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. 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).     

ESR study of primary radical formation during mechanical degradation of poly(2,6-dimethyl-p-phenylene oxide) solid

Radical formation during mechanical degradation of solid poly(2,6-dimethyl-p-phenylene oxide) (PPO) was investigated by electron spin resonance (ESR). The ESR spectrum of PPO fractured at room temperature in air consisted of eight lines with a separation of about 5.5 gauss with g = 2.0043, indicating a small asymmetry. For PPO fractured in liquid nitrogen, a similar spectrum was observed at ?196°C in air or in vacuo. These spectra have been identified as belonging to a 2,6-dimethyl-substituted phenoxy radical and thus indicate the occurrence of main-chain rupture. The phenyl radical which was expected to be formed together with a 2,6-dimethyl-substituted phenoxy radical could not be detected, but at temperatures below ?46°C a small hump was observed at g = 2.034. By subtracting the spectrum observed after decay of this hump from the original one, the resulting curve was the characteristic asymmetric spectrum of a peroxy radical, which was presumably formed by the reaction between a phenyl radical and oxygen. The radical decay curve showed two stepwise-decaying regions; one located in the temperature region between about ?120°C and ?80°C where only a small number of radicals decayed, another located in the temperature region from about ?30°C to 100°C where almost all mechanically formed radicals decayed. The latter radical decay, which occurred considerably below the glass-transition temperature of PPO, was attributed to the molecular motions associated with the mechanical β^* relaxation on the basis of the activation energy and the temperature region.     

Experimental and theoretical investigation of stable diastereomeric conformations of biscarboline amides in solution

Stable axial conformations generally exist only when the single bond's (axis') rotation was sterically hindered in solution. Herein, we firstly show that two stable conformations could be observed in solution by ¹H and ¹³C NMR experiments when the single bond rotates freely. Its coalescence temperature was measured up to 120?°C when we took compound 13 as an example. The ratio of the two stable conformations was computed using quantum methods. The predicted results matched with the experimental results well. The conversion barrier between two conformers was estimated by potential energy scan (PES) and transition state (TS) calculations at the B3LYP/6-311 + G(d) level. Furthermore, its stereochemistry was also well studied by comparing theoretical electronic circular dichroism (ECD) spectra with the experimental one.     

Rate constants for the gas‐phase reactions of a series of C3?C6 aldehydes with OH and NO3 radicals

By using relative rate methods, rate constants for the gas‐phase reactions of OH and NO₃ radicals with propanal, butanal, pentanal, and hexanal have been measured at 296 ± 2 K and atmospheric pressure of air. By using methyl vinyl ketone as the reference compound, the rate constants obtained for the OH radical reactions (in units of 10⁻¹² cm³ molecule⁻¹ s⁻¹) were propanal, 20.2 ± 1.4; butanal, 24.7 ± 1.5; pentanal, 29.9 ± 1.9; and hexanal, 31.7 ± 1.5. By using methacrolein and 1‐butene as the reference compounds, the rate constants obtained for the NO₃ radical reactions (in units of 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹) were propanal, 7.1 ± 0.4; butanal, 11.2 ± 1.5; pentanal, 14.1 ± 1.6; and hexanal, 14.9 ± 1.3. The dominant tropospheric loss process for the aldehydes studied here is calculated to be by reaction with the OH radical, with calculated lifetimes of a few hours during daytime. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 79–84, 2000     

Higher aliphatic polyaldehydes. II. Polymer structure and polymer stability of poly(n-heptaldehyde)

Poly(n-heptaldehyde) has been prepared by anionic and cationic polymerization at ?60°C in methyl cyclohexane. The anionic polymer is more crystalline and of a higher degree of isotactic structure than the somewhat rubbery but still crystalline cationic polymer. The polymers have been acetate-endcapped to improve their thermal stability. Cationic polymer, when endcapped and purified, begins to degrade above room temperature; even crystalline anionic polymer degrades at a reasonable rate at 100°C. The crystallinity of poly(n-heptaldehyde) is caused by crystallization of the acetalic main chain as well as the side chain. Two regions of melting have been recognized by DSC analysis and by microscopic observations. The low melting region between 80 and 100°C has been identified as the melting of the paraffinic side chains of poly(n-heptaldehyde). It consists of three clearly definable endotherms at 78, 87, and 101°C.     

