Refined ab initio 6-31G split-valence basis set optimization of the molecular structures of biphenyl in twisted,planar, and perpendicular conformations

Improved full ab initio optimizations of the molecular structure of biphenyl in twisted minimum energy, coplanar, and perpendicular conformations by use of Poles's GAUSSIAN 82 program have been performed in the 6-31G basis set. These lead to geometries and energies of much higher reliability than our earlier STO-3G results. The torsional angle Φ_min obtained now is 45.41° in close agreement with the recent experimental value of 44.4° ± 1.2°. Calculated CC distances may be converted to experimental ED r_g-values by means of independently determined linear regression correlations with very high statistical confidence, although they agree better with experimental x ray data for coplanar biphenyl without this correction. Calculated intramolecular angles are very similar for both STO-3G and 6-31G basis sets. The calculated torsional energy barrier towards Φ = 90° (ΔE⁹⁰) is 6.76 kJ/mol in close agreement with the experimental-31G value of 6.5 ± 2.0 kJ/mol. For coplanar biphenyl with D_2h-symmetry the calculated torsional energy barrier ΔE⁰ is 13.26 kJ/mol which is surprisingly much higher than the experimental value of 6.0 ± 2.1 kJ/mol. This discrepancy could not be resolved by optimizations assumed for two kinds of distortions of planarity of orthohydrogens from the molecular plane of the coplanar carbon atoms. But for the twisted minimum energy conformation asymmetric bending of ortho-H atoms lead to a torsional angle Φ_min = 44.74° together with a dihedral angle towards ortho-H of 1.22°, and consequently even to an increase of torsional energy barriers to ΔE⁰ = 13.51 and ΔE⁹⁰ = 6.91 kJ/mol.     

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.     

Methyl 4‐O‐β‐l‐fucopyranosyl α‐­d‐­glucopyranoside hemihydrate

The crystal structure of methyl 4‐O‐β‐l ‐fuco­pyran­osyl α‐d ‐gluco­pyran­oside hemihydrate C₁₃H₂₄O₁₀·0.5H₂O is organized in sheets with antiparallel strands, where hydro­phobic interaction accounts for partial stabilization. Infinite hydrogen‐bonding networks are observed within each layer as well as between layers; some of these hydrogen bonds are mediated by water mol­ecules. The conformation of the disaccharide is described by the glycosidic torsion angles: ?_H = ?6.1° and ψ_H = 34.3°. The global energy minimum conformation as calculated by molecular mechanics in vacuo has ?_H = ?58° and ψ_H = ?20°. Thus, quite substantial changes are observed between the in vacuo structure and the crystal structure with its infinite hydrogen‐bonding networks.     

Conformational Flexibility of the Methoxyphenyl Group Studied by Statistical Analysis of Crystal Structure Data

In a large sample of observed methoxyphenyl groups, the twist angle τ about the MeO-C_Ph bond measuring internal rotation of the MeO group shows a continuous distribution with maxima at (0°) (coplanar conformation) and (～90°) (perpendicular conformation). The preferred conformation of methoxyphenyl depends on the nature of the ortho--substituents: In general, it is coplanar in the case of one or two ortho-hydrogens, and perpendicular in the case of two substituents. The internal rotation of the MeO group is accompained by systematic variations in bond angles and bond distances: 1 if MeO is twisted out of plane, the bond angle CH₃? O? C_Ph decreases from 117.7°, until it reaches a minimum of 114.9° at τ = ±90°. The O? C? C angle which is syn to CH₃ for τ = 0° decreases from 124.6° to a minimum of 115.4° at τ = ±180°. These angles changes keep the nonbonded distance CH₃ …? ortho substituent maximal during internal rotation of MeO and tend to minimize the corresponding strain energy. (2) In the perpendicular conformation, the O-atom is ～ 0. 06 Å displaced from the Ph plane, O and CH₃ and being on opposite sides of this plane. In addition, small but systematic increases of bond lengths MeO? C_Ph and CH₃? O are observed. These variations indicate a decrease in conjugation with increasing twist angle. Their interdependence during twisting and the magnitudes of the changes are close values obtained by ab initio calculations.     

