Nucleus-independent Chemical Shifts (nucleus-independent + chemical_shift)

Distribution by Scientific Domains

Selected Abstracts

New insights on the bridge carbon,carbon bond in propellanes: A theoretical study based on the analysis of the electron localization function

Victor Polo
Abstract The nature of the bonding between bridgehead carbon atoms (Ca, Ca,) as well as the ring strain in a family of 10 propellanes formed by three-, four-, or five-member rings: [1.1.1] (I), [2.1.1] (II), [3.1.1] (III), [2.2.1] (IV), [3.2.1] (V), [2.2.2] (VI), [3.3.1] (VII), [3.2.2] (VIII), [3.3.2] (IX), and [3.3.3] (X) are studied by means of the electron localization function (ELF) at the DFT level (B3LYP/cc-pVTZ). The ELF analysis of smaller propellanes (I, II, and III) reveals the coexistence of two resonance forms: one with a nonbonding electron pair partially delocalized between Ca and Ca, atoms outside the cage (ionic) and the other with a bridge bond between the same atoms (covalent). The weights of each form are calculated according to the ELF-basin populations, yielding 94, 88, and 53% for the ionic structure of I, II, and III, respectively, while larger propellanes (IV,X) present only the covalent form. The question of the s-character of the bridge bond is addressed by dissecting the bridge-bond ELF basin into the molecular orbital contributions. Finally, ,-aromaticity associated to surface electron delocalization has been analyzed by means of nucleus-independent chemical shift (NICS) calculations. The results point out that the stability of the fused ring structure of propellanes I, II, and III, can be assigned to the remarkable ,-aromaticity of the involved three-member rings. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source]

Planar mono-, di- aza- and phospha-naphthalene: Structure and aromaticity

Lei Wang
Abstract In this article, we used magnetic criteria, magnetic susceptibility isotropic and nucleus-independent chemical shifts (NICS), calculated with (density functional theory) B3LYP levels at the 6-31G** basis set, to evaluate aromaticity of a set of 29 planar bicyclic ,-electron systems: naphthalene and its mono- and di- aza- and phospha-derivatives. The result showed significant collinearity of the above two magnetic criteria when evaluating N-containing naphthalenes, but did not speak with the same voice when evaluating P-containing naphthalenes. Thus, we could conclude that magnetic susceptibility is a reliable criterion and NICS is not in our case. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source]

The existence of secondary orbital interactions

Chaitanya S. Wannere
Abstract B3LYP/6-311+G** (and MP2/6-311+G**) computations, performed for a series of Diels-Alder (DA) reactions, confirm that the endo transition states (TS) and the related Cope-TSs are favored energetically over the respective exo -TSs. Likewise, the computed magnetic properties (nucleus-independent chemical shifts and magnetic susceptibililties) of the endo - (as well as the Cope) TS's reveal their greater electron delocalization and greater aromaticity than the exo -TS's. However, Woodward and Hoffmann's original example is an exception: their endo -TS model, involving the DA reaction of a syn - with an anti -butadiene (BD), actually is disfavored energetically over the corresponding exo -TS; magnetic criteria also do not indicate the existence of SOI delocalization in either case. Instead, a strong energetic preference for endo -TSs due to SOI is found when both BDs are in the syn conformations. This is in accord with Alder and Stein's rule of "maximum accumulation of double bonds:" both the dienophile and the diene should have syn conformations. Plots along the IRC's show that the magnetic properties typically are most strongly exalted close to the energetic TS. Because of SOI, all the points along the endo reaction coordinates are more diatropic than along the corresponding exo pathways. We find weak SOI effects to be operative in the endo -TSs involved in the cycloadditions of cyclic alkenes, cyclopropene, aziridine, cyclobutene, and cyclopentene, with cyclopentadiene. While the endo -TSs are only slightly lower in energy than the respective exo -TSs, the magnetic properties of the endo -TS's are significantly exalted over those for the exo -TS's and the Natural Bond Orbitals indicate small stabilizing interactions between the methylene cycloalkene hydrogen orbitals (and lone pairs in case of aziridine) with ,-character and the diene , MOs. © 2006 Wiley Periodicals, Inc. J Comput Chem 2007 [source]

Aromatic stabilization in heterofullerenes C48X12 (X,=,N, P, B, Si),

Zhongfang Chen
Abstract B3LYP density functional calculations were performed on two S6 symmetrical isomers (I and II) of C48X12 (X=N, B, P, Si) heterofullerenes, and their global and local aromaticity were evaluated by nucleus-independent chemical shifts (NICS). Despite the unfavorable heteroatom repulsive interactions, isomer II is more stable than I owing to the combination of global and local aromaticity. The latter arises from the presence of triphenylene units in isomer II. The aromatic stabilization effects found in this study should be taken into account when predicting the most stable isomers of heterofullerenes. The same is true for predictions of the isomers of fullerene adducts such as C60Cl12. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Ring Currents as Probes of the Aromaticity of Inorganic Monocycles : P5,, As5,, S2N2, S3N3,, S4N3+, S4N42+, S5N5+, S42+ and Se42+

Frank De Proft Prof.
Abstract Current-density maps were calculated by the ipsocentric CTOCD-DZ/6-311G** (CTOCD-DZ=continuous transformation of origin of current density-diamagnetic zero) approach for three sets of inorganic monocycles: S42+, Se42+, S2N2, P5, and As5, with 6 , electrons; S3N3,, S4N3+ and S4N42+ with 10 , electrons; and S5N5+ with 14 , electrons. Ipsocentric orbital analysis was used to partition the currents into contributions from small groups of active electrons and to interpret the contributions in terms of symmetry- and energy-based selection rules. All nine systems were found to support diatropic , currents, reinforced by , circulations in P5,, As5,, S3N3,, S4N3+, S4N42+ and S5N5+, but opposed by them in S42+, Se42+ and S2N2. The opposition of , and , effects in the four-membered rings is compatible with height profiles of calculated NICS (nucleus-independent chemical shifts). [source]

Inside a Superatom: The M7q (M=Cu, Ag, q=1+, 0, 1,) Case

CHEMPHYSCHEM, Issue 3 2010
Alvaro Muñoz-Castro Dr.
Abstract All-electron relativistic density functional calculations are performed to obtain the electronic structure and nucleus-independent chemical shifts (NICS) of D5hpentagonal-bipyramidal (PBP) Cu7qand Ag7q(q=1+,0,1,) clusters. Scalar and spin,orbit relativistic effects are taken into account at two levels: the two-component zero-order regular approximation (ZORA) Hamiltonian and fully relativistic four-component calculations via the Dirac equation. These clusters are treated by including the spin,orbit effect in the jellium model, within the double-valued point group (D5h*) establishing the symmetry correlations between the molecular and the atomic spinors given by the full rotation group. These clusters show highly spherical aromaticity, which is suggested to increase the hardness of the superatom. Thus, the calculations suggest that the paramagnetic Cu7 and Ag7 clusters can be regarded as pseudohalogens. [source]