Ab Intio Methods Research Articles

Electronic Properties of Aldehyde Complexes Using DFT for Electrooptical Activity

Characterization of materials infer for physical and chemical properties that depend on its molecular structure. Structure of molecule has its dependence on respective electrons of molecule under consideration occupying their positions that correspond to changes in density of electrons. Many theories of its kind were developed to study density of electrons with roots from wavefunction method and electron density method. Wavefunction method has its dependence with linear combination of atomic orbitals, Born approximation, variational principle ,potential energy surfaces for development of Huckel theory, Hartree fock self-consistent theory. Electron density method includes Ab-intio method and density functional theory is possible with Kohenberbg-Kohn existence theorem and Kohn Sham formalism. Density functional studies has diverted attention of researches for properties dependent on structure with use of quantum mechanical descriptors that influence chemical reactivity of molecule forming complexes with properties responsible for electrooptical activity. In the present work complexes with p-anisaldehyde were studied with set of anilines using Gaussian 16 package with B3LYP method. Studies in present work were analyzed from computed infrared spectra responsible for formation of complexes with shifts in wavenumbers; quantum mechanical descriptors for electronic properties. A feature of study is that complexes with p-nitroaniline have greater tendency influence on electronic properties responsible for electrooptical activity due to electrophilic nature.

Conformational inversion-topomerization processes of ethylcyclohexane and 1,2-dimethylcyclohexane: A computational investigation

To deeply reveal the impact of the substituents and their special orientations in ring on conformational behaviors for substituted cyclohexanes, a comprehensive study of ethylcyclohexane, cis-, and trans-1,2-dimethylcyclohexanes has been carried out. All conformational structures for them were captured by the accurate ab intio method, that is, B3LYP/6-311++G(d,p) method was used for geometry optimizations, and MP2/6-311++G(d,p), G4, and CCSD(T)/6-311++G(d,p) methods were applied for the high-level single point energy refinements. Based on CCSD(T)/6-311++G(d,p) quantum results, the conformational populations of minima for these three substituted cyclohexanes were calculated by Boltzmann distribution over 300-2500 K. Additionally, the conformational inversion-topomerization pathways for them were thoroughly investigated. The complete characterization involved in their potential energy surfaces are clearly presented by three or two-dimensional schemes.

Interstellar hydrogen bonding

This paper reports the first extensive study of the existence and effects of interstellar hydrogen bonding. The reactions that occur on the surface of the interstellar dust grains are the dominant processes by which interstellar molecules are formed. Water molecules constitute about 70% of the interstellar ice. These water molecules serve as the platform for hydrogen bonding. High level quantum chemical simulations for the hydrogen bond interaction between 20 interstellar molecules (known and possible) and water are carried out using different ab-intio methods. It is evident that if the formation of these species is mainly governed by the ice phase reactions, there is a direct correlation between the binding energies of these complexes and the gas phase abundances of these interstellar molecules. Interstellar hydrogen bonding may cause lower gas abundance of the complex organic molecules (COMs) at the low temperature. From these results, ketenes whose less stable isomers that are more strongly bonded to the surface of the interstellar dust grains have been observed are proposed as suitable candidates for astronomical observations.

Spectroscopic and structural investigation for the ground and excited states of CaNa+ molecular ion

In the current theoretical study, we investigated several electronic states correlated with the {Ca+Na+} and {Ca++Na} asymptotic limits of different symmetries (Σ+, Π, Δ). Our calculations were based on ab intio method using semi-empirical pseudo-potential theory of both cores Na+ and Ca2+ and Full Configuration Interaction (FCI). Hence, we computed the adiabatic potential energy curves (PECs) and vibrational levels of the ground state along with several higher states of (CaNa)+ molecular ion. From these curves, we extracted all related spectroscopic parameters (De, D0, Te, Re, Be, ωe and ωeχe). Dipolar properties of (CaNa)+ such as Permanent and Transition Dipole Moments (PDM, TDM) were determined and analyzed. Numerous Avoided Crossings (ACs) were detected in PECs and their reflections were clearly observed in PDM and TDM functions. The strong interactions could lead to significant charge or excitation transfer for atom–ion collisions in the diverse charge or excited states.

Structural evolution and solvation of the OH radical in ionized water radical cations (H2O)n(+), n = 5-8.

Structural evolution of ionized water radical cations (H2O)n(+), n = 5-8, is studied by ab intio methods. A structure searching method based on the previous understanding of the hydrogen bond (H-bond) network in neutral and protonated water clusters is found to be effective, covering a wide range of structural isomers of (H2O)n(+). With these local minima, we can analyze both the size and temperature dependence of the structure of (H2O)n(+) and solvation of the OH radical. Agreement between our calculated IR spectra and experimental data in the free OH stretching region confirms that the OH radical preferred to stay on the terminal site of the H-bond network at n = 5 and n = 6. Furthermore, we found that the OH radical began to form H-bonds with water molecules as a H-bond donor at n = 7 and 8. Vibrational signatures of fully solvated OH were found to be located at 3200-3400 cm(-1) coinciding with the additional peaks found in previous experimental data obtained by Mizuse and Fujii.

