Density functional theory study of the reaction mechanism and energetics of the reduction of hydrogen peroxide by ebselen, ebselen diselenide, and ebselen selenol

Density functional theory calculations at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d,p) level have been performed to elucidate the mechanism and reaction energetics for the reduction of hydrogen peroxide by ebselen, ebselen diselenide, ebselen selenol, and their sulfur analogues. The effects of solvation have been included with the CPCM model, and in the case of the selenol anion reaction, diffuse functions were used on heavy atoms for the geometry optimizations and thermochemical calculations. The topology of the electron density in each system was investigated using the quantum theory of atoms in molecules, and a detailed interpretation of the electronic charge and population data as well as the atomic energies is presented. Reaction free energy barriers for the oxidation of ebselen, ebselen diselenide, and ebselen selenol are 36.8, 38.4, and 32.5 kcal/mol, respectively, in good qualitative agreement with experiment. It is demonstrated that the oxidized selenium atom is significantly destabilized in all cases and that the exothermicity of the reactions is attributed to the peroxide oxygen atoms via reduction. The lower barrier to oxidation exhibited by the selenol is largely due to entropic effects in the reactant complex.     

Thermochemical properties of molecules and cations of MgNnHm (n=1,2 and m=1–6): enthalpies of formation, proton affinities and ionization energies

Ab initio molecular orbital calculations are reported for small neutral molecules and cations containing magnesium, nitrogen and hydrogen. Structures have been optimized using gradient techniques at B3LYP/6-31+G(d) and at MP2(full)/6-311++G(d,p). Single-point calculations are reported at QCISD(T)(full)/6-311++G(2df,p) and at CCSD(T)(full)/6-311++G(2df,p) levels using geometries optimized at MP2(full)/6-311++G(d,p). Standard enthalpies of formation at 298 K have been calculated at these two higher levels of theory. Other thermochemical properties calculated include ionization energies and proton affinities. The binding enthalpies of ammonia to Mg⁺, MgNH₂⁺ and MgNH₃⁺ are also reported.     

A theoretical study on the mechanism and thermodynamics of ozone-water gas phase reaction

Ozone water reaction including a complex was studied at the MP2/6-311++G(d,p) and CCSD/6-311++G(2df,2p)//MP2/6-311++G(d,p) levels of theory. The interaction between water oxygen and central oxygen of ozone produces stable H₂O-O₃ complex with no barrier. With decomposition of this complex through H-abstraction by O₃ and O-abstraction by H₂O, three possible product channels were found. Intrinsic reaction coordinate, topological analyses of atom in molecule, and vibrational frequency calculation have been used to confirm the preferred mechanism. Thermodynamic data at T = 298.15 K and atmospheric pressure have been calculated. The results show that the production of hydrogen peroxide is the main reaction channel with ΔG = ?21.112 kJ mol^-1.     

Theoretical study on the hydrogen abstraction reaction of CH3CH2F (HFC-161) with Cl atom

A direct dynamics method is employed to study the hydrogen abstraction reaction of CH₃CH₂F+Cl. Three distinct transition states are located, one for -H abstraction and two for β-H abstraction. The potential energy surface (PES) information is obtained at the QCISD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p) and G2//MP2/6-311G(d,p) level. Based on the QCISD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p) results, the rate constants of the three reaction channels are evaluated by using the canonical variational transition state theory (CVT) with small-curvature tunneling (SCT) contributions over the temperature range of 220–2800 K. The calculated results indicate that -H abstraction dominates the total reaction almost over the whole temperature range.     

