International Journal of Quantum Chemistry

Three representative isomerization reactions (HNC → HCN, CH₃NC → CH₃CN, and N₂H₂ trans → cis and sin) have been studied using both the LCGTO–LSD and LCGTO–NLSD density functional methods and employing a new algorithm for the search and the refinement of the transition‐state structures. The inclusion of the nonlocal corrections and the use of large basis sets improve the reliability of the energetic parameters. Results are in good agreement with previous accurate first‐principle computations and available experimental data. © John Wiley & Sons, Inc.

The interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg²⁺ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas‐phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field–dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the CO molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ω_e, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal–CO bond as arising from σ donation and π back donation.

Paratrimerin C is a natural acridone antioxidant that is also potent as an anti‐UV agent. The photophysical process of Paratrimerin C, including the absorption and emission and the excited state intramolecular proton transfer (ESIPT) mechanism, is studied herein with time‐dependent density functional theory. The solvent effect on ESIPT is studied for polar (water) and non‐polar (benzene) solvents employing the Integral Equation Formalism Polarization Continuum Model and adding up to three explicit molecules of solvent. The normal and tautomer forms of the Paratrimerin C at the ground‐ and excited‐state structures are studied at the M06‐2X/6‐311++G(d,p) level of theory. The potential energy curves along the reaction coordinates indicate that the ESIPT is a barrier‐less reaction, and the variation of O_dO_a distance in the excited state shows significant molecular structure deformation along the proton transfer process following the normal‐to‐tautomerism pathway. The obtained results suggest using of acridone derivatives as efficient and tunable fluorescence molecules besides their biological activities.

The self‐consistent‐charge density‐functional tight‐binding (SCC‐DFTB) method is employed for computing geometric, electronic, and vibrational properties for various topological isomers of small fullerenes. We consider all pentagon/hexagon‐bearing isomers of C₃₈, C₄₀, and C₄₂ as the second part of a larger effort to catalogue the CC distance distributions, valence CCC angle distributions, electronic densities of states (DOSs), vibrational densities of states (VDOSs), and infrared (IR) and Raman spectra for fullerenes C₂₀C₁₈₀ [analogous data for C₂₀C₃₆ were published previously in Małolepsza et al., J Phys Chem A, 2007, 111, 6649]. Common features among the fullerenes are identified and properties characteristic for each specific fullerene cage size are discussed. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Many‐body (diagrammatic) perturbation theory (MBPT), coupled‐pair many‐electron theory (CPMET), and configuration interaction (CI) are investigated with particular emphasis on the importance of quadruple excitations in correlation theories. These different methods are used to obtain single, double, and quadruple excitation contributions to the correlation energy for a series of molecules including CO₂, HCN, N₂, CO, BH₃, and NH₃. It is demonstrated that the sum of double and quadruple excitation diagrams through fourth‐order perturbation theory is usually quite close to the CPMET result for these molecules at equilibrium geometries. The superior reliability of the CPMET model as a function of internuclear separation is illustrated by studying the ¹∑ potential curve of Be₂. This molecule violates the assumption common to nondegenerate perturbation theory that only a single reference function is important and this causes improper behavior of the potential curve as a function of R. This is resolved once the quadruple excitation terms are fully included by CPMET.

Application of the power series for the one‐electron density matrix Gineityte, V., J Mol Struct Theochem 1995, 343, 183 to the case of two interacting molecules is shown to yield a semilocalized approach to investigate chemical reactivity, which is characterized by the following distinctive features: (1) Electron density (ED) redistributions embracing orbitals of the reaction centers of both molecules and of their neighboring fragments are studied instead of the total intermolecular interaction energy; (2) the ED redistributions are expressed directly in the basis of fragmental orbitals (FOs) without passing to the basis of delocalized molecular orbitals (MOs) of initial molecules; (3) terms describing the ED redistributions due to an intermolecular contact arise as additive corrections to the purely monomolecular terms and thereby may be analyzed independently; (4) local ED redistributions only between orbitals of the reaction centers of both molecules are described by lower‐order terms of the power series, whereas those embracing both the reaction centers and their neighborhoods are represented by higher‐order terms. As opposed to the standard perturbative methods based on invoking the delocalized (canonical) MOs of isolated molecules, the results of the approach suggested are in‐line with the well‐known intuition‐based concepts of the classic chemistry concerning reactivity, namely, with the assumption about different roles of the reaction center and of its neighborhood in a chemical process, with the expectation about extinction of the indirect influence of a certain fragment (substituent) when its distance from the reaction center grows, etc. Such a parallelism yields quantum chemical analogs for the classic concepts and thereby gives an additional insight into their nature. The scope of validity of these concepts also is discussed. Applicability of the approach suggested to specific chemical problems is illustrated by a brief consideration of the S_N2 and Ad_E2 reactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 302–316, 2003

