Theoretical study and rate constant calculation for the reactions of SH (SD) with Cl2, Br2, and BrCl

The mechanisms of the SH (SD) radicals with Cl2 (R1), Br2 (R2), and BrCl (R3) are investigated theoretically, and the rate constants are calculated using a dual-level direct dynamics method. The optimized geometries and frequencies of the stationary points are calculated at the MP2/6-311G(d,p) and MPW1K/6-311G(d,p) levels. Higher-level energies are obtained at the approximate QCISD(T)/6-311++G(3df, 2pd) level using the MP2 geometries as well as by the multicoefficient correlation method based on QCISD (MC-QCISD) using the MPW1K geometries. Complexes with energies less than those of the reactants or products are located at the entrance or the exit channels of these reactions, which indicate that the reactions may proceed via an indirect mechanism. The enthalpies of formation for the species XSH/XSD (X = Cl and Br) are evaluated using hydrogenation working reactions method. By canonical variational transition-state theory (CVT), the rate constants of SH and SD radicals with Cl2, Br2, and BrCl are calculated over a wide temperature range of 200-2000 K at the a-QCISD(T)/6-311++G(3df, 2pd)//MP2/6-311G(d, p) level. Good agreement between the calculated and experimental rate constants is obtained in the measured temperature range. Our calculations show that for SH (SD) + BrCl reaction bromine abstraction (R3a or R3a') leading to the formation of BrSH (BrSD) + Cl in a barrierless process dominants the reaction with the branching ratios for channels 3a and 3a' of 99% at 298 K, which is quite different from the experimental result of k3a'/k3' = 54 +/- 10%. Negative activation energies are found at the higher level for the SH + Br2 and SH + BrCl (Br-abstraction) reactions; as a result, the rate constants show a slightly negative temperature dependence, which is consistent with the determination in the literature. The kinetic isotope effects for the three reactions are "inverse". The values of kH/kD are 0.88, 0.91, and 0.69 at room temperature, respectively, and they increase as the temperature increases.     

XPS of oxygen atoms on Ag(111) and Ag(110) surfaces: Accurate study with SAC/SAC‐CI combined with dipped adcluster model

O1s core‐electron binding energies (CEBE) of the atomic oxygens on different Ag surfaces were investigated by the symmetry adapted cluster‐configuration interaction (SAC‐CI) method combined with the dipped adcluster model, in which the electron exchange between bulk metal and adsorbate is taken into account properly. Electrophilic and nucleophilic oxygens (O_elec and O_nuc) that might be important for olefin epoxidation in a low‐oxygen coverage condition were focused here. We consider the O1s CEBE as a key property to distinguish the surface oxygen states, and series of calculation was carried out by the Hartree–Fock, Density functional theory, and SAC/SAC‐CI methods. The experimental information and our SAC/SAC‐CI results indicate that O_elec is the atomic oxygen adsorbed on the fcc site of Ag(111) and that O_nuc is the one on the reconstructed added‐row site of Ag(110) and that one‐ and two‐electron transfers occur, respectively, to the O_elec and O_nuc adclusters from the silver surface. © 2013 Wiley Periodicals, Inc.     

One‐, two‐, and three‐electron bonding: An “in vitro” theoretical study using noninteger nuclear charges evidences the crucial role of electronegativity in the strength of symmetrical bonds

Fictitious hydrogen atoms H*_A of variable nuclear charge 0.5 ≤ Z_A ≤ 2 (and thus of variable electronegativity) are used to study the intrinsic dependency of chemical bonding on electronegativity. Dissociation energy and equilibrium distance are reported for symmetrical 1‐, 2‐ and 3‐electron H*_AH*_A systems and 2‐electron dissymmetrical H*_A‐H ones. Dealing with symmetrical systems, the strongest two‐electron bonds are found for Z_A ≈ 1.2. Oneelectron and three‐electron strongest bonds occur respectively with low (ca. 0.7) and high (ca. 1.7) Z_A values and can become stronger than the corresponding 2‐electron system. Comparison with data on real systems leads to conclude that electronegativity is a prevailing atomic property in the control of the dissociation energy of symmetrical 1‐, 2‐ and 3‐electron bonds. A simplified mathematical model at Hartree‐Fock or Heitler‐London level with a minimal basis set reproduces these trends semi‐quantitatively and provides the overall shape of the dissociation curves. Finally some points are qualitatively discussed from MO analysis, which emphasize the dependence of the bonding/antibonding properties on the nucleus charge Z_A and their occupancy number. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Theoretical calculation of the electronic states with spin‐orbit effects of the molecule NaRb

