Radiative lifetimes of the CO a′3Σ+, b3Σ+, c3Π, d3Δ and B1Σ+ states. Absolute emission cross sections for t

Radiative lifetimes have been measured for the CO a′³Σ⁺(ν′=4–9), b³Σ⁺(ν′= 0), c³Π(ν′=0), d³Δ(ν′=1–16) and B¹Σ⁺(ν′= 0) states. Our experimental values, arranged in the same order, are 7–10 μs, 56 ns, 16 ns, 3–7.5 μs, 34 ns. Some of these values disagree with the results of previous experiments. To our opinion this is due to an incomplete identification of the emission spectrum in regions where many bands may overlap, dependent on the applied spectral resolution. For the a′Σ⁺?a³Π and d³Δ?a³Π emissions effective cross sections for quenching by CO molecules are given. In connection with the identification of the spectrum, absolute emission cross sections for electrons incident on CO have been measured for the b³Σ⁺?a³Π and c³Π?a₃Π transitions. For an electron energy, corresponding to the maximum of the excitation function we find cross sections of 5.94 (?1.2) × 10^?18 cm² and 0.630 (? 0.13) × 10^?18 cm², respectively.     

Laser photolysis of C2H2 and CF3C2H at 193 nm: Production of C2(d3Πg and CH(A2Δ) and their quenching by Xe

The 193 nm laser photodissociation of CH₂H₂ and CF₃C₂H has been studied. With the laser beam focused, C₂(d³Π_g) and CH(A²Δ) radicals were formed by multiphoton processes in both C₂H₂ and CF₃C₂H; however, the one-photon process forming C₂H is still predominant in CF₃C₂H photolysis. The production of C₂(d³Π_g) and CH(A²Δ) emissions is prompt,and the emission intensities show similar (less than quadratic) dependence on laser power whether the radicals are produced from C₂H₂ or CF₃C₂H. In addition, the vibrational distribution of the Swan system is nearly the same in CF₃C₂H as in C₂H₂. The results indicate that the overall photolytic processes are similar in two molecules. Both the C₂(d³Π_g) and CH(A²Δ) emissions are quenched by Xe with rate constants of 4.8×10^?11 and 1.8×10^?11 cm³ molecule^?1 s^?1, respectively.     

Theoretical study of the electronic transition moment for the d 3IIga 3IIu band system of the C2 molecule

Large scale ab initio SCF and CI calculations are employed to study the potential curves for the d ³II_g, a ³II_u and X¹Σ⁺_g states of the C₂ radical. The electronic transition moment R_e′e″ for the Swan bands (d ³II_ga ³II_u) is calculated in various AO and MO basis sets as a function of the internuclear CC distance. The form of the Σ|R_e′e″|² curve is in very good agreement with that obtained recently from measurements of Danylewych and Nicholls and Tatarczyk et al.; the calculated value for Σ|R_e′e″² at 2.44 bohr is found to be 5.2 au² compared to the most recent experimental values of |R_e(r_oo)|² = 3.57 au² of Tatarczyk et al.     

NMR study of (C6H5)3-nPXn (X = Cl,Br, I; n = 0-3) and (C6H6)3-n PXnCr(CO)5 compounds

The ³¹P chemical shift of the (C₆H₅)_3-nPX_n ligands (X = Cl, Br, I; n = 0–3) is dominated by the electronegativity of the substituents. π bonding is only important for derivatives with three strongly electronegative substituents. The ³¹P chemical shift of the corresponding complexes (C₆H₅)_3-nPX_nCr(CO)₅ is governed by the simultaneous effects of the electronegativity, steric hindrance and π bonding. The resonance parameter, δ', indicates an increasing (p_ring → d_p)π and (d_cr→ d_p)π electron delocalization with halogen substitution.     

