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Hyperconjugation

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This review outlines the role of hyperconjugative interactions in the structure and reactivity of organic molecules. After defining the common hyperconjugative patterns, we discuss the main factors controlling the magnitude of hyperconjugative effects, including orbital symmetry, energy gap, electronegativity, and polarizability. The danger of underestimating the contribution of hyperconjugative interactions are illustrated by a number of spectroscopic, conformational, and structural effects. The stereoelectronic nature of hyperconjugation offers useful ways for control of molecular stability and reactivity. New manifestations of hyperconjugative effects continue to be uncovered by theory and experiments. This article is categorized under: Structure and Mechanism > Molecular Structures Software > Molecular Modeling Structure and Mechanism > Reaction Mechanisms and Catalysis
Three isodesmic equations used to calculate substituent stabilization energies (SEax(top), SEeq(middle), ΔSeq‐ax(bottom)) in the equatorial and axial cyclohexyl cations
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Conformational effects on the rate of cationic eliminations reflect stereoelectronics of the developing cation stabilization by C‐Si σ‐donors
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(a) Hybridization for RNH2 and ROH species. (b) Energies of NBOs corresponding to individual lone pairs of O‐ and N‐lone pairs. Gray values for the O‐containing molecules correspond to the high energy p‐lone pair, black values describe σ‐type spn hybrid lone pair. (c) The anti‐symmetric lone‐pair combination MOs in the representative conformations of hydrazine and hydrogen peroxide. Reprinted with permission from Ref.
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(a) The simplified MO description of α‐effect; (b) Formation of X–X bond can lower the lone pair energy and compensate for the expected increase in the donor ability of stereoelectronically coupled lone pairs. Reprinted with permission from Ref.
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Acceptor directionality of C=S (left) and C=O (right) groups in N/O–H…O/S=C hydrogen bonds. Note that the directionality is much closer to the perpendicular approach (coordination with the p‐type lone pair) for C=S. (Reprinted with permission from Ref. . Reproduced with permission of the International Union of Crystallography)
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NBO energies (in a.u., 1 a.u. = 627.5 kcal/mol) of axial and equatorial lone pairs in oxa‐, thia‐, selena‐, and azacyclohexane calculated at the B3LYP/6‐31G** level
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Comparison of acceptor ability (NBO Edel energies in kcal/mol) of C–X bonds in different directions
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Correlation of energy of σ*CX orbitals with electronegativity of element X in substituted ethanes, CH3CH2X
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Correlation of polarization of σ*CX orbitals with electronegativity of element X in substituted ethanes, CH3CH2X. Adopted with permission from Ref.
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Correlation of energy of vicinal NBO σCH → σ*CX interaction, Edel with electronegativity of element X in substituted ethanes, CH3CH2X
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The comparison of possible homoanomeric interaction patterns in six‐membered saturated heterocycles illustrates the connection between stabilizing to nonstabilizing hyperconjugative interactions. The bottom part of the figure shows how hybridization of the lone pair can change the efficiency of through‐space interactions
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Intermolecular positive hyperconjugation in the TS for an SE2 process with retention of configuration. M is an electrofuge, R is an alkyl group, and E is the electrophilic site in the reagent EX
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Top: The antiperiplanar stereoelectronic preference for vicinal conjugation and hyperconjugation. Bottom: Key hyperconjugative interactions between σCH and σ*CH orbitals. (Reprinted with permission from Ref. )
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Newman projections showing the possible conformations in a donor/acceptor substituted ethane molecule. The “main stereoelectronic rule” favors the antiperiplanar conformation
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Vertical π‐ionization potentials of the presented cyclic dienes from UV photoelectron spectroscopy (UV‐PES). The dashed marks indicate the mean π‐ionization potential. (Reprinted with permission from Ref. )
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The significant stabilization of a δ‐cyclohexyl cation by a series of equatorial substituents via double hyperconjugation
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Comparison of normal and extended positive hyperconjugation
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Remote hyperconjugative interactions