ACS symposium

The SCF potential surface of the ground state for NO₂ was calculated by using program JAMOL 3. The McLean–Loew–Berkowitz CGTO 's were used as basis functions. One of the two N? O distance R is fixed to 2.25 a.u. and the other one r and the ONO angle θ are varied from 2.25 to 5.0 a.u. and from 0° to 180°, respectively. The potential surface has the minimum around r = 2.50 a.u. and θ = 120°, where the energy is found to be ?203.954 a.u.     

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. 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.     

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.     

Die Struktur des Spiro-bis(ethylendithia)silans

Contributions to the Chemistry of Silicon Sulfur Compounds. XXXII. The Structure of Spiro-bis (ethylendithia) silane The reaction between sodium methylene dimercaptide with SiCl₄ yielded spiro-bis (ethylendithia)silane. The colorless title compound crystallizes monoclinically with Z = 4 molecules per elementary cell (space group C2/c; a = 1058.4, b = 950.7, c = 1011.9 pm, β = 120.07°; 902 observed unique reflections, R = 0.026). The 4, 4-spiro system nearly is of 222-D₂ symmetry. The twisted five-membered rings as well as the deformation of the spiro angle (ε = 74.4°) result in the deviation from mm2-C₂ symmetry. The mean bond length [d(Si? S) = 211.6pm] was found to be very short. Comparative MNDO/1-calculations of the molecule and for some ortho-acids give no energy minimum for the observed configuration (ε = 74.4°) but show a reduction of the torsional potential to energies comparable to van der Waals interactions, which are caused at the spiro silicon atom by the twisted 5-membered rings.     

Conformational and solid-state studies of diphenyl 1-hydroxy-1-phenylethylphosphonate

The crystal structure of the title compound, diphenyl 1-hydroxy-1-phenylethylphosphonate ( 1 ), was determined by the single-crystal X-ray diffraction method. The crystallographic data for 1 are as follow: C₂₀H₁₉O₄P, M_r = 354.34, monoclinic, P2₁/n, a = 9.787(1) Å, b = 20.235(1) Å, c = 9.797(1) Å, β = 106.18(3)°, V = 1863.3(4) ų, Z = 4, D_calc = 1.26 g/cm³, λ(Mo-Ka) = 0.71073 Å, μ = 1.6 cm⁻¹, F(000) = 744, R = 0.018, and R_w = 0.032 for 2258 observed reflections. The solid-state structure in a dimeric packing mode exhibits intermolecular hydrogen bonding of the type P = O···H–O. Infrared solution studies (CCl₄) indicate that upon high dilution (10⁻⁴ M) the dimers completely dissociate to give conformers with and without intramolecular hydrogen bonds. Theoretical studies (PM3) were undertaken to determine the energy profile about the P–C torsional angle, which exhibited low energy barriers to rotation with no clear minimum energy conformation. © 1996 John Wiley & Sons, Inc.     

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.     

