Is there a general rule for the gauche effect in the conformational isomerism of 1,2-disubstituted ethanes?

The stabilities of the gauche and anti conformations of butane, 1,2-dicyanoethane (DCE), and 1,2-dinitroethane (DNE) have been investigated through theoretical calculations. The gauche effect-the tendency of keeping close vicinal electronegative substituents (thetaX-C-C-X approximately 60 degrees ) in an ethane fragment-is expected to drive the conformational equilibrium of DCE and DNE toward the gauche conformation. It was found that, for butane, where the gauche effect is supposed to be poor/null, the hyperconjugation effect contributes mostly to the anti stabilization in opposition to the traditional sense that the methyl groups repel each other, and this should govern its conformational equilibrium. For DCE the equilibrium was shifted to the anti conformer, essentially due to a gauche repulsion, while for DNE, despite the higher electronic delocalization energies, a predominance of the gauche conformer was obtained, and this was attributed mainly to the attractive dipolar interaction between the two nitro groups. A full orbital energy analysis was performed using the natural bond orbital approach, which showed that bond bending and anti-C-H/C-X* hyperconjugation models, usually applied to explain the origin of the gauche effect in fluorinated derivatives, are not adequate to completely explain the conformational behavior of the titled compounds.     

草酰溴一价正离子(BrCO)+2几何构型及反应活性的DFT研究

用密度泛函方法BHandHIYP以6-311 G(d)和6-311 G(2df）为基组对草酰溴的一价正离子(BrCO)2^ 和中性分子(BrCO)2做了构象分析,结果表明,(BrCO)^ 2和(BrCO)2都具有平面反式和交叉式两种构象。交叉式构象存在超共轭现象。此外,对草酰溴离子、中性分子各解离通道初级反应的Gibbs自由能的计算,发现草酰溴离子C-C键解离通道的反应活性总体上大于中性分子,对该反应通道进一步做了反应机理研究,证实了热力学结论。     

Conformational stability and rotational energy barrier of RC60–C60R dimers: hyperconjugation versus steric effect

The conformational stability of 4, 4?? disubstituted HC₆₀?CC₆₀R and RC₆₀?CC₆₀R dimers were calculated at ONIOM approach (AM1:B3LYP/6-31+G**) and density functional theory (B3LYP/6-31G**). The new evidences for stability and rotational energy barriers of these dimers were obtained by natural bond orbital, natural steric and molecular orbital analyses. Based on B3LYP/6-31G** calculations, except for RC₆₀?CC₆₀R (R?=?hydrogen, tert-butyl and trimethylsilyl) where gauche is the most stable conformer, trans is a global energy minimum. The greater stability of the gauche conformer of HC₆₀?CC₆₀H over trans is the result of hyperconjugation, which dominates the instability caused by the steric effect. By increasing the size of the substituent of HC₆₀?CC₆₀R dimers, the trans becomes sterically unstable but the hyperconjugation of bulky substituents dominates (the trans is global energy minimum). The hyperconjugation stability of RC₆₀?CC₆₀R dimers dominates until R?=?iso-propyl (higher stability of trans). In the case of bulky tert-butyl and trimethylsilyl substituents, the steric energy of trans is large and overweighs the hyperconjugation effect. This favors gauche as the most stable conformer. The calculated rotational energy barrier for HC₆₀?CC₆₀R and RC₆₀?CC₆₀R dimers is less than 7.3 and more than 10?kcal/mol, respectively (depending on substitution).     

Ionization spectroscopy of conformational isomers of propanal: the origin of the conformational preference

Two different conformational isomers of propanal, cis and gauche, are investigated by the vacuum-UV mass-analyzed threshold ionization (VUV-MATI) spectroscopy to give accurate adiabatic ionization potentials of 9.9997 +/- 0.0006 eV and 9.9516 +/- 0.0006 eV, respectively. cis-Propanal, which is the more stable conformer in the neutral state, becomes less stable in the cation compared to gauche-propanal. Vibrational structures revealed in the MATI spectra indicate that cis and gauche isomers undergo their unique structural changes upon ionization. The ionization of gauche-propanal induces a geometrical change along the conformational coordinate, suggesting that the steric effect in the ground state is diminished upon ionization. Natural bonding orbital (NBO) calculations provide the extent of hyperconjugation in each conformational isomer of propanal.     

