High Heat Of Formation Research Articles

Theoretical exploration of energetic molecular design strategy: functionalization of C or N and structural selection of imidazole or pyrazole.

In researching energetic materials with high energy density, it is an effective method to introduce explosophoric groups. In this study, four series of energetic compounds were designed by functionalizing with C- or N-, introducing energetic groups -CH(NO2)2, -CF(NO2)2, -C(NO2)2(NF2), -C(NO2)3, and-CH(NF2)2 into imidazole and pyrazole structures. Density functional theory was employed to optimize the structure of the target compound and subsequently to predict and evaluate its performance based on this. Meanwhile, the sensitivity of the compounds was predicted based on their electrostatic potential analysis. Following analysis of the geometric structure, detonation performance, and sensitivity of the compounds, three factors were discussed: energetic groups, functionalization methods, and skeleton structure differences. The results indicate that C-functionalization has advantages only in density, but N-functionalization is better in thermal stability, heat of formation, and sensitivity. Meanwhile, the data shows that imidazole-based compounds exhibited greater density and detonation performance in the target compounds designed within this study, while pyrazoles have a higher heat of formation and chemical stability. By analyzing the design strategy of C- or N-functionalization of novel high-energy groups on energetic imidazole or pyrazole rings and selecting a more suitable molecular construction strategy, this study provides a theoretical approach for the development of new energetic materials with excellent performance. Gaussian 09 and Multiwfn 3.8 packages are the software used for calculation, and the electrostatic potentials were depicted using the VMD program. In this study, the imidazole and pyrazole derivatives were optimized at the B3PW91/6-311G (d, p) level to acquire the relevant data for the compounds.

Theoretical Investigation of Energetic Materials Based on Imidazole Framework Featuring Azido/Nitro/Nitrato/Fluoro Groups

ABSTRACTDevelopment of new high‐energy materials (HEMs) is one of the thrust areas of research. These compounds are important for various applications such as propellants, gas generations, explosives in mining, construction, civil and military applications, and safety equipment for national security and defense. HEMs based on imidazole frameworks are currently getting research spotlight due to their exceptional detonation performance with optimum sensitivity. This study reports five imidazole derivatives containing azido/nitro/nitrato/fluoro functional groups. These groups are viable options to design promising explosives with moderate sensitivity. Density functional theory (DFT) method is adopted to determine the geometries, thermodynamic properties, detonation properties, and impact sensitivity of the designed molecules. All the compounds have high density in the range of 1.89–2.03 g/cm3 along with high heat of formation. These compounds aid in new strategy to design HEMs with optimum sensitivity.

Polymethylenetetrazole: Synthesis, Characterization, and Energetic Properties.

The tetrazole moiety remains one of the most interesting scaffolds in the development of new high-energy density materials (HEDMs) because of its desired characteristics, such as high nitrogen content and heat of formation (HOF). The combination of several heterocycles with high HOF seems to be a promising strategy for obtaining energetic materials with superior properties. Herein, we report the synthesis and characterization of a tetrazole polymer, polymethylenetetrazole (PMT), as a potential HEDM. The compound was characterized using NMR, IR, and Raman spectroscopy. Its weight average molecular mass was obtained by static light scattering (SLS), and its physical properties by powder XRD analysis. The density, sensitivity to friction (FS), and impact (IS) of the compound were determined as well. The results of the thermal and energetic properties of PMT suggest that this polymer could be an insensitive explosive.

A positional isomer of LLM-116: N—NO2 moiety enhancing the laser ignition combustion performance of nitrogen-rich energetic materials

It has long been a driving force in the development of energetic materials to hunt for novel energetic materials (EMs) with high energy and low sensitivities. Recently, the combination of nitrogen-heterocycles and NNO2 moiety could generate novel energetic derivatives with high energy content and low sensitivity benefiting from the high heat of formation and abundant hydrogen bonding interactions. The critical role of isomerism strategy in tuning the laser ignition combustion performances of EMs is rarely mentioned in the chemical enterprise, although some researches reveal that isomerism strategy has significant effects on their thermal behaviors, mechanical sensitivity and detonation performance. In this work, the positional isomerism of nitramine-pyrazole was investigated, and 3-nitramine-4-nitropyrazole and a series of energetic derivatives were synthesized and well characterized. Their physical properties, energy performance, as well as laser-ignited combustion performance, were further explored. The experimental results showed that all compounds possess excellent detonation pressure, and detonation velocity (P: 26.5∼30.4 GPa, D: 7876∼8377 m·s−1), which were much better than those of trinitrotoluene (TNT) (P: 19.5 GPa and D: 6881 m·s−1). Moreover, all of the resulting energetic derivatives exhibited low mechanical sensitivities (IS>40 J). Interestingly, they also had excellent laser ignited combustion properties owing to the NNO2 moiety attached to nitropyrazole backbone.

