Kinetic study of the ClOO + NO reaction using cavity ring-down spectroscopy

Cavity ring-down spectroscopy was used to study the reaction of ClOO with NO in 50-150 Torr total pressure of O2/N2 diluent at 205-243 K. A value of k(ClOO+NO) = (4.5 +/- 0.9) x 10(-11) cm3 molecule(-1) s(-1) at 213 K was determined (quoted uncertainties are two standard deviations). The yield of NO(2) in the ClOO + NO reaction was 0.18 +/- 0.02 at 213 K and 0.15 +/- 0.02 at 223 K. An upper limit of k(ClOO+Cl2) < 3.5 x 10(-14) cm3 molecule(-1) s(-1) was established at 213 K. Results are discussed with respect to the atmospheric chemistry of ClOO and other peroxy radicals.     

The spectroscopy of OClO in polar liquids

The near-UV A²A₂←X²B₁, transition of OClO is examined in polar solutions. Both the λ_max of the ²A₂ electronic band and the inhomogeneous vibronic widths vary significantly with solvent. The dependence of the absorption maximum on dielectric properties of the solvent can be qualitatively described by continuum solvation models. Changes in the inhomogeneous widths are discussed in terms of inhomogeneity resulting from different solvation geometries. Picosecond time resolved absorption studies following the UV photolysis of OClO at 355 nm indicate a solvent dependence on the excited state reactivity. In many solvents. there is competition between bond breakage (to form ClO and O) and isomerization (to form ClOO). The possible involvement of this OClO heterogeneous photochemistry in stratospheric ozone depletion is described.     

Mass spectrometric and computational study of SnPb in the gas phase

The SnPb molecule has been identified in a Knudsen effusion mass spectrometry experiment. The direct dissociation reaction and two isomolecular exchange reactions involving the Sn(2) and Pb(2) molecules have been studied, in the 1426-1705 K range of temperatures, using both second and third law procedures. The D(degree)0(SnPb,g) has been derived, for the first time, as (122.6+/-4.0) kJ mol(-1). Density functional and ab initio calculations up to the coupled clusters level of theory were also performed. In addition, the anion dissociation energy D(degree)0(SnPb(-),g) of (179.2+/-4.2) kJ mol(-1) was determined using the D(degree)0(SnPb,g) mass spectrometric value derived in this investigation and literature data.     

Kinetics of reactions of OBrO with NO,O3, OClO,and ClO at 240–350 K

Kinetics for the reactions of OBrO with NO, O₃, OClO, and ClO at 240–350 K were investigated using the technique of discharge flow coupled with mass spectrometry. The Arrhenius expression for the OBrO reaction with NO was determined to be k₁ = (2.37 ± 0.96) × 10^?13 exp[(607 ± 63)/T] cm³ molecule^?1 s^?1. The reactions of OBrO with O₃, OClO, and ClO are slow chemical processes at 240–350 K. Upper limit rate constants for the OBrO reactions with O₃, OClO, and ClO at 240–350 K were estimated to be k₂ < 5.0 × 10^?15 cm³ molecule^?1 s^?1, k₃ < 6.0 × 10^?14 cm³ molecule^?1 s^?1, and k₄ < 1.5 × 10^?13 cm³ molecule^?1 s^?1, respectively. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 430–437, 2002     

Fourier transform microwave spectroscopy and Fourier transform microwave-millimeter wave double resonance spectroscopy of the ClOO radical

Pure rotational spectra of the ClOO radical for the (35)Cl and (37)Cl isotopomers have been observed using Fourier transform microwave and Fourier transform microwave-millimeter wave double resonance spectroscopy. The rotational, centrifugal, spin-rotation coupling, and hyperfine coupling constants have been determined by least-squares fits of the observed transition frequencies. The molecular constants indicate that the electronic ground state is 2A". The r(0) structure is determined to be r(0)(ClO)=2.075 A, r(0)(OO)=1.227 A, and theta;(0)(ClOO)=116.4 degrees . Several highly accurate ab initio calculations have also been performed. Some of them turned out to be inaccurate because it is necessary to take into account both static and dynamic electronic correlations. Only multireference (single and double) configuration interaction calculations with large basis sets reproduce the present experimental results. The anharmonic force constants obtained by the ab initio calculations are used to determine the r(e) structure, r(e)(ClO)=2.084(1) A, r(e)(OO)=1.206(2) A, and theta;(e)(ClOO)=115.4(1) degrees . Unique features of the ClOO radical have become clear by the present experiment and the ab initio calculations.     

