Broad-band Photolysis Research Articles

Photoisomerization of (Cyanomethylene)cyclopropane (C5H5N) to 1-Cyano-2-methylenecyclopropane in an Argon Matrix.

Broad-band ultraviolet photolysis (λ > 200 nm) of (cyanomethylene)cyclopropane (5) in an argon matrix at 20 K generates 1-cyano-2-methylenecyclopropane (7), a previously unknown compound. This product was initially identified by comparison of its infrared spectrum to that predicted by an anharmonic MP2/6-311+G(2d,p) calculation. This assignment was unambiguously confirmed by the synthesis of 1-cyano-2-methylenecyclopropane (7) and observation of its authentic infrared spectrum, which proved identical to that of the observed photoproduct. We investigated the singlet and triplet potential energy surfaces associated with this isomerization process using density functional theory and multireference calculations. The observed rearrangement of compound 5 to compound 7 is computed to be endothermic (3.3 kcal/mol). We were unable to observe the reverse reaction (7 → 5) under the photochemical conditions.

Vertical distributions of N<sub>2</sub>O isotopocules in the equatorial stratosphere

Abstract. Vertical profiles of nitrous oxide (N2O) and its isotopocules, isotopically substituted molecules, were obtained over the Equator at altitudes of 16–30 km. Whole air samples were collected using newly developed balloon-borne compact cryogenic samplers over the eastern equatorial Pacific in 2012 and Biak Island, Indonesia, in 2015. They were examined in the laboratory using gas chromatography and mass spectrometry. The mixing ratio and isotopocule ratios of N2O in the equatorial stratosphere showed a weaker vertical gradient than the previously reported profiles in the subtropical and mid-latitude and high-latitude stratosphere. From the relation between the mixing ratio and isotopocule ratios, further distinct characteristics were found over the Equator: (1) observed isotopocule fractionations (ε values) in the middle stratosphere (25–30 km or [N2O] < ca. 260 nmol mol−1) are almost equal to ε values reported from broadband photolysis experiments conducted in the laboratory; (2) ε values in the lower stratosphere (< ca. 25 km or [N2O] > ca. 260 nmol mol−1) are about half of the experimentally obtained values, being slightly larger than those observed in the mid-latitude and high-latitude lower stratosphere ([N2O] > ca. 170 nmol mol−1). These results from the deep tropics suggest the following. (i) The timescale for quasi-horizontal mixing between tropical and mid-latitude air in the tropical middle stratosphere is sufficiently slow relative to the tropical upwelling rate that isotope fractionation approaches the Rayleigh limit for N2O photolysis. (ii) The air in the tropical lower stratosphere is exchanged with extratropical air on a timescale that is shorter than that of photochemical decomposition of N2O. Previously observed ε values, which are invariably smaller than those of photolysis, can be explained qualitatively using a three-dimensional chemical transport model and using a simple model that assumes mixing of “aged” tropical air and extratropical air during residual circulation. Results show that isotopocule ratios are useful to examine the stratospheric transport scheme deduced from tracer–tracer relations.

Untangling the methane chemistry in interstellar and solar system ices toward ionizing radiation: a combined infrared and reflectron time-of-flight analysis.