Externally predictive quantitative modeling of supercooled liquid vapor pressure of polychlorinated-naphthalenes through electron-correlation based quantum–mechanical descriptors

For predicting physico-chemical properties related to environmental fate of molecules, quantitative structure–property relationships (QSPRs) are valuable tools in environmental chemistry. For developing a QSPR, molecular descriptors computed through quantum–mechanical methods are generally employed. The accuracy of a quantum–mechanical method, however, rests on the amount of electron-correlation estimated by the method. In this work, single-descriptor QSPRs for supercooled liquid vapor pressure of chloronaphthalenes and polychlorinated-naphthalenes are developed using molecular descriptors based on the electron-correlation contribution of the quantum–mechanical descriptor. The quantum–mechanical descriptors for which the electron-correlation contribution is analyzed include total-energy, mean polarizability, dipole moment, frontier orbital (HOMO/LUMO) energy, and density-functional theory (DFT) based descriptors, namely, absolute electronegativity, chemical hardness, and electrophilicity index. A total of 40 single-descriptor QSPRs were developed using molecular descriptors computed with advanced semi-empirical (SE) methods, namely, RM1, PM7, and ab intio methods, namely, Hartree–Fock and DFT. The developed QSPRs are validated using state-of-the-art external validation procedures employing an external prediction set. From the comparison of external predictivity of the models, it is observed that the single-descriptor QSPRs developed using total energy and correlation energy are found to be far more robust and predictive than those developed using commonly employed descriptors such as HOMO/LUMO energy and dipole moment. The work proposes that if real external predictivity of a QSPR model is desired to be explored, particularly, in terms of intra-molecular interactions, correlation-energy serves as a more appropriate descriptor than the polarizability. However, for developing QSPRs, computationally inexpensive advanced SE methods such as PM7 can be more reliable than the expensive ab inito methods.

Molecular dynamics simulation of helium behavior in tungsten matrix

The helium behavior in tungsten matrix is investigated by means of molecular dynamics. Firstly, the He-W potential is created by combining the ZBL potential with the data from an ab intio method. The formation energy calculations predict that the most stable configuration for helium in interstitial position is the tetrahedral site, which is in good agreement with recent research results. The helium diffusion is simulated in great detail in a temperature range from 400 K to 1200 K, and the migration energy is obtained to be between the experimental data and the ab intio calculation result. Finally, the mechanism of helium accumulation in its initial stage is investigated from the viewpoint of energy. It is found that as the helium cluster grows, the binding energy of each additional helium atom to the cluster tends to increase, which is conducible to the further growth of the helium cluster.

Ab-initio CI calculations for 3d transition metal L2,3 X-ray absorption spectra of TiCl4 and VOCl3

X-ray-absorption near-edge structures (XANES) at the transition metal (TM) L2,3-edge of TiCl4 and VOCl3 are calculated by the all-electron configuration interaction (CI) method using fully-relativistic molecular spinors with density functional theory (DFT). The electronic excitation from molecular spinors mainly composed of ligand p atomic spinors to those of TM-3d spinors, i.e., the charge transfer, is included by taking additional electronic configurations in the CI. The effects of the ligand field and the charge transfer on the TM-L2,3 XANES are investigated by an ab-intio method. The reduction of inter-electron interaction due to the covalent bonding between TM and ligand atoms has been found. The contribution of charge transferred configuration is small at the initial state of TM-L2,3 XANES while the significant amount of charge transferred configuration is found at the final states. The spectral shapes of TM-L2,3 XANES of both TiCl4 and VOCl3 are strongly modified by the charge transfer effects.

On the Relative Preference of Enamine/Iminium Pathways in an Organocatalytic Michael Addition Reaction

The mechanism of the organocatalyzed Michael addition between propanal and methyl vinyl ketone is investigated using the density functional and ab intio methods. Different modes of substrate activation offered by a secondary amine (pyrrolidine) organocatalyst are reported. The electrophilic activation of enone (P-I) through the formation of an iminium ion, and nucleophilic activation of propanal (P-II) in the form of enamine have been examined by identifying the corresponding transition states. The kinetic preference for the formation of key intermediates is established in an effort to identify the competing pathways associated with the title reaction. A comparison of barriers associated with different pathways as well as intermediate formation allows us to provide a suitable mechanistic rationale for Michael addition reactions catalyzed by a secondary amine. The overall barriers for the C-C bond formation pathways involving enol or iminium intermediates are identified as higher than the enamine pathway. Additionally, the generation of iminium is found to be less favored as compared to enamine formation. The effect of co-catalyst/protic solvent on the energetics of the overall reaction is also studied using the cluster continuum approach. Significant reduction in the activation energies for each step of the reaction is predicted for the solvent-assisted models. The co-catalyst assisted addition of propanal-enamine to methyl vinyl ketone is identified as the most preferred pathway (P-IV) for the Michael addition reaction. The results are in concurrence with the available experimental reports on the rate acceleration by the use of a co-catalyst in this reaction.