Fourier transform infrared spectroscopy and theoretical study of dimethylamine dimer in the gas phase

Dimethylamine (DMA) has been studied by gas-phase Fourier transform infrared (FTIR) spectroscopy. We have identified a spectral transition that is assigned to the DMA dimer. The IR spectra of the dimer in the gas phase are obtained by spectral subtraction of spectra recorded at different pressures. The enthalpy of hydrogen bond formation was obtained for the DMA dimer by temperature-dependence measurements. We complement the experimental results with ab initio and anharmonic local mode model calculations of monomer and dimer. Compared to the monomer, our calculations show that in the dimer the N-H bond is elongated, and the NH-stretching fundamental shifts to a lower wavenumber. More importantly, the weak NH-stretching fundamental transition has a pronounced intensity increase upon complexation. However, the first NH-stretching overtone transition is not favored by the same intensity enhancement, and we do not observe the first NH-stretching overtone of the dimer. On the basis of the measured and calculated intensity of the NH-stretching transition of the dimer, the equilibrium constant for dimerization at room temperature was determined.     

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag₃, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag ₂ ⁺ to be <2.1 eV.     

Electronic spectroscopy and relaxation of gas phase molecular ions

Techniques for obtaining electronic spectra of singly charged cations are described. Jahn-Teller effects in the spectra of polyhalobenzene cations are discussed. Experimental and theoretical methods for studying radiationless transitions in molecular ions are presented. Recent work on the spectroscopy and intramolecular relaxation of doubly-charged cations is reviewed.     

Visible and ultraviolet spectroscopy of gas phase protein ions

Optical spectroscopy has contributed enormously to our knowledge of the structure and dynamics of atoms and molecules and is now emerging as a cornerstone of the gas phase methods available for investigating biomolecular ions. This article focuses on the UV and visible spectroscopy of peptide and protein ions stored in ion traps, with emphasis placed on recent results obtained on protein polyanions, by electron photodetachment experiments. We show that among a large number of possible de-excitation pathways, the relaxation of biomolecular polyanions is mainly achieved by electron emission following photo-excitation in electronically excited states. Electron photodetachment is a fast process that occurs prior to relaxation on vibrational degrees of freedom. Electron photodetachment yield can then be used to record gas phase action spectra for systems as large as entire proteins, without the limitation of system size that would arise from energy redistribution on numerous modes and prevent fragmentation after the absorption of a photon. The optical activity of proteins in the near UV is directly related to the electronic structure and optical absorption of aromatic amino acids (Trp, Phe and Tyr). UV spectra for peptides and proteins containing neutral, deprotonated and radical aromatic amino acids were recorded. They displayed strong bathochromic shifts. In particular, the results outline the privileged role played by open shell ions in molecular spectroscopy which, in the case of biomolecules, is directly related to their reactivity and biological functions. The optical shifts observed are sufficient to provide unambiguous fingerprints of the electronic structure of chromophores without the requirement of theoretical calculations. They constitute benchmarks for calculating the absorption spectra of chromophores embedded in entire proteins and could be used in the future to study biochemical processes in the gas phase involving charge transfer in aromatic amino acids, such as in the mediation of electron transfer or redox reactions. We then addressed the important question of the sensitivity of protein optical spectra to the intrinsic properties of protein ions, including conformation, charge state, etc., and to environmental factors. We report optical spectra for different charge states of insulin, for ubiquitin starting from native and denaturated solutions, and for apo-myoglobin protein. All these spectra are compared critically to spectra recorded in solution, in order to assess solvent effects. We also report the spectra of peptides complexed with metal cations and show that complexation gives rise to new optical transitions related to charge transfer types of excitation. The perspectives of this work include integrative approaches where UV-Vis spectroscopy could, for example, be combined with ion mobility spectrometry and high level calculations for protein structural characterization. It could also be used in spectroscopy to probe biological processes in the gas phase, with different light sources including VUV radiation (to probe different types of excitations) and ultra short pulses with time and phase modulation (to probe and control the dynamics of de-excitation or charge transfer events), and with the derivatization of proteins with chromophores to modulate their optical properties. We also envision that photo-excitation will play an important role in the future to produce intermediates with new chemical and reactive properties. Another promising route is to conduct activated electron photodetachment dissociation experiments.     

