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. 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. 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.     

An ab initio study of the geometries and rotational barriers of 1-, 2-, and 5-phenylimidazole

The torsional barrier was calculated in the 3-21G basis set for 1-, 2-, and 5-phenylimidazole. Full geometry optimization was carried out at inter-ring torsional angles of 0°, 30°, 60°, 90°, 120°, 150°, 180°, and additional intermediate angles. All torsional potential energies were found to be symmetric with respect to the 90° conformation. The 2-phenylimidazole torsional energy exhibits a minimum at 0° (and 180°) and a maximum at 90° with a barrier height of 5.83 kcal/mol relative to the 0° conformation. The minima in the 1- and 5-phenylimidazole torsional potential energies correspond to non-planar conformations, resulting in a double-well potential with maxima at 0° (180°) and 90°. The 1-phenylimidazole minima are located at 46.5 and 133.5°; the 5-phenylimidazole minima, at 35.3 and 144.7°. In the 0° (180°) and 90° conformations, 1-phenylimidazole exhibits torsional barriers of 1.84 and 0.75 kcal/mol, respectively, relative to the energy of the 46.5° conformation. For 5-phenylimidazole, these barriers are 0.94 and 1.89 kcal/mol, relative to the energy of the 35.3° conformation. The energy of 5-phenylimidazole in the 35.3° conformation corresponds to a relative tautomeric energy difference of 1.80 kcal/mol compared to the 0° conformer of the 4-phenylimidazole tautomer.     

Conformational analysis of highly extended poly(ethylene terephthalate) chains by Monte Carlo calculations

The conformational disorder compatible with the highly extended chains found in mesomorphic poly(ethylene terephthalate) has been studied by Monte Carlo calculations on model oligomers confined inside cylindrical tubes. The distribution of torsional angles for such extended chains is characterized by O C C O bonds being always in the trans domain, while the C O C C bonds show an approximately similar probability of being found in trans and gauche states, the probability maxima being centered at 90° and −90° in the latter cases. At variance with the torsional angles of the O C C O and the ester bonds, always very close to 180°, the distributions for all other torsional angles show flat and broad probability maxima, indicating the possibility of substantial deviations from the average value inside each domain. This is also true for the fictitious O C˙˙˙C O bonds across the phenylene rings, for which a nearly trans geometry is preferred in extended conformations.     

Computational thermochemistry of glycolaldehyde

We use a variant of the focal point analysis to refine estimates of the relative energies of the four low‐energy torsional conformers of glycolaldehyde. The most stable form is the cis‐cis structure which enjoys a degree of H‐bonding from hydroxyl H to carbonyl O; here dihedral angles τ₁ (O?C? C? O) and τ₂ (C? C? O? H) both are zero. We optimized structures in both CCSD(T)/aug‐cc‐pVDZ and aug‐cc‐pVTZ; the structures agree within 0.01 Å for bond lengths and 1.0 degrees for valence angles, but the larger basis brings the rotational constants closer to experimental values. According to our extrapolation of CCSD(T) energies evaluated in basis sets ranging to aug‐cc‐pVQZ the trans‐trans form (180°, 180°) has a relative energy of 12.6 kJ/mol. The trans‐gauche conformer (160°, ±75°) is situated at 13.9 kJ/mol and the cis‐trans form (0°, 180°) at 18.9 kJ/mol. Values are corrected for zero point vibrational energy by MP2/aug‐cc‐pVTZ frequencies. Modeling the vibrational spectra is best accomplished by MP2/aug‐cc‐pVTZ with anharmonic corrections. We compute the Watsonian parameters that define the theoretical vibrational‐rotational spectra for the four stable conformers, to assist the search for these species in the interstellar medium. Six transition states are located by G4 and CBS‐QB3 methods as well as extrapolation using energies for structures optimized in CCSD(T)/aug‐cc‐pVDZ structures. We use two isodesmic reactions with two well‐established thermochemical computational schemes G4 and CBS‐QB3 to estimate energy enthalpy and Gibbs energy of formation as well as the entropy of the gas phase system. Our extrapolated electronic energies of species appearing in the isodesmic reactions produce independent values of thermodynamic quantities consistent with G4 and CBS‐QB3. © 2013 Wiley Periodicals, Inc.     

