Weak Attractive Forces Research Articles

Regulation of Peptide Liquid-Liquid Phase Separation by Aromatic Amino Acid Composition.

Membraneless organelles are cellular biomolecular condensates that are formed by liquid-liquid phase separation (LLPS) of proteins and nucleic acids. LLPS is driven by multiple weak attractive forces, including intermolecular interactions mediated by aromatic amino acids. Considering the contribution of π-electron bearing side chains to protein-RNA LLPS, systematically study sought to how the composition of aromatic amino acids affects the formation of heterotypic condensates and their physical properties. For this, a library of minimalistic peptide building blocks is designed containing varying number and compositions of aromatic amino acids. It is shown that the number of aromatics in the peptide sequence affect LLPS propensity, material properties and (bio)chemical stability of peptide/RNA heterotypic condensates. The findings shed light on the contribution of aromatics' composition to the formation of heterotypic condensates. These insights can be applied for regulation of condensate material properties and improvement of their (bio)chemical stability, for various biomedical and biotechnological applications.

Mixed Nanosphere Assemblies at a Liquid-Liquid Interface.

The in-plane packing of gold (Au), polystyrene (PS), and silica (SiO2) spherical nanoparticle (NP) mixtures at a water-oil interface is investigated in situ by UV-vis reflection spectroscopy. All NPs are functionalized with carboxylic acid such that they strongly interact with amine-functionalized ligands dissolved in an immiscible oil phase at the fluid interface. This interaction markedly increases the binding energy of these nanoparticle surfactants (NPSs). The separation distance between the Au NPSs and Au surface coverage are measured by the maximum plasmonic wavelength (λmax) and integrated intensities as the assemblies saturate for different concentrations of non-plasmonic (PS/SiO2) NPs. As the PS/SiO2 content increases, the time to reach intimate Au NP contact also increases, resulting from their hindered mobility. λmax changes within the first few minutes of adsorption due to weak attractive inter-NP forces. Additionally, a sharper peak in the reflection spectrum at NP saturation reveals tighter Au NP packing for assemblies with intermediate non-plasmonic NP content. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) measurements confirm a decrease in Au NP domain size for mixtures with larger non-plasmonic NP content. The results demonstrate a simple means to probe interfacial phase separation behavior using in situ spectroscopy as interfacial structures densify into jammed, phase-separated NP films.

ENHANCED AQUEOUS SOLUBILITY AND IN VITRO DISSOLUTION OF THE ANTI-HYPERLIPIDEMIC AGENT USING SYNTHESIZED SOLID DISPERSION CARRIER

Objective: To improve ATN's solubility, permeability, and dissolution rate of pentaerythritol-eudragit®RS100 co-processed excipients (CE) and their potential as a solid dispersion carrier (ATN-CE-SD). Methods: The ATN-CE-SD was prepared using the solvent evaporation technique. The pure ATN, physical mixture, CE carrier, and optimized ATN-CE-SD was physicochemically characterized using Scanning electron microscopy, Fourier transforms infrared spectroscopy, differential scanning calorimetry, powder x-ray diffractometry, solubility, and in vitro dissolution was used to evaluate solid dispersions. Results: Physical and chemical analysis showed that ATN-CE-SD formed via the involvement of weak intermolecular forces of attraction between CE carrier and ATN. The prepared solid dispersion showed the drug content around ~ 96.94 % w/w, indicating that the solvent evaporation method improved the encapsulation of ATN and, thus, enhanced its drug content. Compared to pure ATN (~ 0.11 mg/ml), ATN-CE-SD (1:2) significantly increased the aqueous solubility by around ~ 25-fold (~ 2.78 mg/ml), indicating solid dispersion improves the solubility of ATN. ATN-CE-SD enhanced the rate of dissolution of ATV (~ 65 %) compared to pure ATN (~ 25 %) and PM (~ 34 %). Likewise, ATN-CE-SD (1:2) improved the rate and extent of ATN (~ 60 %) across the biological membrane compared to pure ATN (~ 22 %) and PM (~ 32 %). The ATN-CE-SD (1:2) improved the dissolution efficiency by around ~ (57.31%) compared to pure ATN (~ 7.02%) and PM (~ 20.43%). According to the study, co-processed excipients could serve as a promising solid dispersion carrier and improve ATN's water solubility, permeability, and dissolution rate. Conclusion: Based on the results, it is possible to use synthetic solid dispersion carriers as alternatives to improve the low water solubility and permeability of ATN.