Effect of Different Concentrations of Co Doping on the Properties of η′‐Cu6Sn5 and Ni3Sn4: First‐Principles Study

In the field of electronic packaging, Ni3Sn4‐based and η′‐Cu6Sn5‐based compounds are common Sn‐based intermetallic compounds into which doping of other appropriate elements has been widely studied. Herein, the structural stability, mechanical properties, and electronic structures of (Ni,Co)3Sn4 and η′‐(Cu,Co)6Sn5 are systematically investigated using first‐principles calculations. By comparing the heat of formation of the doped and undoped intermetallic compounds, it is found that the doped Ni3Sn4 structure has a higher heat of formation and a less stable structure, while the doped η′‐Cu6Sn5 structure has a lower heat of formation and a more stable structure. The doped Ni3Sn4 exhibits an increasing bulk modulus (B) and Young's modulus (E), and the doped η′‐Cu6Sn5 exhibits a gradually increasing bulk modulus (B). These findings suggest that doping improves the rigidity and elastic deformation properties of the two intermetallic compounds. And yet the anisotropy of both intermetallic compounds is decreasing as doping concentration increases. According to the calculations of the electronic structures, the doped (Ni,Co)3Sn4 structure and the doped η′‐(Cu,Co)6Sn5 structure exhibit stronger metallicity. In addition, stronger Co–Sn ionic bonds are formed in the doped Ni3Sn4 and η′‐Cu6Sn5 structures. This suggests that the doped Ni3Sn4 and η′‐Cu6Sn5 structures are harder.

Bistriazolotriazole-tetramine: commendable energetic moiety and cation.

Various 7H,7'H-[6,6'-bi[1,2,4]triazolo[4,3-b][1,2,4]triazole]-3,3',7,7'-tetramine (A) based nitrogen-rich energetic salts were designed and their properties explored. All energetic salts possess relatively high nitrogen content (> 48%), positive heats of formation (> 429kJ/mol) and stability owing to a significant contribution from fused backbone. The cationic component shows a very high heat of formation (2516kJ/mol); therefore, it is highly suitable for enthalpy enhancement in new energetic salts. The cation was paired with the energetic anions nitrate (NO3-), perchlorate (ClO4-), dinitromethanide (CH(NO2)2-), trinitromethanide (C(NO2)3-), nitroamide (NHNO2-), and dinitroamide (N(NO2)2-) to improve oxygen balance and detonation performance. Designed salts show moderate detonation velocities (7.9-8.7 km/s) and pressures (23.8 - 33.1 GPa). The distribution of frontier molecular orbitals, molecular electrostatic surface potentials, QTAIM topological properties, and noncovalent interactions of designed salts were simulated to understand the electronic structures, charge distribution in molecules, hydrogen bonding, and other nonbond interactions. The predicted safety factor (SF) and impact sensitivity (H50) of designed salts suggest their insensitivity to mechanical stimuli. This work explored the 7H,7'H-[6,6'-bi[1,2,4]triazolo[4,3-b][1,2,4]triazole]-3,3',7,7'-tetramine as a suitable cationic component which could be promising and serve exemplarily in energetic materials. The optimization and energy calculations of all the designed compounds were carried out at the B3LYP/6-311 + + G(d,p) and M06-2X/def2-TZVPP levels, utilizing the Gaussian software package. The molecular surface electrostatic potential, quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), and noncovalent interaction (NCI) analysis were performed by employing Multiwfn software. The EXPLO5 (v 7.01) thermochemical code and PILEM web application were used to predict the detonation properties.

