A.A. Dias

Thermal decomposition of methyl 2-azidopropionate studied by UV photoelectron spectroscopy and matrix isolation IR spectroscopy: heterocyclic intermediate vs imine formation.

Methyl 2-azidopropionate (N(3)CH(3)CHCOOCH(3), M2AP) has been synthesized and characterized by different spectroscopic methods, and the thermal decomposition of this molecule has been investigated by matrix isolation infrared (IR) spectroscopy and ultraviolet photoelectron spectroscopy (UVPES). Computational methods have been employed in the spectral simulation of both UVPES and matrix IR spectra and in the rationalization of the thermal decomposition results. M2AP presents a HOMO vertical ionization energy (VIE) of 9.60 ± 0.03 eV and contributions from all four lowest-energy conformations of this molecule are detected in the gas phase. Its thermal decomposition starts at ca. 400 °C and is complete at ca. 650 °C, yielding N(2), CO, CO(2), CH(3)CN, and CH(3)OH as the final decomposition products. Methyl formate (MF) and CH(4) are also found during the pyrolysis process. Analysis of the potential energy surface of the decomposition of M2AP indicates that M2AP decomposes preferentially into the corresponding imine (M2IP), through a 1,2-H shift synchronous with the N(2) elimination (Type 1 mechanism), requiring an activation energy of 160.8 kJ/mol. The imine further decomposes via two competitive routes: one accounting for CO, CH(3)OH, and CH(3)CN (ΔE(G3) = 260.2 kJ/mol) and another leading to CO(2), CH(4), and CH(3)CN (ΔE(G3) = 268.6 kJ/mol). A heterocyclic intermediate (Type 2 mechanism)-4-Me-5-oxazolidone-can also be formed from M2AP via H transfer from the remote O-CH(3) group, together with the N(2) elimination (ΔE(G3) = 260.2 kJ/mol). Finally, a third pathway which accounts for the formation of MF through an M2AP isomer is envisioned.

Measurement of the partial photoionization cross sections and asymmetry parameters of S atoms in the photon energy range 10.0-30.0 eV using constant-ionic-state spectroscopy.

The partial photoionization cross sections and asymmetry parameters of S atoms have been measured using constant-ionic-state (CIS) spectroscopy in the photon energy range 10.0-30.0 eV. The ionizations investigated in these CIS experiments are the (3p)(-1) ionizations S(+)((4)S)<--S((3)P), S(+)((2)D)<--S((3)P), and S(+)((2)P)<--S((3)P). For the first time Rydberg series which converge to the fourth ionization limit have been observed and assignments of these series have been proposed. These correspond to excitations to Rydberg states that are parts of series which converge to the fourth ionization limit, S(+)((4)P)<--S((3)P) (3s)(-1), and autoionize to the lower S(+)((4)S), S(+)((2)D), or S(+)((2)P) states. For each series observed in the CIS spectra photoelectron angular distribution studies, combined with other evidence, has allowed the angular momentum character of the free electron on autoionization to be determined.

Photoelectron spectroscopy of atomic oxygen using the Elettra synchrotron source

Atomic oxygen has been studied using angle resolved photoelectron spectroscopy (PES) and constant-ionic-state (CIS) measurements using radiation from the Elettra synchrotron as the photon source. Relative partial photoionization cross-sections and angular distributions for the O+(4S) ← O(3P) and O+(2D) ← O(3P) ionizations have been measured as a function of photon energy from threshold (13.6 eV) to 19.0 eV. Comparison of the results obtained with recent experimental work performed at lower resolution reveals a number of differences and comparison with results of recent calculations shows the need for the inclusion of coupling intermediate between the j-j and L-S limits in future calculations of photoionization cross-sections and angular distributions. This work has demonstrated the feasibility of and results to be expected from angle resolved PES and CIS measurements on reactive intermediates at Elettra, a third-generation synchrotron source, and further studies on small molecular radicals are proposed.

The Mechanism of Pyrolysis of Benzyl Azide: Spectroscopic Evidence for Benzenemethanimine Formation

We study the gas-phase pyrolysis of benzyl azide (BA, C6H5CH2N3) using ultraviolet photoelectron spectroscopy (UVPES) and matrix-isolation infrared (IR) spectroscopy, together with electronic structure calculations and Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. It is found that BA decomposes via N2 elimination at ca. 615 K, primarily yielding benzenemethaninime. Other end products include HCN and C6H6. N-Methyleneaniline is not detected, although its formation at higher temperature is foreseen by RRKM calculations.

