Stanislav K. Ignatov

Quantum Chemical Study of Trimolecular Reaction Mechanism between Nitric Oxide and Oxygen in the Gas Phase

Singlet and triplet potential energy surfaces of the reaction between molecular oxygen and two nitric oxide(II) molecules were studied by quantum chemical methods (coupled cluster, CASSCF, and density functional theory: B3LYP, TPSS, VSXC, BP86, PBE, B2-PLYP, B2K-PLYP). Elementary steps involving various N2O4 isomers (cyclic, cis-cis-, cis-trans-, trans-trans-ONOONO, cis- and trans-ONONO2, O2NNO2) were considered, as well as weakly bound molecular clusters preceding formation of O2NNO2, and Coupe-type quasi-aromatic hexagonal ring intermediate NO2.O2N. We found that activation energy strongly depends on the conformation of ONOONO peroxide, which is formed barrierlessly. The best agreements with experimental values were achieved by the B3LYP functional with aug-pc3 basis set. The lowest transition state (TS) energies correspond to the following reaction channel: 2NO + O2 (0 kJ/mol) --> cis-cis-ONOONO (-45 kJ/mol) --> TS1 --> NO2.O2N (-90 kJ/mol) --> TS2 --> cis-ONONO2 (-133 kJ/mol)--> TS3 --> trans-ONONO2 (-144 kJ/mol) --> TS4 --> O2NNO2 (-193 kJ/mol). A valley ridge inflection (VRI) point is located on the minimum energy path (MEP) connecting NO2.O2N and cis-ONONO2. The energy landscape between NO2.O2N and CC-TS2 can be classified as a downhill valley-pitchfork VRI bifurcation according to a recent classification of bifurcation events [Quapp, W. J. Mol. Struct. 2004, 95, 695-696]. The first and second transition states correspond to barrier heights of 10.6 and 37.0 kJ/mol, respectively. These values lead to the negative temperature dependence of the rate constant. The apparent activation enthalpy of the overall reaction was calculated to be Delta(r)H(0) = -4.5 kJ/mol, in perfect agreement with the experimental value.

An unexpected mechanism of hydrosilylation by a silyl hydride complex of molybdenum.

Carbonyl hydrosilylation catalyzed by (ArN)Mo(H)(SiH(2)Ph)(PMe(3))(3) (3) is unusual in that it does not involve the expected Si-O elimination from intermediate (ArN)Mo(SiH(2)Ph)(O(i)Pr)(PMe(3))(2) (7). Instead, 7 reversibly transfers β-CH hydrogen from the alkoxide ligand to metal.

Mechanism of Nitric Oxide Oxidation Reaction (2NO + O2 → 2NO2) Revisited.

The reaction between molecular oxygen and two nitric oxide(II) molecules is studied with high-level ab initio wave function methods, including geometry optimizations with coupled cluster (CCSD(T,full)/cc-pCVTZ) and complete active space with second order perturbation theory levels (CASPT2/cc-pVDZ). The energy at the critical points was refined by calculations at the CCSD(T,full)/aug-cc-pCVTZ level. The controversies found in the previous theoretical studies are critically discussed and resolved. The best estimate of the activation energy is 6.47 kJ/mol.

Quantum chemical study of the initial step of ozone addition to the double bond of ethylene.

The mechanisms of the initial step in chemical reaction between ozone and ethylene were studied by multireference perturbation theory methods (MRMP2, CASPT2, NEVPT2, and CIPT2) and density functional theory (OPW91, OPBE, and OTPSS functionals). Two possible reaction channels were considered: concerted addition through the symmetric transition state (Criegee mechanism) and stepwise addition by the biradical mechanism (DeMore mechanism). Predicted structures of intermediates and transition states, the energies of elementary steps, and activation barriers were reported. For the rate-determining steps of both mechanisms, the full geometry optimization of stationary points was performed at the CASPT2/cc-pVDZ theory level, and the potential energy surface profiles were constructed at the MRMP2/cc-pVTZ, NEVPT2/cc-pVDZ, and CIPT2/cc-pVDZ theory levels. The rate constants and their ratio for reaction channels calculated for both mechanisms demonstrate that the Criegee mechanism is predominant for this reaction. These results are also in agreement with the experimental data and previous computational results. The structure of DeMore prereactive complex is reported here for the first time at the CCSD(T)/cc-pVTZ and CASPT2/cc-pVDZ levels. Relative stability of the complexes and activation energies were refined by single-point energy calculations at the CCSD(T)-F12/VTZ-F12 level. The IR shifts of ozone bands due to formation of complexes are presented and discussed.