Kinetic studies of free radical reactions by mass spectrometry. II. The reactions of SO + ClO,SO + OClO and SO + BrO

The rates of several novel elementary reactions involving ClO, BrO and SO free radicals in their ground states were studied in a discharge-flow system at 295 K, using mass spectrometry. The rate constant k₂ was determined from the decay of SO radicals in the presence of excess ClO radicals: The SO + OClO overall reaction has a complex mechanism, with the primary step having a rate constant k₅ equal to (1.9 ± 0.7) × 10^?12 cm³ sec^?1: A lower limit for the rate constant of the rapid reaction of SO radicals with BrO radicals was determined:      

The Cl2 photosensitized decomposition of O3: the reactions of ClO and OClO with O3

Cl₂ was photolyzed in the temperature range 254 – 296 K with 366 nm radiation in the presence of O_3· O₃ was removed with quantum yields of 5.8 ± 0.5, 4.0 ± 0.3, 2.9 ± 0.3 and 1.9 ± 0.2 at 24, 10,0 and—21 °C respectively, independent of the initial O₃ or Cl₂ removal quantum yields were 0.11 ± 0.2 at 24 °C for Cl₂ conversions of about 30%, much higher than expected from mass balance considerations based on the initial quantum yield of 0.089 ± 0.013 for OClO formation at 24 °C. The final chlorine-containing product was Cl₂O₇ which was observed. It was produced at least in part through the formation of OClO as an intermediate which was also observed with an initial quantum yield of Φ_i[;OClO]; = 2.5 × 10³ exp [;—(3025 ± 625)/T]; independent of [O₃] or I_a. The results are consistent with the reaction sequence

The relative importance of the channels for reaction (2) at 296 K are the following: k_2a/k₂ = 0.63; k_2b/k₂ = 0.34; k_2c/k₂ = 0.0032. Also, k_2c/k_2b = 2.5 × 10³ exp [;—(3025 ± 625)/T];. The upper limit for the rate coefficient was found to be less than 1 × 10⁻¹⁸ cm³ s⁻¹ for channels (24a) and (4b):

The addition of nitrogen had no effect, but oxygen reduced —Φ[;O₃]; for unknown reasons and several possibilities are discussed. At temperatures below 296 K the equilibrium

becomes apparent.The reaction of OClO with O₃ was also studied by direct mixing of OClO and O₃ in a quartz vessel in the temperature range 253 – 296 K, and in the Cl₂O₃ system by monitoring OClO decay in the dark in the temperature range 264 – 296 K:

The Arrhenius rate coefficient recommended for reaction (5) is

The low values of k₄ and k₅ obtained in this study indicate that reactions (4) and (5) are probably not important in atmospheric chemistry.     

Structure and properties of FCO,FOO, ClCO and ClOO

High quality SCF-MO calculations for the radicals FCO, FOO, ClCO and ClOO including geometry optimization are presented. One-electron properties and population indices are reported and compared both with experiment where appropriate and amongst themselves. ³⁵Cl-quadrupole coupling constants agree well with the experimental data and the agreement between calculated and observed ESR hfcc is discussed.     

Dissociation routes of protonated toluene probed by infrared spectroscopy in the gas phase

The structures of C(7)H(9)(+) ions generated by protonation of toluene are investigated by means of gas-phase infrared spectroscopy in conjunction with labeling experiments and complementary mass spectrometric studies. In full consistency with previous studies, the unimolecular as well as the multiphoton-induced dissociation of mass-selected C(7)H(9)(+) ions lead to losses of molecular hydrogen and methane. Labeling data clearly imply the occurrence of skeletal rearrangements of protonated toluene to isomeric structures in the course of fragmentation. Complementary reactivity studies indicate, however, that the C(7)H(7)(+) ions generated upon dehydrogenation of C(7)H(9)(+) bear the benzylium structure, rather than that of the more stable tropylium ion. Combination of labeling data and extensive theoretical studies lead to a scheme for the fragmentation of protonated toluene, which can account for all experimental findings reasonably well. As far as infrared spectroscopy of gaseous ions is concerned, the present results confirm the structural predictions derived from theory and provide evidence for the existence of protonated cycloheptatriene but also pose some questions about the comparability of intensities in multiphoton dissociation and linear absorption spectra.     