Acyl- und Alkylidenphosphane. XXXII [1, 2]. Di-cyclo-hexoyl- und Diadamant-1-oylphosphan — Keto-Enol-Tautomerie und Struktur

Acyl- and Alkylidenephosphines. XXXII. Di-cyclohexoyl- and Diadamant-1-oylphosphine – Keto-Enol Tautomerism and Structure Lithium dihydrogenphosphide · DME (¹) [12] and cyclo-hexoyl or adamant-1-oyl chloride react in a molar ratio of 3:2 to give lithium di-cyclo-hexoylphosphide · DME and the corresponding diadamant-1-oylphosphide.2THF (¹) resp. Treatment of these two compounds with 85% tetrafluoroboric acid. diethylether adduct yields di-cyclo-hexoyl- ( 1b ) and diadamant-1-oylphosphine ( 1c ). In nmr spectroscopic studies 1b over a range of 203 to 343 K, a strong temperature dependence of the keto-enol equilibrium is found; thermodynamic data characteristic for the formation of the enol tautomer (ΔH⁰ = ?4.3 kJ. mol^?1; ΔS⁰ = ?9.2 J. mol^?1. K (^?1) are compared of 1,3-diketones. The enol tautomer of diadamant-1-oylphosphine ( E-1c ) as obtained from a benzene solution in thin colourless plates, crystallizes in the monoclinic space group P2₁/c {a = 722.2(2); b = 1085.5(4); c = 2434.8(5) pm; ß = 96.43(2)° at –100 ± 3°C; Z = 4}. An X- ray structure analysis (R_w = 0.033) shows bond lengths and angles to be almost identical within the enolic system (P? C 179/180; C? O 130/129; C? C(adamant-1-yl) 152/153 pm; C? P? C 99°; P? C? O 124°/124°; P? C? C 120°/120°; C? C? O 116°/116°. The geometry of the very strong, but probably asymmetric O‥H‥O bridge is discussed (O? H 120/130, O‥O 245 pm).     

A Molecular orbital study of the rotation about the CC bond in styrene

The geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C_βC₂C₁C₂ dihedral angle-Φ = 0°(cis), 15°, 30°, 60° and 90° — assuming that the vinyl and phenyl groups remain planar, but otherwise with full geometry optimization. Similar calculations have been carried out for 1,3-butadiene and 3-methylene-1,4-pentadiene (MPD) where rotation about 180° generates a different and not the same conformer. The torsional potential energy curve for styrene has a very flat minimum Φ = 0, i.e. the cis structure is the most stable, whereas butadiene and MPD have minima in the region Φ = 37° to 40°, indicative of more stable gauche structures. For styrene the barrier height Φ = 90° is 131.1 KJ mol^?1. These results provide strong support for the potential function obtained by Hollas and Ridley from single level vibronic fluorescence and other spectroscopic data. The distortion of the benzene ring brought about the vinyl group substitution is discussed, also the variation of the C/C and H/C bond lenghts with Φ and the change in charge on the vinyl group and the polarity of the various bonds in the conversion of the cis into the 90° gauche conformer. The stabilization energy for styrene relative to that for benzene has been evaluated according to various criteria, and, in addition, the energy associated with the distortion of the ring.     

Intermolecular interactions in the chiral and racemic forms of 3‐­hydroxy‐2‐(1‐oxoisoindolin‐2‐yl)­butanoic acid derived from threonine

The title compounds, C₁₂H₁₃NO₄, are derived from l ‐threonine and dl ‐threonine, respectively. Hydro­gen bonding in the chiral derivative, (2S/3R)‐3‐hydroxy‐2‐(1‐oxoisoindolin‐2‐yl)­butanoic acid, consists of O—H_acid?O_alkyl—H?O=C_indole chains [O?O 2.659 (3) and 2.718 (3) Å], Csp³—H?O and three C—H?π_arene interactions. In the (2R,3S/2S,3R) racemate, conventional carboxylic acid hydrogen bonding as cyclical (O—H?O=C)₂ [graph set R₂²(8)] is present, with O_alkyl—H?O=C_indole, Csp³—H?O and C—H?π_arene interactions. The COOH group geometry differs between the two forms, with C—O, C=O, C—C—O and C—C=O bond lengths and angles of 1.322 (3) and 1.193 (3) Å, and 109.7 (2) and 125.4 (3)°, respectively, in the chiral structure, and 1.2961 (17) and 1.2210 (18) Å, and 113.29 (12) and 122.63 (13)°, respectively, in the racemate structure. The O—C=O angles of 124.9 (3) and 124.05 (14)° are similar. The differences arise from the contrasting COOH hydrogen‐bonding environments in the two structures.     

