Kinetic isotope effects for the H2 + C2H ↔ C2H2 + H reaction based on the ab initio calculations and a global potential energy surface

In the present paper, kinetic isotope effects of the title reaction are studied with canonical variational transition state theory on the modified Wang Bowman (MWB) potential energy surface (PES) (Chem Phys Lett 2005, 409, 249) and the ab initio calculations at the quadratic configuration interaction (QCISD (T, full))/aug‐cc‐pVTZ//QCISD (full)/cc‐pVTZ level. The calculated rate constants for the isotopic variants of this title reaction on the MWB PES have good agreement with those of the present ab initio calculations over the temperature range of 20–5000 K for the forward reactions and 800–5000 K for the reverse reactions, respectively. In particular, the forward rate constants for the title reaction and its isotopically substituted reactions have negative temperature dependences at about 40 K. Rate expressions are presented for all the studied reactions. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 289–298, 2010     

Electronic spectra of linear isoelectronic clusters C2n+1S and C2n+1Cl+ (n = 0-4): an ab initio study

Structures and stabilities of linear carbon chains C2n+1S and C2n+1Cl+ (n=0-4) in their ground states have been investigated by the CCSD and B3LYP approaches. The CASSCF calculations have been used to determine geometries of selected excited states of both isoelectronic series. Linear C2n+1S cluster has a cumulenic carbon framework, whereas its isoelectronic C2n+1Cl+ has a dominant character of acetylenic structure in the vicinity of terminal Cl. The vertical excitation energies of low-lying excited states have been calculated by the CASPT2 method. Calculations show that the excitation energies have nonlinear size dependence. The 2(1)Sigma+<--X1Sigma+ transition energy in C2n+1S has a limit of 1.78 eV, as the chain size is long enough. The predicted vertical excitation energies for relatively strong 1(1)Pi<--X1Sigma+ and 2(1)Sigma+<--X1Sigma+ transitions are in reasonable agreement with available experimental values. The spin-orbit effect on the spin-forbidden transition in both series is generally small, and the enhancement of the spin-forbidden transition by spin-orbit coupling exhibits geometrical and electronic structural dependence.     

Estimation of the R-H bond energy from the activation energy for the reaction RH + O2 → R· + HO2·

Since the activation energy for the reaction RH + O₂ → R· + HO₂. is very close to its endothermicity, the R-H bond energy can be calculated from the activation energy for free radical formation by the reaction RH + O₂. The relation between E_i and Q_R–H was found empirically after measuring E_i by the method of inhibitors for the oxidation of cyclohexane, n? heptane, and toluene: The values of Q_R–H are calculated from these and earlier experimental data for five hydrocarbons, five phenols, and four aromatic amines.     

Analytical potential from ab initio calculations for the Fe+-H2O and Feo-H2O systems

Analytical intermolecular potentials for the Fe⁺?H₂O and Fe^o?H₂O systems have been determined from ab initio calculations. Interaction energies for a lot of points along the two potential energy surfaces were calculated using Huzinga's MINI ?2 basis set. The results obtained were fitted to an analytical function containing 11 adjustable parameters that we have already used with success for the Fe²⁺?H₂O system. The goodness of the generated intermolecular potentials is discussed.     

Kinetic and thermochemical parameters of chlorine atom transfer reactions from CF3Cl,CF3CF2Cl,CF2ClCF2Cl,and CF2ClCFCl2 in gas phase

The following reactions: (1) were studied over the temperature ranges 533–687 K, 563–663 K, and 503–613 K for the forward reactions respectively and over 683–763 K, for the back reaction. Arrhenius parameters for chlorine atom transfer were determined relative to the combination of the attacking radicals. The ΔH_r°(1) = ?3.95 ± 0.45 kcal mol^?1 was calculated and from this value the ΔH∮(C₂F₅Cl) = ?2.66.3 ± 2.5 kcal mol^?1 and D(C₂F₅-Cl) = 82.0 ± 1.2 kcal mol^?1 were obtained. Besides, the ΔH_r°(2) was estimated leading to D(CF₂ClCF₂Cl) = 79.2 ± 5 Kcal mol^?1. The bond dissociation energies and the heat of formation are compared with those of the literature. The effect of the halogen substitutents as well as the importance of the polar effects for halogen transfer processes are discussed.     

