Vacuum ultraviolet (VUV) photoionization of small water clusters

Tunable vacuum ultraviolet (VUV) photoionization studies of water clusters are performed using 10-14 eV synchrotron radiation and analyzed by reflectron time-of-flight (TOF) mass spectrometry. Photoionization efficiency (PIE) curves for protonated water clusters (H2O)(n)H+ are measured with 50 meV energy resolution. The appearance energies of a series of protonated water clusters are determined from the photoionization threshold for clusters composed of up to 79 molecules. These appearance energies represent an upper limit of the adiabatic ionization energy of the corresponding parent neutral water cluster in the supersonic molecular beam. The experimental results show a sharp drop in the appearance energy for the small neutral water clusters (from 12.62 +/- 0.05 to 10.94 +/- 0.06 eV, for H2O and (H2O)4, respectively), followed by a gradual decrease for clusters up to (H2O)23 converging to a value of 10.6 eV (+/-0.2 eV). The dissociation energy to remove a water molecule from the corresponding neutral water cluster is derived through thermodynamic cycles utilizing the dissociation energies of protonated water clusters reported previously in the literature. The experimental results show a gradual decrease of the dissociation energy for removal of one water molecule for small neutral water clusters (3 相似文献   

Infrared and ultraviolet absorptions of matrix isolated C(6)O(2)

In the course of our research into carbon chains trapped in matrices of molecular oxygen, we encountered an IR absorption line at 2180.4 cm(-1), which we tentatively assigned to linear C(6)O(2). In this article, we describe our attempts to confirm the assignment by partial isotopic substitution of carbon by (13)C and oxygen by (18)O. A detailed analysis of the IR vibration pattern allowed the unambiguous identification of the carrier of the IR absorption as C(6)O(2). The identification work was very much facilitated by the observation that it is possible to produce and destroy C(6)O(2) in a controlled fashion by suitable laser exposures. With the help of this feature, most of the confusing spectral background could be removed. Two infrared absorptions at 2180.4 (nu(5)) and 1817.7 cm(-1) (nu(6)) and ultraviolet absorption at 252 nm were assigned to the C(6)O(2) molecule--all figures are valid for oxygen matrices. The obtained spectral data are compared with results of quantum chemical calculations. DFT B3LYP/6-31G(d) and semiempirical PM3 methods were used for geometry optimization and calculation of vibrational frequencies. CIS and TD-DFT were used to calculate the electronic absorption spectrum.     

Vacuum ultraviolet mcd and absorption spectra of P4 isolated in an ar matrix using synchrotron radiation

The VUV MCD and absorption spectra of P₄ isolated in an argon matrix were obtained using synchrotron radiation. Four well resolved ¹A₁ → ¹T₂^J transitions are observed (J = A, B, C, D from low to high energy). A Jahn-Teller effect is clearly evident in the ¹T₂^C excited state. The ¹T₂^D excited state contains substantial 3d character.     

Infrared spectra of urea isolated in a solid argon matrix

Pure monomeric urea molecules, free of contamination with decomposition products, have been obtained by sublimation of solid urea at ca 30°C and isolating in argon at ca 10K. Very high dilution is required to obtain the pure monomer which shows many spectral features different from that previously reported by King [Spectrochim. Acta28A, 165 (1972)]. The i.r. spectrum of the deutero-analogue has also been measured.     

Vacuum ultraviolet (147 nm) photolysis of 1,2-fluorochloroethane

1,2-Fluorochloroethane was photolyzed at 147 nm in the pressure range of 3.8–20.9 torr. The effects of added NO, H₂S, and large pressures of CF₄ were also investigated. The exponential extinction coefficient at 147 nm and 296°K was found to be 147 ± 4 atm^?1 · cm^?1. The photochemistry in some respects is similar to that of ethyl chloride. The primary processes again appear to involve at least two excited states. One of these yields ethylene by FCl elimination (Φ ? 0.3) and has a lifetime of ～3.2 × 10^?10 sec, with respect to an internal conversion to the vibrationally excited ground state or, more probably, a collisionally induced crossover to a state decomposing mainly by carbon? halogen bond fission. The molecular elimination of HCl, H₂, and small amounts of HF also occurs but not apparently from the same state as does FCl. The quantum yields of products with radical precursors, however, are not large, and hence little, if any, of the FCl and probably none of HCl, H₂, and HF subsequently dissociates. The vinyl fluoride and vinyl chloride, accompanying the elimination of HF and HCl, are postulated as possible sources of the secondary production of acetylene.     