Ab initio studies on polymers. VII. Polyoxymethylene

The structure of an isolated, infinite polyoxymethylene chain has been investigated with the aid of the ab initio crystal orbital method applying a basis set of double-zeta quality. Restricting the primitive unit cell to a single CH₂O group, conformational potential curves as a function of the torsional angle have been evaluated. Only a single minimum closely corresponding to an all-gauche structure was detected. The all-trans conformation is a maximum on the energy curve for simultaneous rotation around C? O single bonds. Detailed geometry optimization in the vicinity of the all-gauche conformation led to the following structure: r_CO = r_OC = 1.425 Å, r_CH = 1.072 Å, ∠HCH = 111.7°, ∠OCO = 110.9°, ∠COC = 115.1°, and τ_OCOC = 70.75°. The computed torsional angle τ_OCOC lies midway between the hexagonal (78.2°) and the orthorhombic (63.5°) modification of solid polyoxymethylene.     

Ab Initio Study of Conformational Properties of Heteroatom-Containing 1,2-Bisketene Analogues

Abstract

Minimum-energy and transition-state geometries of 4-oxobuta-1,3-diene-1-thione, buta-1,3-diene-1,4-dithione, 4-selenoxobuta-1,3-diene-1-thione, 4-selenoxobuta-1,3-diene-1-one, and buta-1,3-diene-1,4-diselenone were calculated using HF, B3LYP, and MP2 levels of theory and 6–31 + G* basis set by rotation around the related ?C?C? single bonds. In all of the above-mentioned molecules, the s-trans conformation was obtained as the most stable conformer with the 180° dihedral angle. In buta-1,3-diene-1,4-dithione, 4-selenoxobuta-1,3-diene-1-thione, and buta-1,3-diene-1,4-diselenone, the s-cis form of these compounds corresponded to the other energy-minimum geometry. Their skew geometries, with torsional angles approximately 100°, were a transition state for conformational interconversion between the two global minima forms. In 4-oxobuta-1,3-diene-1-thione and 4-selenoxobuta-1,3-diene-1-one, geometries with the C?C?C?C dihedral angles about 51 and 43° (respectively) were attributed to the second energy-minimum geometry. Transition-state structures from both molecules were found in the torsional angles at about 0 and 100°.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

GRAPHICAL ABSTRACT     

The shielding of H-α in the stabilized rotameric forms of α,α-di-tert-butylthioacetic esters. The electric and magnetic anisotropic effects of the thione group

The currently accepted geometry of carbonyl magnetic anisotropic effects, if regarded as extendable to the thione group, imply that the more deshielded of the two H-α lines in the ¹H NMR spectra of α,α-di-tert-butylthioacetic esters should be assigned to the ‘180° form’ in which C?S is antiperiplanar to C-α? H. CNDO results, however, indicate the opposite; the line should be assigned to the ‘O° form,’ in which C? S eclipses C-α? H, if the predicted considerable increase in 1s density at H-α on going from the 0° form to the 180° form is taken into account. A change in 1s density at H-α as a result of increased branching of alkylation at C-α is also found. These specific effects must be taken into account when discussing the anisotropic effects of C?O or C?S on H-α shifts.     

Further calculations on the internal rotation spectrum of phenol

The influence of a small deformation of C?O angle in phenol (tilt), into the rotational far-infrared (FIR ) spectrum is analyzed using several approaches. In all of them, the CNDO /2 method is used to determine the potential energy functions. In a first step, the C? O bond and the rotation axis are both supposed to coincide with the C₂ symmetry axis of the phenyl group. With this assumption the torsional frequencies are calculated in both the symmetric and asymmetric rotor approximations. In a second step, the tilt of the C? OH bond is determined theoretically and found to be ?3°, measured from the C₂ symmetry axis, the C? OH bond crossing this axis, Using this second geometry, and taking as the rotation axis the C₂ axis, the torsional frequencies are again determined in both approximations. An improvement of the calculated transition energies is encountered at each stage of the calculation, when compared with experimental data. Finally the importance of the introduction of a tilt into the FIR torsional frequency calculations is discussed.     