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Geminal hyperconjugation in [1.1.1]propellane
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Geminal and vicinal hyperconjugative interactions
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Neutral molecules with negative, neutral and positive hyperconjugation. Note that the NBO interaction energies for neutral hyperconjugation needs to be multiplied by two as it is bidirectional
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Absolute (data below the structures) and relative (data near the arrows) gas phase hydride ion affinities for selected carbocations. All energies are in kcal/mol and taken from Refs. ,
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Positive hyperconjugation in a carbenium cation and its evolution in a nonclassic structure
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Selected patterns of intra‐ and intermolecular negative hyperconjugation
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Contributing resonance structures for positive, negative and neutral hyperconjugation
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Comparison of contributing resonance structures in sacrificial and isovalent hyperconjugation
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NBO analysis of hyperconjugative interactions involved in the conformational profile of ethanal. The combined energies are approximate because interaction with energies below the default NBO threshold of 0.5 kcal/mol were not used in determining the overall balance (reprinted with permission from Ref. )
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(a) Four‐electron destabilizing interaction expressed in terms of nonorthogonal “unperturbed” orbitals (for which there is no imaginable Hermitian perturbation theory). (b) Four‐electron nonstabilizing interaction expressed in terms of orthogonalized unperturbed orbitals (for which there exists a valid Hermitian perturbation theory). (c) Four‐electron stabilizing interaction for a proper three‐term description of orbital energies in terms of Löwdin‐orthogonalized basis orbitals (Reprinted with permission from Refs. 20 and 21)
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Increases stability of acetals and the resurgence of anomeric effect in bis‐peroxides
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A possible way to estimate hyperconjugation in propene through a bond separation reaction
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(a) Description of the vicinal σCH → σ*CH interaction in ethane in terms of resonance theory (“double bond/no bond resonance”). (b) Energy lowering due to hyperconjugative interaction between σC–H and σ*C–X orbitals. (c) NBO plots illustrating overlap of vicinal σCH and σ*CH orbitals in ethane
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Transition from hyperconjugation to conjugation proceeds without a well‐defined border
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Types and examples of delocalizing interactions (hyperconjugative interactions are shown on the gray background)
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(a) Energy lowering due to hyperconjugative interaction between nY and σ*X‐H orbitals in X–H…Y complex. (b) NBO plots illustrating the overlap of the σ*C‐H of fluoroform and the nO orbital of the oxygen atom in water in the fluoroform/ water complex and (c) description of the hyperconjugative nO → σ*C‐H interaction in this complex in terms of resonance theory illustrating effective charge transfer from H‐bond acceptor (water) to H‐bond donor (fluoroform)
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Weakening of the second stereoelectronic effect of 1,2‐Me shift of OOH versions of the trapped CIs. Energies in kcal/mol. Reprinted with permission from Ref.
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Left: Alternative representations of the two stereoelectronic effects for the 1,2‐shift in the Criegee intermediate of the Baeyer‐Villiger rearrangement. Right: strategies for weakening the primary and secondary stereoelectronic effects in the 1,2‐alkyl shift. Reprinted with permission from Ref.
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(a) Increase in efficiency of fragmentation by substituent modification at the alkene terminus. (b) Electronic coupling between nonbonding orbitals in 1,4‐diradicals and β‐heteroatom substituted radicals strengthens in the TS, facilitating C–C bond fragmentation. Additional stabilization due to TB coupling through abreaking bridging bond is shown as ΔE (red). σ and σ* energies in the starting radical are shown in gray
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Simultaneous hyperconjugative alleviation of strain and enhancement of reactivity in cyclooctynes and twisted cyclodecynes for metal‐free click chemistry
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Hyperconjugative assistance to bond formation provided by σ‐acceptors in azide‐alkyne cycloadditions. This example illustrates how intermolecular transfer of electron density can benefit from hyperconjugation. Reprinted with permission from Ref.
