Tomáš Vondrák

Frontier occupied orbitals in methyl-substituted fulvene and dimethylenecyclopentenyltitanium complexes by UV-photoelectron spectroscopy and EHT calculations

Abstract UV photoelectron spectra have been obtained for (η 5 -C 5 (CH 3 ) 5 )(C 5 (CH 3 ) 4 CH 2 )TiCH 3 (Ia), (η 5 -C 5 H(CH 3 ) 4 )(C 5 H(CH 3 ) 3 CH 2 )TiCH 3 (Ib), (η 5 -C 5 (CH 3 ) 5 )(C 5 (CH 3 ) 4 CH 2 )Ti (II), (η 5 -C 5 (CH 3 ) 5 ) (C 5 (CH 3 ) 3 (CH 2 ) 2 )Ti (III) and (η 5 -C 5 H 2 (CH 3 ) 3 ) 2 Ti(CH 3 ) 2 (IV) complexes and analyzed on the basis of extended Huckel-MO calculations. The exomethylene groups in fulvene and dimethylenecyclopentenyl ligands are π-coordinated. The HOMO in Ia, Ib, and III is delocalized over the methylene(s) containing ligand and the central atom, and has a bonding nature. The SOMO in II has predominantly the metal d -character. The calculated MO picture is consistent with the band shapes and relative band intensities observed in the low ionization energy region of the PE spectra.

Electronic structure of halogenoferrocenes studied by He(I) photoelectron spectroscopy

Abstract He(I) photoelectron (PE) spectra are reported for chloroferrocene Fe(η-C 5 H 4 Cl)(η-C 5 H 5 ) and 1,1′-dihalogenoferrocenes Fe(η-C 5 H 4 X) 2 (X = Cl, Br). The difference between the ionization potentials ( IP s) of the e 2g ( d ) and a 1g ( d ) level is not affected by the ring substitution. Only the splitting of the e 1u (π) level of the ligand is observed in the spectra. From the magnitudes of the splittings of this level and halogen non-bonding orbitals it is concluded that there is significant mixing of iron p orbitals with the e 1u (π) level. The spectrum of Fe(η-C 5 H 4 Cl) 2 indicates that there is an interaction between the non-bonding out-of-plane chlorine p orbitals.

Charge distribution in icosahedral carboranes: A UV photoelectron spectroscopic study

Abstract The UV photoelectron (PE) investigation of dicarba- closo -dodecaboranes 1,12-C 2 B 10 H 12 , 1-SH-1,12-C 2 B 10 H 11 , 2-SH-1,12-C 2 B 10 H 11 , 1-X-1,2-C 2 B 10 H 11 (X=SH, phenyl), 3-phenyl-1,2-C 2 B 10 H 11 , 4-X-1,2-C 2 B 10 H 11 , 8-X-1,2-C 2 B 10 H 11 , 9-X-1,2-C 2 B 10 H 11 , 9-X-1,7-C 2 B 10 H 11 , (X = SH,CH 3 S) and 1-SH-1,7-C 2 B 10 H 11 is reported. The first two band systems in the PE spectrum of 1, 12-C 2 B 10 H 11 are assigned to cluster π-bonding orbitals and σBH(CH)-bonding orbitals, respectively. The interaction between exo-substituent and cluster π-orbitals is weak and the sulphur lone pair ionization energy depends simply on the mutual position of substituent and carbon atoms. The electron density at particular vertices in 1,2-C 2 B 10 H 12 decreases in the order 9 > 8 > 4 > 3 > 1.

Electronic structure of bis-cyclopentadienyl-titanium dihalides as indicated by UV photoelectron spectroscopy

Abstract The He(I) photoelectron spectra have been obtained for a series of bis(η 5 -cyclopentadienyl)titanium dihalide complexes (C 5 H 5- n (CH 3 ) n ) 2 TiCl 2 ( n = 0, 1, 3, 4, 5) and (C 5 H 5- n (CH 3 ) n ) 2 (TiBr 2 ( n = 0, 1, 3). From the progressive displacements of the photoionization bands upon methylation it follows that the cyclopentadienyl π-orbitals are ionized at lower energies than halogen lone pairs. The cyclopentadienyl a 1 + b 1 symmetry species lie above the a 2 + b 2 ones. The dichloride and dibromide complexes do not differ in the extent of interaction between halogen p orbitals and cyclopentadienyl π=orbitals.