The gas phase structure of ethynylferrocene using microwave spectroscopy

Gas phase structural parameters for ethynylferrocene have been determined using microwave spectroscopy. Rotational transitions due to a- and b-type dipole moments were measured. Twenty four rotational constants have been determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fits to determine structural parameters and Kraitchman analyses have been used to determine the gas phase structural parameters and the atomic coordinates from the rotational constants. The distance between the Fe atom and the C atoms of the cyclopentadienyl rings is r(Fe-C1) = 2.049(5) A, and the distance between the carbon atoms of the cyclopentadienyl ring is r(C-C) = 1.432(2) A. The ethynyl group is bent away from the Fe atom and out of the plane of the carbon atoms in the adjacent cyclopentadienyl ring by 2.75(6) degrees. Structural parameters were also obtained from DFT calculations and Kraitchman analyses, and the results are compared. Analysis of fit results for 13C isotopic substitution data indicates that the carbon atoms of the two cyclopentadienyl rings are in an eclipsed conformation in the ground vibrational state. Trends in microwave experimental values for the distance from the Fe atom to the center of the cyclopentadienyl ring for a series of substituted ferrocenes have been analyzed. This analysis provides an estimate of the gas phase distance from the Fe atom to the centers of the cyclopentadienyl rings for ferrocene of 1.65(1) A.     

Infrared spectroscopy of ionized corannulene in the gas phase

The gas-phase infrared spectra of radical cationic and protonated corannulene were recorded by infrared multiple-photon dissociation (IRMPD) spectroscopy using the IR free electron laser for infrared experiments. Electrospray ionization was used to generate protonated corannulene and an IRMPD spectrum was recorded in a Fourier-transform ion cyclotron resonance mass spectrometer monitoring H-loss as a function of IR frequency. The radical cation was produced by 193-nm UV photoionization of the vapor of corannulene in a 3D quadrupole trap and IR irradiation produces H, H(2), and C(2)H(x) losses. Summing the spectral response of the three fragmentation channels yields the IRMPD spectrum of the radical cation. The spectra were analyzed with the aid of quantum-chemical calculations carried out at various levels of theory. The good agreement of theoretical and experimental spectra for protonated corannulene indicates that protonation occurs on one of the peripheral C-atoms, forming an sp(3) hybridized carbon. The spectrum of the radical cation was examined taking into account distortions of the C(5v) geometry induced by the Jahn-Teller effect as a consequence of the degenerate (2)E(1) ground electronic state. As indicated by the calculations, the five equivalent C(s) minima are separated by marginal barriers, giving rise to a dynamically distorted system. Although in general the character of the various computed vibrational bands appears to be in order, only a qualitative match to the experimental spectrum is found. Along with a general redshift of the calculated frequencies, the IR intensities of modes in the 1000-1250 cm(-1) region show the largest discrepancy with the harmonic predictions. In addition to CH "in-plane" bending vibrations, these modes also exhibit substantial deformation of the pentagonal inner ring, which may relate directly to the vibronic interaction in the radical cation.     

Observation of methyl iodide clusters in gas phase by infrared cavity ring-down spectroscopy

Infrared spectra of methyl iodide clusters produced in a supersonic jet have been observed in the C–H stretching region by cavity ring-down spectroscopy. The dependence of the spectra on the mixing ratio of CH₃I versus He and on the stagnation pressure has led to a tentative assignment of the absorption peaks to trimer up to pentamer, based on our previous study with matrix isolation technique (Chem. Phys. Lett. 343 (2001) 185). Ab initio calculations at the MP2 level for the trimer and tetramer have shown that two stable isomers exist for the tetramer whereas only one isomer is found to be stable for the trimer. The tentative assignment of the observed spectra has been in qualitative agreement with the results of the calculations. The structure of each isomer and its photochemical relevance are discussed.     

Infrared matrix and microwave spectroscopy of the gas phase vinylchloride-ozone reaction

The gas phase ozonolysis of vinylchloride has been investigated by linear reaction-infrared matrix and -microwave spectroscopy. The reaction is shown to produce formaldehyde and chloroethylene oxide, besides already known products. Implications for the reaction mechanism will be discussed.     

Fragmentation and conformation study of ephedrine by low- and high-resolution mass selective UV spectroscopy

The neurotransmitter molecule, ephedrine, has been studied by mass-selective low- and high-resolution UV resonance enhanced two-photon ionization spectroscopy. Under all experimental conditions we observed an efficient fragmentation upon ionization. The detected vibronic peaks in the spectrum are classified according to the efficiency of the fragmentation, which leads to the conclusion that there exist three different species in the molecular beam: ephedrine-water cluster and two distinct conformers. The two-color two-photon ionization experiment with a decreased energy of the second photon leads to an upper limit of 8.3 eV for the ionization energy of ephedrine. The high-resolution (70 MHz) spectrum of the strongest vibronic peak in the spectrum measured at the fragment (m/z=58) mass channel displays a pronounced and rich rotational structure. Its analysis by the use of a specially designed computer-aided rotational fit process yields accurate rotational constants for the S(0) and S(1) states and the transition moment ratio, providing information on the respective conformational structure.