Sorption characteristics and thermodynamic parameters of bispyribac-sodium and metribuzin on alluvial soil with difference in particle size and pH value

The behavior of bispyribac-sodium and metribuzin herbicides was investigated into alluvial soil. Effect of pH, temperature, and particle size of soil on the sorption process was studied. Sorption isotherms by bulk soil and its sand, silt, clay and humic acid (HA) fractions were quantified using the batch equilibration technique. The results showed that sorption of herbicides tested was significantly affected by temperature and was a spontaneous interfacial process into soil. Freundlich equation accurately predicted the sorption behavior of two herbicides. Thermodynamic parameters summarized that the negative values of the standard free energy changes (ΔG˚) were obtained therefore, the adsorption of tested pesticides in soil was spontaneous and small negative values indicate the sorption is physical in nature involving weak forces of attraction. The negative ΔH˚ value, indicating metribuzin and bispyribac-sodium interactions are exothermic processes and products are energetically stable with a high binding of compound to soil sites. There was an increase in the entropy of tested pesticides-soil systems as shown by positive values of ΔS˚. The highest adsorption in soil was at pH 5 for bispyribac-sodium and at pH 7 for metribuzin. The sorption of metribuzin and bispyribac-sodium were significantly greater on HA fraction followed by clay fraction than the bulk soil. The KF and Kd values increase with decreasing particle size particularly in the case of adsorption of metribuzin.

On the enhanced attractive load capacity of resonant flexural squeeze-film levitators

This paper uses numerical and asymptotic methods to investigate the fluid dynamics underlying the anomalously large attractive forces that were recently observed in squeeze-film levitation systems driven by resonant vibrations of a flexible oscillator. Namely, in a recent experimental study, a thin plastic disk driven near one of its natural frequencies attractively levitated, for the first time, an object weighing several hundred grams. This behavior is in stark contrast with that of rigid-body systems, which produce attractive forces thousands of times weaker and only within a limited range of operating conditions. Flexural systems driven by standing-wave deformations of the oscillator are addressed in this paper in a unifying matched-asymptotic analysis that accounts for effects of fluid viscosity, inertia, and compressibility, as well as pressure variations beyond the outer boundary of the squeeze film. While the weak attractive forces produced by rigid-body systems are known to depend critically on the existence of a net pressure drop across this peripheral region, the present analysis reveals that the augmented attractive load capacity of resonant flexural systems is associated instead with spikes of underpressure near the nodes of the standing wave. Furthermore, the wavenumber, which represents the number of nodes in the waveform, is found to correlate strongly with the attractive load capacity as well as the range of frequencies and oscillator surface areas for which attractive forces can be produced.

Formation behaviour of the kinetic Cucker–Smale model with non-compact support

In this paper, we focus on the formation behaviour of the kinetic Cucker–Smale model for initial datum without compact support for the position variable. Comparing with the case of compact support, the attractive force between particles is weak. First, we obtain the existence and uniqueness of the classical solution to the kinetic Cucker–Smale model by standard approximation method. Second, by using the characteristic flow, we overcome the difficulty brought by the weak attractive force between particles through some estimates and establish the formation behaviour, i.e., consensus of velocity, of the classical solution to the kinetic Cucker–Smale model. Finally, for the measure-valued solution to the kinetic Cucker–Smale model, the formation behaviour is also established.

