Thermal decomposition of allantoin as probed by matrix isolation FTIR spectroscopy

The optimized geometries, energies of the possible conformers of allantoin (2,5-dioxo-4-imidazolidinyl urea, the diureide of glyoxylic acid) as well as the barriers for conformational interconversion have been calculated using the density functional theory [DFT(B3LYP)/6-311++G(d,p)] method. The calculations predicted the existence of four conformers (gC, tT, g′C, and g′T; where the first and second symbols in the name of the conformers designate the conformation around the exocyclic NHC-NHCO and CNH-CO axes, respectively), with the gC form contributing to more than 98% of the population in gas phase at room temperature. This conformer is different from that corresponding to the monomeric unit found in crystalline RS-allantoin (g′C; Mootz, D. Acta Crystallogr.1965, 19, 726), stressing the importance of intermolecular H-bonding in determining the structure of the crystal. Upon sublimation under vacuum (10⁻⁶ mbar), the compound was found to undergo extensive decomposition to urea, isocyanic acid, NH₃, and carbon. The identification of the decomposition products was made by using matrix isolation infrared spectroscopy. In consonance with the theoretical predictions, the allantoin molecules surviving thermal decomposition were found to undergo conformational isomerization and be present in the cryogenic argon matrix in both the gC and g′C conformations. The solid state room temperature infrared spectrum of allantoin was also investigated and assigned.     

Electronic structure and thermal decomposition of 5-methyltetrazole studied by UV photoelectron spectroscopy and theoretical calculations

The electronic properties and thermal decomposition of 5-methyltetrazole (5MTZ) are investigated using UV photoelectron spectroscopy (UVPES) and theoretical calculations. Simulated spectra of both 1H- and 2H-5MTZ, based on electron propagator methods, are produced in order to study the relative tautomer population. The thermal decomposition results are rationalized in terms of G2(MP2) results. 5MTZ yields a HOMO ionization energy of 10.82 ± 0.04 eV and the gas-phase 5MTZ assumes predominantly the 2H-form. Its gas-phase thermal decomposition starts at ca. 195 °C and leads to the formation of N₂,CH₃CN and HCN. N₂ is formed from two competing routes, involving 150.2 and 126.2 kJ/mol energy barriers, from 2H- and 1H-5MTZ, respectively. CH₃CN is formed also from two competing pathways, requiring activation energies of 218.3 (2H-5MTZ) and 198.6 kJ/mol (1H-5MTZ). Conclusions are also drawn in order to explain the formation of HCN from secondary reactions in the thermal decomposition process.     

Thermal decomposition of gaseous phenylnitromethane - product identification by photoelectron spectroscopy

Photoelectron spectroscopy has been employed to study the thermal decomposition of phenylnitromethane in the gas phase. The results indicate at least two processes for the decomposition. Thus at low temperatures (< ca. 400°C) water and benzonitrile are produced, while at high temperatures (> ca. 400°C) water, nitric oxide and benzaldehyde are formed, together with a small amount of carbon monoxide. Possible pathways of reactions are discussed. The thermal decomposition of nitroethane has also been investigated.     

Electronic structure of some penta-atomic heterocyclic molecules studied by gas-phase HeI and HeII photoelectron spectroscopy

Implications on the electronic structure of changing substituents in a series of four penta-atomic heterocyclic compounds, 2,4-thiazolidinedione, rhodanine, pseudothiohydantoin, thiohydantoin, are investigated by gas-phase u.v. photoelectron spectroscopy. Both HeI and HeII spectra are reported and discussed also on the basis of CNDO/2 calculations. The most important interactions occurring in these molecules are the through-space ones between the π orbitals of functional groups in β position. The HOMO in all the molecules has significant contribution from S 3p orbitals.     

Rovibrational photoionization dynamics of methyl and its isotopomers studied by high-resolution photoionization and photoelectron spectroscopy

High-resolution photoionization and pulsed-field-ionization zero-kinetic-energy photoelectron spectra of CH(3), CH(2)D, CHD(2), and CD(3) have been recorded in the vicinity of the first adiabatic ionization threshold following single-photon excitation from the ground neutral state using a narrow-bandwidth vacuum-ultraviolet laser. The radicals were produced from the precursor molecules methyl-bromide, methyl-iodide, dimethyl-thioether, acetone, and nitromethane by 193 nm excimer photolysis in a quartz capillary and were subsequently cooled to a rotational temperature T(rot) approximately equal to 30 K in a supersonic expansion. Nitromethane was identified as a particularly suitable photolytic precursor of methyl for studies by photoionization and threshold photoelectron spectroscopy. Thanks to the cold rotational temperature reached in the supersonic expansion, the rotational structure of the threshold ionization spectra could be resolved, and the photoionization dynamics investigated. Rydberg series converging on excited rotational levels of CH(3) (+) could be observed in the range of principal quantum number n=30-50, and both rotational autoionization and predissociation were identified as decay processes in the threshold region. The observed photoionization transitions can be understood in the realm of an orbital model for direct ionization but the intensity distributions can only be fully accounted for if the rotational channel interactions mediated by the quadrupole of the cation are considered. Improved values of the adiabatic ionization thresholds were derived for all isotopomers [CH(3): 79 356.2(15) cm(-1), CH(2)D: 79 338.8(15) cm(-1), CHD(2): 79 319.1(15) cm(-1), and CD(3): 79 296.4(15) cm(-1)].     

