Ab initio studies of structural features not easily amenable to experiment. 42. Molecular geometries and conformational analysis of methylbutanoate

Conformational energy profiles were calculated for τ₁, the C? C? C?O torsion, and τ₂, the C? C? C? C torsion, of methyl butanoate, using Pulay's ab initio gradient procedure at the 4-21G level with geometry optimization at each point. In addition, the structures of seven conformations were fully relaxed, including the energy minima (τ₁, τ₂) = (0, ?60), (0, 180), (120, 180), (120, ?60), and the maxima (0, 0), (180, 180), and (60, ?60). The calculated geometries confirm the previously formulated rule that, in saturated hydrocarbons, a C? H bond trans to a C? C bond (C? H_s) is consistently shorter than a C? H bond (C? H_a) trans to another C? H bond. Specifically, for X? C(α) (? O)? C(β)? C(γ)? C(δ) systems, the following rules can be formulated, incorporating results from previous studies of butanal, butanoic acid, and 2-pentanone: (1) C(δ)? H_s < C(δ)? H_a in all the conformers in which the δ-methyl group is remote from the ester group; whereas, in all the conformers in which nonbonded interactions are possible between the C(δ)-methyl and the ester groups, the bonding pattern is affected by a C? H ?O?C interaction. (2) In the most stable conformers, (0, 60), C(β)? H_a < C(β)? H_s, and C(γ)? H_a < C(γ)? H_s, regardless of X. (3) The average C? C bonds in the τ₂ = 180° conformers are consistently shorter than those with τ₂ = 60° (compared at τ₁ constant). In the most stable conformations (τ₁ = 0°, τ₂ = 60° or 180°), the bonding sequence is consistently C(α)? C(β) < C(β)? C(γ) < C(γ)? C(δ); whereas, when τ₁ = 120°, C(α)? C(β) < C(β)? C(γ) > C(γ)? C(δ).     

Ab initio studies of structural features not easily amenable to experiment: Part V. Conformational analysis of keto- and enol-acetone

The structures of several conformations of keto- and enol-acetone were determined by unconstrained ab initio geometry refinements using the 4-21G basis set. The geometry of propene was also refined to compare it with enol-acetone. The structural consequences of hyperconjugation for the local geometries of the methyl groups were determined in all conformations. In the most stable form of keto-acetone, one hydrogen atom of each methyl group was found in an eclipsed arrangement with respect to the carbonyl group. The stability of this crowded structure has previously been rationalized in terms of aromatic π-electron delocalization. This result is in contrast to one of two previous gas electron diffraction studies. It is concluded that the electron diffraction data may not contain enough information to determine the exact conformational arrangement of the methyl groups in acetone. The calculated structures are found to be in excellent agreement with experiment. Uncertainties in calculated bond distances and bond angles are on the order of magnitude of 0.01–0.02 Å and 1–2°, respectively.     

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

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 Pulay's 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 40. Structural and conformational investigations of 2-butanone and 2-pentanone

The ab initio gradient refined 4-21G geometries of two conformations of 2-butanone and of six conformations of 2-pentanone are reported. The C---C---C=O torsional energies of both systems were determined with geometry optimization at each point and are compared with those previously calculated for some homologous aldehydes and carboxylic acids. In agreement with the structural trends known for C---H bonds in methyl groups adjacent to C=O, it is found that a C---C bond eclipsing an adjacent C=O bond is more stable and shorter than in a skew position (C---C---C= O = 120°). The sum total of the 4-21G results available for various systems may support the following general rule: in X--- C---C=O systems the C---X bond is relatively short when syn-coplanar with C==O (X---C---C= O = 0°), and relatively long when skew with C=O (X---C---C=O 120°).     

Ab initio studies of structural features not easily amenable to experiment: Part 16. Some characteristic structural aspects of non-cyclic hydrocarbons

The molecular structures of n-propane, n-butane, iso-butane and n-pentane have been determined by geometrically unconstrained ab initio force relaxations on the 4—21G level. Subtle but consistent trends suggest that the local symmetry of methyl groups in the stretched form of hydrocarbons is twofold rather than threefold due to differences in the planar and non-planar CH bonds and in the HCH angles. CH bond distances in the methylene groups are slightly longer than in methyl and increase with the number of adjacent CC bonds that are staggering the CH₂-group. Inner CC bonds in n-butane and n-pentane are slightly longer than those at the end of a chain. Even though most of these features have not yet been observed experimentally they can be expected to describe reasonable trends for the systems studied.     

Ab initio studies of structural features not easily amenable to experiment : Part 24. Molecular structures and conformational analyses of the methyl esters of formic acid, acetic acid and alanine

The molecular structures of four conformations of methyl formate, of two conformations, each, of methyl acetate and methyl alanate have been determined by ab initio force relaxation on the 4-21G level. Relative stabilities and some selected structural aspects are discussed.     

Ab initio studies of structural features not easily amenable to experiment. 18. Conformational analysis and molecular structure of glycine methyl ester

The structures of four conformations of the methyl ester of glycine were determined by standard single-determinant molecular orbital (MO ) calculations using Pulay's force method and the 4-21G basis set. The most stable conformation of this compound has a symmetry plane which contains all the heavy atoms; it is stabilized by hydrogen bonds between the NH₂ group and the carbonyl oxygen; it corresponds to the most stable, stretched form of free glycine. The structural parameters in the different conformations can vary significantly (bond distance by more than 0.02 Å and bond angles by up to 15°). The structural changes which are caused in glycine by esterification are discussed and some of them are interpreted in terms of hyperconjugative π-electron delocalization.     