Vapor-phase photodecomposition of chloropicrin (trichloronitromethane)

Under simulated environmental (atmospheric) conditions, chloropicrin vapor was readily converted in light to phosgene, which remained stable, and nitrosyl chloride which was subsequently photolyzed to nitric oxide and chlorine; a portion of the nitric oxide was oxidized to nitrogen dioxide and dinitrogen tetroxide. The unexpected dependence of the reaction upon oxygen, the initial photoproducts, and the incorporation of ¹⁸O when the photolysis occurred in the presence of ¹⁸O₂ suggest an intramolecular rearrangement involving a trioxazole N-oxide.     

Photochemical synthesis of the cyanodiacetylene HC(5)N: a cryogenic matrix experiment

The reactivity of intermolecular complexes cyanoacetylene:acetylene and dicyanoacetylene:acetylene, trapped in solid argon matrixes at 10 K and irradiated with vacuum UV, has been studied. FTIR measurements, together with (2)H, (15)N, (13)C labeling experiments and with density functional theory (DFT) calculations (B3LYP/aug-cc-pVTZ), pointed to the formation of cyanodiacetylene HC(5)N (cyanobutadiyne). This synthetic route is potentially important for chemical models of the Titan's atmosphere.     

Photochemical dehydration of acetamide in a cryogenic matrix

Vacuum ultraviolet (VUV) irradiation of acetamide has been monitored by Fourier transform infrared spectroscopy in argon matrix at 10 K. Several primary photoproducts, including HNCO ratio CH(4) and CO ratio CH(3)NH(2) molecular complexes, and acetimidic acid, which is reported for the first time, were characterized. The acetimidic acid identification was based on comparison between the experimental and theoretical (B3LYP) infrared spectra. Acetimidic acid is found in argon matrix in the (s-Z)-(E) and (s-Z)-(Z) configurations. It is also an intermediate in the VUV decomposition process, its dehydration leads to the formation of CH(3)CN ratio H(2)O molecular complex. The assignment of the complex was achieved by co-depositing the pairs of respective species and by ab initio calculation.     

Absorption and luminescence spectra and photodecomposition of zinc(II) complex with bis(1,2,3,7,9-pentamethyldipyrrin-8-yl)methane in solution and polymer matrix

New polymeric materials containing dinuclear double-stranded zinc(II) helicate with bis(1,2,3,7,9-pentamethyldipyrrin-8-yl)methane have been prepared on the basis of poly(methyl methacrylate) and tetraethoxysilane. The zinc(II) complex [Zn₂L₂] retains its fluorescence properties in a poly(methyl methacrylate) film. The photostability of [Zn₂L₂] in poly(methyl methacrylate) is higher by a factor of ≈2.5 than in the benzene solution. Conditions have been found for the preparation of [Zn₂L₂]-doped silicate matrix based on tetraethoxysilane via the sol–gel method. The [Zn₂L₂] complex in the tetraethoxysilane solid matrix shows a weak fluorescence with its maximum at λ = 545–549 nm.     

The photodecomposition of platinacycloalkanes in solution: II. 1,4-butanediylplatinum(II) and (IV) compounds

The products of the photolysis of a number of platinacyclopentanes in solution at 25°C under a variety of conditions have been determined. With [I₂P$\text{tCH}{}_2\text{CH}{}_2\text{CH}{}_2\text{C}$H₂(L₂)] (L = PMe₂Ph, PPh₃) in CH₂Cl₂, CH₂Br₂ and (CH₃)₂SO the hydrocarbon products are exclusively ethylene and but-1-ene. Formation of the latter through a 1,3-hydrogen shift is preceded by phosphine ligand dissociation. The photolysis of [ICH₃P$\text{tCH}{}_2\text{CH}{}_2\text{C}$H₂(L₂)] gave methane, ethylene, but-1-ene and n-pentane together with a little n-butane, the methane being formed from internal hydrogen abstraction by the CH₃ group in the excited reactant molecule. Photodecomposition of the platinum(II) compounds [P$\text{tCH}{}_2\text{CH}{}_2\text{CH}{}_2\text{C}$H₂(L₂)] (L = (PMe₂Ph)₂, (PPh₃)₂, Ph₂PCH₂CH₂PPh₂) gave ethylene, but-1-ene, pent-1-ene (with the halogenated solvents) and with some systems appreciable yields of n-butane, the latter being the results of internal abstraction of two hydrogen atoms by the C₄H₈ moiety. The formation of pentene is probably preceeded by the addition of CH₂Cl₂ or CH₂Br₂ to the excited reactant molecule. Addition of diphenylphosphine promotes the production of n-butane.     

