193 nm Laser dissociation of CS2: prompt emission from CS and internal energy distribution of CS(XΣ)

Single and multiple photon processes are identified in the 193 nm excimer laser photolysis of CS₂. CS(X¹Σ⁺, υ = 1 to 5, J = 5 to 45) is observed by dye laser induced fluorescence of the A¹Π ↔ ; X¹Σ⁺ transition, following the single photon 193 nm photolysis of CS₂. Multiple photon 193 nm generation of CS fragment emission from 620 to 170 nm is also reported. A partial assignment of the emission spectrum identifies fluorescence from the CS A′¹Σ⁺ and A¹Π states.     

The A Πu-X Πg emission spectrum of I2

The A ²Π_u-X ²Π_g electronic emission spectrum of I₂⁺ has been recorded at a low rotational temperature in a crossed molecular beam/electron beam apparatus. Six vibrational sequences with five or more members have been assigned to progressions in ν′, giving ω′_e = 122±8 cm⁻¹, but a full vibrational analysis has not been possible. It is not known whether this is due to the relatively poor resolution (≈5 cm⁻¹) at which the spectrum has been recorded or because the A ²Π_u state is perturbed in one or both spin-orbit components. Existing data on the A state of I₂⁺ are reviewed.     

Rotational excitation of ions produced by the He(2 S) Penning ionization of CO and N2

Rotational-state distributions of the CO⁺ (A–X, B–X) and N₂⁺(B–X) emissions produced by the collisions of He(2 ³S) with CO and N₂ were studied in the collision energy (E_R range 100–200 meV. The rotational populations of the emitting states can be fitte by single Boltzmann temperatures (T_R. The T_R (320 ± 30 K) for the ν′ = 3 and 4 levels of the CO⁺ (A²Π) state are nearly independent of, or slightly increase with, E_R, while T_R for the CO⁺(B²Σ⁺, ν′ = 0) state increases rapidly with E_R.The T_R (430 ± 20 K) for the N₂⁺(B²Σ⁺, ν′ = 0) state is nearly independent or slightly decreases with increasing E_R. Interactions providing these trends are discussed.     

Reaction dynamics of the Ca(D2, PJ) + CH3I → CaI + CH3 system: chemiluminescence, energy disposal and product polarization

The Ca(¹D₂, ³P_J) + CH₃ → CaI(A,B) + CH₃ reactions system has been studied by measuring its chemiluminescence under beam-gas conditions. Absolute values of the state-to-state reaction cross-sections were determined at low collision energy . In addition, the electronic branching ratio and product energy disposal have been determined for each metastable reaction. The major changed observed in the chemiluminescence when comparing the Ca(¹D₂) reaction versus that of Ca(³P_J) is the total yield associated with the former reaction. To the best of our spectral resolution neither the electronic branching ratio e.g. CaI(A)/CaI(B) nor the internal CaI energy disposal change significantly as the metastable Ca(¹D₂)/Ca(³P_J) ratio is varied. In spite of the fact that the Ca(³P_J) reaction is less exoergic, the CaI product appears with a higher fraction of internal energy than that of Ca(¹D₂) reaction. Thus, the fraction of the total energy appearing in CaI internal energy amounts to 57.5% in the Ca(³P_J) reaction while it is 19.3% only for the Ca(¹D₂) reaction. This difference is discussed in the light of a distinct mechanism associated with the attack of the excited Ca atom into the C---I bond. No significant chemiluminescence yield was found for the energetically open CaCH^*₃ channels.

The product chemiluminescence polarization was also measured as a function of the metastable concentration. A significant degree of polarization was found depending upon the specific electronic excitation. The analysis of the polarization emission associated to the parallel CaI(X ²Σ⁺ ← B ²Σ⁺) emission led into a strong polarization of the product rotational angular momentum. The comparison of the product rotational alignment for the kinematically identical Ca(¹D₂, ³P_J, ¹P₁) + CH₃ → CaI^* (B₂Σ⁺) + CH₃ reaction system showed that the CaI rotational polarization diminishes in the ³P_J → ¹D₂ → ¹P₁ sequence, e.g. as the reaction exothermicity increases. In addition the degree of polarization associated with other emission bands as for example CaI(X ²Σ⁺ ← A ²Π_1/2) indicates the presence of a parallel transition which was been interpreted as mixing of Hund's case (a) and (c) appropriate for this heavy CaI diatom produced with a high rotational excitation.     