A density functional theory investigation of the substituent effect on acyclovir and guanine derivatives for applications on energetic materials

The main challenge in designing new energetic materials is to find a good balance between four seemingly incompatible requirements, namely, high-energy content, low sensitivity, low production costs and less-polluting content. Fused nitrogen heterocycles of imidazole and pyrimidine, such as acyclovir and guanine, may offer interesting features due to the combination of a coplanar framework and a large conjugate system, which contribute to a reduced sensitivity, and a number of energetic bonds that can be increased by the introduction of explosophore substituents. In this work, to evaluate the potential of acyclovir and guanine derivatives as energetic materials, density functional theory (DFT) calculations were carried out to investigate the influence of the type and position of the explosophore substituent groups –NO2, –NHNO2, –N3, –ONO2, –CN, −N=N−, and −N=N(O)− on the energetic properties and chemical reactivity of 91 acyclovir- and guanine-based molecules, including thirty one nitramines, three nitroheterocycles, seventeen azides, seventeen nitrate esters, seventeen nitriles, three azo and three azoxy compounds. Several molecular properties were computed, including the chemical reactivity, the heat of formation and the detonation velocities and pressures using semiempirical equations. Among the molecules with no bridge groups, we found that, except for cyano group, position 4 were the most stable for acyclovir derivatives, whereas, except for the azido group, position 2 and 5 provided the most stable compounds for guanine derivatives. Among the bridged derivatives, depending on the molecule and positions, the nitrate esters and the nitro derivatives were more stable. In comparison with the parent compounds, calculations showed that the heat of formation (HOF) increased the most with azido and cyano groups, the density increased substantially with nitrate esters, nitro and nitramino groups, and the detonation velocities and pressures increased the most with nitrate ester, nitro and nitramino groups. Although azo groups resulted in higher HOFs than azoxy groups, azoxy derivatives showed superior values in terms of density, heat of maximum detonation, detonation velocity and pressure. Four nitrate esters (GD134, GD245, AZOXYGD13 and AZOXYGD25) displayed higher values of detonation velocity and pressure than RDX. The designed nitramines are less sensitive to impact than RDX. Except for GD134 and GD245, all guanine-based nitrate esters, with no bridge linkages, are expected to be less sensitive to impact than TNT. Due to the combination of good performance and stability, the compounds GD25, GD13, GD45, GD34, and GD14 have considerable potential as energetic materials. Therefore, their synthesis and further investigation are recommended.

Adsorption structure of macrocyclic energetic molecule DOATF on Au(111)

Furazan macrocyclic compound 3,4:7,8:11,12:15,16-tetrafurazan-1,9-dioxazo-5,13- diazocyclohexadecane (DOATF) is an ideal energetic material with high heat of formation. Here, using scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM), we investigated the adsorption structure of DOATF molecules on Au(111) surface, which shows the four furanzan rings in the STM images and a bright protrusion off the center of the molecule in the nc-AFM images. Combined with density functional theory (DFT) calculations, we confirmed that the bright feature in the nc-AFM images is an N–O coordinate bond pointing upwards in one of the two azoxy groups; while the other N–O bond pointing towards the Au(111) surface. Our work contributes for a deeper understanding of the adsorption structure of macrocyclic compounds, which would promote the designing of DOATF-metal frameworks.

Electronic structure and detonation property prediction of pentazolate derivatives: Aminopentazole, diaminopentazole cations, azopentazole, and 1,2-diazopentazole

In this study, various polynitrogen pentazolate derivatives: aminopentazole, diaminopentazole cations, azopentazole (N12), and 1,2-diazopentazole (N14), were designed and computed. Based on quantum chemistry calculations and wavefunction analyses, the equilibrium geometry, bond orders, atoms-in-molecules (AIM) analysis, atomic charges, electrostatic potential distribution, frontier molecular orbitals, and many indices to represent aromaticity were determined and represented. In addition, their densities, heats of formation, detonation properties, and impact sensitivities were calculated using thermodynamics methods. These polynitrogen pentazolate derivatives could exhibit high density (1.668–1.965 g·cm−3); high heat of formation (2.96–15.86 kJ·g−1); outstanding detonation performance (D: 9070–12,243 m·s−1, P: 34.8–70.0 GPa); in particular, N14 (D: 12,243 m·s−1, P: 70.0 GPa); acceptable thermostability and sensitivity (h50: 12–49 cm). The excellent energetic performance demonstrates that polynitrogen pentazolate derivatives are promising energetic materials. The synthesis of aminopentazole and diaminopentazole cations may be achieved via an amination agent and the synthesis of N12 and N14 using an oxidizing agent.

