Maurizio Guerra

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.

Experimental and theoretical study of intense shake-up structures in the XPS spectra of nitrobenzenes and nitrosobenzenes

Abstract The N 1s and O 1s energy regions of the XPS spectra of para-substituted nitrobenzenes and nitrobenzenes [ p -X-C 6 H 4 -NO 2 : X = H, OH, OCH 3 , NH 2 and N(CH 3 ) 2 ; p -X-C 6 H 4 -NO: X = H, Cl, OCH 3 and N(CH 3 ) 2 ] present, close to the main lines, one (NO 2 -derivatives) or two (NO-derivatives) shake-up peaks whose intensity increases steadily with the electron-releasing ability of the substituent. All main energy and intensity trends experimentally observed are correctly reproduced by CNDO/S-Cl calculations, both within given compounds and along the two series. The origin of the satellites and the differences noted between the two series are discussed. The present experimental and theoretical results are in complete agreement with our qualitative explanation of the multi-peak structures previously observed in nitroanilines and other polar aromatic nitro-derivatives.

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.

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).

Shake-up transitions in the XPS spectra of benzonitriles and benzaldehydes

Abstract Shake-up peaks of low-to-moderate intensity accompany core ionisation at the acceptor group of some parasubstituted benzonitriles, X C6 H4 CN [X = H, OCH3, NH2 and N(CH3)2] and benzaldehydes X C6H4 CHO [X = H and N(CH3)2]. Their main energy and intensity trends are reproduced by CNDO/S-CI calculations. The model previously proposed to explain the intense shake-up structure of nitrobenzenes and nitrosobenzenes applies equally well to the present compounds. The trends observed between and along series of compounds are discussed. In benzonitriles the dominant factor determining the energy and intensity of the main satellite line is the energy of the HOMO.

Electronic structure of tert-butyl halides and group 14 derivatives: Electron affinities and dissociative electron attachment

Abstract Electron transmission (ET) and dissociative attachment (DA) spectroscopies are applied to the tert-butyl halides and to derivatives where the central carbon atom is replaced by a heavier group 14 element. In the tert-butyl halide series, the electron affinity increases with increasing size of the halogen atom, while in the group 14 analogues the electron affinity reaches a minimum value for the silicon derivatives. Bound-state and continuum multiple scattering Xα (MS-Xα) calculations reproduce very closely the energies of the resonances observed in the ET spectra. The DA spectra show that the ground anion state of the tert-butyl halides (except for the fluorine derivative) can decay following a dissociative channel, with halogen anion production. In the group 14 derivatives this mechanism is much less active. In some of the compounds investigated, dissociative capture of thermal electrons is also observed.

Electron transmission spectroscopy and MSXα study of closed-shell and open-shell metallocenes

Abstract The electron transmission spectra of ferrocene, chromocene and vanadocene, in the 0–6 eV energy range, are presented and interpreted with the aid of MSXα calculations. Spin—spin interactions induce the formation of sizeably split negative states when an electron is captured into the empty e 1g (d) orbital of the open-shell chromocene and vanadocene.

Spin adducts of group IVB organometallic radicals with Thioketones. an esr study

Abstract The radical adducts resulting from the reaction between group IVB organometallic radicals MR 3 (M = Si, Ge, Sn; R = alkyl or aryl) and a number of thioketones of the chromone and flavone series have been investigated by ESR spectroscopy. The spectral parameters obtained for these species have been compared with those of similar adducts of the corresponding ketones. Both classes of radicals adopt an orthogonal conformation, and the differences between the former and the latter paramagnetic species can be accounted for by the different electronegativity of oxygen and sulphur. the effects of substitution of the heterocyclic oxygen with a sulphur or selenium atom on the spin density distribution are likewise explained. INDO calculations have been carried out on the model systems CH 2 XSiH 3 (X = O, S).

Intense shake-up satellites in the XPS spectra of planar and sterically hindered N-piperidyl-nitrobenzenes and -nitrothiophens

Abstract The shake-up satellites present in the O 1s and N 1s energy regions of the XPS spectra of the planar 4-N-piperidyl-nitrobenzene ( 1 ) and 2-N-piperidyl-5-nitrothiophen ( 3 ), and of the sterically hindered 2-N-piperidyl-nitrobenzene ( 2 ), 2-N-piperidyl-3-nitrothiophen ( 4 ) and 3-N-piperidyl-2-nitrothiophen ( 5 ) and, for comparison, of 2-bromo-3-nitrothiophen ( 6 ) have been analyzed with the help of CNDO/S CI calculations. The spectra of the planar derivatives closely resemble these of the related compounds previously analysed, the main satellites deriving from the transition between the highest occupied molecular orbital (HOMO, localized at the donor-ring moiety) towards the lowest unoccupied one (LUMO, localized at the nitro group). Calculations indicate that in the ortho derivatives new relaxation processes are allowed upon N 1s /NO 2 ionization. In 2 , where the angle of rotation θ is closer to 90° than 60°, the NR 2 lone-pair orbital relaxes through space towards the core-ionized N/NO 2 atom. In 4 and 5 , where 45° ⩽ θ ⩽ 60°, both through-ring and through-space relaxations are present. The assignment of the main N 1s /NO 2 line in 1 is discussed.