MASS SPECTROMETRY OF ALIPHATIC AZIDES

Electron impact (EI) mass spectra of N3CH2COOH, N3CH2COCH3 and N3CH2CH2OH are reported. Molecular ions of these compounds are observed. The fragmentation mechanisms observed for each compound are discussed. Copyright

A study of the NO radical with PE and CIS spectroscopy: investigation of NO(b3 Π, 3p) and NO(b3 Π, 4p) Rydberg states

Angle resolved constant-ionic-state (CIS) and photoelectron (PE) spectra have been recorded for the NO radical from 13.2 to 30.0 eV. CIS spectra obtained for selected vibrational components of the first PE band in the photon energy range 13.5–15.7 eV are dominated by a sharp, intense structure arising from 5σ → np Rydberg resonances, which are parts of series which converge to NO+(b3Π). Spectra of the first PE band of NO were recorded at the observed resonance photon energies to study the effect on the PE vibrational envelopes and the asymmetry parameter β. The evidence obtained, supported by results of Franck–Condon simulations, has allowed assignment of the bands in the CIS spectrum associated with NO(b3Π, 3p) and NO(b3Π, 4p) resonances. This has led to a correction of some of the published assignments of these bands, which have been observed by other methods. A 2π → kσ shape resonance has also been observed centred at 14 eV and its effect on the vibrational envelope and asymmetry parameter of the first PE band has been investigated.

A Study of H2O2 with Threshold Photoelectron Spectroscopy (TPES) and Electronic Structure Calculations: Redetermination of the First Adiabatic Ionization Energy (AIE)

In this work, hydrogen peroxide has been studied with threshold photoelectron (TPE) spectroscopy and photoelectron (PE) spectroscopy. The TPE spectrum has been recorded in the 10.0-21.0 eV ionization energy region, and the PE spectrum has been recorded at 21.22 eV photon energy. Five bands have been observed which have been assigned on the basis of UCCSD(T)-F12/VQZ-F12 and IP-EOM CCSD calculations. Vibrational structure has only been resolved in the TPE spectrum of the first band, associated with the X̃(2)Bg H2O2(+) ← X̃(1)A H2O2 ionization, on its low energy side. This structure is assigned with the help of harmonic Franck-Condon factor calculations that use the UCCSD(T)-F12a/VQZ-F12 computed adiabatic ionization energy (AIE), and UCCSD(T)-F12a/VQZ-F12 computed equilibrium geometric parameters and harmonic vibrational frequencies for the H2O2 X̃(1)A state and the H2O2(+) X̃(2)Bg state. These calculations show that the main vibrational structure on the leading edge of the first TPE band is in the O-O stretching mode (ω3) and the HOOH deformation mode (ω4), and comparison of the simulated spectrum to the experimental spectrum gives the first AIE of H2O2 as (10.685 ± 0.005) eV and ω4 = (850 ± 30) and ω3 = (1340 ± 30) cm(-1) in the X̃(2)Bg state of H2O2(+). Contributions from ionization of vibrationally excited levels in the torsion mode have been identified in the TPE spectrum of the first band and the need for a vibrationally resolved TPE spectrum from vibrationally cooled molecules, as well as higher level Franck-Condon factors than performed in this work, is emphasized.

A study of the CF radical with PE and CIS spectroscopy: investigation of Rydberg states above the first ionization threshold

The CF radical has been studied with photoelectron (PE) and constant-ionic-state (CIS) spectroscopy using synchrotron radiation. By scanning the photon energy in the region between the first and second ionization onsets, while monitoring the intensity of selected vibrational components in the first photoelectron band, excitations to Rydberg states, which are part of a series which converge to the second ionization limit, were revealed. By comparing PE spectra recorded at selected resonance positions with calculated Franck–Condon PE vibrational envelopes to establish the excited state vibrational numbering, fitting the observed resonance positions to Rydberg series, and comparing the results obtained with results from a preceding paper on the isoelectronic molecule NO (1), the structure obtained could be assigned to excitation to np Rydberg states with a CF+(a3Π) core. The Rydberg series fits led to an improved adiabatic ionization energy (AIE) from CF(X2Π) to the CF+(a3Π) state of (13.942 ± 0.003) eV.