High-resolution Fourier-transform IR spectroscopic determination of impurities in silicon tetrafluoride and silane prepared from it

The impurity compositions of silicon tetrafluoride and silane prepared from it have been determined by high-resolution Fourier-transform IR spectroscopy. In the spectra of SiF4 samples differing in purity, we have identified rovibrational bands arising from Si2F6O, SiF3OH, HF, SiF3H, SiF2H2, SiH3F, CH4, CO2, and CO impurities. Their detection limits lie in the range 9 × 10−5 (CO2) to 3 × 10−3 mol % (Si2F6O). In the spectra of SiH4 samples of different purity, we have detected CH4, CO2, SiF3H, SiF2H2, and SiF4 impurities. Their detection limits lie in the range 8 × 10−5 (CO2) to 1 × 10−3 mol % (SiF4).

Structure, Energy, and Vibrational Frequencies of Oxygen Allotropes On (n ≤ 6) in the Covalently Bound and van der Waals Forms: Ab Initio Study at the CCSD(T) Level.

Recent experiments on the UV and electron beam irradiation of solid O2 reveals a series of IR features near the valence antisymmetric vibration band of O3 which are frequently interpreted as the formation of unusual On allotropes in the forms of weak complexes or covalently bound molecules. In order to elucidate the question of the nature of the irradiation products, the structure, relative energies, and vibrational frequencies of various forms of On (n = 1-6) in the singlet, triplet, and, in some cases, quintet states were studied using the CCSD(T) method up to the CCSD(T,full)/cc-pCVTZ and CCSD(T,FC)/aug-cc-pVTZ levels. The results of calculations demonstrate the existence of stable highly symmetric structures O4 (D3h), O4 (D2d), and O6 (D3d) as well as the intermolecular complexes O2·O2, O2·O3, and O3·O3 in different conformations. The calculations show that the local minimum corresponding to the O3···O complex is quite shallow and cannot explain the ν3 band features close to 1040 cm(-1), as was proposed previously. For the ozone dimer, a new conformer was found which is more stable than the structure known to date. The effect of the ozone dimer on the registered IR spectra is discussed.

Unexpected features of stretched Si–H⋯Mo β-agostic interactions

Coupling of silanes with the imido group of (Ar′N)2Mo(PMe3)3 gives either the silanimine dimer (ArN–SiHCl)2 or Si–H agostic silylamido complexes which do not exhibit the commonly expected correlation between the nature of the substituents on silicon, the degree of Si–H addition and the value of the Si–H coupling constant.

Integral Intensities of Absorption Bands of Silicon Tetrafluoride in the Gas Phase and Cryogenic Solutions: Experiment and Calculation

The spectral characteristics of the SiF4 molecule in the range 3100–700 cm−1, including the absorption range of the band ν3, are studied in the gas phase at P = 0.4–7 bar and in solutions in liquefied Ar and Kr. In the cryogenic solutions, the relative intensities of the vibrational bands, including the bands of the isotopically substituted molecules, are determined. The absorption coefficients of the combination bands 2ν3, ν3 + ν1, ν3 + ν4, and 3ν4 are measured in the solution in Kr. In the gas phase of the one-component system at an elevated pressure of SiF4, the integrated absorption coefficient of the absorption band ν3 of the 28SiF4 molecule was measured to be A(ν3) = 700 ± 30 km/mol. Within the limits of experimental error, this absorption coefficient is consistent with estimates obtained from independent measurements and virtually coincides with the coefficient A(ν3) = 691 km/mol calculated in this study by the quantum-chemical method MP2(full) with the basis set cc-pVQZ.

Quantum-chemical study of structural, spectral, and electrooptical parameters of fluorosilanes SiH4− x Fx(x=0–4)

The possibility of calculating the molecular structure, vibrational frequencies, and electrooptical parameters of the silane molecule SiH4 and the silicon tetrafluoride molecule SiF4 is analyzed by using the ab initio quantum-chemical method (the MP2 perturbation theory) and the density functional theory (the B3LYP functional) with a wide variation of basis sets up to the cc-pV5Z set. The results obtained are used as reference values for studing mixed fluorosilanes SiH4−xFx (x=1–3) and for refining their geometry, the vibrational frequencies, and the absolute intensities of absorption bands. Based on the results of high-level quantum-chemical calculation corrected against the experimental data for SiH4 and SiF4, the structural parameters of mixed flouro-silanes are estimated and their vibrational frequencies are calculated and assigned for the first time. The quantum-chemical estimates of the absolute intensities of IR spectra are for the first time presented for all mixed fluorosilanes.

Identification of the products of partial hydrolysis of silicon tetrafluoride by matrix isolation IR spectroscopy

Matrix isolation Fourier transform IR spectroscopy has been used for studying the products and mechanism of the silicon tetrafluoride reaction with water at various component ratios and reaction durations. Assignment of new bands in the spectrum confirms the earlier assumptions of first the formation of a molecular complex with water and later of trifluorosilanol, which finally condenses to give hexafluorodisiloxane.