Rotational Energy Barriers Research Articles

New Insights on the Vibrational Dynamics of 2-Methoxy-, 4-Methoxy- and 4-Ethoxy-Benzaldehyde from INS Spectra and Periodic DFT Calculations.

The dynamics of 2-methoxybenzaldehyde, 4-methoxybenzaldehyde, and 4-ethoxybenzaldehyde in the solid state are assessed through INS spectroscopy combined with periodic DFT calculations. In the absence of experimental data for 4-ethoxybenzaldehyde, a tentative crystal structure, based on its similarity with 4-methoxybenzaldehyde, is considered and evaluated. The excellent agreement between calculated and experimental spectra allows a confident assignment of the vibrational modes. Several spectral features in the INS spectra are unambiguously assigned and torsional potential barriers for the methyl groups are derived from experimental frequencies. The intramolecular nature of the potential energy barrier for methyl rotation about O–CH3 bonds compares with the one reported for torsion about saturated C–CH3 bonds. On the other hand, the intermolecular contribution to the potential energy barrier may represent 1/3 of the barrier height in these systems.

Nuclear magnetic resonance free ligand conformations and atomic resolution dynamics.

Knowledge of free ligand conformational preferences (energy minima) and conformational dynamics (rotational energy barriers) of small molecules in solution can guide drug design hypotheses and help rank ideas to bias syntheses towards more active compounds. Visualization of conformational exchange dynamics around torsion angles, by replica exchange with solute tempering molecular dynamics (REST-MD), gives results in agreement with high-resolution H nuclear magnetic resonance (NMR) spectra and complements free ligand conformational analyses. Rotational energy barriers around individual bonds are comparable between calculated and experimental values, making the in-silico method relevant to ranking prospective design ideas in drug discovery programs, particularly across a series of analogs. Prioritizing design ideas, based on calculations and analysis of measurements across a series, efficiently guides rational discovery towards the "right molecules" for effective medicines.

Ferroelectric-relaxor boundary in La-modified Pb(Mg1/3Nb2/3)O3-xPbTiO3 crossover showing enhanced dielectric and piezoelectric properties

The dielectric and piezoelectric properties of La-modified Pb(Mg1/3Nb2/3)O3-xPbTiO3 crossover ceramics were systematically studied. In this crossover region, there is a ferroelectric-relaxor boundary separating the ferroelectric and relaxor states, which is determined by the spontaneous transition from relaxor to ferroelectric state with decreasing temperature. At the ferroelectric-relaxor boundary, the maximum permittivity increases from 18,200 to 23,500, and piezoelectric coefficient (d33) increases from 300 pC/N to 450 pC/N accompanied by the electrostrain improved from 0.09% to 0.12%, as compared with adjacent ferroelectric specimen. These enhancements may originate from the low energy barrier for polarization rotation induced by the existence of spontaneous transition and the coexistence of macrodomain and polar nanodomain configuration. This work indicates that the ferroelectric-relaxor boundary in crossover region may become an effective approach to enhance the dielectric and piezoelectric properties of ferroelectric materials.

Free H‐Bonding Interaction Sites in Rigid‐Chain Polymers and Their Filling Approach: A Molecular Dynamics Simulation Study

AbstractHydrogen bond (H‐bond) plays an important role in structure evolution and properties of various polymers. Due to directivity and saturation of H‐bond, its formation is influenced by macromolecular chain conformation but the relationship is still not well understood up to now. In this paper, amorphous models of polyamide 66 (PA66), polyamide 6T (PA6T), and poly(p‐phenylene‐terephthamide) (PPTA) are built to study influence of chain rigidity on H‐bonding formation by utilizing Molecular Dynamics simulation. Compared with PA66 and PA6T, it is found that chain rigidity of PPTA is more remarkable and corresponding conformation adjustment is hindered more significantly, which leads to considerable weak H‐bonds and free H‐bonding sites. After introducing benzimidazole moiety into PPTA, more H‐bonds form due to more potential H‐bonding sites in benzimidazole units. However, the newly added H‐bonding donor and acceptor are in the co‐ring, and there is a high rotational energy barrier for benzimidazole ring, both of which hinder the formation of strong H‐bonds. Therefore, a strategy is proposed to fill free H‐bonding sites in rigid‐chain polymers through adding oligomers with high movement ability. Thus, the H‐bonds in rigid‐chain polymers are effectively enhanced due to bridging effect of oligomers and tensile modulus of the aramid is improved significantly.