Ground state C2 density measurement in carbon plume using laser-induced fluorescence spectroscopy

The temporal evolution and spatial distribution of C₂ molecules produced by laser ablation of a graphite target is studied using optical emission spectroscopy, dynamic imaging and laser-induced fluorescence (LIF) investigations. We observe peculiar bifurcation of carbon plume into two parts; stationary component close to the target surface and a component moving away from the target surface which splits further in two parts as the plume expands. The two distinct plumes are attributed to recombination of carbon species and formation of nanoparticles. The molecular carbon C₂ moves with a faster velocity and dies out at ~ 800 ns whereas the clusters of nanoparticle move with a slower velocity due to their higher mass and can be observed even after 1600 ns. C₂ molecules in the d³Π_g state were probed for laser-induced fluorescence during ablation of graphite using the Swan (0,0) band at 516.5 nm. The fluorescence spectrum and images of fluorescence d³Π_g − a³Π_u(0,1)(λ = 563.5 nm) are recorded using a spectrograph attached to the ICCD camera. To get absolute ground state C₂ density from fluorescence images, the images are calibrated using complimentary absorption experiment. This study qualitatively helps to get optimum conditions for nanoparticle formation using the laser ablation of graphite target and hence deducing optimum conditions for thin film deposition.     

The energy balance and branching ratios associated with the chemiluminescent reaction Si(3P) + N2O(1Σ) → SiO*(a3Σ+, b3Π, A1Π) + N2(υ″ ⩾ 5) — possible formation of vibrationally excited N2

Silicon atoms react under single collision conditions with N₂O to yield chemiluminescent emission corresponding to the SiO a³Σ⁺?X¹Σ⁺ and b³Π?X¹Σ⁺ intercombination systems and the A¹Π?X¹Σ⁺ band system. A most striking feature of the SiN₂O reaction is the energy balance associated with the formation of SiO product molecules in the A¹Π and b³Π states. A significant energy discrepancy ( = 10000 cm^? = 1.24 eV) is found between the available energy to populate the highest energetically accessible excited-state quantum levels and the highest quantum level from which emission is observed. It is suggested that this discrepancy may result from the formation of vibrationally excited N₂ in a concerted fast SiN₂O reactive encounter. Emission from the SiO a³Σ⁺ (A¹Π) and b³Π(A¹Π, E¹Σ₀⁺) triplet-state manifold results primarily from intensity borrowing involving the indicated singlet states. Perturbation calculations indicate the magnitude of the mixing between the b³Π, A¹Π and E¹Σ₀⁺ states ranges between 0.5 and 2%. On the basis of these calculations, the branching ratio (excited triplet)/(excited singlet) is found to be well in excess of 500. An approximate vibrational population distribution is deduced for those molecules formed in the b³Π state. The present studies are correlated with those of previous workers in order to provide an explanation for diverse relaxation effects as well as observed changes in the ratio of a³Σ⁺ to b³Π emission as a function of pressure and experimental environment. Some of these effects are attributable to a strong coupling between the a³Σ⁺ and b³Π state. Based on the current results, there appears to be little correlation between either (1) the branching ratio for excited state formation or (2) the total absolute cross section for excited-state formation and (3) the measured quantum yield for the SiN₂O reaction. Implications for chemical laser development are considered.     

The crystal and molecular structure of tetraethylammonium hydrotris(3,5-dimethylpyrazolyl)boratotricarbonylmolybdenum(0), [NEt4][Mo(CO)3HB(C5H7N2)3]

The crystal and molecular structure of tetraethylammonium hydrotris(3,5-dimethylpyrazolyl)boratotricarbonylmolybdenum(0), [N(C₂H₅)₄][Mo(CO)_3^? HB(3,5-Me₂pz)₃] has been determined from intensity data collected using counter methods. The salt crystallizes in space group Pna2₁ with parameters a = 18.038(6), b = 9.956(3), c = 16.881(3) Å, V = 3031.4(20) ų, Z = 4, d_calc = 1.33 g/ml and d_obs = 1.33 g/ml. Final convergence yielded a conventional R = 0.042 and a “goodness of fit” of 2.07. The steric pocket formed by the 3-methyl hydrogens of the pyrazolyl moiety is discussed.     

