Antonija Lesar

Triazole, Benzotriazole, and Naphthotriazole as Copper Corrosion Inhibitors: I. Molecular Electronic and Adsorption Properties

The gas-phase adsorption of 1,2,3-triazole, benzotriazole, and naphthotriazole-considered as corrosion inhibitors-on copper surfaces was studied and characterized using density functional theory (DFT) calculations. We find that the molecule-surface bond strength increases with increasing molecular size, thus following the sequence: triazole<benzotriazole<naphthotriazole. This trend is explained in terms of molecular electronegativity and chemical hardness, which decrease monotonously as the molecular size increases. While the electronegativity of triazole is almost degenerate with the work function of Cu(111) surface, the electronegativity of larger acenotriazoles is smaller. The difference in electronegativity between the Cu(111) and the acenotriazoles thus increases with increasing the molecular size, which, together with decreasing the molecular hardness, results in larger molecule-to-metal electron charge transfer and stronger molecule-surface bonds.

MASS SPECTROMETRIC INVESTIGATION OF THE EVAPORATION PROPERTIES OF LEAD OXIDE

Partial pressures of molecular species over solid PbO were measured in the temperature interval between 890 K and 1100 K using Knudsen-cell mass spectrometry. The assignment of ions is discussed in detail. Structural and vibrational parameters of the gas phase for (PbnOn) (n = 1,2,3,4) were calculated by the ab initio molecular orbital method at different approximation levels. Third law enthalpies of dissociation of Pb2O2 and Pb4O4 to monomeric PbO were obtained as (250.6 ± 6) kJ/mol and (835.0 ± 15) kJ/mol, respectively. The possibility of direct measurement of the dissociation pressure of PbO by a mass spectrometer is demonstrated.

15N and 18O kinetic isotope effects in the thermal decomposition of N2O catalyzed by bromine

15N and 18O kinetic isotope effects (KIEs) in the thermal decomposition of N2O catalyzed by bromine were experimentally determined in the temperature range 773–873 K, resulting in KIE (15N)=−2.07+4020/T and KIE (18O)=−0.41+3290/T. For theoretical interpretation, based on the Bigeleisen formalism, the following planar transition states were taken into account: trans (N–N–O–Br)‡, trans (Br–N–N–O)‡, and branched (N–N<OBr)‡. In addition to KIE, activation energy according to the Sanderson bond‐energy–bond‐order relationship and pre‐exponential factor were calculated as subsidiary parameters in selecting an appropriate transition state among all probable configurations. The result reveals that it is meaningless to speculate whether the Br atom approaches the central or the terminal N atom of the N2O molecule, since both transition states could generate acceptable values of the kinetic isotope effects, activation energy, and pre‐exponential factor.

Nitrogen-15 and oxygen-18 kinetic isotope effects in the catalytic decomposition of N2O over MgO

The 15N kinetic isotope effects (KIE) in the catalytic decomposition of nitrous oxide on MgO powder were determined in the temperature range 675–875 K, and the following temperature dependence was found: KIE(15N)=(0.231±0.149)+(1353±114)/T. At initial pressures of N2O between 40 and 60 kPa, the reaction is of the order of +1 in N2O and has an apparent activation energy of 125±4 kJ mol−1. According to the Bigeleisen formalism, the rate-determining and isotope fractionation governing step was well represented by two transition states: a bent product-like NNO and a fork-type NNOO.

Structure, stability, and spectroscopic properties of H-bonded complexes of HOSO and CH3SO with H2O.

Quantum chemical calculations have been carried out to investigate the structure and stability of 1:1 and 1:2 HOSO-water and CH3SO-water complexes. All of the geometries have been optimized at the DFT and at the CCSD levels of theory using 6-311++G(2df,2pd) and aug-cc-pVDZ basis sets, respectively. The energetics of the hydrogen-bonded complexes are reported at G4 and CBS-QB3 levels of theory. A general characteristic future of the minimum-energy structure complexes is cyclic double H bonding for 1:1 complexes and cyclic triple H bonding for 1:2 complexes. Calculations predict relative large binding energies of 8.2 and 16.8 kcal mol(-1) for 1:1 and 1:2 HOSO-water complexes, respectively, at the CBS-QB3 level of theory. CH3SO-water complexes have somewhat lower stability; the binding energy of 3.8 kcal mol(-1) for the 1:1 CH3SO-water complex increases to 9.5 kcal mol(-1) for the 1:2 complex. The calculated shifts in vibrational frequencies due to complex formation show that the frequencies and intensities of H-bonded OH stretching regions are most affected by complex formation. The large frequency shift is mainly oriented to these OH bonds involved in H-bonding interactions. Vertical electronic excitation energies and the corresponding oscillator strengths have been calculated for the representative radical-water complexes using the TDDFT method and aug-cc-pVTZ basis set. No significant excitation energy difference was observed between the low-lying electronic states of either HOSO within the HOSO-water complexes or CH3SO within the CH3SO-water 1:1 complexes.

