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Gauche effect

The gauche effect is a stereoelectronic phenomenon in organic chemistry observed in 1,2-disubstituted ethanes where the gauche conformation—characterized by a dihedral angle of approximately 60° between substituents—is energetically preferred over the anti conformation (dihedral angle ≈180°), defying classical steric repulsion expectations.^[1] This preference arises primarily from stabilizing interactions involving adjacent lone pairs or polar bonds, such as in molecules with electronegative substituents like fluorine, and is most pronounced in the gas phase for compounds like 1,2-difluoroethane, where the gauche form dominates with a dihedral angle of about 73°.^[2] The effect was first identified in 1960 through infrared and Raman spectroscopy of 1,2-difluoroethane by Klaboe and Nielsen, who noted the unexpected stability of the gauche conformer relative to the anti form, with subsequent electron diffraction and microwave studies confirming this preference even in the gas phase.^[2] The term "gauche effect" was coined around 1972 by S. Wolfe to describe the tendency for maximal gauche interactions between adjacent electron pairs and polar bonds, extending beyond fluoro compounds to systems like fluoroamines and certain carbohydrates.^[2]^[1] For larger halogens (Cl, Br, I) in XCH₂CH₂X, the gauche preference diminishes due to increasing Pauli repulsion, shifting stability toward the anti conformer despite hyperconjugative stabilization of gauche.^[3] Two primary explanations account for the gauche effect: hyperconjugation and bent bonds. In the hyperconjugation model, vicinal electron delocalization—such as σ(C-H) donation into σ*(C-F)—stabilizes the gauche arrangement by lowering the energy through orbital overlap, with natural bond orbital analyses showing this interaction as the dominant factor in 1,2-difluoroethane. The bent bond model posits that distorted C-X bonds (X = electronegative atom) lead to partial double-bond character and angular strain relief in the gauche form, though computational studies emphasize hyperconjugation over steric or bent-bond contributions alone.^[1] This effect influences reactivity and stereochemistry in biomolecules, such as stabilizing gauche orientations in sugars and nucleic acids.^[1]

Definition and Overview

Core Definition

The gauche effect refers to the unexpected energetic preference for the gauche conformation, characterized by a dihedral angle of approximately 60°, over the anti conformation at approximately 180° in molecules featuring adjacent electronegative substituents, contrary to expectations based on typical steric repulsion that would favor the anti arrangement.^[1] This phenomenon arises in vicinal (1,2-) disubstituted ethane-like systems of the general form X-CH₂-CH₂-X, where X denotes electronegative atoms or groups such as fluorine (F), oxygen (O), or nitro (NO₂), and the gauche form is stabilized relative to the anti by 0.5–2 kcal/mol.^[1] To contextualize this preference, key terms in conformational isomerism must be clarified: the gauche (or synclinal) conformation features substituents separated by a torsional angle of about 60°; the anti (or antiperiplanar) conformation has them at 180°; the syn (or cis) conformation aligns them at 0°; and eclipsed conformations occur when bonds on adjacent carbons overlap directly, typically at 0°, 120°, or other multiples leading to torsional strain.^[2] In a Newman projection looking along the C-C bond of a generic X-CH₂-CH₂-X molecule, the gauche conformation depicts the two X substituents rotated by 60° relative to each other, with the front carbon's X group positioned between the back carbon's hydrogen atoms, minimizing but not eliminating steric interactions while allowing electronic stabilization; conversely, the anti conformation shows the X groups directly opposite at 180°, maximizing their separation but lacking the favorable orbital alignments that favor gauche in electronegative cases.^[1] This stereoelectronic stabilization, often involving hyperconjugation, underpins the effect without overriding it in all systems.^[1]

