Self-consistent-field potential energy surface in three dimensions for the Cl + H2 → ClH + H chemical reaction

Non-empirical self-consistent-field calculations have been carried out for 38 points on the potential surface for the Cl + H₂ → ClH + H chemical reaction. A basis set of seven s, five p, and one d functions on chlorine and three s and one p on each hydrogen atom was used. The least energy path occurs for the linear Cl---H---H arrangement. A much higher barrier is found for the approach of Cl along the H---H perpendicular bisector. The linear barrier height is predicted to be 26.2 kcal/mole and the saddle point occurs for R(Cl---H) ≈ 1.46 Å, R(H---H) ≈ 0.94 Å. The experimental activation energy is 5.5 kcal/mole. It seems likely that a general feature of the Hartree-Fock approximation is an overestimation of barrier heights. The exothermicity is calculated to be −6.7 kcal/mole, compared to the near Hartree-Fock result −2.3 kcal/mole and experiment −3.0 kcal/mole.     

A theoretical study of the excited electronic states of the SiCN system

Ab initio methods have been used to study the lowest-lying electronic states of the SiCN radical, which has two stable linear isomers in its electronic ground state, SiCN and SiNC. Vertical excitation energies and oscillator strengths have been computed for a number of states lying up to 8 eV. The geometries of the lowest-lying doublet and quartet states have been determined. The lowest-lying excited doublet state of SiNC (1²Σ⁺, 4.0 eV) arises from a HOMO–LUMO excitation (3π → 10σ), although the 1²Δ state (9σ → 3π) is very close in energy. In the case of the SiCN isomer the lowest excited state is 1²Δ, which arises from an excitation from the highest occupied σ orbital into the HOMO (9σ → 3π) and lies 3.6 eV above the ground state. SiCN should present very strong absorptions at 4.9 and 6.1 eV whereas SiNC should have relatively strong absorptions in the region of 5.7–5.9 eV. The smallest adiabatic energy gaps with respect to the ground state of SiNC and SiCN are very close (about 2.8 eV) and the excited state is the same 1²A′, which has angular equilibrium geometries for both isomers. We have determined accurate values for enthalpies of formation of the two linear doublet forms and .     

The effects of saturation and velocity selective population in two-step 6S1/2 → 6P3/2 → 6D5/2 laser excitation in cesium

Excited states population distributions created by two-step 6S_1/2 → 6P_3/2 → 6D_5/2 laser excitation in room temperature cesium vapor were quantitatively analyzed applying absorption and saturation spectroscopy. A simple method for the determination of the excited state population in a single excitation step that is based on the measurements of the saturated and unsaturated absorption coefficients was proposed and tested. It was shown that only ≈ 2% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-line laser. With complete saturation of the second excitation step, the population amounting to only ≈ 1% of the ground state can be eventually created in the 6D_5/2 state. The fluorescence intensity emerging at 7P_3/2 → 6S_1/2 transition, subsequent to the radiative decay of 6D_5/2 population to the 7P_3/2 state, was used to assess the efficiency of the population transfer in the chosen two-step excitation scheme. The limitations imposed on the sensitivity of such resonance fluorescence detector caused by velocity-selective excitation in the first excitation step were pointed out and the way to overcome this obstacle is proposed.     

S2 → S0 fluorescence in an aromatic thioketone, xanthione

In addition to the red phosphorescence (T₁(³ A₂n, π^*) → S₀) xanthione exhibits in solution an emission with a maximum at ≈ 23 000 cm⁻¹ and φ_f(298°) = 5 × 10⁻³. It is shown that this emission is fluorescence from the second excited singlet state (S₂ (¹A₁ π, π^*) → S₀).     

The n → π* transition in nitroso compounds: A TD-DFT study

The visible spectra of a series of various nitroso dyes have been evaluated by using time-dependent density functional theory approach and explicitly taking into account bulk solvent effects. Using the PBE0 functional with the 6-311++G(3d,3p) atomic basis set, the agreement between theoretical and experimental n → π* transition energies is excellent for all nitroso species. The mean absolute error is limited to 0.05 eV and the maximal deviation is 0.21 eV for a set of 22 cases. As a blind test, we have predicted the absorption spectra for unknown species, in order to assess the impact of the atom bound to the NO group.     