Linewidths of C2H2 perturbed by H2: experiments and calculations from an ab initio potential

In this work we present a theoretical and experimental study of the acetylene-hydrogen system. A potential surface considering rigid monomers has been obtained by ab initio quantum chemistry methods. This 4-dimensional potential is further employed to compute, using the close-coupling approach and the coupled-states approximation, pressure broadening coefficients of C(2)H(2) isotropic Raman Q lines over a temperature range of 77 to 2000 K. Experimental data for the acetylene nu(2) Raman lines broadened by molecular hydrogen are obtained using stimulated Raman spectroscopy. The comparison of theoretical values with experimental data at 143 K is promising. Approximations to increase the computational efficiency are proposed.     

Semiempirical (AM1) and ab initio calculations of 2-X-adenine (X = H, F, Cl) and β-D-1-amino-2,3-didehydro-1,2,3-trideoxyribofuranose

We performed semiempirical (AM1) and ab initio (3-21G and 4-31G^**) calculations, with complete optimization of geometry, for 2-X-adenine (X = H, F, Cl) and selected conformers of β-D-1-amino-2,3-didehydro-1,2,3-trideoxyribo-furanose. Together (as 2-X-2',3'-didehydro-2',3'-dideoxyadenosine), or in separate conjunction with similar fragments, these structures belong to a group of 2',3'-dideoxynucleosides with potential anti-HIV activity.

The 2-X-adenine molecules are basically flat. For the halogenated species, especially the chlorine derivative, the results of the semiempirical method differ from the ab initio findings more widely than for adenine.

The results for the ribose derivative show the existence of conformers differing very little in energy. Ring puckering does not appear to depend on exocyclic torsion angles, structures with different conformations around the exocyclic C-C bond having similar ring conformations in all the cases analysed. The AMI method yielded geometries similar to those obtained ab initio with 3-21G.     

Investigation on ab initio calculations of the electron donor-acceptor complex H3N·SiH3Cl and the hypervalence of Si in it

The formation and the presence of hypervalent Si in the electron donor-acceptor complex H₃N·SiH₃Cl have been investigated by ab initio calculation. The results show that there is a 0.707eV decrease of energy when the complex H₃N·SiH₃Cl is formed from NH₃ and H₃SiCl, the interaction potential between the donor NH₃ and the acceptor H₃SiCl belongs to the Morse type, and the bond angle A(H-Si-Cl) versus bond length d(N-Si) presents a linear relation. The results also show that the interaction is mainly from giving the lone pair electrons in HOMO of NH₃ to LUMO of H₃SiCl, in which the 2p₂ of N and the 3d₀ of Si play important role. Bond N-Si is a weak n-σ* type dative one.     

Flexible H2O2 in water: electronic structure from photoelectron spectroscopy and ab initio calculations

The effect of hydration on the electronic structure of H(2)O(2) is investigated by liquid-jet photoelectron spectroscopy measurements and ab initio calculations. Experimental valence electron binding energies of the H(2)O(2) orbitals in water are, on average, 1.9 eV red-shifted with respect to the gas-phase molecule. A smaller width of the first peak was observed in the photoelectron spectrum from the solution. Our experiment is complemented by simulated photoelectron spectra, calculated at the ab initio level of theory (with EOM-IP-CCSD and DFT methods), and using path-integral sampling of the ground-state density. The observed shift in ionization energy upon solvation is attributed to a combination of nonspecific electrostatic effects (long-range polarization) and of the specific interactions between H(2)O(2) and H(2)O molecules in the first solvation shell. Changes in peak widths are found to result from merging of the two lowest ionized states of H(2)O(2) in water due to conformational changes upon solvation. Hydration effects on H(2)O(2) are stronger than on the H(2)O molecule. In addition to valence spectra, we report oxygen 1s core-level photoelectron spectra from H(2)O(2)(aq), and observed energies and spectral intensities are discussed qualitatively.     

