Torsional Scanning Research Articles

Benchmarking Quantum Mechanical Levels of Theory for Valence Parametrization in Force Fields.

A wide range of density functional methods and basis sets are available to derive the electronic structure and properties of molecules. Quantum mechanical calculations are too computationally intensive for routine simulation of molecules in the condensed phase, prompting the development of computationally efficient force fields based on quantum mechanical data. Parametrizing general force fields, which cover a vast chemical space, necessitates the generation of sizable quantum mechanical data sets with optimized geometries and torsion scans. To achieve this efficiently, choosing a quantum mechanical method that balances computational cost and accuracy is crucial. In this study, we seek to assess the accuracy of quantum mechanical theory for specific properties such as conformer energies and torsion energetics. To comprehensively evaluate various methods, we focus on a representative set of 59 diverse small molecules, comparing approximately 25 combinations of functional and basis sets against the reference level coupled cluster calculations at the complete basis set limit.

An automated protocol to construct flexibility parameters for classical forcefields: applications to metal-organic frameworks.

In this work, forcefield flexibility parameters were constructed and validated for more than 100 metal-organic frameworks (MOFs). We used atom typing to identify bond types, angle types, and dihedral types associated with bond stretches, angle bends, dihedral torsions, and other flexibility interactions. Our work used Manz's angle-bending and dihedral-torsion model potentials. For a crystal structure containing N atoms in its unit cell, the number of independent flexibility interactions is 3(N atoms - 1). Because the number of bonds, angles, and dihedrals is normally much larger than 3(N atoms - 1), these internal coordinates are redundant. To reduce (but not eliminate) this redundancy, our protocol prunes dihedral types in a way that preserves symmetry equivalency. Next, each dihedral type is classified as non-rotatable, hindered, rotatable, or linear. We introduce a smart selection method that identifies which particular torsion modes are important for each rotatable dihedral type. Then, we computed the force constants for all flexibility interactions together via LASSO regression (i.e., regularized linear least-squares fitting) of the training dataset. LASSO automatically identifies and removes unimportant forcefield interactions. For each MOF, the reference dataset was quantum-mechanically-computed in VASP via DFT with dispersion and included: (i) finite-displacement calculations along every independent atom translation mode, (ii) geometries randomly sampled via ab initio molecular dynamics (AIMD), (iii) the optimized ground-state geometry using experimental lattice parameters, and (iv) rigid torsion scans for each rotatable dihedral type. After training, the flexibility model was validated across geometries that were not part of the training dataset. For each MOF, we computed the goodness of fit (R-squared value) and the root-mean-squared error (RMSE) separately for the training and validation datasets. We compared flexibility models with and without bond-bond cross terms. Even without cross terms, the model yielded R-squared values of 0.910 (avg across all MOFs) ± 0.018 (st. dev.) for atom-in-material forces in the validation datasets. Our SAVESTEPS protocol should find widespread applications to parameterize flexible forcefields for material datasets. We performed molecular dynamics simulations using these flexibility parameters to compute heat capacities and thermal expansion coefficients for two MOFs.

TrIP─Transformer Interatomic Potential Predicts Realistic Energy Surface Using Physical Bias.

Accurate interatomic energies and forces enable high-quality molecular dynamics simulations, torsion scans, potential energy surface mappings, and geometry optimizations. Machine learning algorithms have enabled rapid estimates of the energies and forces with high accuracy. Further development of machine learning algorithms holds promise for producing universal potentials that support many different atomic species. We present the Transformer Interatomic Potential (TrIP): a chemically sound potential based on the SE(3)-Transformer. TrIP's species-agnostic architecture, which uses continuous atomic representation and homogeneous graph convolutions, encourages parameter sharing between atomic species for more general representations of chemical environments, maintains a reasonable number of parameters, serves as a form of regularization, and is a step toward accurate universal interatomic potentials. TrIP achieves state-of-the-art accuracies on the COMP6 benchmark with an energy prediction of just 1.02 kcal/mol MAE. We introduce physical bias in the form of Ziegler-Biersack-Littmark-screened nuclear repulsion and constrained atomization energies. An energy scan of a water molecule demonstrates that these changes improve long- and near-range interactions compared to other neural network potentials. TrIP also demonstrates stability in molecular dynamics simulations, demonstrating reasonable exploration of Ramachandran space.