Metallorganische lewissäuren : XX. Reaktionen von (π-C5H5)(CO)2LM-X (L = CO, PR3; M = Mo, W; X = BF4, PF6, AsF6, SbF6) mit Schwefel-haltigen liganden

The compounds (π-C₅H₅)(CO)₂LM-X (L = CO, PR₃; M = Mo, W; X = BF₄, PF₆, AsF₆, SbF₆) react with H₂S, p-MeC₆H₄SH, Ph₂S and Ph₂SO(L′) to give ionic complexes [(π-C₅H₅)(CO)₂LML′]⁺ X⁻. Also sulfur-bridged complexes, [(π-C₅H₅)(CO)₃W---SH---W(CO)₃(π-C₅H₅)]⁺ AsF₆⁻ and [(π-C₅H₅)(CO)₃M-μ-S₂C=NCH₂Ph-M(CO)₃(π-C₅H₅)], have been obtained. Reactions with SO₂ and CS₂ have been examined.     

The chemical production of electronically excited states in the H/NF2 system

A mixture of NF₃ and Ar is passed through an rf discharge in a flow-system to produce, among other species, F and NF₂. When H₂, D₂, or CH₄ are added downstream, reactions with F atoms produce vibrationally excited HF or DF together with H, D, or CH₃. The latter free radicals can react with NF₂, probably by an elimination reaction to produce electronically excited NF: NF₂(²B₁) + H(D, CH₃) → HF^*(DF^* + NF(a¹Δ). A vibrational-to-electronic energy transfer process between the products of this reaction then produces the next higher state of NF: HF(ν 2) + NF(a¹Δ) → HF(ν−2) + NF(b¹Σ⁺). A similar transfer process has also been found between the electronically excited a¹Δ states of O₂ and NF: O₂(a¹Δ) + NF(a¹Δ) → O₂(X³Σ⁻) + NF(b¹Σ⁺). The H or D atoms but not the CH₃ radicals are then found to react with either NF(a¹Δ) or NF(X³Σ⁻) to produce electronically excited N(₂D) atoms, which in turn react with the NF(a¹Δ) molecules to produce N₂(B³Π_g). The observed nitrogen first positive radiation has been demonstrated to be produced entirely by this reaction mechanism rather than by the N(⁴S) recombination that accounts for the Rayleigh afterglow. In addition, the occurrence of the reaction N(₂D) + N₂O → NO(B²Π_r) + N₂ (X¹Σ⁺_g) has been verified. Finally we have observed emission at 3344 Å, which we attribute to the NF(A³Π), which has not been previously reported.     

Identification of a new progression of the Cl2 molecule with Σu symmetry

Synchrotron radiation is used to excite selectively the chlorine molecule in a Ne buffer gas. Due to the fast relaxation induced by the buffer gas, in the excitation spectrum of the D′→A′ emission at 258 nm, a new progression is observed. It is attributed to the 3 ¹Σ_u⁺ state which is the result of an avoided crossing between the Rydberg state π_g→5pπ and the valence state (1441) (σ_g→σ_u). It is characterized by T_e=83251 cm⁻¹, ω_e=783 cm⁻¹, ω_ex_e=29.6 cm⁻¹ and r_e=1.844 Å.     

The vibrational state distributions in both products of the reaction: O(P) + NO2 → O2 + NO

A laser pulse-and-probe method has been used to determine the nascent vibrational populations in NO(v=0–4) and O₂(v=6–11) formed in the thermal reaction: O(³P) + NO₂ → O₂(v) + NO(v). A frequency-tripled Nd: YAG laser is used to photolyse NO₂, diluted tenfold in Ar, and laser-induced fluorescence spectroscopy in the NO A ²Σ⁺-X ²Π and O₂ B ³Σ⁻_u -X ³Σ⁻_g electronic band system is used both to follow the kinetics of individual vibrational states and to determine the nascent vibrational distributions. The majority of the NO product is formed in v = 0 and the average vibrational yield is ≈ 4.6%. The O₂ populations fall monotonically from v = 6 to 11 in a distribution close to what is expected on prior grounds. Based on a surprisal analysis, the average vibrational energy yield in O₂ is ≈ 26%. The nature of the reaction dynamics is discussed.     

