Stereoelectronic Aspects of the Anomeric Effect in Fluoromethylamine

High-level ab initio calculations have been made for fluoromethylamine to study structural and energetic effects of the relative orientation of the N lone pair to the C? F bond. The anti-conformer (N lone pair anti-planar to the C? F bond) corresponds to the global energy minimum. It has the longest C? F distance, the shortest C? N distance, and is 7.5 kcal·mol^?1 more stable than the related perpendicular conformation (lone pair perpendicular to the C? F bond). The syn-conformation also shows hallmarks of the anomeric effect: long C? F bond, short C? N bond, and energetic stability when allowance is made for the two pairs of eclipsed hydrogens. The transition state for N inversion is close to the syn-structure; rotation about the C? N bond is strongly coupled with this inversion process. Small bond distance changes of ca. 0.02 Å between parallel and perpendicular conformations are associated with dissociation energy differences of ca. 30 kcal·mol^?1. Various criteria for assessing the strength of the anomeric effect are discussed.     

The effect of alkyl substitution on the conformation of intramolecularly hydrogen bonded β-γ (enolic) unsaturated alcohols

The equilibrium between associated (OH ⋯ π) and free OH conformations of β-γ unsaturated alcohols has been found to be highly dependent upon the position of alkyl substitution. Two types of intramolecularly hydrogen bonded conformations can be identified which are separated by a difference in OH stretching frequency of approximately 10 cm⁻¹. The occurence of either conformation is dependent upon methyl substitution at the β carbon and the conformations are mutually exclusive. The intensity ratio of the free and intramolecularly hydrogen bonded OH stretching bands are dependent upon the alkyl chain length and also the primary or secondary nature of the alcohol. In both cases, changes in relative intensity result from steric interactions. Increased alkyl chain length decreases the relative intensity of the intramolecularly hydrogen bonded band, whilst a change from primary to secondary alcohol increases the relative intensity of the intramolecularly hydrogen bonded band.     

Ab initio study of the internal rotation in peroxyformic acid

The internal rotation in peroxyformic acid was investigated using the ab initio SCF MO LCAO method with the STO-3G and 4-31G basis-sets and with experimental and optimum values of the geometrical parameters. Both basis sets yield a rather flat double-minimum potential for the COOH torsion, the trans planar form being lightly preferred in comparison with the cis one. The effects on the internal rotation of the geometry relaxation, of the intramolecular hydrogen bond and of the methyl group of peroxyacetic acid are also discussed.     

Contributions of resonance,hybridization, and nonbonded interactions to the structure of butadiene

The fully optimized geometries of four rotational conformers of butadiene have been calculated ab initio in order to investigate the relative importance of resonance, hydridization and nonbonded interactions. The results indicate that the short central CC bond is primarily due to hybridization, although resonance makes a minor contribution. The resonance stabilization of the planar conformers is responsible for the relatively large barrier to internal rotation, whereas the nonbonded interactions are only important in destabilizing the sterically crowded cis conformer.     

Crystal Structure and Quantum Chemical Ab Initio Calculations of Ibotenic Acid,an Excitatory Amino Acid Receptor Agonist

Ibotenic acid, a constituent of Amanita muscaria, is a potent NMDA receptor agonist. The structure of the ibotenic acid zwitterion monohydrate in the crystalline state has been determined by X-ray crystallography. The crystal structure has two ibotenic acid monohydrate formula units in the asymmetric unit and features an extensive hydrogen bond network. The ibotenic acid zwitter-ions in the crystalline state are compared to the in vacuo structure calculated by quantum chemical ab initio calculations at the HF/6-31+G* level and the effects of the hydrogen bond network on the structures in the solid state are discussed. The calculated potential energy curve with respect to side-chain orientation displays a single energy minimum. The conformations corresponding to the solid-state conformations are calculated to be ca. 2 kcal/mol higher in energy than the minimum-energy conformation in vacuo.     

