The thermodynamic stabilities and heats of formation of tricyclic C12H12 tetraenes 1–6 as well as of a truncated tetrahedron hydrocarbon isomer 7 were computed by various density functional methods in conjunction with a polarized double-ζ basis set. As the DFT stabilities of 1–7 differ significantly from the MM2, MM3, MM4 and AM1 results, we conclude that these empirical and semiempirical methods are inappropriate to study such polycyclic hydrocarbons. Compound 6 with only endocyclic double bonds, a potential synthetic precursor of 7, is found to be less favorable energetically than the other isomers. Although the Csp3–Csp3 single bonds in 1–6 are rather long (1.601 to 1.620 Å) due to a combination of ring strain and hyperconjugation, the small nucleus-independent chemical shift (NICS) values of 1 and 6 confirm the expectation that cyclic electron delocalization is lacking. In contrast, NICS is unusually large in the cage center (–14.6) of 7, but this is due to the cumulative diatropic influence of the four cyclopropane rings.
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXT1,2-Dilithioethane. A molecular orbital studyAlexander J. Kos, Eluvathingal D. Jemmis, Paul von Rague Schleyer, Rolf Gleiter, Ursula Fischbach, and John A. PopleCite this: J. Am. Chem. Soc. 1981, 103, 17, 4996–5002Publication Date (Print):August 1, 1981Publication History Published online1 May 2002Published inissue 1 August 1981https://pubs.acs.org/doi/10.1021/ja00407a003https://doi.org/10.1021/ja00407a003research-articleACS PublicationsRequest reuse permissionsArticle Views123Altmetric-Citations55LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts
Eher die Ausnahme als die Regel sind ideale Tetraederwinkel in Organoelementverbindungen. Die Abweichungen können beträchtlich sein, wie die Aufweitung des C‐C‐B‐Winkels in Triethylboran auf fast 120° (Strukturbild rechts) zeigt. Dies läßt sich mit einer hyperkonjugativen Wechselwirkung des leeren Bor‐p‐Orbitals mit den bindenden Orbitalen der α‐C‐H‐Bindungen erklären, was durch ab‐initio‐Berechnungen und Strukturbestimmungen anderer Verbindungen mit dem Strukturelement Et‐X (X = Hauptgruppenelement) gestützt wird. magnified image
ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructures of complex beryllium hydrides and fluorides, LiBeX3 and Li2BeX4Ernst Ulrich Wuerthwein, Mary Beth Krogh-Jespersen, and Paul von Rague SchleyerCite this: Inorg. Chem. 1981, 20, 11, 3663–3667Publication Date (Print):November 1, 1981Publication History Published online1 May 2002Published inissue 1 November 1981https://pubs.acs.org/doi/10.1021/ic50225a016https://doi.org/10.1021/ic50225a016research-articleACS PublicationsRequest reuse permissionsArticle Views64Altmetric-Citations25LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts
Although surprising, the nonplanarity of C(2h) C(4)F(4) is not unique. While C(6)F(6) is planar, other members of the (CF)(n) family, for example, C(5)F(5)(-), C(6)F(6)(-), C(7)F(7)(-), and triplet C(7)F(7)(-) are not. C(2h) C(4)F(4) is not aromatic, as claimed (see above), but its antiaromaticity is reduced relative to the planar D(2h) form due to decreased pi antibonding and enhanced cross-ring pi overlap. The nonplanar C(2h) geometry also benefits from the relief of repulsive FC-CF bond eclipsing interactions.
Theoretical computations give new insights into the energetic, geometric, and magnetic properties of highly reactive o ‐benzyne. Comparisons with experimental NMR data for the species trapped in a hemicarcerand agree best for the geometry optimized at the Becke3LYP/6‐311 + G ** density functional level, which has more acetylenic (cyclohexa‐3,5‐dienyne) than cumulenic character.
Geometries, energies, vibrational frequencies, and magnetic properties have been computed at the B3LYP level with the 6-31G and 6-311+G basis sets for a family of endohedral closo-boranes, -alanes, and -gallanes Ng@A(12)H(12)(2-) with noble gas atoms (Ng) located in the centers of icosahedral [B(12)], [Al(12)], and [Ga(12)] clusters. The endohedral structures of most of the systems are minima lying above separated Ng + A(12)H(12)(2-) by 166 (He@B(12)H(12)(2-)) and 403 (Ne@B(12)H(12)(2-)) kcal/mol for boranes; 29 (He@Al(12)H(12)(2-)), 63 (Ne@Al(12)H(12)(2-)), 154 (Ar@Al(12)H(12)(2-)), and 189 (Kr@Al(12)H(12)(2-)) kcal/mol for alanes; and 39 (He@Ga(12)H(12)(2-)), 71 (Ne@Ga(12)H(12)(2-)), and 213 (Ar@Ga(12)H(12)(2-)) kcal/mol for gallanes. Three types of transition states are found for the exit of Ng from a cage: via an edge (TS-1), through a face (TS-2), and via a more extensive deformation through a pentagonal cage "neck" (TS-3). The most favorable exit path depends on the rigidity of the cage, the exothermicity of the dissociation, and the relationship between the size of the internal cavity of the cage and the Ng atomic radius. Ng exit via TS-3 is preferred for He@Al(12)H(12)(2-), Ne@Al(12)H(12)(2-), He@Ga(12)H(12)(2-), Ne@Ga(12)H(12)(2-), Ar@Al(12)H(12)(2), and Kr@Al(12)H(12)(2-). Helium exits via a cage edge (TS-1) for He@B(12)H(12)(2-), while for Ne@B(12)H(12)(2-) the neon exits via a triangular face (TS-2). Exit barriers (H(exit)(double dagger)) are high enough (30-60 kcal/mol) for all helium clusters and for Ne@Al(12)H(12)(2-) and Ne@Ga(12)H(12)(2-) to ensure the kinetic stability of these systems. The barriers for Ar@Al(12)H(12)(2-) and Kr@Al(12)H(12)(2-) decrease to 10-15 kcal/mol, while Ne@B(12)H(12)(2-) has a very low exit barrier and is not expected to be stable kinetically. There is a linear dependence of Ng@A(12)H(12)(2-) cage size on the Ng atomic radii; that is, the heavier Ng atoms "bulge" the cages. Nucleus independent chemical shifts (NICS) indicate that all three A(12)H(12)(2-) anions are aromatic but the alanes are the least so. A face- or edge-coordinated external Li(+) cation has a moderate effect on the structure and vibrational and magnetic properties of the helium-containing clusters, i.e., Li[He@A(12)H(12)](-). In contrast, for systems with very large exothermicities of Ng exit, Li(+) complexation promotes their dissociation. Thus, the internal atom Ne exits from the cage of Li[Ne@B(12)H(12)](-) and the salt dissociates into Ne + LiB(12)H(12)(-) without barrier. Systems with two Li(+) ions located initially above opposite cage faces (Li(2)[Ng@A(12)H(12)]) undergo complex intramolecular rearrangements leading to destruction of the icosahedral closo structures.
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