Pure methane (CH4/CD4) ices were exposed to three ionizing radiation sources at 5.5 K under ultrahigh vacuum conditions to compare the complex hydrocarbon spectrum produced across several interstellar environments. These irradiation sources consisted of energetic electrons to simulate secondary electrons formed in the track of galactic cosmic rays (GCRs), Lyman α (10.2 eV; 121.6 nm) photons simulated the internal VUV field in a dense cloud, and broadband (112.7-169.8 nm; 11.0-7.3 eV) photons which mimic the interstellar ultra-violet field. The in situ chemical evolution of the ices was monitored via Fourier transform infrared spectroscopy (FTIR) and during heating via mass spectrometry utilizing a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer with a photoionization source (PI-ReTOF-MS). The FTIR analysis detected six small hydrocarbon products from the three different irradiation sources: propane [C3H8(C3D8)], ethane [C2H6(C2D6)], the ethyl radical [C2H5(C2D5)], ethylene [C2H4(C2D4)], acetylene [C2H2(C2D2)], and the methyl radical [CH3(CD3)]. The sensitive PI-ReTOF-MS analysis identified a complex array of products with different products being detected between experiments with general formulae: CnH2n+2 (n = 4-8), CnH2n (n = 3-9), CnH2n-2 (n = 3-9), CnH2n-4 (n = 4-9), and CnH2n-6 (n = 6-7) from electron irradiation and CnH2n+2 (n = 4-8), CnH2n (n = 3-10), CnH2n-2 (n = 3-11), CnH2n-4 (n = 4-11), CnH2n-6 (n = 5-11), and CnH2n-8 (n = 6-11) from broadband photolysis and Lyman α photolysis. These experiments show that even the simplest hydrocarbon can produce important complex hydrocarbons such as C3H4 and C4H6 isomers. Distinct isomers from these groups have been shown to be important reactants in the synthesis of polycyclic aromatic hydrocarbons like indene (C9H8) and naphthalene (C10H8) under interstellar conditions.

Gas-phase and matrix-isolation photochemistry of methyl thioglycolate, CH 3 OC(O)CH 2 SH: Influence of the presence of molecular oxygen in the photochemical mechanisms

The photochemistry of methyl thioglycolate (MTG), CH3OC(O)CH2SH, in gas phase and in matrix isolation conditions was studied by means of FTIR spectroscopy, and the influence of the presence of molecular oxygen on the photochemical mechanisms was investigated. The UV–vis broad-band photolysis of MTG in gas phase originates CH3OC(O)CH3 and S8, while in matrix conditions the photoproducts are CH3OH, H2CS, and CO, interacting inside the matrix cage. The photolysis of MTG in the presence of O2 conducts to the formation of SO2, independent of the experimental conditions, being CH3OH, HC(O)OH, and CO the other photoproducts for the gas-phase photolysis, and H2CO, CH4, and CO2 for the experiments performed in Ar matrix conditions.

A Photo-Triggered Traceless Staudinger-Bertozzi Ligation Reaction.

The use of light to control the course of a chemical/biochemical reaction is an attractive idea because of its ease of administration with high precision and fine spatial resolution. Staudinger ligation is one of the commonly adopted conjugation processes that involve a spontaneous reaction between azides and arylphosphines to form iminophosphoranes, which further hydrolyze to give stable amides. We designed an anthracenylmethyl diphenylphosphinothioester (1) that showed promising Staudinger ligation reactivity upon photo-excitation. Broadband photolysis at 360-400 nm in aqueous organic solvents induced heterolytic cleavage of its anthracenylmethyl-phosphorus bond, releasing a diphenylphosphinothioester (2) as an efficient traceless Staudinger-Bertozzi ligation reagent. The quantum yield of such a photo-induced heterolytic bond-cleavage at the optimal wavelength of photolysis (376 nm) at room temperature is ≥0.07. This work demonstrated the feasibility of photocaging arylphosphines to realize the photo-triggering of the Staudinger ligation reaction.

Infrared Spectra of CH3–MH through Methane Activation by Laser-Ablated Sn, Pb, Sb, and Bi Atoms

Methane activation has been carried out by laser-ablated Sn, Pb, Sb, and Bi atoms. All four metals generate the insertion complex (CH(3)-MH), but subsequent H-migration from C to M to form CH(2)-MH(2) and CH-MH(3) complexes is not observed. Our previous and present experimental and computational results indicate that the higher oxidation state complexes become less favored with increasing atomic mass in groups 14 and 15, which is opposite the general trend found for transition metals. The C-H bond insertion evidently occurs during reaction on sample condensation, and the product dissociates on broad-band photolysis afterward. The insertion complex contains a near right angle C-M-H moiety because of high p contribution from the metal center to the C-M and M-H bonds unlike many transition-metal analogues. The computed methylidene structures for these main group metals are not agostic possibly because of the absence of valence d-orbitals.

