M.L. Costa

A study of the phenyl radical by vacuum ultraviolet photoelectron spectroscopy

Abstract Part of a photoelectron band assigned to the phenyl radical, produced from the F + C 6 H 6 reaction, has been recorded. The adiabatic ionization energy was measured as 8.32±0.04 eV. Two vibrational components separated by 2790±100 cm −1 were observed in the C 6 H 5 band corresponding to excitation of a CH stretching mode in the ion. For C 6 D 5 this separation decreases to 2370±110 cm −1 . Ab initio CI calculations on the low-lying C 6 H + 5 states, 3 B 1 and 1 A 1 , show that state of C 6 H + 5 is 1 A 1 and the first adiabatic ionization energy is estimated as 8.0±0.1 eV. Evidence is presented to show that the observed photoelectron band should be assigned to the ionization of C 6 H + 5 (a 3 B 1 ) ← C 6 H 5 (X 2 A 1 ). In the F + C 6 D 6 reaction, a photoelectron band assigned to C 6 D 6 F is observed at 7.80±0.02 eV which is seen only very weakly in the F + C 6 H 6 reaction.

Toxoplasma gondii prevalence in cats from Lisbon and in pigs from centre and south of Portugal.

Toxoplasmosis is an important zoonosis worldwide. Here we determined the presence of Toxoplasma gondii antibodies in sera and T. gondii DNA in faeces of 215 domestic cats from veterinary clinics in the Lisbon area; 44 (20.5%) had anti-T. gondii IgG antibodies by the modified agglutination test (cut-off 1:40) and DNA was detected in 16 (35.6%) of 45 cat faeces tested. Risk factor analysis indicated increase of seroprevalence with age of the cats. Sera and tissues of 381 pigs from a slaughterhouse were also tested for T. gondii infection; 27 (7.1%) of the 381 pigs were seropositive. T. gondii DNA was demonstrated in diaphragms and/or brains of seven (35.0%) of 20 anti-T. gondii seropositive pigs tested by the B1 nested-PCR. Results indicate very high prevalence of T. gondii DNA in the faeces (oocysts) of definitive hosts and relatively low, but still worrying, seroprevalence of T. gondii antibodies in pigs destined for human consumption.

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.

Competing channels in the thermal decomposition of azidoacetone studied by pyrolysis in combination with molecular beam mass spectrometric techniques.

The thermal decomposition of azidoacetone (CH3COCH2N3) was studied using a combined experimental and computational approach. Flash pyrolysis at a range of temperatures (296-1250 K) was used to induce thermal decomposition, and the resulting products were expanded into a molecular beam and subsequently analyzed using electron bombardment ionization coupled to a quadrupole mass spectrometer. The advantages of this technique are that the parent molecules spend a very short time in the pyrolysis zone (20-30 mus) and that the subsequent expansion permits the stabilization of thermal products that are not observable using conventional pyrolysis methods. A detailed analysis of the mass spectra as a function of pyrolysis temperature revealed the participation of five thermal decomposition channels. Ab initio calculations on the stable structures and transition states of the azidoacetone system in combination with an analysis of the dissociative ionization pattern of each channel allowed the identity and mechanism of each channel to be elucidated. At low temperatures (296-800 K) the azide decomposes principally by the loss of N2 to yield the imine (CH3COCHNH), which can further decompose to CH3CO and CHNH. At low and intermediate temperatures a process involving the loss of N2 to yield CH3CHO and HCN is also open. Finally, at high temperatures (800-1250 K) a channel in which the azide decomposes to a stable cyclic amine (CO(CH2)2NH) (after loss of N2) is active. The last channel involves subsequent thermal decomposition of this cyclic amine to ketene (H2CCO) and methanimine (H2CNH).

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.

Matrix-isolation FTIR study of azidoacetone and azidoacetonitrile

Azidoacetonitrile (N3CH2CN) and azidoacetone (N3CH2COCH3) are studied by matrix-isolation FTIR spectroscopy in solid neon, argon, and nitrogen. The IR spectra calculated using the density-fuctional theoretical method are discussed in comparison with the experimental data. Significant broadening of the recorded azide bands indicate an awkward fit of these compounds into the solid environment. The strongest absorption is observed for both compounds in the regions of asymmetric and symmetric stretches of the N3 azide group. Strong band splittings in the N3 asymmetric stretch region can be most likely explained by very strong Fermi resonances with the CN stretch and combinations and overtones of the numerous lower-frequency vibrational modes.