The potential energy curves of the molecule NaRb have been calculated for the 60 low‐lying electronic states in the Ω‐representation. Using an ab‐initio method the calculation is based on nonempirical pseudo‐potential in the interval 3.0a_o ≤ R ≤ 44.0a_o of the internuclear distance. The spin‐orbit effects have been taken into account through a semiempirical spin‐orbit pseudo‐potential added to the electrostatic Hamiltonian with Gaussian basis sets for both atoms. The spectroscopic constants have been calculated for 42 states and the components of the spin‐orbit splitting have been identified for the states (1, 2, 5)³Π and (1, 2)³Δ. The comparison of the present results with those available in literature shows a good agreement, whereas the other results, to the best of our knowledge, are given here for the first time. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Theoretical study of self‐exchange electron‐transfer reactions for the M(H2O)\documentclass{article}\pagestyle{empty}\begin{document}$^{2+/3+}_{6}$\end{document} (M = V,Cr, Mn,and Fe) systems

Based on an activation model, a available scheme to calculate the rate of the electron‐transfer reaction between transition‐metal complexes in aqueous solution is presented. Ab initio technique is used to determine the electron‐transfer reactivity of the type M(H₂O)$^{2+/3+}_{6}$ of transition‐metal complexes at the UMP2/6‐311G level. The activation parameters and activation energies of the electron‐transfer systems are obtained via the activation model. An alternative determining method of the potential energy surface (curve) slope at the crossing point is given in which the inner‐sphere contribution of potential energy surface slope is expressed as the sum of two separate reactants. Theoretical self‐exchange rate constants for M(H₂O)$^{2+/3+}_{6}$ (M = V, Cr, Mn, and Fe) systems are obtained at 298 K and zero ionic strength. The calculated results of the activation energy, electronic transmission factor, and electron‐transfer rate are compared with the corresponding quasi‐experimental values as well as those obtained from other methods, and better agreements are found. The present results indicate that the scheme can adequately describe the self‐exchange reactions involved in this study. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 32–41, 2000     

CHF_2CF_2CH_2OCHF_2与OH自由基反应机理的理论研究

采用从头算和密度泛函方法研究了多通道反应CHF2CF2CH2OCHF2+OH→产物的反应机理.首先在BMK/6-311+G(d,p)水平下优化了稳定点的几何构型并计算了振动频率;然后在BMC-CCSD水平下,对势能面进行高水平能量校正.结果表明,此反应存在提氢和取代两类反应通道,但是无论从动力学还是从热力学分析,提氢反应通道才是主要的反应通道,且从-CH2-基团上提取氢原子的提氢通道是主要的反应通道.     

Amide–π interactions between formamide and benzene

High‐level ab initio calculations have been carried out using a formamide–benzene model system to evaluate amide–π interactions. The interaction energies were estimated as a sum of the CCSD(T) correlation contribution and the HF energy at the complete basis set limit, for the geometries of the model structures at the energy minimum obtained by potential energy surface (PES) scans. NH/π geometry in a face‐on configuration was found to be the most attractive among the various geometries considered, with interaction energy of ?3.75 kcal/mol. An interaction energy of ?2.08 kcal/mol was calculated for the stacked N/Center type geometry, where the nitrogen atom of formamide points directly toward the center of the aromatic ring. The weakest C?O/π geometry, where a carbonyl oxygen atom points toward the plane of the aromatic ring, was found to have energy minimum at an intermolecular distance of 3.67 Å from the PES, with a repulsive interaction energy less than 1 kcal/mol. However, if there are simultaneous attractive interactions with other parts of the molecule besides the amide group, the weak repulsion could be easily overcome, to give a C?O/π geometry interaction. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Competitive interaction between halogen and hydrogen bonds in NH2Br‐HOX (X = F,Cl, and Br) complex

