1
|
Farcasiu M, Farcasiu D, Conlin RT, Jones M, Schleyer PR. Fate of 1-adamantyl carbene. Evidence for the formation of the bridgehead olefin 3-homoadamantene. J Am Chem Soc 2002. [DOI: 10.1021/ja00805a060] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
|
2
|
Krogh-Jespersen K, Cremer D, Dill JD, Pople JA, Schleyer PR. Aromaticity in small rings containing boron and carbon, ((CH)2(BH)n, n = 1,2): comarisons with isoelectronic carbocations. The decisive roles of orbital mixing and nonbonded 1,3-interactions in the structures of four-membered rings. J Am Chem Soc 2002. [DOI: 10.1021/ja00400a018] [Citation(s) in RCA: 135] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
|
3
|
Kaiser RI, Le TN, Nguyen TL, Mebel AM, Balucani N, Lee YT, Stahl F, Schleyer PR, Schaefer HF. A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons--pathways to hydrogen deficient hydrocarbon radicals in combustion flames. Faraday Discuss 2002:51-66; discussion 121-43. [PMID: 11878006 DOI: 10.1039/b101967h] [Citation(s) in RCA: 83] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Crossed molecular beam experiments on dicarbon and tricarbon reactions with unsaturated hydrocarbons acetylene, methylacetylene, and ethylene were performed to investigate the dynamics of channels leading to hydrogen-deficient hydrocarbon radicals. In the light of the results of new ab initio calculations, the experimental data suggest that these reactions are governed by an initial addition of C2/C3 to the pi molecular orbitals forming highly unsaturated cyclic structures. These intermediates are connected via various transition states and are suggested to ring open to chain isomers which decompose predominantly by displacement of atomic hydrogen, forming C4H, C5H, HCCCCCH2, HCCCCCCH3, H2CCCCH and H2CCCCCH. The C2(1 sigma g+) + C2H4 reaction has no entrance barrier and the channel leading to the H2CCCCH product is strongly exothermic. This is in strong contrast with the C3(1 sigma g+) + C2H4 reaction as this is characterized by a 26.4 kJ mol-1 threshold to form a HCCCCCH2 isomer. Analogous to the behavior with ethylene, preliminary results on the reactions of C2 and C3 with C2H2 and CH3CCH showed the H-displacement channels of these systems to share many similarities such as the absence/presence of an entrance barrier and the reaction mechanism. The explicit identification of the C2/C3 vs. hydrogen displacement demonstrates that hydrogen-deficient hydrocarbon radicals can be formed easily in environments like those of combustion processes. Our work is a first step towards a systematic database of the intermediates and the reaction products which are involved in this important class of reactions. These findings should be included in future models of PAH and soot formation in combustion flames.
Collapse
Affiliation(s)
- R I Kaiser
- Department of Chemistry, University of York, York, UK YO10 5DD
| | | | | | | | | | | | | | | | | |
Collapse
|
4
|
Charkin OP, Klimenko NM, Moran D, Mebel AM, Charkin DO, Schleyer PR. Theoretical study of icosahedral closo-borane, -alane, and -gallane dianions (A(12)H(12)(2-); A = B, Al, Ga) with endohedral noble gas atoms (Ng = He, Ne, Ar, and Kr) and their lithium salts (Li[Ng@A(12)H(12)](-) and Li(2)[Ng@A(12)H(12)]). Inorg Chem 2001; 40:6913-22. [PMID: 11754272 DOI: 10.1021/ic010573s] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
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.
Collapse
Affiliation(s)
- O P Charkin
- Institute of Atomic and Molecular Sciences, Academia Sinica, P.O. Box 23-166, Taipei 10764, Taiwan, ROC
| | | | | | | | | | | |
Collapse
|
5
|
Martin NH, Brown JD, Nance KH, Schaefer HF, Schleyer PR, Wang ZX, Woodcock HL. Analysis of the origin of through-space proton NMR deshielding by selected organic functional groups. Org Lett 2001; 3:3823-6. [PMID: 11720545 DOI: 10.1021/ol016500u] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
GIAO-HF and IGLO-DFT computations of isotropic magnetic shieldings were used to map the NMR shielding environments of small molecules exemplifying selected organic functional groups. Two different probes were employed: a methane molecule and NICS (nucleus-independent chemical shifts) based on computed absolute isotropic shieldings. The reason for the different results obtained using these two probes is perturbation of the wave function by the proximity of methane to the pi bond, as analyzed by the localized orbital contributions to the shieldings. [structure: see text]
Collapse
Affiliation(s)
- N H Martin
- Department of Chemistry, University of North Carolina at Wilmington, Wilmington, NC 28403-5932, USA.
