Relative Stability Of Various Isomers Research Articles

Hydrogen bonding-assisted transformations of cyclic chalcones: E/Z-isomerization, self-association and unusual tautomerism

Condensation of 1- or 2-indanones and 1,3-indandiones with aromatic or heteroaromatic carbaldehydes gives products that demonstrate various transformations and effects of general interest for organic chemistry. In the present review, phenomena such as tautomerism, E/Z-isomerization, π-conjugation, hydrogen bonding, and homo- and heteroassociation are considered. The relative stability of various isomers of cyclic chalcones is shown to be determined by the intramolecular hydrogen bonding in their molecules and the formation of associates. The same effects also result in the formation of unusual, otherwise unstable tautomers, such as 2H-indazoles. The hydrogen bonding-assisted keto-enol and E/Z-isomerization are analyzed. A novel, specific type of conjugation for the studied compounds was proposed and termed ‘roundabout’ conjugation.The bibliography includes 101 references.

Localized-orbital locator (LOL) profiles of transition-metal hydride and dihydrogen complexes,

A bond descriptor based on the kinetic-energy density, the localized-orbital locator (LOL), is used to characterize the nature of the chemical bond in transition-metal hydride and dihydrogen complexes. Cationic complexes of the iron triad [MH3(PMe3)4]+(M = Fe, Ru, Os) serve as model compounds for transition-metal hydrogen bonding, since these complexes not only present examples for hydride as well as dihydrogen complexes, but for certain representatives, the two different types of metal–hydrogen bonds are realized within the same molecule. Both types of ligands show characteristic LOL profiles: (3,–3) Γ attractors in close vicinity to the H-atom for hydride ligands, and (3,–3) Γ attractors located between the two atoms for a dihydrogen ligand with νΓ-values of 0.8 and 0.9, respectively. In-between structures combine elements of the hydride and dihydrogen ligands. Relativistic effects on the relative stability of various isomers for the set of model compounds have been evaluated.

Minimum Polarizability Principle Applied to Lowest Energy Isomers of Some Gaseous All-Metal Clusters

In comparison with the minimum energy criterion as an indicator of the most stable state, the minimum polarizability and maximum hardness principles have been examined to describe the relative stability of various isomers of nine gaseous all-metal clusters M4X- (Cu4Na-, Cu4Li-, Al4Cu-, Ag4Li-, Au4Li-, Ag4Na-, Au4Na-, Al4Ag-, Al4Au-) on the basis of MP2 calculations. In these species, there are two lowest energy isomers with near isoenergy that sometimes make it very difficult to determine which of them is more stable when we depend only on the minimum energy criterion. According to the minimum polarizability principle, however, the square-pyramidal structure is always more stable than the planar isomer at various computational levels, which was also confirmed by the results from the minimum energy principle that sometimes requires higher computational precision. Thus, there is an indication that, at least for our present cluster system, the minimum polarizability principle is less dependent on the computational levels compared to the minimum energy principle.

A DFT study on the dimerization of C62, H2C62, and F2C62

On the basis of calculations using the density functional theory, we show that C(62), a recently synthesized nonclassical fullerene, will presumably undergo dimerization with various isomers at elevated temperatures. This is shown by calculating the dimerization energy and the activation barrier of the dimerization. Eight possible isomers of the dimer were identified, all of which are more stable than the two isolated monomers. The relative stability of various isomers depends upon the kind of C=C bonds within the four-membered carbon ring involved in the dimerization. In addition, similar calculations were performed for the monomers and dimers of H(2)-C(62) and F(2)-C(62). Six isomers were identified for each of the dimers. Although less pronounced than the case of the C(62) dimer, all isomers of the H(2)-C(62) dimer are appreciably more stable than the individual monomers. Although a large steric repulsion due to F atoms significantly reduces the stability of F(2)-C(62) dimer, its two isomers are still more stable than separate monomers.

DFT Study on the Stabilities of the Heterofullerenes Sc3N@C67B, Sc3N@C67N, and Sc3N@C66BN

On the basis of calculations using density functional theory, we investigated the relative stabilities of all isomers of Sc3N@C67B and Sc3N@C67N as well as those of stable isomers of Sc3N@C66BN. As a result, we predict that Sc3N@C68 can be doped substitutionally with a boron atom much better than C60. This effect can be ascribed to the favorable electrostatic attraction between the encased Sc3N cluster and the polar C-B bonds of the fullerene cage, which show the important role played by the encapsulated atoms in stabilizing the fullerene. A difference in the interaction also determines the regiospecificity of Sc3N@C67B. On the contrary, N-doping of the fullerenes forming Sc3N@C67N is much less favorable than that in C60 or C70. A judicious choice of stable isomers of Sc3N@C66BN among a vast number of possible isomers indicates that Sc3N@C68 can also be doped with a pair of B and N atoms better than C60 under the simultaneous existence of B and N sources. Relative stabilities of various isomers of the BN-substituted fullerenes can be understood in terms of the combined electrostatic effects in the B- and N-substitutions of Sc3N@C68 complemented by a specific local preference in the N-substitution and the formation of a B-N bond.

