Role of Molecular Singlet Oxygen in Photochemical Degradation of NTO: DFT Study

NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, may be released to the environment and dissolved in surface water and groundwater due to its good water solubility. Singlet oxygen is an important reactive oxygen species produced in the aquatic environment under sunlight irradiation. A detailed investigation of the possible mechanism for NTO decomposition in water induced by singlet oxygen as one of the pathways for NTO environmental degradation was performed by a computational study at PCM(Pauling)/M06-2X/6-311++G(d,p) level. Decomposition of NTO was found to be a multistep process that may begin with singlet oxygen attachment to the carbon atom of the CN double bond. The formed intermediate undergoes cycle opening, and nitrogen gas, nitrous acid, and carbon (IV) oxide elimination. Isocyanic acid, arisen transiently, hydrolyzes into ammonia and carbon (IV) oxide. The obtained results show a significant increase in reactivity of the anionic form of NTO as compared to its neutral form. The calculated activation energies and high exothermicity of the studied processes support the contribution of singlet oxygen to NTO degradation into low-weight inorganic compounds in the environment

Structure and Energetics of (111) Surface of γ‑Al₂O₃: Insights from DFT Including Periodic Boundary Approach

The (111) surface of γ-alumina has been reexamined, and a new (111) surface model has been suggested. The local structure of this new surface of γ-alumina, (111)_n, has been optimized by the density functionals along with the full electron basis sets by using periodic boundary condition. This newly modeled (111)_n surface is characterized by the same stability as that of the (110) surface, and its surface energy amounts to 2.561 J/m², only about 0.002 J/m² larger than that of (110). Three different types of the tricoordinated Al centers have been identified on (111)_n. Molecular orbital (MO) analysis and the population analysis demonstrate that one type of Al, Al­(I), is a nonpaired electron center. The singly occupied MO on Al­(I) center is expected to play an important role in the catalytic activities of the γ-alumina. Moreover, the neighboring Al (Al­(III)) on the (111)_n surface provides suitable acceptance position for the electron donating groups. The defected surfaces of (111)_n are found to be having a similar stability. The detachment of Al­(I) from the (111)_n surface results in disappearance of the nonpaired electron centers. Meanwhile, the attachment of Al­(I) on (111)_n surface will produce rich nonpaired electron centers on this new surface. Therefore, this newly defined surface is expected to attract the research interests in the catalytic activities of γ-alumina

Toward Selection of Efficient Density Functionals for van der Waals Molecular Complexes: Comparative Study of C–H···π and N–H···π Interactions

We have evaluated the performance of two of the recently developed density functionals (M06-2X and B2PLYP-D), which are widely used, by considering three important prototype systems, including benzene–acetylene, benzene–methane, and benzene–ammonia, possessing C–H···π or N–H···π interactions. Computational results are compared with the available experimental data. Considered density functionals are from two different classes: hybrid meta density functional (M06-2X) and double hybrid density functional (B2PLYP-D). The performance of a range of basis sets (6-31G­(d), 6-31+G­(d), 6-31+G­(d,p), 6-311G­(d,p), 6-311+G­(d,p), aug-cc-pVXZ (X = D, T, Q)) with the above-mentioned two density functionals was evaluated. Comparison of the results includes Pople’s basis sets versus Dunning’s correlation consistent basis sets with the M06-2X and B2PLYP-D functionals considered in this study. The basis set effect on geometrical parameters, dissociation energies, and selected vibrational frequency shifts was thoroughly analyzed. We have addressed whether the counterpoise corrections with geometry optimizations and vibrational frequencies are important. Our computational study reveals that calculations carried out with smaller basis sets very well reproduce the reported experimental values of dissociation energies. The present study also shows that using the very large Dunning’s correlation consistent basis set worsens the results. The necessity of including counterpoise correction for binding energies depends on the system and the type of method used. In general, vibrational frequency calculations using these DFT functionals generate characteristic red shifts for the C–H···π or N–H···π interactions in the complexes