The crystal and molecular structure of dicarbonylbis(diphenylethylphosphine) platinum(0) Pt(CO)2[P(C6H5)2(C2H5)]2

The complex dicarbonylbis(diphenylethylphosphine)platinum, Pt(CO)₂[P(C₆H₅)₂(C₂H₅)]₂, crystallizes in either of the enantiomorphous space groups P3₁21 (No. 152) and P3₂21 (No. 154) with cell dimensions a = 10.64(1), c = 22.06(1) Å, U = 2163 ų; p_c = 1.564 g/cm³ for Z = 3, p_m = 1.55(3) g/cm³. The intensities of 1177 independent reflections have been determined by counter methods with MoKα monochromatized radiation. The structure has been solved by the heavy atom method. The refinement, carried out by full-matrix least squares down to a final R factor of 0.042, has enabled the absolute configuration of the crystal sample (space group P3₁21) to be ascertained. The molecule is roughly tetrahedral, and has the metal atom lying on a two-fold axis of the cell. Bond parameters are: PtC = 1.92(2) Å, PtP = 2.360(4) Å, CPtC = 117(1)° and PPtP = 97.9(2)°. The PtC₂ and PtP₂ moieties make a dihedral angle of 86.0(3)°. The overall C₂ symmetry of the molecule is probably only a statistically averaged situation, a disorder in the PtCO interactions being apparent from the orientations of the thermal ellipsoids of the C and O atoms.     

Polarized crystal spectra of the mixed metal salt [(CH3)3NH]MnxCu1−xCl3·2H2O

The electronic absorption spectra in two polarizations are reported for crystals of the dichroic salt, TMAMn_xCu_1?xCl₃·2H₂O where TMA represents the trimethylammonium cation, (CH₃)₃NH⁺. Although TMACuCl₃·2H₂O is monoclinic, the mixed metal salts in which x ≥ 0.20 adopt the orthorhombic structure of TMAMnCl₃·2H₂O. The bands observed in the near ir region are adequately explained as d-d transitions of the Cu(II) ion in D_2h symmetry. Other polarized bands which occur in the visible region and are neither Mn(II) nor Cu(II) d-d transitions are discussed.     

Λ-doublet populations in CH(A2Δ) produced in the 193 nm multiphoton dissociation of (CH3)2CO, (CD3)2CO, (CH3)2S and CH3NO2

Unequal intensities of the Λ-doublet components were observed in the CH(A²Δ-X²Π) emission following the multiphoton dissociation of (CH₃)₂CO, (CH₃)₂S and CH₃NO₂ by an ArF laser (193 nm). The power dependence of the emission intensity was estimated to be cubic (3.1±0.2) when the laser power was below ≈ 8×10¹⁷ photons cm^?2 pulse^?1. The Λ-doublet populations depended on the rotational quantum number N′ and the preferred level changed at N′ = 20. A similar behavior was observed for the CD(A²Δ) from (CD₃)₂CO. Rotational distributions show bimodal behavior, having a hump around N′ = 13 in CH(A²Δ) and N′ = 11 in CD(A²Δ). These trends indicate that the CH(A²Δ) is produced through multiple processes where stepwise mechanisms are operative via either CH₂ or CH₃, or both radicals as intermediates.     

Fourier transform infrared spectroscopic analyses of the v12 fundamentals of C2H4 and C2D4

High-resolution infrared studies of isotopic ethylenes below 2000 cm^?1 have been commenced with a Nicolet FTIR spectrometer. Accurate vibration and rotation parameters for the v₁₂ fundamentals of C₂H₄ and C₂D₄ are determined from spectra recorded with 0.05 cm^?1 resolution. Excellent band contour simulations confirm that these bands are unperturbed throughout their range.     

Etude du mecanisme de photolyse des complexes cis- et trans-[PtCl2(C2H4)(4-CH3C5H4N)]. Influence de la concentration et de la longueur d'onde

Photodimerization, photoisomerization and photosubstitution quantum yields are measured for cis- and trans-[PtCl₂(C₂H₄)(4-CH₃C₅H₄N)], at various concentrations and wavelengths. Dissociation of the platinumethylene bond o?curs with a quantum yield nearly unity when the cis-compléx is irradiated in the charge transfer bands 5d → π^*(C₂H₄). Dissociation is also observed, but with a lower efficiency, at longer wavelengths. A cis → trans-photoisomerization reaction, probably via a low energy dd excited state is observed at 313,366 and 405 nm, with a constant quantum yield.     