Contrast with Standard Conformational Preferences

In non-polar hydrocarbon systems like n-butane, the antiperiplanar (anti) conformation is thermodynamically favored over the synclinal (gauche) conformation by approximately 0.9 kcal/mol, owing to reduced steric interactions between the terminal methyl groups.^[4] This energy difference arises primarily from van der Waals repulsions, which are minimized when the bulky substituents are positioned farther apart in the extended anti arrangement.^[5] Consequently, the gauche form experiences a steric penalty of about 0.9 kcal/mol relative to the anti, making the latter the predominant conformer at equilibrium (with an anti:gauche population ratio of roughly 4.6:1 at room temperature).^[4] The rotational energy profiles of simple alkanes exemplify these standard preferences. In ethane, rotation about the C-C bond yields a potential energy surface with equivalent staggered minima (dihedral angle of 60°) separated by eclipsed transition states (dihedral angle of 0°), where the barrier height is approximately 2.9 kcal/mol due to torsional strain from overlapping electron densities in adjacent C-H bonds.^[6] For n-butane, the profile is more complex, featuring two types of staggered minima—at 180° (anti, global minimum) and ±60° (gauche, local minima)—interconnected by eclipsed maxima at 0°, 120°, and 240°. The barrier for gauche-to-anti interconversion is around 3–4 kcal/mol, while the central eclipsed barrier (at 0°) is higher, approximately 5–6 kcal/mol, reflecting both torsional strain and additional steric effects.^[7] These conformational behaviors are governed by two key energetic factors: torsional strain, which penalizes eclipsed arrangements by forcing bonds into partial overlap and increasing electron repulsion, and van der Waals repulsions (steric strain), which escalate in gauche conformations as non-bonded atoms approach within their repulsive potential wells.^[5] Torsional strain dominates the barriers between minima, while van der Waals interactions dictate the relative stabilities of the staggered forms.^[6] A qualitative potential energy diagram for n-butane rotation about the central C-C bond highlights the anti-favored profile: energy starts at the low anti minimum (0° relative to 180°), rises sharply through an eclipsed barrier (~3.8 kcal/mol to the gauche), reaches a shallow gauche minimum (+0.9 kcal/mol above anti), and climbs to a higher central barrier (~6 kcal/mol at 0°).^[7] This contrasts sharply with the gauche effect, where the energy profile inverts, positioning the gauche as the global minimum and destabilizing the anti relative to it.^[8] In systems bearing electronegative substituents, this inversion disrupts the normative anti preference, favoring gauche arrangements instead.^[9]

Historical Development

Initial Observations

The gauche effect was first observed in the 1950s through microwave spectroscopy studies of ethylene glycol (HO-CH₂-CH₂-OH), where the gauche conformation was found to be the preferred form despite expectations of anti preference due to dipole-dipole repulsions between the hydroxyl groups.^[10] In the 1960s, more definitive evidence emerged from vibrational spectroscopy of 1,2-difluoroethane by Klaboe and Nielsen, where infrared and Raman spectra revealed the gauche conformer as more stable in the liquid and solid states, with the trans and gauche forms nearly equally stable in the gas phase (energy difference ≈0 kcal/mol).^[11] Subsequent analysis of rotational constants from microwave spectra further confirmed the gauche preference in the gas phase, with a dihedral angle of about 73° for the F-C-C-F bond.^[2] Confirmation of gauche stability in fluoroethane derivatives came from ¹⁹F NMR studies in 1965, which showed vicinal coupling constants consistent with predominant gauche populations in solution.^[12] These empirical findings from the 1950s and 1960s laid the groundwork for recognizing the gauche effect as a general phenomenon in systems with electronegative substituents.

Key Theoretical Advancements

In the 1970s, theoretical understanding of the gauche effect shifted from purely steric interpretations to electronic factors, with early ab initio molecular orbital calculations demonstrating a preference for the gauche conformation in 1,2-difluoroethane. Radom, Lathan, Hehre, and Pople's 1972 study using STO-3G basis sets calculated the gauche form to be more stable than the trans by approximately 1.4 kcal/mol, attributing this to favorable orbital interactions rather than steric repulsion alone.^[13] This work marked a pivotal advancement, highlighting the role of delocalization in overriding classical conformational expectations. Around 1972, S. Wolfe coined the term "gauche effect" in a comprehensive review, formalizing it as a stereoelectronic phenomenon arising from maximizing gauche orientations between adjacent electron pairs or polar bonds, with hyperconjugative donations such as n_N → σ_C-F or σ_C-H → σ_C-F providing stabilization.^[1] This framework linked the gauche preference in 1,2-difluoroethane to similar lone pair interactions in hydrazines, where the barrier to rotation reflects stereoelectronic control. The 1990s saw expansions of the model to incorporate dipole-dipole interactions, particularly in vicinal dinitro compounds like 1,2-dinitroethane, where the gauche conformer benefits from attractive electrostatic alignment of nitro group dipoles. Theoretical analyses, including MP2 calculations, showed that while hyperconjugation contributes, the dominant stabilization in such systems stems from minimized dipole repulsion in the gauche versus anti form, broadening the effect's applicability to highly polar substituents.^[14] Post-2010 refinements have further clarified the hyperconjugative origins across halogens using advanced Kohn-Sham molecular orbital (KS-MO) analyses. A 2021 study on XCH₂CH₂X (X = F–I) revealed that vicinal hyperconjugative interactions (e.g., σ_C-H → σ_C-X) universally favor the gauche conformation, with the effect strongest for fluorine due to the most accessible σ_C-F acceptor orbital, thus unifying electronic mechanisms while quantifying their halogen dependence.^[3]

