Glass and subglass transitions in a series of poly(itaconate ester)s with methyl-terminated poly(ethylene oxide) side chains

A series of poly(itaconate ester)s containing methyl-terminated poly(ethylene oxide) side chains with lengths ranging from 1 to 5 ethylene oxide units has been synthesized. Both heat capacity C_p and dynamic mechanical measurements have been carried out on these polymers using differential scanning calorimetry (DSC) and torsional braid analysis (TBA), respectively. The resulting data for this polymer series are discussed, and comparisons are made with work previously published for the corresponding di-n-alkyl itaconate ester polymers where appropriate.     

Matrix IR spectra and normal coordinate analysis for methanesulfenyl chloride and isotopic analogs

The IR spectra (4000-250 cm⁻¹) of CH₃SCl and CD₃SCl in solid argon have been obtained. Fundamental vibrations, except the torsional vibrations, have been assigned. Normal coordinate analysis has been carried out omitting the torsional modes.     

Preparation of activated carbon from date pits: Effect of the activation agent and liquid phase oxidation

Two series of activated carbons have been prepared from date pits; series C, using carbon dioxide as activating agent, and series S, prepared by activation with steam under the same experimental conditions. The obtained samples were oxidized with nitric acid in order to introduce more oxygen surface groups. The surface area and porosity of the parent and oxidized activated carbons were studied by N₂ adsorption at 77 K and CO₂ adsorption at 273 K. The oxygen surface complexes were characterized by temperature-programmed decomposition (TPD). The results show that carbon dioxide and steam activations produce microporous carbons with an increasing amount of CO evolving groups when increasing the burn-off. On the other hand, oxidation with nitric acid increases the amount of CO and CO₂ evolved by the decomposition of surface oxygen groups, this increase being related to the development of porosity in the carbon with the degree of activation and to the activating agent used (CO₂ versus steam).     

Torsional potentials and conformational structures in the halogen substituted molecules HOCCH2X,HOCCHX2, XOCCH2X,XOCCHX2, XH2CCOCH3, X2HCCOCH3, (XH2C)2CO, (X2HC)2CO,and (X3C)2CO (X = Cl,Br) as determined by molecular-mechanics calculations

Parameter values of the non-blonded atom⋯atom interaction Br⋯OC have been established from gas-phase data. Conformational energies, structures, torsional barrier heights and torsional force constants of the title compounds have been estimated. For BrOCCH₂Br, (Cl₃C)₂CO and (ClH₂C)₂CO the calculated results are compared with gas-phase observations. It is suggested that hexachloroacetone ought to be reinvestigated by electron diffraction.     

Infrared and Raman spectra, ab initio calculations and vibrational assignment for 3-fluoro-1-butyne

The infrared (3500-80 cm⁻¹) and Raman (3500-20 cm⁻¹) spectra of 3-fluoro-1-butyne, CH₃CHFCCH, have been recorded for the gas and solid. Additionally, the Raman spectrum of the liquid has also been recorded to aid in the vibrational assignment. Ab initio electronic structure calculations of energies, geometrical structures, vibrational frequencies, infrared intensities, Raman activities and the potential energy function for the methyl torsion have been calculated to assist in the interpretation of the spectra. The fundamental torsional mode is observed at 251 cm⁻¹ with a series of sequence peaks falling to lower frequency. The three-fold methyl torsional barrier is calculated to be 1441 ± 20 cm⁻¹ (4.12 ± 0.06 kcal mol⁻¹) where the uncertainty is partly due to the uncertainty in values of the V₆ term. A complete vibrational assignment is proposed based on band contours, relative intensities, and ab initio predicted frequencies. Several fundamentals are significantly shifted in the condensed phases compared to values in the vapor state.     