Dihydrogen trioxide clusters, (HOOOH)n (n = 2-4), and the hydrogen-bonded complexes of HOOOH with acetone and dimethyl ether: implications for the decomposition of HOOOH

Hydrogen-bonded gas-phase molecular clusters of dihydrogen trioxide (HOOOH) have been investigated using DFT (B3LYP/6-311++G(3df,3pd)) and MP2/6-311++G(3df,3pd) methods. The binding energies, vibrational frequencies, and dipole moments for the various dimer, trimer, and tetramer structures, in which HOOOH acts as a proton donor as well as an acceptor, are reported. The stronger binding interaction in the HOOOH dimer, as compared to that in the analogous cyclic structure of the HOOH dimer, indicates that dihydrogen trioxide is a stronger acid than hydrogen peroxide. A new decomposition pathway for HOOOH was explored. Decomposition occurs via an eight-membered ring transition state for the intermolecular (slightly asynchronous) transfer of two protons between the HOOOH molecules, which form a cyclic dimer, to produce water and singlet oxygen (Delta (1)O 2). This autocatalytic decomposition appears to explain a relatively fast decomposition (Delta H a(298K) = 19.9 kcal/mol, B3LYP/6-311+G(d,p)) of HOOOH in nonpolar (inert) solvents, which might even compete with the water-assisted decomposition of this simplest of polyoxides (Delta H a(298K) = 18.8 kcal/mol for (H 2O) 2-assisted decomposition) in more polar solvents. The formation of relatively strongly hydrogen-bonded complexes between HOOOH and organic oxygen bases, HOOOH-B (B = acetone and dimethyl ether), strongly retards the decomposition in these bases as solvents, most likely by preventing such a proton transfer.     

An Ab Initio Study of the Complexes Formed by the Positive Acetylene Ion with the Hydrogen and Nitrogen Molecules and the Argon Atom

The complexes formed by the positive acetylene ion with the hydrogen molecule, the nitrogen molecule, and the argon atom are investigated with ab initio calculations using the 6-311G** and the 6-31+G(2df,2pd) basis sets. MP2/6-311G** energies and optimum geometries are obtained, as well as single-point MP3, MP4, and QCISD(T) energies with the MP2/6-311G** optimized geometries. Single-point calculations are performed with the 6-31+G(2df,2pd) basis set at MP2/6-311G** optimized geometries.     

Weakly bound complexes of N2O: an ab initio theoretical analysis toward the design of N2O receptors

Ab initio calculations at MP2/6-311++G(2d,2p) and MP2/6-311++G(3df,3pd) computational levels have been used to analyze the interactions between nitrous oxide and a series of small and large molecules that act simultaneously as hydrogen bond donors and electron donors. The basis set superposition error (BSSE) and zero point energy (ZPE) corrected binding energies of small N2O complexes (H2O, NH3, HOOH, HOO*, HONH2, HCO2H, H2CO, HCONH2, H2CNH, HC(NH)NH2, SH2, H2CS, HCSOH, HCSNH2) vary between -0.93 and -2.90 kcal/mol at MP2/6-311++G(3df,3pd) level, and for eight large complexes of N2O they vary between -2.98 and -3.37 kcal/mol at the MP2/6-311++G(2d,2p) level. The most strongly bound among small N2O complexes (HCSNH2-N2O) contains a NH..N bond, along with S-->N interactions, and the most unstable (H2S-N2O) contains just S-->N interactions. The electron density properties have been analyzed within the atoms in molecules (AIM) methodology. Results of the present study open a window into the nature of the interactions between N2O with other molecular moieties and open the possibility to design N2O abiotic receptors.     

A theoretical study of the H-abstraction reactions from HOI by moist air radiolytic products (H, OH, and O (3P)) and iodine atoms (2P(3/2))