Zur Konformation des Bicyclo[2.2.2]octan-Systems

Crystals of bicyclo [2.2.2] octane-1, 4-dicarboxylic acid are monoclinic, a = 6.01 Å, b = 16.73 Å, c = 10.29 Å, β = 115.08°, space group P2₁/c, with 4 molecules in the unit cell. The structure was solved with the help of direct methods and refined by full-matrix least-squares analysis of the three-dimensional intensity data. Within experimental error the bicyclo [2.2.2]-octane (BCO) skeleton has apparent D_3h-symmetry, corresponding to the totally eclipsed conformation. Analysis of the thermal ellipsoids in terms of the translational and rotational motion of the BCO skeleton leads to an r.m.s. amplitude of 5.9 ± 0.2° for rotation about the threefold axis. On the assumption that the bond lengths remain effectively constant during a torsional vibration of BCO, the potential energy surface has been calculated for a range of semi-empirical potential functions. These calculations show that the energy minimum may be slightly displaced from D_3h symmetry, but if so the barrier between the two such equivalent minima is only about 0.1 kcal mole^?1. The energy eigenvalues and eigenfunctions for a typical variation of potential energy vs torsion angle have been calculated. From the form of the eigenfunction of the ground vibrational state we conclude that BCO has effective D_3h symmetry at all temperatures as far as diffraction methods are concerned.     

A Reinvestigation of the Structure and Torsional Potential of N2O5 by Gas‐Phase Electron Diffraction Augmented by Ab Initio Theoretical Calculations

Gaseous N₂O₅ consists of two NO₂ groups bonded to a bridging O‐atom to form a nonlinear N−O−N moiety. The NO₂ groups undergo slightly hindered internal rotation around the bonds to the bridge so that instantaneous composition of the gaseous system is characterized by molecules with all combinations of torsion angles. In an earlier investigation, an attempt was made to determine the coefficients for an empirical form of the double‐rotor torsional potential, and the bond lengths and bond angles measured subject to assumptions that the structure of the O−NO₂ groups was invariant to torsion angle and that these groups had C_2v symmetry. The system has now been reinvestigated in terms of a more realistic model in which this symmetry restriction was relaxed, account was taken of structural changes in the NO₂ groups with torsion angle as predicted by ab initio theory at the B3LYP/6‐311+G* level, and a more convenient form of the torsional potential was assumed. The most stable conformation has C₂ symmetry with torsion angles τ₁ (defined as ∢(N−O−N=O₄)) equal to τ₂ (defined as ∢(N−O−N=O₆)) equal to 33.7°; because of the broad potential minimum in this region, the uncertainty in these angles is difficult to estimate, but is probably 3 – 4°. The results for the bond lengths and bond angles for the most stable conformation are r_g(N−O)=1.505(4) Å, r_g(N=O)=1.188(2) Å, ∢_α(N−O−N)=112.3(17)°, ∢_α(O=N=O)=134.2(4)°, 〈∢_α(O−N=O)〉=112.8(2)°. The difference between the symmetry‐nonequivalent O−N=O angles is estimated to be ca. 6.7° with the larger angle positioning the two N=O bonds on different NO₂ groups nearest each other. These average values are similar to those obtained in the original study. The main difference is found in the shape of the torsional potential, which at τ₁/τ₂=0/0 has a saddle point in the present work and a substantial peak in the earlier. The implication of the torsion‐angle findings for electron‐diffraction investigations of this type is discussed.     

Spectres RMN et analyse conformationnelle de germa-4 dioxannes-1,3

Methods for the calculation of the torsional angle of the C? C linkage

1 Voir Réf. 1.

are applied to some 4-germa-1,3-dioxanes. It is thereby shown that 4,4-diethyl-2trichloromethyl-4-germa-1,3-dioxane in CCl ₄ and C ₆ D ₆ adopts the chair conformation, with the equatorial C? Cl ₃ group distorted by the presence of the germanium: the torsional angle of the Ge-CH ₂ -CH ₂ -O-fragment is 45°. The most stable 6-alkyl derivatives ( cis isomers) have the same conformation; the less stable trans - tert -butyl isomer prefers a skew-boat form compatible with a torsional angle of about 60°.     