The effect of reagent translational energy in the reaction O(D2) + H2 → OH(Π) + H

Quasiclassical trajectory calculations have been performed to determine the effect of reactant collision energy on product state distributions in the reaction O(¹D) + H₂ → OH(²Π) + H. The product vibrational distribution becomes more excited as the collision energy is increased. This is not due to an increase in the cross section for collinear abstraction. A detailed analysis has shown that strong O---H₂ repulsion, which occurs during the insertion of the O into the H---H bond, converts the kinetic energy of the reacting system to vibrational motion of the intermediate.     

Lifetime measurements of electronically excited C3(Πu) radicals in different vibrational states

The radiative lifetimes of nine vibrational levels of the C₃(¹Π_u) radical were obtained from decay time studies of the C₃(¹Π_u → ¹Σ⁺_g) fluorescence induced by a tunable dye laser. The lifetimes of the different vibronic levels were found to be constant within the experimental error limits, namely, τ_o = (200 ± 10) ns. The collisional deactivation of the C₃(¹Π_u) states by helium gives rate constants between 2.5 and 4 in 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ units.     

Laser induced photodissociation spectroscopy: Br2

The continuous absorption spectrum of molecular bromine has been examined using laser induced photodissociation spectroscopy. In this technique, Br₂ molecules are photolyzed using a flashlamp-pumped dye laser; the atomic products of the dissociation are then monitored by time-resolved resonance absorption spectroscopy in the vacuum ultraviolet. The relative absorptivities for the transitions B³Π_o⁺u ← X¹Σ⁺_g and ¹Π_1u ← X¹Σ⁺_g have been obtained at 18350, 21010 and 22125 cm⁻¹.     

Study of the N2 bΠu state via 1 + 1 multiphoton ionization

Medium-resolution spectra of the N₂ b¹Π_u-X¹Σ_g⁺ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b ¹Π_u state: Δν₀ = 0.50 ± 0.05 cm⁻¹, Δν₁ = 0.28 ± 0.02 cm⁻¹, Δν₂ = 0.65 ± 0.06 cm⁻¹, Δν₃ = 3.2 ± 0.5 cm⁻¹, Δν₄ = 0.60 ± 0.07 cm⁻¹, and Δν₅ = 0.28 ± 0.02 cm⁻¹. From these linewidths, predissociation lifetimes τ_ν were obtained: τ₀ = 16 ± 3 ps, τ₁ > 150 ps, τ₂ = 10 ± 2 ps, τ₃ = 1.6 ± 0.3 ps, τ₄ = 9 ± 2 ps, and τ₅ > 150 ps. Band origins and rotational constants for the b ¹Π_uν = 0 and 1 levels were determined for the ¹⁴N₂ and ¹⁴N¹⁵N molecules.     

A study on copper(II)-Schiff base-azide coordination complexes: Synthesis, X-ray structure and luminescence properties of [Cu(L)(N3)]X (L = Schiff bases; X = ClO4, PF6)