Carbonylvanadium complexes of the tripod ligands MeC(CH2PPh2)3 and P(CH2CH2PPh2)3

UV irradiation of [Et₄N] [V(CO)₆] in the presence of the tripod ligands (L) MeC(CH₂PPh₂)₃ (cp₃) and P(CH₂CH₂PPh₂)₃ (pp₃) yields [Et₄N] [V(CO)₅L], cis-[Et₄N] [V(CO)₄L] and mer-[Et₄N] [V(CO)₃L] (where the meridional configuration for L = cp₃ is uncertain). Except for [Et₄N] [V(CO)₅cp₃], all these species were isolated. The complexes are characterized by their IR, ³¹P and ⁵¹V NMR spectra.     

Dual luminescences in [W(Cp)(CO)3]2 and [Mo(Cp)(CO)3]2

Cyclohexane solutions of [W(Cp)(CO)₃]₂ and [Mo(Cp)(CO)₃]₂ exhibit weak bimodal emission spectra when excited With 354 nm picosecond pulses, but do not luminesce when pumped at 530 nm. Picosecond lifetimes characterize the short-wavelength, emission bands, which may originate from metal-cyclopentadienyl CT excited states.     

Luminescence in near-thermal charge exchange. III. CH(+) and CD(+) A → X emission from He+ C2H2 and He+ + C2D2

A pulsed ICR cell fitted with synchronous photon counting equipment is used to investigate the emission produced between 185 and 500 nm by near-thermal charge exchange between He⁺ and C₂H₂ (C₂D₂). The emission bands observed are A ²Δ → X²π and (weakly) B²Σ^? → X²π in CH(CD) and A ¹π → X¹Σ in CH⁺(CD⁺). Wavelength measurements on the bandheads of the (0,0) and (0,1) bands of CD⁺ A → X are used to evaluate vibrational constants of CH⁺(CD⁺) X¹Σ⁺. The results are (in cm^?1): ω_e = 2869 ± 27 (2106 ± 20); ω_eχ_e = 65 ± 13 (35 ± 7). These constants are used to calculate Morse-potential Franck—Condon factors and vibrational branching ratios for CH⁺ and CD⁺ A → X emission. The spectral distributions and the (relatively low) absolute emission rates produced by He⁺/C₂H₂(C₂D₂) charge exchange are briefly discussed in the light of presently available information on the charge transfer reaction and on the excited states of C₂H_2?⁺     

An unusual oxidative addition-ligand elimination reaction. Preparation and structure of RhCl2(PMe2Ph)2(C3Ph3)

The sym-triphenylcyclopropenium cation (C₃Ph₃⁺) stabilized as the Cl^? or PF₆^? salt, undergoes facile reactions at room temperature with trans-Rh(CO)Cl(PMe₂Ph)₂ to produce complexes which result from the oxidative cleavage of the ring and decarbonylation of the organometallic reactant. The product of the C₃Ph₃⁺Cl^? reaction has been fully characterized by X-ray analysis and is shown to be RhCl₂(PMe₂Ph)₂(C₃Ph₃).     

Crystal and molecular structure of (η3-allyl)carbonylchlorobis(dimethylphenylphosphine)- iridium(III) hexafluorophosphate, [Ir(η3-C3H5)Cl(CO)(P(CH3)2(C6H5))2][PF6

