J.O. Williams

Ab initio studies of structural features not easily amenable to experiment: Part III. The influence of lone pair orbital interactions on molecular structure☆

Abstract The characteristic structural asymmetries and distortions of AXYB systems in which an electron lone pair is at Y are discussed on the basis of the completely relaxed ab initio equilibrium geometries of a number of representative systems including various conformations of methanediol, hydrazine, 1,2-dimethylhydrazine and of compounds with CH 3 groups adjacent to OH, OCH 3 , NH, NCH 3 and C(π). It is found that, regardless of quantitative overlap and energy gap factors, all calculated trends in the relative extensions of bond distances and bond angles can be correlated in every detail to qualitative predictions based only on the orientational aspects of orbital interaction concepts.

Ab initio studies of structural features not easily amenable to experiment: Part 8. The structural consequences of the anomeric effect in compounds with disubstituted tetrahedral carbon atoms

Abstract The geometries of three conformations of FCH 2 OH and four conformations each of NH 2 CH 2 NH 2 and NH 2 CH 2 OH are completely refined by ab initio calculations on the 4–21G level. It is found that most characteristic structural and conformational properties of such systems can be reliably predicted on the basis of a simple anomeric orbital interaction model. The extension of this model to all compounds in which two electronegative substituents with non-bonding lone pairs or bonding π-electrons are attached to the same tetrahedral carbon atom, including polymer systems such as proteins, seems to be useful.

Ab initio studies of structural features not easily amenable to experiment: Part 20. Structural aspects of ethyl groups

Abstract The molecular structures of a number of stable conformations of ethanol, ethylamine, methylethyl ether, methylethylamine and of the ethyl anion have been determined by ab initio geometry optimizations using Pulays Force method on the 4–21G level. The calculated geometries characterize the extent to which structural groups in a molecule are sensitive to asymmetries in their environment. Characteristic structural trends are consistently found for the CH bond distances and CCH angles in the C2H5 groups of trans-ethanol, trans-methylethyl ether and in the ethyl anion. They differ from those previously found for C2H5 groups in hydrocarbons. There is qualitative disagreement between the trends calculated for CH bond distances in trans-ethanol and trans-methylethyl ether and those found in the microwave substitution structures of these compounds. Since the substitution parameters are unresolved because of relatively large experimental or model uncertainties, it is presently impossible to decide whether this discrepancy is the result of computational or experimental deficiency. The methyl groups in methylethyl ether and methylethylamine exhibit the characteristic structural distortions which are usually found for CH3 groups adjacent to electron lone pairs. The CC bond distances in C2H5 in the systems studied here are sensitive to the conformational arrangement of ethyl relative to the rest of a system in a way which can be rationalized by orbital interactions involving antibonding orbitals on sp3-hybridized carbon atoms. The calculated conformational stabilities agree qualitatively with experimental trends, except in the case of ethanol where the trans — gauche energy difference is small (about 0.1 kcal mol−1) and within the uncertainties of the calculations. Our conformational energies for CH3CH2NH2 are in disagreement with a previous ab initio investigation based on a comparison of unoptimized standard geometries. In general, the agreement between calculated structural parameters and corresponding reliable experimental values is very good in all comparable cases.

Ab initio studies of structural features not easily amenable to experiment: Part 7. Molecular structure and conformational analysis of urea and carbamic acid

The geometries of four conformations of urea (one planar, three non-planar) and of the cis- and trans-forms of carbamic acid were determined by unconstrained ab initio geometry relaxation on the 4-21G level. The structural effects of multiple bonding, internal hydrogen bonding and anomeric orbital interactions are discussed. The results confirm the strong dependence of the primary molecular structure of a system on its conformational state and document the inaccuracy of the existing so called “standard geometries” for organic molecules.

Ab initio studies of structural features not easily amenable to experiment: Part 13. Structural consequences of methyl hyperconjugation in some CH3C(X)Y systems

Abstract The molecular structures of several conformations of acetaldehyde, acetic acid and isobutene were completely refined by standard single determinant ab initio procedures on the 4–21G level. Calculated and experimental parameters are in close agreement in all the cases in which the experimental values can be considered as reliably established. In some examples, however, some essential structural features that should be found in CH 3 C(=X)Y systems have not yet been resolved by any experimental technique. The structural consequences for methyl hyperconjugation in the various systems are discussed. The study demonstrates that high quality ab initio geometries can provide information which is complementary to experimental results. The calculations are particularly useful in discussing discrepancies between microwave and electron diffraction structures and in interpreting microwave substitution structure uncertainties which are often difficult to determine accurately.

Ab initio studies of structural features not easily amenable to experiment: Part 14. Complete ab initio equilibrium structures of some conformations of dimethoxymethane

Abstract The molecular structures of five conformations of dimethoxymethane (CH 3 OCH 2 -OCH 3 ) were determined by geometrically unconstrained ab initio force relaxation on the 4—21G level. The results are consistent with the expected structural effects of anomeric orbital interactions.

Ab initio studies of structural features not easily amenable to experiment: Part 6. Quantitative estimate of the effect of bond delocalization on structure and hyperconjugative interaction of the amide group

Abstract The structural changes, which occur in the amide unit when the NH 2 -group is twisted out of plane by rotation about the NC bond, have been determined by comparing the completely relaxed ab initio geometries of planar and perpendicular formamide and acetamide. In the perpendicular conformation, in which the π-electron amide resonance is uncoupled, the NC bond distance is 0.080.09 A longer than in the planar form; the CO bond distance is about 0.01 A shorter; NH distances are about 0.01 A longer; and HNC angles are 510° smaller, whereas the CNO angle is relatively constant. Because of the apparent invariance of CH 3 -hyperconjugation effects in planar and perpendicular acetamide, it is tentatively postulated that anomeric orbital interactive effects (involving the lone pair on NH, the CO π-electron pair and antibonding π*-group-orbitals on C(α) in NHC(HR)C(O)), which should be an important factor in determining peptide chain conformation, do not vary significantly with small deviations from amide group planarity.