Darstellung,Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalyse von vier bindungsisomeren Tetrachlorodirhodanoosmaten(IV)

Preparation, Crystal Structures, Vibrational Spectra, and Normal Coordinate Analysis of Four Linkage Isomeric Tetrachlorodirhodanoosmates(IV) By treatment of cis- or trans-[OsCl₄I₂]^2? with (SCN)₂ in dichloromethane the linkage isomers cis-[OsCl₄(NCS)₂]^2? ( 1 ), trans-[OsCl₄(NCS)(SCN)]^2? ( 2 ), cis-[OsCl₄(NCS)(SCN)]^2? ( 3 ) and trans-[OsCl₄(SCN)₂]^2? ( 4 ) are formed which have been separated by ion exchange chromatography on diethylaminoethyl cellulose. The X-Ray structure determinations on single crystals of cis-(Ph₄As)₂[OsCl₄(NCS)₂] (triclinic, space group P1 , a = 10.019(5), b = 11.702(5), c = 21.922(5) Å, α = 83.602(5)°, β = 85.718(5)°, γ = 73.300(5)°, Z = 2), trans-(Ph₄As)₂[OsCl₄ · (NCS)(SCN)] (monoclinic, space group P2₁/c, a = 18.025(5), b = 11.445(5), c = 23.437(5) Å, β = 94.208(5)°, Z = 4), cis-(Ph₄As)₂[OsCl₄(NCS)(SCN)] (triclinic, space group P1 , a = 10.579(5), b = 11.682(5), c = 22.557(5) Å, α = 81.073(5)°, β = 85.807(5)°, γ = 87.677(5)°, Z = 2) and trans-(Ph₄As)₂ · [OsCl₄(SCN)₂] (triclinic, space group P1 , a = 10.615(5), b = 11.691(5), c = 11.907(5) Å, α = 111.314(5)°, β = 96.718(5)°, γ = 91.446(5)°, Z = 1) reveal the complete ordering of the complex anions. The via N or S coordinated thiocyanate groups are located nearly direct above one of the cis-positioned Cl ligands with Os? N? C angles of 171.2° and 174.3° ( 1 ), 162.3° ( 2 ), 172° ( 3 ) and Os? S? C angles of 108.3° ( 2 ), 105.7° ( 3 ) and 105.5° ( 4 ). Using the molecular parameters of the X-Ray determinations the low temperature (10 K) IR and Raman spectra of the (n-Bu₄N) salts of all four linkage isomers are assigned by normal coordinate analyses based on a modified valence force field. The valence force constants are f_d(OsN) = 1.59 ( 1 ), 1.67 ( 2 ), 1.60 ( 3 ) and f_d(OsS) = 1.27 ( 2 ), 1.31 ( 3 ) and 1.32 mdyn Å^?1 ( 4 ). Taking into account increments of the trans influence a good agreement between observed and calculated frequencies is achieved.     

A Thermodynamic Study of Bromo-Lanthanum (III) Complexes from EMF Measurements

The stability constants (for the formation) of LaBr²⁺ and LaBr₂⁺ ions were obtained potentiometrically at various ionic strengths at 20°, 25°, 30°, and 35°. The molal free energy was given by ΔG°=-2,444.04+2.67T log T+1.39T. The thermodynamic quantities for the formation of LaBr²⁺ were evaluated as ΔH_f° = ?207.9, ΔG_f° = ?199.1 kcal mole^?1 and S° = ?29.5 cal. deg^?1 mole^?1 at 25°.     

Effects of thermal history,crystallinity, and solvent on the transitions and relaxations in poly(bisphenol-A carbonate)