Direct determination of U(VI) in TBP-kerosene extracts using first derivative ultraviolet spectroscopy

Summary The extractive first order derivative spectrophotometry is a selective method for the separation and determination of U(VI) using tri-n-butyl phosphate (TBP), which combines the roles of solvent and complexing agent. The complex is formed by extracting U(VI) from an aqueous 6M sodium nitrate solution at initial pH 3.0 into a 25% solution of TBP in kerosene. This extraction also separates U(VI) from many diverse ions that interfere. After extraction, the determination of uranium shows good accuracy and precision with relative standard deviation of 1.5% (n = 5) at 20 ppm using zero-order spectrum at l_max = 250 nm. Calibration curve was also found to obey Beer's law in the range of 10-100 ppm with 3.33 ppm detection limit. However, these accuracy and precision have been improved to give relative standard deviation of 0.7% (n = 5) at 20 ppm with a lower detection limit of 2.24 ppm using the first-derivative spectrum at l = 263 nm comparing to the normal one.     

Amplified emission of phthalocyanine isolated in cryogenic matrices

Laser induced fluorescence spectroscopy of free-base (H(2)Pc) and zinc (ZnPc) phthalocyanines trapped in rare gas and nitrogen matrices reveals a quite unexpected phenomenon with a moderate increase in the laser intensity. In all matrices except Xe, a huge increase occurs in the intensity of an emission band near 755 nm when pumping the S(1) <-- S(0) transition. The band involves a vibrational mode of the ground state, located at 1550 and 1525 cm(-1) for H(2)Pc and ZnPc, respectively. Many of the characteristics of amplified emission (AE) are exhibited by this vibronic transition. Excitation scans recorded for the AE band yield greatly enhanced site selectivity compared to what is obtained in normal fluorescence excitation scans.     

The photodecomposition of platinacycloalkanes in solution.: I. 1,3-propanediylplatinum(IV) compounds

The gaseous products of the photolysis at 25°C of the platinacyclobutane compounds [X₂$\text{PtCH}{}_2\text{CH}{}_2\text{C}$H₂(N-N)] where X = Cl, Br and N-N = 1,10-phenanthroline, 2,2′-bipyridine, (CH₂NMe₂)₂, (C₅H₅N)₂ in several solvents, in the absence and presence of various additives, have been determined. With solvents of relatively low dielectric constant (e.g. CH₂Cl₂), over 85 mol % of the hydrocarbon products was propene, the formation of which appears to involve a direct transfer of a hydrogen atom between neighbouring groups in the ring. With solvents of relatively high dielectric constant (MeCN, Me₂SO) in the presence of species, e.g. I^?, SbPh₃, having a high trans effect, cyclopropane is the main volatile product. The effect of added halide ion and of the mixed solvents Me₂SO/PhMe and Me₂SO/PhSH indicates that ionisation of the platinacyclobutane and the formation of platinum substituted propyl ion-radicals precede the formation of cyclopropane (and the small amounts of ethylene produced).The photolysis of [X₂$\text{PtCH}{}_2\text{CH}{}_2\text{C}$H₂(MeCN)₂] in methyl cyanide solution in the presence of Et₃RNX′ (X′ = Cl, R = H; X′ = Br, R = Et) gives appreciable amounts of ethylene in the products (up to 25 mol %). It is suggested that the halide ions add to the platinum to give negatively charged platinacyclobutane species, the photodecomposition of which may give C₂H₄.     

High-performance liquid chromatographic assay of erythromycin from biological matrix using electrochemical or ultraviolet detection

Two chromatographic methods were developed for the determination of erythromycin A (EA) residues in animal tissues (muscle, liver, kidney and fat of cattle, pigs and poultry) and cow's milk. In addition to a more traditional method using electrochemical detection, we developed an original alternative method based on UV detection at 236 nm, by pretreating to create a chromophore in the molecule. An internal standard was used with both methods to check the variability of the analytical system. Analysis times and performance were compared. The recovery of EA from various matrices was greater than 95%. For both methods the quantification limit for EA was 0.25 microgram ml-1 for plasma, 0.025 microgram g-1 for milk and 0.125 microgram g-1 for the other biological matrices. The methods can be used to check for EA residues in these matrices; in fact, the statutory maximum residue limits (MRLs) of EA are 0.4 microgram g-1 in muscle, kidney, liver and fat of beef cattle, sheep, pigs and poultry, and 0.04 microgram g-1 in cow's and sheep's milk.     