Synthesis, dynamic 1H NMR and theoretical study of aryl-nitrogen single bond rotational energy barriers in highly functionalized 4H-chromenes

The reactive intermediate was generated by reaction between 2,6-dimethylphenyl isocyanide and dialkyl acetylenedicarboxylates to react with β-diketones such as 1,3- cyclohexanedione or 5,5- dimethyl-1,3-cyclohexanedione to produce the dialkyl 2-(2,6-dimethylphenylamino)-5-oxo- 5,6,7,8-tetrahydro-4H-chromene-3,4-dicarboxylate in fairly high yields 2a-c. The 1 H NMR spectra of these compounds exhibited dynamic effects that are attributed to restricted rotation around the aryl-nitrogen single bond. The calculated rotational energy barrier (∆G ≠ ) for their interconversion of these compounds equals (57.2, 54.0 and 55.7)±2 kJ.mol -1 , respectively. In addition, theoretical study on the basis of rotation around the aryl-nitrogen single bond was investigated using ab intio method at HF/6-31G level theory. The theoretical rotational energy barrier for these interconversion were in a good agreement with the experimental rotational energy emerged from dynamic 1 H NMR data.

Basis set and electron correlation effects in all-electron ab initio calculations of the static dipole polarizability of small cadmium selenide clusters, (CdSe) n, n = 1,2,3,4

We have calculated static dipole polarizabilities for small cadmium selenide clusters relying on finite-field Møller–Plesset perturbation theory and coupled cluster calculations. In addition to the conventional ab intio methods we have also used easily accessible density functional theory (DFT) based approximations. To avoid systematic errors, we have employed two sequences of purpose-oriented basis sets built upon drastically different substrates. Our findings reveal significant patterns in the performance of conventional ab intio and DFT methods. Of particular importance is the systematic divergence of the performance of the B3LYP and B3PW91 DFT methods: α ¯ ( B3LYP ) > α ¯ ( B3PW91 ) and ∣Δ α(B3LYP)∣ > ∣Δ α(B3PW91)∣ for (CdSe) n , 2 ⩽ n ⩽ 4.

Can the Methoxyradical CH3O Act as Sink for Cl and ClO in the Atmosphere?

Several reaction pathways on the potential energy surface for the reaction CH3O + ClO are investigated using ab intio methods. As a result we find two reaction pathways to be most probable: (i) the formation of CH2O + HOCl via hydrogen abstraction from the methoxy radical and (ii) the formation of the low-lying adduct CH3OOCl. The other adduct CH3OClO is calculated to lie about 25 kcal/mol higher in energy than CH3OOCl. CH3OOCl can be considered to be a sink for Cl and ClO radicals on the ground-state surface. We have also performed MRD-CI calculations for the excited states of CH3OOCl and CH3OClO to study their photochemistry. In CH3OOCl we calculated a strong transition (11a → 14a) at 4.80 eV corresponding to a σ(Cl−O)−σ*(Cl−O) excitation. We find all low-lying excited singlet and triplet states in the energy range between 2.5 and 5.5 eV to be highly repulsive toward Cl−O and O−O cleavage, and thus photodissociation pathways forming the reactive radicals Cl and ClO from CH3OOCl are very probable.

Theoretical Study of the SC3Hm+Systems,m= {3, 4}

A comprehensive theoretical study of the potential energy surfaces of the lowest-lying doublet and quartet electronic states of the SC3H4+ system has been performed by means of a combination of density functional and ab intio methods. The potential energy surfaces corresponding to the lowest doublet and quartet electronic states have turned out to be extremely complex. Twenty-six doublet species and twenty-four quartet species have been identified, some of them having several stable conformations. The quartet states are normally much higher in energy than the doublet states. The absolute minimum is the methylthioketene cation, but the 2H-thiete cation, the thioacrolein cation, and the methylthiirene cation lie very close in energy; all of them are doublet states. The most stable quartet species is the thioxyallyl cation. In addition, a rather detailed analysis of the fragmentation of this system has been made. All of the products that are obtained in the S+ + CH3CCH reaction, which is an important step in...