Theoretical exploration of polynitrogen compounds N6, N8, N10, and N6 ions based on N3 − and cyclo-N5 −

ABSTRACT Polynitrogen energetic materials have become an important branch of high-energy-density materials. In this work, a thorough theoretical exploration of polynitrogen compounds N6, N8, N10, and N6 ions was conducted based on N3 − and pentazolate anion (cyclo-N5 −). Based on quantum chemistry calculations and wavefunction analyses, the equilibrium geometry, bond orders, atoms-in-molecules analysis, atomic charges, electrostatic potential distribution, frontier molecular orbitals, and many indices to represent aromaticity were determined and represented. In addition, their densities, heat of formation, detonation properties, and impact sensitivities were calculated using thermodynamics methods. These polynitrogen compounds could exhibit good density (1.578–1.814 g cm−3); high heat of formation (7.45–8.05 kJ g−1); excellent detonation performance (D: 9206–10413 m s−1, P: 34.8–44.7 GPa). The detonation performance demonstrates that polynitrogen compounds are promising energetic materials.

FLIGHT DEMONSTRATION OF A GAP/N2O DIRECT INJECTION GAS-HYBRID ROCKET SYSTEM USING A SMALL ROCKET

This paper presents the first trial of a flight demonstration experiment using a direct-injection gas-hybrid rocket (DIGHR) system, with glycidyl azide polymer (GAP) as the solid fuel and nitrous oxide (N<sub>2</sub>O) as the oxidizer. In this work, the combustion characteristics of the DIGHR system were studied. GAP is a high-energy material with characteristics such as self-decomposition, high density, and high heat of formation. N<sub>2</sub>O has high vapor pressure. Therefore, the system does not need to use additional pressurized gas to oxidize the combustion chamber. Consequently, the DIGHR system is characterized by a high-density specific impulse that enables the development of a small thruster system. In this study, a 400 N thrust class DIGHR was developed to use in the flight demonstration and a ground test was conducted before the flight test. The performance of a DIGHR system using gaseous oxygen (GOX) was previously reported, in which all of the experiments confirmed smooth ignition and stable combustion. In addition, the efficiency of the characteristic velocity (C*) decreased to approximately 85% in comparison to a system using GAP/GOX. One reason for this is that the motor sizes were different. The second reason is the use of liquid N<sub>2</sub>O as an oxidizer. However, these experimental results support the use of a small rocket at an altitude of 100 m. Therefore, a GAP/N<sub>2</sub>O flight motor was designed and developed for a small hybrid rocket. The DIGHR was launched from the ground to the sea and the experiment was successfully performed.

Boosting the Energetic Performance of Trinitromethyl-1,2,4-oxadiazole Moiety by Increasing Nitrogen-Oxygen in the Bridge.

The trinitromethyl moiety is a useful group for the design and development of novel energetic compounds with high nitrogen and oxygen content. In this work, by using an improved nitration method, the dinitromethyl precursor was successfully nitrated to the trinitromethyl product (2), and its structure was thoroughly characterized by FTIR, NMR, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Compound 2 has a high density (1.897 g cm−3), high heat of formation (984.8 kJ mmol−1), and a high detonation performance (D: 9351 m s−1, P: 37.46 GPa) that may find useful applications in the field of high energy density materials.

A hybrid of tetrazolium and pentazolate: An energetic salt with ultrahigh nitrogen content and energy

As a full nitrogen energetic anion, pentazolate (cyclo-N5ˉ) holds great promise in the fields of propellants and explosives. Nowadays, nonmetallic pentazolate salts have received extensive attention as excellent nitrogen-rich energetic materials for their high enthalpies of formation, good oxygen balance, and eco-friendly decomposition products. In this study, a 1,4,5-triaminotetrazolium-based pentazolate salt (TATe+N5ˉ, 8) with a nitrogen content of up to 90.30% was designed and synthesized. Its crystal structure indicates that a large number of hydrogen bonds form a hydrogen-bonded network, and the crystal has a mixed stacking pattern. TATe+N5ˉ, which has a relatively high density (1.64 ​g·cm−3), high heat of formation (861.9 ​kJ·mol−1), and excellent detonation performances (detonation velocity: 9487 ​m·s−1, detonation pressure: 32.5 ​GPa), provides new insights into the stability of ultrahigh-nitrogen compounds.