An intermolecular potential function for the Fe(II)–H2O system from ab initio calculations

An intermolecular potential for the Fe(II)–H₂O system has been determined from ab initio calculations which have been obtained with Huzinaga's MINI -2 basis set. Interaction energies for more than 100 points of the potential energy surface were fitted to an analytical function that contains 11 adjustable parameters. The goodness of the fitting and its applicability to the study of Fe(H₂O) clusters and to Monte Carlo simulations are discussed.     

Low-resolution microwave,infrared and Raman spectra,ab initio calculations and conformational analysis of 1-bromo-2-methoxyethane

The microwave spectrum of gaseous 1-bromo-2-methoxyethane, BrCH₂CH₂OCH₃, has been recorded from 18.0 to 27.0 GHz at low resolution. The Raman spectra of the gas, liquid, and solid along with the infrared spectra of the gas and solid have been recorded from 3500 to 50 cm^–1. A comparison of the vibrational spectra obtained for the fluid phases with those obtained for the annealed solid indicates the presence of more than one conformer in the fluid phases. The presence of two asymmetric rotors allows for five possible conformations, a majority of which are present in the gaseous phase. It is concluded that the conformation present in the solid phase is that of the gauche/trans (GT) form, where the first term refers to the carbon-bromine bond and the second term to the carbon-oxygen bond (methoxy group). In the gas phase the major absorption in the microwave spectrum is a result of the trans/trans (TT) form although it is certain that other conformations are also present. From a variable temperature study of the Raman spectrum of the liquid, the enthalpy difference between the conformers of the—OCH₃ group was found to be 850 ±115 cm^–1 (2.43±0.33 kcal/mol) whereas the difference between the conformers of the—CH₂Br group as found to be 510±24cm^–1 (1.46±0.07 kcal/mol). The conformational energy differences, vibrational frequencies, and structural parameters have been obtained from ab initio calculations with the STO-3G^* basis set, and these theoretical values are compared to the experimental values. All of these results are compared to similar data for some corresponding molecules.Taken in part from the thesis of R. A. Larsen which was submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree.     

Vacuum ultraviolet spectra of CClF2CH2Cl and CClF2CHCl2, and Rydberg assignment based on ab initio molecular orbital calculations

The absorption spectra of CClF₂CH₂Cl and CClF₂CHCl₂ are measured in the spectral range 115–200 nm. The bands observed are assigned on the basis of ab initio molecular orbital calculations. The strong absorption bands in the 115–150 nm range are assigned to the transitions from lone pair orbitals of chlorine atoms to the Rydberg states. The population of the electrons in each molecular orbital (MO) is calculated to clarify the MO character. The four highest MOs for CClF₂CH₂Cl and six for CClF₂CHCl₂, which contribute to the absorption spectra in the 115–150 nm range, are typical chlorine lone pair orbitals, and are responsible for the spectra in this range.     

β‐Aminoacrolein: An ab initio,AIM and NBO study

The molecular structure and the intramolecular hydrogen bonding of β‐aminoacrolein and its simple derivatives were investigated at the MP2 and B3LYP levels of theory using the standard 6‐311++G(d, p) basis set. The “atoms in molecules” or AIM theory of Bader which is based on topological properties of the electron density (ρ), was used. Additionally, an analysis of the critical points was performed to study the nature hydrogen bonding in these systems. Natural bond orbital (NBO) analysis was also carried out for to better comprehend the nature of the intramolecular interactions in β‐aminoacrolein and its derivatives. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Collision energy dependence of the O(1D) + HCl --> OH + Cl(2P) reaction studied by crossed beam scattering and quasiclassical trajectory calculations on ab initio potential energy surfaces