Two Schiff bases N,N′-(bis(pyridin-2-yl)benzylidene)propane-1,3-diamine (pbpd) and N,N′-(bis(pyridin-2-yl)formylidene)butane-1,4-diamine (pfbd) have been prepared and used to synthesize copper(II) complexes. Four complexes of the type [Cu(L)(N₃)]X (1–4) [L = pbpd; X = ClO₄ (1); L = pbpd; X = PF₆ (2); L = pfbd; X = ClO₄ (3); L = pfbd; X = PF₆ (4)] have been synthesized and characterized on the basis of microanalytical, spectroscopic, magnetic, electrochemical, luminescence and other physicochemical properties. Two representative complexes of the series, 2 and 3, have been characterized by single crystal X-ray diffraction measurements which reveal that in each complex the copper(II) ion assumes a distorted trigonal bipyramidal environment through coordination of the metal centre by two pyridine N atoms and two imine N atoms of the Schiff base with the fifth position occupied by a N atom of a terminal . They display intraligand ¹(π–π^*) fluorescence at room temperature and intraligand ³(π–π^*) phosphorescence in glassy solutions (MeOH at 77 K). A band (492 nm) observed for the complexes in their solid-state emission spectra is an excimeric emission arising due to an aromatic π–π interaction. Electrochemical electron transfer study reveals Cu^II–Cu^I reduction in methanolic solutions.     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Structural and spectroscopic effects of vibronic coupling in the C2F radical

The lowest three (1²A^′,2²A^′,1²A^′′) potential energy surfaces of the C₂F radical have been studied at the ab initio level, using Multi Reference Configuration Interaction techniques. For linear geometries, the three surfaces correlate with a ²Π and a ²Σ⁺ state, which are very close in energy. Only the X²A^′ ground state was found to be bent, with R_CC=1.271 Å; R_CF=1.276 Å; CCF=165°, and a barrier to linearity of 275 cm⁻¹. The spin–rovibronic energy levels up to J=7/2 have been calculated using a recently developed method [Carter et al., Mol. Phys. 98 (2000) 1967]. Almost all of the resulting levels arise from a strong mixture of two out of three electronic states and their assignment is intrinsically ambiguous. A partial characterization, based on the shape of the vibronic wavefunctions, has been made.     

Mixed-metal cluster chemistry Part 20. Syntheses, crystal structures and electrochemical studies of W2Ir2(μ-L)(CO)8(η-C5H4Me)2 (L=dppe, dppf)

The 60-electron tetrahedral clusters W₂Ir₂(μ-L)(CO)₈(η⁵-C₅H₄Me)₂ [L=dppe (2), dppf (3)] have been prepared from reaction between W₂Ir₂(CO)₁₀(η⁵-C₅H₄Me)₂ (1) and the corresponding diphosphine in 52 and 66% yields, respectively. A structural study of 2 reveals that three edges of a WIr₂ face are spanned by bridging carbonyls, that the iridium-ligated diphosphine coordinates diaxially and that the tungsten-bound methylcyclopentadienyls coordinate axially and apically with respect to the plane of bridging carbonyls. A structural study of 3 reveals that the dppf ligand bridges an Ir---Ir bond which is also spanned by a bridging carbonyl; tungsten-ligated methylcyclopentadienyl ligands and terminal carbonyls result in electronic asymmetry (17e and 19e iridium atoms) in the electron-precise cluster. Both clusters show two reversible one-electron oxidation processes and an irreversible two-electron reduction; the dppf-containing cluster 3 has a further, irreversible, one-electron oxidation process. UV–vis-NIR spectroelectrochemical studies of the 2→2⁺→2²⁺ progression reveal the appearance of a low-energy transition on oxidation to 2⁺ which persists on further oxidation to 2²⁺.     

Ab initio molecular orbital study of potential energy surface for the H2NO(B1)→NO(Π)+H2 reaction

The mechanism of the H₂NO(²B₁)→NO(²Π)+H₂ reaction has been examined using ab initio molecular orbital methods. Ground-state and first-excited-state potential surfaces were plotted at the FOCI/cc-pVTZ level of theory as functions of two appropriate internal degrees of freedom. A conical intersection was found on the C_s pathway that is symmetric with respect to the plane perpendicular to the molecular plane of C_2v H₂NO(²B₁). It is therefore considered that trajectories that start from H₂NO(²B₁) towards the product region detour around the conical intersection, pass through the neighborhood of the transition state that is located at the saddle point on the C_s pathway, and finally reach the products, NO(²Π)+H₂. Thus we can explain the mechanism of the H₂NO(²B₁)→NO(²Π)+H₂ reaction, which has remained unclear to date.     