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

The Control of Intramolecular Through-Bond and Through-Space Coupling in Single-Molecule Junctions

Target Designing Phase Transition Materials through Halogen Substitution.

Crystalline materials have received extensive attention due to their extraordinary physical and chemical properties. Among them, phase transition materials have attracted great attention in the fields of photovoltaic, switchable dielectric devices, and ferroelectric memories, etc. However, many of them suffer from low phase transition temperatures, which limits their practical application. In this work, we systematically designed crystalline materials, (TMXM)2 PtCl6 (X=F, Cl, Br, I) through halogen substitution on the cations, aiming to improving phase transition temperature. The resulting phase transition of (TMXM)2 PtCl6 (X=F, Cl, Br, I) get a significant enhancement, compared to the parent compound [(CH3 )4 N]2 PtCl6 ((TM)2 PtCl6 ). Such phase transition temperature enhancement can be attributed to the introduction of halogen atoms that increase the potential energy barrier of the cation rotation. In addition, (TMBM)2 PtCl6 and (TMIM)2 PtCl6 have a low symmetry and crystallize in the space group C2 /c and P21 21 21 , respectively. This work highlights the halogen substitution in designing crystal materials with high phase transition temperature.

Illuminating the dark conformational space of macrocycles using dominant rotors.

Three-dimensional conformation is the primary determinant of molecular properties. The thermal energy available at room temperature typically equilibrates the accessible conformational states. Here, we introduce a method for isolating unique and previously understudied conformations of macrocycles. The observation of unusual conformations of 16- to 22-membered rings has been made possible by controlling their interconversion using dominant rotors, which represent tunable atropisomeric constituents with relatively high rotational barriers. Density functional theory and in situ NMR measurements suggest that dominant rotor candidates for the amino-acid-based structures considered here should possess a rotational energy barrier of at least 25 kcal mol-1. Notable differences in the geometries of the macrocycle conformations were identified by NMR spectroscopy and X-ray crystallography. There is evidence that amino acid residues can be forced into rare turn motifs not observed in the corresponding linear counterparts and homodetic rings. These findings should unlock new avenues for studying the conformation-activity relationships of bioactive molecules.

Molecular dynamics investigation of the structural flexibility of H2O2 and H2S2 in response to medium polarity

Hydrogen peroxide (H2O2) and hydrogen persulfide (H2S2) are the simplest members of the peroxide and persulfide families, and both are signaling molecules that regulate several biological functions. Rotation about the O–O and S–S bonds impacts the strength of these molecules as H-bond donors or acceptors and tunes their polarity and stability by converting the polar equilibrium skewed structure (ϕHOOH ~ 112°, ϕHSSH ~ 91°) to the nonpolar less stable trans (ϕ = 180°) or the most polar and least stable cis (ϕ = 0°) conformers. The change in geometry in response to the structure and polarity of the microenvironment is an important feature that would influence how these molecules interact with or permeate through biological systems. Consistent with larger energy barriers for rotation about the S–S bond, quantum chemical calculations with MP2(full)/6–311++G(3df,3pd) reveal that when interacting with a single water, ethanol, or n-pentane molecule in the gas phase, the geometry of H2S2 is more rigid (ϕHSSH = 91 ± 3°) than that of H2O2 (ϕHOOH = 112 ± 8°). Molecular dynamics simulations of a single H2O2 or H2S2 in liquid water (ε = 80), ethanol (ε = 25), or n-pentane (ε = 2) reveal that, compared to the gaseous monomer, H2O2 maintains a more polar geometry in ethanol and water. Interestingly, the distribution of ϕHOOH significantly shifts towards the nonpolar conformation as solvent polarity decreases; the fraction of structures with ϕHOOH ≥ 160° is 0.10, 0.14, and 4.30% in water, ethanol, and n-pentane, respectively. The high-energy cis conformation of H2O2 was negligible in all solvents, especially in n-pentane. The equilibrium geometry of H2S2 is hardly influenced by solvent polarity and both cis and trans conformations are essentially absent in the three solvents. In contrast to rigid H2S2, H2O2 is thus a flexible molecule whose geometry depends on the environment polarity, a property that enables stabilizing H2O2 in different microenvironments and possibly plays a role in how biological systems discriminate between H2O2 and the rigid and polar close analog, H2O.