Thermal decomposition of methyl nitrite in shock waves studied by laser probing

The unimolecular decomposition of methyl nitrite in the temperature range 680–955 K and pressure range 0.64 to 2.0 atm has been studied in shock-tube experiments employing real-time absorption of CW CO laser radiation by the NO product. Computer kinetic modeling using a set of 23 reactions shows that NO product is relatively unreactive. Its initial rate of production can be used to yield directly the unimolecular rate constant, which in the fall-off region, can be represented by the second-order rate coefficient in the Arrhenius form: A RRKM model calculation, assuming a loose CH₃ONO^≠ complex with two degrees of free internal rotation, gives good agreement with the experimental rate constants.     

Molecular complexes of the hydrogen halides studied by matrix isolation infrared spectroscopy

Spectroscopic studies of base—hydrogen halide complexes are reviewed, including previously unpublished data for complexes of hydrogen chloride and hydrogen bromide with a variety of bases in argon matrices. The variation of the HX stretching relative frequency shift with the hydrogen halide and with the medium (gas phase, argon matrix or nitrogen matrix) and correlations of the HX stretching and hydrogen bond bending frequencies with the proton affinity of the base and with the hydrogen bond stretching force constant or dissociation energy of the complex are discussed.     

Contrasting behavior in azide pyrolyses: an investigation of the thermal decompositions of methyl azidoformate, ethyl azidoformate and 2-azido-N, N-dimethylacetamide by ultraviolet photoelectron spectroscopy and matrix isolation infrared spectroscopy

The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCH=NH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations.     

Characterization of rotameric mixtures in o- and m-substituted benzaldehydes by matrix isolation IR spectroscopy

The cis and trans conformers of benzaldehydes substituted in the o- or m-positions by Cl or CN can be differentiated by IR spectroscopy in Argon matrices. Partial photochemical rotamerization allows assignment of the IR bands and establishment of their relative intensities in pairs of rotamers, which permits, in turn, quantitation of equilibrium compositions prior to photolysis. The observed spectra and equilibrium compositions are in very good agreement with the predictions of vibrational spectra and free energy differences from B3LYP/6-31G calculations. The present work represents the first attempt to quantitate the small contributions that the cis rotamers make to the room-temperature equilibrium compositions of the two o-substituted benzaldehydes. The results for the m-substituted derivatives are compared to earlier estimates based on other methods.     

Low temperature adsorption of CO on plycrystalline molybdenum,studied by X-ray and vacuum UV photoelectron spectroscopy

The adsorption of CO on a polycrystalline Mo film at ～80°K has been studied by X-ray and UV photoelectron spectroscopy (XPS and UPS). Two adsorption states were revealed by XPS, the ratio of the O(1s) intensities from these two states was about 4 : 1 at saturation coverage (P_CO ～ 10^?6 torr). Broad resonances corresponding to the MO's of CO(ads) are observed in the UPS spectrum. On warming to room temperature chemical shifts of about 1.2 and 1.9 eV to lower binding energies were observed for the O(1s) and C(1s) signals of the major CO(ads) component. The minor state desorbed as anticipated from previous adsorption studies. The XPS spectrum observed at room temperature was virtually identical to that previously reported for CO adsorbed at room temperature. On warming the ‘orbital’ of adsorbed CO revealed by UPS is replaced by a structure apparently characteristic of carbon and oxygen atoms. These results suggest that low temperature adsorption takes place predominantly into a molecular state which on warming to room temperature dissociates.     