The dynamics of the O(1D) + HCl --> OH + Cl(2P) reaction are investigated by a crossed molecular beam ion-imaging method and quasiclassical trajectory calculations on the three ab initio potential energy surfaces, the ground 1(1)A' and two excited (1(1)A' and 2(1)A') states. The scattering experiment was carried out at collision energies of 4.2, 4.5, and 6.4 kcal/mol. The observed doubly differential cross sections (DCSs) for the Cl(2P) product exhibit almost no collision energy dependence over this inspected energy range. The nearly forward-backward symmetric DCS indicates that the reaction proceeds predominantly on the ground-state potential energy surface at these energies. Variation of the forward-backward asymmetry with collision energy is interpreted using an osculating complex model. Although the potential energy surfaces obtained by CASSCF-MRCI ab initio calculations exhibit relatively low potential barriers of 1.6 and 6.5 kcal/mol for 1(1)A' and 2(1)A', respectively, the dynamics calculations indicate that contributions of these excited states are small at the collision energies lower than 15.0 kcal/mol. Theoretical DCSs calculated for the ground-state reaction pathway agree well with the observed ones. These experimental and theoretical results suggest that the titled reaction at collision energies less than 6.5 kcal/mol is predominantly via the ground electronic state.     

The electronic structure of 2-chloro-2′-deoxyadenosine (drug) studied by Cl-NQR spectroscopy and ab initio calculations

Electronic structure and conformation of 2-chloro-2′-deoxyadenosine (2CldAdo) and 2-chloroadenosine (2ClAde) have been studied by ³⁵Cl-NQR spectroscopy and ab initio calculations by the B3LYP/6-31G^* method.

The usefulness of NQR spectroscopy and ab initio calculations for the investigation of redistribution of electron density and structural changes in 2CldAdo and 2ClAde was assessed and a comparison of the electron distribution in the unsubstituted 2ClAde (monomer, dimer) and the substituted 2ClAdo was made.     

Semiempirical and ab initio calculations on the alkaline hydrolysis of the β-lactam ring Influence of the solvent

We used semiempirical and ab initio calculations to investigate the nucleophilic attack of the hydroxyl ion on the β-lactam carbonyl group. Both allowed us to detect reaction intermediates pertaining to proton-transfer reactions. We also used ab initio calculations and the PM3 semiempirical method to investigate the influence of the solvent on the process. The AMSOL method predicts the occurrence of a potential energy barrier of 20.7 kcal mol⁻¹ due to the desolvation of the hydroxyl ion in approaching the β-lactam carbonyl group. Using the supermolecular approach and a water solvation sphere of 20 molecules around the solute, the potential energy barrier is lowered to 17.5 kcal mol⁻¹. Ab initio calculations using the SCRF method predict a potential energy barrier of 13.6 kcal mol⁻¹. These three values, especially the last two, are very close to the experimental value of 16.7 kcal mol⁻¹.     

Vibrational spectra and structure of CH3Cl:H2O, CH3Cl:HDO, and CH3Cl:D2O complexes. IR matrix isolation and ab initio calculations

The infrared spectra of CH3Cl + H2O isolated in solid neon at low temperatures have been investigated. The CH3Cl + H2O system is remarkable because of its propensity to form CH3Cl:H2O and CH3Cl:(H2O)n (n > or = 2) complexes. We focus here on the CH3Cl:H2O species. Low concentration studies (0.01-0.5%) and subsequent annealing lead to formation of the 1:1 CH3Cl:H2O complex with O-H. . .Cl-C or O. . .H-C intermolecular hydrogen bonds. Vibrational modes of this complex have been detected. In addition, spectra of D2O + CH3Cl and HDO + CH3Cl have also been recorded. A detailed vibrational analysis of partially deuterated species shows that HDO is exclusively D bonded to CH3Cl. This is a consequence of the preference for HDO to form a deuterium bonding complex rather than a hydrogen bonding one.     

