Journal of Theoretical and Computational Chemistry

The reaction mechanism of SiHF radical with HNCO has been investigated by the B3LYP method of density functional theory (DFT), while the geometries and harmonic vibration frequencies of reactants, intermediates, transition states and products have been calculated at the B3LYP/6-311++G** level. To obtain more precise energy result, stationary point energies were calculated at the CCSD(T)/6-311++G**//B3LYP/6-311++G** level. In temperature range of 100 K to 1900 K, the statistical thermodynamics and Eyring transition state theory with Winger correction are used to study the thermodynamic and kinetic characters of the channel with low energy barrier at 1.0 Atm. SiHF + HNCO → IM8 → TS8 → SiFNHCHO(P3) was the main channel with low potential energy in the singlet state, SiFNHCHO was the main product. The analyses for the combining interaction between SiHF radical and HNCO with the atom-in-molecules (AIM) theory have been performed. There are three reaction channels in the triplet.

In the present work, molecular orbital calculations using cluster models were performed within density functional theory (DFT) in order to study the adsorption of an Al atom on regular and defective graphene. Depending on the theoretical treatment of electronic exchange and correlations effects, different bonding results for the adsorption on the perfect surface are obtained. On the other hand, they are very similar for Al adsorbed on a carbon monovacancy. On regular graphene, the adsorption is exothermic when the Perdew, Burke and Ernzerhof (PBE) functional is used and endothermic with the Becke, 3-parameter, Lee–Yang–Parr (B3LYP) functional. Regarding the defective graphene surface, it was shown that the carbon atoms of concave angles in the vacancy are the most reactive to a radical attack. The adsorption of an Al atom on the vacancy restores the trigonal symmetry lost after the extraction of the C atom from regular graphene. Complementary calculations performed at PBE level on both regular and defective surfaces imposing periodic conditions qualitatively support the results obtained with the cluster model.

B3LYP/maug-cc-pVTZ and MP2/maug-cc-pVTZ calculations show that 3,4-dihydro-quinazoline and its 2- and/or 4-methyl and -phenyl substituted derivatives in solution (chloroform, DMSO, methanol) are only by 0.3 kcal/mol–2.2 kcal/mol more stable than the respective tautomeric 1,4-dihydroquinazolines (the available literature experimental stability data are not coherent). In the gas phase, 2- and/or 4-substituted tautomers of 3,4-dihydroquinazoline are also energetically preferred (B3LYP/maug-cc-pVTZ, MP2/maug-cc-pVTZ and CCSD/cc-pVDZ calculations lead to the same conclusion). In vacuum, 1,4-dihydro tautomer is by 0.1 kcal/mol–0.2 kcal/mol more stable only for unsubstituted dihydro-quinazoline. The observed tautomeric preference was found almost independent on substitution and solvent used. The optimization procedure used shows that the pyrimidine ring in dihydropyrimidines studied is not planar. Noncoplanarity of the 2-phenyl ring and C=N bond in the respective compounds studied is responsible for the weakened conjugation of these two moieties.

The analysis of π/π and H/π interactions in complexes are a challenging aspect of theoretical research. Due to the different approximations of different levels of theory, results tend to be inconsistent. We compared the reliabilities of HF, SVWN, M06L, PW91, BLYP, B3LYP, BHandHLYP, B97D, MP2, and DFTB-D approaches in researching π/π and H/π interactions by calculating the binding energies of five benzene-containing dimers. The effects of 6-31+G**, 6-311++G** and 6-311++G(2df,2p) basis sets on the results were analyzed too. We found that the DFTB-D and B97D methods combined with the 6-311++G** basis set perform well for dimers that contain π/π and H/π interactions. With high efficiency and satisfactory precision, DFTB-D is helpful for the calculation of complexes containing π/π and H/π stacking. We further calculated the structures and properties of phenylalanine-containing dimers using the DFTB-D and B97D methods. The properties of low energy conformers such as rotational constants, dipole moments and molecular orbitals were also analyzed. These data should be helpful for research into systems that contain π/π and H/π stacking.

Thermodynamic stability of neutral carbon nanoclusters of C_m, up to the size of C₅₀ have been investigated using density functional theory. Four different symmetry point groups, C_(m/2)h, acetylenic D_(m/2)h, cumulenic D_(m/2)h and D_mh, have been considered for the clusters and the C_(m/2)h has been found to be the most stable isomer for the studied clusters. Peierls transition is also studied for acetylenic D_(m/2)h and cumulenic D_(m/2)h point groups. It has been revealed that the relative stability of clusters converges to a constant value for clusters larger than C₃₈. The changes of calculated structural parameters for these clusters, such as bond lengths, have been in agreement with their relative stability.

