron MO picture cannot offer a satisfactory description of antiaromaticity, since both the alpha-MOs and pi-MOs energies contribute to E (d)(sub CBD) with an overwhelming influence of the alpha electrons.

The characterization of dimethylethyleneguanidino (DMEGN) reveals an unsymmetrical intramolecular hydrogen bridge. It is stated that, a new proton sponge provides a new crossbar in the ladder of highly basic neutral organic compounds.

The merits and limitations of Koopmans' approximation in estimating the vertical and adiabatic ionization potentials are examined. It is reported that serious discrepancies are found between the G2(MP2), G2, and G3 adiabatic ionization potentials AIDP for a series of molecules NHnF(sub 3-n) (n= 1-3) and the experimental data.

The article discusses recent advancement in determination of absolute proton affinities which is measure of gas-phase basicity of neutral organic molecules. The measurement of proton affinities has been performed by computational chemistry studies of organic molecules. It also highlights importance of determination of proton affinity which forms smallest and lightest part of atomic nucleus with its significant role in chemistry, biochemistry and molecular biology.

The correlation energy in planar heteroatomic open chain polyene systems involving N, O, and F atoms is considered by the CASSCF and CASPT2 methods employing a number of the cc-pVmZ (VDZ, VTZ, etc.) correlation consistent basis sets. A thorough study of the smallest molecules shows that the nondynamical correlation energy is virtually independent of the quality of the basis set. In contrast, the dynamical correlation energy is very sensitive to the basis set and, in estimating reliable dynamical correlation effects for larger systems, one has to rely on adequate extrapolation formulas to obtain the infinite basis set limit. We find that a method recently proposed by Truhlar offers economical yet reasonable estimates of the complete basis set results. Investigation of the sensitivity of the results to the choice of active space and the comparison to single reference MP2 calculations indicate that such extrapolations offer a good general method for saturating the basis set in multireference calculations. Thus a simple refinement of the conventional multireference coupled cluster method is proposed. It is also shown that both nondynamical and dynamical correlation energies follow very simple additivity rules in linear and branched planar chain heteroatomic polyenes, making possible their prediction in very large systems without calculation. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a theoretical study on the additivity of deprotonation energies in aromatics. Homodesmic chemical reaction; Application to large compounds; Intrinsic acidity of substituted benzene.