Narrow Results

Novel nonlinear optical (NLO) molecules are designed in order to meet their tremendous demand in the field of optics and electronics. The first attempt of the structural tailoring of disperse orange 3 (DO3)-an azodye is made to develop nineteen derivative molecules (D1-D19). Two approaches were opted for preparing 4 groups of molecules. The first one was the extension of $\pi$-conjugated system and the second strategy was the use of diverse electron donor and acceptor groups to develop unique donor-$\pi$-acceptor systems. Density functional theory (DFT) calculations were performed for in silico characterization of the studied molecules. The polarizability (${\alpha }_o$) and first-order hyperpolarizability (${\beta }_o$) values gave the insight of nonlinear optical response. All the designed molecules with extended conjugation and unique electron donor and acceptor group combination showed remarkably high ${\alpha }_o$ and ${\beta }_o$ values. The highest hyperpolarizability value (42477.48 a.u.) with several thousand increases than ${\beta }_o$ (7113.25 a.u.) of reference DO3, was depicted by D19 of the designed derivatives. This can be attributed to greater intramolecular charge transfer (ICT) in it. Various interaction studies were made and global reactivity descriptors (GRDs) were calculated to determine their chemical nature and stability. The outcome of our study suggests that the designed molecules are potential candidates for NLO applications, like energy conversion for producing tunable lasers and high-resolution spectroscopic studies.

Nitrosourea (NU) and hydroxyurea (HU) are recognized as chemotherapeutic agents. Their efficiency is restricted by the risk of misuse and the release of trace amounts of un-metabolized chemicals into the environment. Numerous potential negative effects may arise from the use of these drugs. Nanomaterials for drug detection are essential in pharmaceutical research, particularly cancer therapeutic applications such as HU and NU. This study sought to investigate the sensitivity of the C${}_{24}$N${}_{24}$ nanocage in detecting HU and NU via density functional theory (DFT). The interactions between HU/NU drugs and the C${}_{24}$N${}_{24}$ nanocage were investigated using optimized geometries, adsorption energies, FMO, NCI, NBO and QTAIM analyses via DFT and TD-DFT at the B3LYP-D3/6-31G(d,p) theoretical level. The adsorption energy estimations of –24.47 kcal/mol for the NUG complex and –19.90 kcal/mol for the HUB complex indicate that the HU/NU medicines are strongly adsorbed onto the C${}_{24}$N${}_{24}$, and the process is exothermic. NCI and QTAIM analyses have shown noncovalent interactions, primarily van der Waals forces, between C${}_{24}$N${}_{24}$ and HU/NU drugs. When HU/NU interacts with the C${}_{24}$N${}_{24}$ surface, new energy levels are generated in the C${}_{24}$N${}_{24}$ PDOS. Upon evaluating the $E_{\text{g}}$ value, sensitivity and recovery time as parameters of the nanocage’s sensing efficacy, it was determined that the HUB complex exhibits the best conductivity (5.67 × 10${}^{12}$ S/m), fine sensitivity (0.2560) and most stability due to its small energy gap of 1.67 eV value. The complex NUG has the lowest recovery time with a value of 5.15 × 10${}^{-17}$ s. As a result of its recovery time, the C${}_{24}$N${}_{24}$ nanocage is highly desirable for its potential application as an HU/NU drug sensor. This demonstrates that HU/NU drugs can be efficiently identified by the C${}_{24}$N${}_{24}$ nanocage. Our findings indicate that the C24N24 nanocage may enhance drug detection (HU/NU), indicating possible pathways for further advancement.

Accurate quantum computational chemistry has evolved dramatically. The size of molecular systems, which can be studied accurately using molecular theory is increasing very rapidly. Theoretical chemistry has opened up a world of new possibilities. It can treat real systems with predictable accuracy. Computational chemistry is becoming an integral part of chemistry research. Theory can now make very significant contribution to chemistry.

