José Paulo Santos

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We present theoretical results for the photoabsorption spectrum of an atom in parallel electric and magnetic fields, using the R-matrix method combined with quantum-defect theory. We introduce a radial basis set which is complete and orthonormal over a semi-infinite interval [r0,(infinity)), to allow calculations to be performed for high Rydberg states in nonhydrogenic atoms without encountering problems due to linear dependence of the basis set. The nonhydrogenic character of the spectra is analyzed for Li and Rb, and a comparison is made with previous high-precision experiments which shows that the theoretical results agree very well with experiment.

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We present a first theoretical determination of the hyperfine coupling constants of a spherical top molecule using ab initio methods. The scalar and tensorial contributions to the spin-rotation constants and the diamagnetic and paramagnetic contributions to the nuclear shielding constant are calculated for the ³²SF₆ molecule. The corrections to the spin-rotation constants due to nuclear Thomas precession are evaluated and discussed. Our results are compared with previously reported experimental values.

We present a first theoretical determination of the hyperfine coupling constants of a spherical top molecule using <i>ab initio</i> methods. The scalar and tensorial contributions to the spin-rotation constants and the diamagnetic and paramagnetic contributions to the nuclear shielding constant are calculated for the ³²SF₆ molecule. The corrections to the spin-rotation constants due to nuclear Thomas precession are evaluated and discussed. Our results are compared with previously reported experimental values.

We derive a theoretical expression for the two-photon emission rate of two-electron systems, in a form suitable for easy implementation in numerical calculations. Racah algebra techniques were used to extended previous work on two-photon emission in hydrogen-like systems to more complex ones. The obtained expression is, as far as we are aware, the first general expression that gives the spontaneous two-photon decay rates of helium-like systems for any combination of multipoles.

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Benzyl azide and the three methylbenzyl azides were synthesized and characterized by mass spectrometry (MS) and ultraviolet photoelectron spectroscopy (UVPES). The electron ionization fragmentation mechanisms for benzyl azide and their methyl derivatives were studied by accurate mass measurements and linked scans at constant B/E. For benzyl azide, in order to clarify the fragmentation mechanism, labelling experiments were performed. From the mass analysis of methylbenzyl azides isomers it was possible to differentiate the isomers ortho, meta and para. The abundance and nature of the ions resulting from the molecular ion fragmentation, for the three distinct isomers of substituted benzyl azides, were rationalized in terms of the electronic properties of the substituent. Concerning the para-isomer, IRC calculations were performed at UHF/6-31G(d) level. The photoionization study of benzyl azide, with He(I) radiation, revealed five bands in the 8-21 eV ionization energies region. From every photoelectron spectrum of methylbenzyl azides isomers it has been identified seven bands, on the same range as the benzyl azide. Interpretation of the photoelectron spectra was accomplished applying Koopmans' theorem to the SCF orbital energies obtained at HF/6-311++G(d, p) level.

Benzyl azide and the three methylbenzyl azides were synthesized and characterized by mass spectrometry (MS) and ultraviolet photoelectron spectroscopy (UVPES). The electron ionization fragmentation mechanisms for benzyl azide and their methyl derivatives were studied by accurate mass measurements and linked scans at constant B/E. For benzyl azide, in order to clarify the fragmentation mechanism, labelling experiments were performed. From the mass analysis of methylbenzyl azides isomers it was possible to differentiate the isomers ortho, meta and para. The abundance and nature of the ions resulting from the molecular ion fragmentation, for the three distinct isomers of substituted benzyl azides, were rationalized in terms of the electronic properties of the substituent. Concerning the para-isomer, IRC calculations were performed at UHF/6-31G(d) level. The photoionization study of benzyl azide, with He(I) radiation, revealed five bands in the 8-21 eV ionization energies region. From every photoelectron spectrum of methylbenzyl azides isomers it has been identified seven bands, on the same range as the benzyl azide. Interpretation of the photoelectron spectra was accomplished applying Koopmans' theorem to the SCF orbital energies obtained at HF/6-311++G(d, p) level.

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We present a systematic study of atomic binding energies, in the Dirac–Fock approximation, for the Lithium (3 electrons) to the Dubnium (105 electrons) isoelectronic series. In each series we have considered all atomic numbers from the one corresponding to the neutral atom up to Z=118. We have obtained the ground state configurations for several heavy ions with charge larger than one.