Quenching of the translationally hot and thermalized NH(c Π) radicals by HN3

The cross section for the quenching of NH(c ¹Π, ν = 0) by HN₃ was measured by using a pulsed laser technique. A single rotational level of NH(c ¹Π, ν = 0) was formed by exciting NH(a ¹Δ, ν = 0) with a frequency doubled dye laser. NH(a¹Δ) was produced by photolyzing HN₃ with a XeCl excimer laser. The time profiles of the NH(c-a) fluorescence were measured at various pressures of HN₃. Experiments were performed both in the presence and in the absence of He buffer gas. In the absence of He, the NH radicals were found to be translationally hot; the average velocity was 3800±600 m s⁻¹. The quenching cross sections for the translationally hot and thermalized NH(c) radicals by HN₃ were determined to be (28±5) × 10⁻¹⁶ and (85±3) × 10⁻¹⁶ cm², respectively. No rotational level dependence could be observed in the quenching of the hot NH(c) radicals.     

Lifetime measurements of the excited states of N2 AND N2 by laser-induced fluorescence

Absorption and fluorescent scattering of nitrogen laser radiation by a low-pressure RF laboratory plasma (n_e = 10¹² cm⁻³) has been observed for the first negative system of N₂⁺. A 67±1 ns lifetime of N₂⁺ (B ²Σ_u⁺) was experimentally measured from the laser-induced fluorescence. In addition, enough collisionally excited N₂ (B ³Π_g) was produced to observe laser-induced fluorescence for the second positive system of N₂. The lifetime of N₂ (C ³Π_u) was found to be 41±2 ns. The measured lifetimes are in good agreement with published values calculated from theory.     

The tungsten-tungsten triple bond---18. Bridging and terminal allyl ligands in complexes of the formula W2(R)2(NMe2)4, where R = C3H5 and C4H7

The reaction between RMgCl (two equivalents) and 1,2-W₂Cl₂(NMe₂)₄ in hydrocarbon solvents affords the compounds W₂R₂(NMe₂)₄, where R = allyl and 1− and 2-methyl-allyl. In the solid state the molecular structure of W₂(C₃H₅)₂(NMe₂)₄ has C₂ symmetry with bridging allyl ligands and terminal W---NMe₂ ligands. The W---W distance 2.480(1) Å and the C---C distances, 1.47(1) Å, imply an extensive mixing of the allyl π-MOs with the WW π-MOs, and this is supported by an MO calculation on the molecule W₂(C₃H₅)₂(NH₂)₄ employing the method of Fenske and Hall. The most notable interaction is the ability of the (WW)⁶⁺ centre to donate to the allyl π^*-MO (π₃). This interaction is largely responsible for the long W---W distance, as well as the long C---C distances, in the allyl ligand. The structure of the 2-methyl-allyl derivative W₂(C₄H₇)₂(NMe₂)₄ in the solid state reveals a gauche-W₂C₂N₄ core with W---W = 2.286(1) Å and W---C = 2.18(1) Å, typical of WW and W---C triple and single bonds, respectively. In solution (toluene-d₈) ¹H and ¹³C NMR spectra over a temperature range −80°C to +60°C indicate that both anti- and gauche- W₂C₂N₄ rotamers are present for the 2-methyl-allyl derivative. In addition, there is a facile fluxional process that equilibrates both ends of the 2-methyl-allyl ligand on the NMR time-scale. This process leads to a coalescence at 100°C and is believed to take place via an η³-bound intermediate. The 1-methyl-allyl derivative also binds in an η¹ fashion in solution and temperature-dependent rotations about the W---N, W---C and C=C bonds are frozen out at low temperatures. The spectra of the allyl compound W₂(C₃H₅)₂(NMe₂)₄ revealed the presence of two isomers in solution—one of which can be readily reconciled with the presence of the bridging isomer found in the solid state while the other is proposed to be W₂(η³-C₃H₅)₂(NMe₂)₄. The compound W₂R₂(NMe₂)₄ where R = 2,4-dimethyl- pentadiene was similarly prepared and displayed dynamic NMR behaviour explainable in terms of facile η¹ = η³ interconversions.