Role of Hydroxyl Radical in Degradation of NTO: DFT Study

Hydroxyl radicals are important reactive oxygen species produced in the aquatic environment under sunlight irradiation. Many organic pollutants may be decomposed as they encounter hydroxyl radicals, due to their high oxidative ability. NTO (5-nitro-1,2,4-triazol-3-one), an energetic material used in military applications, may be released to the environment and dissolved in surface water and groundwater due to its good water solubility. A detailed investigation of the possible mechanism for NTO decomposition in water induced by hydroxyl radical as one of the pathways for NTO environmental degradation was performed by computational study at the PCM/M06-2X/6-311++G(d,p) level. Decomposition of NTO was found to be a multistep process that may begin with an addition of hydroxyl radical to the carbon atom of CN double bond and consequent release of a nitrite radical. The formed intermediate undergoes a series of chemical transformations that include the attachments of hydroxyl radical to carbon atoms, the transfer of hydrogen to hydroxyl radical, isomerization, and bond cleavage, leading to low-weight inorganic compounds, such as ammonia, nitrogen gas, nitrous acid, nitric acid, and carbon(IV) oxide. The anionic form of NTO is more reactive toward interaction with the hydroxyl radical as compared with its neutral form. Calculated activation energies and high exergonicity of the studied process support the significant contribution of the hydroxyl radical to NTO mineralization in environment

Electron Attachment to the Cytosine-Centered DNA Single Strands: Does Base Stacking Matter?

Electron attachment to the trimer of nucleotide, dGpdCpdG, has been investigated by a quantum mechanical approach at a reliable level of theory. The study of the electron attached dGpdCpdG species demonstrates that cytosine contained DNA single strands have a strong tendency to capture low-energy electrons and to form electronically stable cytosine-centered radical anions. The comparative study of the model molecules pdCpdG and dGpdCp reveals that base stacking has little contribution to the adiabatic electron affinity (AEA) of cytosine in DNA single strands. Additionally, the base–base stacking does not affect the vertical detachment energy (VDE) of the cytosine-centered radicals. Intrastrand H-bonding is found to be critical in increasing the values of the AEA and VDE. However, base–base stacking is revealed to be important in enlarging the vertical electron affinity (VEA) of cytosine. The electron attachment to the cytosine moiety intensifies the intrastrand H-bonding between the neighboring G and C bases. This process disrupts the base–base stacking interaction in the radical anion of dGpdCpdG

On the Stability of Perfluoroalkyl-Substituted Singlet Carbenes: A Coupled-Cluster Quantum Chemical Study

A series of trifluoromethyl-substituted carbenes R–C(:)–CF₃ (R = NMe₂, OMe, F, PMe₂, P­(NMe₂)₂, P­(N­(Pr-i)₂)₂, SMe, Cl); (dimethylamino)­(perfluoroalkyl)­carbenes Me₂N–C­(:)–R (R = CF₃, C₂F₅, <i>n</i>-C₃F₇, <i>i</i>-C₃F₇, and <i>t</i>-C₄F₉) and symmetrically substituted carbenes R–C(:)–R (R = NMe₂, OMe, F, PMe₂, SMe, Cl) have been investigated by means of quantum chemistry methods. Different levels of approximation were used, including the CCSD­(T) approach also known in quantum chemistry as the “golden standard”, in combination with three different basis sets (TZVP, cc-pVDZ, cc-pVTZ). Relative stabilities of carbenes have been estimated using the differences between the singlet and triplet ground state energies (Δ<i>E</i>_ST) and energies of the hydrogenation reaction for the singlet and triplet ground states of the carbenes. The latter seem to correlate better with stability of carbenes than the Δ<i>E</i>_ST values. The ¹³C NMR chemical shifts of the methylidene carbon indicate the more high-field chemical shift values in the known, isolable carbenes compared to the unstable ones. This is the first report on the expected chemical shifts in the highly unstable singlet carbenes. Using these criteria, some carbene structures from the studied series (as, for instance, Me₂N–C­(:)–CF₃, Me₂N–C­(:)–C₃F₇-<i>i</i>) are proposed as good candidates for the experimental preparation

Geometric, Magnetic, and Adsorption Properties of Cross-Linking Carbon Nanotubes: A Computational Study