This review will focus on our recent developments in the theoretical and computational methodology for the study of molecular structure and molecular interactions. We are aiming at developing accurate molecular theory on systems containing hundreds of atoms. We continue our research in the following three directions: (i) development of new ab initio theory, particularly multireference-based perturbation theory, (ii) development of exchange and correlation functionals in density functional theory, and (iii) development of molecular theory including relativistic effects.

We have enjoyed good progress in each of the above areas. We are very excited about our discoveries of new theory and new algorithms and would like to share this enthusiasm with readers.

The dissociation and isomerization reactions of N₁₁ isomers, including the two structures 1 and 3 previously studied as well as the three new structures 2, 4, and 5, were investigated by the density functional theory (DFT) at the B3LYP/6-31G(d), B3LYP/6-311G(d), and B3LYP/6-311+G(3df)//B3LYP/6-311G(d) levels of theory. The results indicate that, similar to previous results on N₉ and N₁₀ isomers, the barrier heights for structures 1 and 2 to lose N₂ are about 10–15 kcal/mol whereas the barrier heights for structures 1–3 to lose N₃ are about 25–30 kcal/mol. Therefore, it seems that N₂ is easier to be eliminated than is N₃ from the relatively larger nitrogen isomers. In addition, for structures 1 and 3, both dissociation and isomerization can occur in the N8–N9 bond, due partly to its character in having an aromatic bond. Moreover, the isomerization is preferred compared with dissociation because of its relatively lower energy barrier height.

The Beijing Density Functional (BDF) program package is such a code that can perform nonrelativistic, one-, two-, and four-component relativistic density functional calculations on medium-sized molecular systems with various functionals in most compact and yet sufficient basis set expansions. The mergence of different approaches in a single code facilitates direct and systematic comparisons between different Hamiltonians, since they share all the same numerical and technical issues. In this account, the methodologies adopted in the code will be discussed in great detail and some applications of the code will be briefly presented.

Density functional method was applied to the study of the highly efficient primary explosive 2-diazo-4,6-dinitrophenol (DDNP) in both gaseous tautomers and its bulk state. Two stable tautomers were located. It was found that the structure (I) with open diazo, i.e. with linear CNN, is more stable than that with diazo ring tautomer (II) of DDNP. The structure I is in good agreement with the structure in the bulk. The lattice energy is -89.01 kJ/mol, and this value drops to -83.29 kJ/mol when a 50% correction of the basis set superposition error was adopted. The frontier bands are quite flat. The carbon atoms in DDNP make up the upper valence bands. While the lower conduction bands mainly consist of carbon and diazo N atoms. The bond populations of C–N bonds (both C–Nitro and C–Diazo) are much less than those of the other bonds and the detonation may be initiated through the breakdown of C–N bonds.

As a first step toward the understanding of the aminolysis reaction of β-sultam compounds, the ammonolysis and the effect of a second ammonia on the ammonolysis reactions of N-methyl β-sultam have been studied using Density Functional Theory (DFT) method at the B3LYP/6-31G* level. The exploration of the reaction processes proposed two different mechanisms: concerted and stepwise mechanisms. There is one pathway in concerted mechanism and two pathways in stepwise mechanisms: pathways a and b. The calculations of reaction energy barriers show that the nonconcerted route is the more favored one. Solvent effects were assessed by the PCM method. The results show that the pathway a in channel II is the most favorable in both cases. The presence of solvent disfavors the reaction, and the participation of ammonia in the ammonolysis reaction plays a positive role and reduces the active energy greatly. All transition states in the assisted ammonolysis are 45–65 kJ/mol lower than those for the non-assisted reaction. The results also show that the ammonolysis reaction have a higher energy barrier than the alcoholysis reaction. This low reactivity of amines is also observed in the reactions of N-benzoyl β-sultam and p-nitrophenyl toluene-p-sulfonate where there is a distinct preference towards oxygen nucleophiles.