The rate constants of the reactions of HOI molecules with H, OH, O ((3)P), and I ((2)P(3/2)) atoms have been estimated over the temperature range 300-2500 K using four different levels of theory. Geometry optimizations and vibrational frequency calculations are performed using MP2 methods combined with two basis sets (cc-pVTZ and 6-311G(d,p)). Single-point energy calculations are performed with the highly correlated ab initio coupled cluster method in the space of single, double, and triple (pertubatively) electron excitations CCSD(T) using the cc-pVTZ, cc-pVQZ, 6-311+G(3df,2p), and 6-311++G(3df,3pd) basis sets. Reaction enthalpies at 0 K were calculated at the CCSD(T)/cc-pVnZ//MP2/cc-pVTZ (n = T and Q), CCSD(T)/6-311+G(3df,2p)//MP2/6-311G(d,p), and CCSD(T)/6-311++G(3df,3pd)//MP2/6-311G(d,p) levels of theory and compared to the experimental values taken from the literature. Canonical transition-state theory with an Eckart tunneling correction is used to predict the rate constants as a function of temperature. The computational procedure has been used to predict rate constants for H-abstraction elementary reactions because there are actually no literature data to which the calculated rate constants can be directly compared. The final objective is to implement kinetics of gaseous reactions in the ASTEC (accident source term evaluation code) program to improve speciation of fission products, which can be transported along the reactor coolant system (RCS) of a pressurized water reactor (PWR) in the case of a severe accident.     

PF(X~3∑~-)的结构、势能函数与光谱常数

运用多种方法、多种基组对PF(X3∑-)的平衡结构进行优化计算.用QCISD/6-311G(df)方法得到的平衡结构为RPF=0.158 9 nm,与实验值RPF=0.158 97 nm进行比较,最为接近,得出QCISD/6-311G(df)基组为最优基组;然后对PF(X3∑-)进行单点能扫描计算,用正规方程组拟合Murrell-Sorbie函数,得到相应电子态的势能函数解析式,由势能函数计算了与PF(X3∑-)态相对应的光谱常数,结果与实验数据较为一致.这些数据为反应动力学提供了理论依据.     

High-level ab initio versus DFT calculations on (H2O2)2 and H2O2–H2O complexes as prototypes of multiple hydrogen bond systems

The performance of B-LYP, B-P86, B3-LYP, B3-P86, and B3-PW91 density functionals to describe multiple hydrogen bond systems was studied. For this purpose we have chosen the dimers of hydrogen peroxide and the hydrogen peroxide–water complexes. The geometries and vibrational frequencies obtained with a 6-311+G(d,p) basis set were compared with those obtained at the MP2 level using the same basis set expansion. The corresponding dimerization energies were obtained using a 6-311+G(3df,2p) basis set and compared with those obtained using the G2(MP2) theory. Red shiftings of the OH donor stretching frequencies were predicted by all approaches investigated; however, in all cases, the DFT values were sizably larger than the MP2 ones. Similarly, the blue shifting of the torsion of the hydrogen peroxide subunit was larger when evaluated at the DFT level. All functionals reproduced the G2(MP2) relative stabilities of the different local minima quite well. With the exception of the B-LYP and B3-PW91 approaches, all functionals yielded binding energies which deviated from the G2(MP2) values by less than 0.5 kcal/mol, provided that G2-type basis sets were used and that the corresponding BSSE corrections were included. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1124–1135     

NX(a 1Δ, X=F、Cl、Br)分子结构与ωe～Re关联模型

基于不少双原子分子的稳定激发态系列中存在已知ωe而未给出Re的现象,本文提出了ωe～Reα=C的理论模型,对近60个双原子分子的光谱数据进行了论证,并与量子力学计算结果进行了比较.结果表明,该模型具有通用性与可靠性.结合NX(a 1Δ)替代O2(a 1Δg)的新激光系统可能性研究需要,应用CIS、B3LYP与MCSCF方法,在6-311＋g(3df)基水平计算了NX(X=F、Cl、Br)第一激发态（a 1Δ）的结构,导出了解析势能函数.     

Accurate calculation of absolute one-electron redox potentials of some para-quinone derivatives in acetonitrile

Standard ab initio molecular orbital theory and density functional theory calculations have been used to calculate absolute one-electron reduction potentials of several para-quinones in acetonitrile. The high-level composite method of G3(MP2)-RAD is used for the gas-phase calculations and a continuum model of solvation, CPCM, has been employed to calculate solvation energies. To compare the theoretical reduction potentials with experiment, the reduction potentials relative to a standard calomel electrode (SCE) have also been calculated and compared to experimental values. The average error of the calculated reduction potentials using the proposed method is 0.07 V without any additional approximation. An ONIOM method in which the core is studied at G3(MP2)-RAD and the substituent effect of the rest of the molecule is studied at R(O)MP2/6-311+G(3df,2p) provides an accurate low-cost alternative to G3(MP2)-RAD for larger molecules.     

