László Szepes

Ultraviolet photoelectron spectra of group IV hexamethyl derivatives containing a metal-metal bond

Abstract The He(I) photoelectron spectra of some organometallic compounds of general formula (CH 3 ) 3 M—M′(CH 3 ) 3 (M  M′ C, Si, Ge, and Sn) are presented and assigned by comparison with those of simple related molecules and with the aid of CNDO/2 calculations. The HOMO is highly localized at the central metal—metal bond, and its energy is linearly related to the M′—M+ ionic bond dissociation energy.

Binding Energies and Isomerization in Metallocene Ions from Threshold Photoelectron Photoion Coincidence Spectroscopy

Metallocene ions (Cp(2)M(+), M = Cr, Co, Ni) were studied by threshold photoelectron photoion coincidence spectroscopy (TPEPICO) to investigate the mechanism, energetics, and kinetics of the ionic dissociation processes. The examined energy-selected Cp(2)M(+) ions fragment by losing the neutral cyclopentadienyl ligand. In addition, CH and C(2)H(2) losses appear as minor channels, while the cobaltocene ion also loses an H atom. A possible isomerization pathway has also been observed for Cp(2)Ni(+), yielding a complex with pentafulvalene (C(10)H(8)) with a loss of H(2). In order to determine the 0 K appearance energies for the CpM(+) fragment ions, the asymmetric time-of-flight peak shapes and the breakdown diagrams of the energy-selected metallocene ions were modeled by both the rigid activated complex (RAC) Rice-Ramsperger-Kassel-Marcus (RRKM) theory and the simplified statistical adiabatic channel model (SSACM). The following appearance energies were obtained with SSACM, which is more reliable for loose transition states: 10.57 ± 0.14, 11.01 ± 0.13, and 10.18 ± 0.13 eV for M = Cr, Co, and Ni, respectively. These values combined with the corresponding adiabatic ionization energies yield M-Cp bond dissociation energies in Cp(2)M(+) ions of 5.04 ± 0.16, 5.77 ± 0.15, and 3.96 ± 0.15 eV. Density functional calculations at the B3LYP/6-311G(d,p) level of theory were used to determine the structures of these complexes and to provide parameters necessary for the analysis of the experimental data. The trends in the M-Cp bond energies can be related to the electronic structures of the metallocene ions based on a simple molecular orbital picture.

ARENE-TRANSITION METAL COMPLEXES AS PRECURSORS OF HARD COATINGS PREPARED BY THE CHEMICAL VAPOUR DEPOSITION TECHNIQUE

Abstract Chemical aspects of hard coating preparation by chemical vapour deposition were studied in the temperature range 300–600°C using bis(arene)chromium and (cyclopentadienyl)zirconium or hafnium tetrahydroborate compounds as precursors. Chromium carbide was deposited from bis(benzene)chromium (I), bis(toluene)chromium (II) and bis(p-xylene)chromium (III); another set of experiments, thin films of zirconium carbide-boride as well as zirconium and hafnium carbide from (cyclopentadienyl)zirconium tris(tetrahydroborate) (IV) bis(cyclopentadienyl)zirconium bis(tetrahydroborate) (V) and bis(cyclopentadienyl)hafnium bis(tetrahydroborate) (VI) were prepared. In both cases mass spectrometry fragmentation patterns have been invoked to rationalize trends in the deposition temperature and chemical composition of the obtained thin films.

Photoelectron He(I) Spectra of small silanes

Abstract The photoelectron He(I) spectra of methyl-, dimethyl-, trimethyl- and ethylsiliane are reported and assigned with the help of CNDO/2 calculations. For each silane the ordering of the valence MOs obtained from calculations is, very close to that of the corresponding alkane, and is in agreement with spectral evidence. Participation of silicon d orbitals in bonding is not substantial but is more important in the outermost SiC and SiH MOs than in the π MOs mainly localized on the alkyl groups.

GEOMETRY AND ELECTRONIC STRUCTURE OF BIS(TETRAHYDRIDOBORATO)BIS(CYCLOPENTADIENYL)ZIRCONIUM(IV)

Abstract The geometry and the electronic structure of the title compound was investigated by UV photoelectron spectroscopy, X-ray diffraction and ab initio quantum-chemical methods. The photoelectron spectrum was recorded both at He(I) and He(II) photon energies, which was assigned on the basis of the quantum-chemical calculations and comparison with analogous complexes. Full geometry optimization was performed at the Hartree–Fock and at the second-order many-body perturbation levels of theory. The obtained geometry is in good agreement with the anticipated bidentate ligation of the BH4 groups. The measured Zr–B distance from the X-ray studies provides further evidence for the above mentioned bonding mode of the tetrahydridoborato ligands. The ionization energies of the complex were calculated using the outer valence Green’s function method.