Intraoperative control by Schanz-screws is inaccurate to achieve the exact amount of correction in de-rotational osteotomies.

The accuracy of intraoperative control of correction commonly is achieved by K-wires or Schanz-screws in combination with goniometer in de-rotational osteotomies. The purpose of this study is to investigate the accuracy of intraoperative torsional control in de-rotational femoral and tibial osteotomies. It is hypothesized, that intraoperative control by Schanz-screws and goniometer in de-rotational osteotomies around the knee is a safe and well predictable method to control the surgical torsional correction intraoperatively. 55 consecutive osteotomies around the knee joint were registered, 28 femoral and 27 tibial. The indication for osteotomy was femoral or tibial torsional deformity with the clinical occurrence of patellofemoral maltracking or PFI. Pre- and postoperative torsions were measured according to the method of Waidelich on computed tomography (CT) scan. The scheduled value of torsional correction was defined by the surgeon preoperatively. Intraoperative control of torsional correction was achieved by 5mm-Schanz-screws and goniometer. The measured values of torsional CT scan were compared to the preoperative defined and intended values and deviation was calculated separately for femoral and tibial osteotomies. The surgeon's intraoperative measured mean value of correction in all osteotomies was 15.2° (SD 4.6; range 10-27), whereas the postoperatively measured mean value on CT scan was 15.6 (6.8; 5.0-28.5). Intraoperatively the femoral mean value measured 17.9° (4.9; 10-27) and 12.4° (1.9; 10-15) for the tibia. Postoperatively the mean value for femoral correction was 19.8 (5.5; 9.0-28.5) and 11.3 (5.0; 5.0-26.0) for tibial correction. When considering a deviation of plus or minus 3° to be acceptable femorally 15 osteotomies (53.6%) and tibially 14 osteotomies (51.9%) fell within these limits. Nine femoral cases (32.1.%) were overcorrected, four cases undercorrected (14.3%). Four tibial cases of overcorrection (14.8%) and 9 tibial cases of undercorrection (33.3%) were observed. However, the observed difference between femur and tibia regarding the distribution of cases between the three groups did not reach significance. Moreover, there was no correlation between the extent of correction and the deviation from the intended result. The use of Schanz-screws and goniometer in de-rotational osteotomies as an intraoperative control of correction is an inaccurate method. Every surgeon performing derotational osteotomies must consider this and include postoperative torsional measurement in his postoperative algorithm until new tools or devices are available to guarantee a better intraoperative accuracy of torsional correction. Observational study. III.

Decoupling Planarizing and Steric Energetics to Accurately Model the Rigidity of π-Conjugated Polymers.

The π-conjugated backbone of semiconducting polymers gives rise to both their electronic properties and structural rigidity. However, current computational methods for understanding the rigidity of polymer chains fail in one crucial way. Namely, standard torsional scan (TS) methods do not satisfactorily capture the behavior of polymers exhibiting a high degree of steric hindrance. This deficiency in part stems from the method by which torsional scans decouple energy related to electron delocalization from that related to nonbonded interactions. These methods do so by applying classical corrections of the nonbonded energy to the quantum mechanical (QM) torsional profile for polymers that are highly sterically hindered. These large corrections to the energy from nonbonded interactions can substantially skew the calculated QM energies related to torsion, resulting in an inaccurate or imprecise estimation of the rigidity of a polymer. As a consequence, simulations of the morphology of a highly sterically hindered polymer using the TS method can be highly inaccurate. Here, we describe an alternative, generalizable method by which the delocalization energy can be decoupled from the energy associated with nonbonded interactions─the "isolation of delocalization energy" (DE) method. From torsional energy calculations, we find that the relative accuracy of the DE method is similar to the TS method (within 1 kJ/mol) for two model polymers (P3HT, PTB7) when compared to quantum mechanical calculations. However, the DE method significantly increased the relative accuracy for simulations of PNDI-T, a highly sterically hindered polymer (8.16 kJ/mol). Likewise, we show that comparison of the planarization energy (i.e., backbone rigidity) from torsional parameters is significantly more precise for both PTB7 and PNDI-T when using the DE method as opposed to the TS method. These differences affect the simulated morphology, with the DE method predicting a significantly more planar configuration of PNDI-T.