New scale factors for harmonic vibrational frequencies using the B3LYP density functional method with the triple-zeta basis set 6-311+G(d,p)

We have calculated optimal frequency scaling factors for the B3LYP/ 6-311+G(d,p) method for fundamental vibrational frequencies on the basis of a set of 125 molecules. Using the new scaling factor, the vibrational frequencies calculated with the triple-zeta basis set 6-311+G(d,p) give significantly better accuracy than those calculated with the double-zeta 6-31G(d) basis set. Scale factors were also determined for low-frequency vibrations using the molecular set of 125 molecules and for zero-point energies using a smaller set of 40 molecules. We have studied the effect on the calculated vibrational frequencies for various combinations of diffuse and polarization functions added to the triple-zeta 6-311G basis set. The 6-311+G(d,p) basis set is found to give almost converged frequencies for most molecules, and we conclude that our optimum scaling factors are valid for the basis sets 6-311G(d,p) to 6-311++G(3df,3pd). The new scale factors are 0.9679 for vibrational frequencies, 1.0100 for low-frequency vibrations, and 0.9877 for zero-point vibrational energies.     

A microwave spectroscopic and quantum chemical study of 3-butyne-1-selenol (HSeCH2CH2C[triple bond]CH)

The microwave spectrum of 3-butyne-1-selenol has been studied by means of Stark-modulation microwave spectroscopy and quantum chemical calculations employing the B3LYP/aug-cc-pVTZ and MP2/6-311++G(3df,3pd) methods. Rotational transitions attributable to the H80SeCH2CH2C[triple bond]CH and H78SeCH2CH2C[triple bond]CH isotopologues of two conformers of this molecule were assigned. One of these conformers possesses an antiperiplanar arrangement for the atoms Se-C-C-C, while the other is synclinal and seems to be stabilized by the formation of a weak intramolecular hydrogen bond between the hydrogen atom of the selenol group and the pi electrons of the CC triple bond. The energy difference between these conformers was determined to be 0.2(5) kJ/mol by relative intensity measurements, and the hydrogen-bonded form was slightly lower in energy.     

Direct ab initio dynamics calculation of the reaction rates of CH3OCl with OH

A direct ab initio dynamics method was carried out for the reaction CH3OCl + OH --> products. Three abstraction channels from chlorine atom, in-plane hydrogen, and out-of-plane hydrogen atoms at the CH3 group have been found. The optimized geometries and frequencies of the stationary points and the minimum-energy paths (MEPs) were calculated at the MP2/6-311G(d,p) level. To improve the reaction enthalpy and potential barrier, single-point calculations were made at three higher levels of theory, the approximate QCISD(T)/6-311++G(3df,2pd), G3, and G3(MP2) levels. Furthermore, the rate constants for three abstraction channels were evaluated using canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) over a wide temperature range of 220-2000 K at above three higher theory levels, respectively. The calculated rate constants as well as branching rates are in reasonable agreement with the experimental values in the temperature region 250-341 K. The present results indicate H-abstraction especially from out-of-plane hydrogen is the main reaction pathway, while Cl-abstraction is much less competitive.     

Sugars in the gas phase: The conformational properties of erythrose, threose, and erythrulose characterized by quantum chemistry calculations

The structural properties of the three open chain C₄H₈O₄ sugars, i.e. two aldoses (erythrose and threose) and one ketose (erythrulose), have been investigated by DFT and ab initio calculations to get accurate structures and relative energies. The structure of all the conformers predicted within 10 kJ/mol has been optimized at the B3LYP/6-311++G(d,p) level of the theory. Two types of intramolecular hydrogen bonds have been clearly identified. They are related to the hydroxyl and to the carbonyl oxygen atoms and are of weak and middle strength, respectively. The most stable structures have been optimized at the B3LYP/6-311++G(2df,p) and at the MP2/6-311++G(2df,p) levels of the theory in order to calculate accurate rotational parameters and dipole moment for their future detection in the microwave range in the gas phase. Their corresponding harmonic IR spectra have also been calculated and their fingerprint signature is discussed in the region of the OH stretching vibrations, of the torsion of the C–O bonds and of the deformation of the C–C skeleton.     