Cross-linking carbon nanotubes (CLCNTs) composed of three axially confined single-walled carbon nanotubes (SWCNTs) of the (10,0) type are investigated by plane-wave density functional theory (DFT). Three CLCNT models, differing from each other by the structure of the contact regions of the three SWCNT constituents, are explored in terms of their geometric, electronic, and magnetic properties. Various magnetic phases, as obtained by combining finite SWCNTs in ferromagnetic (FM) or antiferromagnetic (AFM) coordination, are distinguished. The characteristics of these phases are shown to depend on the contact region geometry which plays an essential role in defining the order of their stabilities. For a selected CLCNT, adsorption of hydrogen atoms is discussed. The magnetic features of the CLCNTs turn out to hold the key for understanding the site dependence of the hydrogen atom adsorption energies

Binding of Alkali Metal Ions with 1,3,5-Tri(phenyl)benzene and 1,3,5-Tri(naphthyl)benzene: The Effect of Phenyl and Naphthyl Ring Substitution on Cation−π Interactions Revealed by DFT Study

The effect of substitution of phenyl and naphthyl rings to benzene was examined to elucidate the cation−π interactions involving alkali metal ions with 1,3,5-tri­(phenyl)­benzene (TPB) and 1,3,5-tri­(naphthyl)­benzene (TNB). Benzene, TPB, and four TNB isomers (with ααα, ααβ, αββ, and βββ types of fusion) and their complexes with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ were optimized using DFT approach with B3LYP and M06-2X functionals in conjunction with the def2-QZVP basis set. Higher relative stability of β,β,β-TNB over α,α,α-TNB can be attributed to <i>peri</i> repulsion, which is defined as the nonbonding repulsive interaction between substituents in the 1- and the 8-positions on the naphthalene core. Binding energies, distances between ring centroid and the metal ions, and the distance to metal ions from the center of other six-membered rings were compared for all complexes. Our computational study reveals that the binding affinity of alkali metal cations increases significantly with the 1,3,5-trisubstitution of phenyl and naphthyl rings to benzene. The detailed computational analyses of geometries, partial charges, binding energies, and ligand organization energies reveal the possibility of favorable C–H···M⁺ interactions when a α-naphthyl group exists in complexes of TNB structures. Like benzene-alkali metal ion complexes, the binding affinity of metal ions follows the order: Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺ for any considered 1,3,5-trisubstituted benzene systems. In case of TNB, we found that the strength of interactions increases as the fusion point changes from α to β position of naphthalene

Interaction of 2′-Deoxyadenosine with <i>cis</i>-2-Butene-1,4-dial: Computational Approach to Analysis of Multistep Chemical Reactions

The computational analysis of multistep chemical interactions between 2′-deoxyadenosine and <i>cis</i>-2-butene-1,4-dial has been performed. The applied protocol includes generation of a multistep Gibbs free-energy reaction profile (PCM/M05-2X/6-311+G­(d) level) for the transformations of the reagents to products, followed by evaluation of the rate constants, construction of the corresponding kinetic equations, and solving them. Such a procedure allows one to significantly extend the number of experimentally determined steps by addition of the ones computationally predicted. The primary products of the reaction are found to be four diastereomeric adducts characterized by virtually the same stability. The acid-catalyzed dehydration of these adducts leads to a more stable secondary product. Computational verification of UV and NMR spectra has also been performed. It has been revealed that simulated UV and NMR spectra of primary and secondary 2′-deoxyadenosine adducts of <i>cis</i>-2-butene-1,4-dial are in agreement with the experimental observations

How the “Liquid Drop” Approach Could Be Efficiently Applied for Quantitative Structure–Property Relationship Modeling of Nanofluids

The main goal of this paper is the evaluation of the applicability of the geometrical “liquid drop” model (LDM) to describe physicochemical properties of nanofluids in quantitative structure–property relationship (QSPR) modeling. LDM-based descriptors are size-dependent, which allows them to be applied for a series of nanoparticles with the same chemical composition but different sizes. Thermal conductivity of nanofluids as the target property was investigated. Random forest regression as a nonparametric approach was utilized to determine important structural features of nanofluids responsible for enhancing their thermal conductivity