In situ detection of atomic and molecular iodine using Resonance and Off-Resonance Fluorescence by Lamp Excitation: ROFLEX

Abstract. We demonstrate a new instrument for in situ detection of atmospheric iodine atoms and molecules based on atomic and molecular resonance and off-resonance ultraviolet fluorescence excited by lamp emission. The instrument combines the robustness, light weight, low power consumption and efficient excitation of radio-frequency discharge light sources with the high sensitivity of the photon counting technique. Calibration of I2 fluorescence is achieved via quantitative detection of the molecule by Incoherent Broad Band Cavity-enhanced Absorption Spectroscopy. Atomic iodine fluorescence signal is calibrated by controlled broad band photolysis of known I2 concentrations in the visible spectral range at atmospheric pressure. The instrument has been optimised in laboratory experiments to reach detection limits of 1.2 pptv for I atoms and 13 pptv for I2, for S/N = 1 and 10 min of integration time. The ROFLEX system has been deployed in a field campaign in northern Spain, representing the first concurrent observation of ambient mixing ratios of iodine atoms and molecules in the 1–350 pptv range.

Experimental and theoretical characterization of molecular complexes formed between OCS and XY molecules (X, Y = F, Cl and Br) and their role in photochemical matrix reactions

Molecular complexes between OCS and ClF, Cl(2), Br(2), or BrCl have for the first time been isolated in solid matrixes at low temperature, and characterized by their IR spectra. 1 : 1 adducts between OCS and ClF, Cl(2), or BrCl were formed by broad-band UV-visible photolysis of matrix-isolated FC(O)SCl, ClC(O)SCl or BrC(O)SCl, respectively. Co-deposition of gaseous mixtures of OCS and ClF or Br(2) diluted with Ar on a CsI window cooled to approximately 15 K led to the identification of 1 : 1 complexes, while similar mixtures of OCS and Cl(2) gave rise mainly to 1 : 2 species. The differences in the structures and stoichiometries of the molecular complexes may well be responsible for the different reaction channels observed for the photochemical matrix reactions between OCS and Cl(2) or Br(2). The structures, energy differences, vibrational spectra, and bonding properties of all the possible complexes formed between OCS and XY (XY = ClF, Cl(2), Br(2), or BrCl) have been studied with different theoretical approximations.

Interconversion of Nitrenes, Azirenes, and Diradicals: Rearrangement of 3-Isoquinolylnitrene to o-Cyanophenylketenimine and 1-Cyanoisoindole

Photolysis of tetrazolo[1,5-b]isoquinoline/3-azidoisoquinoline 22T/22A generates 3-isoquinolylnitrene 23, which has been characterized together with a diradical species (25) by Ar matrix ESR spectroscopy. Photolysis at lambda > 300 nm generates azirene 24, characterized by IR spectroscopy, whereas further broad-band UV photolysis destroys the azirene to produce o-cyanophenylketenimine 17. The use of 15N-labeled tetrazole/azide 22T'/22A' demonstrates rapid equilibration of two regioisomeric 15N-labeled azirenes 24' and 24'' prior to formation of 17. Flash vacuum thermolysis (FVT) of 22T/22A affords 1-cyano-2H-isoindole 27 in quantitative yield. FVT of 15N-labeled tetrazole/azide 22T'/22A' causes scrambling of 15N label in the 1-cyano-2H-isoindole product. It is concluded that the interconversion of azirenes 24 takes place via the unobserved diazacycloheptatetraene/diazacycloheptatrienylidene 32/33, and that the rearrangement of azirene to ketenimine 17 and 1-cyanoisoindole 27 takes place via reversion to nitrene 23 followed by ring opening to diradical 25.