Theoretical calculations have been carried out to study the mechanism of intermolecular hydroacylation of acetylene with acetaldehyde. The results indicate that the generally accepted mechanism, C–H oxidative addition followed by hydrometallation and reductive elimination, is the most favorable pathway. The intermolecular hydroacylation reaction of acetylene is shown to suffer from three problems: (1) the formation of a stable bis-acetylene intermediate, which leads to a facile dimerization side-reaction; (2) higher activation barrier of the rate-determining C–H (carbonyl) bond activation; and (3) "fatal" easy decarbonylation from hydrometalated intermediate and subsequent reductive elimination to give decarbonylation product. Our calculations indicate that the above three problems can all be overcome by the use of a tethered carbonyl substrate with a chelation-forming basic donor ligand such as alkylthio group.

The first structural characterization for the twelve binuclear alkaline-earth metal compounds M₂(ηⁿ-N₅)₂ (M=Be, Mg; n = 1, 2) and Ca₂(ηⁿ-N₅)₂(n = 2, 5) have been optimized with local energy minimum by density functional theory (DFT). The most energetically favored structures in M₂(ηⁿ-N₅)₂(M=Be, Mg, Ca) are of D_2d symmetry Be₂(η¹-N₅)₂, Mg₂(η²-N₅)₂ and Ca₂(η²-N₅)₂ and the metal–metal distances are 2.03 Å for Be-Be, 2.77 Å for Mg-Mg and 3.72 Å for Ca-Ca, which are significantly shorter than the experiment values of weakly bound bare diatomic Be₂, Mg₂ and Ca₂.^1,2Ca₂(η⁵-N₅)₂ (D_5d or D_5h) is the only stable specie with sandwiched structure, bearing an even shorter Ca-Ca distance of 3.66 Å, and lying 24 kcal/mol higher in energy than the D_2d structure. The dissociation enthalpies of the twelve M₂(ηⁿ-N₅)₂(M=Be, Mg, Ca) to two M(ηⁿ-N₅) fragments are predicted to be 72.6–73.1, 41.2–43.8, and 27.4–29.7 kcal/mol, respectively, implying a substantial metal–metal bonding. Natural bond orbital (NBO) analysis suggests that metal–metal bonds are of σ-bond. The natural charge of the alkali earth metal atom in the twelve M₂(ηⁿ-N₅)₂ species is larger than +0.88, which is consistent with the +1 oxidation state of the metal atoms. Nucleus-independent chemical shift (NICS) values confirm that the planar exhibits characteristics of aromaticity for these M₂(ηⁿ-N₅)₂ species.

In this paper, we have performed the optimized structures of the red emitting material, 4-(dicyanomethylene)-2-methyl-6-[p-(dimethyl amino) styryl]-4H-pyran (DCM), with different polarity solvent environments by using the density functional theory (DFT) method, B3LYP/6-31G*. The time-dependent density functional theory (TD-DFT) and the polarizable continuum model (PCM) have been used to obtain the optical properties in the solvent environment. It has been observed that when the solvent polarity increases, the DCM molecule exhibits the red shift in the maximum absorption wavelength and enhances the oscillator strength (f). The solvent polarity also enhances the electron transfer ability from the electron-donating dimethylamine group (-N(CH₃)₂) to the electron-withdrawing =C(CN)₂ group. The S₀ → S₁ transition of DCM is found to be π–π*. The maximum absorption wavelengths of different solvent environments are found to be consistent with the reported experimental results.

The Oxidation potentials of pyrogallol and some of its derivatives in aqueous solutions have been calculated. The calculations have been performed using ab initio molecular orbital calculations (HF), and density functional theory (DFT) with the inclusion of entropic and thermochemical corrections to yield free energies of redox reactions. The polarizable continuum model is used to describe the solvent. It was also obtained experimentally with the aid of an electrochemical technique (cyclic voltammetry). The theoretical and experimental values for the oxidation potential of the studied pyrogallol and some derivatives are in excellent agreement with each other and there is only a discrepancy of 0.025 V and 0.020 V for B3LYP and HF methods, respectively, between experimental and theoretical results. The agreement mutually verifies the accuracy of the experimental method and the validity of the applied mathematical model.