Behavioral evaluation on the engineering properties of lignin-stabilized loess: Reuse of renewable materials

Lignin as an eco-friendly material is being studied in the field of soil modification. To examine its feasibility and performance in solidifying loess, this study carried out a series of laboratory tests on physical, chemical, and mechanical properties of lignin-stabilized loess, including Atterberg limits, standard Proctor compaction, unconfined compressive strength, dynamic triaxial torsional shear, pH, conductivity, X-ray diffraction, and scanning electron microscope test, taken into account the effects of curing time and freeze–thaw cycles, analyzed the solidification mechanism. The results showed that the Atterberg limits, maximum dry density, and optimum moisture content were not affected by the lignin content. The strength of stabilized loess showed increased first and then decreased with the growth of lignin content, and the maximum strength was achieved at the lignin content of 1.5%, strength increased by nearly 60%. The strength of the test sample decreased significantly under freeze–thaw cycles, but after 6 cycles, the strength tended to stabilize. The pH value showed a declining trend with the growth of lignin content, while the conductivity showed an opposite trend. The Attberg limits, pH, and conductivity of the stabilized loess were hardly affected by freeze–thaw action. In addition, the results of X-ray diffraction indicated that there was no new mineral composition generated in the stabilized loess.

Open Force Field BespokeFit: Automating Bespoke Torsion Parametrization at Scale.

The development of accurate transferable force fields is key to realizing the full potential of atomistic modeling in the study of biological processes such as protein-ligand binding for drug discovery. State-of-the-art transferable force fields, such as those produced by the Open Force Field Initiative, use modern software engineering and automation techniques to yield accuracy improvements. However, force field torsion parameters, which must account for many stereoelectronic and steric effects, are considered to be less transferable than other force field parameters and are therefore often targets for bespoke parametrization. Here, we present the Open Force Field QCSubmit and BespokeFit software packages that, when combined, facilitate the fitting of torsion parameters to quantum mechanical reference data at scale. We demonstrate the use of QCSubmit for simplifying the process of creating and archiving large numbers of quantum chemical calculations, by generating a dataset of 671 torsion scans for druglike fragments. We use BespokeFit to derive individual torsion parameters for each of these molecules, thereby reducing the root-mean-square error in the potential energy surface from 1.1 kcal/mol, using the original transferable force field, to 0.4 kcal/mol using the bespoke version. Furthermore, we employ the bespoke force fields to compute the relative binding free energies of a congeneric series of inhibitors of the TYK2 protein, and demonstrate further improvements in accuracy, compared to the base force field (MUE reduced from 0.560.390.77 to 0.420.280.59 kcal/mol and R2 correlation improved from 0.720.350.87 to 0.930.840.97).

Correction of tibial tubercle trochlea groove distance is related to torsional correction in high tibial derotational osteotomy.

