Relaxation mechanism of the 1Δg state of liquid O2

Collisional deactivation of the first excited electronic ¹Δ_g(υ = 0) state of O₂ involves intersystem crossing to higher vibrational levels (υ < 5) of the electronic ground state ³Σ^?_g. It is followed by rapid vibrational-vibrational energy exchange which populates the first excited ³Σ^?_g(υ = 1) vibrational level. The suggested relaxation mechanism is supported by experimental results on the time dependence of the populations of the ¹Δ_g(υ = 0) and ³Σ^?_g(υ = 1) states in liquid natural O₂ and ¹⁸O₂.     

CEPA calculations on open-shell molecules. III. Potential curves for the six lowest excited states of He2 in the vicinity of their equilibrium distances

CEPA-PNO and PNO-CI calculations have been performed for the potential energy curves of the He ₂ ⁺ ground state and the six lowest excited states of He₂ in the range of 1.4 a₀ ≤R ≤ 3.5 a₀. The calculated equilibrium distances as well as the spectroscopic constants are in very good agreement with molecular constants as derived experimentally from the rotation-vibration spectrum of He₂ by Ginter, except for thec ³∑ _g ⁺ state. This latter discrepancy is probably due to an “obligatory” hump in thec ³∑ _g ⁺ state occurring at 3.5 a₀ which cannot be properly treated in our calculation. The relative energetic positions of the six lowest states and their ionization energies are reproduced by our calculations with an accuracy of 0–400 cm⁻¹. Extrapolation of our results to infinite basis sets leads to estimates of the dissociation energies of He₂ excited states which cannot be measured spectroscopically because of the humps in all these states.     

Benzonitrile and its van der waals complexes studied in a free jet. III. Enhancement of the intersystem crossing rate in the benzonitrile dimer and other complexes

By using the sensitized phosphorescence spectroscopy, the intensity of the phosphorescence has been recorded upon excitation of the benzonitrile dimer to the S₁ vibronic states in a free jet. The results indicate that the strong vibrational energy dependence of the fluorescence quantum yield, reported previously, is attributable to the increasing rate of intersystem crossing with increasing vibrational energy. Similar behavior is also observed in other van der Waals complexes of benzonitrile though the increase is less obvious. The enhancement of the intersystem crossing can be correlated with the state density of van der Waals modes in the S₁ electronic state. In case of the benzonitrile trimer and benzonitrile-Kr complex, intersystem crossing is found to be fully efficient even without vibrational excitation.     

D+D 2 Quasiclassical rate constant calculations on parallel computers

Summary The calculation of rate constant values of theH+H ₂ reaction for an extended range of excited vibrational states of the diatomic molecule and temperatures is relevant to the modeling ofH ^– sources. To investigate the effect of isotopic substitutions on the efficiency of vibrational deexcitation processes, we extended the calculations to theD+D ₂ system. These calculations were carried out using a program restructured to run on a shared memory vector and parallel computer. The dependence of the efficiency of vibrational deexcitation processes from both the initial vibrational state and temperature of reactants is reported. Restructuring strategies adopted for implementing the program on both shared and distributed memory computers as well as speedups achieved on both types of machines are also discussed.     

Chromium(III) complexes ofd(−)tartaric andl(−) mandelic acids

The binary complexes of anhydrous chromium(III) chloride withd(−) tartaric acidl(−) mandelic acids have been characterized by elemental analyses, magnetic susceptibility, vibrational, electronic and circular dichroism spectra. The magnetic susceptibility data are close to the spin only value for a d³ chromium(III) ion. Three (Cr−Cl) vibrational modes in the region 420–290 cm⁻¹ are observed for the formed complexes indicatingC ₂ local symmetry of ligand atoms around the chromium(III) rather thanC ₃, which would allow two modes. In the visible spectra, two peaks in the 21052–22222 and 15384–16129 cm⁻¹ range are observed and are assigned to the⁴ A _2g → ⁴ T _1g (F) and⁴ A _2g → ⁴ T _2g transitions. The parameters (Dq, B,β ₃₅) place the ligands in the higher end of the spectrochemical series and provide reassurance that the hydroxy acid oxygen complexes to chromium(III) ion. The Cotton effects observed in the spin-forbidden band are assigned to the² E(² E _g ),² A ₂(² T _1g ) and² E(² T _1g ), while that in the spin-allowed band are a results of the splitting of the⁴ A _2g (⁴ T _2g ) to⁴ A ₁(⁴ T _2g) and⁴ E(⁴ T _2g ) transitions. The tartaric acid chelates are likely to befac in terms of ligand carboxylate and/or hydroxy groups since stronger and better defined Cotton effects are observed while mandelic acid chelates are weak suggesting formation of themer structure. TMC 2633     

