An alternative approach to the problem of assessing destabilization energies (strain energies) in cyclic hydrocarbons

George, Philip; Trachtman, Mendel; Bock, Charles W.; Brett, Alistair M.

doi:10.1016/0040-4020(76)80043-9

Cited by 356 publications

(226 citation statements)

References 4 publications

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“…These were defined originally [39] as having: "1) equal numbers of carbon atoms in their various states of hybridization in reactants and products and 2) equal numbers of carbon atoms with zero, one, two, and three H atoms attached in reactants and products." While George et al [41] stated that "these conditions imply that there are equal numbers of each type of CC bond (sp 3 -sp 3 , sp 3 -sp 2 , etc.) and that the various types of CÀH bonds are matched as closely as possible," this statement is not true for general hydrocarbon reactions.…”

Section: Resultsmentioning

confidence: 98%

The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations

Wodrich

Wannere

et al. 2007

Chemistry A European J

189

203

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Branched alkanes like isobutane and neopentane are more stable than their straight chain isomers, n-butane and n-pentane (by 2 and 5 kcal mol(-1), respectively). Electron correlation is largely responsible. Branched alkanes have a greater number of net attractive 1,3-alkyl-alkyl group interactions, there are three such stabilizing 1,3 "protobranching" dispositions in isobutane, but only two in n-butane. Neopentane has six protobranches but n-pentane only three. Propane has one protobranch and is stabilized appreciably, by 2.8 kcal mol(-1), relative to methane and ethane. This value per protobranch also applies to the n-alkanes and cyclohexane. Consequently, energy evaluations employing alkane reference standards, for example, of small ring strain and stabilizations due to conjugation, hyperconjugation, and aromaticity, should be corrected for protobranching, for example, by employing Pople's isodesmic bond separation reaction method. This reduces the ring strain of cyclopropane to 19.2 from the conventional 27.7 kcal mol(-1), while the stabilization energies of alkenes and alkynes due to hyperconjugation (5.5 and 7.7 kcal mol(-1) for propene and propyne) and conjugation (14.8 and 27.1 kcal mol(-1) for butadiene and butadiyne) are considerably larger than the traditional estimates. Widely diverging literature evaluations of benzene resonance energy all give approximately 65 kcal mol(-1) after adjusting for conjugation, hyperconjugation, and protobranching "contaminations." The BLW (block localized wavefunction) method, which localizes pi bonds and precludes their interactions, largely confirms these stabilization estimates for hyperconjugation, conjugation, and aromaticity. Protobranching is seriously underestimated by theoretical computations at the HF and most DFT levels, which do not account for electron correlation satisfactorily. Such levels give bond separation energies, which can differ greatly from experimental values.

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Section: Resultsmentioning

confidence: 98%

The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations

Wodrich

Wannere

et al. 2007

Chemistry A European J

189

203

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“…The reaction enthalpies of homodesmotic reactions [24] given in Equations (1)-(3) were calculated at the G3A C H T U N G T R E N N U N G (MP2)//B3 LYP/6-31G(d) level of theory. [25,26] The calculated strain energy of 42.7 kcal mol À1 for 9 is in excellent agreement with previous experimental estimates of 40-45 kcal mol À1 , [27] while the annellated phosphirane (E)-4 b' has a larger ring-strain energy of 53.9 kcal mol…”

Section: Resultsmentioning

confidence: 99%

Fused Tricyclic Phosphiranes—Analysis of Phosphorus Chemical Shieldings

Couzijn

Ehlers

Slootweg

et al. 2008

Chemistry A European J

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1,2‐Addition of transient W(CO)5‐complexed phosphinidenes exo to hexamethyl Dewar benzene affords the novel 3‐phosphatricyclo[3.2.0.02,4]hept‐6‐ene complexes. The fused tricyclic phosphiranes are obtained as both the Z and the thermally less stable E isomers, the 31P NMR chemical shifts of which differ by about 60 ppm. A computational investigation shows that the phosphorus pyramidalization and the presence of the γ double bond are responsible for this effect. The semiquantitative results contribute to a more systematic understanding of the structural influences on 31P chemical shieldings. The congested double bond of the Z isomer can be epoxidized with m‐chloroperbenzoic acid (MCPBA) to afford a fused tetracyclic P,O bis‐adduct.

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“…In the isodesmic scheme introduced by Hehre et al [27], both sides of the equation include the same number of bonds of given order between elements. The homodesmotic scheme [28], involving the same number of bonds of given order between elements of given hybridization states on each side of the equation, provides a better approach to the cyclicity stabilization value. The hyper-homodesmotic scheme involving the same number of bonds of given substitution pattern on each side of the equation [29] is supposed to give an even more reliable estimate of the energetic aromaticity.…”

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confidence: 99%

Ring carbo-mers: From questionable homoaromaticity to bench aromaticity

Zou

Lepetit

Coppel

et al. 2006

Pure and Applied Chemistry

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Abstract:The title journey is undertaken at the levels of both theory and experiment. Since 1983, homoaromaticity has been shown to play at most a minor role in the stability of Scott's [N]pericyclyne hydrocarbons-the first ring carbo-mers of cycloalkanes. This statement has been systematically refined for N = 3-6 by using both classical theoretical tools and newly designed tools based on electron localization function (ELF) analysis. The compatibility of the [5]-and [6]-pericyclyne cores with vertex functionalities was established by the synthesis of 20 oxy (carbo-cyclitol) derivatives. The stereoisomeric resolution of two of them has been achieved. pericyclynes are actually potential precursors of the carbo-benzenes. Criteria based on density functional theory (DFT) calculations (magnetic, energetic, structural/"electronic") show that the aromaticity of carbo- [N]annulenic species is comparable to that of their parent molecule. This has been challenged by the synthesis of several novel carbo-benzenic molecules with various substitution patterns. The theory-experiment interplay is pursued by considering ring carbo-mers of other conjugated ring systems such as radialenes. The second carbo-mers (butadiyndiyl-expanded rings) of [3]radialene and benzene are also envisioned. Hexaphenyl-carbo 2 -benzene has been observed by 1 H NMR and UV-vis spectroscopy.

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An alternative approach to the problem of assessing destabilization energies (strain energies) in cyclic hydrocarbons

Cited by 356 publications

References 4 publications

The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations

The Concept of Protobranching and Its Many Paradigm Shifting Implications for Energy Evaluations

Fused Tricyclic Phosphiranes—Analysis of Phosphorus Chemical Shieldings

Ring carbo-mers: From questionable homoaromaticity to bench aromaticity

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