The anomeric effect: Ab‐initio studies on molecules of the type XCH₂OCH₃

Abstract

The anomeric effect of the functional groups X = C s N , CECH, COOH, COO-, 0-CH3, NH2, and NH: has been studied with ab initio techniques. Geometry effects upon rotation around the central C-0 bond in X-CH2- have been compared in the various compounds. The energy differences between the conformers with a gauche and trans ( X -C -0 -C) arrangement were calculated at the 6-31G* level in the fully optimized 4-21G geometries. Energy differences calculated at the 4-21G level appeared not to be reliable, especially for the groups X that contain non-sp3 hybridized atoms. The 6-31G* energy differences indicate a normal anomeric effect for X = COO-, 0-CH3, and NH,(g') (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, CEN, and C=CH (ca. 6 kJ/mol). For X = NH2(t) and NH: a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur a t the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X-C -0 -C follows roughly the same trend.

On the nature of the anomeric effect in XHn—CH2—OH systems, with X = F, O, N, C

“…(19). Using a 4-21G basis set, they found s-NH,-CHrOCH3 to be 0.78 kcal/mol more stable at 0" than at 120°, in qualitative agreement with our result for s-NH,-CH,-OH (0.06 kcal/mol).…”

Anab initio interpretation in gas phase and aqueous solution of the generalized anomeric effect in R?O?CR2?NR2 (R = H, CH3)

“…1f, 1g Altona et al 17 reported reverse anomeric effects in X-CH 2 -O-CH 3 (X = NH 2 (t), NH + 3 ) in agreement with the experimental evidence.…”

“…For an extensive set of anomeric compounds of the type X-CH 2 -O-CH 3 , an analogous conclusion was achieved because 6-31G * //4-21G calculations were stated as the minimum level nec- essary to predict energy differences in agreement with experimental information. 17 Even more, for 2 and other compounds containing the NH 2 group, it had been shown that polarization functions are needed, at least on the N, to eliminate the excessive tendency of the NH 2 group to adopt planar conformations during the geometric optimization. 17 Although there are not available data for all the compounds, the relative energies obtained with the 3-21G basis set behave in a similar way to that All the values are in kcal/mol.…”

“…17 Even more, for 2 and other compounds containing the NH 2 group, it had been shown that polarization functions are needed, at least on the N, to eliminate the excessive tendency of the NH 2 group to adopt planar conformations during the geometric optimization. 17 Although there are not available data for all the compounds, the relative energies obtained with the 3-21G basis set behave in a similar way to that All the values are in kcal/mol. Dipole moments (µ) are in debyes.…”

Study of Thermodynamic and NMR Properties of Some Cyclohexane Derivatives

“…The structures found, in all cases, correspond to minima in the hipersurface of the potential energy, since no imaginary frequencies were found after vibrational frequencies calculations. The Natural Bond Orbitals (NBO) analyses were performed to derive population analysis 17 in an attempt to explain the stereo-electronic effects in these of molecules. Since NMR is one of the most efficient tool used in experimental methods [18][19] , to determine the molecular and electronic structure, theoretically-simulated RMN spectra were calculated for all species aiming to obtain their chemical shifts and coupling constants, and see whether the calculated structures suffer any abnormal effect in their chemical shifts and/or either normal or reverse Perlin effect which might be affecting their chemical shifts and bond lengths.…”