The electrode potentials of (E)-3-(4,5-dihydroxy-2-(phenylsulphonyl) phenyl) acrylic acid (DPA), as a new caffeic acid derivative, in aqueous solution have been calculated. DPA has two geometric structures, cis and trans. Since the cis structure of caffeic is unstable, it cannot be found in nature, but in this research, its electrode potential have been calculated theoretically. The calculations have been performed using ab initio molecular orbital calculations (HF), and density functional theory (DFT) with the inclusion of entropic and thermochemical corrections to yield free energies of redox reactions. The electrode potential was also obtained experimentally by means of an electrochemical technique (cyclic voltammetry) and it was 335 mV for trans structure. The theoretical and experimental values for the electrode potential of the studied molecule are in excellent agreement. Geometric parameters and vibrational frequencies values of DPA and (2E)-3-(3,4-dioxo-6-(phenylsulfonyl) cyclohexa-1,5-dienyl)acrylic acid (DPDA is the oxidized form of DPA), were computed using same methods. The calculated IR spectrum of DPA used for the assignment of IR frequencies was observed in the experimental FT-IR spectrum. Correlations between theoretical and experimental vibrational frequencies of DPA molecule were 0.996. The agreement mutually verifies the accuracy of the experimental method and the validity of the applied mathematical model.

Triphenyldichlorophosphorane (Ph₃PCl₂) in solution was studied with cluster model of discrete water molecules and the Onsager reaction field model. The geometry of Ph₃PCl₂–H₂O cluster is optimized with DFT-PW91 and DZVP basis sets. A weak hydrogen bond is formed between Cl and H atom of the water molecule. The geometry data of Ph₃PCl₂ molecule is altered due to the presence of water molecule. The bond length of P–Cl₂ increases from 2.501 Å to 2.832 Å, and the P–Cl₁ bond length decreases from 2.113 Å to 2.064 Å. Three C–P bonds have slightly changed. The geometrical parameters of the three benzene rings do not change much upon clustering. To compare with solution structure, gas structure of Ph₃PCl₂ also was investigated. Four stationary structures were found. The trigonal bipyramidal structure C is a penta-coordinated structure and the most stable one. This is in agreement with the previous research. The trigonal bipyramidal structure C and the ionic structure A has no imaginary frequency. The structure B and structure D have an imaginary frequency of -41.9 cm^-1 and -7.4 cm^-1, respectively.

The transition states for the H₂NO decomposition and rearrangements mechanisms have been explored by the CBS-Q method or by density functional theory. Six transition states were located on the potential energy surface, which were explored with the Quadratic Complete Basis Set (CBS-Q) and Becke's one-parameter density functional hybrid methods. Interesting deviations between the CBS-Q results and the B1LYP density functional theory lead us to believe that further study into this system is necessary. In the efforts to further assess the stabilities of the transition states, bond order calculations were performed to measure the strength of the bonds in the transition state.

The adsorption of CN on Cu(111) has been investigated using density functional theory calculations based on plane-wave expansion and pseudo-potential treatment. Calculations within the generalized gradient approximation predicted a preference for CN in the fcc C-down site. No stationary points corresponding to pure parallel mode were found. But the tilted mode was found to be achievable. The calculated vibrational frequencies of CN were used to correctly discriminate between the adsorption sites.

A series of atomic energy formulas that relate atomic energies to the electrostatic potentials V₀ at nuclei are obtained by a series of polynomial and series fits of V₀ versus nuclear charge (Z). Density functional and Hartree–Fock V₀ are used for a series of fits that involve an isoelectronic series of anions, cations, and neutral ground state atoms to approximate atomic energies. Comparisons to the exact energies were performed in order to demonstrate the efficacy of the rigorous expressions.