The following system of nomenclature for the transitions and relaxations in polycarbonate has been proposed: α = T_g = 150, β = 70, γ = ?100, and δ = ?220°C (frequency range of 10–50 Hz). The three component peaks of the γ relaxation are denoted by γ₁, γ₂, and γ₃ relaxations correspond to phenylene, coupled phenylene-carbonate, and carbonate motions, respectively. Dynamic mechanical analysis of poly(bisphenol-A carbonate) using the DuPont 981–990 DMA system shows that the magnitude of the β relaxation depends upon the thermal history of the polycarbonate; annealing greatly reduces the intensity of the β relaxation. A relaxation map constructed for the β relaxation gives an activation energy of 46 kcal/mol. Exposure of polycarbonate to methylene chloride vapor for various times shows that after an induction period of about 5 min the intensity of the γ₃ relaxation at ?78°C decreases whereas the intensity of the γ₁ relaxation of ?30°C is unaffected and the ratio E″(γ₁)/E″(γ₃) increases linearly with the square root of time. This has been ascribed to the interaction of methylene chloride on the carbonate group in polycarbonate. Thermal crystallization of polycarbonate does not affect the positions of the γ relaxation and the glass transition peaks, but merely reduces their intensity. The glass transition peak intensity falls off sharply in comparison to the γ relaxation intensity. Both the γ₃ and γ₁ peaks in polycarbonate have been observed simultaneously for the first time by dynamic mechanical analysis. Impact strength measurements show that methylene chloride treatment of polycarbonate results in a change in mode of failure from ductile to brittle with a resultant 40-fold reduction in impact energy for fracture. Thermally crystallized polycarbonate exhibits brittle fracture with very low force and energy at break.     

Influence of an identified dimer vibration on the emission spectrum of [2,2]paracyclophane

The emission spectrum of polycrystalline [2,2]paracylophane shows a resolved vibronic structure with a 241 cm^?1 progression at He temperatures. The dependence of the energy of this mode upon selective deuteration in combination with results from FIR and Raman spectra could be used to identify the mode as a torsional dimer vibration. The emission spectra could be simulated assuming a linear coupling of the torsional mode to the electronic transitions with coupling strengths of S = 10 (fluorescence) and S = 13 (phosphorescence). This corresponds to an equilibrium displacement of the benzene rings under electronic excitation by a torsional angle of 10.6° (S₁) and 12.1° (T₁), in addition to the small torsion in the ground state S₀ by about 3°.     

Equilibrium anionic polymerization of β-methoxypropionaldehyde

Anionic polymerization of β-methoxypropionaldehyde (MPA) was carried out in tetrahydrofuran (THF) by using benzophenone–monolithium complex as an initiator. An equilibrium between polymerization and depolymerization was observed at a temperature range of ?90 to ?70°C. From the temperature dependence of the equilibrium monomer concentration, thermodynamic parameters for the polymerization of MPA in THF were evaluated as follows: ΔH_ss = ?4.8 ± 0.2 kcal/mole, ΔH_SS = ?22.4 ± 1.3 cal/mole-deg, and (T_c)_ss = ?59°C. The thermodynamic change upon the conversion of liquid monomer to condensed polymer was computed from both the partial mixing energy of MPA with THF and the linear relationship between the equilibrium volume fraction of MPA monomer and that of the resulting polymer: ΔH_1c = ?4.7 ± 0.2 kcal/mole, ΔS_1c = ?19.5 ± 1.3 cal/mole-deg, and (T_c)_1c = ?35°C.     

Very-low-pressure pyrolysis of N-methyl aniline and N,N-dimethyl aniline. Enthalpy of formation of the anilino and N-methyl anilino radicals

The kinetics of the thermal unimolecular decompositions of N-methyl aniline and N,N-dimethyl aniline into anilino and N-methyl anilino radicals, respectively, have been studied under very low-pressure conditions. The enthalpies of formation of both radicals, ΔH°_f,298°K(Ph?H,g) = 55.1 and ΔH°_f,298°K(Ph?Me,g) = 53.2 kcal/mol, which have been derived from the experimental data, lead to BDE(PhNH-H) = 86.4 ± 2, BDE[PhN(Me)-H] = 84.9 ± 2 kcal/mol and to a value of 16.4 kcal/mol for the stabilization energy of the PhNH radical (relative to MeNH). These results are discussed in connection with earlier work. At high temperatures, the anilino radical loses HNC and forms the very stable cyclopentadienyl radical, a decomposition comparable to that of the phenoxy radical.