Formation of New Halogenothiocarbonylsulfenyl Halides, XC(S)SY, through Photochemical Matrix Reactions Starting from CS2and a Dihalogen Molecule XY (XY = Cl2, Br2, or BrCl)

Isolation of a dihalogen molecule XY (XY=Cl2, Br2, or BrCl) with CS2 in a solid Ar matrix at about 15 K leads, by broad-band UV-vis photolysis (200<or=lambda<or=800 nm), to a variety of products depending on the natures of X and Y. These products have been identified on the basis of the IR spectra of the matrices. In addition to the familiar species SCCl2, ClCS*, Cl*...SCS, CCl4, *CCl3, :CCl2, SCl2, SCBr2, CBr4, *CBr3, BrC(S)Cl, BrCCl3, and :CBrCl, the following new molecules have also been identified as products of the various photoreactions: syn-ClC(S)SCl, anti-ClC(S)SCl, syn-BrC(S)SBr, anti-BrC(S)SBr, syn-ClC(S)SBr, anti-ClC(S)SBr, syn-BrC(S)SCl, anti-BrC(S)SCl, ClC(S)S*, BrCS*, and Br*...SCS. The IR spectra of these hitherto unknown species have been interpreted with reference to the predictions of ab initio (HF and MP2) and density functional theory (DFT) calculations. The results are analyzed in relation to the reaction pathways accessed by matrix photolysis.

Formation of CH3TiX, CH2TiHX, and (CH3)2TiX2 by Reaction of Methyl Chloride and Bromide with Laser-Ablated Titanium Atoms: Photoreversible α-Hydrogen Migration

The simple methylidene (CH2=TiHX) and Grignard-type (CH3TiX) complexes are produced by reaction of methyl chloride and bromide with laser-ablated Ti atoms and isolated in a solid Ar matrix, and they form a persistent photoreversible system via alpha-hydrogen migration between the carbon and titanium atoms. The Grignard-type product is transformed to the methylidene complex upon UV (240 nm < lambda < 380 nm) irradiation and vice versa with visible (lambda > 530 nm) irradiation. More stable dimethyl dihalide complexes [(CH3)2TiX2] are also identified, whose relative concentration increases upon annealing and at high methyl halide concentration. The reaction products are identified with three different groups of absorptions on the basis of the behaviors upon broadband photolysis and annealing, and the vibrational characteristics are in a good agreement with DFT computation results.

Hydrogen Elimination from Ethylene by Laser-Ablated Zr Atoms: An Infrared Spectroscopic Investigation of the Reaction Intermediates in a Solid Argon Matrix

Intermediates of the hydrogen elimination reaction of ethylene by laser-ablated Zr atoms have been isolated in an argon matrix and investigated by means of infrared spectroscopy. The strong absorptions in the region of 1500−1660 cm-1 are due to the Zr−H stretching modes of several intermediates. There are at least three different groups of product absorptions based on the behaviors upon broad-band photolysis and annealing. Among them, the insertion and dihydrido complexes, presumed in previous studies of the hydrogen elimination of ethylene by Zr atoms, are identified. In addition, the remaining Zr−H stretching absorptions, along with absorptions in the C⋮C stretching and other regions, suggest that ethynyl zirconium trihydride is also formed in the hydrogen elimination reaction. Formation of this compound is consistent with the fact that C2H2 and C2D2 as well as CHCD are produced in the reaction of Zr + CH2CD2. Density functional theory calculations for the reaction intermediates are also carried out, and the molecular properties and vibrational characteristics are compared with the experimental values.

Matrix studies of the thermal and photolytic reactions of Ga vapour species with H2O: formation and characterisation of the adducts Ga·OH2and Ga2·OH2and the photoproducts HGaOH, GaOH and Ga(μ-H)(μ-OH)Ga

IR and UV–vis spectra have been used to chart the thermal and photolytic reactions occurring during or after the codeposition of Ga vapour with an excess of H2O-doped Ar. Following the formation of the loosely bound adducts Ga⋯OH2 and Ga2⋯OH2, detected after deposition of the matrix, UV irradiation (200 ≤ λ ≤ 400 nm) causes Ga⋯OH2 to rearrange with insertion of the metal atom into an O–H bond forming the Ga(II) hydride HGaOH, while visible irradiation (λ = ca. 580 nm) causes Ga2⋯OH2 to rearrange with the formation of cyclic Ga(μ-H)(μ-OH)Ga. Broad-band photolysis (200 ≤ λ ≤ 800 nm) results in the destruction of both these products, with HGaOH decaying to GaOH. Characterisation of the various molecules by IR spectroscopy has been underpinned by reference (i) to the effects of isotopic change (1,2H, 16,18O and 69,71Ga) and (ii) to the results of density functional theory (DFT) calculations. The properties of the molecules are compared with those of related species, and the mechanisms of the photochemical changes are discussed.