High tibial osteotomy with internal tibial derotation (high tibial derotation osteotomy = HTDRO) is a common surgical treatment in patients with patellofemoral malalignment alone or in combination with patellofemoral instability. Operative techniques and theoretical calculations may assume that correction of the tibial tubercle-trochlear groove (TTTG) distance is related to the amount of torsional correction. The purpose of this investigation was to predict the change in TTTG distance in HTDRO through a clinical study. Twenty-one consecutive cases of derotational HTO were evaluated by torsional CT scanning in terms of the pre- and postoperative tibial torsion and TTTG distance. Changes in the TTTG distance were related to the changes in the amount of torsional correction. The change in patellar height was measured pre- and postoperatively, and the Caton-Deschamps Index (CDI) was calculated. The mean change in tibial torsion was 13.9°, and the mean change in the TTTG distance was 6.3mm. A strong relationship (0.90) between the change in torsion and the change in TTTG distance from pre- to postoperative status was found (p < 0.001). No statistically significant change in CDI could be seen between the preoperative [mean value (MV) 1.0] and postoperative (MV 1.1) periods. In patients with patellofemoral instability or patellofemoral maltracking with both a high tibial external torsion and a high TTTG distance, a derotational HTO can correct both bony deformities. Patella height does not change significantly with this surgical technique. With 1° of torsional correction, the TTTG distance decreases 0.45mm with our surgical technique of derotational HTO.

Microwave-assisted synthesis, characterizations, antimicrobial activities, and DFT studies on some pyridine derived Schiff bases

This study reports a joint experimental, theoretical and microbiological investigation on the (E)-N,N-dimethyl-4-((pyridine-2-ylmethylene)amino)aniline (5), (E)-N,N-dimethyl-4-((pyridine-4-ylmethylene)amino)aniline (6) and (E)-N,N-dimethyl-4-((pyridine-3-ylmethylene)amino)aniline (7). These compounds were synthesized with microwave method and their structures characterized by FT-IR, 1H-NMR, 13C-NMR, and elemental analysis tecniques. In the theoretical studies, torsional barriers analysis, ground state structure, Fourier Transform Infrared spectra (FT-IR), and Nuclear Magnetic Resonance spectra (NMR) of 5, 6, and, 7 were calculated by Density Functional Theory (DFT) computations. The conformers obtained from the torsional barrier scanning were optimized by B3LYP/6-31G(d,p) level. The harmonic vibrational frequencies, potential energy distribution (PED), infrared intensities, and NMR chemical shifts of the most stable conformers were determined using the B3LYP/6-311++G(d,p). Theoretically, predicted spectral data were compared with experimental results. Antimicrobial studies of the synthesized compounds were performed against various microbial strains. Antimicrobial activities of 5, 6, and, 7 were tested against selected bacteria and yeast through minimum inhibitory concentration (MIC) and diffusion method. Compound 7 was found to be the most active against bacteria and yeast, while compound 5 was found to be moderately active. Compounds 6 (against S. aureus and C. albicans) and, 7 were found to have a very high minimum inhibitory concentration, ranging between 1.95 and 7.81 g/mL (against P. aeruginosa and E. coli). Compounds (6 and 7) showed zone of inhibition values in the range of 10–20 mm against other bacteria except L. monocytogenes and S. thyphimurium.

TorsionNet: A Deep Neural Network to Rapidly Predict Small-Molecule Torsional Energy Profiles with the Accuracy of Quantum Mechanics.

Fast and accurate assessment of small-molecule dihedral energetics is crucial for molecular design and optimization in medicinal chemistry. Yet, accurate prediction of torsion energy profiles remains challenging as the current molecular mechanics (MM) methods are limited by insufficient coverage of drug-like chemical space and accurate quantum mechanical (QM) methods are too expensive. To address this limitation, we introduce TorsionNet, a deep neural network (DNN) model specifically developed to predict small-molecule torsion energy profiles with QM-level accuracy. We applied active learning to identify nearly 50k fragments (with elements H, C, N, O, F, S, and Cl) that maximized the coverage of our corporate compound library and leveraged massively parallel cloud computing resources for density functional theory (DFT) torsion scans of these fragments, generating a training data set of 1.2 million DFT energies. After training TorsionNet on this data set, we obtain a model that can rapidly predict the torsion energy profile of typical drug-like fragments with DFT-level accuracy. Importantly, our method also provides an uncertainty estimate for the predicted profiles without any additional calculations. In this report, we show that TorsionNet can accurately identify the preferred dihedral geometries observed in crystal structures. Our TorsionNet-based analysis of a diverse set of protein-ligand complexes with measured binding affinity shows a strong association between high ligand strain and low potency. We also present practical applications of TorsionNet that demonstrate how consideration of DNN-based strain energy leads to substantial improvement in existing lead discovery and design workflows. TorsionNet500, a benchmark data set comprising 500 chemically diverse fragments with DFT torsion profiles (12k MM- and DFT-optimized geometries and energies), has been created and is made publicly available.

