Self-consistent Isodensity Polarized Continuum Model Research Articles

A comprehensive study of the structure, tautomeric properties, and conformational flexibility of 3-Hydroxy-propeneselenal

In the present work, a conformational analysis of 3-Hydroxy-propeneselenal is performed using several computational methods, including HF, DFT (B3LYP), and G2MP2 levels of theory. The relative electronic energies, the molecular structure, and intramolecular hydrogen bond energies are reported. Twenty different conformers were obtained, the most stable ones being planar. The conformational preference of this molecule was found to be mainly determined by the formation of Se...H-O and Se-H...O intramolecular hydrogen bonds, which are assisted by π-electron resonance. The atoms in molecules (AIM) theory of Bader, which is based on topological properties of the electron density, was used to analyze critical points and to study the nature of hydrogen bonds in these systems. NMR and natural bond orbital (NBO) analyses were also performed for a better understanding of the nature of intramolecular interactions in 3-Hydroxy-propeneselenal. Furthermore, calculations for all the possible conformations of the title compound in solution were also carried out at the B3LYP/6-311 ++G(d,p) level of theory (in three models such as polarizable continuum model (PCM), IEFPCM, and self −consistent isodensity polarized continuum model (SCIPCM). The aim of this work is to highlight the efficiency and suitability of silicadiphenic acid as a supported solid acid catalyst for the synthesis of bis-(indolyl)methane derivatives from a one-pot condensation reaction of various substituted aldehydes and indole in acetonitrile at 60°C.

Theoretical Study on the Self- and Water-Assisted Proton Transfer Reaction of Urazole

The proton transfer reactions of urazole have been investigated theoretically by using the density functional theory at the B3LYP/6-311++G(d,p) level and single-point MP2 calculations with the same basis set. The influence of the solvent on the proton transfer reactions of urazole has been examined using the self-consistent isodensity polarized continuum model in water. Also the proton transfer in the mono- and dihydrated forms of urazole have been investigated at the same level in the gas phase. It was found that the proton transfer to the carbonyl oxygen of the hydrazyl proton of urazole is the easiest in the gas phase and in water. The same proton transfer is also thermodynamically the easiest among water-assisted proton transfer reactions of urazole. In addition, the barrier heights for both H2O-assisted and (H2O)2-assisted reactions are significantly lower than those for the self-assisted tautomerization reactions of urazole.

DFT, FT-IR and FT-Raman investigations of 1-methyl-2-imidazolecarboxaldehyde

The FT-IR and FT-Raman spectra of 1-methyl-2-imidazolecarboxaldehyde were recorded in the region 4000–400cm−1 and 3500–50cm−1, respectively. Optimized geometric parameters, conformational equilibria, normal mode frequencies, and corresponding vibrational assignments of title molecule were examined by means of density functional theory (DFT) method with B3LYP/6-311++G(d,p) basis set. All vibrational frequencies were assigned in detail with the help of total energy distribution (TEDs). The results showed that the DFT/B3LYP method predics vibrational frequencies and the structural parameters effectively. Furthermore, solvent effects are investigated in different solvents (chloroform, dimethylsulfoxide and water) using the self-consistent isodensity polarizable continuum model (SCI-PCM). All results indicate that the combination of SCI-PCM model and DFT/B3LYP/6-311++G(d,p) calculation could give excellent explanations of the solvent effects. Intermolecular hydrogen bonding between 1-methyl-2-imidazolecarboxaldehyde and water was investigated using quantum chemical methods. DFT calculations were used to optimize the adducts of heterocycle with the single water molecule.

Formal Redox Potentials of Organic Molecules in Ionic Liquids on the Basis of Quaternary Nitrogen Cations as Adiabatic Electron Affinities