Matrix Infrared Spectra and DFT Calculations of the Reactive MHx (x = 1, 2, and 3), (H2)MH2, MH2+, and MH4- (M = Sc, Y, and La) Species

Laser-ablated Sc, Y, and La react with molecular hydrogen to give MH, MH2+, MH2, MH3, and MH4- (M = Sc, Y, and La) during condensation in excess argon for characterization by matrix infrared spectroscopy. Annealing forms the dihydrogen complex (H2)MH2, which can be reduced to MH4- by electron capture. The (HD)MHD complex exchanges hydrogen positions on broadband photolysis to form primarily the (D2)MH2 complex. Doping the samples with CCl4 to capture ablated electrons markedly increases the MH2+ infrared band intensities and decreases the MH4- absorptions. Further annealing produces higher (H2)2MH2 complexes, which also exchange hydrogen positions. The reaction products are identified by deuterium and deuterium hydride isotopic substitution. DFT and MP2 theoretical calculations are employed to predict geometries and vibrational frequencies of these novel molecules, complexes, anions, and cations. Charged species from laser-ablation contribute more to the spectra of group 3 reaction products than for any o...

Neon Matrix Infrared Spectra and DFT Calculations of Tungsten Hydrides WHx (x = 1−4, 6)

Laser-ablated tungsten atoms react with molecular hydrogen upon condensation in excess neon. The tungsten hydrides WH, WH2, WH3, WH4, and WH6 are identified by isotopic substitution (D2, HD) and by density functional theory calculations of vibrational fundamentals. The WH diatomic gives a 1860.2 cm-1 band, WH2 gives a strong 1831.9 cm-1 absorption, and WH3 gives a sharp 1894.6 cm-1 peak. Absorptions due to WH4 appear at 1920.1 and 525.2 cm-1, increase on annealing in solid neon, and support a tetrahedral structure. Sharp new absorptions at 2021.2, 2004.4, 1953.8, 1927.5, 1080.3, and 840.7 cm-1 are assigned to WH6, based on deuterium isotopic shifts and comparison with frequencies computed by DFT for the distorted trigonal prism structure predicted earlier to be the global minimum energy structure for WH6. The bands of WH6 increase on annealing, decrease on broadband photolysis, and restore on further annealing. This is the first experimental identification of the WH2, WH4, and WH6 hydride molecules, and W...

Formation and Characterization of the (η2-H2)CrO2, (η2-H2)2CrO2 and HCrO(OH) Molecules

Chromium dioxide dihydrogen and bis-dihydrogen complexes: (η2-H2)CrO2 and (η2-H2)2CrO2 have been prepared by co-condensation of the CrO2 molecules generated from laser ablation of CrO3 with H2 in excess argon at 11 K. Broad-band photolysis of the (η2-H2)CrO2 complex induced a photoisomerization to produce the HCrO(OH) molecule. In addition, laser-ablated Cr atoms reacted with H2/O2 mixtures to give primarily the CrO2 and (η2-O2)CrO2 molecules, while the (η2-H2)CrO2, (η2-H2)2CrO2 and HCrO(OH) molecules were also produced on annealing and photolysis. The aforementioned species were unambiguously characterized by FTIR spectroscopy and density functional theoretical calculations.