Kinetic investigation of electronic energy transfer processes following the pulsed dye-laser generation of excited atomic barium, Ba[6s6p(P1)], in the presence of atomic strontium

The collisional behaviour of Ba[6s5d(³D_J)], 1.151 eV above the 6s²(¹S₀) electronic ground state, in the presence of atomic strontium, has been investigated in the ‘long-time domain' (ca. 100 μs–1 ms) following the pulsed dye-laser excitation of barium vapour at elevated temperature at λ = 553.5 nm (Ba[6s6p(¹P₁)] ← Ba[6s²(¹S₀)]. Ba(³D_J) is subsequently produced from the short-lived ¹P₁ state (τ_e = 8.37 ± 0.38 ns) by a number of radiative and collisional processes. It may then be monitored in the ‘long-time domain' by atomic spectroscopic marker methods involving either collisional activation of Ba(³D_J) by Ba(¹S₀) and He buffer gas to yield Ba[6s6p(³P_J)] with subsequent emission from the ³P₁ state (τ_e = 1.2 ± 0.1 μs): Ba[6s6p(³P₁)] → Ba[6s²(¹S₀)] + hv (λ = 791.1 nm). Alternatively, emission from Ba(¹P₁) may be monitored at long times following the generation of this short-lived state by energy pooling following self-annihilation of Ba(³D_J) + Ba(³D_J) from Ba[6s6p(¹P₁)] → Ba[6s²(¹S₀)] + hv (λ = 553.5 nm). The generation of Ba(³D_J) in the presence of atomic strontium yields emission in the long-time domain from Sr[5s5p(³P₁)] (τ_e = 19.6 μs): Sr[5s5p(³P₁)] → Sr[5s²(¹S₀)]  + hv (λ = 689.3 nm). Whilst the decay profiles at short times are complex in form, at long times all these atomic profiles show first-order kinetic removal with the decay coefficients for λ = 791.1 nm, 689.3 nm and 553.5 nm emissions in the ratio 1 : 2 : 2, consistent with overall third-order activation of the form: Ba(³D_J) + Ba(³D_J) + Sr(¹S₀) → Sr(³P_J) + 2Ba(¹S₀). The mechanism is modelled in detail, including measurement of integrated emission intensities, yielding kinetic data for fundamental collisional processes. The overall rate constant for the third-order collisional activation of Sr[5s5p(³P_J])from 2Ba[6s5d(³D_J)] + Sr[5s²(¹S₀)] takes the upper limit of 5.8 × 10⁻²⁷ cm⁶ atom⁻² s⁻¹ (T = 900 K). The rate constant for the two body collisional quenching of Ba[6s5d(³D_J)] by ground state atomic strontium, Sr[5s²(¹S₀)], is found to be (2.0 ± 0.1) × 10⁻¹² cm³ atom⁻¹ s⁻¹ (T = 900 K).     

Implications of hyperconjugative effects on bond lengths of allylic systems. An NBO investigation

The influence of hyperconjugative interactions on bond lengths of some allylic compounds (H₂CCH–CH₂–M(CH₃)₃; M=C, Si, Ge) has been investigated through NBO calculations using ab initio and density functional methods. The optimized structural parameters, at the B3LYP/6-31+G(d,p) and HF/6-31+G(d,p) levels, showed a good agreement with the resonance theory. Partial geometry optimization with orbital interactions removed confirmed the observations and revealed that σ→σ* interactions, together with the more common σ→π* ones, play an important role in determining the variations in bond lengths on going from C to Ge.     

Dynamic studies of surface reactions with microscopic techniques

Work function changes which are combined with surface processes can be investigated and used to characterize adsorption states during heterogeneous catalytic reactions. The oscillatory behaviour of the CO-oxidation was studied on palladium and platinum single crystal surfaces as well as on platinum field emitter tips. On macroscopic crystals the reaction was simultaneously characterized by mass spectrometric analysis and by work function measurements via a Kelvin probe with an area of ≈ 30 mm² being probed. In a second series of experiments photoelectron emission microscopy (PEEM) was applied to follow surface processes with a spatial resolution of ≈ 1 μm. Finally, Pt surfaces were investigated with a lateral resolution of ≈ 20 Å by using field emission microscopy (FEM).

