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Homodesmotic Reactions (homodesmotic + reaction)

Distribution by Scientific Domains

Chemistry	100%

Selected Abstracts

,,, Energy Separation in Homodesmotic Reactions

CHEMPHYSCHEM, Issue 12 2005
Georg Hohlneicher Dr.
Abstract A well-established quantity for specifying the aromaticity or antiaromaticity of cyclic conjugated molecules is the so-called aromatic stabilization energy (ASE), which can be derived,either experimentally or theoretically,from appropriate homodesmotic reactions. To gain further insight into the origin of aromaticity, several schemes have been devised to partition ASE into nuclear and electronic as well as , and , contributions, some of which have resulted in contradictory statements about the driving force of aromatic stabilization. Currently, these contradictions have not been resolved and have resulted in a confusing distinction between two different types of aromaticity: extrinsic and intrinsic aromaticity. By investigating different homodesmotic reactions we show that, in contrast to ASE itself, the individual contributions that enter the ASE can strongly depend on the type of reaction. Caution is therefore advised if conclusions or physical interpretations are derived from the individual components. The contradictions result from the fact that some reactions suffer from an imbalance in the number of interaction terms at the two sides of the reaction equation. The concept of isointeractional reactions is introduced and results in the elimination of the imbalance. For these reactions, the contradictions disappear and the distinction between intrinsic and extrinsic aromaticity becomes unnecessary. As far as the ,,, partitioning is concerned, several schemes proposed in the literature are compared. Contradictory results are obtained depending on the partitioning scheme and reaction used. In this context, it is demonstrated that for the partitioning of the electron,electron interaction, the scheme introduced by Jug and Köster is the one that is most theoretically grounded. [source]

Ab initio and density functional theory studies of the structure, gas-phase acidity and aromaticity of tetraselenosquaric acid

CHINESE JOURNAL OF CHEMISTRY, Issue 6 2000
Li-Xin Zhou
Abstract Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, ,G*) and aromaticity of tetraselenosquaric acid (3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2 C4 Se4)are reported. The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutanediselenol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is -77.4 (MP2(fu)/6,311+G** /RHF/6 - 311 + G** ) and - 54.8 kJ/mol (B3LYP/6 - 311 + G** //B3LYP/6 -311 + G**). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) -19.13 (CSGT(IGAIM)-RHF/6,311 + G**// RHF/6,311 + G** and -32.91 (4,·10,6 m,3/mol)(CSGT(I-GAIM)-B3LYP/6 - 311 + G* * //B3LYP/6 - 311 + G**). Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas-phase acidity is ,G1*(298k) = 1257.7 and ,G*2 (298k) = 1617.1 kJ/mol. Hence, tetraselenosquaric acid is the strongest acid among the three squaric acids (3, 4-dihydroxy-3-cyclobutene-1, 2-dione, H2 C4 3,4-dithiohydroxy-3-cyclobutene-1,2-dithione, H2C4 S4, 3, 4-diselenyl-3-cyclobutene-1,2-diselenone, H2C4Se4). [source]

Theoretical studies on four-membered ring compounds with NF2, ONO2, N3, and NO2 groups

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2008
Xiao-Wei Fan
Abstract Density functional theory (DFT) method has been employed to study the geometric and electronic structures of a series of four-membered ring compounds at the B3LYP/6-311G** and the B3P86/6-311G** levels. In the isodesmic reactions designed for the computation of heats of formation (HOFs), 3,3-dimethyl-oxetane, azetidine, and cyclobutane were chosen as reference compounds. The HOFs for N3 substituted derivations are larger than those of oxetane compounds with ONO2 and/or NF2 substituent groups. The HOFs for oxetane with ONO2 and/or NF2 substituent groups are negative, while the HOFs for N3 substituted derivations are positive. For azetidine compounds, the substituent groups within the azetidine ring affect the HOFs, which increase as the difluoroamino group being replaced by the nitro group. The magnitudes of intramolecular group interactions were predicted through the disproportionation energies. The strain energy (SE) for the title compounds has been calculated using homodesmotic reactions. For azetidine compounds, the NF2 group connecting N atom in the ring decrease the SE of title compounds. Thermal stability were evaluated via bond dissociation energies (BDE) at the UB3LYP/6-311G** level. For the oxetane compounds, the ONO2 bond is easier to break than that of the ring CC bond. For the azetidine and cyclobutane compounds, the homolysises of CNX2 and/or NNX2 (X = O, F) bonds are primary step for bond dissociation. Detonation properties of the title compounds were evaluated by using the Kamlet,Jacobs equation based on the calculated densities and HOFs. It is found that 1,1-dinitro-3,3-bis(difluoroamino)-cyclobutane, with predicted density of ca. 1.9 g/cm3, detonation velocity (D) over 9 km/s, and detonation pressure (P) of 41 GPa that are lager than those of TNAZ, is expected to be a novel candidate of high energy density materials (HEDMs). The detonation data of nitro-BDFAA and TNCB are also close to the requirements for HEDMs. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source]

Inductive effect of uncharged groups: dependence on electronegativity

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 7 2006
Otto Exner
Abstract Substituent effects in rigid non-conjugated systems were followed on the series of 3-substituted 1-fluoro-bicyclo[1.1.1]pentanes and 2-substituted 1-fluoroethanes in the fixed ap conformation. Their energies were calculated within the framework of the density functional theory at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-311++G(3df,3pd) level and the substituent effects were expressed in terms of isodesmic homodesmotic reactions. The results were confirmed by the energies of 1,4-disubstituted bicyclo[2.2.2]octanes reported in the literature and calculated at a lower level. Interaction of two common substituents of low or medium polarity cannot be described as the classical inductive effect by one term, proportional in all series, but an additional parameter is necessary, which depends only on the first atom of the substituent and may be identified with its electronegativity. The second term decreases with the distance more steeply than the first term and is always much less important. Nevertheless its statistical significance was proved by several sensitive tests at the highest level used in statistics. When one of the substituents is charged (or at least strongly polar as NO2 or CN), the first term is much increased and the second becomes less significant or insignificant. Therefore, the standard definition of the inductive effect with a uniform, universally valid constant can be retained as far as one treats only the ionization equilibria, both in solution and in the gas phase, or kinetics with a strongly polar transition state. In contrast to the firm statistical proofs, the physical meaning of the electronegativity term was not established. Any relation to various group electronegativities does not exist, similarity to the 13C NMR shifts is merely qualitative. Copyright © 2006 John Wiley & Sons, Ltd. [source]