Simultaneous gauche and Anomeric Effects in α-Substituted Sulfoxides

α-Substituted sulfoxides can experience both gauche and anomeric effects, since these compounds have the geometric requirements and strong electron donor and acceptor orbitals which are essential to make operative the hyperconjugative nature of these effects. Indeed, the title effects were calculated to take place for 1,3-oxathiane 3-oxide in polar solution, where dipolar effects are absent or at least minimized, while only the gauche effect is present in 2-fluorothiane 1-oxide. Since the fluorine atom is a suitable probe for structural analysis using NMR, the (1)J(CF) dependence on the rotation around the F-C-S═O dihedral angle of (fluoromethyl)methyl sulfoxide was evaluated; differently from 1,2-difluoroethane and fluoro(methoxy)methane, this coupling constant is at least not exclusively dependent on dipolar interactions (or on hyperconjugation). Because of the nonmonotonic behavior of the (1)J(CF) rotational profile, this coupling constant does not appear to be of significant diagnostic value for probing the conformations of α-fluoro sulfoxides.     

Gauche effect in 1,2-difluoroethane. Hyperconjugation, bent bonds, steric repulsion

Natural bond orbital deletion calculations show that whereas the gauche preference arises from vicinal hyperconjugative interaction between anti C-H bonds and C-F* antibonds, the cis C-H/C-F* interactions are substantial (approximately 25% of the anti interaction). The established significantly >60 degrees FCCF dihedral angle for the equilibrium conformer can then be rationalized in terms of the hyperconjugation model alone by taking into account both anti interactions that maximize near 60 degrees and the smaller cis interactions that maximize at a much larger dihedral angle. This explanation does not invoke repulsive forces to rationalize the 72 degrees equilibrium conformer angle. The relative minimum energy for the trans conformer is the consequence of a balance between decreasing hyperconjugative stabilization and decreasing steric destabilization as the FCCF torsional angle approaches 180 degrees . The torsional coordinate is predicted to be strongly contaminated by CCF bending, with the result that approximately half of the trans --> gauche stabilization energy stems from mode coupling.     

Role of negative hyperconjugation and anomeric effects in the stabilization of the intermediate in SNV reactions

The role of negative hyperconjugation and anomeric and polar effects in stabilizing the XZHCbetaCalphaYY'- intermediates in SNV reactions was studied computationally by DFT methods. Destabilizing steric effects are also discussed. The following ions were studied: X = CH3O, CH3S, CF3CH2O and Y = Y' = Z = H (7b-7d), Y = Y' = H, Z = CH3O, CH3S, CF3CH2O (7e-7i), YY' = Meldrum's acid-like moiety (Mu), Z = H, (8b-8d), and YY' = Mu, Z = CH3O, CH3S, CF3CH2O (8e-8i). The electron-withdrawing Mu substituent at Calpha stabilizes considerably the intermediates and allows their accumulation. The hyperconjugation ability (HCA) (i.e., the stabilization due to 2p(Calpha) --> sigma*(Cbeta-X) interaction) in 8b-8d follows the order (for X, kcal/mol) CH3S (8.5) > CF3CH2O (7.6) approximately CH3O (7.5). The HCA in 8b-8d is significantly smaller than that in 7b-7d due to charge delocalization in Mu in the former. The calculated solvent (1:1 DMSO/H2O) effect is small. The stability of disubstituted ions (7e-7i and 8e-8i) is larger than that of monosubstituted ions due to additional stabilization by negative hyperconjugation and an anomeric effect. However, steric repulsion between the geminal Cbeta substituents destabilizes these ions. The steric effects are larger when one or both substituents are CH3S. The anomeric stabilization (the energy difference between the anti,anti and gauche,gauche conformers) in the disubstituted anions contributes only a small fraction to their total stabilization. Its order (for the following X/Z pairs, kcal/mol) is CF3CH2O/CH3S (8i, 4.9) > CF3CH2O/CH3O (8h, 3.9) > CH3O/CH3S (8g, 3.3) > CH3S/CH3S (8f, 2.9) > CH3O/CH3O (8e, 2.4). Significantly larger anomeric effects of ca. 8-9 kcal/mol are calculated for the corresponding conjugate acids.     

Conformation of 2,2,2-trifluoroethanol and the solvation structure of its 2-fluoropyridine clusters

The conformation of 2,2,2-trifluoroethanol (TFE) in the 2-fluoropyridine-(TFE)(m)-(H(2)O)(n) clusters in a supersonic jet has been investigated with fluorescence-detected infrared spectroscopy and quantum chemical calculations. It is common to the observed clusters that they form chain structures containing the weak interaction of the pyridyl CH with the fluorine or oxygen atom in the terminal TFE. The detectable conformation of TFE is gauche only even in the case of the existence of the strong base such as 2-fluoropyridine. This result is explained by the change in hyperconjugation among several dominant orbitals. The preference of the terminal TFE in the mixed clusters with TFE and water solvents is observed, which is ascribed to the stronger cooperative effect of TFE than water.     

