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Paul Von Ragué Schleyer - One of the best experts on this subject based on the ideXlab platform.
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Hyperconjugation in hydrocarbons: Not just a “mild sort of conjugation”
Pure and Applied Chemistry, 2013Co-Authors: Paul Von Ragué SchleyerAbstract:This article emphasizes two underappreciated aspects of Hyperconjugation in hydrocarbons, two-way Hyperconjugation and Hyperconjugation in tight spaces. Nonplanar polyenes (e.g., cyclooctatetraene (D 2d ), biphenyl (D 2 ), styrene (C 1 )), the nonplanar rotational transition states (TSs) of planar polyenes (e.g., perpendicular 1,3-butadiene), as well as the larger nonplanar Huckel or Mobius annulenes, are stabilized by effective σ-electron delocal- ization (involving either the C-C or C-H bonds) via two-way Hyperconjugation. The collec- tive consequence of two-way Hyperconjugation in molecules can be nearly as stabilizing as π-conjugation effects in planar polyenes. Reexamination of the σ- vs. π-bond strength of ethylene results in surprising counterintuitive insights. Strained rings and cages (e.g., cyclo- propane and tetrahedrane derivatives, the cubyl cation, etc.) can foster unexpectedly large Hyperconjugation stabilizations due to their highly deformed ring angles. The thermochemi- cal stabilities of these species rely on a fine balance between their opposing destabilizing geometrical features and stabilizing hyperconjugative effects in tight spaces (adjustable via substituent effects). We hope to help dispel chemists' prejudice in viewing Hyperconjugation as merely a "mild" effect with unimportant consequences for interpreting the structures and energies of molecules.
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Hyperconjugation in hydrocarbons not just a mild sort of conjugation
Pure and Applied Chemistry, 2013Co-Authors: Paul Von Ragué SchleyerAbstract:This article emphasizes two underappreciated aspects of Hyperconjugation in hydrocarbons, two-way Hyperconjugation and Hyperconjugation in tight spaces. Nonplanar polyenes (e.g., cyclooctatetraene (D 2d ), biphenyl (D 2 ), styrene (C 1 )), the nonplanar rotational transition states (TSs) of planar polyenes (e.g., perpendicular 1,3-butadiene), as well as the larger nonplanar Huckel or Mobius annulenes, are stabilized by effective σ-electron delocal- ization (involving either the C-C or C-H bonds) via two-way Hyperconjugation. The collec- tive consequence of two-way Hyperconjugation in molecules can be nearly as stabilizing as π-conjugation effects in planar polyenes. Reexamination of the σ- vs. π-bond strength of ethylene results in surprising counterintuitive insights. Strained rings and cages (e.g., cyclo- propane and tetrahedrane derivatives, the cubyl cation, etc.) can foster unexpectedly large Hyperconjugation stabilizations due to their highly deformed ring angles. The thermochemi- cal stabilities of these species rely on a fine balance between their opposing destabilizing geometrical features and stabilizing hyperconjugative effects in tight spaces (adjustable via substituent effects). We hope to help dispel chemists' prejudice in viewing Hyperconjugation as merely a "mild" effect with unimportant consequences for interpreting the structures and energies of molecules.
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Why cyclooctatetraene is highly stabilized: The importance of "two-way" (double) Hyperconjugation
Journal of Chemical Theory and Computation, 2012Co-Authors: Judy I Wu, Israel Fernández, Yirong Mo, Paul Von Ragué SchleyerAbstract:Despite its highly nonplanar geometry, the tub-shaped D2d cyclooctatetraene (COT) minimum is far from being an unconjugated polyene model devoid of important π interactions. The warped skeleton of D2d COT results in the large stabilization (41.1 kcal/mol) revealed by its isodesmic bond separation energy (BSE). This originates largely from the “two-way” Hyperconjugation, back and forth across the C–C single bonds, between the CC/CH σ(σ*) and the C═C (π*)π orbitals. These hyperconjugative effects compensate for the substantial, but not complete, loss of π conjugation upon ring puckering. C–C single bond rotation of 1,3-butadiene involves a similar interplay between π conjugation and “two-way” double Hyperconjugation and serves as a simple model for the inversion of D2d to D4h COT. The perpendicular rotational transition states of many other systems, e.g., the allyl cation, styrene, biphenyl, and ethene, are stabilized similarly by “two-way” Hyperconjugation.