Driving torsion scans with wavefront propagation.

The parameterization of torsional/dihedral angle potential energy terms is a crucial part of developing molecular mechanics force fields. Quantum mechanical (QM) methods are often used to provide samples of the potential energy surface (PES) for fitting the empirical parameters in these force field terms. To ensure that the sampled molecular configurations are thermodynamically feasible, constrained QM geometry optimizations are typically carried out, which relax the orthogonal degrees of freedom while fixing the target torsion angle(s) on a grid of values. However, the quality of results and computational cost are affected by various factors on a non-trivial PES, such as dependence on the chosen scan direction and the lack of efficient approaches to integrate results started from multiple initial guesses. In this paper, we propose a systematic and versatile workflow called TorsionDrive to generate energy-minimized structures on a grid of torsion constraints by means of a recursive wavefront propagation algorithm, which resolves the deficiencies of conventional scanning approaches and generates higher quality QM data for force field development. The capabilities of our method are presented for multi-dimensional scans and multiple initial guess structures, and an integration with the MolSSI QCArchive distributed computing ecosystem is described. The method is implemented in an open-source software package that is compatible with many QM software packages and energy minimization codes.

Trigonostemons G and H, dinorditerpenoid dimers with axially chiral biaryl linkage from Trigonostemon chinensis.

Trigonostemons G and H, two novel dimeric dinorditerpenoids, were isolated from the stem barks of Trigonostemon chinensis. Their planar structures and relative configurations were established by extensive analysis of spectroscopic data. Trigonostemons G and H possess a homodimeric biaryl skeleton obtained from two rearranged chiral nonracemic abietane-type dinorditerpenes through an axially chiral biaryl 11,11'-linkage. Torsional scan and computation of the transition states were carried out to estimate the rotational energy barrier, and the axial chirality (aS) was determined by time-dependent density functional theory (TDDFT) electronic circular dichroism (ECD) calculations. The positive n-π* ECD transitions of the isolated carbonyl chromophore above 300 nm could be used to determine the central chirality of trigonostemon G independently by ECD calculations of the diastereomers.

Towards the Analogy of Electrostatic and Electromagnetic Transducers

Electromechanical, or mechatronic, transducers exchange energy between the mechanical and electrical domain via electrostatic and electromagnetic transduction principles, which are the basis for actuators and sensors. This paper reveals the simple analogy between electrostatic and electromagnetic transducers. Summarizing the well-known Gauß, Ampère’s, Ohm’s, Chua’s, Newton’s, Hook’s and the damping laws into a signal-flow diagram with linearized coefficients, the model shows the transducers reciprocity and physical behavior of damped spring-mass mechatronic systems. The presented modeling approach simplifies the derivation of transfer functions, the pull-in phenomenon, and the coupling factors for the design of feedback methods for mechatronic systems. The analogies are shown by comparing an electrostatic and an electromagnetic parallel-plate actuator, represented by a torsional MEMS scanning mirror and a hybrid reluctance fast steering mirror, respectively. The paper discusses the actuators performance and compares the transducer coefficients and the intrinsic stiffness regarding pull-in.

