Reaction of photogenerated fluorine atoms with dopant molecules in solid argon

The products of reactions of dopant CH₄ molecules with F atoms diffusing in solid argon at 20–30 K were identified by ESR and FTIR spectroscopy. The F atoms stabilized in the matrix were generated by UV photolysis of Ar?CH₄(CD₄)?F (1000∶1∶1) samples at 13 K. Subsequent heating above 20 K results in thawing off diffusion of the F atoms and formation of products of their reaction with CH₄: radical-molecular complexes^·CH₃?HF (^·CD₃?DF) and radicals^·CH₃ (^·CD₃). The ESR spectra of the radicala are similar to those observed for matrix-isolated^·CH₃. The^·CH₃?HF complexes are characterized by the IR band of HF stetching vibration at 3764 cm^?1. Two additional splittings on the H (a _H·=2 G) and F(a _F=16G) nuclei of the HF molecule appeal in the ESR spectrum of the complex. The latter splitting is retained in the^·CD₃?DF complex, whereA _D· <0.3G The rate constant of the reaction CH₄+F→^·CH₃+HF is equal to ?10^?25 cm³s^?1 at 20 K. Its activation energy (1.7±0.2 kcal mol^?1) is ?0.5 kcal mol^?1 greater than that in the gas phase. The collinear C_3v-configuration of the^·CH₃?HF complex, which is similar to the configuration of the reagents in the transition state of the reaction considered, was established by the comparison of the exprrimental constants of hyperfine coupling with the results of the quantum-chemical calculation.     

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon

Isolated radicals^.NH₂ and radical-molecule complexes^.NH₂−HF, which are products of the reactions of mobile fluorine atoms with NH₃ molecules in solid argon, were identified by EPR spectroscopy. The isotropic HFC constants of the complex (a _N=1.20,a _H=2.40, anda _F=0.70 mT) were determined experimentally. The constant of isotropic HFC with the nucleus of hydrogen atom of the HF molecule is less than 0.1 mT. This assignment was confirmed in the experiments on isotope substitution of atoms (H→D),¹⁴N→¹⁵N) in the NH₃ molecule. According to quantum-chemical calculations, the free complex^.NH₂−HF has a planar structure withC ₂, summetry and a binding energy of 12 kcal mol⁻¹. Optimization of the arrangement of the complex in the crystal showed that its structure is only slightly distorted in the Ar lattice so that the equilibrium configuration is close to that obtained from gas-phase calculations. Different ratios of relative intensities of the proton triplet lines in the EPR spectra of isolated^.NH₂ radicals and^.NH₂−HF complexes were qualitatively explained by different heights of the barriers to rotation of the NH₂ fragment in the Ar lattice.     

Reactions of photogenerated fluorine atoms with contaminant molecules trapped in solid argon 5. EPR spectroscopy of FC<Subscript>60</Subscript> · radical in solid argon

Reactions of photogenerated fluorine atoms with C₆₀ fullerene molecules isolated in solid argon were studied by EPR spectroscopy in the temperature range from 15 to 25 K. Highly resolved anisotropic EPR spectrum of the FC₆₀ ^⋅ radical was obtained for the first time. The spectrum is characterized by low anisotropy of the g-tensor and by axially symmetric HFC tensor on ¹⁹F nuclei. The parameters of the HFC tensor for ¹⁹F magnetic nuclei were determined. The isotropic HFC constant A _iso equals 202.8 MHz and the anisotropic magnetic dipole-dipole interaction constant A _dip equals 51.8 MHz. Quantum chemical calculations of FC₆₀ ^⋅ radical showed that the PBE1/Λ22m method (PBE1 functional and the correlation triple-zeta basis sets augmented with polarization functions on inner atomic shells) provides good agreement between the theoretical magnetic parameters and experimental data. Specific features of the spin density distribution in the FC₆₀ ^⋅ radical are discussed. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 424–428, March, 2007.     

Reactions of silicon atoms with methane and silane in solid argon: a matrix-spectroscopic study

The reaction of silicon atoms with methane (1) and silane (7) in argon at 10 K has been studied. Methane (1) is found to be inert to silicon atoms, but reaction occurs upon photochemical excitation under formation of methylsilylene (2). On the contrary, silane (7) is spontaneously converted into a mixture of silylsilylene (10) and disilene (12). Subsequent irradiation generates the butterfly-type disilyne Si(2)H(2) (14), together with a third photoproduct, which we assume to be the doubly bridged Si(2)H(4) isomer 13. The structural elucidation of the new species is based on the comparison of the experimental IR and UV/Vis spectra with data from density functional theory calculations. The results are supported by isotopic labeling studies.     