Pyrazole bridges ensure highly stable and insensitive bistetrazoles

• Insertion of a pyrazole-bridge between two tetrazole units increases the thermal stability. • Pyrazole-bridged bistetrazoles - high heats of formation, and nitrogen and insensitivity. • Mild, green halogenation at the 4-position was possible using Oxone® and halide salts. • Compounds 10 and 11 have good detonation properties relative to TATB and RDX. In the quest for efficient and thermally stable insensitive materials, the inclusion of a bridging molecule between symmetric or asymmetric azole units has recently emerged as a fruitful approach. However, the synthesis of new compounds with high density in combination with high thermal stability and insensitivity often occurs only with difficulty. Now a new class of pyrazole-bridged bistetrazole and bis(1-hydroxytetrazolyl) compounds was synthesized and fully characterized by NMR, IR spectroscopy, and elemental analysis. Thermal behavior was investigated by using differential scanning calorimetry (DSC), and thermal gravimetric analysis (TGA). The solid-state structures of the diammonium salt ( 9 ) based on 4-nitro-1 H -pyrazole, and 5,5′-(4-chloro-1 H -pyrazole-3,5-diyl)bis(1 H -tetrazole) ( 18 ) were determined by single-crystal X-ray diffraction. These pyrazole-bridged bistetrazole compounds exhibit excellent thermal stabilities with high decomposition temperatures, moderate heats of formation, high nitrogen content, and insensitivities to impact and friction. Additionally, introduction of a halogen atom at the 4-position of the pyrazole ring gives rise to a new active heterocycle which holds considerable promise as a suitable substrate for further transformation.

Nitrogen-rich ion salts of 1-hydroxytetrazole-5-hydrazide: a new series of energetic compounds that combine good stability and high energy performance.

High-efficiency explosives that combine high stability and excellent energy performance are one of the key directions of energetic materials research. In this study, a novel monocyclic hydroxytetrazole derivative (3) with high stability was prepared, and a series of insensitive energetic ionic salts were derived from it. Benefiting from their outstanding performance in terms of density, 3D hydrogen bonding and π-electron interactions, these salts are excellent in both detonation performance (D = 8709 to 9314 m s-1 and P = 29.9 to 35.6 GPa) and thermal stability (Td = 193.0-232.2 °C). The hydrazine salt (2) exhibits high detonation properties (D = 9314 m s-1 and P = 35.6 GPa), due to its high density (ρ = 1.71 g cm-3) and high heat of formation (ΔfH = 563.2 kJ mol-1 = 3.19 kJ g-1). In addition, the high thermal stability (Td = 232.0 °C) and low mechanical sensitivity (IS = 30 J and FS = 360 N) of 2 are also unmatched by HMX and TKX-50. These improved properties demonstrate the great promise of 2 as an insensitive high-energy explosive.

Achieving short ignition delay and high specific impulse with cyanoborohydride-based hypergolic ionic liquids

A new family of HILs, based on substituted 1H-1,2,4-triazol-4-ium, pyrrolidinium, ammonium and pyridinium cations and a cyanoborohydride anion, is introduced, with higher heats of formation, heats of combustion and specific impulse compared to dimethylhydrazine.

Computational assessment of nitrogen-enriched, stable and insensitive tris(1,2,4,5-tetrazin-3-yl)amine building block for energetic applications