A CD probe with a tailored cantilever for 3D-AFM measurement

A critical dimensional (CD) probe, such as a boot shaped probe, is widely used for measuring the sidewall profiles of nanostructures in atomic force microscopy (AFM). However, conventional CD probes usually have high spring constants that restrict their ability to operate in critical dimension scanning modes. Furthermore, the flexure spring constant of the tip is much smaller than the torsion spring constant of the cantilever, which probably makes the sidewall measurement unstable. Here, we proposed a method of tailoring the probe where the cantilever was trimmed to reduce the spring constant. The simulation results, using a finite element method (FEM), illustrated that both the flexure and torsion spring constants decreased nearly ten times. Meanwhile, the torsional resonant frequency decreased to below 1 MHz, which could benefit the torsional oscillation scanning. By using a focused ion beam (FIB), a CD probe with a rectangular cantilever was reprocessed. It was mounted to a home-built three-dimensional AFM system (3D-AFM), and had the flexure and torsion sensitivities precisely calibrated. The ratio of the flexure sensitivity to torsion sensitivity showed good accordance with the simulation results. Additionally, a grating standard sample was measured using the tailored probe under a modified step-in scanning mode, and the error due to torsional deformation of the tip was compensated. The measurement results demonstrated the validity of the tailored CD probe in 3D metrology.

Atropisomerism by Design: Discovery of a Selective and Stable Phosphoinositide 3-Kinase (PI3K) β Inhibitor.

Atropisomerism is a type of axial chirality in which enantiomers or diastereoisomers arise due to hindered rotation around a bond axis. In this manuscript, we report a case in which torsional scan studies guided the thoughtful creation of a restricted axis of rotation between two heteroaromatic systems of a phosphoinositide 3-kinase (PI3K) β inhibitor, generating a pair of atropisomeric compounds with significantly different pharmacological and pharmacokinetic profiles. Emblematic of these differences, the metabolism of inactive ( M)-28 is primarily due to the cytosolic enzyme aldehyde oxidase, while active ( P)-28 has lower affinity for aldehyde oxidase, resulting in substantially better metabolic stability. Additionally, we report torsional scan and experimental studies used to determine the barriers of rotation of this novel PI3Kβ inhibitor.

The Retinal Energy Landscape as a Function of the Rhodopsin Photocycle

Retinal is the covalently bound inverse-agonist of the prototypical G protein-coupled receptor, rhodopsin. It comprises a β-ionone ring and polyene chain covalently bound to Lys296 of rhodopsin by a protonated Schiff base (PSB). During the course of rhodopsin activation, retinal initially undergoes an 11-cis → all-trans isomerization, followed by a deprotonation of the Schiff base. Using quantum chemical calculations at the MP2 level of theory and solid state NMR spectroscopy we demonstrate substantial differences in retinal structure and dynamics between the protonated and deprotonated species. The delocalization of positive charge from the PSB results in perturbations of the entire retinal moiety upon deprotonation. For example, methyl rotation barriers are shifted as much as 200% [1]. Surprisingly, deprotonation of retinal drastically affects the energetics of β-ionone ring rotation, producing an extra minimum in the C5=C6-C7=C8 torsional energy surface. This results in a proton affinity (PA), and hence pKa, that depends on β-ionone ring orientation. Specifically, the PA of retinal is lowered for non-planar conformations of the β-ionone ring, in turn lowering the pKa and facilitating the deprotonation required for formation of the Meta I pre-activated state of rhodopsin. In order to extend these calculations to account for interactions within the rhodopsin binding pocket we have used the QM data, including MP2 level torsion scans of every dihedral angle, to refine new retinal force fields for both protonation. This data opens the door to future molecular dynamics studies of retinal proteins that include the activated Meta II state. [1] B. Mertz et al. (2011) Biophys. J. 101: L17.