He I photoelectron spectroscopic study of closo-dicarbadodecaboranes and their 9-halogeno derivatives

Abstract He I photoelectron spectra of the closo -carboranes 1,2-C 2 B 10 H 12 and 1,7-C 2 B 10 H 12 , and their 9-halogeno derivatives are reported. The low ionization energy bands are gathered into two groups. The MNDO quantum chemical method provides the same picture and indicates that these bands correspond predominantly to the BH (CH) bonding molecular orbitals. Bromine and iodine bring about an almost uniform increase in ionization energies whereas the shape of the low ionization energy features of the chloro derivatives differ from those of the parent compounds. This effect is interpreted by the conjugative interaction of cluster π-type orbitals with the chlorine non-bonding orbitals.

Electronic structure of ferrocence derivatives studied by He(I) photoelectron spectroscopy and CNDO /2 method

Abstract The effect of substituents in the Cp ligands on the electronic structure has been studied for the 1,1′-disubstituted ferrocenes Fe(CpX) 2 , with X = C 2 H 5 , OCH 3 , CN, COCH 3 , COOCH 3 , OOCCH 3 , CH 2 C 6 H 5 , or C 6 H 5 , by UV photoelectron spectroscopy and by CNDO /2 calculations. The energy gap between the 2 E 2g and 2 AT 1g ion states, 0.36 eV in the parent ferrocene, is affected only by the COCH 3 and COOCH 3 substituents, which lower it to 0.22 and 0.28 eV, respectively. Splitting of e 1u (π) level due to the lowering of the symmetry is the only effect observed in the photoelectron spectra. There is a strong conjugation between the phenyl and cyclopentadienyl β-orbitals in 1,1′-diphenylferrocene. The changes in the a 1g ( d ) ionization energy calculated by the ΔSCF method using CNDO /2 total energies are in a good agreement with the experimental data.

Orbital interactions in 1,3-dithiane derivatives studied by UV photoelectron spectroscopy and MNDO method

Abstract He(I) photoelectron spectra of 1,3-dithiane, 5-hydroxy-1,3-dithiane, 1,3-dithiane-5-one, 1,3-dithiane-1,3-dioxide, 1,3-dithiane-1,3-dioxide-5-one, and 1,4-dithiane are reported. Photoelectron spectra reveal a strong interaction between sulfur non-bonding orbitals and orbitals of the carbonyl group. MNDO calculations show a considerable mixing between the antisymmetric combination of sulphur non-bonding orbitals and the carbonyl non-bonding orbital. The photoelectron spectrum indicates that two conformations of 1,3-dithiane-1,3-dioxide are in equilibrium in the gas phase.

He I photoelectron spectra of [3]-ferrocenophane and its 7-oxa derivative

Abstract He I photoelectron spectra of [3]-ferrocenophane and 6,8-dimethyl-7-oxa-[3]-ferrocenophane have been measured. In comparison with ferrocene the energy gap between 2E′2(a′12e′23) and 2A′1(a′11e′24) is decreased to 0.28 and 0.22 eV due to the destabilization of the a′1(d)-level originating in its more pronounced mixing with ligand orbitals. The increase of photoionization cross section of the a′1(d)-level in the 6,8-dimethyl-7-oxa-[3]-ferrocenophane spectrum supports this interpretation. The band shapes and ionization potentials of ligand π-levels are strongly influenced by the hydrocarbon bridge.

Substituent effects on ionization energies in some 4r-substituted cyclohexanones

Abstract A series of 4R-substituted cyclohexanones were investigated by gas-phase UV photoelectron spectroscopy in order to study the influence of the substituent in the ionization energy of the carbonyl non-bonding orbital. A plot of the IEs vs. the substituents Tafts σI indicated a different behaviour of the alkyl substituted compounds with respect to the others. A sharp linear correlation between IEs and conformational energies for the former compounds is found and discussed in terms of ring flattening and angle deformation along the series -H, -Me, -Et and -t-Bu. Conjugative interactions between polar substituents and the nO(C=O) orbital can explain the departure from linearity of the IEs vs. σIs correlation.