This paper, based on the density functional theory (DFT), mainly studies the distribution of Al in seven nonequivalent T sites as well as the acidity of the corresponding Brønsted acids (B-acids). The distribution of the acid sites in the double-Al models is also investigated. Using ONIOM (B3LYP/6-31g(d,p):AM1) method, substitution energies, proton affinities and frequencies are calculated to characterize the Al distribution and Brønsted acidity. The result shows that T6 is the most destabilized site after substitution, while T7 buried in the framework is the most favorable for Al. Besides, Al4–O4–Si5 and Al2–O1–Si1 are the most readily sites to form B-acids, and the acid strength of Al4–O3–Si3 is the strongest. In the case that there are two B-acid sites on the 10-membered ring, one of the B-acid sites will be most likely to form at the Al4–O4–Si5 site, and the other one forms either at the other Al4–O4–Si5 or at Al2–O1–Si1 site on the opposite side.

A theoretical study of hydrogen bonds and dihydrogen bonds formed by ethyl cation, hydrocarbons and magnesium hydride is presented with calculations performed at the BHandHLYP/6-31G(d,p) level of theory. The structural results and IR analyses demonstrated great insights, mainly the strengthening and weakness of the CC bond of the ethyl cation and π or pseudo-π bonds, respectively. The interaction strength was measured through the supermolecule as well as by means of additional approaches. The QTAIM calculations were applied to characterize not only the intermolecular interactions but specifically the covalent character in the H⁺ ⋯ π, H⁺ ⋯ pseudo-π and H⁺ ⋯ H contacts. The NBO calculations were useful to interpret the polarization on the CC bond and whether this effect is related with the bond length reduction as well as increase of charge density and frequency shifts.

The population transfer process between the vibrational levels of the HF molecule is investigated by solving the time-dependent Schrödinger equation. The two vibrational levels of υ = 0 and 2 are set to be the initial and target states (an overtone transition). The transition processes under the one- and two-photon resonance conditions are considered, respectively. The influences of the laser intensity, frequency and pulse duration on population transfer are explored. It is found that the one-photon transition process is adiabatic, while the two-photon transition process is diabatic. Other vibrational levels, such as υ = 1 and 3, may participate into the two-photon transition process. Compared to the one-photon transition, a much longer pulse duration is required for the two-photon transition to achieve a high population-transfer efficiency, and the two-photon transition is more sensitive to the laser intensity. Almost a complete population transfer can be achieved for the two-photon resonance case, when the intensity is over 0.0091 a.u. with the appropriate duration (1180 fs) and frequency (0.017674 a.u.).

A series of carbazole-based organic dyes such as CzFCA, CzTCA, CzSeCA, CzFRA, CzTRA and CzSeRA with introduction of various linkers (furan, thiophene and selenophen) and acceptor (cyanoacrylic acid and rhodanine-3-acetic acid) groups, were designed and theoretically investigated by density functional theory (DFT) and time dependent DFT (TD-DFT). The ground state structures and absorption spectra (both in vacuum and solvated form) were optimized by using DFT-B3LYP/6-31G(d,p), TD-B3LYP/6-311G(d,p) and PCM-TD-B3LYP/6-311G(d,p) levels of theory. Substitution of linker groups from furan to selenophen, resulted in red-shift of absorption band. CzTRA and CzSeRA exhibited superior electron injection capabilities. The finding of this study can be helpful to obtain dye-sensitized solar cell (DSSC) with superior conversion efficiency.

In this study, the dispersion of aggregated boron nitride nanotubes (BNNTs) in aqueous triton X-100 surfactant solution is studied using molecular dynamic simulation. The results indicate that how in the presence of the surfactant, a space between two BNNTs is created, which leads to the dispersion of the BNNTs. The radial distribution functions (RDFs) of the atoms of BNNTs and hydrophilic and hydrophobic segments of the surfactant respect to atoms of water molecules show that in the presence of the surfactant, a layer of water molecules is located in the neighborhood of the BNNTs and then hydrophobic and hydrophilic segments of the surfactant reside at more distances of the BNNTs. In the absence of the surfactant, the hydrogen bond between nitrogen atom of the BNNT and hydrogen atom of water molecules is established and the distance between water molecules and the BNNTs is decreased with increase of the surfactant concentration. The obtained results for the surfactant radius of gyration and the interfacial angle between two BNNTs reveal more information about the arrangement of the surfactants around the BNNTs in the presence and in the absence of water molecules.