Chemical origin of blue- and redshifted hydrogen bonds: intramolecular hyperconjugation and its coupling with intermolecular hyperconjugation

Upon formation of a H bond Y...H-XZ, intramolecular hyperconjugation n(Z)-->sigma*(X-H) of the proton donor plays a key role in red- and blueshift characters of H bonds and must be introduced in the concepts of hyperconjugation and rehybridization. Intermolecular hyperconjugation transfers electron density from Y to sigma*(X-H) and causes elongation and stretch frequency redshift of the X-H bond; intramolecular hyperconjugation couples with intermolecular hyperconjugation and can adjust electron density in sigma*(X-H); rehybridization causes contraction and stretch frequency blueshift of the X-H bond on complexation. The three factors--intra- and intermolecular hyperconjugations and rehybridization--determine commonly red- or blueshift of the formed H bond. A proton donor that has strong intramolecular hyperconjugation often forms blueshifted H bonds.     

Hyperconjugative stabilization in alkyl carbocations: direct estimate of the beta-effect of group-14 elements

DFT calculations at the BP86/TZ2P level have been carried out for the primary, secondary, and tertiary carbenium ions [H2C-CH(EH3)2](+) (1a-e), [HC{CH(EH3)2}2](+), (2a-e), and [C{CH(EH3)2}3](+) (3a-e) for E = C, Si, Ge, Sn, Pb. The nature of the interaction between the carbenium center H(2-n)C(+) and the substituents {CH(EH3)2}m has been investigated with an energy decomposition analysis (EDA) aiming at estimating the strength of the pi hyperconjugation which electronically stabilizes the carbenium ions. The results of the EDA show that the calculated DeltaEpi values can be used as a measure for the strength of hyperconjugation in carbenium ions arising from the interactions of saturated groups possessing pi orbitals. The theoretical data suggest that the ability of sigma C-E bonds to stabilize positive charges by hyperconjugation follow the order C < Si < Ge < Sn < Pb. Hyperconjugation of C-Si bonds is much stronger than hyperconjugation of C-C bonds while the further rising from silicon to lead is smaller and has about the same step size for each element. The strength of the hyperconjugation in primary, secondary, and tertiary alkyl carbenium ions does not increase linearly with the number of hyperconjugating groups; the incremental stabilization becomes smaller from primary to secondary to tertiary cations. The effect of hyperconjugation is reflected in the shortening of the C-C bond distances and in the lengthening of the C-E bonds, which exhibits a highly linear relationship between the calculated C-C and C-E distances in carbocations 1-3 and the hyperconjugation estimated by the DeltaEpi values.     

Agostic Interactions in Early Transition-Metal Complexes: Roles of Hyperconjugation,Dispersion, and Steric Effect

The agostic interaction is a ubiquitous phenomenon in catalytic processes and transition-metal complexes, and hyperconjugation has been well recognized as its origin. Yet, recent studies showed that either short-range London dispersion or structural constraints could be the driving force, although proper evaluation of the role of hyperconjugation therein is needed. Herein, a simple variant of valence bond theory was employed to study a few exemplary Ti complexes with α- or β-agostic interactions and interpret the agostic effect in terms of the steric effect, hyperconjugation, and dispersion. For the complexes [MeTiCl₃(dmpe)] and [MeTiCl₃(dhpe)] with α-agostic interactions, hyperconjugation plays the dominant role with comparable magnitudes in both systems, but dispersion is solely responsible for the stronger agostic interaction in the former compared with the latter. For the complexes [EtTiCl₃(dmpe)] and [EtTiCl₃(dhpe)] with β-agostic interactions, however, hyperconjugation and dispersion play comparable roles, and the weaker steric repulsion leads to a stronger agostic effect in the former than in the latter. Thus, the present study clarifies the variable and sensitive roles of steric, hyperconjugative, and dispersion interactions in the agostic interaction.     

Chemical Reactivity Controlled by Negative Hyperconjugation: A Theoretical Study

Negative hyperconjugation is a general phenomenon that can be observed in many areas of chemistry. The knowledge of its impact on structural parameters and conformational issues is well established, but little is known about its importance for chemical reactivity. Here we present a systematic study of different aspects of negative hyperconjugation on the reactivity of complex heterocyclic systems using density functional theory. Intermediates from the reaction of nitrogen‐based nucleophiles with bis(1,3,4‐thiadiazolo)‐1,3,5‐triazinium halides serve as benchmark systems to demonstrate the effects of negative hyperconjugation on bond lengths, on the relative stability of conformational isomers and transition structures and, most importantly, on the different reaction pathways of these species. The computational results provided here are in part supported by experiments reported elsewhere.     