| | | | | | | | | | | | | |
Collapse
|
6
|
Alder RW, Harvey JN, Schleyer PR, Moran D. T(h)-symmetrical N8(C=C)6 and P8(C=C)6; an extraordinary contrast in heterofullerene stability. Org Lett 2001; 3:3233-6. [PMID: 11594802 DOI: 10.1021/ol016163p] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
[structure: see text]. T(h)-symmetrical P8(C=C)6, 1, is predicted to be a remarkably stable small heterofullerene with carbon atoms less pyramidal than in C60. T(h)-N8(C=C)6, 2, in sharp contrast, is strongly destabilized relative to T(h)-(HC)8(C=C)6. The causes of this extraordinarily large difference (nearly 1000 kJ x mol(-1) between 1 and 2 are explained.
Collapse
Affiliation(s)
- R W Alder
- School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
| | | | | | | |
Collapse
|
7
|
Abstract
Density-functional theory calculations predict that three borocarbon units with planar pentacoordinate carbons -C3B3-, -C2B4-, and -CB5-, can replace the -(CH)3- subunits in aromatic or even in antiaromatic hydrocarbons to construct "hyparenes" (families of molecules with planar pentacoordinate carbons). These borocarbon units contribute two, one, and zero electrons, respectively, to the parent pi system. Depending on the choice of these units, the hyparenes (judging from computed proton and nucleus-independent chemical shifts), can maintain or can interconvert the aromatic or antiaromatic character of the parent compounds. The hyparenes are low-lying local minima with normal carbon-boron, boron-boron, and carbon-carbon bond lengths. The multicenter bonding in the hyparenes involves contributions of partial sigma and partial pi bonds to the planar pentacoordinate carbons; the octet rule is not violated. Borocarbon species, for which there is some mass spectrometric evidence, might be observed and identified, for example, in matrix isolation by vibrational spectroscopy.
Collapse
Affiliation(s)
- Z X Wang
- Center for Computational Quantum Chemistry, Computational Chemistry Annex, University of Georgia, Athens, GA 30602-2525, USA.
| | | |
Collapse
|
8
|
Affiliation(s)
- P R Schleyer
- University of Georgia, Athens, Georgia University of Erlangen-Nuremberg, Germany
| |
Collapse
|
9
|
Hofmann M, Fox MA, Greatrex R, Schleyer PR, Williams RE. Empirical and ab initio energy/architectural patterns for 73 nido-6<V>-carborane isomers, from B(6)H(9)(-) to C(4)B(2)H(6). Inorg Chem 2001; 40:1790-801. [PMID: 11312733 DOI: 10.1021/ic000844j] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Qualitative rules governing carbon and bridge-hydrogen placement permit the prediction of the most stable isomeric structures for the various carboranes. Seventy-three isomeric boron hydride and carborane structures, from B(6)H(9)(-) to C(4)B(2)H(6), were computed at the ab initio MP2(fc)/6-31G level to determine their relative stabilities quantitatively. Specific architectural features, recognized to be unfavorable, were assigned "energy penalty" values that allow the projection of comprehensive thermodynamic stability values via a simple additivity procedure. These values match the ab initio results with surprising precision. Our study includes Siebert's nido-2,3,5-C(3)B(3)H(7) and Wrackmeyer's nido-2,4-C(2)B(4)H(8) nido-6<V> carboranes, which contain "unusual" CH-B-bridge hydrogens.