Molecular polarizabilities of some energetic compounds.

The dependence of sensitivity of an explosive on its molecular structure may be mainly attributed to the molecular deformability, which can be expressed by some characteristic parameters, resonance energy for aromatic an explosive, strain energy for a strained-ring or strained-cage explosive, large π-π separation energy for a large π-π linked-explosive, bond rotational energy barriers of C-NO2, N-NO2, O-NO2 for C-NO2, N-NO2, O-NO2 bond-based explosives, and so on. Molecular polarizability of an explosive is also an important molecular deformability index, which can be effectively used to compare impact sensitivities of explosive's isomers, isoelectronic species, and similar structures. Interestingly, comparing the molecular polarizabilities under external electric fields with different energy levels of isomeric N20(Ih) and N20(D3d) clusters and the Mo2N20 and Re2N20 complex compounds, it is found that there are different energy thresholds of significant molecular expansion.

Computational development and validation of a representative MDI-BDO-based polyurethane hard segment model.

Segmented polyurethanes show extraordinary physicochemical properties, mainly owing to the nature and the chemistry of the hard segment domains. There are yet many inexplicable physiochemical properties of MDI-BDO-based hard polyurethane segments such as the geometry, cis-trans isomerism, electronic structure, chemical reactivity, the inter-hard-segment interactions, and the photo-response. In the present study, it was attempted to develop and validate a model system that would facilitate further research on the structural and chemical properties of the MDI-BDO hard segments. It was found that the trans isomer of urethane bond is more stable than the cis isomer, and it is argued here that thermal transformation from trans to cis not possible due to the high rotational energy barrier. The differences between the calculated IR spectra of the cis and trans isomers are proposed as a powerful differentiation tool. The calculated Fukui indices show that cis and trans isomers are different in their chemical reactivity. The findings of the present study suggest intermolecular and intramolecular pi-stacking and highly plausible two significant types of hydrogen bond types between hard segments. In the present study, a model system for MDI-BDO hard segment was developed and successfully validated via computational experiments. Further calculations done with the new model provided an indispensable understanding of the structure, cis-trans isomerism, reactivity, and intermolecular interactions of the MDI-BDO hard segments. The proposed model can be further improved in the future by incorporating suitable soft segments. In summary, the model system developed and validated in the present study has provided new opportunities to understand and further study the structural and chemical features of the hard segments of the MDI-BDO-based polyurethane.