Conformational potential energy surface of BrOONO

Abstract Cis-perp and trans-perp conformations with respect to N–O and O–O bonds were found to be the only stable ones on the BrOONO potential energy surface using the CCSD(T)//B3LYP method with the 6-311G* basis set. The energy for the cis-perp form is 2.0 kcal mol −1 lower than for the trans-perp form while the saddle point connecting the two minima is 9.0 kcal mol −1 above the cis-perp level. A comparison of the relative energetics for stationary points on the BrOONO, ClOONO, and HOONO conformational potential energy surfaces is discussed.

THE EVAPORATION THERMODYNAMICS OF LITHIUM IODIDE. MASS SPECTROMETRIC AND AB INITIO STUDIES

The vaporization of LiI was investigated in the range between 583 and 726 K by Knudsen effusion mass spectrometry (KEMS). The ionization or the appearance energies (IE or AE) of all kinds of ions formed from the equilibrium vapour over solid lithium iodide were determined using Vogt and Pascuals deconvolution method. For the determination of vapour pressure two methods, viz. mass-loss Knudsen effusion and Knudsen effusion mass spectrometry were applied. The mean natural logarithms of the equilibrium vapour pressures (in Pa) of monomer and all kinds of oligomers, that can be detected using KEMS, as a function of temperature, can be expressed as follows: The molecular structure and the harmonic vibrational frequencies of (LiI)n species (n = 1,2,3,4) were predicted using ab initio molecular orbital methods. Both the bond dissociation energies and the enthalpy changes of dissociation of (LiI)n oligomers were evaluated and compared with the measured enthalpy change data. Using the calculated geometry and the vibration frequencies, the thermodynamic functions of (LiI)n could be calculated, making it possible to compare the second and third law thermodynamic data.

Theoretical study on the mechanism of the reaction of CF(3)S with NO(2).

The singlet potential energy surface for the CF3S + NO2 reaction has been theoretically investigated using the B3LYP/6-311+G(3df) level of theory. The geometries, vibrational frequencies, and zero-point energies of all stationary points involved in the title reaction have been examined. More accurate energies of stationary points were obtained using ab initio G3//B3LYP and CBS-QB3 composite methods. The results show that the initial addition of CF3S with NO2 leads to CF3SNO2 or CF3SONO intermediates, which are formed without an electronic barrier. CF3SNO2 can easily isomerizes to CF3SONO, while CF3SONO readily isomerizes to CF3S(O)NO or dissociates to CF3SO + NO, which are the major products of the title reaction. Reaction channels leading to the formation the CF3O + SNO and CF2S + FNO2 products are highly improbable processes due to high energy barriers involved. We have also computed heats of formation for CF3SNO2, CF3SONO, and CF3S(O)NO intermediates. It was found that the most stable is the cis-perpendicular form of CF3SONO isomer with DeltaH(f,0)0 = -243.6 kcal mol-1.

Effect of Halogenation on the Mechanism of the Atmospheric Reactions between Methylperoxy Radicals and NO. A computational Study

The mechanism of the reactions between the halogenated methylperoxy radicals, CHX(2)O(2) (X = F, Cl), and NO is investigated by using ab initio and density functional quantum mechanical methods. Comparison is made with the mechanism of the CH(3)O(2) + NO reaction. The most important energy minima in the potential energy surface are found to be the two conformers of the halogenated methyl peroxynitrite association adducts, CHX(2)OONOcp and CHX(2)OONOtp, and the halogenated methyl nitrates, CHX(2)ONO(2). The latter are suggested to be formed through the one-step isomerization of the peroxynitrite adduct and may lead upon decomposition to carbonylated species, CX(2)O + HONO and CHXO + XNO(2). The ambiguous issue of the unimolecular peroxynitrite to nitrate isomerization is reconsidered, and the possibility of a triplet transition state involvement in the ROONOtp <--> RONO(2) rearrangement is examined. The overall calculations and the detailed correlation with the methyl system show the significant effect of the halogenation on the lowering of the entrance potential energy well which corresponds to the formation of the peroxynitrites. The increased attractive character of the potential energy surface found upon halogenation combined with the increased exothermicity of the CHX(2)O(2) + NO --> CHX(2)O + NO(2) reaction are suggested to be the important factors contributing to the enhanced reactivity of the halogenated reactions relative to CH(3)O(2) + NO. The calculated heat of formation values indicate the large stabilization of the fluorinated derivatives.

Ab initio molecular orbital and density functional characterization of the potential energy surface of the N2O+Br reaction

The lowest doublet (2A′) potential energy surface for the reaction N2O+Br→N2+OBr was investigated using ab initio and nonlocal density functional theory calculations. Geometries, energies and vibrational frequencies for stationary points were evaluated at HF/6-31G(d), MP2/6-31G(d) and B3LYP/6-31G(d) levels of theory. All levels of calculation give a similar geometry for the transition state, but the MP2 barrier is narrower. Intrinsic reaction coordinate (IRC) calculations starting from the transition state and proceeding toward the two local minima confirmed that IRC trajectories reached the reactant and product regions, respectively. Calculations of kinetic isotope effects were also performed. They are influenced by the theoretical level and the B3LYP method gives results in best agreement with experimental data. The HF method predicts the relative values of both the primary and secondary nitrogen kinetic isotope effects less accurately. At the MP2 level of calculations only the oxygen kinetic isotope ef...