Computing the 7Li NMR chemical shielding of hydrated Li+ using cluster calculations and time-averaged configurations from ab initio molecular dynamics simulations

Ab initio molecular dynamics (AIMD) simulations have been used to predict the time-averaged Li NMR chemical shielding for a Li(+) solution. These results are compared to NMR shielding calculations on smaller Li(+)(H(2)O)(n) clusters optimized in either the gas phase or with a polarizable continuum model (PCM) solvent. The trends introduced by the PCM solvent are described and compared to the time-averaged chemical shielding observed in the AIMD simulations where large explicit water clusters hydrating the Li(+) are employed. Different inner- and outer-coordination sphere contributions to the Li NMR shielding are evaluated and discussed. It is demonstrated an implicit PCM solvent is not sufficient to correctly model the Li shielding, and that explicit inner hydration sphere waters are required during the NMR calculations. It is also shown that for hydrated Li(+), the time averaged chemical shielding cannot be simply described by the population-weighted average of coordination environments containing different number of waters.     

Abstraction kinetics of H-atom by OH radical from pinonaldehyde (C10H16O2): ab initio and transition-state theory calculations

The kinetics and abstraction rate coefficients of hydroxyl radical (OH) reaction with pinonaldehyde were computed using G3(MP2) theory and transition-state theory (TST) between 200 and 400 K. Structures of the reactants, reaction complexes (RCs), product complexes (PCs), transition states (TSs), and products were optimized at the MP2(FULL)/6-31G* level of theory. Fifteen transition states were identified for the title reaction and confirmed by intrinsic reaction coordinate (IRC) calculations. The contributions of all the individual hydrogens in the substrate molecule to the total reaction are computed. The quantum mechanical tunneling effect was computed using Wigner's and Eckart's methods (both symmetrical and unsymmetrical methods). The reaction exhibits a negative temperature dependent rate coefficient, k(T) = (1.97 ± 0.34) × 10(-13) exp[(1587 ± 48)/T] cm(3) molecule(-1) s(-1), k(T) = (3.02 ± 0.56) × 10(-13) exp[(1534 ± 52/T] cm(3) molecule(-1) s(-1), and k(T) = (4.71 ± 1.85) × 10(-14) exp[(2042 ± 110)/T] cm(3) molecule(-1) s(-1) with Wigner's, Eckart's symmetrical, and Eckart's unsymmetrical tunneling corrections, respectively. Theoretically calculated rate coefficients are found to be in good agreement with the experimentally measured ones and other theoretical results. It is shown that hydrogen abstraction from -CHO position is the major channel, whereas H-abstraction from -COCH(3) is negligible. The atmospheric lifetime of pinonaldehyde is computed to be few hours and found to be in excellent agreement with the experimentally estimated ones.     

Mono- and poly-ligated complexes of Zn2+: An ab initio analysis of the metal–ligand interaction energy

Detailed investigation of the binding energetics of Zn²⁺ to biologically relevant model ligands has been performed by large basis set restricted Hartree-Fock computations. This list includes neutral and anionic ligands that model the sidechains of the amino acid residues of proteins as well as those involved in binding to the metal during enzymatic activation: water, formaldehyde, formamide, imadazole, methylthiol, and the formate, hydroxyl, methoxy, methylthiolate anions. The decomposition of the intermolecular interaction energy into its components (Coulomb, exchange, polarization, and charge transfer) has been done within the frozen fragment reduced variational space procedure (RVS) developed by Stevens and Fink [W. J. Stevens and W. H. Fink, Chem. Phys. Lett., 139 , 15 (1987)]. The use of the RVS procedure was dictated by the very large magnitudes of the second-order interaction energy terms in the divalent cation complexes and the need to obtain polarization and charge-transfer contributions in a variational sense. The behavior of the interaction energy with radial and angular variation of the approach of the metal to the ligand is explored. In addition, the nonadditive behavior of polyligated complexes is studied for water and formate. This will also provide the data for a subsequent fit to a molecular mechanics procedure that considers the second-order interactions. © 1995 by John Wiley & Sons, Inc.