Gas phase coupling of oscillations as demonstrated by the simultaneous oscillations of two separated macroscopic crystals, was missing at small areas of a field emitter. The PEEM registers local oscillations under conditions where the Kelvin probe fails to register dynamic processes. FEM-data showed that critical radii for nucleation could be as small as few Å.     

Solvent effects on the excited state properties of 2-aminopurine—a theoretical study by the ONIOM and Supramolecular method

In this paper, the micro-solvated effects on the lowest-energy vertical transition state and adiabatic excited states of 2-aminopurine (2Ap) were studied by Supramolecular method (B3LYP/6-31++G(d)) and ONIOM (B3LYP/6-31++G(d):PM3) method. The results are as follows: (1) In 2Ap molecule surrounded by six water molecules the pyramidalization of amino group in 2Ap almost disappears, and the hex-atomic ring is obviously buckled. The adiabatic lowest-energy valence excitation of gaseous 2Ap also causes the disappearance of amino pyramidalization. (2) The energy for lowest-energy singlet π→π^* vertical transition in water is predicted as 3.99 and 4.29 eV by Supramolecular and ONIOM method, respectively. Both values are in good agreement with the reported experimental result, 4.11 eV. The energy for the second lowest-energy n→π^* transition, 4.72 eV, by the Supramolecular method is obviously deviated from the reported experimental value 4.46 eV. The corresponding value given by the two-layer ONIOM method, 4.43 eV, is in good agreement with the experimental value. (3) The optimized energy of the fluorescent emission state (S₁ state) are 3.61 and 3.87 eV by Supramolecular and ONIOM methods, respectively. The calculated oscillator strengths, in both gas and water clusters, were compared with reported experimental and theoretical results. These results indicated that both Supramolecular and ONIOM methods, combined with TD DFT B3LYP/6311++G(d), can provide good results of calculating excited state and spectra properties of 2Ap in condensed phase. This fact encouraged us to extend our study to 2Ap-T base pair and its solvated model so as to obtain the spectra properties of 2Ap in real DNA environment.     

Pathway studies in the fragmentation of Ge(CH3)4 involving Ge(3d,3p,3s) and C(1s) inner-shell photoexcitation in the range 49.5–450 eV

The dissociative multiple photoionization of tetramethylgermane (Ge(CH₃)₄) in the valence, and in the Ge(3d,3p,3s) and C(1s) inner-shell regions has been studied by using time-of-flight mass spectrometry coupled to synchrotron radiation in the range 49.5–450 eV. Total and individual photoion yields have been recorded as a function of the incident photon energy. Several discrete resonances over a structureless giant resonance are observed below the Ge(3p), Ge(3s) and C(1s) threshold regions. The structureless giant resonance corresponding to the Ge(3d) presumably arises from the continuum enhancement caused by the 3d→εf transition. Various monocations of H⁺, H₂⁺, CH_n⁺ (n=0–4), C₂H_n⁺ (n=0–5), GeH_n⁺, GeCH_n⁺, GeC₂H_n⁺, and GeC₃H_n⁺ are detected in the whole energy range. Dissociation processes have also been investigated by photoelectron–photoion and photoion–photoion coincidence methods. The dominant dissociation channel is found to be CH_n⁺–GeCH_n⁺ in the whole energy examined. Specific energy dependence of dissociation processes is observed in the Ge(3p) and Ge(3s) regions. With the help of ab initio HF/6-311++G(2df,p) calculation, we roughly estimated the photoabsorption positions and symmetries for the discrete core hole states.     