Central‐Atom Size Effects on the Methyl Torsions of Group XIV Tetratolyls

The Group XIV tetratolyl series X(C₆H₄‐CH₃)₄ (X=C, Si, Ge, Sn, Pb) were studied by using inelastic neutron scattering to measure the low‐energy phonon spectra to directly access the methyl‐group torsional modes. The effect of increased molecular radius as a function of the size of the central atom was shown to have direct influence on the methyl dynamics, reinforced with the findings of molecular dynamics and contact surface calculations, based upon the solid‐state structures. The torsional modes in the lightest analogue were found to be predominantly intramolecular: the Si and Ge analogues have a high degree of intermolecular methyl–methyl group interactions, whilst the heaviest analogues (Sn and Pb) showed pronounced intermolecular methyl interactions with the whole phonon bath of the lattice modes.     

The visible spectrum of jet-cooled CF3NO

The electronic origin of the à A″ ← X̃ A′ transition of trifluoronitrosomethane (CF₃NO) has been reassigned to the very weak feature near 717.9 nm in the fluorescence excitation spectrum of the jet-cooled molecule. The prominent torsional progression has been reinpreted and the height of the threefold torsional barier in the Ã( n,π^* ) state has been revised to 601.5 ± 10 cm⁻¹.     

Electronic structure and photoelectron spectrum Assignment of allene

In this paper a series of ab initio SCF and configuration calculations were reported forthe ground state and excited states X~2E, A~2E,~2B_2 and ~2A_1 of allene.For ground state X~2E Jahn-Teller distorsion was discussed and a twisted angle of 50° and a torsional barriers of 0.21—0.51 eVwere derived.Based on calculated results,the experimental photoelectron spectrum of allene has beenassigned.     

Crystalline structure and OH torsional motion in calcium-strontium arsenate apatites

The crystalline structure of Ca_10?xSr_x(AsO₄)₆(OH)₂ has been studied, and the lattice parameters determined. It has been found that the unit cell expands with x. Geometric parameters of the unit cell, which are defined in relation to the hindered rotation of the OH group around the c axis, have been also obtained. From the ir data, the torsional potential function has been calculated in first and second approximations. A method for computing that function in any order approximation is given. A semiempirical curve is found, relating the stretching and torsional motions for both the OH and OD groups.     

The methyl and methoxyl torsional modes and the lattice vibration in the low-frequency Raman spectrum of 1,4-dimethoxybenzene

The low-frequency (10–450 cm^?1) Raman spectra of solid (at 300 K and 130 K) and liquid (at 335 K) 1,4-dimethoxybenzene-d₀ and 1,4-dimethoxybenzene-d₅ have been measured. The methyl nad methoxyl torsional transitions have been identified and the corresponding torsional barriers calculated. Upon deuleration the methyl torsional barrier is reduced by 450 cm^?1, implying a coupling between the methyl torsion and a low-frequency ring mode. As far as the torsions are considered, the internal dynamic situation in 1,4-dimethoxybezene resembles that in amisole. A tentative assignment of the observed lattice bands in given. Certain changes in the spectrum when going from the solid to the melt are attributed to the coexistence of both cis and trans conformers in the liquid state.     

NMR studies of conformations and dynamic processes—1 : [24]cyclophanes with ethylene bridges

The conformations and dynamic processes in a series of relatively unstrained [2₄]paracyclophanes (with one, two or four -CH₂-CH₂-bridges) and some closely related compounds have been analysed. Their ¹NMR spectra have been recorded at low temperatures and the temperature dependence rationalised as being due to essentially two types of dynamic process-the torsional motion around the sp³-sp³ C-C bonds in the bridges, and the rotation around the sp²-sp³ C-C bonds adjacent to the benzene rings. The barriers to the former process are similar for the series of cyclophanes 1–6 and are due to steric and electronic interactions in the syn-oriented transition states. In cyclophanes 7–9, in which anti-orientations of the aromatic rings are possible, the barriers are lower. The latter process, involving the rotation of the benzene rings, becomes important at temperatures below 150 K and has not been further analysed.     