In this work, we report an approach for designing a series of energetic compounds derived from nitrogen-rich tris(1,2,4,5-tetrazin-3-yl)amine backbone. Based on available synthetic method, nitrogen content (70.8%), heat of formation (1371 ​kJ·mol-1), density (1.67 ​g·cm-3), detonation parameters (7.75 ​km·s-1, 24.30 ​GPa), and experimentally reported high decomposition temperature (231 ​°C), less sensitivity to accidental impact (42 ​J) and friction (360 ​N) data, tris(1,2,4,5-tetrazin-3-yl)amine found to be an efficient building block for designing energetic materials. The explosophoric functional groups –NO2, –NH2, –ONO2, –NHNO2, and N3 were introduced in the tris(1,2,4,5-tetrazin-3-yl)amine framework to further improve energetic properties. All tris(1,2,4,5-tetrazin-3-yl)amine derivatives have a much higher heat of formation than TNT (2,4,6-trinitrotoluene) and RDX (1,3,5-trinitro-1,3,5-triazinane). Crucially, higher detonation velocities (>9.10 ​km·s-1) and detonation pressures (>35.20 ​GPa) were calculated for –NO2, –ONO2, and –NHNO2 substituted compounds compared to RDX. Considering the experimental and computed stability of tris(1,2,4,5-tetrazin-3-yl)amine towards high temperatures and mechanical stimuli, the entire designed molecules are expected to be less sensitive and stable for energetic applications. These advantages make tris(1,2,4,5-tetrazin-3-yl)amine an effective building block. There is no denying that the insertion of energetic functional groups will undoubtedly benefit the development of secondary explosives and propellants.

Combination multi-nitrogen with high heat of formation: theoretical studies on the performance of bridged 1,2,4,5-tetrazine derivatives.

A series of bridged tetrazine derivatives (BDDT) were designed by using different bridges to connect two molecules of 1,2,4, 5-tetrazine oxides and then combining different substituents. At the same time, we used DFT-wB97/6-31 + G** method to regularly predict the HOMO-LUMO, heats of formation (HOF), detonation properties, thermal stability, and thermodynamic property orbitals of BDDT compounds. By studying the comprehensive relationship between different substituents and bridging and performance, it is shown that -N(NO2)2 and -C(NO2)3 are not only excellent groups to improve the heat of formation and detonation properties, but also can cause the compound to have a superior oxygen balance. And that the incorporation of the -N = N- and -NH-N = N- is helpful to enhance their thermal stabilities and HOF. -CH2-CH2- and -CH2-NH- are good for improving the HOMO-LUMO energy gaps. Performances with positive HOF (1170-1590kJmol-1), remarkable density (1.88-1.93gcm-3), outstanding detonation properties (D = 9.15-9.80kms-1, P = 38.24-44.40 GPa), and acceptable impact sensitivity lead C5, D8, E5, E7, F5, and F7 to be the potential candidates of HEDMs.

Evolving the Scope of 5,5’‐Azobistetrazoles in the Search for High Performing Green Energetic Materials

AbstractThe azotetrazole moiety represents a great platform for energetic materials, it offers a planar and nitrogen‐rich backbone, combined with a high heat of formation, which easily can be functionalized and tuned. Herein, we start from sodium 5‐aminotetrazolate and obtain two isomers by substitution reaction with 2‐chloroethanol. Azidoethyl and nitratoethyl substituted azo‐ tetrazoles were finally synthesized by oxidative azo coupling of the respective N‐ethyl functionalized 5‐aminotetrazole precursors using tert‐butyl hypochlorite as reagent. All compounds were analyzed through multicore NMR and IR spectroscopy as well as mass spectrometry. All solid compounds were further investigated using low‐temperature X‐ray crystallography. The purity was verified by CHNO elemental analysis and the decomposition temperature (DTA) and sensitivities toward impact, friction and electrostatic discharged were determined. Based on the CBS‐4M calculation results, the energetic properties were calculated using the EXPLO5 code.

1,3,4,5-Tetraamino-1,2,4-triazolium Cation: An Energetic Moiety.

The amination of 3,4,5-triamino-1,2,4-triazole with O-tosylhydroxylamine yielded the nitrogen-rich 1,3,4,5-tetraamino-1,2,4-triazolium cation as its tosylate salt. Subsequent metathesis reactions produced energetic salts with various energetic anions, including perchlorate, nitrate, nitrotetrazolate, and bistetrazolate diolate. All energetic salts possess relatively high heats of formation, thermal sensitivities, and detonation velocities and pressures. The prepared energetic salts were characterized chemically using single-crystal X-ray crystallography, elemental analysis, and 1H NMR, 13C NMR, and IR spectroscopy and energetically by measuring their thermal, impact, and friction sensitivities. 15N NMR was carried out on the tosylate salt. Energetic performances were determined by a combined experimental-computational method using calculated heats of formation and experimental crystal densities.