Donor-acceptor properties of isonitriles studied by photoelectron spectroscopy and electron transmission spectroscopy

Some alkyl and aryl isonitriles, considered as CO analogue sigma-donor and pi-acceptor ligands in transition metal chemistry, were studied by means of HeI photoelectron spectroscopy and electron transmission spectroscopy, in order to evaluate their donor-acceptor properties from the measured ionization energies (IE) and vertical electron attachment energies (VAE). The investigated molecules were 2-propyl, 1-butyl, tert-butyl, 1-pentyl, cyclohexyl, 2,6-dimethylphenyl, 4-methoxyphenyl and 4-chorophenyl isonitrile. By interpreting the spectra on the basis of literature data and quantum chemical calculations, the spectral features associated with the molecular orbitals mainly involved in coordination and back-donation were identified. The results show that the IE (10.62-10.95 eV) of the sigma electron pair (n(c)) responsible for the sigma-donor capability is substantially lower than that of CO. The VAEs of the empty pi* orbitals involved in the d/pi* back-donation indicate that aryl isonitriles are better acceptors (VAE <0.3 eV) than their aliphatic counterparts (VAE >2.7 eV). In the case of aryl derivatives, the pi-donor ability could also play some role in metal-ligand bonding (IE 8.74-9.34 eV). Isonitrile coordination characteristics are also compared with those of CO, N(2) and CH(3)CN.     

Intermediates of photolysis of 1,3,2,4-benzodithiadiazines studied by matrix isolation spectroscopy and quantum chemistry

Photolysis of 12-electron, formally antiaromatic, 1,3,2,4-benzodithiadiazine (1a) and its tetrafluoro-substituted derivative (1b) was studied in glassy matrices at 77 K and in an argon matrix at 14 K by UV, IR, and ESR spectroscopy. In these matrices, a diamagnetic species (3a,b) is formed as a persistent species. It is product of the intramolecular transformations of heterocycles 1a,b and the precursor of 1,2,3-benzodithiazolyl radicals (2a,b). The nitrenoid structure, intermediate between the structures of singlet 1,2,3-benzodithiazol-2-ylnitrene RS—N: and the corresponding thiazyl RSN, was ascribed to intermediate 3a,b in agreement with the data of IR spectroscopy.     

Thermal oxidation of poly(1-trimethylsilylprop-1-yne) studied by IR spectroscopy

Thermal oxidation of poly(1-trimethylsilylprop-1-yne) was studied by IR spectroscopy in the 20—245 °C temperature interval. In the 20—160 °C temperature range, the reaction proceeds predominantly at the C—Me group as revealed by the decrease in the intensity of the bands of the methyl group bound to the C atom and the appearance of the bands of the hydroperoxide and methylene groups. The decomposition of hydroperoxides produces aldehydes and ethers. At 160—200 °C, oxidation occurs via two routes: at the C—Me and C=C groups, while the Me₃Si group remains unchanged. At 230—240 °C, the rate of the reaction occurring at the C=C bond is higher than the rates of the processes involving the MeC and Me₃Si groups. The relative content of the structural units was calculated for the samples oxidized at different temperatures. Plausible mechanisms of thermal oxidation of poly(1-trimethylsilylprop-1-yne) were considered on the basis of the data obtained.     

Identification of the products of partial hydrolysis of silicon tetrafluoride by matrix isolation IR spectroscopy

Matrix isolation Fourier transform IR spectroscopy has been used for studying the products and mechanism of the silicon tetrafluoride reaction with water at various component ratios and reaction durations. Assignment of new bands in the spectrum confirms the earlier assumptions of first the formation of a molecular complex with water and later of trifluorosilanol, which finally condenses to give hexafluorodisiloxane.     

Thermolysis of fluorinated single-walled carbon nanotubes: identification of gaseous decomposition products by matrix isolation infrared spectroscopy

The thermal decomposition of fluorinated single-walled carbon nanotubes (F-SWNTs), known to result in pristine SWNTs, has been investigated by freezing the gaseous products formed at temperatures between 50 and 500 degrees C under high vacuum in an argon matrix at 10-20 K and analyzing the trapped species by IR spectroscopy. The major products of F-SWNT decomposition are carbonyl fluoride (COF2) below 300 degrees C and CF4 above 300 degrees C. For comparison, graphite fluoride is stable thermally up to 300 degrees C under these conditions, and the major gas-phase species at temperatures below 500 degrees C are CF4 and the CF3 radical. F-SWNTs are thermally less stable than graphite fluoride, and etching of the nanotubes is observed at lower thermolysis temperatures.     

Photo-oxidation and ozonization of poly(3-hexylthiophene) thin films as studied by UV/VIS and photoelectron spectroscopy

The kinetics and the mechanisms of degradation of thin P3HT layers have been investigated quantitatively for ozonization and photo-oxidation. Both, decay kinetics and product evolution of the polymer degradation are monitored by in situ UV/VIS and X-ray photoelectron spectroscopy (XPS). The degradation pathways of ozonization and photo-oxidation of P3HT turn out to be significantly different. Ozone attacks the thiophene units mainly by direct addition to the double bonds, leading to the loss of UV/VIS absorption, while the aliphatic side chains [1] are hardly affected. During photo-oxidation, the polymer is primarily attacked at the alkyl side chain which leads to the formation of reactive peroxide species. These subsequently cause the oxidation of sulfur and concomitantly the destruction of the thiophene ring, resulting in the loss of absorption. From the kinetics of the blue shift of the optical absorption it is concluded that the polymer is mainly attacked at the terminal thiophene rings the case of photo-oxidation whereas ozone attacks positions more or less randomly distributed along the chain. The rate of photo-oxidation under AM 1.5 conditions is at least one order of magnitude faster than the decomposition of P3HT by ozone.     