Theoretical Mechanisms

Hyperconjugation Explanation

The hyperconjugation model posits that the gauche effect arises primarily from stereoelectronic stabilization through delocalization of electron density from filled σ orbitals of C-H bonds into adjacent empty σ* antibonding orbitals of C-X bonds, where X is an electronegative substituent such as fluorine. This interaction is particularly favorable in the gauche conformation because the relevant C-H and C-X bonds adopt an antiperiplanar arrangement (effective overlap angle ≈0°), enabling optimal orbital alignment for electron donation, whereas in the anti conformation, the alignment is gauche (≈60°), resulting in poorer overlap.^[15]^[16] The stabilization energy from these hyperconjugative interactions can be approximated using second-order perturbation theory as \Delta E_{\text{hyper}} \approx \sum \frac{n_i |H_{ij}|^2}{\Delta E_j}, where n_i is the occupancy of the donor orbital, |H_{ij}| is the matrix element reflecting orbital overlap (proportional to the interaction strength), and \Delta E_j is the energy gap between donor and acceptor orbitals. In the gauche arrangement, the superior antiperiplanar overlap leads to greater stabilization despite similar \Delta E_j values.^[15] In systems like ethylene glycol (HO-CH₂-CH₂-OH), the gauche preference also involves vicinal lone pair-σ* hyperconjugation, where oxygen lone pairs donate into adjacent C-H or C-O σ* orbitals, providing significant stabilization that favors the gauche over the anti form.^[17] Natural bond orbital (NBO) analyses confirm this mechanism, revealing greater hyperconjugative delocalization energies in the gauche conformer of 1,2-difluoroethane than in the anti conformer, underscoring the dominant role of these interactions in driving the stereoelectronic preference.^[16]^[15] Bent bond models offer a complementary geometric perspective but are secondary to this orbital-based explanation.^[1] There is ongoing debate regarding whether hyperconjugation or electrostatic effects are the primary driver, with some computational studies supporting hyperconjugation.^[16]

Bent Bond and Electrostatic Models

The bent bond theory provides an alternative explanation for the gauche preference in compounds like 1,2-difluoroethane, where the presence of electronegative substituents increases the p-character of the C-X bonds in the gauche conformation. This rehybridization results in a shortening of the central C-C bond to approximately 1.50 Å (150 pm) in the gauche form, compared to 1.514 Å (151.4 pm) in the anti conformer, and a widening of the C-C-X bond angle by about 3°.^[15] These geometric changes minimize steric repulsion and stabilize the structure through better orbital overlap, contrasting with the standard expectation of longer bonds for more s-character hybrids. Complementing the bent bond approach, the electrostatic model attributes the gauche stabilization to favorable polarization interactions, such as 1,3 C···F attractions in 1,2-difluoroethane. Studies using interacting quantum atoms (IQA) analysis estimate electrostatic contributions stabilizing the gauche over the anti by several kcal/mol, with one report indicating this as the primary mechanism instead of hyperconjugation.^[18] Other point-charge analyses on 1,2-dihaloethanes indicate that electrostatic contributions account for approximately 70% of the gauche preference in 1,2-difluoroethane, with the remainder from bent bond distortions.^[19] Such combined approaches better capture the interplay of geometric and charge-based factors in highly electronegative systems. However, these models are less effective for substituents with lower polarity, such as chlorine, where the gauche effect diminishes due to reduced dipole strength.