Calcination process and kinetic carbonation effect on the hydrated and anhydrate phases of the OPC matrix at early age of hydration

ABSTRACT This study aims to improve the understanding of the effect of both calcination temperature and the carbonation process on cement pastes by analyzing the Fourier-transform infrared spectroscopy (FTIR) principal vibration modes. In this regard, six samples of cement pastes, identified as (CS1, CS2, CS3, CS4, CS5 and CS6), mixed with white sugar with different concentrations (X = 0, 0.2, 0.4, 0.6, 0.8, and 1.1 %), respectively, were prepared. These blended cement samples were heated (calcined) at a temperature of about 600°C. Moreover, an additional sugar-free sample tagged as CS0 was prepared and cured at room temperature for 7 days. Afterward, these samples were subjected to FTIR spectroscopy and X-ray diffraction to investigate the effect of the calcination and the carbonation process on the principal mode of vibrations and the microstructure of the ordinary Portland cement (OPC) matrix as well. The characteristics of the FTIR principal modes of vibrations were investigated. Besides, the relaxation time and the rotational energy barrier were calculated for different sugar concentrations. This study revealed that the kinetic reaction of the carbon contained in the different samples with the C3S, C2S, C–S–H and CH phases leads to the formation of calcium carbonate CaCO3, where the rate of formation depends upon the carbonation rate of samples. The critical variation for the relaxation time and rotational energy barrier clearly appeared at X = 0.6%. Moreover, the kinetic carbonation of both clinker phases and the hydrated phases in the OPC matrix system mainly depends upon the rate of the polymerization of the silicate group inside the matrix of OPC.

Synthetic, spectroscopic, structural, and electrochemical investigations of ferricenium derivatives with weakly coordinating anions: ion pairing, substituent, and solvent effects.

A facile and effective strategy for the preparation of a series of ferricenium complexes bearing either electron-donating or electron-withdrawing substituents with weakly coordinating anions such as [B(C6F5)4]- or SbF6- is reported. These systems were thoroughly investigated for their ground state electronic structures in both solution and solid states using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies as well as single crystal X-ray crystallography and electrochemical measurements. The X-ray structures of the six electron-deficient ferricenium derivatives are of particular interest as only a handful (∼5) of such derivatives have been structurally characterized to date. Comparison of the structural data for both neutral and oxidized derivatives reveals that the nature of the substituents on the cyclopentadienyl (Cp) ligands displays a more significant impact on the metal-ligand separations (FeCt) in the oxidized species than in their neutral analogs. Our 1H-NMR measurements corroborate that in the neutral ferrocene derivatives, electron-donating ring substitutions lead to a greater shielding of the ring protons while electron-withdrawing groups via induction deshield the nearby ring protons. However, the data for the paramagnetic ferricenium derivatives reveals that this substitutional behavior is more complex and fundamentally reversed, which is further supported by our structural studies. We ascribe this reversal of behavior in the ferricenium derivatives to the δ back-donation from the iron atom into the Cp rings which can lead to the overall shielding of the ring protons. Interestingly, our NMR results for the electron-deficient ferricenium derivatives in solution also indicate a direct correlation between the solvent dielectric constant and the energy barrier for rotation around the metal-ligand bond in these systems, whereas such a correlation is absent or not significant in the case of the electron-rich ferricenium species or the corresponding neutral ferrocene analogs. In this work, we also present the electrochemical behavior of the corresponding ferricenium/ferrocene redox couples including potential values (E1/2), peak-to-peak separation (ΔE1/2), and diffusion coefficients (D) of the redox active species in order to provide a concise outline of these data in one place. Our electrochemical studies involved three different solvents and two supporting electrolytes. Notably, our findings point to the significant effect of ion-pairing in lowering the energy necessary for reduction of the ferricenium ion and E1/2 in lower-polarity media. This has significant implications in applications of the ferrocene or ferricenium derivatives as redox agents in low-polarity solvents where an accurate determination of redox potential is critical.