The NH₂Br‐HOX (X = F, Cl, and Br) complexes have been investigated with quantum chemical calculations at the MP2/aug‐cc‐pVTZ level. Five isomers are observed for the Cl and Br complexes, whereas only two isomers are found for the F complex. The geometrical, energetic, and spectroscopic parameters have been analyzed for these complexes. The hydrogen‐bonded complexes are more stable than the halogen‐bonded ones. In most complexes, the associated O? H and O? X bonds are elongated and show a red shift, whereas the distant bonds are contracted and exhibit a blue shift. The complexes have been analyzed with natural bond orbital and atoms in molecules. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

A comparative study of open‐close and related rotamers methods to evaluate the intramolecular hydrogen bond energies in 3‐imino‐propen‐1‐ol and its derivatives

MP2 study of O? H…N intramolecular hydrogen bond (IMHB) in 3‐imino‐propen‐1‐ol and its derivatives were performed and their IMHB energies were obtained using the related rotamers and open‐close methods. Also the topological properties of electron density distribution and charge transfer energy associated with IMHB were gained by quantum theory of atoms in molecules and natural bond orbital theory, respectively. The computational results reveal that the related rotamers method energies are well correlates with geometrical parameters, topological parameters at hydrogen bond and ring critical points, integrated properties, proton transfer barrier and charge transfer energy of O? H…N unit. Surprisingly, it was found that the open‐close hydrogen bond energies cannot represent good linear correlations with these parameters. Consequently, we extrapolate a number of equations that can be used in estimation of O? H…N IMHB energy in complex biological systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Theoretical Study on Gas—phase Pyrolytic Reactions of N—Ethyl,Isopropyl and N—t—Butyl Substituted 2—Aminopyrazine

Density functional theory (DFT) and ab initio methods were used to study gas‐phase pyrolytic reaction mechanisms of iV‐ethyl, N‐isopropyl and N‐t‐butyl substituted 2‐aminopyrazine at B3LYP/6–31G* and MP2/6–31G*, respectively. Single‐point energies of all optimized molecular geometries were calculated at B3LYP/6–311 + G(2d,p) level. Results show that the pyrolytic reactions were carried out through a unimolecular first‐order mechanism which were caused by the migration of atom H(17) via a six‐member ring transition state. The activation energies which were verified by vibrational analysis and correlated with zero‐point energies along the reaction channel at B3LYP/6–311 + G(2d,p) level were 252.02 kJ. mo^?1 (N‐ethyl substituted), 235.92 kJ‐mol^?1 (N‐t‐isopropyl substituted) and 234.27 kJ‐mol^?1 (N‐t‐butyl substituted), respectively. The results were in good agreement with available experimental data.     

Theoretical study of the inner‐sphere energy barrier of the transition‐metal complex M(H2O) in electron‐transfer process

Two theoretical models, a reorganization model and an activation model, are presented for accurately determining the energy barrier of the type M(H₂O) of the transition‐metal complexes in the electron‐transfer process. Ab initio calculations are carried out at UMP2/6‐311G level for several redox pairs M(H₂O) (M=V, Cr, Mn, Fe, and Co) to calculate their inner‐sphere reorganization energies and activation energies according to the models presented in this article. The values of theoretical inner‐sphere reorganization energies and activational energies are comparable with the experimental results obtained from the vibration spectroscopic data. The theoretical reorganization energy of the every redox pair is four times as much as its activation energy, which agrees with Marcus' electron‐transfer theory. The fact proved that the theoretical models presented in this article are scientific and available for studying the electron‐transfer process of the transition‐metal complex. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 119–126, 1999     