Ab initio and semiempirical conformational and configurational analysis of N-2-(1,4-dioxane)-N’-(4-methylbenzenesulfonyl)-O-(4-methylphenoxy)isourea

The conformational, configurational behavior and the structure of N-2-(1,4-dioxane)-N’-(4-methylbenzenesulfonyl)-O-(4-methylphenoxy) isourea 1 has been studied using ab initio and semiempirical calculations (AM1 and PM3). The endo-anomeric effect and hydrogen bonds control the population of dioxane ring conformers or anomers but not the configuration interconversion of the imine of imidoyl moiety. Ab initio and AM1 and PM3 calculations demonstrate that the dioxane ring adopts a chair conformation, that the imidoyl amino group prefers an axial conformation and that the tosyl and tolyl groups about the C=N bond retain an E configuration. The computational analysis of 1 complements the X-ray findings.     

An examination of some effects of OH rotation in phenol and p-nitrophenol

A computational analysis of phenol and p-nitrophenol has been carried out with the objective of determining the extent to which the conjugation between the -OH group and the aromatic ring is affected by a 90° rotation of the former. The ab initio SCF-MO GAUSSIAN 82 procedure was used to compute optimized geometries, electronic densities and electrostatic potentials. The effect of rotation upon the structures and distributions of π electronic charge are rather small. However, the electrostatic potential reveals that as a consequence of rotation around the COH bond, there should be an overall preference for the ortho positions as the initial sites for electrophilic attack. The -NO₂ group deactivates the aromatic ring toward electrophiles and increases the positive character of the hydroxyl hydrogen.     

Molecular orbital studies of conformers and the barrier to internal rotation in 1,3-butadiene

The potential curve for rotation around the central bond in 1,3-butadiene has been estimated by ab initio calculations using Gaussian-type basis functions. The calculations, which also include limited geometry variation during rotation, suggest that in the SCF approximation the second stable form of the molecule is a gauche conformation rather than a cis. The predicted energy difference between the planar trans ground state and the stable gauche form is 2.7 kcal/mole and the barrier to internal rotation is found to be 6.0 kcal/mole using a (9,5) basis for carbon and 4s functions on hydrogen.     

Conformational preferences of the axial ligands in some metalloporphyrins. A theoretical study

The conformational preferences of the axial ligands have been determined for several metalloporphyrins MPL and MPLL′ (M = Mo, Fe; P = porphine dianion; L and L′ being the axial ligands). For MoP(C₂H₂) a qualitative analysis indicates that the conformation with the acetylenic bond eclipsing two Mo-N bonds will be favored. Ab initio SCF calculations indicate that:

1. iron porphyrins with an axial imidazole ligand show a flat potential energy curve for the rotation of the imidazole ligand;
2. iron porphyrins with a dioxygen ligand prefer the staggered conformation with the O-O bond projecting along the bisectors of the Fe-N bonds;
3. in the cis-dinitrosyl molybdenum porphyrin, the nitrosyl ligands should be eclipsed with respect to the Mo-N_pyr bonds.

These theoretical predictions are compared with the experimental structures from the literature.     

6β‐Hydroxy‐5β‐methyl‐20‐oxo‐19‐norpregn‐9(10)‐en‐3β‐yl acetate

In the title compound, C₂₃H₃₄O₄, which is an intermediate in the synthesis of pregnane derivatives with a modified skeleton that show potent abortion‐inducing activity, the conformation of ring B is close to half‐chair due to the presence of both the C=C double bond and the axial 5β‐methyl group. Rings A and C have conformations close to chair, while ring D has a twisted conformation around the bridgehead C—C bond. Molecules are hydrogen bonded via the hydroxyl and acetoxy groups into infinite chains. Quantum‐mechanical ab initio Roothan Hartree–Fock calculations show that crystal packing might be responsible for the low values of the angles between rings A and B, and between ring A and rings C and D, as well as for a different steric position of the methyl ketone side chain compared to the geometry of the free molecule.     