Polar inelastic profiles in fast-atom diffraction at surfaces

Elastic diffraction of fast atoms at crystal surfaces under grazing incidence $\ensuremath{\theta}\ensuremath{\approx}1$\ifmmode^\circ\else\textdegree\fi{} has strong similarities with atomic diffraction at thermal energies discovered almost hundred years ago. Here, we focus on the polar scattering profile, which does not exhibit diffraction features but shows well-defined elastic and inelastic components that are found to be essentially independent of the crystallographic axis. The width ${\ensuremath{\sigma}}_{\ensuremath{\theta}}$ of the inelastic component is very sensitive to the weak attractive forces responsible for the physisorption. This effect is visible on an energy range almost ten times larger than the depth $D$ of the physisorption well. Experimental data are analyzed using a binary collision model with a Morse potential where the width ${\ensuremath{\sigma}}_{\ensuremath{\theta}}$ of the scattering profile is connected to the classical energy loss and is governed by the surface stiffness, defined as the logarithmic derivative of the interaction potential along the surface normal. The main outcome is that the weak attractive forces make the mean surface potential almost twice harder at low energy.

Host-Guest Complexation Between Cyclodextrins and Hybrid Hexavanadates: What are the Driving Forces?

Host-guest complexes between native cyclodextrins (α-, β- and γ-CD) and hybrid Lindqvist-type polyoxovanadates (POVs) [V6 O13 ((OCH2 )3 C-R)2 ]2- with R = CH2 CH3 , NO2 , CH2 OH and NH(BOC) (BOC = N-tert-butoxycarbonyl) were studied in aqueous solution. Six crystal structures determined by single-crystal X-ray diffraction analysis revealed the nature of the functional R group strongly influences the host-guest conformation and also the crystal packing. In all systems isolated in the solid-state, the organic groups R are embedded within the cyclodextrin cavities, involving only a few weak supramolecular contacts. The interaction between hybrid POVs and the macrocyclic organic hosts have been deeply studied in solution using ITC, cyclic voltammetry and NMR methods (1D 1 H NMR, and 2D DOSY, and ROESY). This set of complementary techniques provides clear insights about the strength of interactions and the binding host-guest modes occurring in aqueous solution, highlighting a dramatic influence of the functional group R on the supramolecular properties of the hexavanadate polyoxoanions (association constant K1:1 vary from 0 to 2 000 M-1 ) while isolated functional organic groups exhibit only very weak intrinsic affinity with CDs. Electrochemical and calorimetric investigations suggest that the driving force of the host-guest association involving larger CDs (β- and γ-CD) is mainly related to the chaotropic effect. In contrast, the hydrophobic effect supported by weak attractive forces appears as the main contributor for the formation of α-CD-containing host-guest complexes. In any cases, the origin of driving forces is clearly related to the ability of the macrocyclic host to desolvate the exposed moieties of the hybrid POVs.

Fabrication of 2D-MoSe2 incorporated NiO Nanorods modified electrode for selective detection of glucose in serum samples

Layered molybdenum diselenide (MoSe2) nanosheets were formed by the weak Van der Waals forces of attraction between Se and Mo atoms. MoSe2 has a larger space between the adjacent layers and smaller band gaps in the range of 0.85 to ~ 1.6 eV. In this study, MoSe2 nanosheets decorated nickel oxide (NiO) nanorods have been synthesized by hydrothermal method using sodium molybdate and selenium metal powder. NiO/MoSe2 composite formation was confirmed by powder X-ray diffraction analysis. In addition, the presence of MoSe2 nanosheets on NiO nanorods were confirmed by field emission scanning electron microscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The Nyquist plots of NiO/MoSe2 coated glassy carbon electrode (GCE) was indicated that it had lower charge transfer resistance compared to NiO/GCE and MoSe2/GCE. Furthermore, as-prepared NiO/MoSe2/GCE was used to detect glucose in alkaline solution by cyclic voltammetry and amperometry techniques. The NiO/MoSe2/GCE was exhibited a linear response for the oxidation of glucose from 50 µM to 15.5 mM (R2 = 0.9842) at 0.5 V by amperometry. The sensor response time and the limit of detection were found to be 2 s and 0.6 µM for glucose. Moreover, selectivity of the NiO/MoSe2 sensor was tested in the presence of common interferent molecules such as hydrogen peroxide, fructose, lactose, ascorbic acid, uric acid, and dopamine. It was found that NiO/MoSe2/GCE did not respond to these interfering biomolecules. In addition, NiO/MoSe2/GCE had shown high stability, reproducibility and repeatability. Finally, the practical application of the sensor was demonstrated by detecting glucose in human blood serum with the acceptable recovery.