Ab initio study of coinage metal telluride clusters (M 2Te) n (M=Cu, Ag, Au; n=2, 3)

Coinage metal telluride clusters (M 2Te) n ( n=2, 3; M=Cu, Ag, Au) are studied by ab initio calculations including electron correlation at the Møller–Plesset second-order perturbation theory (MP2) level. The relativistic and non-relativistic pseudopotentials are employed to describe the core electrons. Various stable isomers are found for clusters, and the energy separations are rather small with energy hypersurfaces that are very shallow. The electron correlation and relativistic effects on geometrical structures and relative stabilities of various isomers are discussed.

Structures, energetics, and spectra of OH−(H2O)n and SH−(H2O)n clusters, n=1–5: Ab initio study

OH − ( H 2 O ) n and SH−(H2O)n (n=1–5) clusters have been studied using ab initio calculations. The relative stabilities of various isomers of SH−(H2O)n are quite different from those of the corresponding isomers of OH−(H2O)n (n=3–5) at 0 K, and are almost the same as those of the corresponding isomers of OH−(H2O)n at 298 K. That is, the relative stabilities of various isomers of SH−(H2O)n and OH−(H2O)n at 298 K are quite different from those at 0 K due to the entropy effect. The ionization potential, charge-transfer-to-solvent energy, and OH stretching vibrational spectra are reported to facilitate future experimental work.

Electronic properties of polyoxometalates: a DFT study of alpha/beta-[XM(12)O(40)](n-) relative stability (M=W, Mo and X a main group element).

Keggin heteropolyanions [XM(12)O(40)](n-) have various isomeric structures, alpha and beta being the most common. Conventionally, the alpha structure appears to be the most stable, but calculations carried out at the DFT level for X = P(V), Si(IV), Al(III), As(V), Ge(IV), and Ga(III) and M = W(VI) and Mo(VI) show that this stability depends on several factors, particularly on the nature of the heteroatom (X) and the total charge of the cluster. In this paper, we apply the clathrate model to the Keggin molecule to carry out a fragment-interaction study to elucidate when and why the traditional relative stability of various isomers can be inverted. The fully oxidized anions that have inverted the traditional stability trend in this series are [AlW(12)O(40)](5-) and [GaW(12)O(40)](5-), both of which contain a third-group heteroatom and an overall charge of -5. beta-isomers are always more easily reduced than alpha-isomers. This experimental observation suggests that reduction favors the stability of beta-isomers and one of the most important results of this study is that the alpha/beta inversion is achieved in most cases after the second reduction. The alpha- and beta-isomers may have different properties because the energy of the LUMO, a symmetry-adapted d(xy)-metal orbital, is different.

Boron−Nitrogen (BN) Substitution Patterns in C/BN Hybrid Fullerenes: C60-2x(BN)x (x = 1−7)

Semiempirical AM1 and MNDO and density functional theory (B3LYP/3-21G and 6-31G*) are used to examine the relative stability of various isomers of successive BN substituted fullerenes C60-2x(BN)x, where x=1−7. It is found that stability is enhanced by keeping BN units and BN filled hexagons adjacent to one another. Successive BN substitution prefers N site attachment to the existing BN chain. The localization of MOs shows that lone-pairs of nitrogen atoms reside “inside” the cage, which may be the reason for outward displacement of N atoms. Geometric parameters and charge distribution of the carbon region of hybrid fullerenes are not much perturbed by BN substitution. Band gap (HOMO−LUMO gap), ionization potential, and electron affinities strongly depend on the number of BN units and filling of the hexagons. Partially BN filled hexagons or unsaturated BN fullerenes have a stronger effect than completely filled hexagons on perturbing these properties.

Overlap control and stability of polyhedral molecules. closo-Carboranes

closo-Carboranes that can be formally divided into rings and caps follow a six-electron rule. The relative stability of various isomers within a given carborane depends on the size of the ring on which the polyhedral structure is based. With three-and four-membered rings the CH group fits in as the best cap; the overlap of the orbitals of CH with the orbitals of three-and four-membered borocycles is favourable. Thus 1,5-C2B3H5 is more stable than 1,2-C2B3H5. The BH group with more diffuse orbitals overlaps better with the orbitals of a five-membered ring;1,7-C2B5H7 is less stable than 1,2-C2B5H7, which, in turn, is less stable than 2,4-C2B5H7. The orbitals of BH are not diffuse enough to overlap favourably with the orbitals of a six-membered ring; C2B6H8 prefers a dodecahedron to a hexagonal bipyramid. Caps with more diffuse orbitals such as BeH, Li, and transitional-metal fragments should stabilize polyhedral based on six-membered rings.

Quantum chemical calculations of fluoroethylene molecules by the CNDO and INDO methods

A quantum chemical calculation of seven fluoroethylene molecules and two fluorochloroethylene molecules and two fluorochloroethylene molecules by the CNDO and INDO methods has made it possible to explain the experimentally observed change in reactivity and direction of addition reactions at the double bond. The data of a calculation of the relative stability of various isomers also agree with the experimental results.