Theoretical study of 1,2‐rearrangement in silylmethanethiol

The 1,2‐rearrangements in silylmethanethiol were studied by ab initio molecular orbital theory. The structures of reactants, transition states, and products were fully optimized at the MP2(full)/6‐31G(d) levels. Based on the MP2(full)/6‐31G(d) geometries, harmonic frequencies were obtained. Energies were computed at the G3 level of theory with MP2(full)/6‐31G(d) zero‐point corrections. The results indicate that the 1,2‐rearrangement in silylmethanethiol may occur via two pathways. Pathway A involves the 1,2‐migration of mercapto group from carbon to silicon via a double three‐membered ring transition state, forming methylsilanethiol. The barrier for reaction A is 275.0 kJ/mol. Pathway B involves the 1,2‐migration of silyl group from carbon to sulfur via a four‐membered ring transition state, forming methylthiosilane. The barrier for reaction B is 262.3 kJ/mol. Thermodynamic and kinetic properties of the reactions were analyzed over a temperature range of 300–1,300K. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Analysis of the intermolecular interactions between CH3OCH3, CF3OCH3, CF3OCF3, and CH2F2, CHF3

The intermolecular interaction energy curves of CH(3)OCH(3)-CH(2)F(2), CF(3)OCH(3)-CH(2)F(2), CF(3)OCF(3)-CH(2)F(2), CH(3)OCH(3)-CHF(3), CF(3)OCH(3)-CHF(3), and CF(3)OCF(3)-CHF(3) complexes were calculated by the MP2 level ab initio molecular orbital method using the 6-311G** basis set augmented with diffuse polarization functions. We investigate the fluorine substitution effects of both methane and dimethyl ether on intermolecular interactions. In addition, orientation dependence of intermolecular interaction energies is also studied with utilizing eight types of orientations. Our analyses demonstrate that partial fluorinations of methane make electrostatic interaction dominant, and consequently enhance attractive interaction at several specific orientations. On the contrary, fluorine substitutions of dimethyl ether substantially decrease the electrostatic interaction between ether and CH(2)F(2) or CHF(3); thus, there is no such characteristic interaction between the C-H of fluorinated methane and ether oxygen of CF(3)OCF(3) as conventional hydrogen bonding, due to reduced polarity of fluorinated ether. The combination of different pairs of the electrostatic interaction is therefore responsible for the intermolecular interaction differences among the complexes investigated herein and also their orientations.     

Theoretical study of the intermolecular hydrogen bond interaction for furan-HCl and furan-CHCl_3 complexes

The importance of intermolecular interactions in biology and material science has prompted chemists to explore the nature of the variety of such interactions. The strongest of these interac-tions are the hydrogen bonds, which play an important role in determining the molecular confor-mation, crystal packing, and the structure of biological systems such as nucleic acids. Extensive experimental and theoretical efforts[1—5] have been devoted to the studies of this type of interac-tions, such as …     

Effect of water on zinc (II), cadmium (II) complexes with pyridylimidazole: Theoretical study of stability and electronic spectrum

The geometry structures of complexes such as [Zn(PIm)₂(H₂O)] and [Cd(PIm)₂(H₂O)₂] [PIm = (2‐(2′‐pyridyl) imidazole)] are optimized by density functional theory (DFT) B3LYP methods. On the basis of their stable structures, the stability of the coordinated water existing in the complexes is analyzed quantitatively in terms of the interaction between the central metal and the coordinated water. The interaction energy of the Zn pyridylimidazole complex increased obviously by considering the intermolecular hydrogen bond (O? H…N). The theoretical calculation well explained penta‐ and hexa‐coordinated conformation, respectively, in Zn and Cd pyridylimidazole complexes. The spectral properties of the Zn Cd complexes have been studied by time‐dependent density functional theory (TD‐DFT). The calculation results show that the coordinated waters in Cd complexes have little effect on their spectral properties. While the axially coordinated waters in Zn pyridylimidazole cause a red shift in the absorption wavelength and change the pattern of charge transfer as a result of the effect of polarization from intermolecular hydrogen bond. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006