Formal redox potentials E°' involving neutral species R and radical anions R(•-) in ionic liquids (ILs) composed of ammonium, pyridinium, and imidazolium cations are discussed from the point of view of the adiabatic electron affinity as a molecular property. The dependence of the 1,4-benzoquinone (BQ)/BQ(•-) redox process in CH2Cl2 and CH3CN is primarily investigated over a wide concentration range of ILs as the supporting electrolyte. A logarithmic relationship involving a positive shift of E°' with increasing concentration is obtained when the concentration is changed from 0.01 to 1.0 M. The relationship of E°' at IL concentrations greater than 1.0 M gradually reaches a plateau and remains there even for the neat ILs. It is found that the E°' values in the neat ILs are not influenced by the measurement conditions, and that they remain considerably dependent on the nature and concentration of the electrolyte when measured using the traditional method involving molecular solvents combined with a supporting electrolyte (0.1-0.5 M). The difference in the E°' values observed in the ammonium and pyridinium ILs is only several millivolts. In addition, ESR and self-consistent isodensity polarized continuum model calculation results reveal that the potential shift toward positive values upon the transition from molecular solvents containing ILs to neat ILs is adequately accounted for by changes in the electrostatic interaction of R(•-) taken into the cavity composed of the solvent and IL. On the other hand, the first reduction waves of quinones, electron-accepting molecules, and polynuclear aromatic hydrocarbons are reversibly or quasi-reversibly observed in the ILs. The electrochemical stability of the ILs is exploited in the facile measurement of these quasi-reversible waves at quite negative potentials, such as for the naphthalene (NP)/NP(•-) couple. Notably, the E°' values obtained in the ammonium ILs correlate well with the calculated standard redox potentials and are linearly fitted with high correlation over all classes of compounds using a single regression equation based on Koopmans' theorem.

Quantum chemical studies on molecular conformations, energetic and intramolecular hydrogen bonding in ground and electronic excited state of (thioxosilyl) ethyleneselenol

In this work, a conformational analysis of (thioxosilyl) ethyleneselenol was performed using several computational methods, including density-functional theory (DFT) (B3LYP), MP2 and G2MP2. Harmonic vibrational frequencies were estimated at the same levels to confirm the nature of the stationary points found and also to account for the zero point vibrational energy correction. MES-1 and TES-1 conformers exhibit hydrogen bonding. This feature, although is not the dominant factor in the stability of conformers, appears to be of foremost importance to define the geometry of the molecule. Two intramolecular hydrogen bonds established between the polar groups were identified by the structural geometric parameters. These involved the thiol and selenol functional groups and were identified and characterized by the frequency shift in their stretching vibration modes. Furthermore, the excited-state properties of intramolecular hydrogen bonding have been investigated theoretically using the time-dependent DFT method. The influence of the solvent on the stability order of conformers and the strength of intramolecular hydrogen bonding was considered using the PCM (polarizable continuum model), SCI-PCM (self consistent isodensity-polarizable continuum model) and IEF-PCM (integral equation formalism-polarizable continuum model) methods. The “atoms in molecules” theory of Bader was used to analyze critical points and to study the nature of hydrogen bond in these systems. Natural bond orbital (NBO) analysis was also performed for better understanding the nature of intramolecular interactions. The calculated highest occupiedmolecular orbital and lowest unoccupied molecular orbital energies show that charge transfer occur within the molecule. Further verification of the obtained transition state structures was implemented via intrinsic reaction coordinate analysis. Calculations of the 1H NMR chemical shift at the GIAO/B3LYP/6–311++G** level of theory are also presented.

CONFORMATIONAL PROPERTIES AND INTRAMOLECULAR HYDROGEN BONDING OF 3-AMINO-PROPENESELENAL: ANAB INITIOAND DENSITY FUNCTIONAL THEORY STUDIES

In the present work a conformational analysis of 3-amino-propeneselenal (APS) was performed using several computational methods, including DFT (B3LYP), MP2 and G2MP2. Harmonic vibrational frequencies were estimated at the same levels to confirm the nature of the stationary points found and also to account for the zero point vibrational energy (ZPVE) correction. Two intramolecular hydrogen bonds (HBs) established between the polar groups were identified by the structural geometric parameters. The excited-state properties of intramolecular hydrogen bonding in hydrogen bonded systems have been investigated theoretically using the time dependent density functional theory (TDDFT) method. The influence of the solvent on the stability order of conformers and the strength of intramolecular hydrogen bonding was considered using the polarized continuum model (PCM), the self-consistent isodensity polarized continuum model (SCI-PCM) and the integral equation formalism-polarizable continuum model (IEF-PCM) methods. The "atoms in molecules" theory of Bader was used to analyze critical points and to study the nature of HB in these systems. Natural bond orbital (NBO) analysis was also performed for better understanding the nature of intramolecular interactions. The calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies show that charge transfer occur within the molecule. Further verification of the obtained transition state structures were implemented via intrinsic reaction coordinate (IRC) analysis. Calculations of the1H NMR chemical shift at GIAO/B3LYP/6–311++G** level of theory are also presented.