Three endohedral fullerenes C₂H₂–C₆₀, C₂H₄–C₆₀, and C₂H₆–C₆₀ are investigated theoretically using density functional theory. Their electronic and structural properties are studied. The calculations suggest that the formations of these complexes are endothermic; the dopant and C₆₀ cage affect each other rarely except for the slight distortion of C₆₀ cage and compression of the hydrocarbon molecules. A small quantity of electron transfer from C₆₀ to the hydrocarbon molecule was also observed. Accordingly, C₆₀ could theoretically be a good container for some small hydrocarbon molecules.

Linkages between vinylidene (=C:) and nitrenes (-N:), through methyne and its derivatives (CX), give allylic carbenonitrenes, C=(X)C–N, as a new brand of reactive intermediates, with conceivable singlet, triplet, or quintet ground states (X=H, 1, X=CH₃, 2, X=COOH, 3, X=F, 4, X=OH, 5, XX=OMe, 6, XX=CF₃, 7, XX=CN, 8, and XX=NH₂, 9). High-spin quintet (⁵A″) ground states are found for 1 and 2 at eight ab initio and DFT levels of theory. At the same levels, triplet (³A″) ground states prevail for 3–8. Low-spin singlet (¹A') input structures of 1, 2, and 4–9 cyclize spontaneously through optimization to their corresponding aromatic X-azacyclopropenylidenes, with multiplicities irrelevant to allylic carbenonitrenes. Researchers may aim for generating 3–8 with triplet states, or even 1 and/or 2 with quintet states, but we do not recommend going for generation of 9 with any multiplicity, and/or formation of 1–8 with singlet states.

Hydrolysis of trans-dichloro(ammine)(quinoline)platinum, a novel potential anticancer drug, is believed to be the key activation step before the drug reaches its intracellular target DNA. To obtain an accurate hydrolysis mechanism for this nonclassical class of square-planar Pt(II) complex, five different models were used at the experimental temperature with the solvent effect B3LYP/PCM using hybrid density functional theory. The stationary points on the potential energy surfaces for the first and second hydrolysis steps, proceeding via a five-coordinate trigonal-bipyramidal (TBP)-like structure of transition state, were fully optimized and characterized. The most remarkable structural variations in the hydrolysis process were found to occur in the equatorial plane of the TBP-like structures of the intermediates and transition states. It was found that the explicit solvent effect originating from the inclusion of extra water molecules into the system is significantly stronger than those arising from the bulk aqueous medium, especially for the first aquation step, which emphasizes the use of appropriate models for these types of problems. The results give detailed energy profiles for the mechanism of hydrolysis of trans-dichloro(ammine)(quinoline)platinum, which may assist in understanding the reaction mechanism of the drug with DNA target and in the design of novel platinum-based anticancer drugs with trans geometries.

The mechanism of DNA damage caused by the isomerization of purine base is studied with density functional theory calculations at the B3LYP/6-311+G(d,p) level. The transition states of all the isomerizations are obtained, and the intrinsic reaction coordinate (IRC) analyses are performed to identify these transition states further. The isomerizations of purine bases can be classified into two types. The first is the hydrogen transfer between atoms, whose transition state includes a four-member ring. The second is the bond N–H rotation about the double bond N=C, and the plane CNH is perpendicular to the molecular plane in its transition state. The hydrogen transfer has higher reaction potential barrier, larger tunnel effect, and smaller equilibrium constant and rate constant than that of the N–H rotation. Effects of the hydration are considered in the framework of the polarizable continuum model (PCM) in SCRF method at the B3LYP/6-311+G(d,p) level. The isomerizations which result in the configuration changes of purine base and bring directly the DNA damage are endothermic and thermodynamic nonspontaneous processes. The probability of DNA damage caused by the guanine isomerization is larger than that by adenine.