Giuseppe Distefano

ETS study of the negative ion states of t-butyl and trimethylsilyl derivatives of ethylene and benzene

Abstract Temporary negative-ion formation in 3,3′-dimethyl-1-butene, trimethylsilylethylene, t -butylbenzene and trimethylsilylbenzene has been studied using electron transmission spectroscopy in the 0–5 eV energy range. The prevailing effect on the π molecular orbitals is stabilization arising from interaction with low-lying unoccupied orbitals of the trimethylsilyl group.

Electron attachment to the fluoro‐, bromo‐, and iodomethanes studied by means of electron transmission spectroscopy and Xα calculations

The electron transmission spectra of the fluoro‐, bromo‐, and iodomethanes (except CH2F2 ) and of some monohaloalkanes are reported. The resonance energies for electron attachment to the halomethanes have also been evaluated by means of bound and continuum multiple scattering Xα calculations, which closely reproduce the low‐energy experimental data. A large electron affinity increase is observed on going from the fluorine to the heavier halogen derivatives, while within each series of halides the electron affinity increases with increasing number of halogen atoms. The ground anion state is found to be stable in CHBr3, CBr4, and in all the iodomethanes except CH3I. The fragment anion production, as a function of the electron impact energy, has been measured for some of the compounds under investigation, in order to obtain experimental information on anion states very close to zero energy or slightly stable, which are not accessible in electron transmission.

Temporary anion states in the chloromethanes and in monochloroalkanes

The resonance energies for electron attachment to the chloromethanes are evaluated by means of bound and continuum multiple scattering Xα calculations. The results closely reproduce the experimental electron transmission spectroscopy data and confirm their previous assignment. Electron transmission and dissociative attachment spectra of monochloroalkanes are also reported, in order to obtain information on the effects of branching at the substituted carbon atom and of alkyl chain length on the resonance and chlorine anion production energies.

Influence of the oxy, aza and thio groups on the electron affinities of carbonyl compounds studied by means of ETS

Abstract The attachment energies (AEs) related to temporary electron capture into the empty π* MOs of some cyclic carbonyl and dicarbonyl derivatives have been measured by ETS. The AE values have been found to depend on the nature of the atom or group X bonded to the carbonyl group(s). Changing X from CH 2 to O and NR results in a large (1–2 eV) destabilization of appropriate MOs, whereas this effect is not observed for X = S. These findings have been discussed on the basis of hetero-atom lone-pair energy, bond distances and involvement in bonding of low-lying empty orbitals. The AE data are consistent with a large lone-pair -π co * charge-transfer interaction when X = O and NR and with a reduction of this effect and 3d-π co * mixing when X = S.

Esca study of some chromium complexes: ionization energies and multi-peak structure of the spectra

Abstract The chemical shifts of the core ionization energies of C 6 H 6 CrC 6 H 6 , C 6 H 6 Cr(CO) 3 , COOCH 3 C 6 H 5 Cr(CO) 3 and Cr(CO) 6 have been measured and information on the bonding of these complexes has been obtained. In the spectra of the complexes studied the main peaks have been frequently found to be accompanied by smaller ones, whose intensities and energies are structure dependent. In an attempt to assign these “extra” peaks, the energy separations from the main peak have been compared with the ultraviolet absorption energies of the neutral molecules.

Electron transmission spectra of selenophene and tellurophene and Xα computations of electron affinities for chalcophenes

Abstract The electron transmission spectra of selenophene and tellurophene were recorded in the 0–6 eV energy range. Two resonances associated with the addition of electrons to the 3b 1 and 2a 2 (π * ) MOs were identified. An additional resonance, which is not present in the spectra of furan and thiophene, was observed in selenophene and tellurophene. Xα calculations indicate that this additional resonance is caused by electron capture into a σ * MO with strong heteroatom “d” character. The energy trends of the anion states in the series of chalcophenes, C 4 H 4 X (X = O, S, Se, Te), are discussed in terms of d/π * mixing and of geometrical variations.

An MS Xα and ETS study of the influence of “d” orbitals on the electron affinities of thio-substituted benzenes

Abstract The electron transmission spectra of the o,o ′-dimethyl substituted benzenes 2,6-(CH 3 ) 2 C 6 H 3 -XR (X = O,S, NR: R = H, CH 3 ) show that the substituents adopt a planar (R = H) or rotated (R = CH 3 ) conformation depending upon their size and they suggest that in the sulphur derivatives the 2 B 1 H anion states are stabilized by S3d orbital participation. MS Xα calculations performed on both the planar and orthogonal forms of the model systems p -(XH) 2 -C 6 H 4 (X = O, S) support this hypothesis and assign the extra resonance present in the ET spectra of the thio derivatives to electron capture into [ σ S-R * MOS. strongly mixed with sulphur 3d orbitals ( = 33% d character).

The nature of the stabilizing effect of thio groups on the Π anion states of benzene studied by means of electron transmission spectroscopy

The ET spectra of monosubstituted benzenes containing the −OH, −OCH 3 , −OC(CH 3 ) 3 , −SH, −SCH 3 , −SC(CH 3 ) 3 and −NHCH 3 groups show that in the non-hindered nitrogen and oxygen derivatives the lone pair/π* interaction produces a significant destabilization of the ring 4b 1 π* MO, while the electron affinity (EA) of the LUMO π* 2a 2 is close to that of benzene. The stabilization of the empty π* MOs produced by the sulphur-containing substituents demonstrates the presence of interacting empty orbitals localized at the substituent. The stabilizing effect of these orbitals does not change significantly with the σ framework of the substituent nor with its torsional angle around the S-ring bond. This is consistent with the S 3d character of the stabilizing orbitals. The EA increase of the LUMO in the rotated −SC(CH 3 ) 3 derivative is mainly ascribed to the reduced interaction between the S lone-pair orbital and the 4b 1 π* MO, and, in part, to σ*/π* interaction.

Ionization energies of selenophen, tellurophen and some of their derivatives

Abstract The ionization energies of selenophen, tellurophen and a number of derivatives have been determined by the electron impact technique. Comparison of the values obtained for tellurophen with those referring to the other 5-membered rings reveals two major differences: (1) the IP of the unsubstituted tellurophen is significantly lower than that of its congeners; (2) the sensitivity to substituent effects is much smaller. The peculiar behaviour of tellurophen with respect to the other congener systems can be explained by admitting that a different molecular orbital is involved in the first ionization process of this ring. This is confirmed by photoelectron spectroscopy measurements.

Electron affinity and dissociative electron attachment in saturated dialkyl group-16 derivatives

Abstract Gas-phase temporary negative-ion formation in the series R 2 X (X = O, S, Se and Te; R = C 2 H 5 or CH 3 ) has been studied using electron-transmission spectroscopy in the 0–5 eV energy range. The ground anion state, associated with electron capture into the lowest-lying σ * orbitals of prevailing heteroatom character, steadily decreases in energy with increasing heteroatom size. The spectra of related derivatives containing SS and SeSe bonds display additional resonances ascribed to empty σ * orbitals of mainly heteroatom—heteroatom character. The dissociative attachment spectra of the compounds display abundant fragmentanion yields below 1 eV electron energy.