The IR spectra of the products of the reaction of gallium atoms with iodine molecules in solid argon

The IR spectra of the reaction of gallium atoms with iodine molecules from the argon matrix are studied. Most of the bands observed have been assigned to stretching vibrations in tri-, di-, and monoiodide. Both the intensity relationship and the semiempirical estimates indicate that the valence angle should be no more than 110° in gallium di-iodide.     

Reactions of fluorine atoms with iodides studied by crossed beam laser-induced fluorescence

The reactions of F atoms with C₂H₅I, C₂F₅I, and n-C₃H₅I were studied by the crossed beam laser-induced fluorescence techniques within the 570–620 nm wavelength region. The vibrational and rotational excitation spectra of the reaction product IF were measured. The relative vibrational population densities of v = 3,4, and 5 vibrational levels, and some of the relative detailed vibrational rate constants, the rotational temperatures, and the mean fractions of rotational energy in individual vibrational states of the reaction product IF were obtained. The reaction mechanism was discussed.     

Reactions of late lanthanide metal atoms and methanol in solid argon: a matrix isolation infrared spectroscopic and theoretical study

The reactions of laser-ablated late lanthanide atoms and methanol were studied using matrix isolation infrared spectroscopy and electronic structure calculations at the density functional theory level. Both terbium and lutetium atoms react with methanol spontaneously to form the CH(3)OTbH and CH(3)OLuH insertion products, which react further with another methanol molecule to give the Tb(OCH(3))(2) and Lu(OCH(3))(2) products as found previously for uranium. The reactions of Dy through Yb and methanol first produce Ln(CH(3)OH) complexes during sample annealing, which isomerize to the CH(3)OLnH insertion products on visible irradiation. The Ln-H stretching frequencies of the CH(3)OLnH molecules exhibit a unique trend from Tb to Lu, which is also reproduced by theoretical calculations at the B3LYP level of theory. Although the CH(3)LnOH molecules are predicted to be more stable than the O-H bond insertion products, formation of the C-O bond insertion isomers is kinetically prohibited as revealed by calculated potential energy surfaces.     

Infrared spectra of some uranium oxyfluoride molecules isolated in solid argon

Reaction of laser-ablated uranium with oxygen/fluorine mixtures or laser-ablated uranium dioxide ceramic with fluorine produces the uranium oxyfluorides molecules UO₂F₂, UO₂F and UOF₄, which have been isolated in solid argon and identified by virtue of the effects of oxygen isotopic substitution on their infrared spectra.     

C-C double- and triple-bond formation from reactions of B atoms with CO: experimental and theoretical characterization of OBBCCO and OBCCBO molecules in solid argon

Reactions of boron atoms with CO molecules in solid argon form the following boron carbonyl species (which have been reported earlier): BCO, BBCO, OCBBCO, B(CO)2, and B4(CO)2. The OCBBCO molecule underwent a photochemical rearrangement where CO was activated to form the OBBCCO and OBCCBO molecules. The new molecules were identified on the basis of isotopic IR studies with 10B, 11B, 13C16O, 12C18O, and carbon dioxide mixtures in addition to comparison with quantum-chemical calculations of isotopic frequencies. Theoretical analyses showed that the OBBCCO and OBCCBO molecules are linear with C-C double and triple bonding, respectively, and lie at a much lower energy than the linear OCBBCO structure.     

Heterogeneous decay of fluorine atoms

The probability of decay of fluorine atoms on the surface of molybdenum glass, quartz, Teflon, copper, nickel, gold and on aluminium alloy at 20–300 °C has been determined.

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Reactions of chromium atoms with chloromethanes

The rate constants of the gas-phase reactions of the chromium atom with CC1⁴, CHC1₃, and CH₂Cl₂ were measured behind shock waves at 800–1400 K. The results are presented in the Arrhenius form (the activation energy is given in kJ/mol):k _{CCl 4} = 10^14.32±0.36exp[-(2.0±7.5)/RT],k _{CHCl 3} = 10^14.72±0.21exp[-(18.5.±4.0)/RT, andk _{CH 2 Cl 2}> = 10^14.33±0.16exp[-(24.1±3.1)/RT]cm³mol^-1s^-1.     