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QUANTITATIVE EVALUATION OF Hyperconjugation IN THE CYCLOPROPYLCARBINYL CATION AND IN CYCLOPROPYLBORANE
Chemical Physics Letters, 1997Co-Authors: Paul Von Ragué Schleyer, Haijun Jiao, Zhenyang LinAbstract:Abstract An orbital deletion procedure (ODP) at HF/6-311G** have been used to evaluate the Hyperconjugation effects in the cyclopropylcarbinyl cation ( 1 ) and in cyclopropylborane ( 2 ), as well as the conjugation effects in the allyl cation ( 3 ) and in vinylborane ( 4 ). The Hyperconjugation (or conjugation) energies have been quantified by ODP in which the critical “vacant” carbocation (or boron) p orbital is “deactivated”. Comparisons between the bisected conformations of 1 with 3 , and 2 with 4 demonstrate that cyclopropane can be just as effective as a π-electron donor as a CC double bond.
Jun Zhu - One of the best experts on this subject based on the ideXlab platform.
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Predicting an Antiaromatic Benzene Ring in the Ground State Caused by Hyperconjugation.
Chemistry an Asian journal, 2019Co-Authors: Yu Zhao, Qiong Xie, Tingting Sun, Jun ZhuAbstract:Benzene, the prototype of aromatics, has six equivalent C-C bonds (1.397 A), which are intermediate between a C-C double bond and a C-C single bond. For over 80 years, chemists have spent much effort on freezing a localized structure to obtain a distorted bond-length alternating benzene ring in the ground state, leading to various localized trisannelated benzene rings. However, most of the central benzene rings are still aromatic or nonaromatic. Here we report an antiaromatic benzene ring caused by Hyperconjugation. Specifically, symmetric annulation of 5,5-difluorocyclopentadiene results in an antiaromatic benzene ring, which is supported by various aromaticity indices, including nucleus-independent chemical shift, anisotropy of the induced current density, π-separated electron-localization function and heat of hydrogenation. Our findings highlight a strong power of Hyperconjugation, a "weak" interaction in organic chemistry, paving the way for designing and realizing more novel (anti)aromatics.
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Probing the Strongest Aromatic Cyclopentadiene Ring by Hyperconjugation
Organometallics, 2018Co-Authors: Qiong Xie, Tingting Sun, Jun ZhuAbstract:Hyperconjugation, an interaction of electrons in a σ orbital or lone pair with an adjacent π or even σ antibonding orbital, can have a strong effect on aromaticity. However, most work on hyperconjugative aromaticity has been limited to main-group substituents. Here, we report a thorough density functional theory study to evaluate the aromaticity in various cyclopentadienes that contain both main-group and transition-metal substituents. Our calculations reveal that the strongest aromatic cyclopentadiene ring can be achieved by the synergy of trans influence and Hyperconjugation caused by transition-metal substituents. Our findings highlight the great power of transition metals and trans influence in achieving hyperconjugative aromaticity, opening an avenue to the design of other novel aromatic organometallics.
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Probing the Most Aromatic and Antiaromatic Pyrrolium Rings by Maximizing Hyperconjugation and Push-Pull Effect
Chemistry an Asian journal, 2018Co-Authors: Tingting Sun, Qiong Xie, Liang Zhao, Jun ZhuAbstract:Hyperconjugation, a weak interaction in organic chemistry, can have a strong effect on aromaticity, leading to the concept of hyperconjugative aromaticity, which was first proposed by Mulliken in 1939. However, most studies are limited to main group chemistry. Here we report the most aromatic and antiaromatic pyrrolium ring by maximizing the Hyperconjugation caused by transition metal fragments and the push-pull effect. Our calculations reveal that the origin of the outperformance of transition metal substituents over main group ones on hyperconjugative aromaticity could be attributed to their higher highest occupied molecular orbitals (HOMOs). Among the group 11 transition metals, a silver substituent results in the best performance. All these findings highlight the magic of the transition metal (silver) and could be particularly helpful for the design of other aromatic and antiaromatic counterparts based on a nonaromatic parent species.
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Probing the Strongest Aromatic Cyclopentadiene Ring by Hyperconjugation
2018Co-Authors: Qiong Xie, Tingting Sun, Jun ZhuAbstract:Hyperconjugation, an interaction of electrons in a σ orbital or lone pair with an adjacent π or even σ antibonding orbital, can have a strong effect on aromaticity. However, most work on hyperconjugative aromaticity has been limited to main-group substituents. Here, we report a thorough density functional theory study to evaluate the aromaticity in various cyclopentadienes that contain both main-group and transition-metal substituents. Our calculations reveal that the strongest aromatic cyclopentadiene ring can be achieved by the synergy of trans influence and Hyperconjugation caused by transition-metal substituents. Our findings highlight the great power of transition metals and trans influence in achieving hyperconjugative aromaticity, opening an avenue to the design of other novel aromatic organometallics
Yi-lei Zhao - One of the best experts on this subject based on the ideXlab platform.