Molecular Examples

1,2-Difluoroethane

1,2-Difluoroethane (F-CH₂-CH₂-F) exemplifies the gauche effect as the simplest molecule where the gauche conformation predominates, contrary to the typical preference for the anti arrangement in unfluorinated ethanes. The F-C-C-F dihedral angle in the gauche form ranges from 68° to 71°, as determined by microwave spectroscopy, NMR in nematic solvents, and electron diffraction.^[20]^[21]^[22] The gauche conformer is more stable than the anti by approximately 0.9 kcal/mol at 298 K, leading to a conformational population of about 90% gauche in the gas phase, as confirmed by microwave spectroscopy and electron diffraction measurements.^[22]^[20] This preference arises partly from hyperconjugative interactions stabilizing the gauche arrangement. Structural parameters reveal a shortened C-C bond length of 150 pm in the gauche form compared to 151.4 pm in the anti, with C-F bonds measuring 135 pm in the gauche conformer; these differences reflect the stereoelectronic influences of the gauche effect.^[22] The rotational spectrum observed via microwave spectroscopy identifies the gauche as the ground state, with no signals from the nonpolar anti conformer, and the torsional frequency for the gauche is approximately 200 cm⁻¹, consistent with vibrational spectroscopic data.^[20]^[23]

Polyfluorinated and Other Electronegative Systems

The gauche effect extends beyond simple 1,2-difluoroethane to polyfluorinated systems, where multiple electronegative substituents amplify the preference for gauche arrangements. In perfluoroethane (CF₃-CF₃), the conformation exhibits an all-gauche preference due to cumulative fluorine interactions, with the F-C-C-F dihedral angle approximately 65°. This cumulative effect stabilizes the gauche form over anti, as revealed by density functional theory calculations on perfluoroalkanes that highlight minimized steric and electrostatic repulsions in the staggered gauche configuration.^[24] In systems with other electronegative substituents, such as oxygen, the gauche effect arises from interactions involving lone pairs. For 1,2-dimethoxyethane (MeO-CH₂-CH₂-OMe), the gauche conformer is favored over the trans by about 0.3-0.5 kcal/mol, attributed to the stabilizing influence of oxygen lone pairs through hyperconjugation and dipole alignment. NMR studies and conformational energy calculations confirm this preference, showing the gauche O-C-C-O dihedral angle dominates in solution, with the energy difference estimated at 0.25–0.5 kcal/mol depending on solvent polarity.^[25] Vicinal dinitro compounds also demonstrate the gauche effect, driven by the polarity of the NO₂ groups. In 2,3-dinitrobutane, the gauche conformation is stabilized relative to the trans by an energy difference of 0.8 kcal/mol, owing to favorable electrostatic interactions between the electron-withdrawing nitro groups in the gauche orientation. Rotational isomerism studies indicate a mixture of conformers, but the gauche form predominates in nonpolar solvents like benzene, where the polarity difference enhances stability.^[26] The gauche effect influences diastereomeric preferences in more complex fluorinated systems. In 1,2-difluoro-1,2-diphenylethane, the erythro diastereomer adopts a gauche F-C-C-F arrangement that is lower in energy than the threo anti conformer by 0.21 kcal/mol, as determined by in silico calculations incorporating the fluorine gauche effect alongside phenyl interactions. This subtle stabilization highlights how the effect can override typical steric preferences in substituted ethanes, with the threo isomer showing mixed rotamers but the erythro gauche favored in solution due to hyperconjugative reinforcement.^[27]