Balancing the atomic waistline: Isodensity‐based scrf radii for main‐group elements and transition metals

AbstractToward cracking the problem of understanding, characterizing, and predicting “solvent‐effect” while the world awaits an effective explicit solvent model, we introduce and justify herein a novel set of atomic radii to be used within the most commonly used continuum reaction field, the polarizable continuum model (PCM). The radial values emerge from a quantitative description of the elemental electronic density distribution and are shown to be accurate in such a self‐consistent reaction field (SCRF); labeled accordingly as isodensity‐based SCRF (IDSCRF) radii. Transition row elements with dynamic oxidation states are addressed through an averaging of the electronic properties from all states in the determination of their effective radii. All results for nonmetal elements have been verified with Guthrie's SAMPLE1 test set and are in quantitative agreement with experimental values from the literature and self‐consistent isodensity polarizable continuum model (SCIPCM) calculations. For the compounds with transition metal elements, our IDSCRF results have been verified with SCIPCM results as there are rarely experimental results available. Finally, explicit solvent particles “solvating” Pd‐ and Ni‐containing homogeneous catalysts are also shown to be in close agreement with the IDSCRF radii calculations. © 2012 Wiley Periodicals, Inc.

FT-IR spectroscopy and DFT calculation study on the solvent effects of benzaldehyde in organic solvents

FT-IR spectra of benzaldehyde in 11 different organic solvents were recorded and analyzed. The density functional theory (DFT) B3LYP/6-31G* method was chosen to calculate the infrared spectrum of benzaldehyde in gaseous state. The electrostatic effects of different solvents in benzaldehyde solutions were calculated using DFT with the self-consistent isodensity polarizable continuum model (SCI-PCM). Two remarkable carbonyl (C O) peaks of benzaldehyde were observed by FT-IR in alcohol solvents, which were caused by different hydrogen bond species and explained by ab initio calculation. The results showed that the combination of SCI-PCM model and ab initio calculation could give excellent agreements with FT-IR spectra of title compound in solutions.

Computational study of the intramolecular proton transfer reactions of 3-hydroxytropolone (2,7-dihydroxycyclohepta-2,4,6-trien-1-one) and its dimers

The proton transfer reaction and dimerization processes of 3-hydroxytropolone (3-OHTRN) have been investigated using density functional theory (DFT) at the B3LYP/6-31+G** level. The influence of the solvent on the proton transfer reaction of 3-OHTRN was examined using the self-consistent isodensity polarized continuum model (SCI-PCM) with different dielectric constants (ε = 4.9, CHCI₃; ε = 32.63, CH₃OH; ε = 78.39, H₂O). The intramolecular proton transfer reaction occurs more readily in the gas phase than in solution. Results also show that the stability of 3-OHTRN dimers in the gas phase is directly affected by the hydrogen bond length in the dimer structure.

A theoretical model study on the cyclic reaction of 4-hydroxybutanal catalyzed by Brønsted acid

For a theoretical model study on the cyclic reaction of 4-hydroxybutanal (4-OH-BL), we have examined five assumed reaction pathways (I–V) by performing the B3LYP calculations in the gas phase and self-consistent isodensity polarized continuum model (SCIPCM)-B3LYP calculations in aqueous solution. Pathways II (4-OH-BL + H+), III (4-OH-BL + H3O+), and IV (4-OH-BL + H3O+ + H2O) represent three models for the cyclic reaction catalyzed by Brønsted acids. The present study leads to the following conclusions concerning the five pathways (mainly on the basis of the calculation results in the solution). The high barrier along pathway I (with no catalyst) implies that the reaction does not occur without a catalyst, and the extremely large stabilization energy of the intermediate implies that pathway II is not a realistic model for the reaction catalyzed by Brønsted acid. Along pathway III, there are two intermediates and a transition state in between, and they are 10–16 kcal/mol lower in energy than the reactants (4-OH-BL + H3O+). Along pathway IV, there is only one intermediate, and it is 20.6 kcal/mol lower in energy than the reactants (4-OH-BL + H3O+ + H2O). Pathways III and IV are predicted to be feasible. Energetically, pathway IV is more favourable than pathway III and it is considered as a rational model for the cyclic reaction of 4-OH-BL catalyzed by Brønsted acid. Our calculations for pathway V (catalyzed by H2O) indicate that the water molecule may also serve as a catalyst for the cyclic reaction. The transition state along pathway V is 20.0 kcal/mol higher in energy than the reactants (4-OH-BL + H2O), and one can clearly see the “proton wire” in its structure. Our calculations show strong solvent effects on energetics of the charged intermediates along pathways II, III, and IV.