Force field and 13C-NMR investigations of substituted cyclopentanes. A concept for the adaption of 13C NMR shifts to varying torsional arrangements in flexible conformers

MM2 Exploration of the conformational space for methylcyclopentane, in contrast to cyclopentanone yields more and flatter minima than known previously. Calculations of cyclopentanes with substituents X = F, Cl, CHMe₂, and CMe₃ with two stable conformations indicate <2° torsional angle changes with the different substituents. Cyclopentanes bearing not more than 2 substituents can accommodate all groups in pseudoequatorial positions without changing the basic envelope and twist chair geometries significantly. A model for ¹³C-shift calculation is proposed in which shift increments for the different torsional arrangements are obtained by linear interpolation between corresponding cyclohexane values. After correction for the nonequivalent carbon shifts in the hydrocarbon itself, again using the linear interpolation, a significant improvement of the shift correlations is observed. For disubstituted cyclopentanes these predict the shifts within ± 1.7 ppm with Me, CHMe₂, CMe₃, Cl, Br and OH as substituents. Configurational assignments are difficult with 1,3-di-substituted cyclopentanes, but straightforward with 1,2-di- and trisubstituted compounds. Thus, due to the presence of smaller torsional angles between, e.g. diequatorial vicinal substituents in the 1,2- cis series as compared to the trans compounds, the latter show deshielding, particularly at C2, by 1–4 ppm. Several epimers are stereo-selectively prepared by suitable ketone reduction and displacement methods.     

Potential energy surfaces for van der waals complexes of rare gases with H2S and H2S2: Extension to xenon interactions and hyperspherical harmonics representation

This article is an account and extension of a series of recent investigations, which using extensive quantum chemical methods provide analytical hyperspherical representations of the potential energy surfaces for the interactions of rare gases with H₂S as a rigid molecule, and H₂S₂, considered as a floppy molecule with respect to torsional mode. For the H₂S‐rare gas systems, the representation is based on a minimal model, here introduced and discussed. For H₂S₂, the study of the interaction with Xe, not considered previously, completes the series. The results are discussed with reference to the properties and trends expected for interactions of van der Waals type. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

General spin structures of organic radicals

Changes of spin structures in the course of chemical reactions have been investigated on the basis of the Heisenberg model, asseming two different exchange integrals. It is found that an axial spin structure changes to another axial spin structure passing through the torsional structure such as the triangular spin arrangement in the cyclic three-electron system. The spin structures of odd polyenic cyclic radicals and the tetrahedral H₄ radical are also torsional spin arrangements.     

Microwave spectrum of propionyl chloride

The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the ³⁵Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the ³⁷Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm^?1. The CH₃ internal rotation frequency was found to be 197 cm^?1. Nuclear quadrupole coupling constants were determined for the ground state of the ³⁵Cl and ³⁷Cl species. From observed A-E splittings of ^bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V₃ = 2480 ± 40 cal mol^?1 (867 ± 14 cm^?1).     

Some features of formation and dissolution of a series of Pu(IV) and Zr alkyl and butyl alkyl phosphates in the system tBP — N-dodecane — Nitric acid — Water

The formation and composition of salts produced on interaction of a series of alkyl- and butylalkylphosphoric acids having alkyl radical chain lengths from C₄ to C₁₀ with Pu(IV) and Zr in organic and aqueous phases of the system TBP — n-dodecane — nitric acid — water have been studied. The composition of compounds was found to depend on the conditions of their formation, being defined first of all by the HNO₃ concentration in aqueous and organic phases.     

Design of Extractants for F-Block Elements in a Series of (2-(Diphenylphosphoryl)methoxyphenyl)diphenylphosphine Oxide Derivatives: Synthesis,Quantum-Chemical,and Extraction Studies

With the aim to find new efficient extractants for recovery of f-block elements from processing wastes of different origin, we have compared a series of phosphoryl-containing podands, including (2-(diphenylphosphorylmethoxy)phenyl)diphenylphosphine oxide 1 and its analogues 5–7, where the ArP(O)Ph₂ group of phosphine oxide type is replaced by phosphonic fragments. Quantum-chemical modelling of the structures of phosphoryl-containing podands 1 and 5–7 has been performed, which was later confirmed by the data of X-ray diffraction. The features of extraction of nitric acid, as well as U(VI), Th(IV), Nd(III), and Ho(III) with compounds 1 and 5–7 from nitric acid media into 1,2-dichloroethane have been studied. The compositions of extracted complexes have been determined.     