Reactions of cerium atoms and dicerium molecules with CO: formation of cerium carbonyls and photoconversion to CO-activated insertion molecules

Reactions of cerium with carbon monoxide molecules in solid argon have been studied using matrix isolation infrared absorption spectroscopy. The cerium carbonyls CeCO and Ce2CO are produced spontaneously on annealing and they are photochemically rearranged to the CCeO and c-Ce2(mu-C)(mu-O) isomers, where Ce and Ce2 are inserted into the CO triple bond. Theoretical calculations indicate that CeCO is an end-on-bonded carbonyl with a quintet ground state, whereas Ce2CO is a rare dinuclear lanthanide carbonyl complex with CO serving as an asymmetrically bridged, side-on ligand. The CCeO molecule was theoretically characterized to have a linear structure with a singlet ground state. Evidence is also presented for the CeCO- anion and other cerium carbonyls with higher coordination numbers.     

Depth-dependent imaging of individual dopant atoms in silicon.

We have achieved atomic-resolution imaging of single dopant atoms buried inside a crystal, a key goal for microelectronic device characterization, in Sb-doped Si using annular dark-field scanning transmission electron microscopy. In an amorphous material, the dopant signal is largely independent of depth, but in a crystal, channeling of the electron probe causes the image intensity of the atomic columns to vary with the depths of the dopants in each column. We can determine the average dopant concentration in small volumes, and, at low concentrations, the depth in a column of a single dopant. Dopant atoms can also serve as tags for experimental measurements of probe spreading and channeling. Both effects remain crucial even with spherical aberration correction of the probe. Parameters are given for a corrected Bloch-wave model that qualitatively describes the channeling at thicknesses 20 nm, but does not account for probe spreading at larger thicknesses. In thick samples, column-to-column coupling of the probe can make a dopant atom appear in the image in a different atom column than its physical position.     

Infrared spectroscopic and density functional theory studies on the reactions of cadmium atoms with carbon monoxide in solid argon

Reactions of laser-ablated cadmium atoms with carbon monoxide molecules in solid argon have been investigated using matrix isolation infrared spectroscopy. On the basis of isotopic substitution, the absorption at 1858.2 cm(-1) is assigned to the C-O stretching of the CdCO molecule, which is formed during the sample deposition. Cadmium di- and tricarbonyls, Cd(CO)n (n = 2, 3), have not been observed under the same experimental conditions. Density functional theory calculations have been performed on the cadmium carbonyls Cd(CO)n (n = 1-3), which lend strong support to the experimental assignments of the infrared spectra. It is predicted that the CdCO molecule is a linear triplet molecule and its formation involves Cd 5s --> 5p promotion. This promotion increases the Cd-CO bonding by decreasing the sigma repulsion and increasing the Cd 5p orbital --> CO pi back-donation. The absence of cadmium di- and tricarbonyls, Cd(CO)n (n = 2, 3), has also been discussed in some detail.     

Reactions of group V metal atoms with water molecules. Matrix isolation FTIR and quantum chemical studies

Laser-ablated group V metal atoms (V, Nb, Ta) were co-deposited with water molecules in excess argon. The V atoms reacted with water to form the inserted HVOH molecule spontaneously. The Nb atoms reacted with water to form the NbOH2 complex and the inserted HNbOH molecule. Broad-band photolysis produced the H2VO and H2NbO molecules as well as the VO and NbO monoxides. For Ta + H2O reactions, neither TaOH2 nor HTaOH was observed, while the H2TaO molecule was produced on annealing, and the H2 elimination process was not observed on photolysis. The aforementioned species were identified via isotopic substitutions as well as density functional calculations. Qualitative analysis of the possible reaction paths leading to the observed products is proposed. The results have been compared with our earlier works concerning the Sc and group IV metal atoms with water reactions in order to observe existent trends for the early transition metal atoms.     

Properties of atoms in molecules

Atomic charges calculated by the population analysis method for three types of semi-empirical wave functions have been compared with charges obtained by integrating the corresponding electronic density functions over individual atomic regions. It was found that the two sets of charges compare quite well for CNDO wave functions and for extended-Hückel functions which are in terms of orthogonalized basis orbitals. However only the CNDO charges are reasonably close to those obtained by integrating near-Hartree-Fock electronic density functions.