The structure of (η³-allyl)carbonylchlorobis(dimethylphenylphosphine)-iridium(III) hexafluorophosphate, [Ir(η³-C₃H₅)Cl(CO)(P(CH₃)₂(C₆H₅))₂][PF₆], has been determined from three-dimensional X-ray data to add support for a proposed mechanism of the oxidative addition of allyl halides to IrX(CO)(PR₃)₂ (X = halide). The compound crystallizes in space group C⁵_2h-P2₁/c with four formula units in a cell of dimensions a = 11.027(1), b = 12.230(2), c = 19.447(5) Å, and β = 103.16(2)⁰. Least-squares refinement of the structure has led to a value of the conventional R index (on F) of 0.066 for the 3018 independent reflections having F²₀>3—(F²₀). The crystal structure consists of discrete, monomericions. The hexafluorophosphate anion is disordered. The coordination geometry around the iridium atom may be described as octahedral, with the chloro ligand trans to the carbonyl group and each phosphorus atom trans to a terminal carbon of the allyl group. Structural parameters: Ir—P = 2.366(4), 2.347(3);Ir—Cl = 2.389(3); Ir—C(allyl) = 2.28(1), 2.24(1),2.25(1); Ir—C (carbonyl) = 1.85(1) Å; P—Ir—P = 105.7(1); C(terminal)—Ir—C(terminal) = 66.2(8); C—C—C = 125(2)^o. The allyl group makes an angle of 126^o with the P—Ir—P plane. Correlations between geometric structure and number of d electrons are noted among several M—C₃H₅-complexes, and are interpreted in the light of theoretical models of the M—C₃H₅- bond.     

Oxo-titanium and -zirconium diphosphines [(η-C5H5)2M(CH2PPh2]2O (with M = Ti,Az) as building blocks for heterobimetallic complexes

Oxo-titanium and -zirconium diphosphines [(η-C₅H₅)₂M(CH₂PPh₂)]₂ (with M = Ti, Zr) were synthesized and treated with [Rh(CO)₂Cl]₂ to give the heterobimetallic d⁰-d⁸ species O[(η-C₅H₅)₂M(CH₂PPh₂)]₂Rh(CO)Cl.     

Crystal and molecular structure of(η-C5H5)W[C3(CME3)2Me](PME3)Cl2, a species with a tetrahedral WC3, core formed from a tungstenacyclobutadiene complex via attack on tungsten by a phosphorus-donor ligand

Mercury(II) chloride in refluxing methanol or acetone cleaves the molybdenum—tin bond of π-methylcyclopentadienylmolybdenum tricarbonyl triphenylstannyl [(η⁵-C₅H₄CH₃)(CO)₃MoSnPh₃] to give (η⁵-C₅H₄CH₃)(CO)₃MoHgCl. The same product was also obtained by reaction of [(η⁵-C₅H₄CH₃)(CO)₃Mo]₂Hg with HgCl₂ in acetone at room temperature. Similar reactions have given bimetallic complexes of the type (η⁵-C₅H₄CH₃)(CO)₃MoHgX (X = Br, I, SCN). The new complexes are air-stable crystalline solids. The structure of the compound (η⁵-C₅H₄CH₃)(CO)₃MoHgCl has been determined. It crystallizes in space group P2₁/c with Z = 4, a 6.613(2), b 13.647(4), c 13.257(4) Å, β 101.85(3)°, D_c 2.81 g/cm³, F(000) = 896, μ(Mo-K_α) 143.56 cm^?1. Final R = 0.055 for 1696 observed reflexions.     

The luminescence of Pr3+ in BaY4Si5O17

The cell constants of four new monoclinic compounds BaR₄X₅O₁₇ (R = Y, Gd; X = Si, Ge) are given. The luminescence of various RE activators in the silicates is reported. Pr³⁺-activated BaY₄Si₅O₁₇ shows efficient ultraviolet 5d → 4f emission and weak 4f → 4f emission (mainly red luminescence from the ¹D₂ level). The 5d → 4f emission is ascribed to Pr³⁺ on Y sites, the 4f → 4f emission to Pr³⁺ on Ba sites. Energy transfer from Pr³⁺ to Gd³⁺ has been observed. Gd³⁺ plays an intermediate role in the energy transfer from Pr³⁺ to Sm³⁺ and to Dy³⁺ in BaGd₄Si₅O₁₇. Upon activation with Tb³⁺ the silicates show characteristic green Tb³⁺ luminescence with a quantum efficiency of 75% for ultraviolet excitation.