A conformational analysis of 4,4'-bipyridinium dication

A conformational analysis of the 4,4'-bipyridinium dication has been investigated using the results from 10 ab initio STO-3G calculations in which the inter-ring bond length was optimized at each torsion angle. The most reliable calculation yields: a lowest energy conformer with an equilibrium torsional angle of 45.3° and inter-ring (R_c-c) bond length of 0.152 nm; a barrier height of 5.86 kJ mol⁻¹. An examination of the fitted Fourier series indicates that half of the theoretical data points effectively reproduced the more extensive surface. Furthermore, a comparison of the calculations and fitted surfaces indicate that a rigid rotor is an excellent approximation for this molecule.     

The role of the torsional potential in relaxation dynamics: a molecular dynamics study of polyethylene

The role of the torsional potential in bulk polymer chain dynamics is investigated via molecular dynamics simulation using polyethylene as a model system. A number of three-fold barrier values, both greater and less than the standard one, were invoked. The one-fold potential that determines the gauche vs trans energy difference was also varied. For each of the selected torsional potentials, the MD volumetric glass transition temperature, T_g, was located. It was found that T_g is quite sensitive to the three-fold barrier magnitude, moving from below 100 K to nearly 400 K as the barrier goes from zero to twice the standard value. However T_g was found to be quite insensitive to the gauche trans energy difference. Details of the conformational dynamics were studied for the case of a zero torsional potential. This included the rate and location of conformational transitions, the decay of the torsional angle autocorrelation function (ACF) and the cooperativity of conformational transitions, all as a function of temperature. The temperature dependence of the conformational transition rate remains Arrhenius at all temperatures. The relaxation time characterizing the torsional angle ACF decay exhibits WLF temperature behavior. The conformational transitions are randomly distributed over the bonds at high temperature, but near T_g they become spatially heterogeneous and localized. The transitions show next-neighbor correlation as well as self-correlated forward-backward transitions. All of these features are similar to those found in previous simulations under the standard torsional potential.     

Limitations of Global Hybrids in Predicting the Geometries and Torsional Energy Barriers of Dimeric Systems and the Role of Hartree Fock and DFT Exchange

Prediction of accurate geometries is a prerequisite for accurate prediction of molecular properties. Impact of Hartree Fock (HF) exchange (a₀) on geometry in the framework of DFT is investigated by monitoring dihedral angles, bond length alternations, and torsional energy barriers of 10 dimeric systems against CCSD (ADZ/ATZ) benchmarks. A strong correlation is observed between the fraction of HF exchange, equilibrium dihedral angles, and the potential energy barriers in global hybrids. Full HF exchange is critical to accurately predict the nonplanarity. Lower fractions of (a₀)/larger DFT exchange (1-a₀) results in overestimation of torsional energy barriers at 90⁰ and underestimation at 0⁰. Large contributions of (1-a₀) in global hybrid functionals tend to overestimate torsional energy barriers (90⁰) and are biased toward planar geometries. However, inclusion of larger fractions of (a₀)/lower (1-a₀) also overestimate the torsional energy barriers in syn-conformations due to the localization errors associated with HF exchange in global hybrids. Hence, irrespective of the fraction of HF/DFT exchange incorporated, global hybrids fail to accurately predict torsional energy barriers at 0⁰ and 90⁰ simultaneously. Long-range corrected (LC) functionals, which employ full HF exchange at longer regions, outperform global hybrid functionals in predicting geometries and torsional energy barriers of the dimeric molecules. The distance dependence of (a₀) thus provides a balanced fraction of HF exchange as the dihedral torsion varies. Impact of range separation parameter on geometries is marginal in altering the planarity/nonplanarity. However, range separation parameter within 0.20–0.40 bohr⁻¹ predicts more reliable torsional energies and geometries. © 2019 Wiley Periodicals, Inc.