Experimental and Theoretical Studies on the Characteristics of Atmospheric Pressure Glow Discharge with Liquid Cathode

This study investigated the characteristics of an atmospheric pressure air-glow discharge with a liquid cathode. Distilled water was utilized as the cathode. The electric field strength, gas temperature as well as the emission intensity of some N₂(C³Π_u → B³Π_g) and OH (A²Σ⁺ → X²Π) bands were measured at a discharge current ranging from 15 to 50 mA. Based upon the data obtained, the reduced electric field strength, E/N, and effective vibrational temperatures for N₂(C³Π_u, X¹Σ _g ⁺ ) and OH (A²Σ⁺) were examined. The electron energy distribution function (EEDF) and some electron parameters (average energy, electron density and rate coefficients) were obtained based on a numerical solution of the Boltzmann kinetic equation. The result showed that the EEDF was not in equilibrium and the effective vibrational temperatures for N₂(C³Π_u, X¹Σ _g ⁺ ) were essentially higher than the gas temperatures.     

Vibrational relaxation and quenching of CO(A1Π, ν = 0–8) in collisions with rare gas atoms

Steady state fluorescence experiments yielded effective cross sections for the vibrational relaxation and quenching in collisions of electronically excited CO molecules, in single vibrational levels ν = 0 to 8 of the A¹Π state, with rare gas atoms. Vibrational relaxation was found to proceed not only by Δν = 1 but also by Δν = 2 and Δν = 3 processes. The relaxation cross sections for the Δν = 1 processes decrease with increasing vibrational quantum number ν. The quenching data give strong evidence for collision induced intersystem crossing to nearby triplet states.     

Multiconfigurational study on the synchronous mechanisms of the ClO self-reaction leading to Cl or Cl<Subscript>2</Subscript>

For studying the adiabatic and nonadiabatic mechanisms of the ClO (X ²Π) + ClO (X ²Π) → ClOOCl → 2Cl (² P _u) + O₂ (X ³Σ _g ⁻) reaction (1) and the ClO (X ²Π) + ClO (X ²Π) → ClOOCl → Cl₂ (X ¹Σ _g ⁺) + O₂ (X ³Σ _g ⁻) reaction (2), we calculated, by partial geometry optimizations under the C₂ constraint, the O–O and O–Cl dissociation potential energy curves (PECs) from the five low-lying states of ClOOCl at the CASPT2 level. The CASSCF minimum-energy crossing point (MECP) between the potential energy surfaces of the 1 ¹A ground state [correlating with the product of reaction (1)] and the 1 ³B state [correlating with the product of reaction (2)] states was also determined. Based on the CAS calculation results (PECs, energies, and spin–orbit coupling at the MECP), we predict that reaction (1) occurs along pathway 1: ClO (X ²Π) + ClO (X ²Π) → ClOOCl (1 ¹A) → 2Cl (² P _u) + O₂ (X ³Σ _g ⁻) and that reaction (2) occurs along pathway 2: ClO (X ²Π) + ClO (X ²Π) → ClOOCl (1 ¹A) → 1 ¹A/1 ³B MECP (142.4 cm⁻¹) → ClOOCl (1 ³B) → Cl₂ (X ¹Σ _g ⁺) + O₂ (X ³Σ _g ⁻). The needed energies (relative to the reactant) for pathways 1 and 2 are predicted to be 5.3 and 11.1 kcal/mol, respectively, which indicates that reaction (1) is more favorable than reaction (2). The present work supports the traditional photochemical model for ozone degradation: ClOOCl (1 ¹A), formed by two ClO (X ²Π), can directly produce O₂ plus two Cl atoms.     