DFT calculations of hyperfine coupling constants of organic π radicals and comparison with empirical equations and experiment

Density functional theory calculations (UB3LYP/EPR‐III) for a series of radicals and radical ions were performed to check the internal consistency of the method and its implications to the theoretical concepts of electron paramagnetic resonance such as π–σ spin polarization, hyperconjugation and phenyl hyperconjugation. In the second part, experimental data for seven radicals (43 hyperfine coupling constants) are compared with calculations, yielding a correlation of r² = 0.97. Copyright © 2016 John Wiley & Sons, Ltd.     

Vibrational spectra and conformations of chloromethyldimethyl- and tris(chloromethyl)phosphine oxides

A study was carried out on the IR spectra of (CH₃)₂P(O)CH₂Cl and (CH₂Cl)₂PO. The frequencies and forms of the normal modes were calculated. The molecular mechanics method was used to calculate the energy of the various conformations of these molecules. (CH₃)₂P(O)CH₂Cl in the liquid and solution is a mixture of trans and gauche conformations, while (CH₂Cl)₃PO is a mixture of trans,gauche,gauche, trans,gauche,gauche, and gauche,gauche,gauche conformations.S. M. Kirov Kazan Chemical Engineering Institute and A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 6, pp. 38–46, November–December, 1989.     

Anti and gauche conformers of an inorganic butane analogue, NH3BH2NH2BH3

The crystal structures of an inorganic butane analogue, NH(3)BH(2)NH(2)BH(3) (DDAB), were determined using single crystal X-ray diffraction, revealing both anti and gauche conformations. The anti conformation is stabilized by intermolecular dihydrogen bonds in the crystal whereas two gauche conformations of DDAB observed in its 18-crown-6 adducts are stabilized by an intramolecular dihydrogen bond. The two gauche conformations show rotational isomerization but whether they are a pair of enantiomers is yet to be defined.     

A theoretical study of rhodium(I) catalyzed carbonylative ring expansion of aziridines to beta-lactams: crucial activation of the breaking C-N bond by hyperconjugation

The regioselectivity and enantiospecificity of the [Rh(CO)2Cl]2-catalyzed carbonylative ring expansions of N-tert-butyl-2-phenylaziridine to yield 2-azetidinone and the lack of reactivity of N-tert-butyl-2-methylaziridine along this process were investigated at the B3LYP/6-31G(d) (LANL2DZ for Rh) theory level taking into account solvent effects. According to our results, the regioselectivity in the ring expansion of N-tert-butyl-2-phenylaziridine and the unreactivity of N-tert-butyl-2-methylaziridine experimentally observed are determined by the different degree of activation of the breaking C-N bond in the initial aziridine-Rh(CO)2Cl complex due to its hyperconjugation interaction with the substituent on the carbon atom. When a phenyl substituent is present its hyperconjugation interaction with the C(alpha)-N bond facilitates the insertion of the metal atom into this bond. On the other hand, when the substituent is a methyl group, a larger stability of the initial complex along with a lower stabilization through hyperconjugation of the TS for insertion of the Rh atom into the C(alpha)-N bond make the ring expansion of N-tert-butyl-2-methylaziridine unviable. The enantiospecificity experimentally observed is also reproduced by our calculations given that the stereogenic center is never perturbed to change its configuration.     

Direct estimate of the strength of conjugation and hyperconjugation by the energy decomposition analysis method