Reactions of laser-ablated Y and La atoms with H 2O Infrared spectra and density functional calculations of the HMO, HMOH and M(OH) 2 molecules in solid argon

Reactions of laser-ablated Y and La atoms with water molecules have been studied using matrix-isolation FTIR spectroscopy and density functional calculations. The reaction products were identified based on isotope labeled experiments and density functional calculations. For both Y and La atom reactions with H 2O, the major reaction products are the HMOH, MO and M(OH) 2 (M=Y and La) molecules. In addition, HMO and HM(OH) 2 were produced on matrix sample annealing or UV broadband photolysis. Evidence is also presented for the formation of the HYOH + species. The potential energy surfaces for the Y, La+H 2O reactions have been calculated to interpret the reaction mechanisms.

Reactions of group V metal atoms with water molecules. Matrix isolation FTIR and quantum chemical studies.

Laser-ablated group V metal atoms (V, Nb, Ta) were co-deposited with water molecules in excess argon. The V atoms reacted with water to form the inserted HVOH molecule spontaneously. The Nb atoms reacted with water to form the NbOH2 complex and the inserted HNbOH molecule. Broad-band photolysis produced the H2VO and H2NbO molecules as well as the VO and NbO monoxides. For Ta + H2O reactions, neither TaOH2 nor HTaOH was observed, while the H2TaO molecule was produced on annealing, and the H2 elimination process was not observed on photolysis. The aforementioned species were identified via isotopic substitutions as well as density functional calculations. Qualitative analysis of the possible reaction paths leading to the observed products is proposed. The results have been compared with our earlier works concerning the Sc and group IV metal atoms with water reactions in order to observe existent trends for the early transition metal atoms.

Photochemistry of (η5-C5H5)(η5-C4H4N)Fe and (η5-C5H5)(η1-N-C4H4N)Fe(CO)2 in Low-Temperature Matrixes and Room-Temperature Solution. Evidence for a Photoinduced Haptotropic Shift of the π-Coordinated Pyrrolyl Ligand

Azaferrocene, (η5-C5H5)(η5-C4H4N)Fe, undergoes a η5 → η1 haptotropic shift of the pyrrolyl ligand upon long-wavelength photolysis (λexc > 495 nm) both in alkane solvents at room temperature and in frozen matrixes at 12 K. Room-temperature photolysis (λexc > 495 nm) in CO-saturated cyclohexane solution generated (η5-C5H5)(η1-N-C4H4N)Fe(CO)2. Irradiation with λexc = 532 nm also produced an allyl monocarbonyl species, exo-(η5-C5H5)(η3-C-C4H4N)Fe(CO), identified by IR spectroscopy. In CO-doped matrixes at 12 K both (η5-C5H5)(η1-N-C4H4N)Fe(CO) and (η5-C5H5)(η1-N-C4H4N)Fe(CO)2 are formed following broad-band irradiation (λexc > 495 nm) of (η5-C5H5)(η5-C4H4N)Fe, in a ratio dependent on the concentration of CO in the matrix. Initial irradiation with λexc = 538 nm followed by broad-band photolysis (λexc > 495 nm) in CO-doped matrixes formed additional monocarbonyl species, exo-(η5-C5H5)(η3-C-C4H4N)Fe(CO), and a species absorbing at 1962 cm-1, which is either the appropriate endo-isomer or aza-allyl species. Laser ...

NMR studies of the structure, isomerism and photochemical reactions of trans-[W(CO)4(η2-alkene)2] complexes

Abstract UV broad-band photolysis of W(CO)6 in the presence of propene and 1-butene has been used to synthesise the bis-alkene complexes trans-[W(CO)4(η2-CH2CHCH3)2] 1 and trans-[W(CO)4(η2-CH2CHCH2CH3)2] 2. The 1H- and 13C-NMR spectra are discussed in detail in relation to the structure and isomerism of these alkene compounds. On the evidence of the spectra measured at different temperatures, the barrier to alkene rotation is ca. 46 and 48 kJ mol−1 in 1 and 2, respectively. The NMR method has been used for the identification and characterisation of the unstable alkene=propene or 1-butene complexes [W(CO)5(η2-alkene)] and cis-[W(CO)4(η2-alkene)2] and of hydrido derivatives formed in photochemical reactions of trans-[W(CO)4(η2-alkene)2] occurring in solution at ca. 200 K.