Collapse
Affiliation(s)
- M Hofmann
- Loker Hydrocarbon Research Institute, University of Southern California, University Park, Los Angeles, California 90089-1661, USA
| | | | | | | | | |
Collapse
|
10
|
Mauksch M, Schleyer PR. Effective monkey saddle points and berry and lever mechanisms in the topomerization of SF(4) and related tetracoordinated AX(4) species. Inorg Chem 2001; 40:1756-69. [PMID: 11312730 DOI: 10.1021/ic990500x] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The topomerization mechanisms of the SF(4) and SCl(2)F(2) sulfuranes, as well as their higher (SeF(4), TeF(4)) and isoelectronic analogues PF(4)(-), AsF(4)(-), SbF(4)(-), SbCl(4)(-), ClF(4)(+), BrF(4)(+), BrCl(2)F(2)(+), and IF(4)(+)), have been computed at B3LYP/6-31+G and at B3LYP/6-311+G. All species have trigonal bipyramidal (TBP) C(2)(v)() ground states. In such four-coordinated molecules, Berry rotation exchanges both axial with two equatorial ligands simultaneously while the alternative "lever" mechanism exchanges only one axial ligand with one equatorial ligand. While the barrier for the lever exchange in SF(4) (18.8 kcal mol(-1)) is much higher than that for the Berry process (8.1 kcal mol(-1)), both mechanisms are needed for complete ligand exchange. The F(ax)F(ax) and F(eq)F(eq) isomers of SF(2)Cl(2) have nearly the same energy and readily interconvert by BPR with a barrier of 7.6 kcal mol(-1). The enantiomerization of the F(ax)F(eq) chiral isomer can occur by either the Berry process (transition state barrier 8.3 kcal mol(-1)) or the "lever" mechanism via either of two C(s)() transition states, based on the TBP geometry: Cl(ax) <--> Cl(eq) or F(ax) <--> F(eq) exchanges with barriers of 6.3 and 15.7 kcal mol(-1), respectively. Full scrambling of all ligand sites is possible only by inclusion of the lever mechanism. Planar, "tetrahedral", and triplet forms are much higher in energy. The TBP C(3)(v) structures of AX(4) either have two imaginary frequencies (NIMAG = 2) for the X = F, Cl species or are minima (NIMAG = 0) for the X = Br, I compounds. These "effective monkey saddle points" have degenerate modes with two small frequencies, imaginary or real. Although a strictly defined "monkey saddle" (with degenerate frequencies exactly zero) is not allowed, the flat C(3)(v) symmetry region serves as a "transition state" for trifurcation of the pathways. The BPR mechanism also is preferred over the alternative lever process in the topomerization of the selenurane SeF(4) (barriers 5.9 vs. 12.1 kcal mol(-1)), the tellurane TeF(4) (2.1 vs. 6.4), and the interhalogen cations ClF(4)(+) (2.5 vs 14.8), BrF(4)(+) (4.7 vs. 11.3), BrF(2)Cl(2)(+) (14.6 vs. 17.4), and IF(4)(+) (1.4 vs. 6.0), as well as for the series PF(4)(-) (7.0 vs. 9.0), AsF(4)(-) (9.3 vs. 17.2), and SbF(4)(-) (3.8 vs. 5.3 kcal mol(-1)), all computed at B3LYP/6-311+G with the inclusion of quasirelativistic pseudopotentials for Te, I, and Sb. The heavier halogens increasingly favor the lever process, where the barrier (2.6 kcal mol(-1)) pertaining to the effective monkey saddle point (C(3)(v) minimum for SbCl(4)(-)) is less than that for the Berry process (8.2 kcal mol(-1)).
Collapse
Affiliation(s)
- M Mauksch
- Computer Chemie Centrum, Universität Erlangen-Nürnberg, Nägelsbachstrasse 25, 91052 Erlangen, Germany
| | | |
Collapse
|
11
|
Wang ZX, Manojkumar TK, Wannere C, Schleyer PR. A Theoretical Prediction of Potentially Observable Lithium Compounds with Planar Tetracoordinate Carbons. Org Lett 2001; 3:1249-52. [PMID: 11348206 DOI: 10.1021/ol015573a] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Several potentially experimentally accessible lithiated heterocyclic and heteroatom compounds with planar tetracoordinate carbons (ptC) have been predicted computationally. These utilize the strong electron-donating ability and the bridging proclivity of lithium to achieve the ptC preferences. As the p orbitals on the central carbons are only partially occupied, their electronic structures are similar to those of the related carbenes, e.g. imidazole-2-ylidene, rather than to the other ptC compounds such as dilithiocyclopropane.
Collapse
Affiliation(s)
- Z X Wang
- Computational Chemistry Annex, Department of Chemistry, University of Georgia, Athens, Georgia 30602-2525, USA
| | | | | | | |
Collapse
|
12
|
Schleyer PR, Schaefer HF. Structural and Mechanistic Organic Chemistry: A Tribute to Professor Norman L. Allinger. Mol Phys 1996. [DOI: 10.1080/00268979609484524] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
|
13
|
|
14
|
Brown HC, Rothberg I, Schleyer PR, Donaldson MM, Harper JJ. The slow rates of solvolysis of endo-5,6-trimethylene-endo-(2 and 8)-norbornyl tosylates: steric hindrance to ionization and the foote-schleyer correlation. Proc Natl Acad Sci U S A 1966; 56:1653-60. [PMID: 16591398 PMCID: PMC220149 DOI: 10.1073/pnas.56.6.1653] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Affiliation(s)
- H C Brown
- R. B. WETHERILL LABORATORY, PURDUE UNIVERSIT
| | | | | | | | | |
Collapse
|