Vacuum ultraviolet absorbance of alkanes: an experimental and theoretical investigation

The electronic absorption spectra of alkanes are known to be broad and lack discrete band structures. Previous studies have suggested their HOMO-LUMO energy gaps could be used to understand the absorbance edges of these spectra. With the advent of a new benchtop vacuum ultraviolet (VUV) spectroscopic absorption detector, it is now possible to collect VUV absorption spectra (from 120 to 240 nm) for an extended range of structures under an inert gas phase environment more conveniently. The previously reported hypothesis was revisited and its limits were explored using a combination of new extended experimental VUV spectral data and theoretical quantum mechanics calculations. It was pointed out from this study that the first strong excitations of alkanes are not always the HOMO-LUMO transition. As a result, the HOMO-LUMO energy gap of alkanes should not be used directly as a universal and reliable parameter to understand their experimental absorbance edges. VUV spectral data for a larger variety of alkanes structures were reported and the relation between these structures and spectra was discussed.

     

Use of CdSe/ZnS luminescent quantum dots incorporated within sol-gel matrix for urea detection

In this work, urea detection techniques based on the pH sensitivity of CdSe/ZnS QDs were developed using three types of sol-gel membranes: a QD-entrapped membrane, urease-immobilized membrane and double layer consisting of a QD-entrapped membrane and urease-immobilized membrane. The surface morphology of the sol-gel membranes deposited on the wells in a 24-well microtiter plate was investigated. The linear detection range of urea was in the range of 0-10 mM with the three types of sol-gel membranes. The urea detection technique based on the double layer consisting of the QD-entrapped membrane and urease-immobilized membrane resulted in the highest sensitivity to urea due to the Michaelis-Menten kinetic parameters. That is, the Michaelis-Menten constant (K_m =2.0745 mM) of the free urease in the QD-entrapped membrane was about 4-fold higher than that (K_m =0.549 mM) of the immobilized urease in the urease-immobilized membrane and about 12-fold higher than that (K_m =0.1698 mM) of the immobilized urease in the double layer. The good stability of the three sol-gel membranes for urea sensing over 2 months showed that the use of sol-gel membranes immobilized with QDs or an enzyme is suitable for biomedical and environmental applications.     

Desorption Dynamics,Internal Energies,and Imaging of Organic Molecules from Surfaces with Laser Desorption and Vacuum Ultraviolet (VUV) Photoionization

There is enormous interest in visualizing the chemical composition of organic material that comprises our world. A convenient method to obtain molecular information with high spatial resolution is imaging mass spectrometry. However, the internal energy deposited within molecules upon transfer to the gas phase from a surface can lead to increased fragmentation and to complications in analysis of mass spectra. Here it is shown that in laser desorption with postionization by tunable vacuum ultraviolet (VUV) radiation, the internal energy gained during laser desorption leads to minimal fragmentation of DNA bases. The internal temperature of laser‐desorbed triacontane molecules approaches 670 K, whereas the internal temperature of thymine is 800 K. A synchrotron‐based VUV postionization technique for determining translational temperatures reveals that biomolecules have translational temperatures in the range of 216–346 K. The observed low translational temperatures as well as their decrease with increased desorption laser power is explained by collisional cooling. An example of imaging mass spectrometry on an organic polymer by using laser‐desorption VUV postionization shows 5 μm feature details while using a 30 μm laser spot size and 7 ns pulse duration. Applications of laser‐desorption postionization to the analysis of cellulose, lignin, and humic acids are briefly discussed.     

ESR study on photodecomposition mechanism of oxidized poly(vinyl alcohol)

Photolysis behavior of oxidized poly(vinyl alcohol) (PVA) in which carbonyl groups were introduced by oxidation with sodium hypochlorite and periodic acid was investigated by using ESR spectrometry. Increased formation of PVA radicals was observed in the sample with low degree of oxidation, which showed an emphasized formation of triplet component radical. On the other hand, the formation of radicals tended to be depressed in the highly oxidized sample, and a singlet component was dominant in the ESR spectrum. By examining infrared and ultraviolet spectra of the oxidized sample, the sample was found to contain two structures, a saturated ketone group and an α,β-unsaturated group; the latter structure was intensified with increasing degree of oxidation. Based on ESR studies of the photoirradiated acetone and mesityl oxide, the models of saturated and α,β-unsaturated ketone structures in the oxidized PVA, respectively a photodecomposition mechanism of oxidized PVA was proposed.