MRD CI study on the low-lying states of AlP

Large-scale MRD CI calculations assign to AlP the ground state X ³Σ⁻ (9σ²3π²) and a close-lying state 1 ³Π (9σ3π³) (T_e = 0.08 eV). Up to transition energies of 2.0 eV, other states are described by the configurations 9σ3π³ (1¹Π), 8σ²3π⁴ (1 ¹Σ⁺), 9σ²3π² (1 ¹Δ and 2 ¹Σ⁺) and 9σ3π²4π (1 ⁵Π). The 2 ³Π state, located at ≈ 2.30 eV, shows a shallow double minimum. Numerous perturbations are expected to induce predissociation upon 2 ³Π. Multiplets arising from the occupation 8σ²3π³4π are clustered in the 3.25–3.50 eV region. Quintet states with the configuration 8σ9σ3π³4π are bound, with T_e values (in eV) of 3.80 (1 ⁵Σ⁺), 4.44 (1 ⁵Δ) and 4.88 (3 ⁵Σ⁻), respectively. The 9σ → 4s Rydberg members ⁵Σ⁻ and ³Σ⁻ lie in the 4.58–4.72 eV energy region. The first ionization potential (ionization to X⁴Σ⁻ of AlP⁺, 9σ → ∞) is estimated to be 7.65 eV. Ionization to the 1 ²Σ⁻ and 1 ²Π states of AlP⁺ is suggested to occur between 8.0 and 8.8 eV. The dipole moments of X ³Σ⁻, 1 ¹Δ and 2 ¹Σ⁺ are close to 1.0 D, whereas the 1 ¹Σ⁺ state has μ = 3.49 D; 1 ³Π and 1 ¹Π have dipole moments from 2.45 to 2.91 D. All low-lying states show a polarity Al⁺P⁻. Finally, the electronic structure and transition energies of AlP are compared with those of the isoelectronic species BN, AIN, and SiP⁺.     

Gas and solution phase chloroiodomethane short-time photodissociation dynamics in the B-band absorption

Resonance Raman spectra were obtained within and to the red of the B-band absorption spectrum of gas phase chloroiodomethane and chloroiodomethane in cyclohexane solvent. The spectra show the fundamental and overtones of the nominal C---I stretch (nν₅) and combination bands of the CH₂ wag (ν₃), I---C---Cl bend (ν₆), and the CH₂ scissor (ν₂) fundamentals with the C---I stretch bands (nν₅). The chloroiodomethane B-band short-time photodissociation dynamics have significant substituent effects relative to the B-band of iodomethane due to the presence of the C---Cl chromophore n(X) → σ^* (C---X) transitions ≈170 nm that are close to the B-band absorption of chloroiodomethane but absent in iodomethane.     

Electronic absorption spectrum of Ba(MnO4)2·3H2O/Ba(ClO4)2·3H2O

Polarized absorption spectra of Ba(MnO₄)₂·3H₂O/Ba(ClO₄)₂·3H₂O mixed single crystals are reported at 4.2°K. Previous ¹T₂ → ¹A₁ assignments for the 5200 Å and 3000 Å absorption bands of MnO₄⁻ are substantiated; further support is provided for the ¹T₁ → ¹A₁ assignment of the 3600 Å absorption band of MnO₄⁻. The site-splitting of the 5200 Å ¹T₂ state is E(¹E)−E(¹A) ≈ −150 cm⁻¹; that of the 3000 Å ¹T₂ state is E(¹E)−E(¹A) ≈ 300 cm⁻¹. A significant e vibronic intensity component is observed in the 5200 Å ¹T₂ state.     

Comparison of zinc and cadmium plasma parameters produced by laser-ablation

We report the measurement of the zinc and cadmium plasma parameters produced by the fundamental, second, and third harmonics of the neodymium-doped yttrium aluminium garnet laser. The excitation temperature has been determined from the Boltzmann plot method, whereas the electron number density is estimated from the Stark broadened profile of several spectral lines. The temporal evolution of the plasma has also been investigated. Besides, we present experimental relative transition probabilities of the Zn (4s5s ³S₁ → 4s4p ³P_0,1,2) and Cd (5s6s ³S₁ → 5s5p ³P_0,1,2) triplets and compare our data with that listed in the National Institute of Standards and Technology database. The experiments have been performed in air but also in He, Ne and Ar atmosphere to study the effects of ambient gas environment on the emission intensity of the atomic and ionic lines and on the plasma parameters.     

The structure of intermediate products of ‘fragmentation’ of 10-bromodihydrocinchonine

10-Bromodihydrocinchonine 1d, similarly to analogical derivatives of other main cinchona alkaloids, transforms into nicinquine and isonicinquine 2d formally loosing its C2 carbon atom in a form of formaldehyde. This reaction was found to proceed via the so-far unstudied intermediate compounds (5a) 4-S-(Z-propenyl)- and (5 4-S-(E-propenyl)-6-R-7-S-(quinolyl-4)-8-oxa-1-R-azabicyclo[4.3.0]nonane which at the same time are products of a novel rearrangement of the parent cinchonine. The stereostructure of these compounds was determined using, mainly, NMR techniques. The energy minima of conformers 5 and 5a were supported by molecular mechanics calculations. The mechanisms for the 1d → 5 → 2d sequence have been discussed. The alkaloid 5 is sterically preferred to its Z-isomer. The accompanying nucleophilic substitution (1d → 6) and elimination (1d → 7) are also stereospecific.     