Modeling of an electrostatic torsional micromirror for laser projection system

This paper presents the Simulink™ model of a new 2D torsional scanning micromirror. This Micro-Opto-Electro-Mechanical-System (MOEMS) is very important towards the integration into a system on a package of a complete laser projection system. Modeling and simulation of the MOEMS is a key point for the development of the proper micromirror electronic conditioning interface thus reducing time to market and production costs. In literature there are not exhaustive examples of complete characterizations of 2D torsional scanning micromirrors. The aim of this work is to develop a Simulink™ model which incorporates the main mechanical and electrostatic parameters of the sensor such as the resonance frequency, the torsional constant and capacitance versus rotation angle characteristic. This model has been successfully verified via experimental measurements and it proved his effectiveness in the development of the relevant electronic conditioning circuitry.

Closing the side-chain gap in protein loop modeling

The success of structure-based drug design relies on accurate protein modeling where one of the key issues is the modeling and refinement of loops. This study takes a critical look at modeled loops, determining the effect of re-sampling side-chains after the loop conformation has been generated. The results are evaluated in terms of backbone and side-chain conformations with respect to the native loop. While models can contain loops with high quality backbone conformations, the side-chain orientations could be poor, and therefore unsuitable for ligand docking and structure-based design. In this study, we report on the ability to model loop side-chains accurately using a variety of commercially available algorithms that include rotamer libraries, systematic torsion scans and knowledge-based methods.

Changes in the Isotropic Shielding of the17O Nucleus upon Torsion in Terminal Oxygen Systems: A Computational Study on Their Origin

We are presenting a computational study on the isotropic shielding, charge, and orbital contributions to the shielding of oxygen in benzaldehydes (Ar-CHO), nitrobenzenes (Ar-NO2), phenyl isocyanates (Ar-NCO), anilides (Ar-NHCOCH3), and N-sulfinylamines (Ar-NSO). In particular, changes upon ortho substitution of the aromatic ring and upon torsion of the unsubstituted parent molecules are examined. The experimentally observed changes in (17)O chemical shift, be they upfield or downfield, upon substitution by ortho-alkyl groups are reproduced well by the calculations. Relaxed torsional scans of the parent systems reveal that (a) charges change as expected from resonance arguments and (b) changes in isotropic shielding are monotonic and in line with changes upon substitution, with N-sulfinylaniline as an exception. In general, the changes in isotropic shieldings are explained in terms of changes in molecular orbitals, their energies, and relative alignments, whose mixing is magnetically active. Thus, for example, the observed deshielding of (17)O upon methyl substitution and upon torsion of benzaldehyde is mainly caused by a contribution from the pi-type oxygen lone pair, yet how these contributions change is fundamentally different. As a consequence, the experimentally observed downfield shift upon methyl substitution cannot be interpreted to imply a change in torsion angle between the phenyl ring and the aldehyde group. For N-sulfinylaniline, the consecutive downfield shifts upon methyl and tert-butyl substitution and the associated changes in torsion angle are in contrast to the 45 degrees maximum in isotropic shielding that is determined from a relaxed torsional scan.

Curing Behaviour of Epoxy Resin with a Diamine Containing Heterocyclic Rings

The curing behaviour of a new epoxy curing agent, 2, 6-di (p-aminophenyl) benzo[1.2.5.4] bisoxazole (DIABO) was discussed. Diglycidyl-4, 5-epoxycyclohexane -1,2-dicarboxylate (TDE) was cured with a mixture of DIABO and diaminodiphenylsulfone (DDS). The aspects investigated including the curing procedure, reaction kinetics parameters and the heat resistance of the product. Average apparent activation energy of the resin's curing reaction was 63.55 kJ/mol, calculated according to the Kissinger and the Ozawa methods. The glass transition temperature was 261 °C and 257 °C determined by torsional braid analysis (TBA) and differential scanning calorimetry (DSC) respectively. The integral procedural decomposition temperature (IPDT) was 814 °C, respectively. Char residue was 44.58% at 572 °C. Compared to cure with TDE/DDS, these properties were improved by 18%, 8%, 30% and 50%, respectively.