Fluoride und Fluorosäuren. IV. Kristallstrukturen von Bortrifluorid und seinen 1:1-Verbindungen mit Wasser und Methanol,Hydroxo- und Methoxotrifluoroborsäure

Fluorides and Fluoro Acids. IV. Crystal Structures of Boron Trifluoride and its 1:1 Compounds with Water and Methanol, Hydroxo- and Methoxotrifluoroboric Acid Solid boron trifluoride displays an enantiotropic phase transition α ? β at ?147°C. A further solid phase, γ-BF₃, is metastable or stable only just below the melting point. Its crystal structure was determined. It is monoclinic with space group P2₁/c, eight molecules in the unit cell and the lattice parameters a = 4.779, b = 14.00, c = 7.430 Å, β = 107.60° at ?131°C. Two independent trigonal planar molecules with a mean B? F bond length of 1.287 Å (1.319 Å after correction for thermal motion) form a three-dimensional packing connection with non-parallel molecular planes across intermolecular B···F contacts of in the average 2.690 Å, by which the boron atoms achieve a total coordination of five fluorine atoms with nearly trigonal bipyramidal geometry. — The crystal structures of hydroxotrifluoroboric acid (BF₃OH₂, monoclinic, P2₁/n, Z = 4, a = 7.641, b = 7.957, c = 4.864 Å, β = 94.80 at ?35°C) and methoxotrifluoroboric acid (BF₃O(CH₃)H, orthorhombic, Pbca, Z = 8, a = 7.054, b = 9.390, c = 11.547 Å at ?40°C) display unlimited three-dimensional and one-dimensional linking, respectively, of the molecules by hydrogen bonds O? H···F.     

PM3 semiempirical study and its comparison with X-ray crystal structure of 2-methyl-4-(4-methoxyphenylazo)phenol

The crystal and molecular structure of 2-methyl-4-(4-methoxyphenylazo)phenol have been determined by X-ray single crystal diffraction technique. The compound crystallizes in the monoclinic space group P2₁/c with a=9.7763(8) Å, b=11.3966(8) Å, c=11.9531(8) Å and β=108.752(6)°. In addition to the molecular geometry from X-ray experiment, its optimized molecular structure has been obtained with the aid of PM3 semiempirical quantum mechanical method, and then the corresponding geometric parameters were compared with those of X-ray crystallography. To determine conformational flexibility and crystal packing effects on the molecules, molecular energy profile of the title compound was obtained with respect to two selected degrees of torsional freedom, which were varied from ?180° to +180° in steps of 10°. Crystal structure of the title compound is a fibroid structure constructed by C–H···O and O–H···N type intermolecular hydrogen bonds. The most favorable conformer of the title compound has been determined by the crystal packing effects and there is no steric hindrance during rotation around the selected torsion angles.     

Liquid-liquid equilibrium data for ternary systems containing organic acid,hydrocarbon and water

Alessi, P., Kikic, I., Fermeglia, M. and Nonino, C., 1984. Liquid-liquid equilibrium data for ternary systems containing organic acid, hydrocarbon and water. Fluid Phase Equilibria, 18: 93–102.Experimental LLE data are presented for the acetic acid-toluene-water system at 60, 70 and 80°C and for the acetic acid-heptane-water, propionic acid-toluene-water, propionic acid-heptane-water and butanoic acid-toluene-water systems at 50, 60 and 70°C. The data are correlated by means of the NRTL and UNIQUAC models and the relevant parameters are given. The UNIFAC method is used to predict the behavior of the systems.     

Microwave spectra of isotopic glycolaldehydes,substitution structure,intramolecular hydrogen bond and dipole moment