The potential energy surfaces and the radiationless dynamics of excited states of benzene and pyrazine

The potential energy surfaces of the lowest excited states of benzene and pyrazine are investigated as a function of some of the symmetry-adapted internal coordinates by means of the INDO/S method. A large stabilization of the T₂ (ππ*) state of pyrazine (≈ 0.5 eV) along the S_8b vibrational coordinate is found. The calculated potential energy in some excited states (T₁ in benzene, T₂ and S₂ in pyrazine) is a very flat function of the S_16b vibrational coordinate, leading to a crossing with the potential energy of the ground state at relatively small excess of vibrational energy (≈ 1 eV). Thus the ν_16b vibrational mode is postulated to play an important role in the radiationless relaxation to the ground states of these systems. No such crossing has been found near the “channel three” threshold of benzene.     

Theoretical investigations of the substituent effect on the absorption and emission properties for a series of platinum (II) complexes supported by tetradentate N2O2 chelates

The photophysical properties, which vary as R is varied, of a series of [Pt(N₂O₂)] complexes bearing bis(phenoxy)bipyridine auxiliaries with different substituents R=H (Pt-H) (1), 4,4′-2NH₂ (Pt-NH₂) (2), 4,4′-2tBu (Pt-tBu) (3), 4,4′-2CN (Pt-CN) (4), and 4,4′-2NO₂ (Pt-NO₂) (5) are investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The solvent effects are discussed in CH₂Cl₂, CH₃CN and CH₃OH solutions, respectively, by polarizable continuum model (PCM). It is anticipated that compared with σ-donor substituents, π-acceptors have more dramatic effects on the electronic and optical properties in this series of complexes. Introduction of π-electron withdrawing substituents on bipyridine ligand will benefit the LLCT (or MLCT) and prohibit the non-radiative pathways via d–d transitions by increasing the energy gap between the HOMO–LUMO and d–d transitions. The results also reveal that the lowest-energy excitations of all complexes show blue-shifts in the polarized solution and when the polarity of the solvent increases from CH₂Cl₂, CH₃CN and CH₃OH, the low-energy broad absorption band exhibit blue-shifts. The lowest-energy excitations and photoluminescence of all complexes are dominated by π(phenoxy)→π^*(bpy/NO₂) (LLCT) excited state mixed with some energetically dπ (Pt)→π^*(bpy/NO₂) (MLCT) transition.     

Rainbow scattering for Ar + Ar and Xe + Xe

Elastic differential scattering measurements have been performed on Ar⁺ + Ar and Xe⁺ + Xe. The rainbow scattering angle is found at τ = Eθ ≈ 115 eV deg for Ar⁺₂ and τ ≈ 93 eV deg for Xe⁺₂. These data are consistent with a potential well depth of 1.25 eV for Ar⁺₂ and 0.97 eV for Xe⁺₂.     

Three-dimensional trajectory study of the I + HI → IH + I, H + I2 reactions

The quasiclassical trajectory calculation of the I + HI collision is performed in order to study the exchange and endothermic reaction channels. The rate constant of the exchange reaction I′ + HI → HI′ + I is found to be equal to 7.7 × 10¹¹ cm³/mol s at room temperature of 300 K. The study of the endothermic reaction I + HI → H + I₂ shows that it takes place only in configurations with large bend angles. Due to the non-collinear character of this reaction its threshold (2 eV) exceeds significantly the threshold of the minimal energy (collinear) reaction path (1.55 eV).     

Shake up in the core ionization of atomic zinc and cadmium

Shake up in atomic zinc and cadmium has been investigated by X-ray photoelectron spectroscopy using Al Kα (1487 eV) radiation. In zinc the 4s → 5s shake up associated with 2p ionization has been found to lie 12.5 eV from the main peak and to have a relative intensity of 7.3%. In cadmium the 5s → 6s shake up accompanying 3d ionization has an intensity of 4.5% and occurs at 11.8 eV from the main peak. The relative intensities reported in this work correspond to the sudden limit values as shown by calculations using equations developed by Thomas in 1984.