Should a franck-condon or a curve-crossing picture be applied to ion-target collisional activation? A study of keV CO<Stack><Subscript>2</Subscript><Superscript>+·</Superscript></Stack>/He collisions by emission spectroscopy

Collision-induced photon emissions (CIE) were observed for keV CO₂^+·/He collisions from 190 to 1020 nm. The emissions were assigned to the Δν=0 band of the CO₂^+· B ²Σ_u⁺ → X ²Π_g electronic transition and the Δν = +3, +2, +1, 0, −1, −2, −3 vibrational transition progression in the CO₂^+· A ²Π_u → X ²Π_g electronic transition. The other peaks arise from the emissions of excited O^· fragment atoms and the target gas. The relative intensities of the CO₂^+· and O^· emissions are independent of the ion translational energy above 3 keV, supporting the curve-crossing mechanism for collisional excitation. Investigation of the relative intensities within the A ²Π_u → X ²Π_g emission of CO₂^+· indicates that the vibrational distribution is well described by the Franck-Condon principle at high collision energy, a consequence of short collision time but not necessarily an indication of vertical transitions. Below 3 keV ion translational energy, vibrational excitation in the A ²Π_u electronic state was observed. The observation is consistent with the explanation that the reaction occurs at small impact parameters, in which short-range, repulsive interactions between the projectile and the target result in direct translational-vibrational excitation.     

Kinetics and mechanism of dehydration of kaolinite,muscovite and talc analyzed thermogravimetrically by the third-law method

Summary The third-law method has been applied to determine the enthalpies, Δ_rH_T⁰, for dehydration reactions of kaolinite, muscovite and talc. The Δ_rH_T⁰values measured in the equimolar (in high vacuum) and isobaric (in the presence of water vapour) modes (980±15, 3710±39 and 2793±34 kJ mol^-1, for kaolinite, muscovite and talc, respectively) practically coincide if to take into account the strong self-cooling effect in vacuum. This fact strongly supports the mechanism of dissociative evaporation of these compounds in accordance with the reactions (primary stages): Al₂O₃·2SiO₂·2H₂O(s)→Al₂O₃(g)↓+2SiO₂(g)↓+2H₂O(g); K₂O·3Al₂O₃·6SiO₂·2H₂O(s) →K₂O(g)↓+3Al₂O₃(g)↓+6SiO₂(g)↓+2H₂O(g) and 3MgO·4SiO₂·H₂O(s) →3MgO(g)↓+4SiO₂(g)↓+H₂O(g). The values of the Eparameter deduced from these data for equimolar and isobaric modes of dehydration are as follows: 196 and 327 kJ mol^-1for kaolinite, 309 and 371 kJ mol^-1for muscovite and 349 and 399 kJ mol^-1for talc. These values are in agreement with quite a few early results reported in the literature in 1960s.     

High-Resolution Experimental Study on Photodissocaition of N2O

We study the photodissociation dynamics of nitrous oxide using the time-sliced ion velocity imaging technique at three photolysis wavelengths of 134.20, 135.30, and 136.43 nm. The O(¹S_J=0)+N₂(X¹∑_g⁺) product channels were investigated by measuring images of the O(¹S_J=0) products. Vibrational states of N₂(X¹∑_g⁺) products were fully resolved in the images. Product total kinetic energy releases (TKER) and the branching ratios of vibrational states of N₂ products were determined. It is found that the most populated vibrational states of N₂ products are v=2 and v=3. The angular anisotropy parameters (β values) were also derived. The β values are very close to 2 at low vibrational states of the correlated N₂(X¹∑_g⁺) products at all three photolysis wavelengths, and gradually decrease to about 1.4 at v=7. This indicates the dissociation is mainly through a parallel transition state to form products at lower vibrational states, and the highly vibrational exited products are from a more bent configuration. This is consistent with the observed shift of the most intense rotational structure in the TKER as the vibrational quantum number increases.     

Quasielastic light scattering study of chemically crosslinked gelatin solutions and gels