Weak Anion Binding to Poly(N-isopropylacrylamide) Detected by Electrophoretic NMR.

Ion specific effects are ubiquitous in solutions and govern a large number of colloidal phenomena. To date, a substantial and sustained effort has been directed at understanding the underlying molecular interactions. As a new approach, we address this issue by sensitive 1H NMR methods that measure the electrophoretic mobility and the self-diffusion coefficient of poly(N-isopropylacrylamide) (PNIPAM) chains in bulk aqueous solution in the presence of salts with the anion component varied from kosmotropes to chaotropes along the Hofmeister series. The accuracy of the applied electrophoretic NMR experiments is exceptionally high, on the order of 10–10 m2/(V s), corresponding to roughly 10–4 elementary charges per monomer effectively associated with the neutral polymer. We find that chaotropic anions associate to PNIPAM with an apparent Langmuir-type saturation behavior. The polymer chains remain extended upon ion association, and momentum transfer from anion to polymer is only partial which indicates weak attractive short-range forces between anion and polymer and, thereby and in contrast to some other ion–polymer systems, the lack of well-defined binding sites.

Polyethylene oxide assisted separation of molybdenite from quartz by flotation

Low flotation efficiency has been suffered in beneficiation of low-grade molybdenum ores due to finer size liberation and crystal anisotropy. As reported, polyethylene oxide (PEO) was proved to improve flotation efficiency of micro-fine molybdenite particles visibly. In this study, PEO was employed to investigate the influence on flotation of micro-fine molybdenite particles from mixed ores and the underlying mechanisms were fully discussed by Atomic Force Microscopy (AFM), Focused Beam Reflectance Measurement (FBRM), Dynamic Light Scattering etc. The results showed that high consumption of kerosene could not achieve a desired recovery in flotation of micro-fine molybdenite from quartz. The introduction of PEO considerably increased flotation efficiency of molybdenite from quartz, and flotation selectivity was improved via pH and stirring speed adjustment. In the presence of PEO, pH affected flocculation capacity of molybdenite slightly while the flocculation capacity of quartz had a strong dependence on the pH. Adsorption amount and hydrodynamic radius of PEO molecules resulted in the phenomenon jointly. Stirring speed had a prominent effect on flocculation, de-flocculation and re-flocculation behaviors of molybdenite and quartz. Higher stirring speed scrubbed quartz from flocs due to weak attractive forces between molybdenite and quartz to reduce heterogeneous flocculation. Moreover, strong attractive forces drove molybdenite flocs reformation when suspensions were re-submitted to low stirring speed while quartz lost the flocculation ability. Hence, higher selectivity was achieved in the flotation of molybdenite from quartz with appropriate pH and stirring speed.

Viologen Tweezers to Probe the Force of Individual Donor-Acceptor π-Interactions.