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

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 CH₃ groups adjacent to OH, OCH₃, NH, NCH₃ 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 14. Complete ab initio equilibrium structures of some conformations of dimethoxymethane

The molecular structures of five conformations of dimethoxymethane (CH₃OCH₂-OCH₃) 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. 38. Structural and conformational investigations of propanoic, 2-methylpropanoic,and butanoic acid

The structures and conformational energies of several conformations of propanoic acid, 2-methylpropanoic acid, and butanoic acid were determined by geometrically unconstrained ab initio gradient geometry refinement on the 4-21G level. The O?C? C? C torsional potentials of propanoic acid and butanoic acid are found to be practically identical. There are energy minima at 0° and 120°, and maxima in the 60° region and at 180°. In 2-methylpropanoic acid there are energy minima at H? C? C?O dihedral angles of 0° and 120°, and maxima at 60° and 180°. The exact positions of the maxima and minima of the H? C? C?O torsional potential of 2-methylpropanoic acid are found to be predictable from propanoic acid rotational-potential parameters. Some conformationally dependent, local geometry trends are discussed.     

Ab initio studies of structural features not easily amenable to experiment: the molecular structures of some hydrogenfluoride clusters

The completely relaxed ab initio structures of some forms of H₂F₂ and H₃F₃ reflect the cooperative nature of hydrogen bonding and can be used to estimate the order of magnitude of the variations in local geometry which are neglected when interactive potentials for HF in clusters or in the liquid state are evaluated with constrained geometries.     

Ab initio studies of structural features not easily amenable to experiment: Part 15. The influence of solvation on molecular structure in some formic acid and carbonic acid hydrates

The completely relaxed ab initio geometries (4—21G) of a trihydrate of carbonic acid and of the monohydrates of cis and trans formic acid are compared with the corresponding unhydrated structures. The maximum structural changes caused by hydration in the free acid structures are of the order of magnitude of 0.03 Å and 3° for bond distances and bond angles, respectively. The corresponding changes in free and complexed water are 0.005 Å and 5°, respectively. The results are significant for the general problem of the transferability of gas phase molecular structures to molecules in solution and for estimates of the uncertainties in theoretical hydration energy surfaces which are generated by using fixed, monomer geometries for water and solvate molecules. Compared with the free geometries, the sum total of the structural changes in some of the systems studied corresponds to energies of several kcal mol^?1.     

Ab initio studies of structural features not easily amenable to experiment. 30. Conformational analysis and molecular structures of propanal and butanal

The geometries of several conformations of propanal and butanal have been refined by geometrically unconstrained ab initio gradient relaxation on the 4-21G level. Both compounds possess energy minima at O? C? C? C torsional angles of 0° and in the 120° region, and energy maxima in the 70° region and at 180°. The structure of the aldehyde functional group is found to be relatively invariant both when different systems or when different conformations of the same system are compared. Conformationally dependent geometrical trends in propanal and butanal are discussed and found to be subtle yet noticeable.     

Ab initio studies of structural features not easily amenable to experiment: The molecular structures of two low-energy forms of unionized serine

The structures of two unionized conformations of serine were refined without geometrical constraints using standard single determinant ab initio procedures on the 4-21G level. The results represent the best now available estimate for the unobserved equilibrium geometry of this system.     

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

The geometries of three conformations of FCH₂OH and four conformations each of NH₂CH₂NH₂ and NH₂CH₂OH 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. 25. Conformational analysis of methyl propanoate and comparison with the methyl ester of glycine

The molecular geometries of three conformations of methyl propanoate (MEP) (C? C? C?O torsions of 0°, 120°, and 180°) and the potential-energy surfaces of MEP (C? C? C?O torsions) and of the methyl ester of glycine (MEG) (N? C? C?O torsions) have been determined by ab initio gradient calculations at the 4-21G level. MEP has conformational energy minima at 0° and 120° of the C? C? C?O torsion, while the 60–90° range and 180° are energy maxima. For MEG there are two minima (at 0° and 180°) and one barrier to N? C? C?O rotation in the 60–90° range. The N? C? C?O barrier height is about twice as high (4 kcal/mol) as the C? C? C?O barrier. The 180° N? C? C?O minimum is characteristically wide and flat allowing for considerable flexibility of the N? C? C?O torsion in the 150–210° range. This flexibility could be of potential importance for polypeptide systems, since the N? C? C?O angles of helical forms are usually found in this region. The molecular structures of the methyl ester group CH₃OC(?O)CHRR′ in several systems are compared and found to be rather constant when R ? H and R′ ? H, CH₃, CH₃CH₂; or when R ? NH₂ and R′ ? H, CH₃, or CH(CH₃)₂.     

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

The structural changes, which occur in the amide unit when the NH₂-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 Å longer than in the planar form; the CO bond distance is about 0.01 Å shorter; NH distances are about 0.01 Å 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 CH₃-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.     

Ab initio studies of structural features not easily amenable to experiment. 32. Conformational analysis and molecular structures of isopropyl and ethyl formate and comparison with spectroscopic data

The geometries of several conformations of ethyl and isopropyl formate were optimized by the ab initio gradient method on the 4-21G level. The calculations are in agreemnt with the existence of two conformers of ethyl formate of nearly equal energy. The COCC torsional angle in one is anti (180°) and in the other is gauche (about 80°). The equilibrium configuration of the isopropyl group in the formate is found to be unsymmetrical, with a COCH torsional angle of about 40°. A second minimum of torsional energy, at COCH = 180°, is 1.2 kcal/mol less stable than the unsymmetrical form. The calculations demonstrate the tranferability of internal rotational-potential parameters and of conformationally dependent geometrical trends between ethyl and isopropyl formate. There is good agreement between the calculated results and empirical potential-energy functions and rotational constants determined from microwave spectroscopy.