Experimental and Computational Evidence

Spectroscopic Measurements

Spectroscopic measurements have provided key empirical evidence for the gauche effect by identifying distinct signatures of preferred conformers in relevant molecules. Infrared (IR) spectroscopy, in particular, reveals differences in vibrational frequencies associated with the C-F stretching modes, which are sensitive to the torsional conformation. In 1,2-difluoroethane, the gauche conformer exhibits a C-F stretch at approximately 1100 cm⁻¹, higher than the ~1050 cm⁻¹ observed for the anti conformer, reflecting the altered bonding environment in the gauche arrangement.^[28] These frequency shifts arise from the stereoelectronic interactions stabilizing the gauche form, with the dominant gauche population leading to intensified bands in the gas-phase IR spectrum.^[29] Nuclear magnetic resonance (NMR) spectroscopy further confirms the gauche preference through vicinal proton-proton coupling constants (³J_HH), which depend on the dihedral angle between coupled hydrogens. For 1,2-difluoroethane in solution, the observed ³J_HH values average around 4-5 Hz, indicative of a predominant gauche population where the H-C-C-H dihedral angle is ~60°, compared to ~12 Hz for the anti conformer with a 180° angle.^[30] These couplings allow estimation of conformer ratios, typically showing 70-80% gauche in 1,2-difluoroethane, directly supporting the energetic favorability of the gauche state.^[29] Microwave spectroscopy distinguishes conformers via rotational constants and dipole moments, providing precise structural parameters for isolated molecules in the gas phase. In ethylene glycol, a classic example exhibiting the gauche effect, the rotational spectrum of the gauche conformer yields constants that confirm its prevalence, with a dipole moment of approximately 2.5 D aligning with the predicted value for the gauche orientation of hydroxyl groups.^[31] This technique isolates the gauche species, revealing no significant anti population under supersonic jet conditions, thus validating the intrinsic stability of the gauche form.^[32] Gas-phase electron diffraction offers direct measurement of molecular geometries, capturing average dihedral angles from scattering patterns. For fluorinated compounds like 1,2-difluoroethane, electron diffraction data indicate a F-C-C-F dihedral angle of 60°-70° for the dominant gauche conformer, with no evidence of substantial anti contribution at room temperature. These angles, determined from radial distribution functions, underscore the gauche preference in isolated molecules, consistent with the effect's intramolecular nature.^[33]

Quantum Chemical Calculations

Early ab initio calculations using Hartree-Fock (HF) theory with the 6-31G* basis set predicted a gauche preference of approximately 1.0 kcal/mol for 1,2-difluoroethane relative to the anti conformer, closely matching experimental estimates from spectroscopic data.^[17] These computations, performed in the late 1980s and early 1990s, provided initial theoretical support for the gauche stabilization without accounting for electron correlation, highlighting the effect's persistence even at lower levels of theory.^[34] Density functional theory (DFT) methods, particularly B3LYP, have since offered improved accuracy for torsional dihedrals in gauche systems, reproducing experimental angles within 5° for 1,2-difluoroethane and related compounds.^[18] More recent DFT studies using functionals like ωB97X-D, as detailed in 2021 analyses of fluorinated ethanes, emphasize hyperconjugation as the dominant mechanism, with dispersion corrections enhancing agreement for delocalized interactions in polyfluorinated systems. Natural bond orbital (NBO) and natural resonance theory (NRT) analyses quantify the hyperconjugative delocalization underpinning the gauche effect, revealing significant stabilization from σ_{C-H} → σ*_{C-F} interactions in the gauche conformer of 1,2-difluoroethane. These donor-acceptor interactions are significantly stronger in the gauche geometry compared to anti, providing a direct measure of the stereoelectronic contribution.^[35] Benchmarking studies comparing second-order Møller-Plesset perturbation theory (MP2) with coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] across halogenated ethanes, such as 1,2-difluoro- and 1,2-dichloroethane, demonstrate that the gauche preference is uniquely pronounced for fluorine due to its electronegativity enhancing orbital overlap. MP2 tends to overestimate the effect by 0.2-0.5 kcal/mol relative to CCSD(T) benchmarks, but both methods confirm the F-specific strength, with errors minimized using augmented basis sets like aug-cc-pVTZ.^[17]