Donor-acceptor (DA) π-interactions are weak attractive forces that are exploited widely in molecular and supramolecular chemistry. They have been characterized extensively by ensemble techniques, providing values for their energies that are useful for the design of soft materials. For implementation of motions or operations based on these DA π-interactions in wholly synthetic molecular machines, the mechanical strength and force associated with their out-of-equilibrium performance are the key parameters, in addition to their energies obtained at thermodynamic equilibrium. In this context, we have used single-molecule force spectroscopy as a nonequilibrium technique to determine the mechanical strength of individual DA π-interactions in solution. We designed and synthesized a molecular tweezer that is able to encapsulate π-donors and also demonstrated a precise opening extension. The mechanical breaking of the noncovalent interactions between viologen units-π-acceptors commonly employed in mechanically interlocked molecules-and several π-donors afforded a characteristic force-distance signature, revealing the opening of individual viologen tweezers with an unambiguous extension. Single-tweezer host-exchange experiments performed in situ demonstrated the sensitivity of the technique. This simple design could be exploited in quantifying the force of a large range of weak noncovalent bonding interactions as well as the potential work that molecular machines can generate at the single-molecule level.

Interplay between surface chemistry and osteogenic behaviour of sulphate substituted nano-hydroxyapatite

Surface potential and chemical compositions of the bioceramics are the core of therapeutic effect and are key factors to trigger the interfacial interactions with surrounding hard and soft tissues to repair and regeneration. Ionic substitution in hydroxyapatite (Hap) lattice significantly influences the zeta potential from −16.46 ± 0.66 mV to −6.01 ± 0.68 mV as well as an average nano-rod length from ~40 nm to ~26 nm with respect to SO42− ion content. Moreover, the surface chemistry of Hap is mainly inter-related to SO42− substitution rate at PO42− site. Specifically, nano-sized feature with lowered negative surface potential influences the protein adsorption via their weak repulsive or attractive forces. Bovine serum albumin (BSA) and lysozyme (LSZ) adsorption studies confirmed the increased affinity to active binding sites of Hap's surface. Further, SO42− ion substituted Hap (SNHA) showed improved in vitro biomineralization activity and alkaline phosphatase activity. Expression of osteogenic biomarkers such as collagen I, V, osteopontin and osteocalcin were evaluated in Saos-2 and MC3T3-E1 cells. Gene expression of these markers was influenced by SO42− ion content in Hap (maximum with 10SNHA). Altogether, these data emphasizes that chemical composition and surface properties are dominant aspect in bioceramic development towards bone regeneration.

Solar Photoelectroreduction of Nitrate Ions on PbI2/CuI Nanocomposite Electrodes

Solar light–driven photoelectrochemical conversion of nitrate ions to nitrite is achieved on copper‐lead iodide nanocomposite electrodes. By optimizing the composition, 1.2 ± 0.05 mA cm−2 photocurrent density is obtained under concentrated solar irradiation. The reduction products are NO2− and N2, at 0.61 V reversible hydrogen electrode (RHE), which is at least 600 mV higher than the redox potential of these processes. The faradaic efficiency of NO2− ion formation is up to 52% while no hydrogen evolution occurs. Optical studies show the possibility of exciting electrons from the valence band of CuI to the conduction band of PbI2 upon illumination, extending the visible light absorption of the nanocomposite photoelectrode. Experimental and theoretical optoelectronic studies indicate that CuI and PbI2 are held together by weak attractive forces (also excluding the formation of a new compound), which opens a new strategy to design metal halide nanocomposite photoelectrodes with tailored properties.

Remarkable enhancement in sensor ability of polyaniline upon composite formation with ZnO for industrial effluents

Polyaniline emeraldine salt and Polyaniline Zinc Oxide composite are comprehensively studied to compare their sensing ability towards ammonia, acetone, methanol and ethanol. Sensing ability is evaluated through thermodynamic, geometric and electronic parameters. A number of orientations are evaluated in search for the lowest energy structure. The comparison of thermodynamic, geometric and electronic parameters of simple and composite sensors confirmed that composite sensor shows better sensing ability than simple PANI. Composite formation between polyaniline and zinc oxide is a thermodynamically feasible process with interaction energy of −42.8 kcal/mol. Both sensor shows highest interaction energy for ethanol among four analytes. Composite sensor shows almost twice the interaction with ethanol, methanol and acetone while 1.5 times the interaction energy for ammonia. The results of run simulations and subsequent calculations showed that in composite sensors, zinc oxide not only enhances the binding power of conducting polymer with analytes but also interacts directly with analytes. Strong hydrogen bonding as well as weak dispersion forces of attraction are responsible for the stability of composite and all complexes, as revealed by non-covalent interaction (NCI) studies. Acetone shows a different behaviour in composite sensor complex than other analytes.