Characterization and Stability Study of Immobilized PQQ‐Dependent Aldose Dehydrogenase Bioanodes

In this paper, glucose oxidizing bioanodes employing immobilized PQQ‐dependent aldose dehydrogenase were prepared and characterized. The enzyme was immobilized on carbon paper in two different polymeric systems: tetrabutylammonium bromide (TBAB) modified Nafion and butanal modified chitosan. Characterization of the bioanodes included electron microscopy, electrochemical evaluation, as well as stability and leaching studies. Results indicate that the operational degradation was the same but the long term storage stability is better in the case of modified Nafion. The performance of the modified Nafion immobilized bioanodes stayed at 70 % of the initial value after 60 days of storing at 4 °C and 25 °C. Compared to TBAB modified Nafion immobilized bioanodes, butanal modified chitosan immobilized bioanodes showed 50 % activity after eight weeks storage at 4°C and one week storage at 25 °C. However, the electrochemical properties of modified chitosan were better.     

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.     

Conformational analysis of N,N-diisopropylamides by combined use of n.m.r. lanthanide-induced shifts and conformational energy calculations

A new method of conformational analysis has been developed, in which energy minimization calculations are combined with lanthanide-induced shift data. First, exhaustive energy calculations are carried out on the free molecules in order to determine the conformations of lowest energy. Then, the coordinates of all low energy conformations or pairs of conformations are used in the pseudocontact shift equation for lanthanide-induced shifts in order to find which of the theoretically obtained conformation(s) gives the best agreement with experiment. The molecules complexed to the lanthanide shift reagent were N,N-diisopropylformamide (DIPF) and N,N-diisopropylacetamide (DIPA). Two different lanthanide shift reagents were used, Eu(fod)₃

1 Fod is the anion of 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedione-d₂₇.

and Pr(fod)₃, in order to check the validity of the method. Proton magnetic resonance spectra were taken at 6 °C in carbon tetrachloride solution. The principal conformation found was different for each amide. DIPF was found to exist as a mixture of I (39 mol%) and II (61 mol%) with Eu(fod)₃, and a mixture of I (37%) and II (63%) with Pr(fod)₃. DIPA was found to exist as a mixture of I (79%) and IV (21%) with Eu(fod)₃ and a mixture of I (87%) and IV (13%) with Pr(fod)₃. For both molecules, the two conformations of lowest computed energy were also the pair which gave the best fit to the lanthanide shift reagent data. The location of the principal magnetic axis of the complex was found to lie between 0° and 14° from the lanthanide atom–oxygen atom bond axis. The technique of combining lanthanide shift reagent data with energy calculations shows great promise in conformational analysis.     

Conformational study of halogen- and hydrogen-substituted polythionylphosphazenes using density functional theory method

The structure and conformational stability of polythionylphosphazenes is investigated by modeling single polymer chains with small mimics. The model compounds are composed of repeat units of the corresponding polythionylphosphazenes. Two of the model compounds have hydrogens and two have chlorines as substituents on phosphorus atoms. The substituents on sulfur may be either fluorine or chlorine. Fully geometry-optimized structures and energies of the stable conformations involving rotations around the P? N bond near the sulfur are obtained using the density functional theory method. The structural and conformational analyses indicate that the rotation around the N? P bond leads to variations in the bond lengths, the SNP bond angle openings, as well as couplings between dihedral angles in different conformations in all model compounds. In addition, the conformational analysis suggests that the minima on the conformational potential energy surface in these compounds may be located in the vicinity of the following values of the NP? NS dihedral angle: -50°, 90° (or 60°), 180°, and 240°. It was found that the values of the conformational energy differences range between less than 1 to 5 kcal/mol. A comparison is made between the structural results obtained using the density functional theory and the ab initio molecular orbital theory for the global minimum structures. © 1995 John Wiley & Sons, Inc.     

Carbon-13 nuclear magnetic resonance spectra and conformations of cis,cis-1,5-cyclooctadiene monoepoxide and cis,syn,cis-1,5-cyclooctadiene diepoxide

The natural-abundance ¹³C NMR spectra of cis,cis-1,5-cyclooctadiene monoepoxide and cis,syn,cis-1,5-cyclooctadiene diepoxide have been investigated over the temperature range of – 10 to – 180°C. Whereas the spectra of the former showed no dynamic NMR effect, two different conformations in the ratio of 3:1 were observed at low temperatures for the latter. The free-energy barrier (ΔG^≠) for conversion of the major conformation to the minor conformation is calculated to be 5.9°0.2 kcal mol^?1 from a line-shape analysis of spectra obtained at intermediate temperatures. It is shown that cis,syn,cis-1,5-cyclooctadiene diepoxide exists in solution in chair (major) and in twist-boat (minor) conformations of slightly different energies. Interconversion paths between these conformations are discussed. The monoepoxide is suggested to have a twist-boat conformation that is rapidly pseudorotating via a boat conformation even at – 180°C.     