Quasielastic light scattering measurements are reported for experiments performed on mixtures of gelatin and glutaraldehyde (GA) in the aqueous phase, where the gelatin concentration was fixed at 5 (w/v) and the GA concentration was varied from 1×10⁻⁵ to 1×10⁻³ (w/v). The dynamic structure factor, S(q,t), was deduced from the measured intensity autocorrelation function, g ₂(τ), with appropriate allowance for heterodyning detection in the gel phase. The S(q,t) data could be fitted to S(q,t)=Aexp(−D _f q ² t)+Bexp(−t/τ_c)^β, both in the sol (50 and 60 ^∘C) and gel states (25 and 40 ^∘C). The fast-mode diffusion coefficient, D _f showed almost negligible dependence on the concentration of the crosslinker GA; however, the resultant mesh size, ξ, of the crosslinked network exhibited strong temperature dependence, ξ∼(0.5−χ)^1/5exp(−A/RT) implying shrinkage of the network as the gel phase was approached. The slow-mode relaxation was characterized by the stretched exponential factor exp(−t/τ_c)^β. β was found to be independent of GA concentration but strongly dependent on the temperature as β=β₀+β₁ T+β₂ T ². The slow-mode relaxation time, τ_c, exhibited a maximum GA concentration dependence in the gel phase and at a given temperature we found τ_c(c)=τ₀+τ₁ c+τ₂ c ². Our results agree with the predictions of the Zimm model in the gel case but differ significantly for the sol state. Received: 25 May 1999 /Accepted in revised form: 27 July 1999     

Theoretical treatment of predissociation of the CO (3sσ) B and (3pσ) C1Σ+ Rydberg states based on a rigorous adiabatic representation

Ab initio multireference configuration interaction calculations for adiabatic potential curves, nonadiabatic couplings 〈φ _i (R,r)|d/dR|φ _j (R,r)〉 and 〈φ _i (R,r)|d²/dR ²|φ _j (R,r)〉, and nuclear kinetic energy corrections 〈dφ _i (R,r)/dR|dφ _i (R,r)/dR〉 for the (3sσ) B and (3pσ) C¹Σ⁺ Rydberg states of the CO molecule have been carried out. The energy positions and predissociation linewidths for the observed vibrational levels of these two states have been determined in a rigorous adiabatic representation by the complex scaling method employing a basis of complex scaled harmonic vibrational functions in conjunction with the Gauss-Hermite quadrature method to evaluate the complex Hamiltonian matrix elements. The present treatment correctly reproduces the observed trends in energies and line broadening for vibrational levels of the B¹Σ⁺ state and represents an improvement over the previous treatment in literature. The errors in the determined spacings of the v = 0–4 vibrational levels of the C¹Σ⁺ state are less than 2% compared with measured data. The predissociation linewidths for the v=3,4 levels of the C¹Σ⁺ state are found to be 4.9 and 8.9 cm⁻¹, respectively, in good agreement with the observed values. Received: 23 March 1998 / Accepted: 27 July 1998 / Published online: 9 October 1998     

Theoretical study on thermal decomposition of azoisobutyronitrile in ground state

Since the first discovery of azoalkanes in 1909,the studies of the chemistry of azoalkane radicals havegone through a long history and many significativeresults have been gotten during the past 30 years[1,2].These versatile compounds lose nitrogen thermally orphotochemically under a wide variety of conditions:R─N═N─R ? 2R?+ N2; hence, they are probablythe cleanest and most convenient sources of variousradicals and biradicals of nearly any desired structure.Several reviews on the applicati…     

Excited State Dynamics of Isocyano Rhenium(I) Phenanthroline Complexes from Time-Resolved Spectroscopy

The photophysical processes in a series of isocyano Re(I) phenanthroline complexes {[Re(CNR)_n(CO)_4-n(phen)](PF₆); n=2, 3, 4, R=2,6-(ⁱPr)₂C₆H₃- or ^tBu- (n=2)} in acetonitrile have been studied by resonance Raman spectroscopy, transient resonance Raman spectroscopy, and femtosecond / nanosecond transient spectroscopy to elucidate the nature of their electronic transitions and emissive excited state(s). The kinetics of the intersystem crossing, vibrational relaxation and radiative decay of the metal-to-ligand charge transfer {MLCT [dπ(Re)→π*(phen)]} excited state have also been determined.     

