Aromatic Stabilization Energy Research Articles

The aromatic diboracyclopropenyl (diboriranyl) anion; CB2H3 −: An ab initio study

The most stable structure of CB2H3−, as established computationally, is the aromatic diboracyclopropenyl (diboriranyl) anion (5), while open-chainC2v, isomer H2BCBH (7) is only 3 kcal/mol higher in energy at the QCISD(T)/6-311 +G**//MP2/6-31+G*+ZPE (HF/6-31 +G*). The 47-kcal/mol barrier between cyclic,5, and open-chain,7, structures suggests that both of them may be observed. The aromatic stabilization energy of the diboriranyl anion (18 kcal/mol) is half the value in the isoelectronic cyclopropenium ion, C3H3+. The computed, by IGLO method (5a), and experimental (6a) chemical shifts,δ(13C) andδ(11B), agree within 4 ppm range. The theoretical vibrational frequencies of the most stable isomers,5 and7, are presented for experimental verification of these species.

Theoretical study of metastable N 2CO isomers. New candidates for high energy materials?

The geometries, energies and frequencies of singlet and triplet N 2CO isomers were computed ab initio at the HF/6-31G *, MP2/6-31+G * and at the BECKE3LYP/6-311+G * levels. Single point energies were calculated at the QCISD(T)/6-311+ G *//MP2/6-31+G * level and at the CCSD(T)/6-311+G(2D)//BECKE3LYP/6-311+G * level. The most stable bound N 2CO isomer is the unknown three-membered cyclic C 2v structure (diazirinone); its aromatic stabilization energy is only 7 kcal/mol. Dissociation of diazirinone into N 2 and CO is exothermic by 96–100 kcal/mol, but the 27 kcal/mol barrier is sufficiently high to permit observation. The open-chain NNCO triplet lies 5 kcal/mol above diazirinone and ≈ 35 kcal/mol below the spin-allowed dissocition into X 1Σ + g N 2 and 3Π r CO. While the known nitrosyl cyanide (NCNO) is 11 kcal/mol less stable than diazirinone, the dissociation into CN and NO radicals is endothermic by 47–50 kcal/mol.

Aromatic character of graphite and carbon nanotubes

Degrees of aromatic stabilization were estimated graph-theoretically for a single graphite sheet and infinitely long graphitic tubules. Graphite was found to be as aromatic as fully benzenoid hydrocarbons. All graphitic tubules have essentially the same aromatic stabilization energy per carbon atom as a single graphite sheet. This is closely associated with the fact that graphitic tubules can change their diameters so as to form a closely packed multilayered carbon nanotube. Metallic tubules are only slightly less aromatic than semiconductive ones.

Comparison of computational methods applied to oxazole, thiazole, and other heterocyclic compounds

AbstractA variety of computational methods, including the semiempirical techniques AM1, PM3, and MNDO, and the thermochemical basis sets of Benson and Stine, was used to calculate and compare heats of formation (ΔHf°) data for optimized geometries of a variety of aromatic and nonaromatic heterocycles. Detailed analyses, including 6‐31G* and MP2/6‐31G* ab initio calculations, were performed for the oxazole and thiazole heterocycles. The results indicate a scatter among the methods sensitive to the nature of the heterocycle. This was in particular evident in the oxazole molecule, where AM1 gave a singularly high value of ΔHf° consistent with longer calculated bond lengths, particularly about the oxygen atom. Aromatic stabilization energy appears to be addressed differently among the employed methods. Implications of this contrast applied to calculation of macromolecular systems containing heterocyclic units are discussed.