Monte Carlo simulations on short single-stranded oligonucleotides. I. Application to RNA trimers

Monte Carlo (MC) structural simulation of short RNA sequences has been carried out by random variations of the nucleotide conformational angles (i.e., phosphodiester chain torsional angles and sugar pucker pseudorotational angles). All of the chemical bond lengths and valence angles remained fixed during the structural simulation, except those of the sugar pucker ring. In this article we present the simulated structures of RNA trimers—r(AAA) and r(AAG)—obtained at 11°C and 70°C. The influence of various initial conformations (selected as starting points in the MC simulations) on the equilibrium conformations has been discussed. The simulated conformational angles have been compared with those estimated by nuclear magnetic resonance (NMR) spectroscopy. For both of the oligonucleotides studied here, the most stable structures are helical conformations with stacked bases, at 11°C and 70°C. However, when the starting point is a stretched chain, it is found that r(AAA) adopts a reverse-stacked structure at low temperature (11°C), in which the A3 base is located between the A1 and A2 bases. Although the energies of these conformations (helical and reverse stacked) are very close to each other, the potential barrier between them is extremely high (close to 30 kcal/mol). This hinders the conformational transition from one structure to the other at a given temperature (and in the course of a same MC simulation). However, it is possible to simulate this structural transition by heating the reverse-stacked structure up to 500°C and cooling down progressively to 70°C and 11°C: Canonical helical structures have been obtained by this procedure. © 1994 by john Wiley & Sons, Inc.     

Urea and CaCl2 as inhibitors of coal low-temperature oxidation

Thermal analysis was used to study the influence of CaCl₂ and urea as possible chemical additives inhibiting coal oxidation process at temperatures 100?C300?°C. Weight increase due to oxygen chemisorption and corresponding amount of evolved heat were evaluated as main indicative parameters. TA experiments with different heating rates enabled determination of effective activation energy E _a as a dependence of conversion. In the studied range of temperatures, the interaction of oxygen with (untreated) coal was confirmed rather as a complex process giving effective activation energies changing continuously from 70?kJ?mol^?1 (at about 100?°C) to ca. 180?kJ?mol^?1 at temperatures about 250?°C. The similar trend in E _a was found when chemical agents were added to the coal. However, while the presence of CaCl₂ leads to higher values of the effective activation energies during the whole temperature range, urea causes increase in E _a only at temperatures below 200?°C. Exceeding the temperature 200?°C, the presence of urea in the coal induces decrease in activation energy of the oxidation process indicating rather catalysing than inhibiting action on coal oxidation. Thus, CaCl₂ can only be recommended as a ??real?? inhibitor affecting interaction of coal with oxygen at temperatures up to 300?°C.     

Ab initioSTO-3G optimization of planar,perpendicular, and twisted molecular structures of biphenyl

The complete molecular structure of biphenyl, characterized by 12 independent parameters, has been derived by ab initio gradient techniques using a STO -3G basis set for coplanar, perpendicular, and minimum energy conformations with the constraint of planar phenyl ring units and a C₂ symmetry axis along the CC interring bond. The minimum torsional angle obtained was ? = 38.63° with torsional energy barriers of 8.59 and 10.04 kJ/mol for ? = 0° and ? = 90°, respectively.     

离子液体存在下脂肪醛的环化三聚反应

Aliphatic aldehydes such as ethanal,propanal,n-butanal,isobutyraldehyde,n-valeraldehyde,isovaleraldehyde,n-hexanal and n-octanal were converted into the corresponding 2,4,6-trialkyl-1,3,5-trioxanes through cyclotrimer-ization in the presence of the ferric chloride based ionic liquids at room temperature without solvent in high selec-tivity.The effects of different ionic liquids,acidity of ionic liquids and temperature on cyclotrimerization were alsostudied.The results showed that the ferric chloride based ionic liquids(apparent molar fraction of FeCl_3(x(FeCl_3)=0.62))were a kind of efficient catalysts for the cyclotrimerization of aliphatic aldehyde which could be separatedconveniently from the reaction mixture and recycled without loss of catalytic activity.The conversion of isobu-tyraldehyde and the selectivity to 2,4,6-triisopropyl-1,3,5-trioxane were 91.1% and 99.8% respectively under opti-mum reaction condition(isobutyraldehyde 25.0 g,[Et_3NH]Cl/FeCl_3(x(FeCl_3)=0.62)1.0 g,25 ℃for 1 h).     