Ground and excited states calculations of 7-phenylamino-substituted coumarins

A theoretical calculation of the ground and first excited states of two 7-phenylamino-substituted coumarin compounds are performed. In order to study the effect of phenyl substituted in amino group and fluorine atoms substituted in methyl group, 7-amino-4-methyl coumarin (C120) and 7-amino-4-(trifluoromethyl) coumarin (C151) were also studied. The geometries of the ground state and the first ground state were optimized using density function theory and configuration interaction singles levels of theory. Molecular orbitals (MO) of the ground and first excited states of the four coumarin compounds were obtained to explain the change of the structures. ZINDO and TD-DFT methods with different basis sets were applied to predict the UV absorption spectra. The solvent effect had also been taken into account using self-consistent isodensity polarized continuum model. The predicted spectra are in agreement with the experimental data.

UV–visible, 1H and 13C NMR spectroscopic studies on the interaction between protons or alkaline earth metal ions and the benzoate ion in acetonitrile

The interaction between protons (H +) or alkaline earth metal ions (Mg 2+, Ba 2+) and the benzoate ion has been examined in acetonitrile by means of UV–visible, 1H and 13C NMR spectroscopy. The assignments of 13C NMR spectra for the benzoate ion (L −), benzoic acid (HL), and the protonated benzoic acid (H 2L +) in acetonitrile were properly performed by means of theoretical calculations, including the influence of the acetonitrile solvent by using the self-consistent isodensity polarized continuum model and the polarized continuum model. It is demonstrated that “strong” interactions of alkaline earth metal ions (M 2+ = Mg 2+, Ba 2+) with the benzoate ion (L −) lead to the formation of mono-charged species, ML +, as well as non-charged species, ML 2, in acetonitrile solution: the strong ability in coordination for alkaline earth metal ions is verified.

The effects of conformation and solvation on optical rotation: Substituted epoxides

The vapor-phase optical rotation (or circular birefringence) of (S)-1,2-epoxybutane, (S)-epichlorohydrin, and (S)-epifluorohydrin has been measured at the nonresonant excitation wavelengths of 355 nm and 633 nm by means of Cavity Ring-Down Polarimetry (CRDP). Complementary solution-phase studies were performed in a wide variety of dilute solvent media to highlight the pronounced influence of solute-solvent interactions. Density functional theory calculations of optical activity have been enlisted to unravel the structural and electronic provenance of experimental observations. Three stable, low-lying conformers have been identified and characterized for each of the targeted chiral species, with thermal (relative population weighted) averaging of their antagonistic chiroptical properties allowing specific rotation values to be predicted under both isolated and solvated conditions. For (S)-epichlorohydrin and (S)-epifluorohydrin, a self-consistent isodensity polarizable continuum model (SCI-PCM) has been exploited to gain further insight into the underlying nature of solvation effects.

The anionic chain process mechanism for reactions of perchlorofluoroethanes with nucleophiles in solution: a theoretical study

AbstractAnionic chain process mechanism (including reactions (1)–(4)) suggested by the experimental investigations for the reactions of nucleophile (CH3O−) with perchlorofluoroethanes CF2ClCCl3 (1), CF2ClCCl2F (2), and CF3CCl3 (3) in solution is examined by performing calculations using the B3LYP method and the SCIPCM (self‐consistent isodensity polarizable continuum) model for simulating solution effects. The SCIPCM‐B3LYP calculations indicate that anionic species have large solvation energies and solvation energy values for different kinds of anions are quite different, which effects changes in ΔH's for the reactions in solution. Competition between anionic hyperconjugation and solvation leads to negative ΔH values for reactions (2) from 1 and 2 in solution. Reactions (3) and (4) from 1, 2, and 3 in solution are predicted to be exothermic or highly exothermic. Since the ΔH values for reactions (1) and (2) from 1 and 2 in solution are negative or small positive values, the reactions of CH3O− with 1 and 2 in solution proceed via the anionic chain process. For 3 in solution, reactions (1) and (2) are endothermic while reactions (3) and (4) are exothermic or highly exothermic. The reactions of CH3O− with 3 in solution may proceed via the anionic chain process. All these conclusions are in agreement with the experimental indications for reactions of nucleophiles with 1, 2, and 3 in solution. Copyright © 2007 John Wiley & Sons, Ltd.

