Marco V. Andreocci

UV photoelectron spectra and pseudopotential ab initio calculations of (CH3)3PbCCCH3

Abstract Gas-phase UV photoelectron spectroscopy (UPS) and pseudopotential ab initio calculations were used to analyse the electronic structure of (CH3)3PbCCCH3, with particular interest in the possible σ-π interaction between the π(CC) system and the σ(PbC) bonds. The results obtained, compared with the UPS data already known for the tin analogue, and with the pseudopotential calculations performed during the present work for both compounds, showed that the bonding is similar in the two molecules. In particular the σ-π interaction is quite weak and the HOMO has mainly π(CC) character. The present interpretation is supported by the analysis of the NMR results.

Ultraviolet photoelectron spectroscopic investigation of electronic structure of some organic thio- and isothio-cyanates and their selenium analogues

He(I) photoelectron spectra of CH3SCN and CH3SeCN exhibit a series of five ionization bands < 15 eV, the first four being assigned to a sequence of n, π and σ orbitals of the S(Se)CN group on the grounds both of empirical correlations with simpler molecules, and of semi-empirical and ab initio quantum mechanical calculations. Theoretical calculations are reported and discussed also for the isomeric CH3NCS molecule, already reported in the literature, but re-examined here for comparison. In phenyl-substituted thio- and isothio-cyanates and their selenium analogues, photoelectron spectra are dominated by the presence of strong benzenoid bands, yet some general trends related to structural effects can be recognized.

Electronic structure of tetrahedral tin acetylides by pseudopotential ab initio calculation and gas-phase UV photoelectron spectroscopy

Abstract The electronic structure of highly symmetric tin acetylides was investigated by means of ab initio pseudopotential calculations and gas-phase UV photoelectron spectroscopy. Good agreement between calculated energy levels and experimental ionization energies was found, which allowed to the assignment of the outermost molecular orbitals.

Electronic structure of alkyl-lead(IV) acetylides by gas-phase UV photoelectron spectroscopy and pseudopotential calculations

Gas-phase UV photoelectron spectra and pseudopotential calculations were used to elucidate the electronic structure of a series of alkyl lead acetylides, a trimethylsilyl-trimethylplumbyl-acetylide and bis(trimethylsilyl)butadiyne. The analysis of the results obtained, along with the comparison with previous data on tin analogues, confirmed the presence of a ?/? conjugation between the ?C?C and the ? orbitals of the metal-alkyl moieties. The ionization of the 5d pseudo-valence orbitals of Pb was observed and compared with the calculated net atomic charge on Pb. A spin-orbit effect was found in the ionization of valence MOs with significant contribution of 6p Pb orbitals.

Theoretical study of the most stable conformations of NN′-dimethylthiourea, NNN′-trimethylthiourea, and the methyl ester of dithiocarbazic acid

CNDO/2 Calculations on NN′-dimethylthiourea, NNN′-trimethylthiourea, and the methyl ester of dithiocarbazic acid are reported. The results are in satisfactory agreement with experimental data (n.m.r. spectra) on the equilibrium conformations found in solutions of these systems.

UV Photoelectron Spectra and Pseudopotential “ab initio” Calculations of Some 4-Membered Cyclic Amides of Group XIV Elements

Gas-phase He I and He II photoelectron spectroscopy and Pseudopotential “ab initio” calculations were used to determine the electronic structure of some 4-membered cyclic amides containing Si, Sn and Pb. The IE splitting on the non-bonding nitrogen-localized m .o .s ., nNasym(a2) and nNsym(b2), due to the “through space” interaction is critically affected by the planar ring molecular structure and the coordination of the silicon and tin atoms of the ring. The pseudopotential “ab initio” model resulted successful in describing the electronic structure of the molecules containing heavy atoms, at a Koopmans’ approximation level.

He(I) photoelectron spectra of tetracarbonyliron complexes of Group 5 ligands and of olefinic ligands

The photoelectron spectra of some five-co-ordinate trigonal-bipyramidal complexes [Fe(CO)4L](L = PMe3, PPh3, AsPh3, or pyridine in an axial position, or CH2CH–CN and CH2CH–CHO in the equatorial plane) have been measured. The spectra have been assigned by comparing them with one another and with those available for [Fe(CO)5] and [Fe(CO)4(η-C2H4)]. CNDO calculations carried out for [Fe(CO)4(py)] and [Fe(CO)4(η2-CH2CH–CN)] are of only limited help in making assignments. The substituent effect on the stability of the Fe-L bond is discussed. The stability order obtained is the same as that found previously via kinetic studies.

Valence-shell photoionization spectra of some methyl dihalogenophosphates

He(I) photoelectron spectra of PX2O(OMe)(X = F, Cl, or Br), PCl2O(OEt), and PCl2(OMe)S are reported, and the peaks are assigned on the grounds of systematic comparisons throughout the investigated series, with the parent POX3(PSX3) compounds, and with results from CNDO/2 calculations. Twelve out of the 19 (for the methyl compounds) valence orbitals can thus be identified, with characterizing contributions from specific bond orbitals or groups of bond orbitals, including (in approximate order of decreasing orbital energy)πPO or πPS > πX∼πOMe∼σCH(higher levels)∼σPOOζσPS > σPX > σCH(lower levels)∼σOC. Evidence is given for the inductive effect of halogens, and for some splitting and mixing effects related to the low symmetry of the investi-gated molecules.