Influencing Factors

Solvent and Environmental Effects

The gauche effect exhibits significant sensitivity to solvent polarity, as the conformers typically possess differing dipole moments that lead to preferential solvation of one over the other. In nonpolar environments, the intrinsic stereoelectronic preference for the gauche conformation dominates, but increasing solvent polarity can modulate this equilibrium by stabilizing the conformer with the lower dipole moment through reduced electrostatic repulsion or enhanced solvation. For instance, in 2,3-dinitro-2,3-dimethylbutane, the gauche:anti population ratio is 79:21 in benzene but shifts to 42:58 in carbon tetrachloride, with benzene favoring the gauche more than the slightly less polar CCl4, demonstrating how subtle differences in solvent polarity alter the balance via dipole interactions.^[26] This polarity dependence aligns with electrostatic models where solvation screens the unfavorable dipole-dipole repulsions in the gauche form more effectively in polar media.^[1] In protic solvents, hydrogen bonding further influences the gauche preference, particularly in systems like ethylene glycol where intermolecular O-H···O interactions stabilize the gauche conformation relative to the anti. NMR studies reveal a strong gauche bias in ethylene glycol across a range of solvents, with protic environments such as water enhancing this preference through competitive hydrogen bonding that favors the geometry allowing optimal solute-solvent interactions. Intramolecular hydrogen bonding also contributes in low-polarity protic media, reinforcing the gauche orientation by positioning the hydroxyl groups for effective O-H···O contacts. These effects highlight how protic solvents amplify the stereoelectronic gauche stabilization via network formation. Comparisons between gas and solution phases underscore environmental modulation, with typical 10–20% shifts in gauche populations due to solvation disrupting isolated-molecule preferences. In 1,2-difluoroethane, gas-phase electron diffraction indicates approximately 60% gauche conformer at room temperature, reflecting the unperturbed hyperconjugative and electrostatic drivers.^[22] In polar solvents like chloroform, the gauche population decreases due to partial screening of favorable interactions. In crystalline solids, packing forces occasionally override the gauche preference, imposing anti conformations despite the intrinsic tendency, though such cases remain rare. Analysis of crystal structures in the Cambridge Structural Database shows that while most electronegative-substituted ethanes retain gauche geometries, intermolecular interactions in densely packed lattices can enforce extended anti forms to minimize steric clashes or optimize hydrogen bonding networks.^[36] For example, certain polyfluorinated systems adopt anti in crystals due to lattice constraints, contrasting their solution-phase behavior.^[37]

Temperature and Substituent Variations

The stability of the gauche conformation in the gauche effect is predominantly enthalpic, but entropy contributions favor the anti conformer, leading to temperature-dependent variations in conformer populations. Substituent electronegativity strongly influences the magnitude of the gauche effect, with highly electronegative groups enhancing gauche stabilization through hyperconjugative or electrostatic interactions. For fluorine (Pauling electronegativity χ = 4.0), the gauche conformer is favored over anti by ΔE ≈ 0.9 kcal/mol in 1,2-difluoroethane. In contrast, for iodine (χ = 2.5), the effect is absent, and the anti conformer is preferred by approximately 2.0 kcal/mol in 1,2-diiodoethane.^[22]^[38] The gauche effect is a vicinal phenomenon that diminishes with increasing chain length beyond 1,2-positions, as the stereoelectronic interactions weaken. For example, in 1,3-difluoropropane, the conformation reverts to anti preference for the relevant bonds, lacking the strong gauche stabilization observed in 1,2-difluoroethane. Steric bulk from additional substituents can counteract the gauche effect by increasing repulsion in the gauche arrangement. In 2,3-difluorobutane, the presence of methyl groups reduces the gauche stabilization by approximately 0.3 kcal/mol compared to 1,2-difluoroethane, shifting the equilibrium toward the anti conformer.^[39]

Anomeric Effect

The anomeric effect describes the unexpected preference for an electronegative substituent, such as oxygen or halogen, to adopt an axial orientation at the anomeric carbon (the carbonyl-derived carbon) in six-membered pyranose rings of carbohydrates, exemplified by the intrinsic stability of the α-anomer of D-glucopyranose over the β-anomer in the gas phase, though in aqueous solution the β-anomer predominates (∼63% β vs ∼37% α) due to solvation effects. This stereoelectronic phenomenon, first identified in the mid-1950s, counteracts the typical steric preference for equatorial substituents in chair conformations and provides stabilization energies of approximately 1–2 kcal/mol for such axial arrangements in sugar systems. The effect is particularly pronounced in glycosides and related acetals, where the axial positioning enhances molecular stability despite potential 1,3-diaxial interactions. The underlying mechanism shares conceptual overlap with the gauche effect through hyperconjugative interactions, specifically the donation from a lone pair on the ring oxygen (n_O) into the antibonding orbital of the anomeric C–X σ* bond (n_O → σ*_{C-X}), which aligns optimally in the axial conformation. This delocalization lowers the energy of the axial isomer by facilitating electron withdrawal from the electronegative X group (e.g., OR or F), a process less effective in the equatorial position due to suboptimal orbital overlap. While early explanations invoked dipole-dipole repulsions, modern analyses emphasize this hyperconjugation as a primary driver, though electrostatic contributions also play a role in polar environments. A representative example is observed in fluoroglycosides, such as α-fluoroglucopyranosides, where the highly electronegative fluorine exhibits a strong axial preference that overrides the destabilizing A^{1,3} strain from vicinal axial hydrogens or substituents, leading to selective formation and isolation of axial anomers under equilibrating conditions.^[40] This defiance of steric expectations underscores the effect's magnitude, with computational and experimental studies confirming axial stabilization on the order of 1.5–3 kcal/mol in such systems, influencing glycoside synthesis and biological mimicry.^[40] In distinction to the gauche effect, which pertains to torsional preferences favoring gauche over anti conformations in acyclic chains bearing adjacent electronegative atoms, the anomeric effect manifests as a positional bias (axial vs. equatorial) within the constrained geometry of cyclic structures like pyranoses.^[41] This cyclic context amplifies the stereoelectronic control, linking the two phenomena as manifestations of similar n → σ* interactions but adapted to ring topology.^[41]