Agglomeration Characteristics of Various Inclusions in Al-killed Molten Steel Containing Rare Earth Element

The nozzle-clogging problem is closely related to the agglomeration behavior of inclusions in steel-containing rare earth (RE). The present study first optimized the thermodynamic model by supplementing thermodynamic data of the RE compound to study the formation of various inclusions in the Ce-Al-O-S system. Based on that, laboratory-scale experiments with different Ce and Al contents were designed to obtain samples containing a single type of inclusions, viz., Al2O3, Ce-Al-O, Ce2O3, and Ce-O-S, respectively. Additionally, the agglomeration behavior of inclusions was observed in situ by confocal laser scanning microscopy (CLSM), and the calculated attractive forces were used to compare the agglomeration tendency of the four inclusions. The results show that the agglomeration of Al2O3 inclusions is attributed to the strongest attractive force (1.0 × 10−15N to 3.0 × 10−14N) among the four kinds of inclusions. Although Ce-Al-O and Ce2O3 inclusions have the weaker attractive forces (1.3 × 10−16N to 2.0 × 10−14N), they can also agglomerate together and form large-sized clusters. The attractive forces between Ce-O-S inclusions are the weakest (2.1 × 10−18N to 6.0 × 10−16N), that is, this kind of inclusion is more difficult to agglomerate than other inclusions. The study can provide theoretical support for improving the nozzle-clogging problem of RE-containing steel from the inclusion-controlling perspective.

A Cause of Force of Gravity between Bodies

An isolated electric charge, in the form of an indestructible spherical shell of radius a, has its straight and radial electric fields pulling the surface charge equally outwards, directed from the center, to maintain an equilibrium. Any imbalance in the field lines of force causes the charge to move, with acceleration, in the direction of the resultant force. The field lines of adjacent charges become curved to make for the force of repulsion or attraction. It is proposed that the electric field lines from a charge open out slightly, on encountering a like charge in space, thereby reducing the force of repulsion. The field lines close in slightly, against an unlike charge, to increase the force of attraction. Thus, there is a net force of attraction between two neutral bodies composed of equal numbers of positive and negative charges. For a neutral body, the strong electrical forces of repulsion and attraction, proportional to the magnitude of a charge, in accordance with Coulomb’s law, cancel out everywhere. The weak gravitational forces of attraction, proportional to the mass of a charge, which is also proportional to the square of magnitude of a charge, in accordance with Newton’s law, remain and add up. It is shown that gravity is a pulling force of attraction and an electrical property emanating from a body, not a result of curvature of empty space surrounding the body, as envisaged by the theory of general relativity.

On the Origin of the Negative Thermal Expansion Behavior of YCu

Among the intermetallics and alloys, YCu is an unusual material because it displays negative thermal expansion without spin ordering. The mechanism behind this behavior that is caused by the structural phase transition of YCu has yet to be fully understood. To gain insight into this mechanism, we experimentally examined the crystal structure of the low-temperature phase of YCu and discuss the origin of the phase transition with the aid of thermodynamics calculations. The result shows that the high-temperature (cubic CsCl-type) to low-temperature (orthorhombic FeB-type) structural phase transition is driven by the rearrangement of three covalent bonds, namely, Y-Cu, Y-Y, and Cu-Cu, which compete for the bonding energy and phonon entropy. At low temperatures, the mixing of Y and Cu does not take place easily because of the weak attractive force between these atoms expected from the small negative mixing enthalpy. This causes all three interactions to take part in the bonding, and Y and Cu are segregated to form an FeB-type structure, which is stabilized by internal energy. At higher temperatures, Cu ions are bound loosely with Y ions due to the large Y-Cu distance (3.01 Å), which results in large vibration entropy and stabilizes a CsCl-type crystal structure. In addition, the CsCl-type structure is reinforced by the Y-Y interaction between next-nearest neighbors, resulting in a smaller unit cell volume. The crystal structure has the simple cubic framework of Y containing Cu ions bound loosely at the cavity sites. The calculated frequency of the Y-like phonon modes is much higher than that of the Cu-like modes, indicating the presence of Y-Y covalent interactions in the CsCl-type phase.