An ab initio study of the relationship between the O?H bond length and the harmonic and anharmonic stretching force constants for planar molecules containing C?OH,N?OH,and O?OH groups

The O? H bond length and the quadratic, cubic, and quartic stretching force constants, calculated ab initio using the unscaled 4-31G basis set with full geometry optimization, are reported for 30 planar conformers of ten molecules contaning either the C? OH, N? OH, or O? OH group. The data are analyzed in terms of the general form of Clark's equation, and the power functions and exponential functions proposed by Herschbach and Laurie. In the case of the quadratic constants, significant trends are found in the values of the parameters depending on whether the O? H group is bonded to carbon, nitrogen, or oxygen, and whether it is non-hydrogen-bonded or involved in intramolecular hydrogen bond formation in four-, five-, or six-membered rings. Using data for diatomic molecules, O? H, and C? H bonds, and the C?O and C? C bonds in planar monosubstituted carbonyl compounds, the parameter d_ij in the power function equation for quadratic constants, which can be regarded as the distance of closest approach of the two nuclei, is shown to increase progressively along the series (i) diatomic molecule; (ii) similar bond in a polyatomic environment with one of the two atoms covalently bonded to a neighboring atom; (iii) as in (ii) but with the second atom hydrogen bonded; and (iv) with both atoms covalently bonded to neighboring atoms.     

Conformational analysis—CXVII : Methyl ethyl disulfide. Molecular mechanics and molecular orbital calculations

The conformation of methyl ethyl disulfide was investigated by molecular mechanics calculations using a recently developed force field for sulfur-containing alkanes. The results indicate that in the gas phase the molecule exists predominantly in two conformations, both with the CSSC dihedral angle gauche (84°), and the SSCC dihedral angle either gauche (72°) or trans (179°), and the methyl protons staggered. Ab initio molecular orbital calculations using an STO-3G basis set were employed to corroborate that these two conformations are of roughly equal stability, and that the next most stable conformation (by 0.6 kcal/mole) has the SSCC dihedral angle gauche (295°) with the terminal methyls proximal. In contrast to earlier CNDO/2 (spd) predictions, the SSCC cis conformer is the least stable, and no sizable attractive S?HC nonbonded interactions are discerned. Reasons for this are traced to a failure of the CNDO/2 method, which is especially serious when d orbitals are included in the basis set (spd) and the rigid rotor approximation is used. The present results are found to be consistent with recent electron diffraction, IR, Raman spectroscopic and X-ray diffraction data. The conformation of diethyl disulfide was also investigated by molecular mechanics calculations, and again gauche and trans SSCC arrangements are predicted to be preferred.     

Theoretical study of “protonated pyruvate”: A methylhydroxycarbene—carbon dioxide complex—implications for the decarboxylation of pyruvic acid

Two conformers of protonated pyruvate, CH₃C⁺(OH)COO, with the OH group either trans or cis to the methyl group and the carboxylate group in the C? C? C plane have been studied using the ab initio SCF/3-21G method, as well as by some semiempirical AM1 calculations. Both ab initio SCF and AM1 curves for the potential energy as a function of the C? COO distance exhibit a minimum corresponding to a complex of methylhydroxycarbene, CH₃COH, associated with carbon dioxide, but only the AM1 curves predict an inner minimum corresponding to a covalently bonded protonated pyruvate molecule with a C? COO distance of 1.6–1.7 Å. The two models also disagree on the dissociation pathway for pyruvic acid, with the AM1 calculations predicting formation of acetyl and HOCO radicals while the ab initio method predicts dissociation into methylhydroxycarbene and carbon dioxide following an initial intramolecular proton transfer. The weakly bound complexes of methylhydroxycarbene and carbon dioxide have been studied in some detail using ab initio SCF and MP2 methods in conjunction with 6-311G** basis sets, obtaining equilibrium geometries and vibrational frequencies. In addition, the lactone-type isomer of protonated pyruvate, which contains a C? C? O ring, was also studied. The conclusions of these calculations are consistent with those from earlier work using the smaller 3-21G basis set. The most stable complex is predicted to occur between trans-methylhydroxycarbene and carbon dioxide where substantial stabilization is provided by an OH ? OC hydrogen bond. © 1993 John Wiley & Sons, Inc.     