The molecular structures, relative energies, vibrational frequencies, and electron affinities for the Cl₂O_n/Cl₂O (n = 1–4) systems have been investigated using hybrid Hartree–Fock/density functional theories (BHLYP and B3LYP) and pure density functional theories (BP86 and BLYP). The three different types of neutral/anion energy differences reported in this research are the adiabatic electron affinity (EA_ad), the vertical electron affinity (EA_vret), and the vertical detachment energy (VDE). The basis set used in this work is of double‐ζ plus polarization quality with additional s‐ and p‐type diffuse functions, and it is denoted DZP++. The geometries are fully optimized by all four DFT methods. We have predicted a number of possible low‐lying local minima, including some that are unprecedented. Most strikingly, in several cases structures that have been observed in the laboratory turn out not to be lowest in energy for a particular chemical composition. Two structures are predicted with a formally seven‐coordinate chlorine atom. The global minima for the Cl₂O_n/Cl₂O (n = 1–4) systems are ClOCl (C_2v), ClClO⁻ (C_s), ClOOCl (C₂), OOCl (C_2v), ClOOOCl (C₂/C₁), ClO(O₂)Cl⁻ (C_s), trans‐ClO(O₂)OCl (C_i), and trans‐ClO(O₂)OCl⁻ (C_i), respectively. The relative energies of the different minima are reported. Five of the 42 structures predicted here have been determined experimentally. Our theoretical geometries and vibrational frequencies are carefully compared with the limited available experimental results, and the BHLYP functional in general provides the best agreement. The ClClO⁻ and ClClO ground states might be regarded as Cl⁻ … ClO and Cl⁻ … ClO₂ complexes, respectively. The adiabatic electron affinities, obtained at the favored DZP++ BLYP level of theory, are 3.12 eV for Cl₂O, 3.96 eV for Cl₂O₂, 3.66 eV for Cl₂O₃, and 4.15 eV for Cl₂O₄. The adiabatic EAs for the first three systems are similar to those of Br₂O_n (3.14, 3.80, and 3.46 eV with BLYP for Br₂O_n, n = 1–3), and this may reflect the geometric similarity between these bromine and chlorine species. But, EA_ad for Br₂O₄ (1.97 eV) is different from Cl₂O₄ because the neutral and anionic bromine species have different global minimum geometries from those for Cl₂O₄. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Due to the environmental problems caused by the existing Halon substitutes, it is essential to explore new extinguishants with better environmental friendliness. In this study, in order to evaluate the practicability of cis‐1,1,1,4,4,4‐hexafluoro‐2‐butene (HFO‐1336mzz(Z)) as a potential Halon substitution product, the thermal decomposition mechanism and fire‐extinguishing performance of HFO‐1336mzz(Z) were studied by using density functional theory calculation and experimental analysis. The computational results show that thermal decomposition of HFO‐1336mzz(Z) would result in some products that can further react with active OH˙ and H˙ radicals, which are indispensable reactants in the flame and combustion. Moreover, during the interaction between HFO‐1336mzz(Z) and flame, the fire‐extinguishing radical CF₃˙ would be produced, indicating the chemical‐extinguishing mechanism and the pronounced fire‐extinguishing performance of HFO‐1336mzz(Z). To explore its actual fire‐extinguishing effect, the fire‐extinguishing concentration (FEC) of HFO‐1336mzz(Z) on methane‐air flame was measured in cup‐burner. The FEC value of HFO‐1336mzz(Z) is 6.84% in volume, which is lower than those of HFC‐125 and HFC‐116, and is slightly higher than that of HFC‐236fa. Both the experimental and theoretical results suggest that HFO‐1336mzz(Z) can be a promising candidate for Halon substitute.