Importance of the Single Amino Acid Potential in Water for Secondary and Tertiary Structures of Proteins

The structure of a protein molecule is considered to be primarily determined by the inter-amino-acid nonbonded interactions, such as hydrogen bonds. However, the conformational space of the polypeptide chain should be simultaneously restricted by the intrinsic conformational preferences of the individual amino acids. We present here precise single amino acid potential (SAAP) surfaces for glycine (For-Gly-NH(2)) and alanine (For-Ala-NH(2)) in water (epsilon = 78.39) and ether (epsilon = 4.335), which were calculated at the HF/6-31+G(d,p) level applying the self-consistent isodensity polarizable continuum model (SCIPCM) reaction field with geometry optimization in the corresponding solvents. The obtained Ramachandran potential surfaces in water showed distinct potential wells in the alpha- and beta-regions. The profiles were in almost perfect agreement with the Ramachandran plots of glycine and alanine residues in folded proteins, suggesting the Boltzmann distributions on the SAAP surfaces. Molecular simulations of polyalanines (For-Ala(n)-NH(2); n = 3-5) by using the SAAP force field equipped with the SCIPCM potentials revealed that the polyalanines readily form 3(10)-helical structures in water but not in vacuo. In ether (hydrophobic environments), the helical structures were relatively stable, but the most stable structure was assigned to a different one. These results indicated that the intrinsic conformational preferences of the individual amino acids (i.e., the SAAPs) in water are of significant importance not only for describing conformations of a polypeptide chain in the random coil state but also for understanding the folding to the secondary and tertiary structures.

Predictions of the electronic absorption and emission spectra of luciferin and oxyluciferins including solvation effects

The ground and excited state properties of luciferin (LH 2) and oxyluciferin (OxyLH 2), the bioluminescent chemical in the firefly, have been characterized using the configuration interaction singles (CIS) and time dependent density functional (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin are predicted with a self-consistent isodensity polarized continuum model of the solvent using both the configuration interaction singles model and time dependent density functional theory. The S 0 → S 1 vertical excitation energies in the gas phase and in water are obtained with both methods. Optimizations of the excited state geometries permit the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both the absorption and emission spectra on proceeding from the gas phase to aqueous solution also are predicted.

Density Functional Studies on Conformational Behaviors of Glycinamide in Solution

Conformational behaviors and relative stabilities for four glycinamide conformers in aqueous solution have been investigated at the B3LYP/6-311++G** level of theory employing the self-consistent isodensity polarized continuum (SCIPCM) model within the framework of the self-consistent reaction field (SCRF) theory. Overall, the peptide bond (C1-N4) in all of the conformers is strengthened while the C1=O5 bond is weakened upon solvation though the structural parameters have no significant changes except for the dihedral angles in conformer IIA. The relative stabilities among glycinamide conformers still remain as that in the gas phase except that two minor conformers reverse their order. The characteristic frequencies of amide I, primarily involving the C1=O5 stretching mode, are red-shifted in all conformers, and IR intensities of mostly absorption bands are calculated to become more intense in going from the gas phase to solution. The microscopic model, which is simulated by studying the interactions of four glycinamide conformers with one water molecule in the gas phase, has also been performed to explore the microhydrated effects. All of the results, which are based on the SCIPCM model, microscopic model, and combination of them as well, are in agreement with one another, especially for the prediction of the relative stabilities among all of the glycinamide conformers. Comparison of the SCIPCM model employed in the present study and Onsager model used previously indicates that the latter may be used to study larger analogous systems that are too expensive to treat with the SCIPCM model computationally. Additionally, the tautomeric equilibria between glycinamide and glycinamidic acid have been assessed with and without the participation of one water molecule. Calculated results show that the presence of solvent disfavors the tautomeric process, whereas the water-assisted proton transfer plays a positive role in the tautomeric process.