Autoionization and neutral dissociation of superexcited HI studied by two-dimensional photoelectron spectroscopy

Two-dimensional photoelectron spectroscopy of hydrogen iodide (HI) has been performed in the photon energy region of 11.10-14.85 eV, in order to investigate dynamical properties on autoionization and neutral dissociation of Rydberg states HI*(RA) converging to HI+(A 2Sigma1/2(+)). A two-dimensional photoelectron spectrum exhibits strong vibrational excitation of HI+(X 2Pi) over a photon energy region from approximately 12 to 13.7 eV, which is attributable to the autoionizing feature of the 5 dpi HI*(RA) state. A noticeable set of stripes in the photon energy region of 13.5-14.5 eV is assigned as resulting from autoionization of the atomic Rydberg states of I* converging to I+ (3P0 or 3P1). The formation of I* is understood in terms of predissociation of multiple HI*(RA) states by way of the repulsive Rydberg potential curves converging to HI+(4Pi1/2).     

3s Rydberg and cationic States of pyrazine studied by photoelectron spectroscopy

We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-1) cationic states of pyrazine [1,4-diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D0 and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0 and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1-D0 and 3s(pi-1)-3s(n-1) conical intersections were estimated. From the line width in the D1 <-- S0 spectrum, we estimated the lifetime of D1 to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(pi-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1 observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.     

Kinetic study of light-induced polymerization by real-time UV and IR spectroscopy

Real time ultraviolet (RTUV) spectroscopy was used to study the photolysis kinetics of a radical-type morpholino initiator, during the polymerization of a multiacrylate monomer exposed to UV radiation in bulk, in solution, in a polyurethane-acrylate resin, and in a poly(methyl methacrylate) matrix. The photolysis rate constant k was determined from the exponential loss profile recorded; it was found to vary between 0.1 and 3s^?1, depending on the light intensity and on the monomer concentration. The quenching of the photoinitiator excited states by the acrylate monomer was shown to be an important deactivation pathway which substantially reduces the rate of initiation. The observed influence of the film thickness and photoinitiator concentration on the k value were accounted for by the internal filter effect. Conversion versus time curves were recorded by real time infrared (RTIR) spectroscopy for the various systems examined, thus allowing a direct comparison of both the actual polymerization rate and the residual unsaturation content of the cured polymer. Various factors were shown to be responsible for the early stop of the polymerization, such as depletion of the photoinitiator, O₂ inhibition, or vitrification of the polymer. The photoinitiated cationic ring-opening polymerization of a cycloaliphatic diepoxy monomer was also studied in real time by RTUV and RTIR spectroscopy. Despite a very fast photolysis of the triarylsulphonium initiator, the polymerization of the epoxy monomer developed less rapidly than for the acrylic monomer, with shorter kinetic chain lengths. A linear relationship was found to exist between the decay rate constant and the light intensity, for both the radical and the cationic photoinitiators, as expected for a direct photolysis process.     

The Ni + O2 reaction: a combined IR matrix isolation and theoretical study of the formation and structure of NiO2

The reaction of Ni atoms with molecular oxygen has been reinvestigated experimentally in neon matrices and theoretically at the DFT PW91PW91/6311G(3df) level. Experimental results show that i) the nature of the ground electronic state of the superoxide metastable product is the same in neon and argon matrices, ii) two different photochemical pathways exist for the conversion of the superoxide to the dioxide ground state (involving 1.6 or 4 eV photons) and iii) an important matrix effect exists in the Ni + O(2)--> Ni(O(2)) or ONiO branching ratios. Theoretical results confirm that the electronic ground state of the metastable superoxide corresponds to the singlet state, in agreement with former CCSD(T) calculations, but in contradiction with other recent works. Our results show that the ground electronic state of the dioxide is (1)Sigma(+)(g) with the lowest triplet and quintet states at slightly higher energy, consistent with the observation of weak vibronic transitions in the near infrared. The potential energy profiles are modelled for the ground state and nine electronic excited states and a pathway for the Ni(triplet) + O(2)(triplet) --> Ni(O(2)) or ONiO (singlet) reaction is proposed, as well as for the Ni(O(2)) --> ONiO photochemical reaction, accounting for the experimental observations.