Variable-temperature <sup>1</sup>H-NMR Studies on Three C-glycosylflavones Exhibiting Rotational Isomerism

Rotational isomerism has been observed on some C-glycosyl flavonoids with special chemical strutures. It may bring rotamers arising from restricted rotation about one single bond. C-glycosyl flavonoids showed a wide range of biological activities. This emphasizes the significance to understand the conformational stability of C-glycosyl flavonoids in drug development. Three C-glycosyl flavonoids, vaccarin (1), isovitexin-2”-O-arabinoside (2) and spinosin (3), were found to exist as rotamers in the light of their featured behavior in nuclear magnetic resonance spectroscopy (NMR). Some separated signals were observed in their ¹H-NMR and ¹³C NMR spectra. Variable-temperature (VT) ¹H-NMR experiments were then conducted to study the observation and to determine their rotational energy barriers and then evaluate their conformational stability. The temperature varied from 298K to 363K and separated signals merged at high temperature. With the help of Eyring equation, the rotational energy barriers of compounds 1-3 were calculated and found much lower than 24 kcal/mol. When the energy barrier is high enough (> 24 kcal/mol) and the time scale for interconversion (halflife > 1000 s) is long enough for isolation of individual rotamers, the isomers are termed atropisomers, which thus will display axial charity. According to this standard, the compounds 1-3 may not exhibit atropisomerism and isolation of rotamers should be difficult. Their relatively low energy barriers indicate they can be developed as rapidly equilibrating mixture.

Encapsulating N-Heterocyclic Carbene Binuclear Transition-Metal Complexes as a New Platform for Molecular Rotation in Crystalline Solid-State.

In crystalline solids, molecules generally have limited mobility due to their densely packed environment. However, structural information at the molecular level may be used to design amphidynamic crystals with rotating elements linked to rigid, lattice-forming parts, which may lead to molecular rotary motions and changes in conformation that determine the physical properties of the solid-state materials. Here, we report a novel design of emissive crystalline molecular rotors with a central pyrazine rotator connected by implanted transition metals (Cu or Au) to a readily accessible enclosure formed by two N-heterocyclic carbenes (NHC) in discrete binuclear complexes. The activation energies for the rotation could be tuned by changing the implanted metal. Exchanging Cu to Au resulted in an ∼4.0 kcal/mol reduction in the rotational energy barrier as a result of lower steric demand by elongation of the axle with the noble metal, and a stronger electronic stabilization in the rotational transition state by enhancement of the d-π* interactions between the metal centers and the pyrazine rotator. The Cu(I) rotor complex showed a greater electronic delocalization than the Au(I) rotor complex, causing a red-shifted solid-state emission. Molecular rotation-induced emission quenching was observed in both crystals. The enclosing NHC rotors are easy to prepare, and their rotational motion should be less dependent on packing structures, which are often crucial for many previously documented amphidynamic molecular crystals. The platform from the encapsulating NHC cationic metal complexes and the metal-centered rotation-axis provide a promising scaffold for a novel design of crystalline molecular rotors, including manipulation of rotary dynamics and solid-state emission.

Design and development of high-power piezoelectric ceramics through integration of crystallographic texturing and acceptor-doping

This study demonstrates the integrated approach based upon texturing and acceptor doping for realizing a high-power piezoelectric ceramic with combined soft and hard properties. The textured Mn-doped 0.24 Pb(In1/2Nb1/2)O3-0.42 Pb(Mg1/3Nb2/3)O3-0.34 PbTiO3 (PIN-PMN-PT) ceramic exhibits enhanced piezoelectric coefficient d33 and electromechanical coupling factor k31 in comparison with random counterpart. This enhanced piezoelectric response originates from the combined intrinsic high piezoelectric properties of <001>-oriented grains, and reduced energy barrier for polarization rotation in textured ceramics. The BaTiO3 (BT) template in textured ceramics increases the tetragonality degree which results in improved coercive field Ec but decreased mechanical quality factor Qm in comparison with random counterpart. The decreased Qm values of textured ceramics are related to the crystallographic dependence of Qm and the enhanced domain mobility due to the existence of small size domains. The textured ceramic with 2 vol.% BT content exhibited an excellent combination of soft and hard piezoelectric properties, measured to be: d33 = 517 pC/N, Qm = 1147, Ec = 10.0 kV/cm, and tan δ = 0.49%, which is highly promising for high power piezoelectric applications.