Cuprophilic Interactions in and between Molecular Entities.

Despite being weak attractive forces, closed-shell metallophilic interactions play important roles in the Group 11 metal complexes on their diverse structural and physical features. A plethora of experimental and computational studies has thus been dedicated to such weak attractive d10 -d10 interactions, particularly aurophilic and argentophilic interactions. Although d10 -d10 CuI -CuI forces had been recognized for four decades, cuprophilic interactions are less explored and they are best evidenced by single-crystal X-ray crystallographic analysis on CuI complexes and aggregates thereof, by which precise information about the Cu⋅⋅⋅Cu contacts, shorter than the sum of two van der Waals radii (3.92 Å) between the copper centers concerned can be obtained. Based on recently compelling experimental and spectroscopic evidence for intra- and intermolecular cuprophilic interactions in copper chemistry, the present Minireview summarizes recent progress in the past three decades in the synthesis and structures of multinuclear homometallic copper complexes, whereby supported and unsupported d10 -d10 CuI -CuI interactions are at work.

Molecular Structure of 1-Isocyano-1-silacyclopent-3-ene: A Combined Microwave Spectral and Theoretical Study.

The microwave spectrum of 1-isocyano-1-silacyclopent-3-ene has been obtained from broad-band chirped-pulse Fourier transform microwave spectroscopy. The rotational constants (RCs) for the standard abundant isotopic species are A = 3328.4182(23), B = 1017.69404(53), and C = 1012.33297(58) MHz. The symmetric quartic centrifugal distortion constants, using the Ir representation in CS symmetry for ΔJ,ΔJK, ΔK, and δJ, have been evaluated; similarly, the 14N nuclear quadrupole coupling has been determined. Several singly substituted isotopologues observed in natural abundance enabled most of the heavy atom substructure to be determined. The five-membered ring is close to planar, but the orientation of the isocyanate unit, derived from the N13CO spectrum, unexpectedly lies above the ring center in a cis C2,5-Si-N═C conformation. Our initial equilibrium structural searches led to a trans orientation of the C2,5-Si-N═C unit, i.e., bending away from the ring. When the cis conformation was applied, the final equilibrium structure, assuming CS symmetry, gave RC values of 3221.3 ( A), 1037.0 ( B), and 1031.3 ( C) MHz, very close to the MW values. This enabled the full-equilibrium structure to be determined with confidence. The principal bond lengths were 1.7157 (Si-N), 1.8696 (Si-C), 1.1998 (N═C), and 1.1737 (C═O) Å, with angles 163.3 (Si-N═C), 178.1 (N═C═O), 96.5 (C-Si-C), and 118.7° (C-C═C), respectively. The extensive widening of the SiNC angle is particularly notable; the SiNCO unit has a trans dihedral angle. The cis orientation implies a (weak) attractive force between the ring and isocyanate groups by a through-space interaction. An atoms in molecule study, where the local minima of electron density are determined, fails to disclose the exact nature of the interaction; however, a highly polarized skeleton was obtained. A systematic theoretical study of the Si-N═C angle potential energy surface (PES) relative to the ring gave a very shallow double minimum with the barrier being less than 1 cm-1; a polynomial fit to the surface shows major contributions of both harmonic and quartic components. A similar study of the XSiN angle, where X is at the ring center, also gave a PES with considerable quartic character.