Quantum chemical study of hydrogen abstraction reactions of the ethynyl radical with hydrogen compounds (C2H+HX)

We have theoretically investigated the hydrogen abstraction reactions of ethynyl radical with simple hydrogen compounds, C₂H+HX, using quantum chemical computations. Computations have been performed using the density functional theory with the recently proposed MPW1K functional and the 6-311++G(3df,2p) basis set. An analysis of the resulting energy barriers for hydrogen abstraction reactions has been carried out using the bond dissociation energy of the breaking X–H bond and DFT-based reactivity parameters to rationalize the reaction behavior.     

An exploration of conformational search of leucine molecule and their vibrational spectra in gas phase using ab initio methods

An extensive exploration of the conformational space has been carried out to characterize all possible gas phase structures of leucine. A total of 324 unique trial structures for canonical leucine were generated by considering all possible combinations of single bond rotamers. All trial structures were optimized at the B3LYP/6-311G* level of the DFT method. A total of 77 unique and stationary canonical conformers were found. Further, 15 most stable conformers were reoptimized at B3LYP/6-311++G** level and their respective relative energies, vertical ionization energies, hydrogen bonding patterns, rotational constants and dipole moments were calculated. A single point energy calculations for leucine conformers have also been done at both B3LYP/6-311++G(2df, p) and MP2/6-311++G(2df, p) levels. The good agreement between our estimates of rotational constants for two most stable conformers and available experimental measurements supports the reliability of the B3LYP/6-311++G** level of theory for describing the conformational behavior of leucine molecule. The proton affinity and gas phase basicity were also determined. Using the statistical approach, conformational distributions at various temperatures have also been performed and analyzed. Vibrational spectra were also calculated. It is also observed that zwitterions of leucine are not stable in gas phase.     

On the existence and lifetimes of Cu2+ complexes with water, ammonia, and hydrogen cyanide

High-level ab initio calculations have been carried out to evaluate the lifetimes of complexes formed by the association of Cu2+ to water, ammonia, and hydrogen cyanide. The corresponding binding energies were evaluated at the CCSD(T)6-311+G(3df,2pd) level of theory. The potential-energy curves corresponding to their dissociation into Cu+ + L+ (L=H2O, NH3, and HCN) were obtained at the CCSD(T)6-311+G(3df,2p) level on BHLYP6-311+G(d,p) optimized geometries. Lifetimes were calculated using the exterior complex scaling and the semiclassical WKB methods. Although all the complexes investigated are thermodynamically unstable with respect to their dissociation into Cu+ + L+ in a typical Coulomb explosion, the activation barrier is high enough to accommodate several vibrational resonances, with very large lifetimes. As a matter of fact, if the three complexes are produced in the lowest vibrational levels, they behave as totally bound (with almost infinite lifetime) species.     

Inverse hydrogen bonds between SiH_4 and hydrides of Na,Mg and Be

The optimized geometries of the three complexes between MeHn (Me=Na,Mg,Be;n=1 or 2) and SiH4 have been calculated at the B3LYP/6-311++g**,MP2/6-311++g(3df,3pd) and MP2/aug-cc-pvtz levels,respectively.The red-shift inverse hydrogen bonds (IHBs) based on Si-H,an electron donor,were reported.The calculated binding energies with basis set super-position error (BSSE) correction of the three complexes are-5.98,-8.65 and-3.96 kJ mol-1 (MP2/6-311++g(3df,3pd)),respectively,which agree with the results obtained via M...