Cis Effect in Alkenes

The cis effect in alkenes refers to the energetic preference for the cis configuration over the trans in disubstituted alkenes bearing electronegative substituents, such as halogens. This phenomenon is exemplified by 1,2-difluoroethene, where the cis isomer is more stable than the trans by approximately 1 kcal/mol, as determined by both experimental measurements and high-level quantum chemical calculations.^[42]^[43] In contrast to typical alkenes, where trans isomers are favored due to reduced steric repulsion, the cis preference arises specifically from electronic interactions involving the electronegative groups. The mechanism underlying the cis effect involves two primary interpretations: hyperconjugative delocalization and electrostatic attractions. In the hyperconjugation model, the lone pairs on the electronegative substituents (e.g., fluorine) donate electron density into the antibonding π* orbital of the C=C bond, stabilizing the cis geometry where this overlap is more favorable due to better alignment.^[44] Alternatively, electrostatic models emphasize attractive interactions between the partially negative substituents in the cis arrangement, outweighing steric repulsion, as supported by natural bond orbital analyses showing significant charge-transfer contributions.^[43] These effects are more pronounced with highly electronegative atoms like fluorine, enhancing the delocalization or attraction. In perfluoroalkenes, such as cis-1,2-difluoroethene and related polyfluorinated systems, the cis motif is consistently favored, reflecting amplified electronic stabilization from multiple fluorine substituents. This contrasts sharply with non-fluorinated conjugated systems like 1,3-butadiene, where the s-trans conformer predominates by about 3 kcal/mol due to steric and conjugative preferences that disfavor the cis-like s-cis form.^[42]^[45] The cis effect holds synthetic relevance in reactions like olefin metathesis involving fluorinated alkenes, where it influences stereoselectivity by favoring Z (cis) products through lower-energy transition states in metallacyclobutane intermediates stabilized by fluorine's electronic effects. For instance, ruthenium- and molybdenum-catalyzed cross-metathesis of fluoroalkenes often yields Z-selective outcomes, enabling precise control in the synthesis of fluorinated olefins for materials and pharmaceuticals.^[46]

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NMR spectra of 1,2-difluoroethane in nematic solvents - ScienceDirect
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Conformational analysis. 7. 1,2-Difluoroethane. An electron ...
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The torsional potential of perfluoro n-alkanes: A density functional ...
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NMR Studies and Conformational Energy Calculations of 1,2 ...
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Rotational isomerism in 2,3-dinitro-2,3-dimethylbutane
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NMR Spectroscopy :: 5-HMR-5 Vicinal Proton-Proton Coupling 3JHH
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In Quest of the Missing C 2 H 6 O 2 Isomers in the Interstellar Medium
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Conformation of ethylene glycol from the rotational spectra of the ...
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Structure and conformation of 1,4-difluorobutane as determined by ...
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The conformation of 1,2-difluoroethane - Henry Rzepa's Blog
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Small Molecule Conformational Preferences Derived from Crystal ...
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Crystal packing, hydrogen bonding, and the effect of crystal forces ...
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1,2-Diiodoethane: gas-phase molecular structure and ...
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A Computational Study of Vicinal Fluorination in 2,3‐Difluorobutane ...
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The Synthesis and Glycoside Formation of Polyfluorinated ...
May 25, 2022 · Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position.
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Anomeric and Gauche Effects - ACS Publications
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On the Origin of cis-Effect in 1,2-Difluoroethene
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10.6: Butadiene is Stabilized by a Delocalization Energy
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