The C 1s and N 1s photoelectron spectra of gas-phase 5-aminotetrazole (5ATZ) were recorded using synchrotron radiation, with the aim of evaluating 1H/2H tautomer population ratios. The core-electron binding energies (CEBEs) were estimated from computational results, using the delta self-consistent-field (ΔSCF) approach. Simulated spectra were generated using these CEBEs and the results from Gaussian-n (Gn, n=1, 2 and 3) and Complete Basis Set (CBS-4M and CBS-Q) methods. Results reveal the almost exclusive predominance of the 2H-tautomer, with a 1H/2H ratio of ca. 0.12/0.88, taken from a gross analysis of the XPS C 1s spectrum, recorded at 365 K.

The C 1s and N 1s photoelectron spectra of gas-phase 5-aminotetrazole (5ATZ) were recorded using synchrotron radiation, with the aim of evaluating 1H/2H tautomer population ratios. The core-electron binding energies (CEBEs) were estimated from computational results, using the delta self-consistent-field (ΔSCF) approach. Simulated spectra were generated using these CEBEs and the results from Gaussian-n (Gn, n=1, 2 and 3) and Complete Basis Set (CBS-4M and CBS-Q) methods. Results reveal the almost exclusive predominance of the 2H-tautomer, with a 1H/2H ratio of ca. 0.12/0.88, taken from a gross analysis of the XPS C 1s spectrum, recorded at 365 K.

The relative populations of the 1H- and 2H-tautomer of gas-phase 5-methyltetrazole (5MTZ) have been assessed through core-level photoelectron spectroscopy, and compared with the results obtained from Gaussian-n (Gn, n = 1, 2 and 3) and Complete Basis Set methods (CBS-4M and CBS-Q). The C 1s and N 1s core‚Äìelectron binding energies (CEBEs) for each ionization site of both tautomers have been computed using the Œîself-consistent-field (ŒîSCF) approach. The C 1s and N 1s XPS spectra, obtained at 313 K, yield a 1H/2H tautomer ratio of ca. 0.16/0.84 and 0.21/0.79, respectively.

Chemical Physics Letters, 516 (2011) 149-153. doi:10.1016/j.cplett.2011.10.001

The relative populations of the 1H- and 2H-tautomer of gas-phase 5-methyltetrazole (5MTZ) have been assessed through core-level photoelectron spectroscopy, and compared with the results obtained from Gaussian-n (Gn, n&#xa0;=&#xa0;1, 2 and 3) and Complete Basis Set methods (CBS-4M and CBS-Q). The C 1s and N 1s core–electron binding energies (CEBEs) for each ionization site of both tautomers have been computed using the Δself-consistent-field (ΔSCF) approach. The C 1s and N 1s XPS spectra, obtained at 313&#xa0;K, yield a 1H/2H tautomer ratio of ca. 0.16/0.84 and 0.21/0.79, respectively.

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Ab initio and density functional calculations have been performed to study the benzyl azide, 2-, 3- and 4-methyl benzyl azides. Several molecular properties, such as conformational equilibrium, optimal geometry, and vibrational frequencies, have been computed for these molecules. Ionisation energies were also computed.

Ab initio and density functional calculations have been performed to study the benzyl azide, 2-, 3- and 4-methyl benzyl azides. Several molecular properties, such as conformational equilibrium, optimal geometry, and vibrational frequencies, have been computed for these molecules. Ionisation energies were also computed.

An extensive conformational analysis was carried at ab initio and DFT levels of theory on two molecules - methyl 2-azidopropionate (N3CH3CHCOOCH3) and methyl 3-azidopropionate (N3CH2CH2COOCH3). In each case, the lowest energy conformers were characterized and the energy barriers between them were estimated. Ionization energies and vibrational frequencies were also computed, in order to support future spectroscopic studies with ultraviolet photoelectron spectroscopy (UVPES) and matrix isolation infrared spectroscopy (Matrix Isolation FTIR).

Ab initio calculations have been performed to study the methyl azidoformate (N3COOCH3) and the ethyl azidoformate (N3COOCH2CH3). Several molecular properties, such as conformational equilibrium, optimal geometry, and vibrational frequencies, have been computed for these molecules. Ionization energies based on Koopman's theorem were also computed.