Heterocyclic aromatic anions with 4n + 2 .pi.-electrons

Equilibrium acidities in DMSO for several cyclic carboxamides, thiocarboxamides, esters, and sulfones that form anions possessing 4n+2 electrons have been measured. Aromatic stabilization energies (ASEs) for these anions have been estimated by comparing their pK HA values with those of open-chain analogues. The ASEs (kcal/mol) are 8.3 for N-methylindolin-2-one, 15.5 for N-methylindoline-2-thione, 7.1 for 2-oxo-2,3-dihydrobenzo[b]furan, 8.5 for 2-oxo-2,3-dihydrobenzo[b]thiophene, 11.4 for 3-phenyl-2H-thiopyran 1,1-dioxide, and 23 for cyclopentadiene. These values need to be corrected, however, for the effects of cyclization on pK HA values, which are about 3 kcal/mol for carboxamides and 5 kcal/mol for esters

Comparisons of the acidities and homolytic bond dissociation energies of the acidic nitrogen-hydrogen and carbon-hydrogen bonds in diphenylmethanes and carbazoles

The acidities and homolytic bond dissociation energies (BDEs) of the acidic H-C or H-N bonds in six α-substituted diphenylmethanes, Ph 2 CHG, nine remotely substituted diphenylamines, four bridged diphenylamines (iminodibenzyl, iminostilbene, phenothiazine, phenoxazine), three carbazoles, indole, and pyrrole are reported. The near constancy of the increased acidity for 9-G-fluorenes compared to α-G-diphenylmethanes, which represents the approximate relative aromatic stabilization energies of the fluorenide anions, indicates that electronic effects on 9-GFl − and Ph 2 CG − ions do not differ greatly. On the other hand, steric effects in Ph 2 CG • radicals cause destabilizing effects when G is SPh, CO 2 Et, or SO 2 Ph, whereas, except for SO 2 Ph, these functions are stabilizing in 9-GFl • radicals

Theoretically-derived, energy-based criteria for aromaticity

Previous energy-based criteria for aromaticity developed from one electron eigenvalues (using methods ranging from Huckel to ab initio calculations) have not correlated well with experimentally derived criteria such as χ or chemical reactivity data, and are particularly poor for heteroaromatic compounds. Previous work has shown that criteria derived from the lowest π molecular orbitals of heteroaromatic compounds correlate well with χ and chemical reactivity, prompting us to modify Kollmar's procedure for determining aromatic stabilization energies by localizing the lowest π orbital rather than the entire π bonding manifold. This modified procedure was implemented with both the natural (NLMO) and Boys orbital localization methods, and was applied to a set of ten five-membered ring heteroaromatics and to pyridine and its diazine derivatives. The new criterion ΔπL correlated well with χ (R2 = 0.86 (NLMO); R2 = 0.92 (Boys)) for five-membered rings, and with theoretical measures of aromaticity for both sets of heteroaromatics. The Boys procedure gave better overall correlations and was markedly superior for pyrazole and pyridazine.

How Do Proton Chemical Shifts Reflect Aromaticity ? Graph Theory of the Secondary Magnetic Field Due to Ring Currents

Abstract A secular determinant for a polycyclic conjugated molecule, combined with a test dipole, is definable as a function of the applied magnetic field. It was found that this determinant can be expanded graph-theoretically into a characteristic polynomial. A secondary magnetic field is brought about by so-called ring currents. A new formula for evaluating the secondary field was derived by applying Newton’s method to this polynomial. This formula indicates that the secondary field at any point in the molecular plane is given exactly as a sum of the contributions of all π-electron circuits. The contribution of each π-electron circuit is expressible in terms of a pure geometric factor and an aromatic stabilization energy assigned to the circuit. The secondary field and then the proton chemical shifts can thus be related to aromaticities of individual π-electron circuits. Magnetotropism of most polycyclic conjugated molecules can be rationalized in this manner. π-Electron circuits which share the conjugated atom nearest to a given proton dominate the chemical shift. In general, a peripheral π-electron circuit contributes modestly to the overall thermodynamic stability of a molecule, but contributes much to the secondary fields at attached protons.

Hexasilabenzene (Si6H6). An ab initio theoretical study of its aromaticity and relative stability

Ab initio calculations show that hexasilabenzene has approximately half the aromatic stabilization energy of benzene, and lies very close in stability to its valence Si6H6 isomers, this being in sharp contrast with the fact that benzene is by far the most stable C6H6 isomer.

Quantitative measure of the aromatic stabilization energy of 2-pyridone and related compounds

Consideration of tautomeric equilibria allows the estimation of the aromatic stabilization energy of 2-pyridone as 25 kcal mol–1: the method is applied to related compounds.