Cyclopropyl Homoconjugation—Experimental Facts and Interpretations

Childs, Ronald F.; Cremer, Dieter; Elia, George

doi:10.1002/0470023481.ch8

Cited by 26 publications

(45 citation statements)

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“…For an accurate description of 1 in benzene solution, we have applied the NMR/ab initio/IGLO method (IGLO:individual gauge for localized orbitals), which is perfectly suited for the determination of molecular geometries in solution. − The method is based on the sensitivity of calculated NMR chemical shifts with regard to the geometry used. − Previous investigations have shown that reasonable δ( 29 Si) NMR chemical shifts are obtained if the geometry of the Si compound in question has been optimized at the Hartree−Fock (HF) or second-order Møller−Plesset (MP2) level using a DZ+P basis sets. − Since the calculated NMR chemical shifts clearly depend on the geometry, an agreement between experimental and theoretical shifts not only means a clear identification but also a geometical determination of the molecule in question …”

Section: Nmr/ab Initio/iglo Methods and Determination Of Molecular Ge...mentioning

confidence: 99%

Nature of the Si(SiMe₃)₃⁺Cation in Aromatic Solvents

Ottosson

Cremer

1996

Organometallics

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The silylium cation Si(SiMe 3 ) 3 + (1) was investigated by HF, B3LYP, PISA-HF, SCRF, and the NMR/ab initio/IGLO approach in the gas phase and in benzene solution employing the 6-31G(d) basis set. In the gas phase, the δ( 29 Si) value of the central Si atom of 1 is 920 ppm relative to that of TMS according to IGLO/[7s6p2d/5s4p1d/3s1p]//HF/6-31G(d) calculations, which is in line with the corresponding δ( 29 Si) values calculated for other silyl-substituted silylium cations and results from large paramagnetic contributions to the δ( 29 Si) shift value. In benzene solution, 1 forms the Wheland σ-complex Si(SiMe 3 ) 3 -C 6 H 6 + (3), in which the silylium cation character of 1 is lost despite the fact that 3 is the weakest silylium cationbenzene complex so far investigated. According to NMR/ab initio/IGLO calculations, complex 3 is characterized by a weak covalent SiC bond of 2.293 Å, a coordination energy of 13.6 kcal/mol, and a δ( 29 Si) value of 111 ppm. Steric blocking by three SiMe 3 groups is not sufficient to prevent a silylium cation from interacting with aromatic solvent molecules.

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Section: Nmr/ab Initio/iglo Methods and Determination Of Molecular Ge...mentioning

confidence: 99%

Nature of the Si(SiMe₃)₃⁺Cation in Aromatic Solvents

Ottosson

Cremer

1996

Organometallics

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“…Since the first synthesis of 1a by Zimmerman et al, more than four decades ago, semibullvalene has fascinated chemists because it undergoes a degenerate boat Cope rearrangement with an enthalpic barrier of only Δ H ⧧ = 5.0 ± 0.2 kcal/mol. , The low barrier height is one reason why 1 might be a good candidate for undergoing this reaction rapidly at cryogenic temperatures by tunneling…”

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confidence: 99%

Calculations Predict That Carbon Tunneling Allows the Degenerate Cope Rearrangement of Semibullvalene to Occur Rapidly at Cryogenic Temperatures

2010

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Calculations on the role of tunneling in the degenerate Cope rearrangements of semibullvalene (1) and barbaralane (3) predict that, at temperatures below 40 K, tunneling from the lowest vibrational level should make the temperature-independent rate constants k ) 1.43 × 10 -3 s -1 and k ) 7.28 × 10 -9 s -1 , respectively. An experiment, using semibullvalene-2(4)-d 1 , is proposed to test the prediction of rapid tunneling by 1 at cryogenic temperatures.

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“…Various carbenes have been computationally explored to extend the concept of homoaromaticity to neutral structures . After considering several examples, Cremer et al concluded that the bicyclo[3.2.1]octadienyl anion, 20 (Figure ), is most likely a potential contender for homoaromaticity . Brown and Occolowitz proposed that the stability of the bicyclo[3.2.1]octadienyl anion, 20a and 20b , can be explained by invoking the possibility of a π-delocalized resonance structure, 20c (Figure ).…”

Section: Resultsmentioning

confidence: 99%

The Design of “Neutral” Carbanions with Intramolecular Charge Compensation

2016

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Strategies to construct zwitterionic anions from the parent anions are proposed. Two principles are employed; the cationic counterpart is (a) attached as a substituent or (b) inserted as an integral part at a remote location in the assembly. The optimized geometries reveal that a striking similarity exists between the zwitterions and the respective precursor parent anion. The computed vibrational frequencies emphasize that these novel entities are minima on their respective potential energy surfaces. A substantial HOMO-LUMO gap indicates that the proposed structures do not show instability in their respective electronic states and that the higher energy configuration states do not contribute to the ground state viability. The separation of charge between the monopoles in these zwitterions is demonstrated by moderately large nonzero dipole moments. Significant large energy barriers for rearrangement to the closely related positional isomers, demonstrated in a few cases, advocate the thermal stability (associated with spectroscopic viability) of the novel molecules. The donor capacity (basicity) of the anionic subunit in these zwitterions is comparable to that of the respective parent anions. Since the qualitative and quantitative features in the designed charged compensated complexes are conserved as anions, these molecules may perhaps be employed in synthetic organic or organometallic chemistry.

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Cyclopropyl Homoconjugation—Experimental Facts and Interpretations

Cited by 26 publications

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Nature of the Si(SiMe₃)₃⁺Cation in Aromatic Solvents

Nature of the Si(SiMe₃)₃⁺Cation in Aromatic Solvents

Calculations Predict That Carbon Tunneling Allows the Degenerate Cope Rearrangement of Semibullvalene to Occur Rapidly at Cryogenic Temperatures

The Design of “Neutral” Carbanions with Intramolecular Charge Compensation

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Cyclopropyl Homoconjugation—Experimental Facts and Interpretations

Cited by 26 publications

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Nature of the Si(SiMe3)3+Cation in Aromatic Solvents

Nature of the Si(SiMe3)3+Cation in Aromatic Solvents

Calculations Predict That Carbon Tunneling Allows the Degenerate Cope Rearrangement of Semibullvalene to Occur Rapidly at Cryogenic Temperatures

The Design of “Neutral” Carbanions with Intramolecular Charge Compensation

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Nature of the Si(SiMe₃)₃⁺Cation in Aromatic Solvents

Nature of the Si(SiMe₃)₃⁺Cation in Aromatic Solvents