Relaxor behaviors enhance piezoelectricity in lead-free BiFeO3-BaTiO3 ceramics incorporated with a tiny amount of Bi(Mg1/2Ti1/2)O3 near the morphotropic phase boundary

The relaxor behaviors near the morphotropic phase boundary (MPB) can reduce the energy barrier for polarization rotation, which is an effective strategy to improve the piezoelectric properties of materials. In this work, the phase structure, microstructure, and piezoelectric properties of (0.69-x)Bi1.02FeO3-0.31BaTiO3-xBi(Mg1/2Ti1/2)O3 (0.00 ≤ x ≤ 0.08) ceramics were systematically studied. The addition of a tiny amount of Bi(Mg1/2Ti1/2)O3 was found to cause the phase to transform from R-phase-dominant ferroelectrics (FEs) to PC-phase-dominant relaxor ferroelectrics (RFEs). The dielectric and electrical data reveal that nonergodic relaxor ferroelectrics (NE-RFEs) near the MPB were favorable for domain switching, which contributed to the enhanced piezoelectric and electrostrain properties of d33 = 182 pC/N, TC = 475 °C, and S = 0.21% at x = 0.04. Finally, a model of domain growth induced by an electric field was proposed for FEs and NE-RFEs.

Molecular rotors with designed polar rotating groups possess mechanics-controllable wide-range rotational speed

Molecular rotors with controllable functions are promising for molecular machines and electronic devices. Especially, fast rotation in molecular rotor enables switchable molecular conformations and charge transport states for electronic applications. However, the key to molecular rotor-based electronic devices comes down to a trade-off between fast rotational speed and thermal stability. Fast rotation in molecular rotor requires a small energy barrier height, which disables its controllability under thermal excitation at room temperature. To overcome this trade-off dilemma, we design molecular rotors with co-axial polar rotating groups to achieve wide-range mechanically controllable rotational speed. The interplay between polar rotating groups and directional mechanical load enables a “stop-go” system with a wide-range rotational energy barrier. We show through density functional calculations that directional mechanical load can modulate the rotational speed of designed molecular rotors. At a temperature of 300 K, these molecular rotors operate at low rotational speed in native state and accelerates tremendously (up to 1019) under mechanical load.

Microscopic Mechanism of Van der Waals Heteroepitaxy in the Formation of MoS2/hBN Vertical Heterostructures.

Recent studies have revealed that van der Waals (vdW) heteroepitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to the formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes a strong limitation on the crystal orientation of grains is still not understood yet. In this work, we have focused on investigating the microscopic mechanism of the self-alignment of MoS2 grains in vdW epitaxial growth on hBN. Using the density functional theory and the Lennard–Jones potential, we found that the interlayer energy between MoS2 and hBN strongly depends on the size and crystal orientation of MoS2. We also found that, when the size of MoS2 is several tens of nanometers, the rotational energy barrier can exceed ∼1 eV, which should suppress rotation to align the crystal orientation of MoS2 even at the growth temperature.

A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...

Hydration-Induced Disorder Lowers the Energy Barriers for Methyl Rotation in Drug Molecules.

The thermally activated dynamics of methyl groups are important for biochemical activity as they allow for a more efficient sampling of the energy landscape. Here, we compare methyl rotations in the dry and variously hydrated states of three primary drugs under consideration to treat the recent coronavirus disease (COVID-19), namely, hydroxychloroquine and its sulfate, dexamethasone and its sodium diphosphate, and remdesivir. We find that the main driving force behind the considerable reduction in the activation energy for methyl rotations in the hydrated state is the hydration-induced disorder in the methyl group local environments. Furthermore, the activation energy for methyl rotations in the hydration-induced disordered state is much lower than that in an isolated drug molecule, indicating that neither isolated molecules nor periodic crystalline structures can be used to analyze the potential landscape governing the side group dynamics in drug molecules. Instead, only the explicitly considered disordered structures can provide insight.