A way to compare experimental and SCRF electronic static dipole polarizability of pure liquids

When the polarizability tensor is computed through Self-Consistent Reaction Field Models (the interactions between solute and solvent are incorporated into calculation), there still exists a need to take into account the modifications of the applied macroscopic fields at the local site of the solvated molecule (the phenomenon historically indicated as ‘local field effects’). Another way is to remove local field factors from experimental mean polarizability ( α ̄ ) and polarizability anisotropy (Δ α) values. The present paper focuses on this way. Seven liquids (CS 2, C 6H 6, (CH 3) 2CO, CHCl 3, CH 2Cl 2, CH 3CN, CCl 4) have been studied. In a first step, the α tensor components have been computed at the B3LYP and HF levels with Sadlej basis set using a finite field procedure. The solvent effect is introduced into the calculation through the Self-Consistent Isodensity Polarizable Continuum Model (SCIPCM). In a second step, experimental literature values of refractive index, density, isothermal compressibility coefficient and depolarization ratio have been carefully chosen and introduced in modified experimental equations (where local field factors have been removed) to determine α ̄ and Δ α. Concerning α ̄ , there is a good agreement between experimental and B3LYP values for dipolar molecules. In the case of non-dipolar molecules calculated values are always lower than experimental ones. Such a behaviour had not been observed in a previous study concerning molecules in the gas state, therefore it seems that the SCIPCM induces a deficiency over calculated α ̄ (0) in the case of non-dipolar molecules.

Theoretical study of aqueous uranyl carbonate (UO 2CO 3) and its hydrated complexes: UO 2CO 3· nH 2O ( n=1–3)

Extensive ab initio calculations have been carried out to study the structure and bonding of hydrated UO 2CO 3 complexes using state-of-the-art techniques. The structures of aqueous UO 2CO 3 and its hydrated complexes have been further studied by considering the solvent as a polarizable continuum dielectric. The calculations have been carried out using polarization continuum, self-consistent isodensity polarizable continuum model, and conductor like screen models. The calculated uranyl frequencies of the hydrated UO 2CO 3 have been compared with the available vibrational frequencies and the nature of water binding has been analyzed using energy decomposition techniques.

Electronic and steric substituent influences on the conformational equilibria of cyclohexyl esters: the anomeric effect is not anomalous!

The cyclohexyl esters of a series of carboxylic acids, RCO(2)H, spanning a range of electronegativities and quotients of steric hindrance for the R substituent (R=Me, Et, iPr, tBu, CF(3), CH(2)Cl, CHCl(2), CCl(3), CH(2)Br, CHBr(2), and CBr(3)) were prepared. Their conformational equilibria in CD(2)Cl(2) were examined by low-temperature (1)H NMR spectroscopy to study the axial or equatorial orientation of the ester functionality with respect to the adopted chair conformation of the cyclohexane ring. The ab initio and DFT geometry-optimized structures and relative free energies of the axial and equatorial conformers were also calculated at the HF/6-311G**, MP2/6-311G**, and B3LYP/6-31G** levels of theory, both in the gas phase and in solution. In the latter case, a self-consistent isodensity polarized continuum model was employed. Only by including electron correlation in the modeling calculations for the solvated molecules was it possible to obtain a reasonable correlation between DeltaG degrees (calcd) and DeltaG degrees (exp). Both the structures and the free energy differences of the axial and equatorial conformers were evaluated with respect to the factors normally influencing conformational preference, namely, 1,3-diaxial steric interactions in the axial conformer and hyperconjugation. It was assessed that hyperconjugative interactions, sigma(C-C)/sigma(C-H) and sigma*(C-O), together with a steric effect--the destabilization of the equatorial conformer with increasing bulk of the R group--were the determinant factors for the position of the conformational equilibria. Thus, because hyperconjugation is held responsible as the mitigating factor for the anomeric effect in 2-substituted, six-membered saturated heterocyclic rings, and since it is also similarly responsible, at least partly, in these monosubstituted cyclohexanes for a preferential shift towards the axial conformer, the question is therefore raised: can the anomeric effect really be construed as anomalous?