Drawing behavior of linear polyethylene. II. Effect of draw temperature and molecular weight on draw ratio and modulus

The drawing behavior of linear polyethylene homopolymers with weight-average molecular weights (M?_w) from 101,450 to ca. 3,500,000 has been studied over the temperature range 75°C to the melting point. In all cases 1-cm gauge length samples were drawn in an Instron tensile testing machine at a constant cross-head speed of 10 cm/min. With the exception of the lowest molecular weight polymer, it was found that increasing the draw temperature led to substantial increases in the maximum draw ratio which could be achieved, and that this increased monotonically with increasing draw temperature. Measurements of the Young's modulus of the drawn materials showed, however, that the unique relationship between modulus and draw ratio previously established for drawing at 75°C was not maintained to the highest draw temperatures. The highest draw temperature at which this relation held was found to be strongly molecular weight dependent, increasing from ca. 80 to ca. 125°C when M?_w increased from 101,450 to 800,000. In all cases conditions could be found for drawing samples to draw ratios of 20 or more with correspondingly large values of the Young's modulus.     

Solid-state polymerization of oxetanes. I. Lattice parameters and packing properties of the monomers

The crystallographic unit cells of melt-crystallized 3,3-bischloromethyloxetane and 3,3-bisbromomethyloxetane were determined by the Weissenberg method. The two isomorphous lattices are triclinic with two molecules in the unit cell. 3,3-Bisfluoromethyloxetane forms plastic crystals in the temperature range between ?36°C and +22°C, as shown by differential calorimetry and NMR broad-line spectroscopy. The Debye-Scherrer diagram and the general physical properties indicate the formation of a face-centered cubic lattice. No correlation between the lattice parameters of the monomer and polymer can be found On the basis of these results, the question is raised as to whether a topochemical polymerization of bishalomethloxetanes, i.e., a solid-state polymerization without destruction of the crystal lattice, can take place at all. The halomethyl side groups of the oxetanes can be shown to possess different conformations in monomer and polymer crystals, so that a conformational change of the groups and rearrangement of the molecules must take place during polymerization. Therefore, a topochemical mechanism for the solid-state polymerization of bishalomethyloxetanes seems to be impossible.     

13C, 1H and 31P dynamic NMR studies of tetraalkyldiphosphanes: Barrier to internal rotation about the P?P Bond in tetraisopropyldiphosphane

The ¹H, ³¹P and ¹³C NMR spectra of tetramethyldiphosphane (1), tetraethyldiphosphane (2) and tetraisopropyldiphosphane (3) have been studied in the temperature range 30 to ?150°C and at magnetic induction up to 5.87 T. In the range ?100 to ?135 °C, the ¹H and ¹³C spectra of 3 show important changes which can be attributed to freezing the interconversion between two equivalent non-trans conformations. The line shape analysis of the ¹³C signals leads to ΔG^≠ = 29.4 kJ mol^?1 at ?113 °C for the dynamic process involved. The spin coupling parameters ¹J(PP) and N(PC) = ¹J(PC) + ²J(PC) observed for 1 in the temperature range 30 to ?120 °C show variations which could be due to a preference for the trans conformation in this diphosphane.     

Probing the weakest bond and the cleavage of p‐chlorobenzaldehyde diperoxide energetic molecule via quantum chemical calculations and theoretical charge density analysis

A high‐level ab initio Hartree‐Fock/Møller‐Plesset 2 and density functional theory quantum chemical calculations were performed on p‐chlorobenzaldehyde diperoxide energetic molecule to understand its bond topological, electrostatic, and energetic properties. The optimized molecular geometry for the basis set 6‐311G** exhibit chair diperoxide ring and planar aromatic side rings. Although the diperoxide ring bear same type of side rings, surprisingly, both the rings are almost perpendicular to each other, and the dihedral angle is 96.1°. The MP2 method predicts the O? O bond distance as ～1.466 Å. The charge density calculation reveals that the C? C bonds of chlorobenzaldehyde ring have rich electron density and the value is ～2.14 e Å^?3. The maximum electron density of the O? O bonds does not lie along the internuclear axes; in view of this, a feeble density is noticed in the ring plane. The high negative values of laplacian of C? C bonds (approximately ?22.4 e Å^?5) indicate the solidarity of these bonds, whereas it is found too small (approximately ?1.8 e Å^?5 for MP2 calculation) in O? O bonds that shows the existence of high degree of bond charge depletion. The energy density in all the C? C bonds are found to be uniform. A high electronegative potential region is found at the diperoxide ring which is expected to be a nucleophilic attack area. Among the bonds, the O? O bond charge is highly depleted and it also has high bond kinetic energy density; in consequence of this, the molecular cleavage is expected to happen across these bonds when the material expose to any external stimuli such as heat or pressure treatment. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011