Photophysical Properties of Pyrazinopsoralen,A New Monofunctional Psoralen*

Abstract— Pyrazinopsoralen (PzPs), a new monofunctional psoralen, has a UV absorption spectrum similar to other psoralens except that it absorbs more strongly in the long-UVA than 8-methoxypsoralen. The solvent effects on the UV absorption and fluorescence emission spectra indicate that the lowest excited singlet state is the π,π* state like other psoralen derivatives. It shows a much lower fluorescence quantum yield (0.0008 in ethanol at room temperature) than the other psoralens as expected by the increased proximity effect (vibronic perturbation) due to close ¹(n,π*) to ¹(π,π*) states. The fluorescence lifetime was 1.05 ns in methylcyclohexane with a single exponential decay, while more than two components were observed in other solvents with the short-lived component being the major (>95%). The triplet state of PzPs could not be detected by phosphorescence, laser flash excitation (T-T absorption) and singlet oxygen formation probably due to very low φ_isc, or short lifetime of the triplet state (τ_T) caused by the fast T₁→ S₀ intersystem crossing.     

Spectrum of states in icosahedral structures of the electronic configuration gn (N = 1–7). 1. Invariant expansion for integrals and energies

The energy spectrum of the states that appear in structures of icosahedral (I,I_h symmetry with open electronic shells g^N (dim g = 4; N = 1–7) is reported. The energies are obtained in terms of integral invariants (reduced matrix elements of electron-electron interaction) H^k (g, g). The latter are analogs of the Slater-Condon parameters F^k(l,l) for atoms with the l^N electronic configuration. A similar representation is proposed for the integrals mm’≨’) of electron-electron interaction on the 4-fold degenerate g orbitals in the “standard” representation. The relation between the terms of the g^N(I,I_h) configuration and the parent states of the orthogonal group O⁺(4) is discussed. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 1, pp. 3–13, January–February, 1997.     

Ab initio study of the molecular vibrations and electronic structure of the X,B, D and F2Σ+ states of AlO

Molecular vibrations and electronic structure of the X²Σ⁺, B²Σ⁺, D²Σ⁺, and F²Σ⁺ states of AlO are studied by carrying out ab initio configuration interaction calculations and molecular vibration calculations using accurate potential energy functions. An avoided crossing between the D²Σ⁺ and F²Σ⁺ potential energy curves occurs in the neighborhood of 4.0 a₀ and results in irregular vibrational levels of the D and F²Σ⁺ states. The vibrational constants for the F²Σ⁺ state are predicted from the vibrational levels not involved in the irregularity. Configuration mixing is important in describing the B, D, and F²Σ⁺ states. The F²Σ⁺ state at and around its well minimum and the D and F²Σ⁺ states in the avoided crossing region are characterized in terms of their main configurations and dipole moment functions.     

Role of spin-orbit coupling in processes of synthesis and photodegradation of ozone

Based on comparison of the results ofab initio and semiempirical configuration-interaction calculations, we have plotted the potential energy surface for the ground state and a number of low-lying excited states of ozone which correlate with the three lowest dissociation limits. We calculate the matrix elements for the spin-orbit coupling operator at points of singlet-triplet term crossing. Based on this, we evaluate the nonadiabatic decomposition process occurring with a change in spin on absorption of ozone in the Huggins band. We also predict a triplet-singlet nonadiabatic transition in the triplet channel for recombination of the speciesO( ³ P)+O ₂(X ³ Σ _g ⁻ ) with formation of ozone in the ground state. Cherkassy Engineering and Technological Institute, 460 Bul’var Shevchenko, 257006 Cherkassy, Ukraine. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 33, No. 4, pp. 219–223, July–August, 1997.     

Characterizing the Intramolecular H-bond and Secondary Structure in Methylated GlyGlyH<Superscript>+</Superscript> with H<Subscript>2</Subscript> Predissociation Spectroscopy

We report vibrational predissociation spectra of the four protonated dipeptides derived from glycine and sarcosine, GlyGlyH⁺•(H₂)_1,2, GlySarH⁺•(D₂)₂, SarGlyH⁺•(H₂)₂, and SarSarH⁺•(D₂)₂, generated in a cryogenic ion trap. Sharp bands were recovered by monitoring photoevaporation of the weakly bound H₂ (D₂) molecules in a linear action regime throughout the 700–4200 cm^–1 range using a table-top laser system. The spectral patterns were analyzed in the context of the low energy structures obtained from electronic structure calculations. These results indicate that all four species are protonated on the N-terminus, and feature an intramolecular H-bond involving the amino group. The large blue-shift in the H-bonded N–H fundamental upon incorporation of a methyl group at the N-terminus indicates that this modification significantly lowers the strength of the intramolecular H-bond. Methylation at the amide nitrogen, on the other hand, induces a significant rotation (~110^o) about the peptide backbone.