The microwave spectra of ¹³CH₂OH-CHO, CH₂OH-¹³CHO, and CH₂OH-CH¹⁸O are reported and have been used in combination with previously published data on other monosubstituted glycolaldehydes to determine the substitution structure of the molecule as r(CO) = 1.209 Å, r(C-O) = 1.437 Å, r(C-C) = 1.499 Å, r(O-H) = 1.051 Å, r(C-H_ald) = 1.102 Å, r(C-H_alc) = 1.093 Å, r(O β H) = 2.007 Å, r(O β O) = 2.697 Å, ∠(C-CO) = 122°44', ∠(C-C-H_ald) = 115°16', ∠(C-C-O) = 111°28', ∠(C-O-H) = 101°34', ∠(C-C-H_alc) = 109°13', ∠(H-C-H) = 107°34', ∠(O-H β O) = 120°33', ∠(H β OC) = 83°41', and ∠(O-H, C0) = 24°14'. The intramolecular hydrogen bond and the other structural parameters are discussed and compared to related molecules. The dipole moment is redetermined to be μ_a = 0.262 ±0.002 D, μ_b = 2.33 ± 0.01 D, and μ_tot = 2.34 ± 0.01 D. Relative intensity measurements yielded 195 ± 30 cm^?1 for the C-C torsional fundamental and 260±40 cm^?1 for the lowest in-plane skeletal bending mode. Computations performed by the CNDO/2 method correctly predict the observed cis hydrogen-bonded conformer to be the energetically favoured one and in addition yield some indication of the existence of at least two other non-hydrogen-bonded forms of higher energy.     

Phase equilibria for liquid mixtures of (benzonitrile + a carboxylic acid + water) atT = 298.15 K

(Liquid  +  liquid) equilibrium data are presented for mixtures of {benzonitrile(1)  +  acetic acid or propanoic acid or butanoic acid or 2-methylpropanoic acid or pentanoic acid or 3-methylbutanoic acid(2)  +  water(3)} atT =  298.15 K. The relative mutual solubility of each of the carboxylic acids is higher in the benzonitrile layer than in the aqueous layer. The influence of 3-methylbutanoic acid, pentanoic acid, 2-methylpropanoic acid, and butanoic acid on the solubility of the hydrocarbons in benzonitrile is greater than that of the acetic and propanoic acids. Three three-parameter equations have been fitted to the binodal curve data. These equations are compared and discussed in terms of statistical consistency. The NRTL and UNIQUAC models were used to correlate the experimental tie lines and to calculate the phase compositions of the ternary systems. The NRTL equation fitted the experimental data far better than the UNIQUAC equation. Selectivity values for solvent separation efficiency were derived from the tie line data.     

A comparison of selected force constants derived from ab initioSCF calculations on β-hydroxyacrolein. Acrolein,performic acid,and formic acid

Harmonic force fields have been calculated for the planar hydrogen-bonded ring conformer of β-hydroxyacrolein, cCc, which is the most stable, and the chain conformer, cCt, generated by 180° rotation of O? H about the C? O bond axis. The equilibrium structure obtained using the 4-31 G basis set with full geometry optimization was employed in each case. Selected force constants for the bonds directly concerned in the formation of the ring from the chain structure, and the increments in going from the one to the other, are compared with the values for the corresponding conformers of performic and formic acids. As the ring size increases from four in trans-formic acid, to five in cis–cis-performic acid and to six in the cCc conformer of β-hydroxyacrolein there is a successive increase in the mechanical strength of the hydrogen-bridging unit. The energy changes for the chain → ring conversion do not follow this progression: performic acid is out of order. But, since a force constant is a localized bond property, whereas the energy changes are determined not only by interactions specific to the hydrogendonor and hydrogen-acceptor groups but also by interactions involving more distant parts of the molecule, the force constants for the bonds directly concerned in the formation of the hydrogen bridge provide a less ambiguous basis for comparing the strength of the intramolecular hydrogen bonding.     

Phase equilibria for liquid mixtures of (butanenitrile + a carboxylic acid + water) at 298.15 K

Liquid–liquid equilibrium data are presented for mixtures of (butanenitrile (1)+acetic acid or propanoic acid or butanoic acid or 2-methylpropanoic acid or pentanoic acid or 3-methylbutanoic acid (2)+water (3)) at 298.15 K. The relative mutual solubility of all the carboxylic acids is higher in the butanenitrile layer than in the aqueous layer. The influence of acetic acid and propanoic acid on the solubility of water in butanenitrile is greater than that of the other acids. Three parameter equations have been fitted to the binodal curve data. These equations are compared and discussed in terms of statistical consistency. Selectivity values for solvent separation efficiency were derived from the tie-line data. The NRTL and UNIQUAC models were used to correlate the experimental results and to calculate the phase compositions of the ternary systems. The NRTL equation fitted the experimental data far better than the UNIQUAC equation.