The intrinsic strength of pi interactions in conjugated and hyperconjugated molecules has been calculated using density functional theory by energy decomposition analysis (EDA) of the interaction energy between the conjugating fragments. The results of the EDA of the trans-polyenes H2C=CH-(HC=CH)n-CH=CH2 (n = 1-3) show that the strength of pi conjugation for each C=C moiety is higher than in trans-1,3-butadiene. The absolute values for the conjugation between Si=Si pi bonds are around two-thirds of the conjugation between C=C bonds but the relative contributions of DeltaE pi to DeltaE orb in the all-silicon systems are higher than in the carbon compounds. The pi conjugation between C=C and C=O or C=NH bonds in H2C=CH--C(H)=O and H2C=CH-C(H)=NH is comparable to the strength of the conjugation between C=C bonds. The pi conjugation in H2C=CH-C(R)=O decreases when R = Me, OH, and NH2 while it increases when R = halogen. The hyperconjugation in ethane is around a quarter as strong as the pi conjugation in ethyne. Very strong hyperconjugation is found in the central C-C bonds in cubylcubane and tetrahedranyltetrahedrane. The hyperconjugation in substituted ethanes X3C-CY3 (X,Y = Me, SiH3, F, Cl) is stronger than in the parent compound particularly when X,Y = SiH3 and Cl. The hyperconjugation in donor-acceptor-substituted ethanes may be very strong; the largest DeltaE pi value was calculated for (SiH3)3C-CCl3 in which the hyperconjugation is stronger than the conjugation in ethene. The breakdown of the hyperconjugation in X3C-CY3 shows that donation of the donor-substituted moiety to the acceptor group is as expected the most important contribution but the reverse interaction is not negligible. The relative strengths of the pi interactions between two C=C double bonds, one C=C double bond and CH3 or CMe3 substituents, and between two CH3 or CMe3 groups, which are separated by one C-C single bond, are in a ratio of 4:2:1. Very strong hyperconjugation is found in HC[triple bond]C-C(SiH3)3 and HC[triple bond]C-CCl3. The extra stabilization of alkenes and alkynes with central multiple bonds over their terminal isomers coming from hyperconjugation is bigger than the total energy difference between the isomeric species. The hyperconjugation in Me-C(R)=O is half as strong as the conjugation in H2C=CH-C(R)=O and shows the same trend for different substituents R. Bond energies and lengths should not be used as indicators of the strength of hyperconjugation because the effect of sigma interactions and electrostatic forces may compensate for the hyperconjugative effect.     

Negative hyperconjugation in methyl and silyl anions and its effect on the acidities of CH(4-n)Fn CH(4-n)C1n SiH(4-n)Fn and SiH(4-n)Cln

Group separation reactions calculated using an ab intio molecular orbital calculation at the MP4/6-31 + + G(d,p) level of theory, show the negative hyperconjugation between fluorine atoms to be larger in methanes than in silanes. Stabilisation due to negative hyperconjugation is larger in anions than in identically substituted neutral molecules, e.g. 43.1 kcal mol⁻¹ in CF₃⁻ compared with 26.7 kcal mol⁻¹ in CHF₃. By contrast, in chloro-substituted methanes, silanes, methyl anions and silyl anions, group separation energies are approximately zero, indicating no appreciable negative hyperconjugation. An -chloro substituent is more effective than an -fluoro one at delocalising the negative charge of an anion and, as a consequence, the chloromethanes and chlorosilanes are all more acidic than the identically substitued fluoromethanes and fluorosilanes. For chloro-substituted molecules the acidity is linearly dependent on the number of chlorine atoms; for fluoro-substituted molecules stabilisation by negative hyperconjugation results in each additional fluorine atom increasing the acidity by larger increments.     

Investigation of the diastereoselectivity of tricyclo(5.2.1.0(2,6))decan-10-ones: controversies and agreements

The diastereoselectivities of tricyclo(5.2.1.0(2,6))decan-10-one and its derivatives are controlled by antiperiplanar and vicinal sigma --> piC=O interactions rather than the hyperconjugation effects as reported previously.     

Section C—Heterocyclic Compounds

Abstract

Carbonyl stretching frequencies and apparent molar absorptivities are reported for some α-(alkylthio)-acetates, -propionate and -isobutyrate, in the fundamental and 1st overtone regions and compared to those for the corresponding unsubstituted esters. The I.R. data for α-(alkylthio)-esters indicate the cis-gauche rotational isomerism. Both the unusual solvent effect and the Avco frequency shifts for the title compounds indicate that the cis rotamers arc less polar than the gauche ones. The gauche rotamer (G₁) for the a-(ethylthio)-propionate corresponds to the gauche rotamer (G) for the α-(ethylthio)-acetate while the more crowded gauche rotamer (G₂′) for the a-(ethylthio)-isobutyrate corresponds to the gauche rotamer (G₂) for the α-(ethylthio)-propionate. The larger Δμ_CO values for the cis (C) and gauche (G₂′) rotamers of α-(ethylthio)-isobutyrate in comparison to the cis (C) and gauche (G₁) rotamers of the α-(ethylthio)-propionate are explained in terms of variation in the α-carbonyl angle, changes in mechanical coupling and hybridization. Charge transfer from |Gp_CO to 3d_(s orbitals and |Gp_CO/σ_C-S hyperconjugative interaction are also invoked to explain the obtained results.