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Comparison of homoconjugative and homohyperconjugative effects
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Solvolysis assisted by homoconjugation
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Lone pair/radical stabilization dramatically accelerates the anionic oxy‐Cope rearrangement
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Large rate enhancements of the oxy‐Cope rearrangement provided by anionic TS stabilization
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(a) Changes in HOMO and LUMO upon stretching and twisting of the central σ‐bond. (b) Dominant hyperconjugative interactions which control outward rotation of a donor substituent and inward rotation of an acceptor substituent. In the first case, the key interaction is negative hyperconjugation between the transition state HOMO (a stretched and twisted σ‐orbital) with a substituent empty p‐orbital (same topology is important for an acceptor σ*‐ or a π*‐orbital). In the second case, the key interaction is positive hyperconjugation between the transition state LUMO (a stretched and twisted σ*‐orbital) with a substituent filled p‐orbital (same topology is important for a donor σ‐ or a π‐orbital)
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Examples of of torquoselectivity in stereoselective cyclobutene and oxacyclobutene ring‐openings
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(a) Top: stabilization in p‐benzyne stems from the through‐bond coupling between the two radical centers; bottom: stabilization in the product of Au‐catalyzed Bergman cyclization is dramatically increased. Energies in kcal/mol. (b) Selected NBO interactions (in kcal/mol) stabilizing the positive charge in the product. PR3 group omitted for clarity
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Effect of negative double hyperconjugation on the stability of aryl anions (a) and on basicity of 4‐substituted piperidines (b)
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Effect of negative homohyperconjugation on the stability of aryl anions
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Activation of anomeric effect explains why bis‐peroxides can be more thermodynamically stable than mono‐peroxides
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Electronic and structural differences that account for the extreme weakness of anomeric effect in peroxides
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Changes in the charge distribution in the axial and equatorial conformers of 2‐fluoro‐1,3‐dioxane
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Structural consequences of anomeric effect. (a) Selected bond lengths in cis‐2,3‐dichloro‐2,4‐dioxane. (b) The opposing effects of endo‐ and exo‐anomeric effects on geometries. (c) Selected structural consequences of anomeric effect in OCF moiety. (Reprinted with permission from Ref. )
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Antiperiplanar negative hyperconjugative interaction in the endo‐ and exo‐anomeric effects
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Top: Conformational preferences illustrate the generalized anomeric effect in cyclic and acyclic systems in the gas phase (M06‐2X/6‐311G++(d,p) data in kcal/mol) Bottom: hyperconjugational contribution to the generalized anomeric effect
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(a) Axial preference for acceptor groups at the anomeric positions. (b) Combination of gauche and anomeric effects in control of sugar conformations
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Second order perturbation NBO energies in kcal/mol for important hyperconjugative interactions in axial and equatorial cyclohexyl cations (B3LYP/6‐31G**)
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Axial and equatorial “hyperconjomers” of cyclohexyl cations
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Comparison of the isodesmic bond separation energy of benzene and cyclooctatetraene evaluated using heats of formation. (Reprinted with permission from Ref. )
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Stabilizing neutral hyperconjugation between πC=C and σCH/CC orbitals in (a) tub‐shaped (D2d) cyclooctatetraene and (b) perpendicular 1,3‐butadiene. (Reprinted with permission from Ref. )
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The interplay of hyperconjugative and electrostatic interactions determines orientation of benzylic C–X bonds relative to the adjacent aromatic system
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The relative importance of different eclipsed conformations in substituted allyl fluorides at the B3P86/6‐311G(3d,2p) level. (Reprinted with permission from Ref. . Copyright 2010 John Wiley & Sons, Ltd.)
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The difference between eclipsed and staggered conformers of propene and NBO energies for the hyperconjugative interactions between the alkene and the CH2 group. Reprinted with permission from Ref.
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Experimental heats of hydrogenation for selected carbonyl compounds and alkenes
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(a) Conventional equations for the evaluation of hyperconjugation. (b) Revised bond separation energy (BSE) values for alkene and alkyne hyperconjugation, corrected for protobranching
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Comparison of the enthalpies of hydrogenation ΔHhyd (expt in kcal/mol) of butenes and butynes. According to these estimates, the conjugation energy of 1,3‐butadiene (right) is 3.9 ± 0.1 kcal/mol, but for 1,3‐butadiyne (left), it is zero
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Contrasting effects of alkyl substitution on BDEs for C–H, C–C, C–O and C–F bonds (Reprinted with permission from Ref. . Copyright 2005 American Chemical Society.). The BDEs (in kJ/mol) increase in the more substituted alkyl fluorides, alcohols and ethers but decrease in respective alkanes. Reprinted with permission from Ref.