Electron attachment to the mixed dimers (CH3)3M–M′(CH3)3, with M,M′=Si, Ge, Sn, and correlation with the calculated σ∗ virtual orbital energies

Abstract The energies of vertical electron attachment to the permethylated group 14 dimers (CH3)3M–M′(CH3)3, with M,M′=Si, Ge, Sn, were measured by means of electron transmission spectroscopy. The tin derivatives show the best electron-acceptor properties. The use of Koopmans’ theorem calculations for prediction of the energies of σ ∗ anion states was investigated. The LUMO energies from HF and B3LYP calculations were found to correlate linearly with the experimental energies of the first resonance observed in the present mixed dimers, the homonuclear analogues and the tetramethyl monomers.

He(I)/He(II) ultraviolet photoelectron spectroscopic studies on organosulfur compounds with emphasis on the sulfur–oxygen interaction

Abstract The electronic structure of compounds having the possibility of exhibiting intramolecular sulfur–oxygen non-bonded interaction of the S(II, IV or VI)⋯O(carbonyl or nitro) type and a hypervalent sulfur atom of the O–S–O type — i.e, R–S–Me, R–SO–Me, R–SO2–Me (R=2-nitrophenyl; 2, 7 and 12), R–S–R, R–SO–R, R–SO2–R (R=2-methoxycarbonylphenyl; 5, 10 and 15) and spirosulfurane (16) — have been investigated by the He(I) and He(II) ultraviolet photoelectron spectroscopic method. Reference compounds with R=phenyl or 4-nitrophenyl group (1, 3, 4, 6, 8, 9, 11, 13 and 14) were also studied for the sake of completeness and uniformity. On the basis of the photoelectron spectra and ab initio Hartree–Fock calculations, the sulfur–oxygen non-bonded interaction in compounds 2, 7, 12 and 5, 10, 15 can be attributed to electrostatic interaction. The sulfur–oxygen bond in the sulfoxides and sulfones, which is usually considered as a polarized double bond, seems to be a single bond with zwitterionic character. The photoelectron spectra of spirosulfurane (16) show no similarity to those of the symmetrical disubstituted sulfide, sulfoxide and sulfone (5, 10 and 15).

Photoelectron spectroscopy study of the triphenyl derivatives of the group V elements

Abstract The photoelectron He(I) spectra of several phenyl derivatives of the Group IV elements are reported, and the first few bands are assigned to the corresponding MOs. A sizable interaction among the π orbitals of the rings has been found for the carbon derivatives HCPh 3 , HC(mesityl) 3 and H 2 CPh 2 , but not for the Si, Ge and Sn triphenyl derivatives. The exceptional behaviour of the carbon compounds has been attributed to the short central atom—ring distance, by analogy with findings for the Group V triphenyl derivatives. The charge transfer from the rings towards the Si atom is substantially reduced compared with that in H 3 SiPh.

Rotational isomerism in tetramethyldistibane studied by UV photoelectron spectroscopy

Abstract He(I) photoelectron spectroscopic (PE) measurements and ab initio calculations were performed on tetramethyldistibane. The spectroscopic measurements gave clear evidence for the existence of gauche and anti conformers in the gas phase. The composition of the rotameric mixture is 12% gauche and 88% anti conformers, estimated on the basis of the corresponding PE band areas. The Hartree-Fock calculations were performed using both effective core potential and all-electron 3–21 Gaussian basis sets. Full geometry optimizations were applied, resulting in theoretical evidence for the existence of the two rotamers. The calculated ionization energies seem to necessitate further studies on the assignment of the PE bands related to the antimony lone pairs.

HE(I) PHOTOELECTRON SPECTROSCOPIC STUDY OF HALF- AND BENT SANDWICH TETRAHYDROBORATES

Abstract The He(I) ultraviolet photoelectron spectra of CpZr(BH 4 ) 3 , Cp 2 Zr(BH 4 ) 2 , Cp 2 Hf(BH 4 ) 2 , Cp 2 Ta(BH 4 ) and Cp 2 TaH 3 are reported. A tentative assignment is given for the low energy part of the spectra. At higher energies, cyclopentadienide ionizations merge with the borohydride ionizations resulting in a complicated overlapped band structure. As a consequence no photoelectron spectroscopic evidence can be given concerning the ligation of the tetrahydrborate groups.