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.     

C-F?HO hydrogen bond in 8-fluoro-4-methyl-1-naphthol

The 8-fluoro-4-methyl-1-naphthol forms the intermolecular bifurcated HB, OH?(F, HO) in the crystal. This is quite contrasting to the reported results of the ab initio calculations. On the contrary, the intramolecular hydrogen bond in solution was confirmed by ¹H, ¹³C, and ¹⁹F NMR spectra.     

The rotational barrier in dimethyl acetylene and related molecules

The rotational barriers in dimethyl acetylene (CH₃CCCH₃), diamino acetylene (H₂NCCNH₂), dihydroxy acetylene (HOCCOH), methyl trifluoro methyl acetylene (CF₃CCCH₃), silyl methyl acetylene (SiH₃CCH₃), propene, cis and trans 2-butene and ethane were studied by ab initio molecular orbital methods using various basis sets. The eclipsed structure for dimethyl acetylene and its CF₃ and SiH₃ analogs was found to be the most stable, as has been inferred from experimental work, and the barrier height for these compounds has been predicted. The barriers in the OH and NH₂ substituted acetylenes, propene, butene and ethane were studied in order to more clearly understand the important influences in determining the barrier mechanism; specifically, the delocalized molecular orbital and Pauling VB model have been compared.     

The molecular structure of p-bis(trimethylsilyl)-benzene from gas phase electron diffraction

Nearly regular tetrahedral silicon bond configuration and a considerably distorted ring characterize the p-bis(trimethylsilyl)benzene molecular geometry according to an electron diffraction study. The SiC_methyl bond is longer than the SiC_phenyl bond, in agreement with expectation but contrary to an X-ray diffraction determination. The extent of ring deformation is consistent with the electropositive character of the trimethylsilyl substituent and with the structural variations in other para-disubstituted benzene derivatives. The electron diffraction data are consistent with either free rotation around the SiC_phenyl bonds or with a rotamer deviating by about 15° from the eclipsed form. The following bond lengths (r_g, pm) and bond angles (°) have been determined with parenthesized estimated total errors: (CC)_mean 140.8(3), (C_ipso)(C_orthoC_meta) 1.6(7), (SiC)_mean 188.0(4), (SiC_methyl)(SiC_phenyl) 3.3(7), (CH)_methyl 111.3(3), CC_ipsoC 115.7(6), and C_phenylSiC_methyl 109.2(4).     

Vapour-liquid equilibria. X. The ternary system cyclohexane-methanol-acetone at 293.15 and 303.15 K

Total vapour pressure measurements made by the modified static method for the ternary system cyclohexanemethanolacetone and all the constituent binary systems at 293.15 and 303.15 K are presented. The alcohol high-dilution region of the cyclohexanemethanol system has been thoroughly studied. Different exprerssions for G^E suitable for correlation of these data are tested. The prediction of ternary VLE from the constituent binaries is studied. The accuracy of the prediction of H^E from two (P, x) isotherms is studied for the binary systems. The possibility of predicting the ternary H^E from VLE isotherms is also. Our results are compared with literature data.     

Vibrational and conformational analysis of n-propylamine by means of i.r. spectroscopy and ab initio MO calculations

The gas-phase i.r. absorption spectra of normal and amino-deuterated n-propylamine were observed. Most of the observed bands were assigned with the help of ab initio MO calculations for the normal frequencies. The ab initio force constants were scaled to fit the observed spectrum by a least squares method. The existence of five rotational isomers is suggested from an analysis of the NH₂ wagging and torsion bands. The gauche-conformers about the CN axis are found to occupy about 70 % of all n-propylamine molecules, and the gauche-conformers about the CC axis are found to be more abundant than the trans-conformers.     

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.