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Top: Relative energies of the three difluoroethenes. Bottom: The antiperiplanar hyperconjugation effect (the AP effect), the synperiplanar hyperconjugation effect (the SP effect) and the lone pair delocalization effects (σ or πLP effect) coexist in 1,2‐dihaloethenes (X = halogen)
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(a) The gauche effect for 1,2‐difluoroethane. (b) The overlap of σC‐H and σ*C‐F bonds in the anti geometry. (c) Expanded list of substituents that prefer gauche conformation relative to a σC‐F bond (gas phase energies at B3LYP/ 6–311 + G(d,p) level. (d) solvent effects on the gauche/anti equilibrium in 1,2‐difluorocyclohexane. (e) the strong gauche preference in fluoro compounds with positively charged γ‐substituents
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Stabilizing consequences of fluorine introduction to hydrocarbons
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Hyperconjugative stabilization of the perpendicular (a) ground state of triplet ethylene and (b) rotational singlet ethylene transition state
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Bonds involved in the main hyperconjugative interactions which influence the conformational equilibrium of methylcyclohexane. (Reprinted with permission from Ref. )
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Top: Comparison of hybridization, polarization and HXH bond angles in ethane and ammonia borane (%s in: C–H bonds 23, N–H bonds 21, B–H bonds 28). Calculations are at the B3LYP/6‐311++G(d,p) level. Bottom: Comparison of vicinal hyperconjugation in ethane and ammonia borane according to NBO analysis at B3LYP/6–311++G(d,p) level. The σNH → σ*BH interactions are even smaller (<0.5 kcal/mol). The s‐character (larger font, bold) and the polarization (smaller font) of C–H and B–H bonds are indicated
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Figure Effects of negative hyperconjugation at the direct C–C coupling at anomeric positions
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Experimental (a) and theoretical (b, calculated at B3LYP/6‐31G[d,p] level) data illustrating the differences between axial and equatorial C–H couplings constants in cyclohexane and its heteroatomic analogues
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Conformational effects on direct C–H coupling in esters
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The effect of the adjacent lone pairs on the axial and equatorial 1JCH values in 1,3‐di‐tert‐butyl‐5‐methyl‐1,3‐diazacyclohexane and on the 1JCH value for the central C–H bond in rigid tricyclic ortho amides
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Selected direct C–H coupling constants calculated at B3LYP/6‐31G(d,p) level
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The Perlin effect, where smaller 1JCH values are observed experimentally for axial protons in cyclohexane, results from σCH → σ*CH interactions. The longer and weaker bonds are shown in bold. All data from Ref.
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Factors responsible for structural and spectroscopic consequences of H–bond formation. Different dominating effects give “normal” redshifted H‐bonds and “improper” blueshifted H‐bonds
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Intermolecular hyperconjugation in H‐bonding usually leads to a redshift in the H–X frequency analogous to that observed in the Bohlmann effect
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C–H Bonds adjacent to π‐bonds displaying redshifted IR frequencies
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Bohlmann effect observed in C–H IR‐stretching frequencies reflect stereoelectronic effects in the carbonyl containing compounds. Bond lengths calculated at B3LYP/6‐311G++(d,2p). Values from Ref. . Insert at the bottom: Deactivation of nO → σ*CH interactions by coordination of a Lewis acid at the carbonyl
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Figure Bohlmann effect observed for the vC‐His “isolated” frequencies in imines
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Bohlmann effect reflects stereoelectronic effects in amines, alcohols, and ethers. C–H bonds with the redshifted IR stretching frequencies are shown in red. Bond lengths calculated at the B3LYP/6‐311G++(d,2p) level of theory. vC‐His values from Ref.
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Selected effects of CH/CX hyperconjugation on C–H IR‐stretching frequencies (app = antiperiplanar.) Redshifted H‐bonds are shown in red
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Selected effects of CH/CH hyperconjugation on C–H IR‐stretching frequencies (app = antiperiplanar)
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(a) Resonance structures of cyclopentadienes depicting the hypeconjugative interactions involved. The left structure is antiaromatic in the ground state and aromatic in the excited state, the right is the opposite. (b) Excitation wavelengths (in nm) of four cyclopentadienes calculated at MS‐CASPT2(8in8)/ANO‐RCC‐VTZP level (Reprinted with permission from Refs. )
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σ‐Homoaromaticity (the two‐electron system) and antiaromaticity (the four‐electron system) in six‐membered heterocycles
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MO symmetry effects on hyperconjugation efficiency in cyclic systems
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(a) Stereoelectronic basis for assistance to alkyne bending utilized in TS stabilization in azide‐alkyne cycloadditions. (b) Symmetric bending scan of butyne and 2‐fluorobutyne in the gauche, synperiplanar, and antiperiplanar conformations (reprinted with permission from Ref. )
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Correlation of C–Cl distance with the NBO energies of nN → σ*CCl interaction and the NBO charge at Cl during the process of C–Cl bond stretching in 3‐chloropiperidine
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Changes in HOMO and LUMO energies can amplify stereoelectronic interactions associated with bond breaking in transition states
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The correlations between the differences in stabilization energies of δ‐XHn substituents in the equatorial and axial cyclohexyl cations (Figure ) and electronegativity of the X (second period—blue, third—red, fourth—green). Separate correlations are shown for each period. Calculations were performed at the B3LYP/6‐31G** (B3LYP/6‐311++G**) level (Reprinted with permission from Ref. )
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