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Hyperconjugation promoted by hydrogen bonding between His98/His241 and a carboxyl group contributes to tyrosine decarboxylase catalysis
Catalysis Science & Technology, 2019Co-Authors: Wei Dai, Yi-lei ZhaoAbstract:The utility of the σ → π* Hyperconjugation in pyridoxal 5′-phosphate (PLP)-dependent tyrosine decarboxylase (TyDC) is demonstrated here for the first time. His98 and His241 form hydrogen bonds with the carboxyl group of an external aldimine and contribute to the Hyperconjugation of TyDC from Lactobacillus brevis. The dihedral angle of C–Cα–NSB–C4′ in PLP–substrate covalent complexes was found to be vital to the decarboxylation efficiency of TyDC.
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Hyperconjugation promoted by hydrogen bonding between his98 his241 and a carboxyl group contributes to tyrosine decarboxylase catalysis
Catalysis Science & Technology, 2019Co-Authors: Wei Dai, Yi-lei ZhaoAbstract:The utility of the σ → π* Hyperconjugation in pyridoxal 5′-phosphate (PLP)-dependent tyrosine decarboxylase (TyDC) is demonstrated here for the first time. His98 and His241 form hydrogen bonds with the carboxyl group of an external aldimine and contribute to the Hyperconjugation of TyDC from Lactobacillus brevis. The dihedral angle of C–Cα–NSB–C4′ in PLP–substrate covalent complexes was found to be vital to the decarboxylation efficiency of TyDC.
Gino A. Dilabio - One of the best experts on this subject based on the ideXlab platform.
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Bond strengths: the importance of Hyperconjugation.
Organic letters, 2006Co-Authors: Keith U. Ingold, Gino A. DilabioAbstract:Gronert (J. Org. Chem. 2006, 71, 1209) has challenged the importance of Hyperconjugation in determining C−H bond dissociation enthalpies (BDEs) in alkanes. Electron paramaganetic resonance spectra of H3CCH2•, (H3C)2CH•, and (H3C)3C• show significant positive spin on their β-H3C groups' hydrogens. A 55%/45% partitioning of these spins between Hyperconjugation and spin polarization mechanisms linearly correlates with the C−H BDEs in methane, ethane, propane, isobutane and propene. Hyperconjugation is an important factor determining alkane C−H BDEs.
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bond strengths the importance of Hyperconjugation
Organic Letters, 2006Co-Authors: Keith U. Ingold, Gino A. DilabioAbstract:Gronert (J. Org. Chem. 2006, 71, 1209) has challenged the importance of Hyperconjugation in determining C−H bond dissociation enthalpies (BDEs) in alkanes. Electron paramaganetic resonance spectra ...
Wei Dai - One of the best experts on this subject based on the ideXlab platform.
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Hyperconjugation promoted by hydrogen bonding between His98/His241 and a carboxyl group contributes to tyrosine decarboxylase catalysis
Catalysis Science & Technology, 2019Co-Authors: Wei Dai, Yi-lei ZhaoAbstract:The utility of the σ → π* Hyperconjugation in pyridoxal 5′-phosphate (PLP)-dependent tyrosine decarboxylase (TyDC) is demonstrated here for the first time. His98 and His241 form hydrogen bonds with the carboxyl group of an external aldimine and contribute to the Hyperconjugation of TyDC from Lactobacillus brevis. The dihedral angle of C–Cα–NSB–C4′ in PLP–substrate covalent complexes was found to be vital to the decarboxylation efficiency of TyDC.
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Hyperconjugation promoted by hydrogen bonding between his98 his241 and a carboxyl group contributes to tyrosine decarboxylase catalysis
Catalysis Science & Technology, 2019Co-Authors: Wei Dai, Yi-lei ZhaoAbstract:The utility of the σ → π* Hyperconjugation in pyridoxal 5′-phosphate (PLP)-dependent tyrosine decarboxylase (TyDC) is demonstrated here for the first time. His98 and His241 form hydrogen bonds with the carboxyl group of an external aldimine and contribute to the Hyperconjugation of TyDC from Lactobacillus brevis. The dihedral angle of C–Cα–NSB–C4′ in PLP–substrate covalent complexes was found to be vital to the decarboxylation efficiency of TyDC.
